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Sample records for hf isotopic constraints

  1. The Lu-Hf isotopic composition of CHUR and BSE: Tighter constraints from unequilibrated chondrites

    NASA Astrophysics Data System (ADS)

    Bouvier, A.; Vervoort, J. D.; Patchett, J.

    2007-12-01

    = 0.0339 ± 4, 176Hf/177Hf = 0.282816 ± 32, 147Sm/144Nd = 0.1966 ± 10, and 143Nd/144Nd = 0.512639 ± 28. These last values are concordant with the Sm-Nd CHUR and BSE parameters that are currently widely used [4]. Lu-Hf and Sm-Nd isotopic systems involve refractory and lithophile elements, so that the composition of BSE should coincide with that of the CHUR. The Lu/Hf chondrite range in unequilibrated OC and CC is now constrained by ~7%, equivalent to what is found for the paired Sm-Nd system. To refine the Lu-Hf BSE estimate, we need to determine which chondrites are the best representative of BSE. For Sm-Nd isotope systematics, there is wide overlap between the chondrite groups. In constrast, CC have significantly higher Lu/Hf than OC. If we compare with other refractory and lithophile elements, CV, CK, CM and CO chondrites represent the closest composition with Earth's mantle [5]. From O and Cr isotope constraints [6], EC share a common reservoir of formation with the Earth. We will refine the BSE composition based on these observations and also present supplementary data on CI, CR and unequilibrated EC. [1] Blichert-Toft and Albarède, 1997. EPSL, 148, 243-258. [2] Patchett et al., 2004. EPSL, 222, 29-41. [3] Bizzarro et al., 2003. Nature, 421, 931-933. [4] Jacobsen and Wasserburg, 1980. EPSL, 50, 139-155. [5] Palme, 2001. Philo. Trans. R. Soc. Lond., 359, 2061-2075. [6] Trinquier et al., 2007. APJ, 655, 1179-1185.

  2. Hf and Nd isotopes in marine sediments: Constraints on global silicate weathering

    NASA Astrophysics Data System (ADS)

    Bayon, G.; Burton, K. W.; Soulet, G.; Vigier, N.; Dennielou, B.; Etoubleau, J.; Ponzevera, E.; German, C. R.; Nesbitt, R. W.

    2009-01-01

    The combined use of Lu-Hf and Sm-Nd isotope systems potentially offers a unique perspective for investigating continental erosion, but little is known about whether, and to what extent, the Hf-Nd isotope composition of sediments is related to silicate weathering intensity. In this study, Hf and Nd elemental and isotope data are reported for marine muds, leached Fe-oxide fractions and zircon-rich turbidite sands collected off the Congo River mouth, and from other parts of the SE Atlantic Ocean. All studied samples from the Congo fan (muds, Fe-hydroxides, sands) exhibit indistinguishable Nd isotopic composition (ɛ Nd ~ - 16), indicating that Fe-hydroxides leached from these sediments correspond to continental oxides precipitated within the Congo basin. In marked contrast, Hf isotope compositions for the same samples exhibit significant variations. Leached Fe-hydroxide fractions are characterized by ɛ Hf values (from - 1.1 to + 1.3) far more radiogenic than associated sediments (from - 7.1 to - 12.0) and turbidite sands (from - 27.2 to - 31.6). ɛ Hf values for Congo fan sediments correlate very well with Al/K (i.e. a well-known index for the intensity of chemical weathering in Central Africa). Taken together, these results indicate that (1) silicate weathering on continents leads to erosion products having very distinctive Hf isotope signatures, and (2) a direct relationship exists between ɛ Hf of secondary clay minerals and chemical weathering intensity. These results combined with data from the literature have global implications for understanding the Hf-Nd isotope variability in marine precipitates and sediments. Leached Fe-hydroxides from Congo fan sediments plot remarkably well on an extension of the 'seawater array' (i.e. the correlation defined by deep-sea Fe-Mn precipitates), providing additional support to the suggestion that the ocean Hf budget is dominated by continental inputs. Fine-grained sediments define a diffuse trend, between that for igneous

  3. New Hf isotope data from the Jack Hills zircons: constraints on the Hadean crustal evolution

    NASA Astrophysics Data System (ADS)

    Amelin, Y.; Davis, D.; Lee, D.

    2004-05-01

    Here we present a follow-up of our study of the "older" population of detrital zircons from the Jack Hills metaconglomerate W-74 [1]. We report Lu-Hf data for zircon grains, which have been previously analyzed with a number of techniques: BSE and CL imaging, detailed U-Pb SHRIMP geochronology, trace element concentrations, and oxygen isotopic compositions. After completion of non-destructive SIMS analyses and imaging, the zircons were extracted from the mounts, dissolved and analyzed for U-Pb and Lu-Hf using isotope dilution. Twenty five grains were air abraded before digestion, and eight grains were digested without abrasion. Four grains were cut, and the fragments were analyzed for U-Pb and Lu-Hf separately. The 207Pb/206Pb ages determined by isotope dilution vary between 3788-4186 Ma; the maximum SHRIMP spot 207Pb/206Pb ages of the same grains are between 3871-4276 Ma. The spot 207Pb/206Pb ages averaged over each grain are close to the whole grain isotope dilution values. The U-Pb discordance depends mainly on whether the grains were abraded: the median discordance of 27 abraded grains and fragments is 2.7 (the range is -0.4 to 20.2), whereas the median discordance of 11 unabraded grains and fragments is 66.5 (the range is 20.5 to 83.5). The epsilon176Hf values, calculated using the whole grain TIMS 207Pb/206Pb ages and the 176Lu decay constant of 1.865*10-11, are between -1.4 and -10.6. Using maximum SHRIMP spot 207Pb/206Pb ages and the same decay constant yields the range of epsilon176Hf of 0.1 to -8.6. If the decay constant of 1.983*10-11 is used instead, then the range of epsilon176Hf becomes 4.7 to -5.0 using the whole grain ages, or 6.3 to -3.0 using the maximum SHRIMP spot ages. Grain fragment analyses show internal variations of initial 176Hf/177Hf in three grains out of five. This observation is consistent with multi-episodic zircon growth rather than with ancient Pb loss. In the presentation we shall discuss the prospect of reliable interpretation of

  4. Meteorite zircon constraints on the bulk Lu-Hf isotope composition and early differentiation of the Earth.

    PubMed

    Iizuka, Tsuyoshi; Yamaguchi, Takao; Hibiya, Yuki; Amelin, Yuri

    2015-04-28

    Knowledge of planetary differentiation is crucial for understanding the chemical and thermal evolution of terrestrial planets. The (176)Lu-(176)Hf radioactive decay system has been widely used to constrain the timescales and mechanisms of silicate differentiation on Earth, but the data interpretation requires accurate estimation of Hf isotope evolution of the bulk Earth. Because both Lu and Hf are refractory lithophile elements, the isotope evolution can be potentially extrapolated from the present-day (176)Hf/(177)Hf and (176)Lu/(177)Hf in undifferentiated chondrite meteorites. However, these ratios in chondrites are highly variable due to the metamorphic redistribution of Lu and Hf, making it difficult to ascertain the correct reference values for the bulk Earth. In addition, it has been proposed that chondrites contain excess (176)Hf due to the accelerated decay of (176)Lu resulting from photoexcitation to a short-lived isomer. If so, the paradigm of a chondritic Earth would be invalid for the Lu-Hf system. Herein we report the first, to our knowledge, high-precision Lu-Hf isotope analysis of meteorite crystalline zircon, a mineral that is resistant to metamorphism and has low Lu/Hf. We use the meteorite zircon data to define the Solar System initial (176)Hf/(177)Hf (0.279781 ± 0.000018) and further to identify pristine chondrites that contain no excess (176)Hf and accurately represent the Lu-Hf system of the bulk Earth ((176)Hf/(177)Hf = 0.282793 ± 0.000011; (176)Lu/(177)Hf = 0.0338 ± 0.0001). Our results provide firm evidence that the most primitive Hf in terrestrial zircon reflects the development of a chemically enriched silicate reservoir on Earth as far back as 4.5 billion years ago.

  5. Meteorite zircon constraints on the bulk Lu−Hf isotope composition and early differentiation of the Earth

    PubMed Central

    Iizuka, Tsuyoshi; Yamaguchi, Takao; Hibiya, Yuki; Amelin, Yuri

    2015-01-01

    Knowledge of planetary differentiation is crucial for understanding the chemical and thermal evolution of terrestrial planets. The 176Lu−176Hf radioactive decay system has been widely used to constrain the timescales and mechanisms of silicate differentiation on Earth, but the data interpretation requires accurate estimation of Hf isotope evolution of the bulk Earth. Because both Lu and Hf are refractory lithophile elements, the isotope evolution can be potentially extrapolated from the present-day 176Hf/177Hf and 176Lu/177Hf in undifferentiated chondrite meteorites. However, these ratios in chondrites are highly variable due to the metamorphic redistribution of Lu and Hf, making it difficult to ascertain the correct reference values for the bulk Earth. In addition, it has been proposed that chondrites contain excess 176Hf due to the accelerated decay of 176Lu resulting from photoexcitation to a short-lived isomer. If so, the paradigm of a chondritic Earth would be invalid for the Lu−Hf system. Herein we report the first, to our knowledge, high-precision Lu−Hf isotope analysis of meteorite crystalline zircon, a mineral that is resistant to metamorphism and has low Lu/Hf. We use the meteorite zircon data to define the Solar System initial 176Hf/177Hf (0.279781 ± 0.000018) and further to identify pristine chondrites that contain no excess 176Hf and accurately represent the Lu−Hf system of the bulk Earth (176Hf/177Hf = 0.282793 ± 0.000011; 176Lu/177Hf = 0.0338 ± 0.0001). Our results provide firm evidence that the most primitive Hf in terrestrial zircon reflects the development of a chemically enriched silicate reservoir on Earth as far back as 4.5 billion years ago. PMID:25870298

  6. Meteorite zircon constraints on the bulk Lu-Hf isotope composition and early differentiation of the Earth.

    PubMed

    Iizuka, Tsuyoshi; Yamaguchi, Takao; Hibiya, Yuki; Amelin, Yuri

    2015-04-28

    Knowledge of planetary differentiation is crucial for understanding the chemical and thermal evolution of terrestrial planets. The (176)Lu-(176)Hf radioactive decay system has been widely used to constrain the timescales and mechanisms of silicate differentiation on Earth, but the data interpretation requires accurate estimation of Hf isotope evolution of the bulk Earth. Because both Lu and Hf are refractory lithophile elements, the isotope evolution can be potentially extrapolated from the present-day (176)Hf/(177)Hf and (176)Lu/(177)Hf in undifferentiated chondrite meteorites. However, these ratios in chondrites are highly variable due to the metamorphic redistribution of Lu and Hf, making it difficult to ascertain the correct reference values for the bulk Earth. In addition, it has been proposed that chondrites contain excess (176)Hf due to the accelerated decay of (176)Lu resulting from photoexcitation to a short-lived isomer. If so, the paradigm of a chondritic Earth would be invalid for the Lu-Hf system. Herein we report the first, to our knowledge, high-precision Lu-Hf isotope analysis of meteorite crystalline zircon, a mineral that is resistant to metamorphism and has low Lu/Hf. We use the meteorite zircon data to define the Solar System initial (176)Hf/(177)Hf (0.279781 ± 0.000018) and further to identify pristine chondrites that contain no excess (176)Hf and accurately represent the Lu-Hf system of the bulk Earth ((176)Hf/(177)Hf = 0.282793 ± 0.000011; (176)Lu/(177)Hf = 0.0338 ± 0.0001). Our results provide firm evidence that the most primitive Hf in terrestrial zircon reflects the development of a chemically enriched silicate reservoir on Earth as far back as 4.5 billion years ago. PMID:25870298

  7. Combined Sr, Nd, Pb and Hf isotopic constraints on the origin of Shatsky Rise (NW Pacific)

    NASA Astrophysics Data System (ADS)

    Geldmacher, J.; Heydolph, K.; Murphy, D. T.; Romanova, I.; Mahoney, J. J.; Hoernle, K.

    2012-12-01

    The submarine Shatsky Rise plateau in the northwest Pacific Ocean (ca. 1500 km east of Japan) formed during the Late Jurassic to Early Cretaceous. Based on magnetic reversals combined with bathymetric data, the three main volcanic edifices Tamu, Ori and Shirshov massifs are proposed to have successively formed by massive volcanism along a southwest-northeast moving, rapidly spreading triple junction. To investigate a proposed interaction of a possible mantle plume head with the spreading system, Shatsky Rise was drilled during IODP Expedition 324 in 2009 (Expedition 324 Scientists, 2010). Based on major and trace element compositions, the origin of the vast majority of the recovered rocks can be explained by derivation from a normal mid-ocean ridge basalt (MORB)-like source, although a distinct depletion in heavy rare earth elements implies that melting started at greater depth (Sano et al. in press). A small fraction of samples (all from Ori massif), however, exhibit higher ratios of highly over moderately incompatible trace element ratios indicating an enriched (plume?) source. We present compiled Sr, Nd, Pb and Hf isotope ratios from all three volcanic edifices of Shatsky Rise and will discuss them in the light of the new trace element study. Most isotope data overlap with Pacific MORB composition although regional variations can be seen. Whereas lavas from three drill sites on the oldest edifice, Tamu massif, yield fairly uniform compositions, a wider spread is found for lavas erupted on the younger edifices, Ori and Shirshov, suggesting that the source has become more heterogeneous with time (also consistent with the trace element data). This variation could reflect a decreasing degree of melting (and therefore less homogenization of inherent plume heterogeneities) or less effective stirring and mixing during the interaction of the spreading center with a waning plume head. Interestingly, lavas from the Ori and Shirshov massifs have generally lower 143Nd/144Nd

  8. Hadean crustal evolution revisited: New constraints from Pb-Hf isotope systematics of the Jack Hills zircons

    NASA Astrophysics Data System (ADS)

    Kemp, A. I. S.; Wilde, S. A.; Hawkesworth, C. J.; Coath, C. D.; Nemchin, A.; Pidgeon, R. T.; Vervoort, J. D.; DuFrane, S. A.

    2010-07-01

    Detrital zircon crystals from the Jack Hills metasedimentary belt, Western Australia, are the only surviving vestiges of Hadean crust and represent an extraordinary archive into the nature of the early Earth. We report the results of an in situ isotopic study of 68 Jack Hills zircons in which the Hf and Pb isotope ratios were measured concurrently, allowing a better integration of isotope tracer information ( 176Hf/ 177Hf) with crystallization age ( 207Pb/ 206Pb). These data are augmented by Hf isotope data from zircons of the surrounding Narryer gneisses (3.65-3.30 Ga) and from Neoarchaean granites that intrude the Jack Hills belt. The detrital zircons define a subchondritic ɛHf-time array that attests to a far simpler evolution for the Hadean Earth than claimed by recent studies. This evolution is consistent with the protracted intra-crustal reworking of an enriched, dominantly mafic protolith that was extracted from primordial mantle at 4.4-4.5 Ga, perhaps during the solidification of a terrestrial magma ocean. There is no evidence for the existence of strongly depleted Hadean mantle, or for juvenile input into the parental magmas to the Jack Hills zircons. This simple Hf isotope evolution is difficult to reconcile with modern plate tectonic processes. Strongly unradiogenic Hf isotope compositions of zircons from several Archaean gneiss terranes, including the Narryer and Acasta gneisses, suggest that Hadean source reservoirs were tapped by granitic magmas throughout the Archaean. This supports the notion of a long-lived and globally extensive Hadean protocrust that may have comprised the nuclei of some Archaean cratons.

  9. Mantle evolution on Mars: Constraints from Lu-Hf and Sm-Nd isotope systematics of SNC meteorites

    NASA Astrophysics Data System (ADS)

    Scherer, E. E.; Kurahashi, E.; Mezger, K.

    2012-12-01

    The long-lived 176Lu-176Hf and 147Sm-143Nd isotope systems are commonly employed to track the evolution of complementary mantle and crust reservoirs. The four elements involved are refractory and lithophile, and thus their relative abundances are not expected to have been changed by accretion or core formation. Subsequent silicate differentiation processes, however, e.g., the formation of crust by extraction of melts from the mantle, will fractionate Lu/Hf and Sm/Nd. This typically leaves a depleted mantle with higher Lu/Hf and Sm/Nd values than those of the undifferentiated, presumably chondritic parental reservoir. On the other hand, these same values in crustal rocks tend to be lower than those of their source. (Apparent exceptions are the Martian shergottites, which tend to have lower Lu/Hf as expected, but Sm/Nd higher than their presumed sources. Such decoupling of the two isotope systems may be explained by two-stage melting [e.g., 1, 5].) The ensuing chemical variability among secondary and later generation silicate reservoirs causes their isotopic compositions (e.g., 176Hf/177Hf and 143Nd/144Nd) to diverge from that of the bulk silicate planet over hundreds of millions of years. The resulting isotopic diversity preserved (SNC) meteorites is being used to constrain the differentiation history, melting mineralogy, and dynamics of the Martian mantle [e.g., 1-8]. However, interpretations based on the initial isotope compositions of Hf and Nd strongly depend on the accuracy of crystallization ages. The ages of shergottites in particular are debated (e.g., [3,4,7]). To resolve this issue and gain a better understanding of Martian mantle evolution, we are investigating the Lu-Hf and Sm-Nd systematics of bulk SNC meteorites and constructing internal (mineral) isochrons. Eleven bulk Martian meteorites (5 shergottites, 4 nakhlites, and 2 chassignites) were digested without prior leaching in high-pressure autoclaves for 5 days. Initial ɛ176Hf and ɛ143Nd values

  10. Evolution of granitoids in the Catalina metamorphic core complex, southeastern Arizona: U-Pb, Nd, and Hf isotopic constraints

    NASA Astrophysics Data System (ADS)

    Fornash, Katherine F.; Patchett, P. Jonathan; Gehrels, George E.; Spencer, Jon E.

    2013-06-01

    The Santa Catalina Mountains, SE Arizona, was one of the first metamorphic core complexes to be described. Despite its status as a type example, relatively little is known about precise ages and origins of the intrusive rocks that make up most of the crystalline core. U-Pb and Hf isotopic data by laser ablation-inductively coupled plasma-mass spectrometry from zircons and Nd isotopic results from whole rocks were obtained for 12 granitoids ranging from 1,440 to 26 Ma. Results confirm that the 1.44-Ga Oracle Granite extends through the Catalina Range as variably mylonitic granite and banded gneiss. Laramide intrusions (67-73 Ma) display initial ɛNd values -5 to -8 and ɛHf from -7.5 to -9. Magmatic ages for the prominent white granite sills of the Wilderness suite are 46-57 Ma, in agreement with Terrien (2012), and these granites have initial ɛNd values -8 to -10 and ɛHf from -7 to -14. Lastly, the undeformed Catalina Granite has an age of 26 Ma, with an initial ɛNd and ɛHf of -6 and -8, respectively. Our Nd results agree with limited results from Farmer and DePaolo (89:10141-10160, 1984). Although the Catalina Granite seems to have a significant juvenile component based on Nd and Hf, most of the Laramide and Wilderness intrusions contain Nd and Hf compositions lying close to the evolution of 1.44-Ga Oracle Granites, a fact that is confirmed by the U-Pb data, which show both 1.7- and 1.4-Ga zircon cores in these samples, with 1.4 Ga as the dominant core age. In order to become the dominant source of most of the 72-45-Ma magmas, the Oracle pluton must not only extend across the whole Catalina region, but also have abundant deep-seated equivalents to provide magma sources.

  11. Zircon U-Pb, O, and Hf isotopic constraints on Mesozoic magmatism in the Cyclades, Aegean Sea, Greece

    NASA Astrophysics Data System (ADS)

    Fu, Bin; Bröcker, Michael; Ireland, Trevor; Holden, Peter; Kinsley, Leslie P. J.

    2015-01-01

    Compared to the well-documented Cenozoic magmatic and metamorphic rocks of the Cyclades, Aegean Sea, Greece, the geodynamic context of older meta-igneous rocks occurring in the marble-schist sequences and mélanges of the Cycladic Blueschist Unit is as yet not fully understood. Here, we report O-Hf isotopic compositions of zircons ranging in age from ca. 320 Ma to ca. 80 Ma from metamorphic rocks exposed on the islands of Andros, Ios, Sifnos, and Syros with special emphasis on Triassic source rocks. Ion microprobe (SHRIMP II) single spot oxygen isotope analysis of pre-Cretaceous zircons from various felsic gneisses and meta-gabbros representing both the marble-schist sequences and the mélanges of the study area yielded a large range in δ18O values, varying from 2.7 ‰ to 10.1 ‰ VSMOW, with one outlier at -0.4 %. Initial ɛHf values (-12.5 to +15.7) suggest diverse sources for melts formed between Late Carboniferous to Late Cretaceous time that record derivation from mantle and reworked older continental crust. In particular, variable δ18O and ɛHf( t) values for Triassic igneous zircons suggest that magmatism of this age is more likely rift- than subduction-related. The significant crustal component in 160 Ma meta-gabbros from Andros implies that some Jurassic gabbroic rocks of the Hellenides are not part of SSZ-type (supra-subduction zone) ophiolites that are common elsewhere along the margin of the Pelagonian zone.

  12. Plume-Lithosphere Interaction beneath the Snake River Plain, Idaho: Constraints from Pb, Sr, Nd, and Hf Isotopes

    NASA Astrophysics Data System (ADS)

    Jean, M. M.; Hanan, B. B.; Shervais, J. W.

    2011-12-01

    The Yellowstone-Snake River Plain (YSRP) volcanic province links 17 million years of volcanic activity that extends from the Owyhee Plateau in western Idaho/eastern Oregon to its current terminus underlying the Yellowstone Plateau. This investigation presents new Strontium, Neodymium, Lead, and Hafnium isotopic compositions of 25 basalts that represent four distinct areas of the YSRP (i.e., eastern province, central province, western province, Owyhee Plateau), which transect the ancient cratonic boundary of North America. The purpose of this study is to test and refine models for plume-lithosphere interaction and determines the mantle origin for YSRP basalts. New results shows: (1) low-K tholeiites from the eastern, central, and western SRP have ɛNd (-2 to -5.5), 87Sr/86Sr (0.7060-0.7071) and similar Pb-isotopes [206Pb/204Pb (17.8-18.6), 207Pb/204Pb (15.5-15.66), 208Pb/204Pb (38.4-39.1)]; (2) central SRP tholeiites are enriched in 208Pb/204Pb (~38.5-38.9), relative to eastern SRP basalts and define a 208Pb/204Pb trend, intermediate between the eastern SRP and Craters of the Moon lavas; (3) western SRP high-K basalts are depleted in ɛNd (> -1) and 87Sr/86Sr (0.7050-0.7057), relative to low-K tholeiites, and plot closer to "bulk silicate earth," but are enriched in 206Pb/204Pb (18.66-18.71), and have 207Pb/204Pb (15.62-15.65) and 208Pb/204Pb (39.1-39.2) isotope ratios similar to high-K basalts of Smith Prairie (Boise River Group 2); (4) Silver City basalt (>16.6 Ma) overlaps in Pb-isotope space with Imnaha basalt compositions (Columbia River Basalt Group); (5) new 177Hf/176Hf isotopic data lie above and parallel to the Mantle array in Nd and Hf isotope space and define a linear trend between Leucite Hills lavas and OIB basalts (i.e., Steens and Hawaii); (6) these basalts follow a systematic geographic pattern: eastern and central plain low-K tholeiites have low ɛNd (-3 to -5) and intermediate 206Pb/204Pb (~17.7-18.5), while western plain low-K tholeiites are

  13. Sr-Nd-Hf-Pb isotopic constraints on the origin of silicic lavas in the northern Cascade Arc

    NASA Astrophysics Data System (ADS)

    Martindale, M.; Mullen, E.; Weis, D.

    2015-12-01

    The Cascade Arc is the type-locality for a 'hot' subduction zone, where the downgoing slab is young and subduction is relatively slow; a unique setting for studying the controls on silicic (>56 wt% SiO2) magma genesis [1,2]. We present high precision Sr-Nd-Hf-Pb isotopic and trace element data for silicic lavas and country rocks from the major centres of the Garibaldi Volcanic Belt (GVB) in British Columbia, which are hosted by the Mesozoic Coast Plutonic Complex and accreted Coast Belt terranes. In isotopic plots, the silicic GVB lavas define mixing curves between northern Cascadia Basin sediment [3] and Juan de Fuca MORB. The silicic GVB lavas have lower ɛNd, and higher ɛHf, 87Sr/86Sr, 208Pb/204Pb and 207Pb/204Pb for a given 206Pb/204Pb than co-existing alkalic mafic lavas [2,4] which define a separate isotopic cluster. The alkalic mafic lavas have OIB-like trace element compositions [2,4], but the silicic lavas are calc-alkaline with a typical 'arc' trace element signature. Geochemical systematics suggest that a mafic, isotopically 'depleted' contaminant may be affecting the composition of GVB silicic lavas. However, modelling indicates that slab melts do not constitute a major component of the lavas despite high slab temperatures. Geochemical models also rule out the accreted Coast Belt terranes as an assimilant. However, AFC modelling using 147 Ma Cloudburst quartz diorite [5] as the assimilant can explain both the trace element and isotopic compositional range displayed by GVB silicic magmas, consistent with the Coast Plutonic Complex as a major component of the deep crust in this region. Crustal assimilation would have partially overprinted any alkalic mantle-derived signature of parental magmas, while imparting a calc-alkaline arc signature to resulting silicic magmas. [1] Green & Harry (1999) EPSL, 171; [2] Mullen & Weis (2013) G3, 14; [3] Carpentier et al. (2014) Chem Geol, 382; [4] Mullen & Weis (2015) EPSL, 414; [5] Friedman & Armstrong (1995) GSA

  14. LU-HF Age and Isotope Systematics of ALH84001

    NASA Technical Reports Server (NTRS)

    Righter, M.; Lapen, T. J.; Brandon, A. D.; Beard, B. L.; Shafer, J. T.; Peslier, A. H.

    2009-01-01

    Allan Hills (ALH) 84001 is an orthopyroxenite that is unique among the Martian meteorites in having the oldest inferred crystallization age (approx..4.5 to 4.0 Gyr) [e.g., 1-6 and references therein 7]. Its ancient origin makes this stone a critical constraint on early history of Mars, in particular the evolution of different planetary crust and mantle reservoirs. However, because there is significant variability in reported crystallization ages, determination of initial isotope compositions is imprecise making assessment of planetary reservoirs difficult. Here we report a new Lu-Hf mineral isochron age, initial Hf-176/Hf-177 isotope composition, and inferred Martian mantle source compositions for ALH84001 that place constraints on longlived source reservoirs for the enriched shergottite suite of Martian meteorites including Shergotty, Zagami, NWA4468, NWA856, RBT04262, LAR06319, and Los Angeles. Sm-Nd isotope analyses are under way for the same mineral aliquots analyzed for Lu-Hf. The Lu-Hf system was utilized because Lu and Hf are both lithophile and refractory and are not easily redistributed during short-lived thermal pulses associated with shock metamorphism. Moreover, chromite has relatively modest Hf concentrations with very low Lu/Hf ratios [9] yielding tight constraints on initial Hf-176/Hf-177 isotope compositions

  15. Zircon and baddeleyite from the economic ultramafic-mafic Noril'sk-1 intrusion (Russia): Hf-isotope constraints on source composition

    NASA Astrophysics Data System (ADS)

    Malitch, K. N.; Belousova, E. A.; Badanina, I. Yu.; Griffin, W. L.

    2012-04-01

    subcontinental lithospheric source probably at least Neoproterozoic in age. We propose that the SCLM component is especially prominent in the mineralized portions of the intrusion. This is consistent with the suggestion of Zhang et al (2008) that ancient cratonic lithospheric mantle may have contributed significantly to the PGE and Ni budget of the "fertile" Siberian Large Igneous Province. Small population of zircons from the gabbro-diorite show the least 'radiogenic' Hf-isotope values, indicating the input of a distinctly older lithospheric, possibly crustal, component, being consistent with a hybrid nature of this lithology. Our approach for deciphering the origin of zircon and baddeleyite from mafic and ultramafic rocks provides a unique set of U-Pb and Hf-isotope constraints on temporal evolution and petrologic history of the Noril'sk-1 intrusion. The study was supported by Uralian Branch of Russian Academy of Sciences (12-U-5-1038). Refereneces: Campbell I.H., Czamanske G.K., Fedorenko V.A., Hill R.I., Stepanov V. (1992) Synchronism of the Siberian traps and the Permian-Triassic boundary. Science 255, 1760-1763. Griffin W.L., Wang X., Jackson S.E., Pearson N.J., O'Reilly S.Y., Xu X., Zhou X. (2002) Zircon chemistry and magma genesis, SE China: in-situ analysis of Hf isotopes, Pingtan and Tonglu igneous complexes. Lithos 61, 237-269. Kamo S.L., Czamanske G.K., Krogh T.E. (1996) A minimum U-Pb age for Siberian flood-basalt volcanism. Geochim. Cosmochim. Acta 60, 3505-3511. Malitch K.N., Badanina I.Yu., Belousova E.A., Tuganova E.V. (2012) Results of U-Pb dating of zircon and baddeleyite from the Noril'sk-1 ultramafic-mafic intrusion (Russia). Russian Geology and Geophysics 53(2), 123-130. Zhang M., O'Reilly S.Y., Wang K-L., Hronsky J., Griffin W.L. (2008) Flood basalts and metallogeny: The lithospheric connection. Earth-Science Reviews 86, 145-174.

  16. Magmatism as a response to exhumation of the Priest River complex, northern Idaho: Constraints from zircon U-Pb geochronology and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Stevens, L. M.; Baldwin, J. A.; Crowley, J. L.; Fisher, C. M.; Vervoort, J. D.

    2016-10-01

    Zircon and monazite U-Pb geochronology and zircon Hf isotopes place constraints on the temporal and source relationships between crustal anatexis, magmatism, and exhumation of the Priest River metamorphic core complex, northern Idaho. Granitoids that intruded the migmatitic, pelitic Hauser Lake gneiss include the < 76.5 ± 0.1 Ma Spokane granite, 50.13 ± 0.02 Ma Silver Point quartz monzonite, c. 47.9 Ma Wrencoe granodiorite, < 46.4 ± 1.8 Ma Rathdrum granite, and a < 49.8 ± 0.4 Ma leucocratic dike. Cretaceous magmatism preceded the c. 64 Ma peak metamorphism (recorded by monazite) of the Hauser Lake gneiss, whereas discrete pulses of Eocene magmatic activity post-date the onset of exhumation by 10 Ma. The relative timing of pluton emplacement in the Priest River complex indicates that it was primarily a response to decompression rather than a cause. The mylonitized Silver Point and undeformed Wrencoe plutons bracket the end of a rapid phase of exhumation to c. 50-48 Ma. Zircon εHf(i) values and Lu-Hf isotope evolution indicate that the Silver Point and Wrencoe plutons crystallized from homogeneous magmas sourced from Archean-Proterozoic basement orthogneisses, whereas the Spokane granite and two leucocratic units appear to have been produced by partial melting of the Hauser Lake gneiss. Comparison of the Priest River complex with other deeply exhumed northern Cordilleran complexes indicates variability in the timing and, therefore, relative influences of partial melting and magmatism on the initiation of exhumation, which must be accounted for in numerical models of metamorphic core complex formation and evolution.

  17. Source components and magmatic processes in the genesis of Miocene to Quaternary lavas in western Turkey: constraints from HSE distribution and Hf-Pb-Os isotopes

    NASA Astrophysics Data System (ADS)

    Aldanmaz, Ercan; Pickard, Megan; Meisel, Thomas; Altunkaynak, Şafak; Sayıt, Kaan; Şen, Pınar; Hanan, Barry B.; Furman, Tanya

    2015-08-01

    Hf-Pb-Os isotope compositions and highly siderophile element (HSE) abundance variations are used to evaluate the mantle source characteristics and possible effects of differentiation processes in lavas from western Turkey, where the eruption of Late Miocene to Quaternary OIB-type intraplate mafic alkaline lavas followed pre-Middle Miocene convergent margin-type volcanism. Concentrations of Os, Ir, and Ru (IPGE) in the OIB-type intraplate lavas decrease with fractionation for primitive melts (MgO > 10 wt%), suggesting that these elements reside predominantly in olivine and associated HSE retaining trace phases and behave compatibly during olivine-dominated fractionation. Fractional crystallization trends indicate distinctly lower bulk partition coefficients for IPGE in more evolved lavas, possibly reflecting a change in the fractionating assemblages. Pd and Re in the primitive melts display negative correlations with MgO, demonstrating moderately incompatible behavior of these elements during fractionation, while the significantly scattered variation in Pt against MgO may indicate the effects of micronuggets of a Pt-rich alloy. Os-rich alkaline primary lavas (>50 ppt Os) exhibit a limited range of 187Os/188Os (0.1361-0.1404), with some xenolith-bearing lavas displaying depletions in 187Os/188Os (0.1131-0.1232), suggesting slight compositional modification of primitive melts through contamination with highly depleted, Os-rich mantle lithosphere. More radiogenic Os isotope ratios (187Os/188Os > 0.1954) in the evolved lavas reflect contamination of the magmas by high187Os/188Os crustal material during shallow differentiation. The OIB-type lavas show limited variations in Hf and Pb isotopes with 176Hf/177Hf = 0.282941-0.283051, 206Pb/204Pb = 18.683-19.091, 207Pb/204Pb = 15.579-15.646, 208Pb/204Pb = 38.550-38.993; 176Hf/177Hf ratios correlate negatively with 208Pb*/206Pb*, suggesting the effects of similar mantle processes on the evolution of time-integrated Th/U and Lu/Hf

  18. Re-Os and Lu-Hf isotopic constraints on the formation and age of mantle pyroxenites from the Bohemian Massif

    NASA Astrophysics Data System (ADS)

    Ackerman, Lukáš; Bizimis, Michael; Haluzová, Eva; Sláma, Jiří; Svojtka, Martin; Hirajima, Takao; Erban, Vojtěch

    2016-07-01

    We report on the Lu-Hf and Re-Os isotope systematics of a well-characterized suite of spinel and garnet pyroxenites from the Gföhl Unit of the Bohemian Massif (Czech Republic, Austria). Lu-Hf mineral isochrons of three pyroxenites yield undistinguishable values in the range of 336-338 Ma. Similarly, the slope of Re-Os regression for most samples yields an age of 327 ± 31 Ma. These values overlap previously reported Sm-Nd ages on pyroxenites, eclogites and associated peridotites from the Gföhl Unit, suggesting contemporaneous evolution of all these HT-HP rocks. The whole-rock Hf isotopic compositions are highly variable with initial εHf values ranging from - 6.4 to + 66. Most samples show a negative correlation between bulk rock Sm/Hf and εHf and, when taking into account other characteristics (e.g., high 87Sr/86Sr), this may be explained by the presence of recycled oceanic sediments in the source of the pyroxenite parental melts. A pyroxenite from Horní Kounice has decoupled Hf-Nd systematics with highly radiogenic initial εHf of + 66 for a given εNd of + 7.8. This decoupling is consistent with the presence of a melt derived from a depleted mantle component with high Lu/Hf. Finally, one sample from Bečváry plots close to the MORB field in Hf-Nd isotope space consistent with its previously proposed origin as metamorphosed oceanic gabbro. Some of the websterites and thin-layered pyroxenites have variable, but high Os concentrations paralleled by low initial γOs. This reflects the interaction of the parental pyroxenitic melts with a depleted peridotite wall rock. In turn, the radiogenic Os isotope compositions observed in most pyroxenite samples is best explained by mixing between unradiogenic Os derived from peridotites and a low-Os sedimentary precursor with highly radiogenic 187Os/188Os. Steep increase of 187Os/188Os at nearly uniform 187Re/188Os found in a few pyroxenites may be connected with the absence of primary sulfides, but the presence of minor

  19. Lu-Hf constraints on the evolution of lunar basalts

    NASA Technical Reports Server (NTRS)

    Fujimaki, H.; Tatsumoto, M.

    1984-01-01

    It is shown that a cumulate-remelting model best explains the recently acquired data on the Lu-Hf systematics of lunar mare basalts. The model is constructed using Lu and Hf concentration data and is strengthened by Hf isotopic evidence of Unruh et al. (1984). It is shown that the similarity in MgO/FeO ratios and Cr2O3 content in high-Ti and low-Ti basalts are not important constraints on lunar basalt petrogenesis. The model demonstrates that even the very low Ti or green glass samples are remelting products of a cumulate formed after at least 80-90 percent of the lunar magma ocean had solidified. In the model, all the mare basalts and green glasses were derived from 100-150 km depth in the lunar mantle. The Lu-Hf systematics of KREEP basalts clearly indicate that they would be the final residual liquid of the lunar magma ocean.

  20. Mantle Heterogeneity and Mixing Beneath the Bouvet Triple Junction Region: Hf Isotope Constraints from the Westernmost Southwest Indian Ridge (0-11deg.E)

    NASA Astrophysics Data System (ADS)

    Janney, P. E.; le Roex, A. P.

    2013-12-01

    We have undertaken new Hf (and supplementary Sr, Nd and Pb) isotope and trace element measurements of MORB from the westernmost Southwest Indian Ridge (SWIR; 0 to 11 deg. E) in order to clarify mixing relationships and the effect of the Bouvet and other local hot spots on the composition of the upper mantle beneath the Bouvet Triple Junction (BTJ) region. The new data are fully consistent with the findings of previous studies (le Roex et al., J. Petrol., 1983; Kurz et al., GCA, 1998) that isotopic heterogeneity in this region is largely explained by mixing between a moderately depleted mantle source (i.e., ɛHf ≈ +14, ɛNd ≈ +9, 87Sr/86Sr ≈ 0.7026; 206Pb/204Pb ≈ 18.5) and an enriched component isotopically similar to Bouvet OIB. Unlike the pattern expressed by He isotopes (Kurz et al., GCA, 1998; Georgen et al., EPSL, 2003) the strength of the Bouvet hot spot signature in terms of Hf-Sr-Nd-Pb isotope ratios is not well correlated with distance from Bouvet Island along ridge, except in the most general sense. Some MORB from 0-11E do approach the Hf-Sr-Nd-Pb isotopic composition of Bouvet OIB. However, the most extreme isotopic compositions (with ɛHf values that are slightly lower than, and Nd and Pb isotope compositions that are equivalent to, those of Bouvet OIB), are actually found in MORB from the 11-16E oblique spreading segment of the SWIR (le Roex et al., CMP, 1992; Janney et al., J. Petrol., 2005), located further from Bouvet Island than the segments at 0-11E. The lack of a strong correlation between the radiogenic isotope ratios of SWIR MORB and proximity to the Bouvet hot spot in this region supports the notion that local conditions of melting of a lithologically heterogeneous mixture of enriched, Bouvet hot spot-derived and depleted mantle materials plays the dominant role in controlling the radiogenic isotope composition of western SWIR MORB (le Roex et al., CMP, 1992; Salters & Dick, Nature, 2002). The depleted mantle present beneath the western

  1. Sr-Nd-Hf-Pb Isotopic Constraints on the Role of South China Sea Sediments in Mantle Wedge Metasomatism Beneath the North Luzon Arc

    NASA Astrophysics Data System (ADS)

    Kuo, T.; Yang, H.; Lee, D.; Lai, Y.

    2007-12-01

    Recycling sediments into mantle through subduction zones causes mantle heterogeneity and is critical on chemical evolution of the Earth. Because the compositions of subducted sediments vary significantly between subduction zones, there is a need to characterize the sediments from individual subduction zones and evaluate their contributions to mantle wedge metasomatism. This study investigates the role of South China Sea (SCS) sediments in the chemical characteristics of the North Luzon arc (NLA) magmatism. Thirty-five sediment samples (0-35 Ma) recovered by ODP Leg 184 at sites 1148 and 1147 were analyzed for Sr, Nd, Hf, and Pb isotope ratios and trace element abundances. Results were compared to the data of the North Luzon arc lavas to establish models for mantle wedge metasomatism. The NLA lavas deviate from the terrestrial array to higher 176Hf/177Hf values at a given 143Nd/144Nd value, consistent with involving subducted sediments in source regions. Since Hf in the subducted slabs cannot be transported to mantle wedges by hydrous fluids, slab-derived siliceous melts are the most probable metasomatic agents. This algorithm leads to three metasomatism models: (I) addition of bulk sediments to depleted mantle, (II) depleted mantle metasomatized by sediment-derived melts, and (III) depleted mantle metasomatized by melts derived from sediments and altered oceanic crust (AOC). Although not considered in model calculations, the contributions of sediment-derived and AOC-derived fluids are also addressed qualitatively. The first model results in mixing curves overlapping with the mantle array in Sr versus Nd and Hf isotope plots, inconsistent with the distributions of the NLA lavas, which deviate from mantle array to lower 143Nd/144Nd and 176Hf/177Hf values. The involvement of sediment-derived melts (Model II) leads to source compositions with larger deviation from the NLA lavas in Sr versus Nd and Hf isotope plots, because sediment-derived melts have higher Sr/Nd and

  2. Linking south China to northern Australia and India on the margin of Gondwana: Constraints from detrital zircon U-Pb and Hf isotopes in Cambrian strata

    NASA Astrophysics Data System (ADS)

    Xu, Yajun; Cawood, Peter A.; Du, Yuansheng; Hu, Lisha; Yu, Wenchao; Zhu, Yanhui; Li, Wenchao

    2013-12-01

    sedimentary rocks in the southern part of the South China Craton were derived from a source that lay to the south or southeast, beyond the current limits of the craton and which is no longer preserved nearby. U-Pb ages and Hf isotope data on detrital zircons from the Cambrian sequence define two distinctive age peaks at 1120 Ma and 960 Ma, with ɛHf(t) values for each group identical to the coeval detrital zircons from Western Australia and the Tethyan Himalaya zone, respectively. The circa 1120 Ma detrital zircons were most likely derived from the Wilkes-Albany-Fraser belt between southwest Australia and Antarctica, whereas the circa 960 Ma detrital zircons could have been sourced from the Rayner-Eastern Ghats belt between India and Antarctica. Derivation of detritus from these sources suggests that south China was located at the nexus between India, Antarctica, and Australia, along the northern margin of East Gondwana during the Cambrian.

  3. Coupled Nd-142, Nd-143 and Hf-176 Isotopic Data from 3.6-3.9 Ga Rocks: New Constraints on the Timing of Early Terrestrial Chemical Reservoirs

    NASA Technical Reports Server (NTRS)

    Bennett, Vickie C.; Brandon, alan D.; Hiess, Joe; Nutman, Allen P.

    2007-01-01

    Increasingly precise data from a range of isotopic decay schemes, including now extinct parent isotopes, from samples of the Earth, Mars, Moon and meteorites are rapidly revising our views of early planetary differentiation. Recognising Nd-142 isotopic variations in terrestrial rocks (which can only arise from events occurring during the lifetime of now extinct Sm-146 [t(sub 1/2)=103 myr]) has been an on-going quest starting with Harper and Jacobsen. The significance of Nd-142 variations is that they unequivocally reflect early silicate differentiation processes operating in the first 500 myr of Earth history, the key time period between accretion and the beginning of the rock record. The recent establishment of the existence of Nd-142 variations in ancient Earth materials has opened a new range of questions including, how widespread is the evidence of early differentiation, how do Nd-142 compositions vary with time, rock type and geographic setting, and, combined with other types of isotopic and geochemical data, what can Nd-142 isotopic variations reveal about the timing and mechanisms of early terrestrial differentiation? To explore these questions we are determining high precision Nd-142, Nd-143 and Hf-176 isotopic compositions from the oldest well preserved (3.63- 3.87 Ga), rock suites from the extensive early Archean terranes of southwest Greenland and western Australia.

  4. Zircon U-Pb ages and Hf-O isotopic composition of migmatites from the Zanjan-Takab complex, NW Iran: Constraints on partial melting of metasediments

    NASA Astrophysics Data System (ADS)

    Moghadam, Hadi Shafaii; Li, Xian-Hua; Stern, Robert J.; Ghorbani, Ghasem; Bakhshizad, Farzaneh

    2016-01-01

    We study migmatites and other metamorphic rocks in the Zanjan-Takab region of NW Iran and use these results to report the first evidence of Oligocene core complex formation in Iran. Four samples of migmatites associated with paragneisses, including leucosomes and associated para-amphibolite melanosomes were selected for U-Pb dating and Hf-O isotopic analysis. Zircon cores - interpreted as originally detrital zircons - have variable ages that peak at ca. 100-110 Ma, but their sedimentation age - indicated by the youngest 206Pb/238U ages - is ca. 35-40 Ma. New zircons associated with incipient melting occur as overgrowths around zircon cores and/or as newly grown grains. Morphologies and internal structures suggest that rim growth and formation of new zircons were associated with partial melting. All four samples contain zircons with rims that yield 206Pb/238U ages of 28-25 Ma, indicating that partial melting occurred in Late Oligocene time. δ18O values for zircon rims vary between 8.2 and 12.3‰, significantly higher than expected for mantle inputs (δ18O ~ 6‰) and consistent with equilibrium with surface materials. Zircon rims yield εHf(t) between 2.2 and 12.4 and two-stage Hf model ages of ~ 448-562 Ma, indicating that the region is underlain by Cadomian-Caledonian crust. According to the Hf-O isotopic values, the main mechanism forming zircon rims was dissolution of pre-existing detrital zircons with reprecipitation of new zircon shortly thereafter. Oligocene ages indicate that partial melting accompanied core complex formation in the Zanjan-Takab region. Extension, melting, and core complex formation in south-central Iran are Eocene in age, but younger ages of Oligocene-Miocene in NW Iran and Turkey indicate that extension was distributed throughout the region during Cenozoic time.

  5. TUNGSTEN ISOTOPIC COMPOSITIONS IN STARDUST SiC GRAINS FROM THE MURCHISON METEORITE: CONSTRAINTS ON THE s-PROCESS IN THE Hf-Ta-W-Re-Os REGION

    SciTech Connect

    Avila, Janaina N.; Ireland, Trevor R.; Holden, Peter; Lugaro, Maria; Buntain, Joelene; Gyngard, Frank; Zinner, Ernst; Amari, Sachiko; Cristallo, Sergio; Karakas, Amanda

    2012-01-01

    We report the first tungsten isotopic measurements in stardust silicon carbide (SiC) grains recovered from the Murchison carbonaceous chondrite. The isotopes {sup 182,183,184,186}W and {sup 179,180}Hf were measured on both an aggregate (KJB fraction) and single stardust SiC grains (LS+LU fraction) believed to have condensed in the outflows of low-mass carbon-rich asymptotic giant branch (AGB) stars with close-to-solar metallicity. The SiC aggregate shows small deviations from terrestrial (= solar) composition in the {sup 182}W/{sup 184}W and {sup 183}W/{sup 184}W ratios, with deficits in {sup 182}W and {sup 183}W with respect to {sup 184}W. The {sup 186}W/{sup 184}W ratio, however, shows no apparent deviation from the solar value. Tungsten isotopic measurements in single mainstream stardust SiC grains revealed lower than solar {sup 182}W/{sup 184}W, {sup 183}W/{sup 184}W, and {sup 186}W/{sup 184}W ratios. We have compared the SiC data with theoretical predictions of the evolution of W isotopic ratios in the envelopes of AGB stars. These ratios are affected by the slow neutron-capture process and match the SiC data regarding their {sup 182}W/{sup 184}W, {sup 183}W/{sup 184}W, and {sup 179}Hf/{sup 180}Hf isotopic compositions, although a small adjustment in the s-process production of {sup 183}W is needed in order to have a better agreement between the SiC data and model predictions. The models cannot explain the {sup 186}W/{sup 184}W ratios observed in the SiC grains, even when the current {sup 185}W neutron-capture cross section is increased by a factor of two. Further study is required to better assess how model uncertainties (e.g., the formation of the {sup 13}C neutron source, the mass-loss law, the modeling of the third dredge-up, and the efficiency of the {sup 22}Ne neutron source) may affect current s-process predictions.

  6. Geochronological, geochemical and Nd-Hf isotopic constraints on the petrogenesis of Late Cretaceous A-type granites from the southeastern coast of Fujian Province, South China

    NASA Astrophysics Data System (ADS)

    Zhao, Jiao-Long; Qiu, Jian-Sheng; Liu, Liang; Wang, Rui-Qiang

    2015-06-01

    We present comprehensive petrological, geochronological, major and trace element, and Nd-Hf isotopic data for the Baishishan, Jingangshan, and Wushan granitic plutons on the southeastern coast of Fujian Province, South China, with the aims of elucidating their origin and gaining new insights into the petrogenesis of aluminous A-type granites. Zircon U-Pb ages obtained by laser ablation-inductively coupled plasma-mass spectrometry show that the three investigated plutons were emplaced at 92-86 Ma, indicating that they were generated during a Late Cretaceous magmatic event. The granites from the three plutons are composed mainly of perthite, quartz, plagioclase, and minor biotite; they have high SiO2 contents, and low CaO, MnO, Fe2O3tot, and MgO contents, and show a metaluminous to slightly peraluminous signature. The granites are enriched in some large ion lithophile elements (e.g., Rb, Th, and U) and high field strength elements (e.g., Nb and Ta) with elevated Ga/Al ratios, and spidergrams show strong depletions in Ba, Sr, Ti, and P. Chondrite-normalized REE patterns show relative enrichments in light rare earth elements, flat heavy rare earth element profiles, and strongly negative Eu anomalies. These mineralogical and geochemical characteristics suggest that all three plutons can be classified as aluminous A-type granites. The plutons exhibit nearly identical whole-rock Nd and zircon Hf isotopic compositions, and yield Mesoproterozoic two-stage model ages (1.4-1.1 Ga) for both Nd and Hf isotopes. Based on a synthesis of the geochemical and isotopic data and petrogenetic modelling, we suggest that these A-type granitic rocks were most likely formed by variable degrees of fractional crystallization of magmas produced by the partial melting of a tonalitic to granodioritic source, with plagioclase-rich residual phases in the middle to lower crust, and emplaced at shallow crustal levels along the Changle-Nan'ao Fault. Our data on the Baishishan, Jingangshan, and

  7. Constraints on slab inputs and mantle source compositions in the northern Cascade arc (Garibaldi belt) from Sr-Nd-Pb-Hf isotopes and trace elements in primitive basalts

    NASA Astrophysics Data System (ADS)

    Mullen, E. K.; Carpentier, M.; Weis, D.

    2011-12-01

    The northernmost segment of the Cascade arc, known as the Garibaldi volcanic belt (GVB), extends from Glacier Peak in Washington to the Bridge River cones in British Columbia. GVB primitive basalts display strong arc-parallel geochemical gradients, most prominently a northerly progression from calc-alkaline to highly alkalic compositions, which present an ideal opportunity to address key questions regarding the origin of primary arc basalts including the compositions and relative inputs of sub-arc mantle sources and slab-derived fluids/melts. The gradient in GVB basalt alkalinity was proposed to result from a northerly reduction in slab-derived contributions to the sub-arc mantle wedge, a consequence of the ~4 myr decrease in slab age at the trench [1-3]. As a test of this hypothesis, we have obtained new whole-rock high-precision isotopic (Sr, Nd, Pb, Hf) and trace element data for the GVB basalt suites previously investigated by Green and others. La/Nb decreases from south (4.25 at Glacier Peak) to north (0.78 at Bridge River), confirming a progressive reduction in the "arc signature" (elevated LILE and LREE abundances relative to HFSE and HREE). 87Sr/86Sr ranges from 0.70310 to 0.70396, 206Pb/204Pb from 18.65 to 18.92, ɛNd from 8.5 to 3.8, and ɛHf from 13.3 to 8.7. Our data overlap the Sr-Nd-Pb isotopic compositions of primitive samples at Mt. Baker and Chilliwack batholith [4], but extend to more depleted compositions. GVB basalts are isotopically distinct from other Cascade arc primitive basalts, with systematically lower 208Pb/206Pb at a given 206Pb/204Pb and higher ɛNd at a given 87Sr/86Sr. In Pb-Pb space, GVB basalts define a linear trend extending from Explorer MORB to local subducting sediments drilled at ODP Sites 888 and 1027 in the northern Cascadia basin. We interpret this array as a mixing line reflecting variable sediment input to the mantle. However, Sr, Pb and Nd isotope ratios are only weakly correlated with La/Nb and latitude, whereas 208Pb

  8. Lu-Hf constraints on the evolution of lunar basalts

    SciTech Connect

    Fujimaki, H.; Tatsumoto, M.

    1984-02-15

    Very low Ti basalts andd green glass samples from the moon show high Lu/Hf ratios and low Hf concentrations. Low-Ti lunar basalts show high and variable Lu/Hf ratios and higher Hf concentrations, whereas high-Ti lunar basalts show low Lu/Hf ratios and high Hf concentrations. KREEP basalts have constant Lu/Hf ratios and high but variable Hf concentrations. Using the Lu-Hf behavior as a constraint, we propose a model for the mare basalts evolution. This constraint requires extensive crystallization of the primary lunar magma ocean prior to formation of the lunar mare basalt sources and the KREEP basalts. Mare basalts are produced by the melting of the cumulate rocks, and KREEP basalts represent the residual liquid of the magma ocean.

  9. Zircon U-Pb ages, geochemical and Sr-Nd-Pb-Hf isotopic constraints on petrogenesis of the Tarom-Olya pluton, Alborz magmatic belt, NW Iran

    NASA Astrophysics Data System (ADS)

    Nabatian, Ghasem; Jiang, Shao-Yong; Honarmand, Maryam; Neubauer, Franz

    2016-02-01

    A petrological, geochemical and Sr-Nd-Pb isotopic study was carried out on the Tarom-Olya pluton, Iran, in the central part of the Alpine-Himalayan orogenic belt. The pluton is composed of diorite, monzonite, quartz-monzonite and monzogranite, which form part of the Western Alborz magmatic belt. LA-ICP-MS analyses of zircons yield ages from 35.7 ± 0.8 Ma to 37.7 ± 0.5 Ma, interpreted as the ages of crystallization of magmas. Rocks from the pluton have SiO2 contents ranging from 57.0 to 69.9 wt.%, high K2O + Na2O (5.5 to 10.3 wt.%) and K2O/Na2O ratio of 0.9 to 2.0. Geochemical discrimination criteria show I-type and shoshonitic features for the studied rocks. All investigated rocks are enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs), depleted in high-field strength elements (HFSEs), and show weak or insignificant Eu anomalies (Eu/Eu* = 0.57-1.02) in chondrite-normalized trace element patterns. The Tarom-Olya pluton samples also show depletions in Nb, Ta and Ti typical of subduction-related arc magmatic signatures. The samples have relatively low ISr (0.7047-0.7051) and positive εNd(36 Ma) (+ 0.39 to + 2.10) values. The Pb isotopic ratios show a (206Pb/204Pb)i ratio of 18.49-18.67, (207Pb/204Pb)i ratio of 15.58-15.61 and (208Pb/204Pb)i ratio of 38.33-38.77. The εHf(t) values of the Tarom-Olya pluton zircons vary from - 5.9 to + 8.4, with a peak at + 2 to + 4. The depleted mantle Hf model ages for the Tarom-Olya samples are close to 600 Ma. These isotope evidences indicate contribution of juvenile sources in petrogenesis of the Tarom-Olya pluton. Geochemical and isotopic data suggest that the parental magma of the Tarom-Olya pluton was mainly derived from a sub-continental lithospheric mantle source, which was metasomatized by fluids and melts from the subducted Neotethyan slab with a minor crustal contribution. Subsequent hot asthenospheric upwelling and lithospheric extension caused decompression melting in the final stage of

  10. Origin of Meso-Proterozoic post-collisional leucogranite suites (Kaokoveld, Namibia): constraints from geochronology and Nd, Sr, Hf, and Pb isotopes

    NASA Astrophysics Data System (ADS)

    Jung, S.; Mezger, K.; Nebel, O.; Kooijman, E.; Berndt, J.; Hauff, F.; Münker, C.

    2012-01-01

    Leucocratic granites of the Proterozoic Kaoko Belt, northern Namibia, now preserved as meta-granites, define a rock suite that is distinct from the surrounding granitoids based on their chemical and isotopic characteristics. Least evolved members of this ~1.5-1.6-Ga-old leucogranite suite can be distinguished from ordinary calc-alkaline granites that occur elsewhere in the Kaoko Belt by higher abundances of Zr, Y, and REE, more radiogenic initial ɛNd values and unradiogenic initial 87Sr/86Sr. The leucogranites have high calculated zircon saturation temperatures (mostly > 920°C for least fractionated samples), suggesting that they represent high-temperature melts originating from deep crustal levels. Isotope data (i.e., ɛNdi: +2.3 to -4.2) demonstrate that the granites formed from different sources and differentiated by a variety of processes including partial melting of mantle-derived meta-igneous rocks followed by crystal fractionation and interaction with older crustal material. Most fractionation-corrected Nd model ages (TDM) are between 1.7 and 1.8 Ga and only slightly older than the inferred intrusion age of ca. 1.6 Ga, indicating that the precursor rocks must have been dominated by juvenile material. Epsilon Hf values of zircon separated from two granite samples are positive (+11 and +13), and Hf model ages (1.5 and 1.6 Ga) are similar to the U-Pb zircon ages, again supporting the dominance of juvenile material. In contrast, the Hf model ages of the respective whole rock samples are 2.3 and 2.4 Ga, demonstrating the involvement of older material in the generation of the granites. The last major tectonothermal event in the Kaoko Belt in the Proterozoic occurred at ca. 2.0 Ga and led to reworking of mostly 2.6-Ga-old rocks. However, the presence of 1.6 Ga "post-collisional" granites reflects addition of some juvenile mantle-derived material after the last major tectonic event. The results suggest that similar A-type leucogranites are potentially more

  11. The role of magma mixing in the evolution of high-K calc-alkaline granitoid suites: in situ trace element and Sr-Nd-Hf isotope constraints

    NASA Astrophysics Data System (ADS)

    Laurent, Oscar; Zeh, Armin; Gerdes, Axel; Slaby, Ewa; Villaros, Arnaud

    2015-04-01

    The so-called "I-type", high-K calc-alkaline granitoids are often considered as "hybrid" in origin, i.e. involving both mantle and crustal components in their petrogenesis. The interactions between both components either take place (1) at mantle levels (i.e. enriched mantle source); (2) at emplacement levels (i.e. crustal contamination and/or magma mixing with crustal melts) or (3) both. Magma mixing is, in particular, frequently invoked to explain the compositional range of high-K calc-alkaline granitoid suites, especially phases of intermediate composition (SiO2 = 60-65 wt.%) such as quartz-diorites or granodiorites. We investigated the role of magma mixing in the origin of such rocks using elemental and isotope (Sr-Nd-Hf) chemistry of magmatic minerals (plagioclase, zircon, apatite, titanite, epidote), measured in situ by LA-(MC-)ICPMS, allowing a much greater spatial resolution than classical whole-rock geochemistry. We focused on a suite of late-Archaean (2.69 Ga-old) high-K, calc-alkaline granitoids from the Pietersburg block, northern Kaapvaal Craton (South Africa): the Mashashane, Matlala, Matok and Moletsi plutons. Those plutons range from diorites to monzogranites, emplaced at different crustal levels but all within a relatively short time span and showing evidence for interactions (mingling), both at the outcrop and mineral scale. Hf isotope data on zircon confirm that all rocks are cogenetic (identical ɛHf(t)), but trace element and Sr isotopes in plagioclase point to the involvement of several components in their petrogenesis, at different stages of the magma evolution. The most mafic rocks (diorites) derive from the interaction, at mantle levels, between depleted peridotite and a sedimentary component of quartzofeldspathic nature. The mineral chemistry of more felsic rocks can be explained by (1) differentiation from the diorite magmas through Amp + Plag fractionation; (2) interactions with magmas derived from melting of local crust (tonalites

  12. Growth rate of the preserved continental crust: II. Constraints from Hf and O isotopes in detrital zircons from Greater Russian Rivers

    NASA Astrophysics Data System (ADS)

    Wang, Christina Yan; Campbell, Ian H.; Stepanov, Aleksandr S.; Allen, Charlotte M.; Burtsev, Igor N.

    2011-03-01

    Detrital zircons from the Ob, Yenisey, Lena, Amur, Volga, Dnieper, Don and Pechora rivers have been analyzed for U-Th-Pb, O and Lu-Hf isotopes to constrain the growth rate of the preserved continental crust in Greater Russia. Four major periods of zircon crystallization, 0.1-0.55, 0.95-1.3, 1.45-2.0 and 2.5-2.9 Ga, were resolved from a compilation of 1366 zircon U/Pb ages. The Archean zircons have δ18O values lying between 4.53‰ and 7.33‰, whereas Proterozoic and Phanerozoic zircons have a larger range of δ18O values in each of the recognized U/Pb time intervals with maximum δ18O values up to 12‰. We interpret the zircons with δ18O between 4.5‰ and 6.5‰ to have been derived from a magmatic precursor that contains little or no sedimentary component. The variable δ18O values of the zircons were used to constrain the 176Lu/ 177Hf ratios of the crustal source region of the zircons, which, in turn, were used to calculate Hf model ages (T DMV). The crustal incubation time, the time difference between primitive crust formation (dated by T DMV) and crustal melting (dated by zircon U/Pb age), varies between 300 to 1000 Myr for the majority of analyzed zircon grains, but can be up to 2500 Myr. The average T DMV Hf model age weighted by the fraction of zircons in the river load is 2.12 Ga, which is in reasonable agreement with the area-weighted average of 2.25 Ga. The T DMV Hf model age crustal growth curve for zircons with mantle-like δ18O values (4.5-6.5‰), weighted by area, shows that growth of the Great Russian continental crust started at 4.2 Ga, and that there are two principal periods of crustal growth, 3.6-3.3 Ga and 0.8-0.6 Ga, which are separated by an interval of low but more or less continuous growth. An alternative interpretation, in which the average 176Lu/ 177Hf ratio (0.0115) of the continental crust is used for the Paleoproterozoic zircons from the Lena River, lowers the average T DMV age of these grains by about 500 Myr and delays the onset

  13. Zircon geochronology and Hf isotopic composition of Mesozoic magmatic rocks from Chizhou, the Lower Yangtze Region: Constraints on their relationship with Cu-Au mineralization

    NASA Astrophysics Data System (ADS)

    Yang, Wei; Zhang, Hong-Fu

    2012-10-01

    Zircon U-Pb ages and Hf isotopic compositions of Mesozoic magmatic rocks from the Chizhou Area are systematically investigated to reveal the tectonic setting of magmatism and their relationship with Cu-Au mineralization in the Lower Yangtze River Belt, southeastern China. The samples cover nearly all types of magmatic rocks in a 30 × 50 km2 region, including 6 granite porphyries, 6 dacites and 4 granites. The zircon U-Pb geochronology yields a range of 151-124 Ma, with granite porphyries ranging from 151 to 146 Ma, dacites from 132 to 127 Ma and granites from 127 to 124 Ma, indicating two magmatic episodes of the late Jurassic and the early Cretaceous. The earlier episode mainly formed small granite porphyries (generally < 5 km) and is always associated with porphyry Cu-Au deposits. The later episode began with dacites and was then dominated by large granite intrusions (generally > 10 km), which are barren in mineralization. The ore-barren dacites and the granites (131-124 Ma) are poor in inherited zircons. Zircons in these rocks yield a very large ɛHf(t) variation of - 20.8-0.4, suggesting a mixing between mantle-derived and crustal-derived magmas. By contrast, the ore-bearing porphyries (151-146 Ma) are rich in inherited zircons. The magmatic zircons have ɛHf(t) values of - 8.8-0.9, and the inherited ones yield U-Pb ages of 1156-811 Ma with ɛHf(t) values of 2.5-11.5. The existence of quantitative inherited zircons indicates that the crustal rocks of 1156-811 Ma significantly contribute to the formation of the ore-bearing porphyries, either being source or contamination. Since these inherited zircons are igneous as indicated by their oscillatory zonings, they may derive from components of the Grenvillian oceanic crust (ca. 1100-1000 Ma), i.e. the Neoproterozoic magmatic rocks related to arc (970-890 Ma) and Nanhua rift (ca. 825 Ma). Recent studies reveal that the ore-baring porphyries of the Lower Yangtze River Belt have slab melt features and conclude that

  14. Geological, geochronological, geochemical, and Sr-Nd-O-Hf isotopic constraints on origins of intrusions associated with the Baishan porphyry Mo deposit in eastern Tianshan, NW China

    NASA Astrophysics Data System (ADS)

    Wang, Yinhong; Xue, Chunji; Liu, Jiajun; Zhang, Fangfang

    2016-10-01

    The Baishan porphyry Mo deposit (0.72 Mt; 0.06 % Mo) is located in the interior of the eastern Tianshan orogenic belt in Xinjiang, NW China. The deposit comprises 15 orebodies that are associated with monzogranite and granite porphyry stocks and are structurally controlled by roughly EW-trending faults. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating of the monzogranite and granite porphyry yielded the Middle Triassic age (228 ± 2 to 227 ± 2 Ma), which coincide with the molybdenite Re-Os model ages ranging from 226 ± 3 to 228 ± 3 Ma. The Triassic monzogranite and granite porphyry belong to high-K calc-alkaline series and are characterized by high SiO2 and Al2O3 and low MgO, TiO2, and P2O5 concentrations, with negative Eu anomalies (δEu = 0.55-0.91). The least-altered monzogranite and granite porphyry yield uniform ɛ Nd( t) values from +1.6 to +3.6, and wide (87Sr/86Sr) i ratios ranging between 0.7035 and 0.7071, indicating that they were derived from the lower crust. In situ O-Hf isotopic analyses on zircon using SIMS and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) indicate that the δ18O and ɛ Hf( t) values of zircon from a monzogranite sample vary from 6.1 to 7.3 ‰ and +8.0 to +11.7, respectively, whereas zircon from a granite porphyry sample vary from 6.2 to 6.9 ‰ and +7.3 to +11.2, respectively. The geochemical and isotopic data imply that the primary magmas of the Baishan granite were likely derived from partial melts from the lower crust involving some mantle components. The Baishan Mo deposit and granitic emplacement were proposed to be most likely related to post-orogenic lithospheric extension and magmatic underplating. An extensional event coupled with the rising of hot mantle-derived melts triggered partial melting of the lower crust, as well as provided metals (Mo).

  15. Whole-rock Nd-Hf isotopic study of I-type and peraluminous granitic rocks from the Chinese Altai: constraints on the nature of basement and tectonic setting

    NASA Astrophysics Data System (ADS)

    Yu, Yang; Sun, Min; Long, Xiaoping; Li, Pengfei; Zhao, Guochun; Kröner, Alfred; Broussolle, Arnaud; Yang, Jinhui

    2016-04-01

    Previously published whole-rock Nd isotopic data are often inconsistent with zircon Hf isotopic data for the early Paleozoic granitic rocks (380-478 Ma) in the Chinese Altai, which led to contradictory interpretations for their magma sources and tectonic setting. Nd and Hf isotopic analyses were conducted in this study on the same whole-rock samples of representative early to middle Paleozoic granitic rocks, and the data are used to reconcile the controversies and to discuss the complicated curst-mantle interaction in the accretionary orogenesis. Our samples include I-type granites (Hanasi batholith, Kurmutu pluton, Altay batholith and Kezgar pluton), strongly peraluminous granites (Hemu batholith, Kuwei batholith), and rhyolites (Jiadengyu). The I-type granites are metaluminous to weakly peraluminous and have ɛNd(t) and ɛHf(t) values ranging from -2.6 to 0.7 and from 4.0 to 13.0, respectively. The strongly peraluminous granites and rhyolites have similar ɛNd(t) and ɛHf(t) values ranging from -3.2 to 1.7 and from 2.1 to 15.7, respectively. All samples plot above the Terrestrial Array in a Nd-Hf isotopic diagram, indicating significant Nd-Hf isotopic decoupling in the magma sources. Our sample locations nearly cover the entire Chinese Altai, and their ages range from 445 Ma to 368 Ma, illustrating that the Nd-Hf isotopic decoupling prevailed in the region throughout the early and middle Paleozoic. All samples show flat HREE pattern and have Lu/Hf ratios similar to the average crust, suggesting that isotopic decoupling was not originated from an ancient basement with elevated Lu/Hf ratios. The high ɛHf(t) values and poor correlation between ɛHf(t) and ɛNd(t) values suggest that these rocks represent an early Paleozoic magmatic arc built on a late Neoproterozoic crustal block such as the Lake Zone in western Mongolia. The observed isotopic decoupling is similar to that for those modern island arcs, such as the Lesser Antilles and Sunda, where Nd selectively

  16. Petrogenesis of the Late Triassic volcanic rocks in the Southern Yidun arc, SW China: Constraints from the geochronology, geochemistry, and Sr-Nd-Pb-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Leng, Cheng-Biao; Huang, Qiu-Yue; Zhang, Xing-Chun; Wang, Shou-Xu; Zhong, Hong; Hu, Rui-Zhong; Bi, Xian-Wu; Zhu, Jing-Jing; Wang, Xin-Song

    2014-03-01

    Studies on zircon ages, petrology, major and trace element geochemistry, and Sr-Nd-Hf-Pb isotopic geochemistry of intermediate volcanic rocks from the Southern Yidun arc, Sanjiang-Tethyan Orogenic Belt, SW China have been undertaken in this paper. They are used to discuss the petrogenesis of these rocks and to constrain the tectonic setting and evolution of the Yidun arc. These intermediate volcanic rocks were erupted at ca. 220 Ma (U-Pb zircon ages). Trachyandesite is the dominant lithology among these volcanic rocks, and is mainly composed of hornblende and plagioclase, with minor clinopyroxene and biotite. A hornblende geobarometer suggests that the stagnation of magma in the lower crust, where plagioclase crystallization was suppressed while hornblende crystallized, giving rise to high Sr/Y ratios that are one of the distinguishing features of adakites, after the primary magma originated from the lithospheric mantle wedge. Steeply right-inclined Rare Earth Element (REE) pattern combined with high La/Yb ratios suggests adakitic affinity of these volcanic rocks, implying that slab-melt from the subducting oceanic crust is a necessary component in the primary magma. Besides, trace element geochemistry and isotopic geochemistry also indicate that partial melting of pelagic sediments in the subduction zone and noticeable contamination with the lower crust were involved in the evolution of parental magma of these volcanic rocks. Based on previous work on the Northern Yidun arc and this study, we propose that the subduction was initiated in the Northern Yidun arc and extended to the southern part and that the Northern Yidun arc is an island arc while the Southern Yidun arc represents a continental arc, probably caused by the existence of the Zhongza Massif, that was invoked to be derived from Yangtze Block, as a possible basement of the Southern Yidun arc.

  17. U-Pb zircon age, geochemical and Lu-Hf isotopic constraints of the Southern Gangma Co basalts in the Central Qiangtang, northern Tibet

    NASA Astrophysics Data System (ADS)

    Wang, Ming; Li, Cai; Xie, Chao Ming; Xu, Jian Xin; Li, Xing Kui

    2015-08-01

    A recent study suggests that the central Qiangtang is a key locality to investigate the evolution of the opening and closure of the Paleo-Tethys Ocean. Basalts are commonly associated with supercontinent fragmentation, and they have the potential to indicate the tectonic environment into which they were erupted. In this study, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon dating, whole-rock geochemistry, and zircon Hf isotope analysis were used to study newly discovered basalts in the southern Gangma Co area of central Qiangtang. Dating of magmatic zircons from three basalt samples indicates that the basalts erupted in the Late Devonian to early Carboniferous (early Tournaisian) at 360-350 Ma. The basalts are geochemically similar to within-plate basalts and formed from a depleted mantle source. We suggest that the basalts may have been a product of the breakup of the northern margin of Gondwana in the Late Devonian to early Carboniferous (360-350 Ma), which may be linked to the initial rifting and opening of the Paleo-Tethys Ocean in the Qiangtang area.

  18. Petrogenesis of gold-mineralized magmatic rocks of the Taerbieke area, northwestern Tianshan (western China): Constraints from geochronology, geochemistry and Sr-Nd-Pb-Hf isotopic compositions

    NASA Astrophysics Data System (ADS)

    Tang, Gong-Jian; Wang, Qiang; Wyman, Derek A.; Sun, Min; Zhao, Zhen-Hua; Jiang, Zi-Qi

    2013-09-01

    Many Late Paleozoic Cu-Au-Mo deposits occur in the Central Asian Orogenic Belt (CAOB). However, their tectonic settings and associated geodynamic processes have been disputed. This study provides age, petrologic and geochemical data for andesites and granitic porphyries of the Taerbieke gold deposit from the Tulasu Basin, in the northwestern Tianshan Orogenic Belt (western China). LA-ICP-MS zircon U-Pb dating indicates that the granitic porphyries have an Early Carboniferous crystallization age (349 ± 2 Ma) that is broadly contemporaneous with the eruption age (347 ± 2 Ma) of the andesites. The andesites have a restricted range of SiO2 (58.94-63.85 wt.%) contents, but relatively high Al2O3 (15.39-16.65 wt.%) and MgO (2.51-6.59 wt.%) contents, coupled with high Mg# (57-69) values. Geochemically, they are comparable to Cenozoic sanukites in the Setouchi Volcanic Belt, SW Japan. Compared with the andesites, the granitic porphyries have relatively high SiO2 (72.68-75.32 wt.%) contents, but lower Al2O3 (12.94-13.84 wt.%) and MgO (0.10-0.33 wt.%) contents, coupled with lower Mg# (9-21) values. The andesites and granitic porphyries are enriched in both large ion lithophile and light rare earth elements, but depleted in high field strength elements, similar to those of typical arc magmatic rocks. They also have similar Nd-Hf-Pb isotope compositions: ɛNd(t) (+0.48 to +4.06 and -0.27 to +2.97) and zircons ɛHf(t) (+3.4 to +8.0 and -1.7 to +8.2) values and high (206Pb/204Pb)i (18.066-18.158 and 17.998-18.055). We suggest that the Taerbieke high-Mg andesitic magmas were generated by the interaction between mantle wedge peridotites and subducted oceanic sediment-derived melts with minor basaltic oceanic crust-derived melts, and that the magmas then fractionated to produce the more felsic members (i.e., the Taerbieke granitic porphyries) during late-stage evolution. Taking into account the Carboniferous magmatic record from the western Tianshan Orogenic Belt, we suggest that

  19. Provenance and paleogeography of the Late Cretaceous Mengyejing Formation, Simao Basin, southeastern Tibetan Plateau: Whole-rock geochemistry, U-Pb geochronology, and Hf isotopic constraints

    NASA Astrophysics Data System (ADS)

    Wang, Licheng; Liu, Chenglin; Gao, Xiang; Zhang, Hua

    2014-05-01

    The Late Cretaceous Mengyejing Formation, which contains the only pre-Quaternary potash salt deposit in the Simao Basin, southeastern Tibet, is thought to be genetically related to the Maha Sarakham Formation in the Khorat Basin. The provenance and paleogeography of these two basins have been under debate, although little diagnostic evidence has been previously published. A combined analysis of whole rock geochemistry, zircon U-Pb chronology, and Hf isotopic compositions was performed to characterize the provenance of the Mengyejing Formation. These formation's sandstones are characterized by moderate chemical index of alteration (CIA) values. These values, together with plots of the Th/U-Th ratios, suggest that certain samples have undergone moderate weathering and sedimentary recycling. The major and trace elements (La/Th-Hf, Th/Sc-Zr/Sc, Eu/Eu*-Th/Sc, TiO2-Fe2O3T + MgO, Al2O3/SiO2-Fe2O3T + MgO, K2O/Na2O-SiO2) indicate that the sedimentary sources were felsic rocks from an active continental margin or continental arc with a minor amount of recycled sediment from a passive continental margin. The Mengyejing Formation contains detrital zircons primarily with U-Pb ages of 2.45-2.57 Ga, 1.8-1.9 Ga, 740-880 Ma, 410-470 Ma, and 215-300 Ma. The results reveal that the pre-Devonian zircons are derived from the recycled sediments of the Yangtze block originating in the Qinling Orogenic Belt, and they share this provenance with the coeval sediments in the Khorat Basin. The magmatic rocks of the Ailaoshan and Lincang areas are responsible for supplying the Devonian to Triassic detrital zircons. These provenance data combined with published paleocurrent results suggest that the Simao Basin was situated on the western margin of the Khorat Basin during the Late Cretaceous. The basins were connected when marine incursion occurred. We propose that pre-Devonian materials from the southwestern Sichuan Basin first supplied detritus to the Simao Basin and subsequently to the Khorat

  20. Zircon U-Pb and Lu-Hf isotopic and geochemical constraints on the origin of the paragneisses from the Jiaobei terrane, North China Craton

    NASA Astrophysics Data System (ADS)

    Shan, Houxiang; Zhai, Mingguo; Zhu, Xiyan; Santosh, M.; Hong, Tao; Ge, Songsheng

    2016-01-01

    Clastic sedimentary rocks are important tracers to understand the evolution of the continental crust. Whole-rock major and trace element data, zircon U-Pb dating and Hf isotopic data for the paragneisses from the Jiaobei terrane are presented in this study in order to constrain their protoliths, provenance and tectonic setting. The paragneisses are characterized by enrichment in Al2O3 and TiO2, negative DF (DF = 10.44 - 0.21SiO2 - 0.32Fe2O3T - 0.98MgO + 0.55CaO + 1.46Na2O + 0.54K2O) values and the presence of aluminum-rich metamorphic minerals (e.g., garnet and sillimanite). Together with the mineral assemblages and zircon features, it can be inferred that the protoliths of these rocks are of sedimentary origin. The K-A (A = Al2O3/(Al2O3 + CaO + Na2O + K2O), K = K2O/(Na2O + K2O)) and log(Fe2O3/K2O)-log(SiO2/Al2O3) diagrams indicate that they belong principally to clay-silty rocks with some contributions from graywacke. A series of geochemical indexes, such as the widely employed CIA (CIA = [Al2O3/(Al2O3 + CaO∗ + Na2O + K2O)] × 100; molar proportions) and ICV (ICV = (Fe2O3 + MnO + MgO + CaO + Na2O + K2O + TiO2)/Al2O3) values, and the A-CN-K diagram for the paragneisses indicate relatively weak weathering in the source rocks and negligible post-depositional K-metasomatism. In addition, their REE patterns, low Cr/Zr (0.61-1.99), high Zr/Y (4.81-23.59) and Th/U (3.21-40.67) ratios, the low to moderate contents of Cr (197-362 ppm) and Ni (6.68-233 ppm), and source rock discrimination diagrams collectively suggest that the sediments of the protoliths of the paragneisses in the Jiaobei terrane were derived from the source with intermediate-acidic composition, probably granitic-to-tonalitic rocks. In combination with geochronological and isotopic studies on the paragneisses and the basement rocks in the Jiaobei terrane, it is suggested that the Archean-early Paleoproterozoic granitic rocks in the Jiaobei terrane possibly provided the most important source materials. In

  1. Environmental Hf-Nd isotopic decoupling in World river clays

    NASA Astrophysics Data System (ADS)

    Bayon, Germain; Skonieczny, Charlotte; Delvigne, Camille; Toucanne, Samuel; Bermell, Sylvain; Ponzevera, Emmanuel; André, Luc

    2016-03-01

    The hafnium and neodymium radiogenic isotope systems behave differently during Earth surface processes, causing a wide dispersion of Hf and Nd isotopic compositions in sediments and other sedimentary rocks. The decoupling between Hf and Nd isotopes in sediments is generally attributed to a combination of preferential sorting of zircon during sediment transport and incongruent weathering processes on continents. In this study, we analysed size-fractions of sediment samples collected near the mouth of 53 rivers worldwide to better understand the factors controlling the distribution of Hf and Nd isotopes in sediments. Our results for rivers draining old cratonic areas and volcanic provinces demonstrate that both granite and basalt weathering can lead to significant grain-size dependent Hf isotopic variability. While silt-size fractions mainly plot along the Terrestrial Array, World river clays are systematically shifted towards more radiogenic Hf isotopic compositions, defining together with published data a new Clay Array (εHf = 0.78 ×εNd + 5.23). The Hf-Nd isotope decoupling observed in volcanogenic sediments is best explained by selective alteration of Lu-rich mineral phases (e.g. olivine) and preferential enrichment of resistant unradiogenic minerals, such as spinel and ilmenite, in silt fractions. We also show that the extent to which World river clays deviate from the Clay Array (ΔεHf clay) is not linked to the presence of zircons. Instead, it correlates positively with weathering indices and climatic parameters (temperature, rainfall) of the corresponding drainage basins. Overall, these findings demonstrate that the distribution of Hf-Nd isotopes in clay-size sediments is related to a large extent to weathering conditions on continents, although the precise mechanisms controlling this relationship remain unclear. We finally propose that the Hf-Nd isotope pair proxy could be used in palaeoenvironmental studies to provide semi-quantitative information on

  2. Sr-Nd-Hf isotopes of the intrusive rocks in the Cretaceous Xigaze ophiolite, southern Tibet: Constraints on its formation setting

    NASA Astrophysics Data System (ADS)

    Zhang, Liang-Liang; Liu, Chuan-Zhou; Wu, Fu-Yuan; Zhang, Chang; Ji, Wei-Qiang; Wang, Jian-Gang

    2016-08-01

    The Cretaceous Xigaze ophiolite is best exposed at the central part of the Yarlung-Zangbo Suture Zone, Tibet Plateau. It consists of a thick section of mantle peridotites, but a relatively thin mafic sequence. This study presents geochronological and geochemical data for intrusive dykes (both mafic and felsic) and basalts to revisit the formation setting of the Xigaze ophiolite. The rodingites are characterized by high CaO and low Na2O contents relative to mafic dykes and show big variations in trace element compositions. Both gabbros and diabases have similar geochemical compositions, with MgO contents of 6.42-11.48 wt% and Mg# of 0.56-0.71. They display REE patterns similar to N-MORB and are variably enriched in large ion lithophile elements. Basalts have fractionated compositions and display LREE-depleted patterns very similar to N-MORB. They do not show obvious enrichment in LILE and depletion in high-field-strength elements, but a negative Nb anomaly is present. The studied plagiogranites have compositions of trondhjemite to tonalite, with high Na2O and low K2O contents. They have low TiO2 contents less than 1 wt%, consistent with melts formed by anatexis of gabbros rather than by differentiation of basalts. Zircons from seven samples, including three rodingites, three plagiogranites, and one gabbro, have been dated and yielded U-Pb ages of 124.6 ~ 130.5 Ma, indicating the Xigaze ophiolite was formed during the Early Cretaceous. They have mantle-like δ18O values of + 4.92 ~ + 5.26‰ and very positive εHf(t) values of + 16 ~ + 13.3. Ages of the rodingites and less altered gabbros indicate that serpentinization was occurred at ~ 125 Ma. Occurrence of both gabbroic and diabase dykes within the serpentinites suggests that the mantle lithosphere of the Xigaze ophiolite was rapidly exhumed. Both mafic and felsic dykes have slightly more radiogenic 87Sr/86Sr ratios relative to MORB, but depleted Hf-Nd isotpe compositions. They have a limited range of ε

  3. Zircon U-Pb and Hf-Nd isotopic constraints on the genesis of granites from the vicinity of Bosumtwi crater

    NASA Astrophysics Data System (ADS)

    Losiak, A.; Schulz, T.; Koeberl, C.

    2012-04-01

    The well preserved Bosumtwi crater (Ghana), 10.5 km in diameter and 1.07 Myr old (e.g., Koeberl and Reimold 2005), was excavated in rocks of the Early Proterozoic Birimian Supergroup. These rocks were deposited 2.1-2.15 Gyr ago in a volcanic arc environment and were metamorphosed to greenschist facies during the Eburnean tectono-thermal event (e.g., Jones et al. 1981, Feybesse et al. 2006). The Birimian Supergroup mainly consists of two contemporary units: volcanic belts and sedimentary basins aligned in multiple parallel features. Additionally, numerous granitoid intrusions were emplaced within the Birimian Supergroup (Wright et al. 1985). Two main types of granitoid intrusions are recognized in Ghana: belt granitoids and basin granitoids (Leube et al. 1990). Both types can be distinguished according to petrology, chemistry, and age. Whereas previous studies of target rocks from the Bosumtwi crater focused on metasedimentary rocks (e.g., Karikari et al. 2007), in this study we investigate felsic intrusive bodies. This work will also provide more data on the geological evolution of the Kumasi basin and Ashanti belt regions in Ghana. We analyzed thirteen samples for their major- and trace element compositions, as well as their U-Pb, Lu-Hf, and Sm-Nd systematics. Twelve samples come from three different felsic intrusive bodies. Two intrusions are located to the north of the crater, previously described as basin-type granitoids, whereas one intrusion was classified as belt-type granite (Koeberl and Reimold 2005). One sample represents a mafic (diabase) dyke. Analyses were performed using optical microscopy, XRF, and INAA at the University of Vienna, as well as by MC-ICPMS at the University of Bonn. Additional zircon U-Pb ages were obtained with VG Sector-54 multicollector TIMS at the Massachusetts Institute of Technology. Zircon ages for samples from four different intrusive bodies vary within a narrow range between 2091.96 ± 7 Ma and 2097.70 ± 7 Ma. A whole

  4. Large Nd-Hf isotopic decoupling in Himalayan River Sediments

    NASA Astrophysics Data System (ADS)

    Garcon, M.; Chauvel, C.; France-Lanord, C.

    2011-12-01

    Nd isotopic compositions of river sediments are widely used to trace sediment provenance in the Himalayan mountain range. In contrast, Hf isotopic compositions are not used even though they are excellent proxies to record the history of continental areas (Hawkesworth and Kemp, Chem. Geol. 226, 2006). Here, we focus on the Hf isotopic message carried by Himalayan river sediments and combine it to the more classical Nd isotopes to better understand the behavior of the two systems during erosion. We report Nd-Hf isotopic compositions of bedloads and suspended loads sampled at different depths in the Narayani River in Nepal (upstream of the Ganga floodplain), in the Ganga River in Bangladesh (downstream of the Ganga floodplain) and in the Yamuna River, a major tributary of the Ganga in India. Nd-Hf isotopic compositions of bedloads span a small range of values (-18< ɛNd <-16 and -30< ɛHf <-23) and lie next to the terrestrial array in a ɛHf vs. ɛNd diagram. Nd isotopic compositions are similar to those of the main Himalayan sources. By contrast, suspended loads have much more variable ratios (-19< ɛNd <-10 and -25< ɛHf <-7) and plot well above the terrestrial array in a ɛHf vs. ɛNd diagram. Like oceanic sediments, they are characterized by high ɛHf compared to their ɛNd. We explain this Nd-Hf decoupling by mineralogical sorting, a process that enriches bottom sediments in coarse and dense minerals, such as unradiogenic zircons, while the surface sediments are enriched in fine material with radiogenic Hf signatures. Bedloads and suspended loads, collected at the same sampling site at different depths in the Narayani and Ganga Rivers, share similar ɛNd. However, differences of about 5 ɛNd and 15 ɛHf units are observed between bedload and surface samples in the Yamuna River. In this river, both Nd and Hf isotopic ratios decrease from surface to bottom. We believe that part of the Hf isotopic variability is due to mineralogical sorting but the rest of it

  5. Dating the Indo-Asia collision in NW Himalaya: constraints from Sr-Nd isotopes and detrital zircon (U-Pb) and Hf isotopes of Paleogene-Neogene rocks in the Katawaz basin, NW Pakistan

    NASA Astrophysics Data System (ADS)

    Zhuang, Guangsheng; Najman, Yani; Millar, Ian; Chauvel, Catherine; Guillot, Stephane; Carter, Andrew

    2015-04-01

    The time of collision between the Indian and Asian plates is key for understanding the convergence history and the impact on climatic systems and marine geochemistry. Despite much active research, the fundamental questions still remain elusive regarding when and where the Indian plate collided with the Asian plate. Especially in the west Himalaya, the questions become more complex due to disputes on the amalgamation history of interoceanic Kohistan-Ladakh arcs (KLA) with Karakoram of the Asian plate and the Indian plate. Here, we present a result of multiple-isotopic geochemistry and geochronology study in the Katawaz Basin in NW Pakistan, a remnant oceanic basin on the western Indian plate which was the repository for the sediments eroded from the west Himalaya ( Qayyum et al., 1996, 1997a, 1997b, 2001; Carter et al., 2010), to evaluate the time and character of collision in this region. In this study, we analyzed 22 bulk mudstone samples for Sr-Nd isotopes and 11 medium-grained sandstones for detrital zircon (U-Pb) geochronology and Hf isotopes. We constructed the Cenozoic chronology in the Katawaz Basin based on our newly collected detrital zircon U-Pb ages and fission track ages. We present the first record of Katawaz chronology that constrained the Khojak Formation to be < 40 Ma to < 22 Ma. The result is consistent with the previous nanofossil study that constrained the upper part of underlying Nisai Formation to be the Middle to Late Eocene. Our current study revealed that the Katawaz sedimentary sequence ranges in age from Eocene to the earliest Miocene. The samples from the Nisai Formation show the 87Sr/86Sr - ɛNd values overlapping those of the end member of the Karakoram of Asian origin, revealing the arrival of Asian detritus on the Indian plate prior to 50 Ma. There are two parallel lines of evidence supporting this conclusion: (1) young zircon grains (< 120 Ma), characterizing the KLA and Karakoram, persistently exist throughout the whole sedimentary

  6. Serpentinization Changes Nd, but not Hf Isotopes of Abyssal Peridotites

    NASA Astrophysics Data System (ADS)

    Bizimis, M.; Frisby, C. P.; Mallick, S.

    2015-12-01

    Serpentinization of the oceanic lithosphere is a known sink for fluid mobile elements (B, Cl, Li, Sr, etc.), while high field strength elements (HFSE: e.g., Hf, Zr, Ti, Nb) are thought to be unaffected by it. In contrast, the fate of REE during serpentinization is equivocal. Correlations between REE and HFSE concentrations in abyssal peridotites suggest control by magmatic processes (Niu, 2004, J. Pet), while some LREE enrichments in serpentinized peridotites compared to their clinopyroxene (cpx) and Nd, Sr isotope data (Delacour et al., 2008, Chem. Geol.) imply seawater-derived REE addition to the mantle protolith (Paulick et al., 2006, Chem. Geol). To further constrain peridotite-seawater interaction during serpentinization we compare bulk rock and cpx Hf and Nd isotope data in partially (up to ~70%) serpentinized abyssal peridotites (9-16°E South West Indian Ridge). We also present a new method that improves yields in Hf, Nd and Pb separations from depleted (<0.03 ppm Hf) ultramafic rocks, which includes coprecipitation of metals with Al-Fe hydroxides and ether-HCl liquid-liquid exchange for Fe removal. Nd isotopes in the bulk peridotite are up to 7ɛNd units less radiogenic than their cpx (i.e., the magmatic value) while Hf isotopes remain equal to cpx within 1 ɛHf. Melt-rock reaction by the local lavas cannot generate this decoupling. The largest Nd isotopic difference between cpx and bulk is seen in the most LREE-depleted samples, while refertilized samples show little change. Leaching experiments show that 30-60% of REE are mobilized from the rock, but >90% of Hf, Zr, Ti are retained in the residue. LA-ICPMS data shows that serpentine after olivine typically has higher LREE/HREE ratios than cpx, pronounced negative Ce anomalies, high U, Sr concentrations and low HFSE, unlike the coexisting cpx. These data are consistent with some seawater-derived LREE addition to peridotite during serpentinization, localized in the serpentine and other secondary phases

  7. Temporal-spatial distribution and tectonic implications of the batholiths in the Gaoligong-Tengliang-Yingjiang area, western Yunnan: Constraints from zircon U-Pb ages and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Xu, Yi-Gang; Yang, Qi-Jun; Lan, Jiang-Bo; Luo, Zhen-Yu; Huang, Xiao-Long; Shi, Yu-Ruo; Xie, Lie-Wen

    2012-07-01

    Considerable progress has recently been made regarding temporal and spatial distribution of magmatism in the Lhasa Terrane. However the eastward and southeastward correlation of these Tibetan magmatic suites in western Yunnan and Burma remains poorly constrained. This paper reports zircon U-Pb dating and Hf isotopic compositions of granites in the Gaoligong-Tengliang-Yingjiang area, west Yunnan. It reveals three episodes of plutonism, and more importantly a southwestward magmatic migration. The Gaoligong batholiths in the northeast were mainly emplaced during early Cretaceous (126-121 Ma) and comprised predominantly S-type granites with negative zircon ɛHf values (ɛHf = -2˜-12). The Tengliang granites, situated southwest of the Gaoligong belt, were emplaced in late Cretaceous (68-76 Ma) and also displayed a strong peraluminous affinity and negative ɛHf (-5˜-14), indicating a provenance from a Proterozoic sedimentary source with little mantle contribution. The youngest phase of magmatism (52-66 Ma) occurred in Yingjiang, southwestmost of the study area. It is composed of S-type granites (ɛHf = -2˜-12) in east Yingjiang and I-type granites (ɛHf = -4˜+6) in west Yingjiang, near the China-Burma border. The late Cretaceous-early Cenozoic plutons in the Tengliang and Yingjiang area are thus considered as the northern continuation of the late Cretaceous magmatic arc (west), which comprises I-type granites and andesitic rocks, and of the belt of predominant S-type granites (east) in Burma, Thailand and Malaysia. Such a chemical polarity of the dual I-type and S-type granites is strongly reminiscent of the northern American Cordillera, indicating a Cordilleran-style continental margin during the late Cretaceous-early Cenozoic. While the magmatic arc was related to eastward subduction of the Neo-Tethys beneath the Asian continent, the S-type granites represented the melting products of thickened crust in the hinterland, in response to subduction-induced decrease in

  8. Early Cretaceous arc volcanic suite in Cebu Island, Central Philippines and its implications on paleo-Pacific plate subduction: Constraints from geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Deng, Jianghong; Yang, Xiaoyong; Zhang, Zhao-Feng; Santosh, M.

    2015-08-01

    The Philippine island arc system is a collage of amalgamated terranes of oceanic, continental and island arc affinities. Here we investigate a volcanic suite in Cebu Island of central Philippines, including basalt, diabase dike, basaltic pyroclastic rock and porphyritic andesite. LA-ICP-MS U-Pb geochronology of zircon grains from the porphyritic andesite and pyroclastic rock yielded ages of 126 ± 3 Ma and 119 ± 2 Ma, respectively, indicating an Early Cretaceous age. The age distribution of the detrital zircons from river sand in the area displays a peak at ca. 118 Ma, close to the age of the pyroclastic rock. The early Cretaceous volcanic rocks in the central Philippines were previously regarded as parts of ophiolite complexes by most investigators, whereas the Cebu volcanics are distinct from these, and display calc-alkaline affinity and island arc setting, characterized by high LREE/HREE ratios and low HFSE contents. These features are similar to the Early Cretaceous arc basalts in the Amami Plateau and east Halmahera in the northernmost and southernmost West Philippine Basin respectively. Zircon Hf isotopes of the pyroclastic rocks show depleted nature similar to those of the Amami Plateau basalts, implying the subducted Pacific-type MORB as probable source. Zircon Hf isotopes of the porphyritic andesite show slight enrichment relative to that of the pyroclastic rocks and MORB, indicating subducted sediments as a minor end-member in the source. The Hf isotopic compositions of the volcanic rocks are also reflected in the detrital zircons from the river sands. We propose that the volcanic rocks of Cebu Island were derived from partial melting of sub-arc mantle wedge which was metasomatized by dehydration of subducted oceanic crust together with minor pelagic sediments. Within the tectonic environment of Southeast Asia during Early Cretaceous, the volcanic rocks in Cebu Island can be correlated to the subduction of paleo-Pacific plate. The Early Cretaceous

  9. Formation of intra-arc volcanosedimentary basins in the western flank of the central Peruvian Andes during Late Cretaceous oblique subduction: field evidence and constraints from U-Pb ages and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Polliand, Marc; Schaltegger, Urs; Frank, Martin; Fontboté, Lluis

    2005-04-01

    During late Early to Late Cretaceous, the Peruvian coastal margin underwent fast and oblique subduction and was characterized by important arc plutonism (the Peruvian Coastal Batholith) and formation of volcanosedimentary basins known as the Western Peruvian Trough (WPT). We present high-precision U-Pb ages and initial Hf isotopic compositions of zircon from conformable volcanic and crosscutting intrusive rocks within submarine volcanosedimentary strata of the WPT hosting the Perubar massive sulfide deposit. Zircons extracted from both the volcanic and intrusive rocks yield concordant U-Pb ages ranging from 67.89±0.18 Ma to 69.71±0.18 Ma, indicating that basin subsidence, submarine volcanism and plutonic activity occurred in close spatial and temporal relationship within the Andean magmatic arc during the Late Cretaceous. Field observations, satellite image interpretation, and plate reconstructions, suggest that dextral wrenching movements along crustal lineaments were related to oblique subduction. Wrench tectonics is therefore considered to be the trigger for the formation of the WPT as a series of pull-apart basins and for the emplacement of the Coastal Batholith. The zircon initial ɛHf values of the dated magmatic rocks fall between 5.5 and 7.4, and indicate only very subordinate influence of a sedimentary or continental component. The absence of inherited cores in the zircons suggest a complete lack of old basement below the WPT, in agreement with previous U-Pb and Sr isotopic data for batholithic rocks emplaced in the WPT area. This is supported by the presence of a most likely continuous block of dense (~3.0 g/cm3) material observed beneath the WPT area on gravimetric crustal cross sections. We suggest that this gravimetric anomaly may correspond to a piece of lithospheric mantle and/or oceanic crust inherited from a possible Late Permian-Triassic rifting. Such young and mafic crust was the most probable source for arc magmatism in the WPT area.

  10. U-Pb geochronology and Lu-Hf isotopes of zircons from newly identified Permian-Early Triassic plutons in western Liaoning province along the northern margin of the North China Craton: constraints on petrogenesis and tectonic setting

    NASA Astrophysics Data System (ADS)

    Bai, Xiang; Liu, Shuwen; Wang, Wei; Yang, Pengtao; Li, Qiugen

    2013-04-01

    Mafic to felsic gneisses along the northern margin of the North China Craton (NMNCC), in western Liaoning province, China, were previously assumed to be part of Archean metamorphic basement but are here identified as younger (Permian-Early Triassic) intrusions. LA-ICP-MS zircon U-Pb isotopic dating reveals that the magmatic precursors of the mafic gneisses were emplaced from 295 ± 3 to 259 ± 2 Ma and that the magmatic precursors of the dioritic and monzogranitic gneisses were emplaced at 267 ± 1 and 251 ± 2 Ma, respectively, thus recording a continuum of Permian to Early Triassic magmatism. The mafic and dioritic rocks exhibit zircon ɛHf( t) values from -20.7 to -3.3, suggesting they were mainly derived from a metasomatized lithospheric mantle source, possibly involving some crustal contamination. The monzogranitic rocks display their zircon ɛHf( t) values of +0.9 to +4.7, indicating the acidic magma was derived from partial melting of juvenile crustal materials from the depleted mantle source. Crustal model ages ( T {DM/C}) obtained from zircon Hf isotopes of these monzogranitic rocks range from 976 to 1,215 Ma, with an average of 1,074 ± 32 Ma, possibly implying an episode of Grenvillian crustal growth in western Liaoning province. These new lines of evidence show that the NMNCC witnessed abundant magmatic activity and interaction of the crust and mantle during the Permian and Early Triassic and that the mafic magmatism was earlier than the monzogranitic activity. These findings indicate that the monzogranitic activity was the result of underplating of mafic magma with an enriched mantle source. In the context of regional Late Paleozoic to Early Mesozoic magmatic activity, the Permian magmatism occurred in an Andean-style continental margin setting when the Paleo-Asian oceanic plate was subducted beneath the NMNCC, and in this context, the Late Permian to Early Triassic magmatism may have been linked to post-collisional extension and asthenospheric

  11. U-Pb zircon, geochemical and Sr-Nd-Hf-O isotopic constraints on age and origin of the ore-bearing intrusions from the Nurkazgan porphyry Cu-Au deposit in Kazakhstan

    NASA Astrophysics Data System (ADS)

    Shen, Ping; Pan, Hongdi; Seitmuratova, Eleonora; Jakupova, Sholpan

    2016-02-01

    Nurkazgan, located in northeastern Kazakhstan, is a super-large porphyry Cu-Au deposit with 3.9 Mt metal copper and 229 tonnage gold. We report in situ zircon U-Pb age and Hf-O isotope data, whole rock geochemical and Sr-Nd isotopic data for the ore-bearing intrusions from the Nurkazgan deposit. The ore-bearing intrusions include the granodiorite porphyry, quartz diorite porphyry, quartz diorite, and diorite. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating indicates that the granodiorite porphyry and quartz diorite porphyry emplaced at 440 ± 3 Ma and 437 ± 3 Ma, respectively. All host rocks have low initial 87Sr/86Sr ratios (0.70338-0.70439), high whole-rock εNd(t) values (+5.9 to +6.3) and very high zircon εHf(t) values (+13.4 to +16.5), young whole-rock Nd and zircon Hf model ages, and consistent and slightly high zircon O values (+5.7 to +6.7), indicating that the ore-bearing magmas derived from the mantle without old continental crust involvement and without marked sediment contamination during magma emplacement. The granodiorite porphyry and quartz diorite porphyry are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) and depleted in high-field strength elements (HFSE), Eu, Ba, Nb, Sr, P and Ti. The diorite and quartz diorite have also LILE and LREE enrichment and HFSE, Nb and Ti depletion, but have not negative Eu, Ba, Sr, and P anomalies. These features suggest that the parental magma of the granodiorite porphyry and quartz diorite porphyry originated from melting of a lithospheric mantle and experienced fractional crystallization, whereas the diorite and quartz diorite has a relatively deeper lithospheric mantle source region and has not experienced strong fractional crystallization. Based on these, together with the coeval ophiolites in the area, we propose that a subduction of the Balkhash-Junggar oceanic plate took place during the Early Silurian and the ore-bearing intrusions and associated Nurkazgan

  12. Origin of the granites and related Sn and Pb-Zn polymetallic ore deposits in the Pengshan district, Jiangxi Province, South China: constraints from geochronology, geochemistry, mineral chemistry, and Sr-Nd-Hf-Pb-S isotopes

    NASA Astrophysics Data System (ADS)

    Xu, Bin; Jiang, Shao-Yong; Luo, Lan; Zhao, Kui-Dong; Ma, Liang

    2016-05-01

    The Pengshan Sn and Pb-Zn polymetallic deposits are located in the south margin of the Jiujiang-Ruichang (Jiurui) district of the Middle-Lower Yangtze River Metallogenic Belt in South China. Four large deposits include Huangjinwa, Zengjialong, Jianfengpo, and Zhangshiba, the former three are Sn-dominant deposits which occur as stratiform orebodies in the contact zones of the Pengshan granites and within the country rock strata, whereas Zhangshiba consists of stratiform Pb-Zn orebodies within the Precambrian metasedimentary strata. In this study, we present results on zircon U-Pb ages, major and trace elements, and mineral chemistry as well as Sr-Nd-Hf isotope data of the granites, Pb and S isotopes of both the Sn-dominant and Pb-Zn dominant deposits, and U-Pb dating of cassiterite from the Pengshan district. SHRIMP and LA-ICP-MS zircon U-Pb dating shows that the Pengshan granites were emplaced in the Early Cretaceous (129-128 Ma), which is in good agreement with the U-Pb dating (130-128 Ma) of cassiterite from the Jianfengpo Sn deposit. The Pengshan granites consist mainly of weakly peraluminous highly fractionated I-type affinity granitic rocks. Detailed elemental and isotopic data suggest that the granites formed by partial melting of Mesoproterozoic metamorphic basement materials with minor input of mantle-derived melts. The mineral chemistry of biotite demonstrates that the Pengshan granitic magma had a low oxygen fugacity, thereby precluding the tin dominantly partitioning into the rock-forming silicate minerals and favoring accumulation in the exsolved residual liquid during magma crystallization stages. Sulfur isotopes show a relatively heavy sulfur isotopic composition from 5.8 to 17.6 ‰, and no difference for sulfur isotopes between the Sn deposits (5.8-13.4 ‰, Huangjinwa, Zengjialong, Jianfengpo) and the Pb-Zn deposit (mostly 7.1-13.0 ‰, except for one 17.6 ‰, Zhangshiba). The sulfur isotope data of pyrite from the host sedimentary rocks show

  13. Zircon U-Pb ages, Hf-O isotopes and trace elements of Mesozoic high Sr/Y porphyries from Ningzhen, eastern China: Constraints on their petrogenesis, tectonic implications and Cu mineralization

    NASA Astrophysics Data System (ADS)

    Wang, Fangyue; Liu, Sheng-Ao; Li, Shuguang; Akhtar, Shamim; He, Yongsheng

    2014-07-01

    The relationship between high Sr/Y (adakitic) rocks and Cu mineralization has been long recognized but the mechanism remains unclear. The Cretaceous high Sr/Y porphyries in the Ningzhen area host major Cu polymetallic deposits in the Lower Yangtze River Belt (LYRB) of eastern China. These rocks exhibit some geochemical characteristics (e.g., non-radiogenic Pb isotope ratios) that differ from adakitic rocks from adjacent locations in the LYRB. In this study, we present a study of the zircon U-Pb-Hf-O isotope and trace element compositions for five porphyries from Ningzhen to reveal their petrogenesis and how that correlates with Cu-Fe-Mo mineralization. Zircon U-Pb ages of Anjishan (Cu deposit), Tongshan (Cu-Mo deposit) and Xiangshan (Fe deposit) plutons in the Ningzhen area are 108.8 ± 1.5 Ma, 105-107 Ma and 100-105 Ma, respectively, which are significantly younger than the ore-bearing adakites (140 ± 5 Ma) in the western part of the LYRB. Zircon εHf(t) and δ18O values range from - 23.4 to - 10.6 and from 5.7 to 7.0‰, respectively, falling between subduction-related adakites from the other regions in the LYRB and delamination-related adakitic rocks from the adjacent South Tan-Lu Fault Zone. The similarities of Ce4 +/Ce3 + and Eu/Eu* ratios in zircons from Ningzhen and those from the western LYRB indicate higher oxygen fugacity in their magma sources. Ti-in-zircon thermometer yields magma temperatures of 550 to 700 °C (with an average of ~ 650 °C) for the Ningzhen porphyries, which are significantly lower than those of the South Tan-Lu Fault adakites (> 750 °C), but similar to those for the LYRB adakites (< 700 °C). In summary, the Ningzhen high Sr/Y porphyries have high Mg# (> 50), non-radiogenic Pb, enriched Sr-Nd isotopic compositions, negative zircon εHf(t), mantle-like δ18O values, high oxygen fugacities and low magma temperatures. Mafic rocks that co-exist with ore-bearing porphyries or occur as xenoliths in porphyries are widespread. We proposed

  14. Zircon U-Pb and molybdenite Re-Os geochronology and Sr-Nd-Pb-Hf isotopic constraints on the genesis of the Xuejiping porphyry copper deposit in Zhongdian, Northwest Yunnan, China

    NASA Astrophysics Data System (ADS)

    Leng, Cheng-Biao; Zhang, Xing-Chun; Hu, Rui-Zhong; Wang, Shou-Xu; Zhong, Hong; Wang, Wai-Quan; Bi, Xian-Wu

    2012-10-01

    The Xuejiping porphyry copper deposit is located in northwestern Yunnan Province, China. Tectonically, it lies in the southern part of the Triassic Yidun island arc. The copper mineralization is mainly hosted in quartz-dioritic and quartz-monzonitic porphyries which intruded into clastic-volcanic rocks of the Late Triassic Tumugou Formation. There are several alteration zones including potassic, strong silicific and phyllic, argillic, and propylitic alteration zones from inner to outer of the mineralized porphyry bodies. The ages of ore-bearing quartz-monzonitic porphyry and its host andesite are obtained by using the zircon SIMS U-Pb dating method, with results of 218.3 ± 1.6 Ma (MSWD = 0.31, N = 15) and 218.5 ± 1.6 Ma (MSWD = 0.91, N = 16), respectively. Meanwhile, the molybdenite Re-Os dating yields a Re-Os isochronal age of 221.4 ± 2.3 Ma (MSWD = 0.54, N = 5) and a weighted mean age of 219.9 ± 0.7 Ma (MSWD = 0.88). They are quite in accordance with the zircon U-Pb ages within errors. Furthermore, all of them are contemporary with the timing of the Garzê-Litang oceanic crust subduction in the Yidun arc. Therefore, the Xuejiping deposit could be formed in a continental margin setting. There are negative ɛNd(t) values ranging from -3.8 to -2.1 and relatively high initial 87Sr/86Sr ratios from 0.7051 to 0.7059 for the Xuejiping porphyries and host andesites. The (206Pb/204Pb)t, (207Pb/204Pb)t and (208Pb/204Pb)t values of the Xuejiping porphyries and host andesites vary from 17.899 to 18.654, from 15.529 to 15.626, and from 37.864 to 38.52, respectively, indicative of high radiogenic Pb isotopic features. In situ Hf isotopic analyses on zircons by using LA-MC-ICP-MS exhibit that there are quite uniform and slightly positive ɛHf(t) values ranging from -0.2 to +3.2 (mostly between 0 and +2), corresponding to relatively young single-stage Hf model ages from 735 Ma to 871 Ma. These isotopic features suggest that the primary magmas of the Xuejiping porphyries and

  15. U Pb zircon age, geochemical and Sr Nd Pb Hf isotopic constraints on age and origin of alkaline intrusions and associated mafic dikes from Sulu orogenic belt, Eastern China

    NASA Astrophysics Data System (ADS)

    Liu, Shen; Hu, Ruizhong; Gao, Shan; Feng, Caixia; Qi, Youqiang; Wang, Tao; Feng, Guangying; Coulson, Ian M.

    2008-12-01

    Post-orogenic alkaline intrusions and associated mafic dikes from the Sulu orogenic belt of eastern China consist of quartz monzonites, A-type granites and associated mafic dikes. We report here U-Pb zircon ages, geochemical data and Sr-Nd-Pb-Hf isotopic data for these rocks. The SHRIMP U-Pb zircon analyses yield consistent ages ranging from 120.3 ± 2.1 Ma to 126.9 ± 1.9 Ma for five samples from the felsic rocks, and two crystallization ages of 119.0 ± 1.7 Ma and 120.2 ± 1.9 Ma for the mafic dikes. The felsic rocks and mafic dikes are characterized by high ( 87Sr/ 86Sr) i ranging from 0.7079 to 0.7089, low ɛNd( t) values from - 15.3 to - 19.2, 206Pb/ 204Pb = 16.54-17.25, 207Pb/ 204Pb = 15.38-15.63, 208Pb/ 204Pb = 37.15-38.45, and relatively uniform ɛHf( t) values of between - 21.6 ± 0.6 and - 23.7 ± 1.0, for the magmatic zircons. The results suggest that they were derived from a common enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials before magma generation. Geochemical and isotopic characteristics imply that the primary magma to these rocks originated through partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dikes may have been generated by subsequent fractionation of clinopyroxene, whereas the felsic rocks resulted from fractionation of potassium feldspar, plagioclase and ilmenite or rutile. Both were not affected by crustal contamination. Combined with previous studies, these findings provide new evidence that the intense lithospheric thinning beneath the Sulu belt of eastern China occurred between 119 and 127 Ma, and that this was caused by the removal of the lower lithosphere (mantle and lower crust).

  16. Zircon U-Pb ages, geochemistry, and Sr-Nd-Pb-Hf isotopes of the Nuri intrusive rocks in the Gangdese area, southern Tibet: Constraints on timing, petrogenesis, and tectonic transformation

    NASA Astrophysics Data System (ADS)

    Chen, Lei; Qin, Ke-Zhang; Li, Guang-Ming; Li, Jin-Xiang; Xiao, Bo; Zhao, Jun-Xing; Fan, Xin

    2015-01-01

    Abundant magmatic rocks of various ages are exposed in Gangdese, southern Tibet. These rocks play an important role in understanding the tectonic transformation from the subduction of Neo-Tethyan oceanic crust to the collision of the Indian and Asian continents. Based on zircon U-Pb ages, geochemistry, and Sr-Nd-Pb-Hf isotopic data of the Late Cretaceous to early Oligocene (~ 96-30 Ma) intrusive rocks in the Nuri Cu-W-Mo deposit, we discuss the Late Cretaceous to early Oligocene tectonic transformation of the region and the origin of Oligocene Cu-W-Mo mineralization in southern Gangdese. The Nuri intrusive rocks represent three magmatic episodes: 96-91, 56-52, and 33-30 Ma. The 96-91 and 56-52 Ma rocks have relatively low (87Sr/86Sr)i (0.7041 to 0.7060), and high εNd(t) (+ 3.1 to + 3.5) and εHf(t) values (+ 3.7 to + 15); the 33-30 Ma rocks have relatively high (87Sr/86Sr)i (0.7061 to 0.7063) and Pb isotopes, and low εNd(t) (- 3.8 to - 1.8) and εHf(t) values (+ 0.6 to + 10.1). The three stages of intrusive rocks have geochemical characteristics that are similar to those of coeval rocks in Gangdese. The 96-91 and 33-30 Ma rocks are adakitic, whereas the 56-52 Ma rocks have characteristics of arc calc-alkaline magmatic rocks. The 96-91 Ma rocks were produced by the partial melting of Neo-Tethyan basaltic oceanic crust and minor sediments, whereas the 56-52 Ma rocks were generated by the partial melting of juvenile crust and the 33-30 Ma rocks were formed by the melting of Indian plate lower crust contaminated with overlying mantle materials. On the basis of the regional tectonic and magmatic characteristics, we suggest that Neo-Tethyan oceanic slab subduction and slab roll-back occurred from ~ 100 to 65 Ma, collision between the Indian and Asian continents occurred at 65 to 40 Ma, Neo-Tethyan oceanic slab break-off took place at ~ 50 Ma, and the Indian continent subducted northwards beneath the Asian continent at ~ 30 Ma. From the Late Cretaceous (96-91 Ma) to

  17. Detrital zircon U-Pb age and Hf isotopic composition from foreland sediments of the Assam Basin, NE India: Constraints on sediment provenance and tectonics of the Eastern Himalaya

    NASA Astrophysics Data System (ADS)

    Vadlamani, Ravikant; Wu, Fu-Yuan; Ji, Wei-Qiang

    2015-11-01

    Synorogenic Palaeogene-Neogene sediments of the Assam foreland basin, were derived by erosion of adjacent crustal and orogenic sources following the Greater India-Eurasia collision since ∼55 Ma. To constrain source sediment influx, and its relation to Himalayan tectonics, from pre- to post-collision time, detrital zircon U-Pb geochronology and their Hf isotopic compositions were carried out. The varying detrital zircon spectral patterns analyzed from the Paleogene Jaintia, Barail and Neogene Surma and Tipam Groups, with sediment petrography, track source sediment derived from cratonic India, Gangdese and eastern Transhimalayan batholiths and the eastern Himalaya. These sources are tested against Cenozoic paleopositions proposed for the northeastward motion of the Indian plate. Precollisional cratonic detritus to Middle to Late Eocene Sylhet Formation shifted to Tethyan Himalaya and arc sources of the Gangdese and eastern Transhimalayan batholiths to Late Eocene Kopili and Barail Formations, consistent with the proposed paleoposition proximal to the Indus-Yarlung suture. This Sylhet-Kopili Formation transition, within the Jaintia Group, reflects one of the earliest Himalayan hinterland exhumation stages during the Late Eocene. Major shift in provenance to Higher Himalayan Crystalline and arc detritus is recorded from the Surma Group, constraining Mid Miocene Himalayan tectonic exhumation from the eastern Himalaya. Late Miocene Tipam Group preserves sediment of Higher Himalayan Crystalline detritus, ophiolite and likely Lesser Himalayan rocks.

  18. Neoproterozoic-middle Paleozoic tectono-magmatic evolution of the Gorny Altai terrane, northwest of the Central Asian Orogenic Belt: Constraints from detrital zircon U-Pb and Hf-isotope studies

    NASA Astrophysics Data System (ADS)

    Chen, Ming; Sun, Min; Buslov, Mikhail M.; Cai, Keda; Zhao, Guochun; Zheng, Jianping; Rubanova, Elena S.; Voytishek, Elena E.

    2015-09-01

    The Gorny Altai terrane (GA) is a key area in understanding the crustal evolution of the Central Asian Orogenic Belt (CAOB). This paper reports U-Pb and Hf-isotope data for detrital zircons from Cambrian to early Devonian sedimentary sequences to constrain their provenance, as well as the tectono-magmatic events and crustal growth in this region. Nearly all the detrital zircons are characterized by euhedral to subhedral morphology, high Th/U ratios (ca. 0.1-1.6) and typical oscillatory zoning, indicating a magmatic origin. The three samples from the Gorny Altai Group (middle Cambrian to early Ordovician) yield detrital zircon populations that are composed predominantly of 530-464 Ma grains, followed by a subordinate group of 641-549 Ma old. The Silurian and Devonian samples exhibit similar major zircon populations (555-456 Ma and 525-463 Ma, respectively), but a significant amount of additional 2431-772 Ma zircons occur in the early Devonian sample. Our results suggest that detritus from the nearby Kuznetsk-Altai intra-oceanic island arc served as a unitary source for the Cambrian-Silurian sedimentary sequences, but older detritus from other sources added to the early Devonian sequence. The low abundance of ca. 640-540 Ma detrital zircons may testify that this island arc was under a primitive stage in this period, when mafic volcanic rocks probably dominated. In contrast, the dominant population of ca. 530-470 Ma zircons may indicate an increased amount of granitic rocks in the source area, suggesting that the Kuznetsk-Altai island arc possibly evolved into a mature one in the Cambrian to early Ordovician. The ca. 530-470 Ma detrital zircons are almost exclusively featured by positive εHf(t) values and have two-stage Hf model ages of ca. 1.40-0.45 Ga, indicating that the precursor magmas were sourced predominantly from heterogeneous juvenile materials. We conclude that the late Neoproterozoic to early Paleozoic magmatism in the Kuznetsk-Altai arc made a

  19. Geochronology, elemental and Nd-Hf isotopic geochemistry of Devonian A-type granites in central Jiangxi, South China: Constraints on petrogenesis and post-collisional extension of the Wuyi-Yunkai orogeny

    NASA Astrophysics Data System (ADS)

    Feng, Shang-Jie; Zhao, Kui-Dong; Ling, Hong-Fei; Chen, Pei-Rong; Chen, Wei-Feng; Sun, Tao; Jiang, Shao-Yong; Pu, Wei

    2014-10-01

    Details on processes of the early-middle Paleozoic Wuyi-Yunkai orogeny in South China remain poorly defined. Most Silurian-Devonian granites in South China are S-type or I-type granites, which are suggested to be petrogenetically related to the Wuyi-Yunkai orogeny. This paper firstly reported a systematic study on two Devonian A-type granites in the central Jiangxi Province. LA-ICP-MS zircon U-Pb dating results imply that the Huitong and Epo granites were emplaced at about 415 Ma. Both of the two granites have the petrographic and geochemical characteristics of A-type granites. Interstitial biotites occur along the boundary of euhedral plagioclase and quartz and they formed later than plagioclase and quartz. It implies that the primary magma could have been anhydrous. Biotites from the two granites are Fe-rich and have high Fe2 +/(Fe2 + + Mg2 +) ratios (0.60-0.74). The magmatic temperatures estimated from zircon saturation thermometer are 802-920 °C for the two granites, higher than common I-type and S-type granites. The two granites show high contents of total alkalis (Na2O + K2O = 6.96-9.39 wt.%), high field strength elements (e.g. Zr = 181-437 ppm, Y = 22.1-39.7 ppm, Nb = 18.6-30.3 ppm and Zr + Nb + Ce + Y = 324-555 ppm), rare earth elements (total REE = 155-312 ppm) as well as high Ga/Al ratios (10,000 × Ga/Al = 2.50-3.44). These geochemical characteristics are similar to those of A-type granites. The Huitong and Epo granites have relatively low εNd(t) values of - 10.4 to - 7.7, and low zircon εHf(t) values (peak value of - 8.0). Whole-rock two stage Nd isotopic model ages and zircon Hf isotopic model ages mostly vary from 1.78 Ga to 2.00 Ga. According to these data, we suggest that the two granites might have derived from partial melting of pre-Cambrian sedimentary rocks which had been granulitized during an earlier thermal event. The two granites contain abundant contemporaneous mafic microgranular enclaves (MMEs). The MMEs display igneous textures and

  20. Petrogenesis of the Yaochong granite and Mo deposit, Western Dabie orogen, eastern-central China: Constraints from zircon U-Pb and molybdenite Re-Os ages, whole-rock geochemistry and Sr-Nd-Pb-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Chen, Wei; Xu, Zhaowen; Qiu, Wenhong; Li, Chao; Yu, Yang; Wang, Hao; Su, Yang

    2015-05-01

    The Dabie orogen is among the most famous continent-continent collisional orogenic belts in the world, and is characterized by intensive post-collisional extension, magmatism and Mo mineralization. However, the genetic links between the mineralization and the geodynamic evolution of the orogen remain unresolved. In this paper, the Yaochong Mo deposit and its associated granitic stocks were investigated to elucidate this issue. Our new zircon U-Pb ages yielded an Early Cretaceous age (133.3 ± 1.3 Ma) for the Yaochong granite, and our molybdenite Re-Os dating gave a similar age (135 ± 1 Ma) for the Mo deposit. The Yaochong stock is characterized by high silica and alkali but low Mg, Fe and Ca. It is enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs: Rb, K, Th and U), but strongly depleted in heavy REEs, and high field strength elements (HFSEs: Nb, Ta, Ti and Y). The Yaochong granite has initial 87Sr/86Sr ratios of 0.7087-0.7096, and Pb isotopic ratios of (206Pb/204Pb)i = 16.599-16.704, (207Pb/204Pb)i = 15.170-15.618 and (208Pb/204Pb)i = 36.376-38.248. The granite has εNd(t) of -18.0 to -16.3 and εHf(t) values of -26.5 to -20.0. All these data indicate that the Yaochong granite is a high-K calc-alkaline fractionated I-type granite, and may have originated from partial melting of the thickened Yangtze continental crust. The Mo ores also show low radiogenic Pb isotopes similar to the Yaochong stock. Medium Re content in molybdenite (21.8-74.8 ppm) also suggests that the ore-forming materials were derived from the thickened lower crust with possibly minor mixing with the mantle. Similar to the Eastern Dabie orogen, the thickened crust beneath the Western Dabie orogen may also have experienced tectonic collapse, which may have exerted fundamental geodynamic controls on the two-stage Mo mineralization in the region.

  1. Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution

    USGS Publications Warehouse

    Patchett, P.J.

    1983-01-01

    The 176Lu-176Hf isotope method and its applications in earth sciences are discussed. Greater fractionation of Lu/Hf than Sm/Nd in planetary magmatic processes makes 176Hf 177Hf a powerful geochemical tracer. In general, proportional variations of 176Hf 177Hf exceed those of 143Nd l44Nd by factors of 1.5-3 in terrestrial and lunar materials. Lu-Hf studies therefore have a major contribution to make in understanding of terrestrial and other planetary evolution through time, and this is the principal importance of Lu-Hf. New data on basalts from oceanic islands show unequivocally that whereas considerable divergences occur in 176Hf 177Hf- 87Sr 86Sr and 143Nd l44Nd- 87Sr 86Sr diagrams, 176Hf 177Hf and 143Nd 144Nd display a single, linear isotopic variation in the suboceanic mantle. These discordant 87Sr 86Sr relationships may allow, with the acquisition of further Hf-Nd-Sr isotopic data, a distinction between processes such as mantle metasomatism, influence of seawater-altered material in the magma source, or recycling of sediments into the mantle. In order to evaluate the Hf-Nd isotopic correlation in terms of mantle fractionation history, there is a need for measurements of Hf distribution coefficients between silicate minerals and liquids, and specifically for a knowledge of Hf behavior in relation to rareearth elements. For studying ancient terrestrial Hf isotopic variations, the best quality Hf isotope data are obtained from granitoid rocks or zircons. New data show that very U-Pb discordant zircons may have upwardly-biased 176Hf 177Hf, but that at least concordant to slightly discordant zircons appear to be reliable carriers of initial 176Hf 177Hf. Until the controls on addition of radiogenic Hf to zircon are understood, combined zircon-whole rock studies are recommended. Lu-Hf has been demonstrated as a viable tool for dating of ancient terrestrial and extraterrestrial samples, but because it offers little advantage over existing methods, is unlikely to find

  2. Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution

    NASA Technical Reports Server (NTRS)

    Patchett, P. J.

    1983-01-01

    The Lu-176-Hf-176 isotope method and its applications in earth sciences are discussed with regard to planetary-evolution studies. From new data on basalts from oceanic islands, Hf-176/Hf-177 and Nd-143/Nd-144 are found to display a single linear isotopic variation in the suboceanic mantle, whereas considerable divergences occur in Hf-176/Hf-177-Sr-87/Sr-86 and Nd-143/Nd-144-Sr87/Sr-86 diagrams. With the acquisition of further Hf-Sr-Nd isotopic data, these discordant Sr-87/Sr-86 relationships may allow a distinction between processes such as mantle metasomatism, influence of sea-water altered material in the magma source, or recycling of sediments into the mantle. The best quality Hf isotope data are obtained from granitoid or zircons, and are most suitable for studying ancient terrestrial Hf isotopic variations. Lu-Hf is shown to be a viable method for dating ancient terrestrial and extraterrestrial samples, but is unlikely to find wide application in pure chronological studies because it offers little advantage over existing methods.

  3. Hf isotope compositions of U.S. Geological Survey reference materials

    NASA Astrophysics Data System (ADS)

    Weis, Dominique; Kieffer, Bruno; Hanano, Diane; Nobre Silva, Inês; Barling, Jane; Pretorius, Wilma; Maerschalk, Claude; Mattielli, Nadine

    2007-06-01

    A systematic multi-isotopic and trace element characterization of U.S. Geological Survey reference materials has been carried out at the Pacific Centre for Isotopic and Geochemical Research, University of British Columbia. Values of 176Hf/177Hf are recommended for the following reference materials (mean ±2 SD): G-2: 0.282523 ± 6; G-3: 0.282518 ± 1; GSP-2: 0.281949 ± 8; RGM-1: 0.283017 ± 13; STM-1: 0.283019 ± 12; STM-2: 0.283021 ± 5; BCR-1: 0.282875 ± 8; BCR-2: 0.282870 ± 8; BHVO-1: 0.283106 ± 12; BHVO-2: 0.283105 ± 11; AGV-1: 0.282979 ± 6; and AGV-2: 0.282984 ± 9. Reproducibility is better than 50 ppm for the granitoid compositions and better than 40 ppm for the basaltic/andesitic compositions. For the isotopic analyses acquired early in this project on glass columns, Hf isotopic analyses from several of the reference materials were significantly less reproducible than Nd and Sr isotopic analyses determined from the same sample dissolution. The 176Hf/177Hf ratios for relatively radiogenic compositions (BCR-1, 2; BHVO-1, 2; RGM-1) were shifted systematically toward lower values by 100-150 ppm when a borosilicate primary column was used. Although systematic, the shift for felsic compositions was generally within analytical error, except for GSP-2, which has a very low Hf isotopic ratio, where the shift was to higher 176Hf/177Hf. Trace element and isotopic characterization of the borosilicate glass column, borosilicate frits, and quartz columns reveals extremely variable levels of trace elements. The 176Hf/177Hf ratios for these materials are very unradiogenic (borosilicate glass <0.28220 frit = 0.28193 ± 4). The borosilicate frit material appears to be the most variable in elemental concentration and isotopic composition. The quartz material has very low levels (Hf/177Hf and high Hf concentrations of the borosilicate glass column (16 ppm) and frit material (22 ppm) indicate that only small amounts of such unradiogenic

  4. REE and Hf distribution among mineral phases in the CV-CK clan: A way to explain present-day Hf isotopic variations in chondrites

    NASA Astrophysics Data System (ADS)

    Martin, Céline; Debaille, Vinciane; Lanari, Pierre; Goderis, Steven; Vandendael, Isabelle; Vanhaecke, Frank; Vidal, Olivier; Claeys, Philippe

    2013-11-01

    Chondrites are among the most primitive objects in the Solar System and constitute the main building blocks of telluric planets. Among the radiochronometers currently used for dating geological events, Sm-Nd and Lu-Hf are both composed of refractory, lithophile element. They are thought to behave similarly as the parent elements (Sm and Lu) are generally less incompatible than the daughter elements (Nd and Hf) during geological processes. As such, their respective average isotopic compositions for the solar system should be well defined by the average of chondrites, called Chondritic Uniform Reservoir (CHUR). However, while the Sm-Nd isotopic system shows an actual spread of less than 4% in the average chondritic record, the Lu-Hf system shows a larger variation range of 28% [Bouvier A., Vervoort J. D. and Patchett P. J. (2008) The Lu-Hf and Sm-Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets. Earth Planet. Sci. Lett.273, 48-57]. To better understand the contrast between Sm-Nd and Lu-Hf systems, the REE and Hf distribution among mineral phases during metamorphism of Karoonda (CK) and Vigarano-type (CV) carbonaceous chondrites has been examined. Mineral modes were determined from elemental mapping on a set of five CK chondrites (from types 3-6) and one CV3 chondrite. Trace-element patterns are obtained for the first time in all the chondrite-forming minerals of a given class (CK chondrites) as well as one CV3 sample. This study reveals that REE are distributed among both phosphates and silicates. Only 30-50% of Sm and Nd are stored in phosphates (at least in chondrites types 3-5); as such, they are not mobilized during early stages of metamorphism. The remaining fraction of Sm and Nd is distributed among the same mineral phases; these elements are therefore not decoupled during metamorphism. Of the whole-rock total of Lu, the fraction held in phosphate decreases significantly

  5. A routine high-precision method for Lu-Hf isotope geochemistry and chronology

    USGS Publications Warehouse

    Patchett, P.J.; Tatsumoto, M.

    1981-01-01

    A method for chemical separation of Lu and Hf from rock, meteorite and mineral samples is described, together with a much improved mass spectrometric running technique for Hf. This allows (i) geo- and cosmochronology using the176Lu???176Hf+??- decay scheme, and (ii) geochemical studies of planetary processes in the earth and moon. Chemical yields for the three-stage ion-exchange column procedure average 90% for Hf. Chemical blanks are <0.2 ng for Lu and Hf. From 1 ??g of Hf, a total ion current of 0.5??10-11 Ampere can be maintained for 3-5 h, yielding 0.01-0.03% precision on the ratio176Hf/177Hf. Normalisation to179Hf/177Hf=0.7325 is used. Extensive results for the Johnson Matthey Hf standard JMC 475 are presented, and this sample is urged as an international mass spectrometric standard; suitable aliquots, prepared from a single batch of JMC 475, are available from Denver. Lu-Hf analyses of the standard rocks BCR-1 and JB-1 are given. The potential of the Lu-Hf method in isotope geochemistry is assessed. ?? 1980 Springer-Verlag.

  6. Hf isotope systematics of seamounts near the East Pacific Rise (EPR) and geodynamic implications

    NASA Astrophysics Data System (ADS)

    Zhang, Yu; Meng, Fanxue; Niu, Yaoling

    2016-10-01

    We report new Hf isotopic data for basaltic glasses from seamounts flanking the East Pacific Rise (EPR) between 5° and 15°N that have been previously analyzed for Sr-Nd-Pb isotopes as well as major and trace elements. The Hf isotopic data offer new perspectives on the petrogenesis of these samples in a broader context on mantle dynamics. The Hf isotope compositions show significant correlations with Sr-Nd-Pb isotopes and with both abundances and ratios of incompatible elements. The seamount lavas are thus best interpreted as products of melting-induced mixing in a two-component mantle. The range in composition of EPR seamount lavas cannot be generated by simple mixing of melt and melting of variably heterogeneous mantle in which enriched and depleted materials contribute equally to melting (source mixing). Instead, the trace element and isotope compositions of seamount lavas can be reproduced by melting models in which more enriched, fertile mantle component are preferentially melted during mantle upwelling. At progressively lower degrees of melting, erupted lavas are thus more enriched in incompatible trace elements, have higher 87Sr/86Sr, 208Pb/204Pb ratios and lower 143Nd/144Nd, 176Hf/177Hf ratios. The "EM1" and "pyroxenite" endmember might be the suitable enriched component. The Hf-Nd isotopic variations on global scale might result from the variations in amounts of residual continental lithospheric mantle that detached into upper mantle during continental rifting. The significant correlations of Rb/Sr vs 87Sr/86Sr, Sm/Nd vs 143Nd/144Nd and Lu/Hf vs 176Hf/177Hf give pseudochron ages of 182 ± 33 Ma, 276 ± 50 Ma and 387 ± 93 Ma, respectively. These different "ages" have no significance, but result from melting-induced mixing with the pseudochron slopes controlled by the compositions of enriched component and depleted end-member.

  7. Neutron capture on Pt isotopes in iron meteorites and the Hf-W chronology of core formation in planetesimals

    NASA Astrophysics Data System (ADS)

    Kruijer, Thomas S.; Fischer-Gödde, Mario; Kleine, Thorsten; Sprung, Peter; Leya, Ingo; Wieler, Rainer

    2013-01-01

    The short-lived 182Hf-182W isotope system can provide powerful constraints on the timescales of planetary core formation, but its application to iron meteorites is hampered by neutron capture reactions on W isotopes resulting from exposure to galactic cosmic rays. Here we show that Pt isotopes in magmatic iron meteorites are also affected by capture of (epi)thermal neutrons and that the Pt isotope variations are correlated with variations in 182W/184W. This makes Pt isotopes a sensitive neutron dosimeter for correcting cosmic ray-induced W isotope shifts. The pre-exposure 182W/184W derived from the Pt-W isotope correlations of the IID, IVA and IVB iron meteorites are higher than most previous estimates and are more radiogenic than the initial 182W/184W of Ca-Al-rich inclusions (CAI). The Hf-W model ages for core formation range from +1.6±1.0 million years (Ma; for the IVA irons) to +2.7±1.3 Ma after CAI formation (for the IID irons), indicating that there was a time gap of at least ˜1 Ma between CAI formation and metal segregation in the parent bodies of some iron meteorites. From the Hf-W ages a time limit of <1.5-2 Ma after CAI formation can be inferred for the accretion of the IID, IVA and IVB iron meteorite parent bodies, consistent with earlier conclusions that the accretion of differentiated planetesimals predated that of most chondrite parent bodies.

  8. Petrogenesis and tectonic implications of the high-K Alamas calc-alkaline granitoids at the northwestern margin of the Tibetan Plateau: Geochemical and Sr-Nd-Hf-O isotope constraints

    NASA Astrophysics Data System (ADS)

    Zhang, Qichao; Liu, Yan; Huang, He; Wu, Zhenhan; Zhou, Qing

    2016-09-01

    The Alamas granitoid pluton in the eastern part of the Western Kunlun Orogen, the northwestern margin of the Tibetan Plateau, is composed of quartz diorite. Zircon separates from the pluton has SIMS U-Pb age of ∼446 Ma. Rocks from the pluton have a narrow range of SiO2 (56.84-62.57 wt%), MgO (1.76-2.94 wt%), and total alkalis (Na2O + K2O = 5.14-9.59 wt%), and are metaluminous and high-K calc-alkaline to shoshonitic in composition. They are enriched in light rare earth elements (LREEs), with (La/Yb)N = 14-25, and show weakly negative Eu anomalies. These rocks are relatively enriched in Sr (472-676 ppm) and Ba (435-2388 ppm), and depleted in Nb, Ta, Th, and Ti. Their εNd(t) values range from -6.4 to -8.4, and (87Sr/86Sr)i = 0.7184-0.7200. Zircons from the pluton show εHf(t) values of -1.4 to -8.8, and δ18O = 6.4-9.0‰. Geochemical data indicate that the granitoids were likely derived from the reworking of an ancient, deep crustal source, influenced by a minor mantle-derived component. Magmatic differentiation was dominated by the fractional crystallization of hornblende, biotite, and accessory minerals such as apatite, allanite, and Fe-Ti oxides. In summary, the Late Ordovician Alamas pluton is an I-type granitoid that was emplaced in a post-collisional environment, suggesting that this tectonic stage had already initiated prior to ∼445 Ma.

  9. Subduction Controls of Hf and Nd Isotopes in Lavas of the Aleutian Island Arc

    SciTech Connect

    Yogodzinski, Gene; Vervoort, Jeffery; Brown, Shaun Tyler; Gerseny, Megan

    2010-08-29

    The Hf and Nd isotopic compositions of 71 Quaternary lavas collected from locations along the full length of the Aleutian island arc are used to constrain the sources of Aleutian magmas and to provide insight into the geochemical behavior of Nd and Hf and related elements in the Aleutian subduction-magmatic system. Isotopic compositions of Aleutian lavas fall approximately at the center of, and form a trend parallel to, the terrestrial Hf-Nd isotopic array with {var_epsilon}{sub Hf} of +12.0 to +15.5 and {var_epsilon}{sub Nd} of +6.5 to +10.5. Basalts, andesites, and dacites within volcanic centers or in nearby volcanoes generally all have similar isotopic compositions, indicating that there is little measurable effect of crustal or other lithospheric assimilation within the volcanic plumbing systems of Aleutian volcanoes. Hafnium isotopic compositions have a clear pattern of along-arc increase that is continuous from the eastern-most locations near Cold Bay to Piip Seamount in the western-most part of the arc. This pattern is interpreted to reflect a westward decrease in the subducted sediment component present in Aleutian lavas, reflecting progressively lower rates of subduction westward as well as decreasing availability of trench sediment. Binary bulk mixing models (sediment + peridotite) demonstrate that 1-2% of the Hf in Aleutian lavas is derived from subducted sediment, indicating that Hf is mobilized out of the subducted sediment with an efficiency that is similar to that of Sr, Pb and Nd. Low published solubility for Hf and Nd in aqueous subduction fluids lead us to conclude that these elements are mobilized out of the subducted component and transferred to the mantle wedge as bulk sediment or as a silicate melt. Neodymium isotopes also generally increase from east to west, but the pattern is absent in the eastern third of the arc, where the sediment flux is high and increases from east to west, due to the presence of abundant terrigenous sediment in the

  10. Zircon U-Pb and Lu-Hf isotopic and whole-rock geochemical constraints on the Lanhe and Heichashan Groups: Implications for the Paleoproterozoic tectonic basin evolution of the Lüliang Complex

    NASA Astrophysics Data System (ADS)

    Liu, Chaohui; Zhao, Guochun; Liu, Fulai; Shi, Jianrong; Ji, Lei; Liu, Pinghua; Yang, Hong; Liu, Lishuang; Wang, Wei; Tian, Zhonghua

    2016-10-01

    The Lüliang Complex is located at the western margin of the middle segment of the Trans-North China Orogen, along which the Western and Eastern Blocks collided to form the North China Craton. The complex mainly consists of metamorphosed granitic plutons and supracrustal rocks, of which the latter are subdivided into the Jiehekou, Lüliang, Yejishan, Lanhe and Heichashan Groups. The Lanhe Group is composed of meta-conglomerates, quartzites, and phyllites with minor meta-basalts, whereas the Heichashan Group consists of molasse-like meta-conglomerates and coarse-grained quartzites. Geochemistry of the Yejishan meta-sedimentary rocks indicates weak source weathering and dominantly chemical immature features, whereas the Lanhe and Heichashan samples display opposite features. U-Pb ages of detrital zircons from the Lanhe Groups yield four age peaks at ~ 2180 Ma, ~ 2370 Ma, ~ 2520 Ma and ~ 2700 Ma. The former three peaks coincide with ages of the Chijianling-Guandishan TTG gneisses (2199-2151 Ma) and meta-volcanic rocks from the Lüliang and Yejishan Groups (2213-2156 Ma), age of the Gaijiazhuang porphyritic gneisses (2375-2364 Ma) and age of the Yunzhongshan TTG gneisses (2499 Ma) respectively, whereas detrital zircons forming the oldest age peak were most likely derived from the early Neoarchean crust of the Eastern Block. For the Heichashan Group, the dominant 2.2-2.0 Ga detrital zircons were probably recycled from the underlying Jiehekou Group and the minority is directly derived from the early Paleoproterozoic granitoids in the Lüliang Complex. The youngest detrital zircon age peaks of ~ 2.17 Ga and ~ 1.82 Ga place maximum depositional ages on the Lanhe and Heichashan Groups respectively, whereas the local 1.81-1.79 Ga massive granites place constraint on their minimum depositional ages. Taking into account the lithostratigraphic features, provenance and formation ages, we suggest that the Lanhe Group formed in a shrinked remnant back-arc basin and the Heichashan

  11. Photon scattering experiments off 176Hf and the systematics of low-lying dipole modes in the stable even-even Hf isotopes 176,178,180Hf

    NASA Astrophysics Data System (ADS)

    Scheck, M.; Belic, D.; von Brentano, P.; Carroll, J. J.; Fransen, C.; Gade, A.; von Garrel, H.; Kneissl, U.; Kohstall, C.; Linnemann, A.; Pietralla, N.; Pitz, H. H.; Stedile, F.; Toman, R.; Werner, V.

    2003-06-01

    The low-lying dipole strength distribution in the rare isotope 176Hf was studied in nuclear resonance fluorescence experiments performed at the Stuttgart Dynamitron facility using bremsstrahlung beams with end- point energies of 4.1 and 2.4 MeV. In total, about 55 excited spin-1 states, unknown so far, were observed in the excitation energy range up to 4 MeV. Detailed spectroscopic information has been obtained on excitation energies, spins, decay widths, decay branchings, and transition probabilities. Ascribing a positive parity to all observed K=1 states, the detected total B(M1)↑ strength in the energy range of the scissors mode amounts to 2.56(6) μ2N, nearly as much as for well-deformed midshell rare-earth nuclei. The total strength is higher than in the heavier Hf isotopes 178,180Hf, but fits well into the systematics. The observed low-lying ΔK=0 transitions (with probable E1 character) lie in the energy range around 2 MeV, as expected from the systematics. The excitation probabilities correspond to values which are characteristic for nuclei in the transitional region from deformed rotors to more γ-soft nuclei.

  12. Geodynamic investigation of the processes that control Lu-Hf isotopic differences between different mantle domains and the crust

    NASA Astrophysics Data System (ADS)

    Jones, Rosie; van Keken, Peter; Hauri, Erik; Vervoort, Jeff; Ballentine, Chris J.

    2016-04-01

    The chemical and isotopic composition of both the Earth's mantle and the continental crust are greatly influenced by subduction zone processes, such as the formation of continental crust through arc volcanism and the recycling of surface material into the deep mantle. Here we use a combined geodynamical-geochemical approach to investigate the long term role of subduction on the Lu-Hf isotopic evolution of the mantle and the continental crust. We apply the geodynamic model developed by Brandenburg et al., 2008. This model satisfies the geophysical constraints of oceanic heat flow and average plate velocities, as well as geochemical observations such as 40Ar in the atmosphere, and reproduces the geochemical distributions observed in multiple isotope systems which define the HIMU, MORB and EM1 mantle endmembers. We extend this application to investigate the detail of terrestrial Lu-Hf isotope distribution and evolution, and specifically to investigate the role of sediment recycling in the generation of EM2 mantle compositions. The model has been updated to produce higher resolution results and to include a self-consistent reorganisation of the plates with regions of up-/down-wellings. The model assumes that subduction is initiated at 4.5 Ga and that a transition from 'dry' to 'wet' subduction occurred at 2.5 Ga. The modelling suggests that the epsilon Hf evolution of the upper mantle can be generated through the extraction and recycling of the oceanic crust, and that the formation of continental crust plays a lesser role. Our future intention is to utilise the model presented here to investigate the differences observed in the noble gas compositions (e.g., 40Ar/36Ar, 3He/4He) of MORB and OIB. Brandenburg, J.P., Hauri, E.H., van Keken, P.E., Ballentine, C.J., 2008. Earth and Planetary Science Letters 276, 1-13.

  13. Bulk Sediment Hf-Nd Isotopic Composition Across the EOT, Northern Hemisphere Glaciation?

    NASA Astrophysics Data System (ADS)

    Duggan, B.; Buckley, W. P., Jr.; Bizimis, M.; Scher, H. D.

    2015-12-01

    In recent decades, near and far field proxies of continental ice production indicate the presence of continental ice on Antarctica. Short Antarctic glaciations blinked in and out of existence throughout the middle and late Eocene, culminating in the formation of a continental ice sheet during the Eocene Oligocene Transition (EOT; ~34 Ma). Moreover, the onset of the Antarctic glaciation coincides with pCO2 declining below a critical threshold for the accumulation of a continental ice sheet. New evidence suggesting bipolar glaciation (that is, northern and southern hemisphere) occurred through this period with ice sheets on Greenland and Antarctica. However, the pCO2 threshold for the accumulation of ice on Greenland is not reached until the late Oligocene. Preliminary hafnium-neodymium (Hf-Nd) isotope results of oxyhydroxide leachates from IODP Site U1411 on the Newfoundland Ridge points to increased weathering intensity coinciding with the EOT, marked by less radiogenic Hf isotope compositions. One interpretation of this data is that glaciation of the northern hemisphere (e.g. Greenland) coincides with that of Antarctica during the EOT. Hf-Nd isotopic composition of sediment on the Newfoundland ridge indicates a shift from incongruous chemical weathering to a more congruous mechanical weathering regime (i.e. glaciers). However, it could be suggested that the observed congruous Hf-Nd isotopic signal originates in the southern ocean and has been propagated north from the Antarctic. We are using sediment core from the equatorial Pacific to determine if a signal of glacial weathering could be transmitted though deep waters from Antarctica. The core, IODP Site 1333, is in the equatorial Pacific positioned far from either pole thus, a shift towards a less radiogenic Hf isotopic compositions is not to be expected. The absence of a shift in Hf isotopes in the oxydroxide leachates, or a shift of lesser magnitude, will strengthen the possibility of northern hemisphere

  14. Evolution of continental crust and mantle heterogeneity: Evidence from Hf isotopes

    USGS Publications Warehouse

    Jonathan, Patchett P.; Kouvo, O.; Hedge, C.E.; Tatsumoto, M.

    1982-01-01

    We present initial 176Hf/177 Hf ratios for many samples of continental crust 3.7-0.3 Gy old. Results are based chiefly on zircons (1% Hf) and whole rocks: zircons are shown to be reliable carriers of essentially the initial Hf itself when properly chosen on the basis of U-Pb studies. Pre-3.0 Gy gneisses were apparently derived from an unfractionated mantle, but both depleted and undepleted mantle are evident as magma sources from 2.9 Gy to present. This mantle was sampled mainly from major crustal growth episodes 2.8, 1.8 and 0.7 Gy ago, all of which show gross heterogeneity of 176Hf/177Hf in magma sources from ??Hf=0 to +14, or about 60% of the variability of the present mantle. The approximate ??Hf=2??Nd relationship in ancient and modern igneous rocks shows that 176Lu/177Hf fractionates in general twice as much as 147Sm/144Nd in mantle melting processes. This allows an estimation of the relative value of the unknown bulk solid/liquid distribution coefficient for Hf. DLu/DHf=??? 2.3 holds for most mantle source regions. For garnet to be an important residual mantle phase, it must hold Hf strongly in order to preserve Hf-Nd isotopic relationships. The ancient Hf initials are consistent with only a small proportion of recycled older cratons in new continental crust, and with quasi-continuous, episodic growth of the continental crust with time. However, recycling of crust less than 150 My old cannot realistically be detected using Hf initials. The mantle shows clearly the general positive ??Hf resulting from a residual geochemical state at least back to 2.9 Gy ago, and seems to have repeatedly possessed a similar degree of heterogeneity, rather than a continuously-developing depletion. This is consistent with a complex dynamic disequilibrium model for the creation, maintenance and destruction of heterogeneity in the mantle. ?? 1981 Springer-Verlag.

  15. Petrogenesis and economic potential of the Erhongwa mafic-ultramafic intrusion in the Central Asian Orogenic Belt, NW China: Constraints from olivine chemistry, U-Pb age and Hf isotopes of zircons, and whole-rock Sr-Nd-Pb isotopes

    NASA Astrophysics Data System (ADS)

    Sun, Tao; Qian, Zhuang-Zhi; Li, Chusi; Xia, Ming-Zhe; Yang, Su-Hong

    2013-12-01

    The Erhongwa mafic-ultramafic intrusion is located in the southern margin of the Central Asian Orogenic Belt in northern Xinjiang where many early-Permian mafic-ultramafic intrusions host important Ni-Cu sulfide deposits. In this paper we report zircon U-Pb age, olivine chemistry and integrated whole-rock chemical and isotopic compositions for the Erhongwa mafic-ultramafic intrusion. This intrusion is composed of lherzolites and gabbroic rocks. The U-Pb age of zircon from a large olivine gabbro sample from the intrusion is 283.1 ± 1.5 Ma, which indicates that the Erhongwa intrusion is contemporaneous with the early-Permian sulfide ore-bearing mafic-ultramafic intrusions in the central Tianshan region. Olivine from the Erhongwa intrusion contains up to 89.5 mol% Fo and 3000 ppm Ni, which are the highest among all known early-Permian mafic-ultramafic intrusions in the region. The occurrence of small sulfide inclusions in the most primitive olivine and significant Ni depletion in more fractionated olivine in the Erhongwa intrusion indicate that sulfide segregation took place during olivine fractional crystallization. The Erhongwa intrusive rocks are characterized by light REE enrichment relative to heavy REE, negative Nb anomalies, positive εNd (t = 283 Ma) values from + 6.3 to + 7.7, low initial 87Sr/86Sr ratios from 0.7034 to 0.7036, initial 206Pb/204Pb ratios from 17.8 to 17.9 and zircon εHf values from 8.0 to 15.5. The Erhongwa mafic-ultramafic rocks and coeval A-type granites in the region have similar isotopic compositions but the former have lower Th/Nb ratios than the latter. These similarities and differences are consistent with the interpretation that the Erhongwa magma formed by the mixing of a mafic magma derived from a depleted mantle with a granitic melt derived from a juvenile arc crust. It is deduced that sulfide saturation in the Erhongwa magmatic system was related to the magma mixing event at depth. More significant sulfide mineralization may

  16. The temporal evolution of Hf and Nd isotopes of rhyolites from the Long Valley Caldera System

    NASA Astrophysics Data System (ADS)

    Simon, J. I.; Depaolo, D. J.; Weis, D.; Renne, P. R.; Mundil, R.

    2008-12-01

    Early investigations of magma evolution at Long Valley are based on crystal model ages in which protracted periods of closed system behavior are assumed. Recent studies imply that precaldera rhyolitic extrusions at Long Valley tap discrete magmas that include a mixture of several source components and evolve by open system behavior. In order to track the potentially changing source components of Long Valley magmas, we performed zircon Hf and whole rock Hf and Nd analyses from select rhyolites erupted over the ~2 Ma history of the volcanic field. New Ar/Ar dating of alkali feldspar and obsidian help refine, when necessary, the eruptive history previously provided by K-Ar dating (e.g., Bailey 1989). The radioisotopic tracers, coupled with this improved geochronology, yield a high-resolution temporal record of magma sources before and after caldera collapse. High precision (±0.1 epsilon) isotopic measurements of Hf separated from single large (~10 μg) and multiple size-sorted aliquots of smaller (≤4 to ~0.3 μg) zircon crystals were analyzed by MC-ICPMS. High precision (±0.1 epsilon) isotopic analyses of Hf and Nd separated from whole rock samples were performed by MC-ICPMS and TIMS, respectively. Zircons contained in the ~1712 ka precaldera Glass Mountain rhyolite (OD) exhibit 176Hf/177Hf values ranging from 0.28270 to 0.28278, whereas zircons from pumice in the ~777 ka Bishop Tuff exhibit values from 0.28278 to 0.28285. These zircon separates come from samples in which feldspar and glass Pb isotopic compositions have recently (Simon et al., 2007) been used as evidence for a secular change towards increasing mantle contribution in younger magmas. The ~2.5 epsilon unit increase in ɛHf (i.e., towards more mantle signatures) between the average zircon Hf isotopic compositions of OD and the Bishop Tuff are consistent with the ~2.0 epsilon unit increases in ɛHf and ɛNd between the whole rock values of the two rhyolites found here. Collectively, data from a

  17. The “zircon effect” as recorded by the chemical and Hf isotopic compositions of Lesser Antilles forearc sediments

    NASA Astrophysics Data System (ADS)

    Carpentier, Marion; Chauvel, Catherine; Maury, René C.; Mattielli, Nadine

    2009-09-01

    Oceanic sediments contain the products of erosion of continental crust, biologic activity and chemical precipitation. These processes create a large diversity of their chemical and isotopic compositions. Here we focus on the influence of the distance from a continental platform on the trace element and isotopic compositions of sediments deposited on the ocean floor and highlight the role of zircons in decoupling high-field strength elements and Hf isotopic compositions from other trace elements and Nd isotopic compositions. We report major and trace element concentrations as well as Sr and Hf isotopic data for 80 sediments from the Lesser Antilles forearc region. The trace-element characteristics and the Sr and Hf isotopic compositions are generally dominated by detrital material from the continental crust but are also variably influenced by chemical or biogenic carbonate and pure biogenic silica. Next to the South American continent, at DSDP Site 144 and on Barbados Island, sediments, coarse quartz arenites, exhibit marked Zr and Hf excesses that we attribute to the presence of zircon. In contrast, the sediments from DSDP Site 543, which were deposited farther away from the continental platform, consist of fine clay and they show strong deficiencies in Zr and Hf. The enrichment or depletion of Zr-Hf is coupled to large changes in Hf isotopic compositions (- 30 < ɛHf < + 4) that vary independently from the Nd isotopes. We interpret this feature as a clear expression of the "zircon effect" suggested by Patchett and coauthors in 1984. Zircon-rich sediments deposited next to the South American continent have very low ɛHf values inherited from old zircons. In contrast, in detrital clay-rich sediments deposited a few hundred kilometers farther north, the mineral fraction is devoid of zircon and they have drastically higher ɛHf values inherited from finer, clay-rich continental material. In the two DSDP sites, average Hf isotopes are very unradiogenic relative to

  18. Which minerals control the Nd-Hf-Sr-Pb isotopic compositions of river sediments?

    NASA Astrophysics Data System (ADS)

    Garcon, M.; Chauvel, C.; France-Lanord, C.; Limonta, M.; Garzanti, E.

    2013-12-01

    River sediments naturally sample and average large areas of eroded continental crust. They are ideal targets not only for provenance studies based on isotopic compositions, but also to establish average continental crust isotopic values. However, in large fluvial systems, mineral sorting processes significantly modify the mineralogy, and thus the geochemistry of the transported sediments. We still do not know, in any quantitative way, to what extent mineral sorting affects and fractionates the isotopic compositions of river sediments. Here, we focus on this issue and try to decipher the role of each mineral species in the bulk isotopic compositions of bedloads and suspended loads sampled at the outflow of the Ganga River that drains the Himalayan mountain range. We analyzed Nd, Hf, Sr and Pb isotopic compositions as well as trace element contents of a large number of pure mineral fractions (K-feldspar, plagioclase, muscovite, biotite, magnetite, zircon, titanite, apatite, monazite/allanite, amphibole, epidote, garnet, carbonate and clay) separated from bedload sediments. We combine these data with mineral proportions typical of the Ganga sediments to perform Monte-carlo simulations that quantify the contribution of individual mineral species to the Nd, Hf, Sr and Pb isotopic budgets of bedloads and suspended loads. We show that the isotopic systematic of river sediments is entirely buffered by very few minerals. Despite their extremely low proportions in sediments, zircon and monazite/allanite control Hf and Nd isotopes, respectively. Feldspars, epidote and carbonate buffer the Sr isotopic budget while clay, feldspars and heavy minerals dominate Pb isotopes. We also demonstrate that the observed difference in Hf, Sr and Pb isotopic compositions between bedloads and suspended loads reflects their different mineral proportions. Our findings highlight the need to be very careful about the choice of isotopic compositions measured on sediments when used as source

  19. Lu-Hf and Sm-Nd Isotopic Studies of Shergottites and Nakhlites: Implications for Martian Mantle Sources

    NASA Technical Reports Server (NTRS)

    Debaille, V.; Yin, Q.-Z.; Brandon, A. D.; Jacobsen, B.; Treiman, A. H.

    2007-01-01

    We present a new Lu-Hf and Sm-Nd isotope systematics study of four enriched shergottites (Zagami, Shergotty, NWA856 and Los Angeles), and three nakhlites (Nakhla, MIL03346 and Yamato 000593) in order to further understand processes occurring during the early differentiation of Mars and the crystallization of its magma ocean. Two fractions of the terrestrial petrological analogue of nakhlites, the Archaean Theo's flow (Ontario, Canada) were also measured. The coupling of Nd and Hf isotopes provide direct insights on the mineralogy of the melt sources. In contrast to Sm/Nd, Lu/Hf ratios can be very large in minerals such as garnet. Selective partial melting of garnet bearing mantle sources can therefore lead to characteristic Lu/Hf signatures that can be recognized with Hf-176/Hf-177Hf ratios.

  20. Hf Isotope Geochemistry of USGS Reference Materials and Various Labware: Insight into Potential Contaminant Sources

    NASA Astrophysics Data System (ADS)

    Weis, D.; Nobre Silva, I.; Kieffer, B.; Barling, J.; Pretorius, W.; Maerschalk, C.

    2005-12-01

    We have undertaken a high-precision geochemical and isotopic study of USGS reference materials by HR-ICP-MS, TIMS and MC-ICP-MS, including basalt (BCR-1,2; BHVO-1,2), andesite (AGV-1,2), rhyolite (RGM-1), syenite (STM-1,2), granodiorite (GSP-2), and granite (G-2,3). Only a few 176Hf/177Hf results are published on these materials and with the increased use of MC-ICP-MS it is critical to build a solid reference database. Standard hotplate dissolution was used, except for granitoid compositions where it involved a high-pressure bomb procedure. The reproducibility of 176Hf/177Hf is better than 100 ppm for granitoid compositions (G-2: 0.282523±8; G-3: 0.282505±20; GSP-2: 0.282059±27) and better than 65 ppm for basaltic/andesitic compositions in glassware and better than 30 ppm in teflon (BCR-2: 0.282872±9; BHVO-2: 0.283103±6). Overall, our results agree with the rare published data (BCR-1&2, BHVO-1 and RGM-1). Slight differences appear depending on the chemical procedure used to separate Hf and the type of labware used. There are systematic shifts in 176Hf/177Hf for basaltic compositions towards lower values (by 100-150 ppm) when non-teflon material is used. As a result, we then carried out a systematic trace element and isotopic study of various labware, including borosilicate glass and quartz columns and frits. Maximum concentrations (in ppm) of these materials (in the order listed above) are: Hf=16-0.3-22, Nd=0.8-0.1-23, Sr=8-0.08-16, Pb=1.4-0.5-14. The frit material appears the most variable in elemental concentration and isotopic composition, which might reflect various accumulations resulting from column chemistry. 176Hf/177Hf is 0.282198±4 in borosilicate glass and even lower in some of the frit material (<0.28195). Only a small amount of such unradiogenic material can account for the shifts observed in basaltic rocks. Our systematic study shows that careful analyses of rock reference materials with different compositional matrices are necessary, in

  1. Lu-Hf and PbSL geochronology of apatites from Proterozoic terranes: A first look at Lu-Hf isotopic closure in metamorphic apatite

    NASA Astrophysics Data System (ADS)

    Barfod, Gry Hoffmann; Krogstad, Eirik Jens; Frei, Robert; Albarède, Francis

    2005-04-01

    The mineral apatite is characterized by elevated and highly variable Lu/Hf ratios that, in some cases, allow for single-crystal dating by the Lu-Hf isotopic system. Apatites from the Adirondack Lowlands and Otter Lake area in the Grenville Province, and from the Black Hills, South Dakota, yield Lu-Hf ages that are consistently older than their respective Pb step leaching ages. Isotopic closure for the Lu-Hf system, therefore, occurs before U-Pb system closure in this mineral. In the Adirondack Lowlands, where H 2O activity was low, Lu-Hf systematics of cm-sized apatite crystals remained undisturbed during upper amphibolite facies metamorphism (˜700 to 675 °C) at 1170-1130 Ma. The relatively old Lu-Hf ages of 1270 and 1230 Ma observed for these apatites correlate with decreasing crystal size. In contrast, apatite from the fluid-rich Otter Lake area and Black Hills yields unrealistically low apparent Lu-Hf closure temperatures, implying that in these apatites, fluids facilitated late exchange. The Lu-Hf ages for the metamorphic apatites were thus controlled either by the prevailing temperature and grain size, or by fluid activity.

  2. High precision Lu and Hf isotope analyses of both spiked and unspiked samples: A new approach

    NASA Astrophysics Data System (ADS)

    Lapen, Thomas J.; Mahlen, Nancy J.; Johnson, Clark M.; Beard, Brian L.

    2004-01-01

    The functional form of instrumentally produced mass fractionation associated with MC-ICP-MS analysis is not accurately known and therefore cannot be fully corrected by traditional approaches of internal normalization using power, linear, or exponential mass-bias laws. We present a method for robust correction of instrumentally produced mass-fractionation of both spiked and unspiked samples that can be applied to mass analysis of Hf as well as Nd, Sr, Os, etc. Correction of 176Hf/177Hf for unspiked samples follows a traditional approach of internal normalization using an exponential law, followed by normalization to a standard of known composition, such as JMC-475. For spiked samples, standards are used to characterize a linear instrumental mass-bias coefficient; the mass-bias coefficient is defined by the slope of a tie-line between measured and true values of a standard. This approximation results in identical precision and accuracy of measurements for spiked and unspiked samples (±0.005% 2σ, external reproducibility). The effects of the spike on the 176Hf/177Hf ratio and calculation of the molar spike-sample ratio is determined by a closed-form solution modified from the double-spike approach used for Fe isotope analysis by TIMS [Johnson and Beard, 1999]. The measured 176Lu/175Lu ratios are corrected by doping the sample with Er and using the 167Er/166Er ratio to externally normalize the 176Lu/175Lu ratio using an exponential law. Finally, spike-sample equilibration is confirmed for our sample dissolution protocol through analysis of varying physical mixtures of 1 Ga garnet and hornblende, where all the data lie on a mixing-line, within error, on a 176Lu/177Hf-176Hf/177Hf diagram. Precision of 176Lu/177Hf ratios is determined to be ±0.2% (2σ) for standards and for physical mixtures of garnet and hornblende.

  3. Lu-Hf isotope systematics of fossil biogenic apatite and their effects on geochronology

    NASA Astrophysics Data System (ADS)

    Herwartz, Daniel; Münker, Carsten; Tütken, Thomas; Hoffmann, J. Elis; Wittke, Andreas; Barbier, Bruno

    2013-01-01

    Reliable methods for direct dating of biogenic apatite from pre-Pleistocene fossils are currently not available, and recent attempts using the Lu-Hf decay system yielded highly inaccurate ages for both bones and teeth. The geological processes accounting for this poor accuracy of Lu-Hf chronometry are not yet understood. Here we explore Lu-Hf systematics in fossil bones and teeth in detail, by applying five different sample digestion techniques that are tested on bones and composites of bone and sediment. Our current dataset implies that dissolution methods only slightly affect the resulting Lu-Hf ages, while clear differences between the individual digestion techniques became apparent for element concentrations. By analysing the insoluble leftovers from incomplete sample dissolution, four main reservoirs of Hf in fossil bones were identified: (1) a radiogenic end-member associated with apatite; (2) an unradiogenic end-member represented by the authigenic minerals or the embedding sediment; (3) a highly unradiogenic end-member that can be attributed to detrital zircon; and (4) a moderately soluble phase (probably a Zr(Hf)-phosphate) that yielded very low Lu/Hf but a highly radiogenic Hf isotope composition at the same time. This Zr(Hf)-phase must have been precipitated within the fossil bone sample at a late stage of burial history, thereby incorporating radiogenic 176Hf released from apatite surfaces over geological timescales. A second focus of our study is the effect of different sediment matrices and of crystal size on the preservation of pristine Lu-Hf isotope compositions in bioapatite. Because near-depositional Lu-Hf ages of phosphate fossils have previously been reported for the London Clay (England) and a calcareous marl from Tendaguru (Tanzania), we herein investigate specimens fossilised in carbonate matrices (calcareous marl from Oker, Germany; carbonate concretions from the Santana Formation, Brazil; carbonate from the Eifel, Germany) and argillaceous

  4. Evolution of Eastern Arctic crust revealed from zircon U-Pb, O and Hf isotopic records.

    NASA Astrophysics Data System (ADS)

    Gottlieb, E. S.; Akinin, V. V.; Miller, E. L.

    2015-12-01

    Zircon U-Pb ages, in conjunction with O and Hf isotope geochemistry, obtained from Proterozoic to Cretaceous supracrustal metamorphic to magmatic rocks (total 30 samples) collected from Cretaceous granite-gneiss dome and magmatic arc complexes located along Arctic cost of Chukotka, Neoproterozoic basement exposures on Wrangel Island, and crustal xenoliths-bearing Neogene alkali basalts on Zhokhova Island (De Long archipelago, Russian Arctic) provide new insights about evolution of crust in Arctic Alaska-Chukotka block. The oldest magmatic zircons yield U-Pb ages ranging from 710 to 570 Ma have depleted mantle-like Hf and O isotopic signatures (d18O range predominantly from +5.3 to +6.2 ‰, whereas ɛHf(i) ranges from +8 to +13), suggesting the juvenile crust formation during Cryogenian-Ediacaran, roughly correlated with Rodinia breakup. Zircons from Devonian (390-360 Ma) to Cretaceous (105-88 Ma) arc granites and rhyolites have increasingly heavier O isotopic composition (up to +6.5 to +9.0 ‰), and less radiogenic ɛHf(i) (as low as to -2.5 to -10), suggesting significant anatexis of the eastern Arctic crust associated with Devonian and Cretaceous age pluton forming events.

  5. Mixing and melt sources in the Miocene Aztec Wash pluton (Nevada, USA) as revealed by zircon Hf and O and whole rock Sr, Nd, and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Ryan, M.; Miller, J. S.; Miller, C. F.; Bromley, S.; Davies, G. R.; Schmitt, A. K.

    2011-12-01

    The 15.6 Ma Aztec Wash Pluton (AWP) is one of several Miocene intrusions located within the northern Colorado River extensional corridor. Extensive E-W tilting of fault blocks has exposed the pluton from the roof to 5 km structural depth. Earlier field and petrologic studies subdivided the AWP into two distinct zones: (1) a Granite Zone (GZ) comprised of relatively homogeneous granite with subtle differences in textures and mineralogy; (2) a Heterogeneous Zone (HZ), which interfingers the GZ, contains evidence for mafic and felsic magma input with a wide compositional range (42-78 wt% SiO2), and abundant field evidence for hybridization. Previous whole rock geochemistry and zircon trace element analyses indicated that compositional variation was produced by multi-component mixing between mafic and felsic melts within the HZ. New whole rock Sr, Nd, and Hf isotope data from the HZ show that all rocks (including high-silica granites) formed by mixing Precambrian crust and enriched mantle, with mixtures having a large mantle fraction (≥50%). New Hf (n=189) and O (n=241) isotope analyses of zircon from samples in the HZ confirm these melt sources and provide a broader perspective on hybridization processes within the AWP. Zircon grains from all samples show heterogeneous Hf and O isotopic compositions (-5 to -18 ɛHf; 4.5-7.5% δ18O), but despite the clear signature of Precambrian crust in the whole rock data, obvious Precambrian zircons (or cores) were mostly absent; only one zircon was clearly Precambrian (ɛHf = -25). Resolvable intragrain variability is relatively limited (including the Precambrian grain, which is unzoned). Zircons from hand samples and from compositional groups also show heterogeneous ɛHf and δ18O values, although the spreads are more restricted than in the whole data set (6-10 ɛHf in granites, 5-7 ɛHf in intermediate "hybrids", 5-6 ɛHf in gabbro/diorite sheets). Oxygen isotope values for the zircons also show intra-handsample heterogeneity

  6. The Yellowstone hotspot in space and time: Nd and Hf isotopes insilici magmas

    SciTech Connect

    Nash, Barbara P.; Perkins, Michael E.; Christensen, John N.; Lee,Den-Chuen; Halliday, A.N.

    2006-04-19

    Over the course of its 16 m.y. history, the Yellowstonehotspot has produced silicic magmas exhibiting systematic, and oftensympathetic, variations in isotopic and chemical composition, temperatureand frequency of eruption. Nd and Hf isotopic ratios vary systematicallyfrom initial eruptions at ~;16 Ma, contemporaneous with basalticvolcanism in eastern Oregon and Washington, to the present dayYellowstone Volcanic Plateau. Nd and Hf isotopic ratios co-vary and spanthe range of most terrestrial samples, reflecting mixing of mantle andcrustal sources. Earliest erupted silicic magmas were hot (in excess of1050oC), relatively less evolved and have isotopic ratios within therange of contemporaneous Columbia River flood basalts. The transit of thehotspot across the lithospheric boundary between the western accretedoceanic terrain and the Precambrian craton at 15 Ma is marked by shiftsin eNd from +4 to -11 and in ?Hf from +10 to -10. The duration of thetransit yields a crustal magma source diameter of ~;70 km. In theinterval from 14 to 9 Ma, ?Nd systematically increases from -11 to -7,recording a minimum increase in the mantle component from 5 percent to 30percent. The mantle component could be twice as great, depending upon theisotopic composition of crust and mantle reservoirs. In this sameinterval, peak temperatures of ~;1000oC occurred at 9 Ma. The last 8 m.y.are characterized by less frequent eruption of lower temperature(830-900oC) and more compositionally evolved magmas.

  7. Hf isotopic evidence for a cogenetic magma source for the Bushveld Complex and associated felsic magmas

    NASA Astrophysics Data System (ADS)

    VanTongeren, J. A.; Zirakparvar, N. A.; Mathez, E. A.

    2016-04-01

    Here, we test the hypothesis that the rhyolitic lavas of the Rooiberg Group and granophyres associated with the roof of the Bushveld Complex are differentiation products of Bushveld-age mafic liquids. We present Lu-Hf isotopic compositions in zircons from roof rocks that have been interpreted to represent thermally metamorphosed and remelted Rooiberg Group lavas and from granophyres interpreted to be differentiation products of the cumulate rocks that make up the Bushveld Complex. All of these rocks were found to possess εHf (2.06 Ga) statistically indistinguishable from the intrusion-wide average εHf (2.06 Ga) value of - 8.6 ± 1.2 of the Bushveld Complex. Our results, combined with chronologic and field relations, suggest that the felsic rocks were generated by fractional crystallization of Bushveld mafic liquids, including those that gave rise to the cumulate rocks of the Bushveld Complex.

  8. A chilling perspective on Greenland's early Cenozoic climate from coupled Hf-Nd isotopes

    NASA Astrophysics Data System (ADS)

    Scher, H. D.; Bizimis, M.; Buckley, W. P., Jr.; Duggan, B.; Bohaty, S. M.; Wilson, P. A.

    2015-12-01

    The prevailing view of northern hemisphere glaciation has been of ice sheets forming on Greenland after 2.7 Ma, with iceberg rafting as early as 15 Ma. This view is incompatible with recent results from global climate/ice sheet models that predict northern hemisphere glaciation only after CO2 falls below ~280 ppmv (occurring at 25 Ma) and with recent sediment evidence for Arctic iceberg rafting as early as the middle Eocene. However, the amount of northern hemisphere ice represented by these sediments is ambiguous and global ice budget calculations for the early Cenozoic are controversial. Here we use coupled Hf-Nd isotopes of oxyhydroxides in sediments from the upper Eocene to lower Oligocene section in ODP Site U1411 (Newfoundland Ridge) to determine when the circum-North Atlantic came under the influence of a mechanical weathering regime. Leached oxyhydroxide Hf-Nd isotopes are an indicator of weathering intensity because mechanical weathering by ice sheets mobilizes the zircon-bound Hf reservoir in the crust, which has extreme unradiogenic eHf values. Chemical weathering produces a distinct seawater array on Hf-Nd diagrams, while seawater exposed to the products of mechanical weathering plot on divergent arrays closer to the Terrestrial Array. Hf-Nd isotopes of Site U1411 leachates are grouped in a near vertical trend between the seawater and terrestrial global reference arrays. Within this group there are four distinct arrays that can be delineated by age. Samples that are late Eocene in age fall along an array that is slightly divergent from the seawater array. The aspect of the Hf-Nd isotope data changes permanently after the first step of the EOT, falling along arrays that are systematically offset in the direction of the terrestrial arrays. The steepest array, most proximal to the terrestrial array, is comprised of samples deposited between 33.7 and 32.2 Ma. These results indicate a circum-North Atlantic weathering regime appeared in the earliest Oligocene.

  9. How Recycling of Sediments and Oceanic Crust Have Changed the Nd-Hf Isotopic Composition of the Mantle through Time

    NASA Astrophysics Data System (ADS)

    Garcon, M.; Carlson, R.; Shirey, S. B.; Chauvel, C.; Arndt, N. T.

    2015-12-01

    The Nd and Hf isotopic compositions of modern mantle-derived rocks such as mid-ocean ridge basalts (MORB) and ocean island basalts (OIB), define a linear trend known as Nd-Hf mantle array. This array is thought to reflect mixing between mantle from which crust has previously been extracted and enriched recycled oceanic crustal components of variable ages - including sediments with radiogenic Hf isotopic compositions (Chauvel et al., Nature Geoscience, 2008). Thus the present-day mantle as sampled by MORB and OIB is pervasively contaminated by recycled material. We here develop a model to quantify how the continuous incorporation of recycled material changed the Nd-Hf isotopic composition of the mantle through time. The Nd-Hf isotopic compositions of Archean sediments were decoupled - high ɛHf for a given ɛNd - due to the contribution of radiogenic Hf from cherts. From these observations, the possible compositions for recycled material currently into the mantle can be estimated. Assuming that modern MORB and OIB contain ~3-5% and ~10-15% recycled material, respectively; we calculate that a mantle reservoir free of recycled material would be significantly more radiogenic than the most extreme MORB and would lie well below the Nd-Hf mantle array. Such a Nd-Hf isotopic composition is not sampled by modern mantle-derived rocks. Crustal rocks formed at ~3.8 Gyr (compilation of granites and TTG), however, have low ɛHf for a given ɛNd, consistent with our estimated composition for the mantle without recycled material. We thus suggest that Eoarchean mantle-derived rocks sampled depleted mantle that was not yet contaminated by recycled material and that the slope of the Nd-Hf mantle array changed through time due to continuous recycling of sedimentary material in subduction zones.

  10. Petrogenesis and origin of modern Ethiopian rift basalts: Constraints from isotope and trace element geochemistry

    NASA Astrophysics Data System (ADS)

    Ayalew, D.; Jung, S.; Romer, R. L.; Kersten, F.; Pfänder, J. A.; Garbe-Schönberg, D.

    2016-08-01

    The source of continental rift-related basalts and their relation to rifting processes is a continuous matter of debate. We present major and trace element and Sr, Nd, Hf and Pb isotope data for axial rift basalts from eight volcanic centres (Ayelu, Hertali, Dofan, Fantale, Kone, Bosetti and Gedemsa, from NE to SW) in Afar and Main Ethiopian Rift (MER) to assess their source regions and their genetic relationships. These lavas have geochemical characteristics, i.e., a peak at Ba, Nb and troughs at K and Rb in primitive mantle-normalised multielement diagrams, which are consistent with predominant melting of an amphibole-bearing lithospheric mantle. However, the isotopic compositions for these lavas are heterogeneous (87Sr/86Sr = 0.70354-0.70431, 143Nd/144Nd = 0.51280-0.51294, 176Hf/177Hf = 0.28301-0.28315, 206Pb/204Pb = 18.48-19.31, 207Pb/204Pb = 15.53-15.62, 208Pb/204Pb = 38.61-39.06) and require various mantle reservoirs with distinctive isotopic signatures. The range of isotopic compositions requires the involvement of three distinct source components from the asthenospheric and veined lithospheric mantle. Progressive rifting leads to lithosperic thinning and upwelling of hot asthenospheric mantle, which induces melting of the veined lithospheric mantle. The trace element characteristics of the lavas are dominated by the vein material, which has a higher trace element content than the surrounding mantle. The isotopic composition of the vein material, however, is not very different from the ambient mantle, giving rise of apparent uncoupling of trace element and isotope constraints for the melt source. The uprising basaltic liquids in part inherit a lithospheric trace element signature, while their isotopic compositions are mostly unaffected due to short residence times within the lithosphere in context with progressive rifting and lithospheric thinning. Thus, the geochemical and isotope data are consistent with a multi-component source prevailing beneath the Afar

  11. Hf-Nd-Sr isotopic fingerprinting of mineral dust from Asian and North African deserts

    NASA Astrophysics Data System (ADS)

    Ji, J.; Zhao, W.; Balsam, W.

    2015-12-01

    Mineral dust accounts for more than 50% of the atmospheric dust loading and plays an important role in the marine and terrestrial geochemical cycles. The deserts in North Africa, Northern China and Southern Mongolia are the major sources of mineral dust and have been studied intensively over past decades, especially with Sr, Nd and recently Hf isotopes which are seen as powerful tools to identify source areas. However, the isotopic compositions of dust are highly dependent on particle size hindering the ability to accurately identify dust provenance. The clay fraction (<2 μm) comprises about half of all mineral dust and has unique minerals phases dominanted by clay minerals. Once the clay-sized particles are deflated to the upper troposphere, they are transported over long distances and are removed from the atmosphere mainly by wet deposition. Thus, the clay-sized isotopic fingerprints from deserts may be ideal targets not only for tracking the provenance tracing of long-distance transported mineral dust, but also to provide an unparalleled window for understanding the global dust cycle, especially eolian dust preserved in deep-sea sediments and ice cores. In this work we investigate multivariate joint radiogenic Sr, Nd, and Hf isotopic compositions obtained from complete dissolution of clay-sized fractions of surface sediments from Asian and North African deserts. Asian dust source samples included the ten Northern China deserts and sandy lands - the Taklimakan, Gurbantunggut, Qaidam, Badaim Jaran, Tengger and Mu Us deserts, and the Hobq, Hulun Buirm, Onqin Daga and Horqin sandy land - and Mongolian Gobi desert. North African dust samples were from four transects in the Sahara and Sahel from Mali, Togo, Egypt and Morocco . Our results on the clay-sized isotopic measurements of these samples describe (1) the general characteristics of dusts from the Asian with ɛNd from -17.3 to 0.98, ɛHf from -5.95 to 3.68 and 87Sr/86Sr from 0.710113 to 0.73306, and North

  12. A Sr-Nd-Hf isotope characterization of dust source areas in Victoria Land and the McMurdo Sound sector of Antarctica

    NASA Astrophysics Data System (ADS)

    Blakowski, Molly A.; Aciego, Sarah M.; Delmonte, Barbara; Baroni, Carlo; Salvatore, Maria Cristina; Sims, Kenneth W. W.

    2016-06-01

    Determining the geographical provenance of dust provides crucial insight into the global dust cycle. For the East Antarctic Ice Sheet (EAIS), the importance of Southern hemisphere potential dust sources has been thoroughly investigated using radiogenic isotopes, whereas proximal dust source areas located on the periphery of the ice sheet remain poorly documented from a geochemical standpoint. In this work, we expand the existing isotopic (Srsbnd Nd) catalogue of dust and sand-sized sediments from Victoria Land and the McMurdo Sound sector, and incorporate Hf isotopic data to place additional constraints on dust source identification. The isotopic field for materials considered in this study is characterized by 87Sr/86Sr ratios ranging from 0.703 to 0.783, εNd between -12.01 and 6.36, and εHf from -16.77 to 6.89. As reported in previous works, the data reveal close relationships between Antarctic sediments and distinct parent lithologies; in addition, our findings emphasize the background presence of very fine dusts originating from dominant global sources and regional volcanic activity as barriers to direct source-to-sink comparison of isotopic signatures. Thus, geochemical characterizations of dust sources to the Antarctic ice sheet involving multiple size fractions, including coarser-grained particles more susceptible to short-range transport, can help us to rule out global sources of dust when examining local sediment cores and ice cores.

  13. Petrogenesis of Sveconorwegian magmatism in southwest Norway; constraints from zircon U-Pb-Hf-O and whole-rock geochemistry

    NASA Astrophysics Data System (ADS)

    Roberts, Nick M. W.; Slagstad, Trond; Parrish, Randall R.; Norry, Michael J.; Marker, Mogens; Horstwood, Matthew S. A.; Røhr, Torkil

    2013-04-01

    The Sveconorwegian orogen is traditionally interpreted as a Himalayan-scale continental collision, and the eastward continuation of the Grenville Province of Laurentia; however, it has recently been proposed that it represents an accretionary orogen without full-scale continental collision (Slagstad et al., in press). We suggest that magmatism is one of the key constraints to differentiate between different types of orogens; thus, detailed investigation of the timing and petrogenesis of the magmatic record is a requirement for better understanding of the Sveconorwegian orogen as a whole. Here, we present new U-Pb geochronology, zircon Hf-O isotope, and whole-rock geochemical data to constrain the petrogenesis of the early -Sveconorwegian Sirdal Magmatic Belt (SMB). The SMB is a batholithic-scale complex of intrusions that intrudes into most of the Rogaland-Hardangervidda Block in southwest Norway. Current age constraints put emplacement between ~1050 to 1020 Ma. New ages from the Suldal region indicate that the onset of SMB magmatism can be put back to 1070 Ma, which is some 30-50 Myrs prior to high-grade metamorphism. Average initial ɛHf signatures range from ~0 to 4; these overlap with later post-Sveconorwegian granites and with early-/pre-Sveconorwegian ferroan (A-type) granites. Average δ18O signatures range from ~5.7 to 8.7, except for one anomalous granite at ~11.6. The Hf-O signatures are compatible with a mixed mantle-crustal source. Crustal sources may include ~1500 Ma Telemarkian or ~1200 Ma juvenile crust. Hf-O bulk-mixing modelling using a 1500 Ma crustal source indicates >50 % mantle input. Although much further mapping and geochronological work is required, granitic magmatism appears to have persisted throughout much of the ~1100 to 900 Ma period that spans the Sveconorwegian orogen. This magmatism is consistently ferroan (i.e. dry); however, the SMB marks a clear transition to magnesian (i.e. wet) magmatism, with a return to ferroan magmatism at

  14. Evolution of E 2 transition strength in deformed hafnium isotopes from new measurements on 172Hf,174Hf, and 176Hf

    NASA Astrophysics Data System (ADS)

    Rudigier, M.; Nomura, K.; Dannhoff, M.; Gerst, R.-B.; Jolie, J.; Saed-Samii, N.; Stegemann, S.; Régis, J.-M.; Robledo, L. M.; Rodríguez-Guzmán, R.; Blazhev, A.; Fransen, Ch.; Warr, N.; Zell, K. O.

    2015-04-01

    Background: The available data for E 2 transition strengths in the region between neutron-deficient hafnium and platinum isotopes are far from complete. More and precise data are needed to enhance the picture of structure evolution in this region and to test state-of-the-art nuclear models. In a simple model, the maximum collectivity is expected at the middle of the major shell. However, for actual nuclei, particularly in heavy-mass regions, which should be highly complex, this picture may no longer be the case, and one should use a more realistic nuclear-structure model. We address this point by studying the spectroscopy of Hf as a representative case. Purpose: We remeasure the 21+ half-lives of 172,174,176Hf, for which there is some disagreement in the literature. The main goal is to measure, for the first time, the half-lives of higher-lying states of the rotational band. The new results are compared to a theoretical calculation for absolute transition strengths. Method: The half-lives were measured using γ -γ and conversion-electron-γ delayed coincidences with the fast timing method. For the determination of half-lives in the picosecond region, the generalized centroid difference method was applied. For the theoretical calculation of the spectroscopic properties, the interacting boson model is employed, whose Hamiltonian is determined based on microscopic energy-density functional calculations. Results: The measured 21+ half-lives disagree with results from earlier γ -γ fast timing measurements, but are in agreement with data from Coulomb excitation experiments and other methods. Half-lives of the 41+ and 61+ states were measured, as well as a lower limit for the 81+ states. Conclusions: This work shows the importance of a mass-dependent effective boson charge in the interacting boson model for the description of E 2 transition rates in chains of nuclei. It encourages further studies of the microscopic origin of this mass dependence. New experimental

  15. Detrital zircon evidence for Hf isotopic evolution of granitoid crust and continental growth

    NASA Astrophysics Data System (ADS)

    Iizuka, Tsuyoshi; Komiya, Tsuyoshi; Rino, Shuji; Maruyama, Shigenori; Hirata, Takafumi

    2010-04-01

    We have determined U-Pb ages, trace element abundances and Hf isotopic compositions of approximately 1000 detrital zircon grains from the Mississippi, Congo, Yangtze and Amazon Rivers. The U-Pb isotopic data reveal the lack of >3.3 Ga zircons in the river sands, and distinct peaks at 2.7-2.5, 2.2-1.9, 1.7-1.6, 1.2-1.0, 0.9-0.4, and <0.3 Ga in the accumulated age distribution. These peaks correspond well with the timing of supercontinent assembly. The Hf isotopic data indicate that many zircons, even those having Archean U-Pb ages, crystallized from magmas involving an older crustal component, suggesting that granitoid magmatism has been the primary agent of differentiation of the continental crust since the Archean era. We calculated Hf isotopic model ages for the zircons to estimate the mean mantle-extraction ages of their source materials. The oldest zircon Hf model ages of about 3.7 Ga for the river sands suggest that some crust generation had taken place by 3.7 Ga, and that it was subsequently reworked into <3.3 Ga granitoid continental crust. The accumulated model age distribution shows peaks at 3.3-3.0, 2.9-2.4, and 2.0-0.9 Ga. The striking attribute of our new data set is the non-uniformitarian secular change in Hf isotopes of granitoid crusts; Hf isotopic compositions of granitoid crusts deviate from the mantle evolution line from about 3.3 to 2.0 Ga, the deviation declines between 2.0 and 1.3 Ga and again increases afterwards. Consideration of mantle-crust mixing models for granitoid genesis suggests that the noted isotopic trends are best explained if the rate of crust generation globally increased in two stages at around (or before) 3.3 and 1.3 Ga, whereas crustal differentiation was important in the evolution of the continental crust at 2.3-2.2 Ga and after 0.6 Ga. Reconciling the isotopic secular change in granitoid crust with that in sedimentary rocks suggests that sedimentary recycling has essentially taken place in continental settings rather than

  16. Cathodoluminescence guided zircon Hf isotope depth profiling: Mobilization of the Lu-Hf system during (U)HP rock exhumation in the Woodlark Rift, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Zirakparvar, N. A.

    2015-04-01

    Cathodoluminescence image guided Hf isotope depth profiling by laser ablation of zircons from two quartzofeldspathic host gneisses constrains the Lu-Hf system's behavior during rapid exhumation of (U)HP rocks in the Woodlark Rift, Papua New Guinea. Investigation of the depth profiling technique in individual and composite zircon standards demonstrates that it is possible to resolve ~ 8 μm thick domains in which εHf(present) differs by as little as 4 units. In a metasedimentary gneiss, 2.89 ± 0.29 Ma zircon overgrowths on Cretaceous aged inherited cores have radiogenic εHf(present) indicating growth in a medium that was originally in equilibrium with garnet undergoing recrystallization (the 'garnet effect' of Zheng et al., 2005). In a separate gneiss sample that originated as an exhumation related anatectic melt, 3.66 ± 0.13 Ma zircons lacking inheritance contain sub-domains that differ from each other by > 15 εHf(present). Some of these sub-domains are radiogenic and can be explained by the 'garnet effect', whereas others also contain highly elevated Lu and Yb in addition to their radiogenic Hf compositions, thus necessitating a medium derived from the complete breakdown of garnet. Zircons in this sample also contain non-radiogenic sub-domains that grew in the presence of Hf mobilized from the surrounding rocks of the subducted and metamorphosed remnants of the Australian continental margin. The results confirm that rapid exhumation of (U)HP rocks can result in the following: 1) transmission of radiogenic Hf (and sometimes Lu and the other HREE) from garnet bearing mafic lithologies into the quartzofeldspathic gneisses, and 2) mobilization and transport of unradiogenic Hf present within the quartzofeldspathic remnants of subducted continental crust.

  17. In-situ U Pb geochronology and Hf isotope analyses of the Rayner Complex, east Antarctica

    NASA Astrophysics Data System (ADS)

    Halpin, J. A.; Gerakiteys, C. L.; Clarke, G. L.; Belousova, E. A.; Griffin, W. L.

    2005-02-01

    In-situ zircon U Pb and Hf isotopic analysis via laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) of samples from Kemp and MacRobertson Lands, east Antarctica suggests that the Kemp Land terrane evolved separately from the rest of the Rayner Complex prior to the ca. 940 Ma Rayner Structural Episode. Several Archaean metamorphic events in rocks from western Kemp Land can be correlated with events previously reported for the adjacent Napier Complex. Recently reported ca. 1,600 Ma isotopic disturbance in rocks from the Oygarden Group may be correlated with a charnockitic intrusion in the Stillwell Hills before ca. 1,550 Ma. Despite being separated by some 200 km, THfDM ages indicate felsic orthogneiss from Rippon Point, the Oygarden Group, Havstein Island and the Stillwell Hills share a ca. 3,660 3,560 Ma source that is indistinguishable from that previously reported for parts of the Napier Complex. More recent additions to this crust include Proterozoic charnockite in the Stillwell Hills and the vicinity of Mawson Station. These plutons have distinct 176Hf/177Hf ratios and formed via the melting of crust generated at ca. 2,150 2,550 Ma and ca. 1,790 1,870 Ma respectively.

  18. Isotopic constraints on crustal growth and recycling

    NASA Technical Reports Server (NTRS)

    Jacobsen, Stein B.

    1988-01-01

    The Sm-Nd isotopic data on clastic and chemical sediments are used with the present-day age distribution of continental crustal rocks to estimate the rates of crustal accretion, growth and recycling throughout earth's history. A new method for interpreting Nd model ages on both chemical and clastic sediments is proposed. A general relationship is derived between the mean crustal residence time of material recycled from the crust to the mantle (i.e., sediments), the mean age of the crust, and the crustal growth and recycling rates. This relationship takes into account the fact that the age distribution of material in the continental crust is generally different from the age distribution of material recycled into the mantle. The episodic nature of the present-day age distribution in crustal rocks results in similar episodicity in the accretion and recycling rates. The results suggest that by about 3.8 Ga ago, about 40 percent of the present continental volume was present. Recycling rates were extremely high 3-4 Ga ago and declined rapidly to an insignificant value of about 0.1 cu km/a during most of the Phanerozoic. The Nd model age pattern on sediments suggests a fairly high rate of growth during the Phanerozoic.

  19. Magmatic and crustal differentiation history of granitic rocks from Hf-O isotopes in zircon.

    PubMed

    Kemp, A I S; Hawkesworth, C J; Foster, G L; Paterson, B A; Woodhead, J D; Hergt, J M; Gray, C M; Whitehouse, M J

    2007-02-16

    Granitic plutonism is the principal agent of crustal differentiation, but linking granite emplacement to crust formation requires knowledge of the magmatic evolution, which is notoriously difficult to reconstruct from bulk rock compositions. We unlocked the plutonic archive through hafnium (Hf) and oxygen (O) isotope analysis of zoned zircon crystals from the classic hornblende-bearing (I-type) granites of eastern Australia. This granite type forms by the reworking of sedimentary materials by mantle-like magmas instead of by remelting ancient metamorphosed igneous rocks as widely believed. I-type magmatism thus drives the coupled growth and differentiation of continental crust.

  20. Magmatic and crustal differentiation history of granitic rocks from Hf-O isotopes in zircon.

    PubMed

    Kemp, A I S; Hawkesworth, C J; Foster, G L; Paterson, B A; Woodhead, J D; Hergt, J M; Gray, C M; Whitehouse, M J

    2007-02-16

    Granitic plutonism is the principal agent of crustal differentiation, but linking granite emplacement to crust formation requires knowledge of the magmatic evolution, which is notoriously difficult to reconstruct from bulk rock compositions. We unlocked the plutonic archive through hafnium (Hf) and oxygen (O) isotope analysis of zoned zircon crystals from the classic hornblende-bearing (I-type) granites of eastern Australia. This granite type forms by the reworking of sedimentary materials by mantle-like magmas instead of by remelting ancient metamorphosed igneous rocks as widely believed. I-type magmatism thus drives the coupled growth and differentiation of continental crust. PMID:17303751

  1. Sr-Nd-Pb-Hf isotope results from ODP Leg 187: evidence for mantle dynamics of the Australian-Antarctic Discordance

    NASA Astrophysics Data System (ADS)

    Kempton, P. D.; Pearce, J. A.; Barry, T. L.; Fitton, J. G.; Langmuir, C. H.; Christie, D. M.

    2003-04-01

    New high precision PIMMS Hf and Pb isotope data for 14-28 Ma basalts recovered during ODP Leg 187 are compared with zero-age dredge samples from the Australian-Antarctic Discordance (AAD) in order to extend the characterization of the Indian/Pacific mantle domain boundary from on-axis to older (14-28 Ma) off-axis crust and to address questions relating to the origin and evolution of the AAD. Based on our new Nd-Hf isotope data we demonstrate that Pacific MORB-source mantle (PMM) was present near the eastern margin of the AAD from as early as 28 Ma―its boundary with Indian MORB-source mantle (IMM) coinciding with the eastern edge of a basin-wide arcuate depth anomaly that is centered on the AAD. This observation rules out models requiring rapid migration of Pacific MORB mantle into the Indian Ocean basin since separation of Australia from Antarctica. Although PMM does not occur west of the fracture zone at 127^oE, IMM is recovered along with PMM up to 100 km east of this transform; at two localities, basalts derived from both mantle domains occur within the same site. Even here, however, IMM and PMM retain their compositional distinctiveness, indicating that the boundary between the two mantle domains is remarkably sharp. Hf isotope data also place constraints on the origin of the mantle reservoirs underlying the AAD. Nd-Hf isotope systematics can be explained by a model in which IMM basalts from the AAD are derived from mantle previously processed above a subduction zone. Such mantle could have been generated within the convergent margin that existed off the east coast of Gondwana throughout most of the Paleozoic and Mesozoic Eras and subsequently recycled into the upper mantle. Upwelling of a stagnated, subducted slab beneath the SEIR (Gurnis et al., 1998) is responsible for the progressive displacement of low-eHf (lithosphere-contaminated) IMM by high-eHf (uncontaminated but subduction-processed) IMM. Gurnis, M., R. D. Muller, R.D., &L. Moresi, Science, 279

  2. Changes in erosion and ocean circulation recorded in the Hf isotopic compositions of North Atlantic and Indian Ocean ferromanganese crusts

    USGS Publications Warehouse

    Piotrowski, Alexander M.; Lee, Der-Chuen; Christensen, John N.; Burton, Kevin W.; Halliday, Alex N.; Hein, James R.; Günther, Detlef

    2000-01-01

    High-resolution Hf isotopic records are presented for hydrogenetic Fe–Mn crusts from the North Atlantic and Indian Oceans. BM1969 from the western North Atlantic has previously been shown to record systematically decreasing Nd isotopic compositions from about 60 to ∼4 Ma, at which time both show a rapid decrease to unradiogenic Nd composition, thought to be related to the increasing influence of NADW or glaciation in the northern hemisphere. During the Oligocene, North Atlantic Hf became progressively less radiogenic until in the mid-Miocene (∼15 Ma) it reached +1. It then shifted gradually back to an ϵHf value of +3 at 4 Ma, since when it has decreased rapidly to about −1 at the present day. The observed shifts in the Hf isotopic composition were probably caused by variation in intensity of erosion as glaciation progressed in the northern hemisphere. Ferromanganese crusts SS663 and 109D are from about 5500 m depth in the Indian Ocean and are now separated by ∼2300 km across the Mid-Indian Ridge. They display similar trends in Hf isotopic composition from 20 to 5 Ma, with the more northern crust having a composition that is consistently more radiogenic (by ∼2 ϵHf units). Paradoxically, during the last 20 Ma the Hf isotopic compositions of the two crusts have converged despite increased separation and subsidence relative to the ridge. A correlatable negative excursion at ∼5 Ma in the two records may reflect a short-term increase in erosion caused by the activation of the Himalayan main central thrust. Changes to unradiogenic Hf in the central Indian Ocean after 5 Ma may alternatively have been caused by the expanding influence of NADW into the Mid-Indian Basin via circum-Antarctic deep water or a reduction of Pacific flow through the Indonesian gateway. In either case, these results illustrate the utility of the Hf isotope system as a tracer of paleoceanographic changes, capable of responding to subtle changes in erosional regime not readily resolved

  3. Hadean to Modern Mantle Evolution from a 142Nd-143Nd-176Hf Isotopic Perspective

    NASA Astrophysics Data System (ADS)

    Bennett, V. C.; Brandon, A. D.; Hiess, J.; Wan, Y.; Nutman, A.

    2009-12-01

    A key question in mantle chemistry is the relative roles of early planetary processes, including accretion and intra-mantle differentiation, versus on-going processes such as continental crust extraction and crustal recycling in creating and modifying mantle chemical signatures. Here, using both published and new determinations, we present integrated high-precision 142Nd and 176Hf datasets from four of the oldest (>3.7 Ga) terrestrial rock terranes (Itsaq Complex of southwest Greenland, Napier Complex of East Antarctica, Anshan of China and Narryer Complex of Western Australia). High-precision 142Nd compositions were determined from whole rock powders using TIMS (Triton); Lu-Hf isotopic compositions were measured using LA-MC-ICPMS (Neptune) on U-Pb age dated (using SHRIMP) zircons extracted, for most samples, from the same whole rocks. Significant (0-20ppm) variations in 142Nd compared with modern terrestrial compositions reflect early (>4.4 Ga) formation of high Sm/Nd domains, while 146Sm (T1/2=103 Myr) was actively decaying. In contrast 176Hf compositions for the oldest zircon populations in each rock are all near-chondritic (using CHUR values of Bouvier et al., 2008, EPSL 273: 48-57; and λ176Lu=1.867 X 10-11 yr-1) requiring time-averaged chondritic Lu/Hf ratios; there is no correlation of 176Hf with 142Nd (or 143Nd) signatures. The absence of Lu/Hf fractionation places quantitative limits on the volumes and mean age of Hadean continental crust that could have been formed and preserved into the early Archean and indicates only a minor role for early continental crust extraction in generating Hadean-Eoarchean mantle chemical fractionation. The spatial and time progressive variations in 142Nd compositions of Archean rocks apparent in our new dataset, reflect early formed and persistent domains with variable Sm/Nd. Supplemented by the recent discovery of complementary negative 142Nd anomalies in Proterozoic rocks (Upadhyay et al., 2009, Nature 459, 1118-1121), the

  4. Correlated cosmogenic W and Os isotopic variations in Carbo and implications for Hf-W chronology

    NASA Astrophysics Data System (ADS)

    Qin, Liping; Dauphas, Nicolas; Horan, Mary F.; Leya, Ingo; Carlson, Richard W.

    2015-03-01

    An obstacle for establishing the chronology of iron meteorite formation using 182Hf-182W systematics (t1/2 = 8.9 Myr) is to find proper neutron fluence monitors to correct for cosmic ray modification of W isotopic composition. Recent studies showed that siderophile elements such as Pt and Os could serve such a purpose. To test and calibrate these neutron dosimeters, the isotopic compositions of W and Os were measured in a slab of the IID iron meteorite Carbo. This slab has a well-characterized noble gas depth profile reflecting different degrees of shielding to cosmic rays. The results show that W and Os isotopic ratios correlate with distance from the pre-atmospheric center. Negative correlations, barely resolved within error, were found between ε190Os-ε189Os and ε186Os-ε189Os with slopes of -0.64 ± 0.45 and -1.8(+1.9/-2.1), respectively. These Os isotope correlations broadly agree with model predictions for capture of secondary neutrons produced by cosmic ray irradiation and results reported previously for other groups of iron meteorites. Correlations were also found between ε182W-ε189Os (slope = 1.02 ± 0.37) and ε182W-ε190Os (slope = -1.38 ± 0.58). Intercepts of these two correlations yield pre-exposure ε182W values of -3.32 ± 0.51 and -3.62 ± 0.23, respectively (weighted average ε182W = -3.57 ± 0.21). This value relies on a large extrapolation leading to a large uncertainty but gives a metal-silicate segregation age of -0.5 ± 2.4 Myr after formation of the solar system. Combining the iron meteorite measurements with simulations of cosmogenic effects in iron meteorites, equations are presented to calculate and correct for cosmogenic effects on 182W using Os isotopes.

  5. Isotopic and chemical constraints on mantle-crust evolution

    SciTech Connect

    Jacobsen, S.B. )

    1988-06-01

    A formalism for the general treatment of three-layer mantle-crust evolution models is presented and various published models are shown to be special cases of this more general model. The Sm-Nd, Lu-Hf, and Rb-Sr isotopic present day mass balance for the continental crust-depleted mantle systems is consistent with {approximately}30% of the mantle being depleted. A growth curve for the continental crust is calculated on the basis of total inversion of the Sm-Nd isotopic data for all of Earth history. The curve suggests that by about 3.8 Ga ago, {approximately}40% of the present continental volume was present. Both the estimated continental recycling and addition rates show maxima around 3.0 Ga. The resulting continental addition rates are also very high 4.5-4.0 Ga ago and during the Phanerozoic. The Sm-Nd data are not compatible with a steady state model for the crust over the past 2-3 Ga. The major uncertainty in evaluating crust-mantle evolution models is the extent of exchange between the upper and lower mantle.

  6. Xenon isotopic constraints on the timing of atmospheric volatile recycling

    NASA Astrophysics Data System (ADS)

    Parai, R.; Mukhopadhyay, S.

    2015-12-01

    Constraints on the recycling of atmospheric volatiles into the deep Earth provide important insights into mantle temperature, cooling rate, structure and style of convection over Earth history. Studies of ancient atmospheric gases trapped in Archean cherts show that the Xe isotopic composition of the atmosphere at ~3.5 Ga differed from the modern atmosphere [1]. This suggests the atmosphere evolved in isotopic composition until it reached its present-day composition at some time after 3.5 Ga. The evolution of the atmospheric Xe isotopic composition presents an opportunity to constrain the timing of Xe recycling into the Earth's mantle. Xe isotopes measured in mid-ocean ridge basalts [MORBs; 2,3] and plume-related basalts [4,5] indicate that both the upper mantle and plume source Xe isotopic compositions are dominated by recycled Xe [e.g., 3]. We find that the mantle source Xe isotopic compositions cannot be explained by recycling ancient atmospheric Xe alone; rather, subduction and incorporation of material bearing the modern atmospheric Xe composition must dominate. We note that our findings are consistent with a number of physical reasons that recently-subducted volatiles should be more prevalent than ancient subducted volatiles. First, a higher Archean mantle potential temperature should inhibit early Xe recycling to the deep Earth. Second, since the mantle turnover time scale is estimated to be between a few hundreds of Myr and 1 Gyr, the mantle recycled atmospheric Xe budget should be primarily composed of Xe subducted after ~2.5 Ga, at which point the atmosphere approaches the modern Xe composition [1]. Therefore, even if ancient atmospheric Xe were recycled efficiently to the mantle early in Earth history, the recycled atmospheric Xe budget of the mantle should still be dominated by the modern atmospheric Xe composition. [1] Pujol et al., 2011, EPSL; [2] Tucker et al., 2012, EPSL; [3] Parai and Mukhopadhyay, 2015, G-cubed; [4] Mukhopadhyay, 2012, Nature; [5

  7. Lu-Hf Isotope Systematics of the Nuvvuagittuq Supracrustal Belt (Québec, Canada)

    NASA Astrophysics Data System (ADS)

    Guitreau, M.; Blichert-Toft, J.; Mojzsis, S. J.; Roth, A. S.; Bourdon, B.

    2012-12-01

    The Nuvvuagittuq supracrustal belt (NSB) in northern Québec (Canada) is a mafic terrane enclosed in the westernmost part of the Minto Block of the Superior Province. This locality became famous after the discovery by [1] of 142Nd deficits in the constituent amphibolites. The age of the NSB is a matter of debate because zircon U-Pb and whole-rock 147Sm-143Nd isotope systematics give Eoarchean ages of ~3.8 Ga, while the short-lived 146Sm-142Nd chronometer indicates a Hadean age of ~4.3 Ga. We present new Lu-Hf isotope data for mafic and felsic rocks from the NSB in an attempt to resolve this age issue and, hence, understand the origin of the negative 142Nd anomalies. The Lu-Hf data define a "scatterchron" yielding an Eoarchean age of 3864±70 Ma, which is consistent with 147Sm-143Nd and U-Pb zircon ages but in disagreement with the Hadean age inferred from 146Sm-142Nd systematics. We interpret the Lu-Hf age as the mean emplacement age of the different autochthonous units of the NSB. The observed alignment of the data along a Lu-Hf "scatterchron" precludes a Hadean age for the NSB because their isotopic characteristics appear to be controlled by long-term radiogenic ingrowth. A Hadean derivation should have caused age differences of hundreds of millions of years to manifest as strong deviations from the observed scatterchron. Furthermore, combined Lu-Hf and Sm-Nd data on the same NSB (Ca-poor cummingtonite- and hornblende-bearing) amphibolite samples define a mixing hyperbola at ca. 3800 Ma with end-member compositions representative of compositional groupings identified for these lithologies [2]. Low 142Nd/144Nd values relative to Bulk Silicate Earth are endemic to the "low-TiO2" amphibolite population, and can be attributed to a Hadean multi-stage history of its mantle source as indicated by rare-earth element patterns. The 142Nd deficits could have developed in response to a later re-fertilization episode within a mantle source depleted by primordial crust

  8. Oxygen isotope constraints on the petrogenesis of Aleutian arc magmas

    SciTech Connect

    Singer, B.S.; O'Neil, J.R. ); Brophy, J.G. )

    1992-04-01

    The first measurement of {sup 18}O/{sup 16}O ratios of plagioclase, clinopyroxene, orthopyroxene, and titanomagnetite phenocrysts from modern Aleutian island-arc lavas provides new insight and independent constraints on magma sources and intracrustal processes. Basalts are heterogeneous on the scale of the entire arc and individual volcanic centers. Combined with Sr isotope and trace element data {delta}{sup 18}O{sub plag} values suggest a variable magma source characterized by differences in the mantle wedge or the subducted sediment component along the volcanic front. Seven tholeiitic basalt to rhyodacite lavas from the Seguam volcanic center have nearly identical {delta}{sup 18}O{sub plag} values of 6.0{per thousand} {plus minus} 0.2{per thousand}, reflecting extensive closed-system plagioclase-dominated crystal fractionation. Oxygen isotope thermometry and pyroxene and oxide equilibria indicate that differentiation occurred between 1,150 {plus minus} 100C (basalt) and 950 {plus minus} 100C (rhyodacite). In contrast, {delta}{sup 18}O{sub plag} values of 12 calc-alkalic basaltic andesites and andesites from the smaller Kanaga volcanic center span a broader range of 5.9{per thousand}-6.6{per thousand}, and consist of mostly higher values. Isotopic disequilibrium in the Kanaga system is manifest in two ways: two types of basaltic inclusions with contrasting {delta}{sup 18}O values occur in one andesite, and in two other andesites plagioclase-titanomagnetite and clinopyroxene-titanomagnetite oxygen isotope temperatures are inconsistent.

  9. Zircon U-Pb and Hf Isotopes Provide Insights into Triassic Magmatism in the Chinese Pamir

    NASA Astrophysics Data System (ADS)

    Imrecke, D. B.; Robinson, A. C.

    2015-12-01

    Recent research has improved understanding of Triassic magmatism and sedimentation in the Songpan-Ganzi/Hoh-Xil Terranes of Tibet and the implications for the closure of the Paleotethys ocean (Pullen et al., 2008; Ding et al,. 2013; Zhang et al., 2014). However, our knowledge of the age of magmatism in the laterally equivalent Karakul-Mazar Terrane in the Northern Pamir is limited. While previous investigations indicate Karakul-Mazar igneous bodies have generally documented crystallization ages 225-245 Ma, detrital zircon studies of Late Triassic/Early Jurassic strata within the Northern Pamir and the Tarim Basin record a significant quantity of <220 Ma zircons (Bershaw et al., 2011) sourced from the Pamir. 6 granite samples were analyzed for zircon U-Pb and Hf isotopes, representing plutons distributed across the Chinese Pamir, to determine the distribution of crystallization ages and chemical maturity of the magma source. Analyses yielded 204 Ma and 212-214 Ma zircon U-Pb crystallization ages. The dated samples yield ɛHf(t) values ranging from -6.7 to 9.6. Results show that a large volume of magmatic rocks in the Northern Pamir intruded in the Late Triassic prior to closure of the Paleotethys Ocean at ~200 Ma (Angiolini et al., 2013). Weakly positive and negative ɛHf(t) values indicate a primitive source for the dated magmatic bodies. Additionally, compliation of previously published data with these results suggests two pulses of magmatism, ~210 Ma and 230-245 Ma respectively. Finally, Triassic igneous bodies in the Pamir show similar crystallization ages and chemical signatures compared to plutons in the Songpan-Ganzi/Hoh-Xil Terranes to the east, suggesting lateral continuity of geodynamic processes across the terrane in the Mesozoic.

  10. a Coupled nd and HF Isotopic Study of Isua Archean Rocks and the Differentiation of the Hadean Mantle

    NASA Astrophysics Data System (ADS)

    Rizo Garza, H. L.; Boyet, M. M.; Blichert-Toft, J.; Rosing, M.; Gannoun, A.

    2010-12-01

    The first 500 Ma of Earth’s history is for the most part unknown due to the scarcity of rocks and minerals available for investigation. Instead, early mantle evolution can be inferred from short- and long-lived isotope systematics in Earth’s oldest rocks. While low concentrations of Sm and Nd in zircons make this mineral unsuitable for Nd isotope analysis, its high concentrations of Hf and low abundances of Lu makes it ideal for Hf isotope analysis. Hence, models of the early terrestrial mantle are based on Sm-Nd and Lu-Hf isotope analyses of, respectively, whole rocks and zircons. We here present the first such coupled 146,147Sm-142,143Nd and 176Lu-176Hf study of mafic rocks from the western part of the Isua supracrustal belt (SW Greenland). Lu-Hf and Sm-Nd isotopic compositions were measured by the Lyon Nu Plasma MC-ICPMS and the Clermont-Ferrand Triton TIMS. The Lu-Hf and Sm-Nd isochrons yield identical ages within error: 3.701±0.063 Ga and 3.674±0.035 Ga, respectively. This is in good agreement with minimum zircon U-Pb ages from crosscutting tonalites [1] and, thus, can be taken as the emplacement age. Our results further confirm and extend the existing database of 142Nd excesses in Isua samples (ɛ142Nd<12 ppm relative to the Nd terrestrial standard), supporting the existence of an early-depleted reservoir (EDR) in the Archean [2]. Coupled 146,147Sm-142,143Nd suggest that the EDR differentiated from the Bulk Silicate Earth between ~4.32 and 4.47 Ga and that the Hadean mantle was less depleted (147Sm/144Nd ≈ 0.211) than the present-day MORB source. The intercept of the Sm-Nd whole rock isochron is in accordance with 142Nd results and consistent with a superchondritic initial 143Nd/144Nd ratio (ɛ143Nd= +1.41±0.98). In contrast, the corresponding initial ɛ176Hf = -1.41±0.57 is subchondritic. The correlation between Nd and Hf isotopes in most terrestrial samples [3] attests to coupled fractionation behaviour of Lu/Hf and Sm/Nd during igneous

  11. Stellar Neutron Capture Cross Sections of the Lu and Hf Isotopes

    SciTech Connect

    Wisshak, K.; Voss, F.; Kaeppeler, F.; Kazakov, L.; Krticka, M.

    2005-05-24

    The neutron capture cross sections of 175,176Lu and 176,177,178,179,180Hf have been measured in the energy range from 3 to 225 keV at the Karlsruhe 3.7 MV Van de Graaff accelerator relative to the gold standard. Neutrons were produced by the 7Li(p,n)7Be reaction and capture events were detected by the Karlsruhe 4{pi}BaF2 detector. The cross section ratios could be determined with uncertainties between 0.9 and 1.8% about a factor of five more accurate than previous data. A strong population of isomeric states was found in neutron capture of the Hf isotopes, which are only partially explained by CASINO/GEANT simulations based on the known level schemes.Maxwellian averaged neutron capture cross sections were calculated for thermal energies between kT = 8 keV and 100 keV. Severe differences up to40% were found to the data of a recent evaluation based on existing experimental results. The new data allow for a much more reliable analysis of the important branching in the s-process synthesis path at 176Lu which can be interpreted as an s-process thermometer.

  12. Lu-Hf geochronology on cm-sized garnets using microsampling: New constraints on garnet growth rates and duration of metamorphism during continental collision (Menderes Massif, Turkey)

    NASA Astrophysics Data System (ADS)

    Schmidt, Alexander; Pourteau, Amaury; Candan, Osman; Oberhänsli, Roland

    2015-12-01

    This study shows Lu-Hf geochronology of zoned garnet crystals contained in mica schists from the southern Menderes Massif, Turkey. Selected samples are four 3-5 cm large garnet megacrysts of which several consecutive garnet shells have been sampled with a micro-saw and analyzed for dating. The results are used to extract growth rates of garnet, and also to improve the time constraint for Alpine-aged overprint of the Pan-African basement in the Menderes Massif. Lu-Hf ages of the sampled garnet shells are determined by two-point garnet-only isochrons using the garnets' Lu-depleted rim compositions. This yields a consistent decrease of age information from core to rim segments of individual garnet crystals and the calculated isochron ages propose a time frame of growth between 42.6 ± 1.9 and 34.8 ± 3.1 Ma. Major element profiles in the investigated garnets characterize zoning patterns indicative of prograde conditions: Rayleigh fractionated bell-shaped Mn and decreasing Fe/(Fe + Mg) are recorded by the garnets' core to rim compositions. Therefore the obtained Lu-Hf ages record timing of early prograde growth for the cores of the garnets. Two of the large garnet crystals also yield isochron ages of 58.83 ± 0.69 and 50.16 ± 0.84 Ma in their innermost cores, which appear to record an early nucleation event. This view, however, is not in concordance with the observed major element profiles of these garnets, and therefore is interpreted with caution. Termination of the garnet growth period is determined through the calculation of radial growth rates based on the size of the garnets and the Lu-Hf ages obtained for consecutive shells. Extrapolation of these rates potentially constrains the total duration for garnet growth terminating at 31 ± 6 Ma. Comparison of the growth rates calculated for individual crystals shows a variety of slow and fast growing garnets, and similar results have been previously obtained with the Rb-Sr and Sm-Nd isotope systems. The new data

  13. Using paired U-Pb and Hf isotopes to characterize the Yavapai - Mojave province boundary in Grand Canyon, AZ

    NASA Astrophysics Data System (ADS)

    Holland, M. E.; Karlstrom, K. E.; Doe, M. F.; Gehrels, G. E.; Pecha, M.; Shufeldt, O. P.

    2013-12-01

    Two distinct Proterozoic provinces of southwest Laurentia, the Mojave and Yavapai, are discriminated in terms of their age, isotopic composition, and metamorphic grade. The crystalline basement rocks of the Mojave province preserve an evolved isotopic signature (Nd, Pb, Hf) that suggests Archean crustal material is detected in all isotopic systems, but the origin and tectonic significance of this Archean component, and the nature and location of province boundaries, are debated. Previous models include: 1) subducted Archean detritus as the source of the evolved isotopic signature of the Mojave, 2) a wide isotopically mixed (Pb) zone resulting from rifting and hybridization of older crust, and 3) a distributed tectonic suture centered at the Crystal shear zone in Grand Canyon. U-Pb and Hf isotopic analysis of zircons separated from igneous and metasedimentary lithologies along a transect in Grand Canyon provide new insight into the Mojave province's evolved isotopic composition, and the nature of the Mojave - Yavapai boundary. Comparison of the Hf isotopic composition of zircons separated from 1.75 - 1.71 Ga granodiorite plutons west of river mile 96-98 (Crystal shear zone) characteristically contain Paleoproterozoic grains that yield Archean (2.5 - 3.3 Ga) Hf model ages, as well as xenocrystic Archean grains. In contrast, 1.75 - 1.71 Ga plutons in eastern Grand Canyon have Hf model ages of 1.7 - 1.8 Ga suggesting they were dominantly derived from juvenile 1.7 - 1.8 Ga crust. Vishnu Schist metaturbidites are exposed across the entire Grand Canyon transect and have a uniform bimodal zircon population (~1.85 and 2.48 Ga peaks), with only 13% juvenile 1.75 Ga grains; Hf signatures also are uniform across the transect. These data suggest that: 1) the Mojave province contains a heterogeneous older lower crust containing 1.85 and 2.5 Ga domains, 2) Yavapai crust is dominantly juvenile east of the Crystal shear zone and in central AZ, and 3) juxtaposition of Mojave

  14. Comprehensive Pb-Sr-Nd-Hf isotopic, trace element, and mineralogical characterization of mafic to ultramafic rock reference materials

    NASA Astrophysics Data System (ADS)

    Fourny, Anaïs.; Weis, Dominique; Scoates, James S.

    2016-03-01

    Controlling the accuracy and precision of geochemical analyses requires the use of characterized reference materials with matrices similar to those of the unknown samples being analyzed. We report a comprehensive Pb-Sr-Nd-Hf isotopic and trace element concentration data set, combined with quantitative phase analysis by XRD Rietveld refinement, for a wide range of mafic to ultramafic rock reference materials analyzed at the Pacific Centre for Isotopic and Geochemical Research, University of British Columbia. The samples include a pyroxenite (NIM-P), five basalts (BHVO-2, BIR-1a, JB-3, BE-N, GSR-3), a diabase (W-2), a dolerite (DNC-1), a norite (NIM-N), and an anorthosite (AN-G); results from a leucogabbro (Stillwater) are also reported. Individual isotopic ratios determined by MC-ICP-MS and TIMS, and multielement analyses by HR-ICP-MS are reported with 4-12 complete analytical duplicates for each sample. The basaltic reference materials have coherent Sr and Nd isotopic ratios with external precision below 50 ppm (2SD) and below 100 ppm for Hf isotopes (except BIR-1a). For Pb isotopic reproducibility, several of the basalts (JB-3, BHVO-2) require acid leaching prior to dissolution. The plutonic reference materials also have coherent Sr and Nd isotopic ratios (<50 ppm), however, obtaining good reproducibility for Pb and Hf isotopic ratios is more challenging for NIM-P, NIM-N, and AN-G due to a variety of factors, including postcrystallization Pb mobility and the presence of accessory zircon. Collectively, these results form a comprehensive new database that can be used by the geochemical community for evaluating the radiogenic isotope and trace element compositions of volcanic and plutonic mafic-ultramafic rocks.

  15. Comparative behavior of Sr, Nd and Hf isotopic systems during fluid-related deformation at middle crust levels

    NASA Astrophysics Data System (ADS)

    Luais, Béatrice; Le Carlier de Veslud, Christian; Géraud, Yves; Gauthier-Lafaye, François

    2009-05-01

    We have carried out a comparative Rb-Sr, Sm-Nd and Lu-Hf isotopic study of a progressively deformed hercynian leucogranite from the French Massif Central, belonging to the La Marche ductile shear zone, in order to investigate the respective perturbation of these geochronometers with fluid induced deformation. The one-meter wide outcrop presents a strongly deformed and mylonitized zone at the center, and an asymmetric deformation pattern with a higher deformation gradient on the northern side of the zone. Ten samples have been carefully collected every 10 cm North and South away from the strongest deformed mylonitic zone. They have been analyzed for a complete major, trace element data set, oxygen isotopes, Rb-Sr, Sm-Nd and Lu-Hf isotopic systematics. We show that most of major and trace elements except SiO 2, alkaline elements (K 2O, Rb), and some metal transition elements (Cu), are progressively depleted with increasing deformation. This depletion includes REE + Y, but also HFS elements (Ti, Hf, Zr, Nb) which are commonly considered as immobile elements during upper level processes. Variations in elemental ratios with deformation, e.g. decrease in LREE/MREE- HREE, Nd/Hf, Th/Sr, increase in Rb/Sr, U/Th and constant Sr/Nd, lead to propose the following order of element mobility: U ≫ Th > Sr = Nd ≫ Hf + HREE. We conclude in agreement with previous tectonic and metallogenic studies that trace element patterns across the shear zone result from circulation of oxidizing F-rich hydrothermal fluids associated with deformation. A temperature of the fluid of 470-480 °C can be deduced from the δ 18O equilibrium between quartz-muscovite pairs. Elemental fractionation induces perturbation of the Rb-Sr geochronometer. The well-defined 87Rb/ 86Sr- 87Sr/ 86Sr correlation gives an apparent age of 294 ± 19 Ma, slightly younger than the 323 ± 4 Ma age of leucogranites in this area. This apparent age is interpreted as dating event of intense deformation and fluid circulation

  16. Calcium isotope constraints on the end-Permian mass extinction

    PubMed Central

    Payne, Jonathan L.; Turchyn, Alexandra V.; Paytan, Adina; DePaolo, Donald J.; Lehrmann, Daniel J.; Yu, Meiyi; Wei, Jiayong

    2010-01-01

    The end-Permian mass extinction horizon is marked by an abrupt shift in style of carbonate sedimentation and a negative excursion in the carbon isotope (δ13C) composition of carbonate minerals. Several extinction scenarios consistent with these observations have been put forward. Secular variation in the calcium isotope (δ44/40Ca) composition of marine sediments provides a tool for distinguishing among these possibilities and thereby constraining the causes of mass extinction. Here we report δ44/40Ca across the Permian-Triassic boundary from marine limestone in south China. The δ44/40Ca exhibits a transient negative excursion of ∼0.3‰ over a few hundred thousand years or less, which we interpret to reflect a change in the global δ44/40Ca composition of seawater. CO2-driven ocean acidification best explains the coincidence of the δ44/40Ca excursion with negative excursions in the δ13C of carbonates and organic matter and the preferential extinction of heavily calcified marine animals. Calcium isotope constraints on carbon cycle calculations suggest that the average δ13C of CO2 released was heavier than -28‰ and more likely near -15‰; these values indicate a source containing substantial amounts of mantle- or carbonate-derived carbon. Collectively, the results point toward Siberian Trap volcanism as the trigger of mass extinction. PMID:20421502

  17. Calcium isotope constraints on the end-Permian mass extinction.

    PubMed

    Payne, Jonathan L; Turchyn, Alexandra V; Paytan, Adina; Depaolo, Donald J; Lehrmann, Daniel J; Yu, Meiyi; Wei, Jiayong

    2010-05-11

    The end-Permian mass extinction horizon is marked by an abrupt shift in style of carbonate sedimentation and a negative excursion in the carbon isotope (delta(13)C) composition of carbonate minerals. Several extinction scenarios consistent with these observations have been put forward. Secular variation in the calcium isotope (delta(44/40)Ca) composition of marine sediments provides a tool for distinguishing among these possibilities and thereby constraining the causes of mass extinction. Here we report delta(44/40)Ca across the Permian-Triassic boundary from marine limestone in south China. The delta(44/40)Ca exhibits a transient negative excursion of approximately 0.3 per thousand over a few hundred thousand years or less, which we interpret to reflect a change in the global delta(44/40)Ca composition of seawater. CO(2)-driven ocean acidification best explains the coincidence of the delta(44/40)Ca excursion with negative excursions in the delta(13)C of carbonates and organic matter and the preferential extinction of heavily calcified marine animals. Calcium isotope constraints on carbon cycle calculations suggest that the average delta(13)C of CO(2) released was heavier than -28 per thousand and more likely near -15 per thousand; these values indicate a source containing substantial amounts of mantle- or carbonate-derived carbon. Collectively, the results point toward Siberian Trap volcanism as the trigger of mass extinction. PMID:20421502

  18. In-situ Hf isotope analysis of early Archean zircons in the Acasta Gneisses from the Slave province, Northwestern Canada

    NASA Astrophysics Data System (ADS)

    Iizuka, T.; Komiya, T.; Maruyama, S.; Hirata, T.

    2003-12-01

    Lu-Hf and Sm-Nd isotopic systems of early Archean rocks provide insights into the early crustal evolution and early mantle differentiation of the Earth. The Acasta Gneisses have been established as the oldest known intact terrestrial rocks (Bowring et al., 1999). The Acasta Gneiss Complex comprises mainly of Gray Gneiss (granodioritic gneiss), White Gneiss (tonalitic to granitic gneiss), and Foliated Granite, with many aplite and basaltic intrusions, and the relation between these rocks is very complex. Bowring et al. (1989) carried out the whole-rock Sm-Nd isotopic system measurement of the Acasta gneisses, and demonstrated that the gneisses exhibit a wide range of initial ɛ (Nd) (+3.5 to -4 at 4.0 Ga and +4 to -7 at 3.6 Ga). However, because most of the Acasta gneisses have experienced amphibolite facies metamorphism, it is difficult that the whole-rock isotopic system remains closed. Zircon, which is extremely resistant against erosion and/or metamorphic events, and it can be also dated precisely by U-Pb chronometer. Because of high Hf content (ca. 1 wt%) and low Lu/Hf ratio, zircon has been widely used for the isotopic study using Lu-Hf system, too. Recent Lu-Hf isotopic studies were carried out using a multiplecollector inductively coupled plasma mass spectrometer (MC-ICPMS). Amelin et al. (2000) carried out the Hf isotope analyses of some zircon grains from the Acasta Gneisses using MC-ICPMS. The zircon grains exhibit enriched initial ɛ (Hf) (+0.16 to -4.1 at ca. 3.6 Ga), while other early Archean zircon grains from the Amitsoq gneisses and the Barberton gneisses indicate depleted signature (Amelin et al., 2000). One possible reason is that the zircon grains from the Acasta Gneisses are grown at partial melting of the gneisses and/or underwent isotopic disturbance caused by intrusion of younger granites. Therefore, it is very important to reveal the growth features of zircon, such as oscillatory zoning, in order to derive inherent information of the early

  19. ~100 Ma Lu-Hf eclogite ages from Koralpe and Saualpe (Austroalpine nappes, Austria): New constraints for the kinematics of Eoalpine subduction

    NASA Astrophysics Data System (ADS)

    Miladinova, Irena; Froitzheim, Nikolaus; Nagel, Thorsten; Janák, Marian; Münker, Carsten

    2016-04-01

    The Koralpe and Saualpe complexes are part of the Austroalpine basement nappe system. They represent the largest region in the Eastern Alps exposing high-pressure metamorphic rocks from the Cretaceous Eoalpine orogenic event and also contain the type locality for eclogite. The grade of the Cretaceous metamorphism in the Eastern Alps increases to the southeast, with maximum pressures and temperatures reaching up to 3.5 GPa and 850 °C in the Pohorje Mountains (Janak et al., 2015). The estimated P-T-conditions for the eclogites from Saualpe and Koralpe are 2-2.2 GPa and 600-740 °C (Miller & Thöni 1997, Thöni et al. 2008). Here we present a new Lu-Hf isotopic study of the eclogites from the Hohl locality in the southern Koralpe, and from the Grünburgerbach and Wolfsberger Hütte localities in the southern Saualpe. Two-point isochrones from samples of Hohl and Wolfsberger Hütte based on one whole rock and one garnet separate yield ages of 99.2 ± 1.1 Ma and 101.7 ± 2 Ma, respectively. Two eclogite samples from Grünburgerbach give garnet-omphacite-whole rock ages of 100.3 ± 1 Ma and 101.79 ± 0.92 Ma, identical within error. The garnets in the eclogite from Hohl display a homogenous composition with no zoning of major elements, whereas the garnets of the samples from Grünburgerbach show an enrichment of Mn in the cores and lower contents towards the rims, which indicates prograde garnet growth during increasing P and T. The ages are therefore related to burial during subduction. These new Lu-Hf garnet ages are slightly older than the Lu-Hf garnet age data from Pohorje (~95 Ma; Sandmann et al. 2011, Thöni et al. 2008), which also date burial. If Koralpe/Saualpe and Pohorje would belong to one continuous crustal unit subducted and exhumed "en bloc" in a southeast-dipping subduction zone, the opposite age difference would be expected. Our results show that this is not the case and represent important constraints for a more realistic kinematic model. Janak, M

  20. Distribution coefficients of 60 elements on TODGA resin: application to Ca, Lu, Hf, U and Th isotope geochemistry.

    PubMed

    Pourmand, Ali; Dauphas, Nicolas

    2010-05-15

    Batch equilibration experiments are conducted to measure the distribution coefficients (K(d)) of a large number of elements in nitric, nitric plus hydrofluoric, and hydrochloric acids on Eichrom TODGA extraction chromatography resin. The K(d)s are used to devise a multi-element extraction scheme for high-precision elemental and isotopic analyses of Ca, Hf, Lu, Th and U in geological materials, using high-purity lithium metaborate (LiBO(2)) flux fusion that allows rapid digestion of even the most refractory materials. The fusion melt, dissolved in nitric acid, is directly loaded to a TODGA cartridge on a vacuum chamber for elemental separation. An Ln-Spec cartridge is used in tandem with TODGA for Lu purification. The entire procedure, from flux digestion to preparation for isotopic analysis, can be completed in a day. The accuracy of the proposed technique is tested by measuring the concentrations of Ca (standard bracketing), Hf, Lu, Th and U (isotope dilution), and the isotopic composition of Hf in geostandards (USNM3529, BCR-2, BHVO-1, AGV-1 and AGV-2). All measurements are in excellent agreement with recommended literature values, demonstrating the effectiveness of the proposed analytical procedure and the versatility of TODGA resin. PMID:20298848

  1. Constraints on Lu-Hf and Nb-Ta systematics in globally subducted oceanic crust from a survey of orogenic eclogites and amphibolites

    NASA Astrophysics Data System (ADS)

    Zirakparvar, N. Alex

    2016-04-01

    To further understand Lu-Hf and Nb-Ta systematics in globally subducted oceanic crust, this paper evaluates all available Lu-Hf garnet isochron ages and initial ɛHf values in conjunction with present-day bulk-rock Lu-Hf isotope and trace element (K, Nb, Ta, Zr, and Ti in addition to Lu-Hf) data from the world's orogenic eclogites and amphibolites (OEAs). Approximately half of OEAs exhibit Lu-Hf and Nb-Ta systematics mimicking those of unsubducted oceanic crust whereas the rest exhibit variability in one or both systems. For the Lu-Hf system, mixing calculations demonstrate that subduction-related phase transformations, in conjunction with open system behavior, can shift subducted oceanic crust toward higher Lu/Hf, or toward lower Lu/Hf that can also be associated with unradiogenic ɛHf values. However, evaluation of potential mechanisms for fractionating Nb from Ta is more complicated because many of the OEAs have Nb-Ta systematics that are decoupled from Lu-Hf and the behavior of K, Zr, and Ti. Nonetheless, the global data set demonstrates that the association between unradiogenic ɛHf and elevated Nb/Ta observed in some kimberlitic eclogite xenoliths can be inherited from processes that occurred during subduction of their oceanic crustal protoliths. This allows for a geologically based estimate of the Nb concentration in a reservoir composed of deeply subducted oceanic crust. However, mass balance calculations confirm that such a reservoir, when considered as a whole, likely has a Nb concentration similar to unsubducted oceanic crust and is therefore not the solution to the problem of the Earth's "missing" Nb.

  2. Zircon U-Pb ages and Hf isotopic compositions indicate multiple sources for Grenvillian detrital zircon deposited in western Laurentia

    NASA Astrophysics Data System (ADS)

    Howard, Amanda L.; Farmer, G. Lang; Amato, Jeffrey M.; Fedo, Christopher M.

    2015-12-01

    Combined U-Pb ages and Hf isotopic data from 1.0 Ga to 1.3 Ga (Grenvillian) detrital zircon in Neoproterozoic and Cambrian siliciclastic sedimentary rocks in southwest North America, and from igneous zircon in potential Mesoproterozoic source rocks, are used to better assess the provenance of detrital zircon potentially transported across Laurentia in major river systems originating in the Grenville orogenic highlands. High-precision hafnium isotopic analyses of individual ∼1.1 Ga detrital zircon from Neoproterozoic siliciclastic sedimentary rocks in Sonora, northern Mexico, reveal that these zircons have low εHf (0) (-22 to -26) and were most likely derived from ∼1.1 Ga granitic rocks embedded in local Mojave Province Paleoproterozoic crust. In contrast, Grenvillian detrital zircons in Cambrian sedimentary rocks in Sonora, the Great Basin, and the Mojave Desert, have generally higher εHf (0) (-15 to -21) as demonstrated both by high precision solution-based, and by lower precision laser ablation, ICPMS data and were likely derived from more distal sources further to the east/southeast in Laurentia. Comparison to new and existing zircon U-Pb geochronology and Hf isotopic data from Grenvillian crystalline rocks from the Appalachian Mountains, central and west Texas, and from Paleoproterozoic terranes throughout southwest North America reveals that zircon in Cambrian sandstones need not entirely represent detritus transported across the continent from Grenville province rocks in the vicinity of the present-day southern Appalachian Mountains. Instead, these zircons could have been derived from more proximal, high εHf (0), ∼1.1 Ga, crystalline rocks such as those exposed today in the Llano Uplift in central Texas and in the Franklin Mountains of west Texas. Regardless of the exact source(s) of the Grenvillian detrital zircon, new and existing whole-rock Nd isotopic data from Neoproterozoic to Cambrian siliciclastic sedimentary rocks in the Mojave Desert

  3. Sr-Nd-Pb-Hf isotope systematics of the Hugo Dummett Cu-Au porphyry deposit (Oyu Tolgoi, Mongolia)

    NASA Astrophysics Data System (ADS)

    Dolgopolova, A.; Seltmann, R.; Armstrong, R.; Belousova, E.; Pankhurst, R. J.; Kavalieris, I.

    2013-04-01

    Major and trace element geochemistry including Sr-Nd-Pb-Hf isotopic data are presented for a representative sample suite of Late Devonian to Early Carboniferous plutonic and volcanic rocks from the Hugo Dummett deposit of the giant Oyu Tolgoi porphyry Cu-Au district in South Gobi, Mongolia. Sr and Nd isotopes (whole-rock) show restricted ranges of initial compositions, with positive ɛNdt mainly between + 3.4 and + 7.4 and (87Sr/86Sr)t predominantly between 0.7037 and 0.7045 reflecting magma generation from a relatively uniform juvenile lithophile-element depleted source. Previously dated zircons from the plutonic rocks exhibit a sample-averaged range of ɛHft values of + 11.6 to + 14.5. Depleted-mantle model ages of 420-830 (Nd) and 320-730 Ma (zircon Hf) limit the involvement of pre-Neoproterozoic crust in the petrogenesis of the intermediate to felsic calc-alkaline magmas to, at most, a minor role. Pb isotopes (whole-rock) show a narrow range of unradiogenic initial compositions: 206Pb/204Pb 17.40-17.94, 207Pb/204Pb 15.43-15.49 and 208Pb/204Pb 37.25-37.64, in agreement with Sr-Nd-Hf isotopes indicating the dominance of a mantle component. All four isotopic systems suggest that the magmas from which the large Oyu Tolgoi porphyry system was generated originated predominantly from juvenile material within the subduction-related setting of the Gurvansayhan terrane.

  4. Lu-Hf isotopic memory of plume-lithosphere interaction in the source of layered mafic intrusions, Windimurra Igneous Complex, Yilgarn Craton, Australia

    NASA Astrophysics Data System (ADS)

    Nebel, O.; Arculus, R. J.; Ivanic, T. J.; Nebel-Jacobsen, Y. J.

    2013-10-01

    Most layered mafic intrusions (LMI) are formed via multiple magma injections into crustal magma chambers. These magmas are originally sourced from the mantle, likely via plume activity, but may interact with the overriding lithosphere during ascent and emplacement in the crust. The magma injections lead to the establishment of different layers and zones with complex macroscopic, microscopic and cryptic compositional layering through magmatic differentiation and associated cumulate formation, sometimes accompanied by crustal assimilation. These complex mineralogical and petrological processes obscure the nature of the mantle sources of LMI, and typically have limited the degree to which parental liquids can be fully characterised. Here, we present Lu-Hf isotope data for samples from distinct layers of the Upper Zone of the Windimurra Igneous Complex (WIC), an immense late-Archean LMI in the West Australian Yilgarn Craton. Lu-Hf isotope systematics of whole rocks are well correlated (MSWD=5.6, n=17) with an age of ˜3.05±0.05 Ga and initial ɛHf˜+8. This age, however, is older than whole rock Sm-Nd and zircon U-Pb ages of the intrusion, both of which are ca. 2.8 Ga. Stratigraphically-controlled initial Hf isotope variations (associated with multiple episodes of emplacement at ca. 2.8 Ga) indicate isotope mixing between a near-chondritic and an ultra-radiogenic component, the latter with ɛHf[2.8 Ga]>+15. This Hf isotope mixing creates a pseudochron-relationship at the time of intrusion of ˜250 Myr that is superimposed on subsequent radiogenic ingrowth after crystallisation, generating an age that predates the actual emplacement event. Mixing between late-stage crystallisation products (melt + crystals) from the Middle Zone and replenishing, plume-derived liquids was followed by crystal accumulation in a chemically evolving magma chamber. The ultra-radiogenic Hf isotope endmember in the WIC mantle source requires parent-daughter ratios consistent with very early

  5. A Sr, Nd, Hf, and Pb isotope perspective on the genesis and long-term evolution of alkaline magmas from Erebus volcano, Antarctica

    NASA Astrophysics Data System (ADS)

    Sims, Kenneth W. W.; Blichert-Toft, Janne; Kyle, Philip R.; Pichat, Sylvain; Gauthier, Pierre-Jean; Blusztajn, Jurek; Kelly, Peter; Ball, Lary; Layne, Graham

    2008-11-01

    We report new Nd, Hf, Sr, and high-precision Pb isotopic data for 44 lava and tephra samples from Erebus volcano. The samples cover the entire compositional range from basanite to phonolite and trachyte, and represent all three phases of the volcanic evolution from 1.3 Ma to the present. Isotopic analyses of 7 samples from Mt. Morning and the Dry Valley Drilling Project (DVDP) are given for comparison. The Erebus volcano samples have radiogenic 206Pb/ 204Pb, unradiogenic 87Sr/ 86Sr, and intermediate 143Nd/ 144Nd and 176Hf/ 177Hf, and lie along a mixing trajectory between the two end-member mantle components DMM and HIMU. The Erebus time series data show a marked distinction between the early-phase basanites and phonotephrites, whose Nd, Hf, Sr, and Pb isotope compositions are variable (particularly Pb), and the current 'phase-three' evolved phonolitic lavas and bombs, whose Nd, Hf, Sr, and Pb isotope compositions are essentially invariant. Magma mixing is inferred to play a fundamental role in establishing the isotopic and compositional uniformity in the evolved phase-three phonolites. In-situ analyses of Pb isotopes in melt inclusions hosted in an anorthoclase crystal from a 1984 Erebus phonolite bomb and in an olivine from a DVDP basanite are uniform and identical to the host lavas within analytical uncertainties. We suggest that, in both cases, the magma was well mixed at the time melt inclusions were incorporated into the different mineral phases.

  6. Constraints on Hf and Zr mobility in high-sulfidation epithermal systems: formation of kosnarite, KZr2(PO4)3, in the Chaquicocha gold deposit, Yanacocha district, Peru

    NASA Astrophysics Data System (ADS)

    Deditius, Artur P.; Utsunomiya, Satoshi; Sanchez-Alfaro, Pablo; Reich, Martin; Ewing, Rodney C.; Kesler, Stephen E.

    2015-04-01

    , potentially new geochronological applications of highly insoluble vein kosnarite, including Rb-Sr dating, may provide further age constraints in pervasively altered areas where other isotopic systems might have been reset.

  7. Carbon Isotopic Constraints on Arctic Methane Sources, 2008-2010

    NASA Astrophysics Data System (ADS)

    Fisher, R. E.; Lowry, D.; Lanoiselle, M.; Sriskantharajah, S.; Nisbet, E. G.

    2010-12-01

    Arctic methane source strengths are particularly vulnerable to large changes with year-to year meteorological variations and with climatic change. A global increase in methane seen in 2007 (Dlugokencky et al., 2009) may have been in part be due to elevated wetland emissions caused by a warm, wet summer over large parts of Siberia. In 2010 wildfires over large areas of Russia will have added methane to the Arctic atmosphere. Carbon isotopic composition of methane in air from the Arctic arriving at a measurement station can be used to identify sources of the gas. Measurement of methane δ13C in air close to sources, including wetlands, permafrost, pine forest and submarine methane clathrate has extended the available data of source signatures of methane from northern sources. Keeling plot analysis of diurnal records from field campaigns in Arctic wetlands show that bulk wetland methane emissions are typically close to δ13CCH4 -69±1 ‰. Air samples from Zeppelin (Spitsbergen, Norway), Pallas (Finland) and Barra (Outer Hebrides, Scotland) have been regularly analysed for methane δ13C. Summer campaigns at Zeppelin point to a 13C depleted bulk Arctic source of dominantly biogenic origin, at -67‰. In spring, while the wetlands are still frozen, the source signature is more enriched, -53‰, with trajectory analysis implying a large contribution from onshore gas fields. Arctic methane emissions respond rapidly to warming with strong positive feedbacks. With rapid warming there is the potential to release large stores of carbon from permafrost and methane hydrates. Isotopic data are powerful discriminants of sources. High frequency, ideally continuous, monitoring of methane δ13C from a number of Arctic sites, onshore and offshore, coupled with back-trajectory analysis and regional modelling, will be important if future changes in Arctic source strengths are to be quantified. Reference: Dlugokencky, E. J., et al. (2009), Observational constraints on recent increases

  8. Bulk Chemical and Hf/W Isotopic Consequences of Lossy Accretion

    NASA Astrophysics Data System (ADS)

    Dwyer, C. A.; Nimmo, F.; Chambers, J.

    2013-12-01

    The late stages of planetary accretion involve stochastic, large collisions [1]. Many of these collisions likely resulted in hit-and-run events [2] or erosion of existing bodies' crusts [3] or mantles [4]. Here we present a preliminary investigation into the effects of lossy late-stage accretion on the bulk chemistry and isotopic characteristics of the resulting planets. Our model is composed of two parts: (1) an N-body accretion code [5] tracks the orbital and collisional evolution of the terrestrial bodies, including hit-and-run and fragmentation events; (2) post-processing evolves the chemistry in light of radioactive decay and impact-related mixing and partial equilibration. Sixteen runs were performed using the MERCURY N-body code [5]; each run contained Jupiter and Saturn in their current orbits as well as approx 150 initial bodies. Different collisional outcomes including fragmentation are possible depending on the velocity, angle, mass ratio, and total mass of the impact (modified from [6, 7]). The masses of the core and mantle of each body are tracked throughout the simulation. All bodies are assigned an initial mantle mass fraction, y, of 0.7. We track the Hf and W evolution of these bodies. Radioactive decay occurs between impacts. We calculate the effect of an impact by assuming an idealized model of mixing and partial equilibration [8]. The core equilibration factor is a free parameter; we use 0.4. Partition coefficients are assumed constant. Diversity increases as final mass decreases. The range in final y changes from 0.66-0.72 for approx Earth-mass planets to 0.41-1 for the smallest bodies in the simulation. The scatter in tungsten anomaly increases from 0.79-4.0 for approx Earth-mass to 0.11-18 for the smallest masses. This behavior is similar to that observed in our solar system in terms of both bulk and isotopic chemistry. There is no single impact event which defines the final state of the body, therefore talking about a single, specific age of

  9. Strong Relationship between Hf-Nd-Pb Isotopes in Atlantic Sediments and the Lesser Antilles arc Composition

    NASA Astrophysics Data System (ADS)

    Carpentier, M.; Chauvel, C.; Mattielli, N.

    2006-12-01

    Geochemical variability of lavas from the Lesser Antilles arc is well established and is characterized by a chemical zoning from north to south along the arc. Lavas from the northern part of the arc have usually less radiogenic and less variable Sr and Pb isotopic compositions than those from the south. Possible explanations include a larger contribution from sediments in the source of the southern islands, and/or a north-south change in the chemical composition of the sediments that are subducted beneath the Lesser Antilles arc We conducted a geochemical study (Nd, Hf and Pb isotopic compositions) of Atlantic sediments coming from two different sites drilled during DSDP Leg 78 (site 543, 15.7N) and DSDP Leg 14 (site 144, 9.5N). At site 543, the sedimentary pile has epsilon Nd values between -14.6 and -11 and epsilon Hf between -10.6 and -1. 206Pb/204Pb ratios vary between 19.1 and 19.5. The sediment pile has an overall strong continental signature suggesting that their source is primarily detrital. This is in agreement with the interpretation of White et al. (1985) who suggested that the dominant source was the Archean Guiana Highland drained by the Orinoco River. Further south, at site 144, the succession consists of chalk ooze, marl and clays, and organic-rich black shales. Samples have epsilon Nd between -18.4 and -10 and epsilon Hf between -20.4 and -5.4 and the Pb isotopic compositions are extremely variable. Chalk ooze, marl and clay have 206Pb/204Pb ratios between 18.8 and 20.0, while the black shales have extremely radiogenic compositions with 206Pb/204Pb between 21.6 and 27.7. These compositions reflect the radioactive decay of authigenic uranium concentrated in organic- rich layers characterized by elevated 238U/204Pb ratios (100 up to 600). The isotopic compositions of sediments from both sites are largely influenced by continental input with Nd and Hf isotopes plotting in the continental domain of the "terrestrial array", but the southern site has more

  10. Investigations of the g{sub K}-factors in the {sup 175,177,179}Hf Isotopes

    SciTech Connect

    Yakut, Hakan; Kuliev, Ali; Guliyev, Ekber

    2008-11-11

    In this paper the intrinsic g{sub K} and effective spin g{sub s} factors of the odd-mass {sup 175-179}Hf isotopes have been investigated within the Tamm-Dancoff approximation by using the realistic Saxon-Woods potential. The theoretically calculated g{sub K} and g{sub s}{sup eff} values are compared with experimental data. The comparison of the measured and calculated values of the effective g{sub s} factor shows that the spin polarization explains quite well the observed reduction of g{sub s} from its free-nucleon value.

  11. New insights into magma source compositions: constraints from combined Zr/Hf and Nb/Ta systematics

    NASA Astrophysics Data System (ADS)

    Bennett, S.; Elliott, T.; Blundy, J.

    2003-04-01

    Knowledge of the behaviour of trace elements in the mantle is a pre-requisite for the modelling and ultimately, understanding of mantle processes. As new, improved analytical techniques are developed additional groups of elements can be exploited to this end. One such group are the High Field Strength Elements (HFSE). Conventional wisdom suggests that the HFSE pairs Zr-Hf and Nb-Ta should behave as "geochemical twins" retaining chondritic ratios during petrogenetic processes such as melt generation and fractional crystallisation. However, precise measurements of these elements using Isotope Dilution (ID)- Plasma Ionisation Multi-collector Mass Spectrometry (PIMMS) are able to demonstrate that Zr/Hf and Nb/Ta show considerable variation between samples [1,2,3]. We have also developed new high precision separation and measurement techniques for the HFSE to specifically investigate the origin of the "garnet signature" in MORB and OIB [4,5,6]. Experiments carried out in simple synthetic systems by van Westrenen et al [7] suggest that DZr < DHf for pyropic garnets, such as those found in garnet peridotite, but the reverse, DZr > DHf is true of grossular rich garnets as would be present in a eclogitic source component of recycled crustal origin. This suggests that the HFSE have the potential to distinguish between these two garnet-bearing mantle sources. We will present data on a range of mantle derived melts with variable "garnet signatures" in order to address this issue. [1]Weyer et al. (2003), EPSL 205, 309-324. [2]Büchl et al. (2002), Goldschmidt Abstracts, A108. [3]David et al. (2000), EPSL 178, 285-301. [4]Hirschmann and Stolper (1996), CMP 124, 185-208. [5]Hirschmann (1996), Nature 384, 215-217. [6]Stracke et al. (1999), G-cubed, 1. [7]van Westrenen et al. (1999), Am. Min 84, 838-847.

  12. Re-Os Isotopic Constraints on the Chemical Evolution and Differentiation of the Martian Mantle

    NASA Technical Reports Server (NTRS)

    Brandon, Alan D.; Walker, Richard J.

    2002-01-01

    The (187)Re-187Os isotopic systematics of SNC meteorites, thought to be from Mars, provide valuable information regarding the chemical processes that affected the Martian mantle, particularly with regard to the relative abundances of highly siderophile elements (HSE). Previously published data (Birck and Allegre 1994, Brandon et al. 2000), and new data obtained since these studies, indicate that the HSE and Os isotopic composition of the Martian mantle was primarily set in its earliest differentiation history. If so, then these meteorites provide key constraints on the processes that lead to variation in HSE observed in not only Mars, but also Earth, the Moon and other rocky bodies in the Solar System. Processes that likely have an effect on the HSE budgets of terrestrial mantles include core formation, magma ocean crystallization, development of juvenile crust, and the addition of a late veneer. Each of these processes will result in different HSE variation and the isotopic composition of mantle materials and mantle derived lavas. Two observations on the SNC data to present provide a framework for which to test the importance of each of these processes. First, the concentrations of Re and Os in SNC meteorites indicate that they are derived from a mantle that has similar concentrations to the Earth's mantle. Such an observation is consistent with a model where a chondritic late veneer replenished the Earth and Martian mantles subsequent to core formation on each planet. Alternative models to explain this observation do exist, but will require additional data to test the limitations of each. Second, Re-Os isotopic results from Brandon et al. (2000) and new data presented here, show that initial yos correlates with variations in the short-lived systems of (182)Hf- (182)W and (142)Sm-142Nd in the SNC meteorites (epsilon(sub W) and epsilon(sub 142Nd)). These systematics require an isolation of mantle reservoirs during the earliest differentiation history of Mars, and

  13. ATOMIC AND MOLECULAR PHYSICS: Influence of Isotope Substitution Helium Atom on Partial Cross Sections in He-HF Collisions

    NASA Astrophysics Data System (ADS)

    Yu, Chun-Ri; Zhang, Jie; Chen, Li; Jiang, Gui-Sheng; Huang, Guo-Dong

    2009-11-01

    Close-coupling equation and anisotropic potential developed in our previous research are applied to HF-3He (4He, 6He, 8He, 10He) collision system, and partial cross sections (PCSs) at the incident energy of 40 meV are calculated. By analyzing the differences of these PCSs, change rules of PCSs with the increase of partial wave number, and with the change of the mass of isotope substitution helium atom are obtained. The results show that excitation PCSs converge faster than elastic PCSs for collision energy and each of systems considered here. Also excitation PCSs converge more rapidly for high-excited states. Tail effect is present only in elastic scattering and low-excited states but not in high-excited states. With the increase of the mass of isotope substitution helium atom, converging speed of elastic, total inelastic, and state-to-state excitation PCS slows down, and the maxima of these PCSs undergoes a regular change.

  14. Implications for the evolution of continental crust from Hf isotope systematics of detrital zircons in Archean sandstones

    SciTech Connect

    Stevenson, R.K.

    1989-01-01

    The fractionation of zircons by sedimentary processes into continental margin sandstone deposits results in a biased preservation of pre-existing continental crust in the form of zircon in those sequences. This provides a unique opportunity to distinguish between the contrasting theories of episodic growth versus constant volume of continental crust over geologic time through Hf isotope ratios of detrital zircons. {sup 176}Hf/{sup 177}Hf ratios were determined for detrital zircon fractions from 2.6-3.0 Ga old sedimentary sequences from the Canadian Shield, North Atlantic, Wyoming, and Kaapvaal Cratons. The data strongly suggest inheritance of pre-3.0 Ga zircons only in areas where pre-3.0 Ga old crust exists today, and imply that the quantity of continental crust prior to 3.0 Ga ago was not much greater in extent than the pre-3.0 Ga crust exposed today. Small amounts of continental crust prior to 3.0 Ga ago and rapid addition of continental crust between 2.5 and 3.0 Ga ago are consistent with the episodic growth theory of crustal evolution.

  15. Ancient and modern subduction zone contributions to the mantle sources of lavas from the Lassen region of California inferred from Lu-Hf isotopic systematics

    USGS Publications Warehouse

    Borg, L.E.; Blichert-Toft, J.; Clynne, M.A.

    2002-01-01

    Hafnium isotopic compositions have been determined on a suite of calc-alkaline and high-alumina-olivine tholeiitic lavas from the Lassen region of California and are used, in conjunction with previously published mineralogical, geochemical, and isotopic data, to constrain their petrogenesis. Positive correlation between ??Hf values and geochemical indices of the modern subduction component indicates that the isotopic compositions of the calc-alkaline lavas record addition of radiogenic Hf from the subducted slab. However, the addition of the modern subduction component increases the ??Hf values of most calc-alkaline lavas by <0.5 units over estimates of non-subduction enriched peridotites of the mantle wedge. The Lu-Hf isotopic systematics of the Lassen lavas suggest that the calc-alkaline magmas have equilibrated with garnet at some point in their history, whereas the tholeiitic magmas have not. These observations require the two lava types to be derived from different sources. The isotopic variability of the Lassen lavas cannot be produced by mixing mantle sources inferred to be present in the eastern-central Pacific and western USA with a modern subduction component. Instead, the isotopic variability is consistent with mixing of a depleted mantle source, a more fertile mantle source enriched by an ancient subduction component, and a modern subduction component.

  16. Correlated nucleosynthetic isotopic variability in Cr, Sr, Ba, Sm, Nd and Hf in Murchison and QUE 97008

    NASA Astrophysics Data System (ADS)

    Qin, Liping; Carlson, Richard W.; Alexander, Conel M. O.'D.

    2011-12-01

    Acid leaching of the primitive C-chondrite Murchison and O-chondrite QUE 97008 reveal nucleosynthetic anomalies in Cr, Sr, Ba, Nd, Sm and Hf. The anomalies in all but Cr and Sm are best explained by variable additions of pure s-process nuclides to a background nebular composition slightly enriched in r-process isotopes compared to average Solar System material. Leaching leaves a residue in Murchison that is strongly enriched in s-process nuclides with depletions of over 0.1% in 135Ba and seven parts in 10,000 in 84Sr. If there are p-process anomalies in these two elements, they are lost in the variability caused by different r-, s-process contributions to the normalizing isotopes. The concentration and isotope systematics are consistent with the Ba and Sr isotopic composition in the Murchison residue being strongly influenced by s-process-rich presolar SiC. In general, the nucleosynthetic isotope anomalies are 2- to 5-fold smaller in QUE 97008 than in Murchison. The different magnitudes of isotope anomalies are similar to the difference in matrix abundance between CM and O chondrites consistent with the suggestion that the carriers of nucleosynthetically anomalous material preferentially reside in the matrix and that some of this material has been distributed throughout the O-chondrite minerals as a result of thermal metamorphism. Neodymium, Sm and Hf display variable s-, r-process nuclide abundances as in Ba and Sr, but the anomalies are much smaller (e.g. ɛ 148Nd, ɛ 148Sm = -5.7, 2.1, respectively, in Murchison and -0.43, 0.16, respectively in QUE 97008 residues). After correcting Nd and Sm for s-, r-process variability, Sm in whole rock chondrites shows variable relative abundances of the p-process isotope 144Sm that correlate weakly with 142Nd suggesting that the direct p-process contribution to 142Nd is small (˜7-9%). Nucleosynthetic variability in Nd explains the range in 142Nd/ 144Nd seen between C and O, E-chondrites, but not the difference between

  17. Isotopic constraints on biogeochemical cycling of copper in the ocean.

    PubMed

    Takano, Shotaro; Tanimizu, Masaharu; Hirata, Takafumi; Sohrin, Yoshiki

    2014-12-05

    Trace elements and their isotopes are being actively studied as powerful tracers in the modern ocean and as proxies for the palaeocean. Although distributions and fractionations have been reported for stable isotopes of dissolved Fe, Cu, Zn and Cd in the ocean, the data remain limited and only preliminary explanations have been given. Copper is of great interest because it is either essential or toxic to organisms and because its distribution reflects both biological recycling and scavenging. Here we present new isotopic composition data for dissolved Cu (δ(65)Cu) in seawater and rainwater. The Cu isotopic composition in surface seawater can be explained by the mixing of rain, river and deep seawater. In deep seawater, δ(65)Cu becomes heavier with oceanic circulation because of preferential scavenging of the lighter isotope ((63)Cu). In addition, we constrain the marine biogeochemical cycling of Cu using a new box model based on Cu concentrations and δ(65)Cu.

  18. Miocene climate change recorded in the chemical and isotopic (Pb, Nd, Hf) signature of Southern Ocean sediments

    NASA Astrophysics Data System (ADS)

    VlastéLic, Ivan; Carpentier, Marion; Lewin, ÉRic

    2005-03-01

    The Middle Miocene transition from carbonate to biosilica sedimentation at DSDP site 266 (Australian-Antarctic basin) reflects a global transition toward a colder climate. The 143Nd/144Nd, 176Hf/177Hf, and Al/Ti of bulk sediments display systematic, coupled variations through time, which have been attributed to a change of the detrital source. This change could correspond to a reduction of input from the Antarctic continent, an increase of input from the Kerguelen volcanic province, or both. Mixing models based on Nd isotopes and Al/Ti suggest a 30-40% reduction of Antarctic input and an equivalent increase of Kerguelen input during the Miocene. Reduction of Antarctic input may result from the formation of a stable East Antarctic ice sheet. Consistently, Pb isotopes and trace element systematics suggest a change of weathering style during the Miocene, with an increase in physical weathering, or a reduction of chemical weathering, after 15 Ma. Increase of Kerguelen input may reflect the initiation, or enhancement, of the Antarctic Circumpolar Current (ACC), thus raising the possibility of a simultaneous onset of North Atlantic Deep Water production and the ACC during the Middle Miocene. In addition, large geochemical oscillations occurred during the Pliocene, possibly reflecting fluctuation in strength of the ACC or, alternatively, periods of instability of the Antarctic ice sheet.

  19. Isotopic Constraints on Magmatic Sources at Nyiragongo and Nyamulagira Volcanoes, Virunga Volcanic Province, DR Congo

    NASA Astrophysics Data System (ADS)

    Phillips, E. H. W.; Sims, K. W. W.; Tedesco, D.; Blichert-Toft, J.; Scott, S. R.; Reagan, M. K.

    2015-12-01

    The active volcanoes Nyiragongo and Nyamulagira in the DR Congo have very different physical and geochemical characteristics, despite being situated a mere 15 km apart. Nyiragongo's foiditic lavas are some of the most silica-undersaturated on earth, whereas the highly effusive Nyamulagira erupts primarily basanites and tephrites. To determine the extent and scale of mantle heterogeneities and gain insight into the magmatic sources beneath this portion of the East African Rift, we have measured Hf and Pb isotope compositions for 43 samples from Nyiragongo and Nyamulagira. The Nd and Sr isotope data for the same sample dissolutions are forthcoming. Nyiragongo lavas are clearly distinct from Nyamulagira lavas in terms of their Hf and Pb isotope compositions, suggesting that a long-lived and small-scale heterogeneous mantle source exists beneath these two volcanoes. Nyiragongo lavas have ɛHf ranging from +1.8 to +5.5 with an average of +2.9 (n=29) and 206Pb/204Pb ranging from 19.4049 to 19.7252 with an average of 19.6329 (n=29). Nyamulagira lavas have ɛHf ranging from -0.5 to +1.5 with an average of +0.5 (n=14) and 206Pb/204Pb ranging from 19.2518 to 19.2828 with an average of 19.2663 (n=13). Nyiragongo lavas erupted in 2002 or later have amongst the highest 206Pb/204Pb within this suite of samples. We note that Chakrabarti et al. (2009, Chem Geol 259) measured bulk silicate earth-like Nd and Sr isotope compositions for Nyiragongo lavas and proposed a primitive mantle/bulk-earth plume source for this volcano. Our new Hf isotope compositions for Nyiragongo, however, are higher than bulk silicate earth, suggesting a more depleted source for these highly alkaline lavas. We also note that the He isotope compositions of olivine and clinopyroxene from Nyiragongo lavas (R/Ra = 6.7-8.5; Pik et al., 2006, Chem Geol 226; Tedesco et al., 2010, J Geophys Res 115) are inconsistent with a long-term bulk silicate earth-like source.

  20. Oxygen isotope constraints on the origin of island arc granitoids

    NASA Astrophysics Data System (ADS)

    Perez, R. J.; Cavosie, A. J.; Valley, J. W.

    2007-12-01

    Granitic intrusions in island arcs constitute additions of juvenile crust from oceanic environs that ultimately get accreted to continents. The genesis of island arc granitoids is thus important to studies of the growth of oceanic and continental crust. Puerto Rico (USA) is a composite island arc terrane that preserves a record of plutonism from 85 Ma to 38 Ma (Cavosie et al., 2007 AGU). Mid-crustal granitoid plutons are exposed (~1 to 500 km2), but their origins are unknown, as no suspected parental magmas associated with the plutons (e.g., gabbro) are exposed. This study uses petrography, WR major elements, and oxygen isotopes of WR and zircon from granitoids and xenoliths to place better constraints on the origin of granitoid in the Greater Antilles island arc. WR δ18O analyses were made with laser fluorination by IRMS at the Univ. of Wisconsin (uncertainties = 0.10 to 0.20‰, 2sd). The main plutons (Caguas, Rio Blanco, San Lorenzo, Utuado, Vieques) yield primitive δ18O(WR) values, ranging from 6.24 to 7.72‰ over a range of wt.% SiO2= 58.03 to 66.54. Smaller stocks (<20 km2) yield higher δ18O(WR) values, ranging from 7.47 to 10.27‰. Qualitative petrographic analysis reveals that granitoids with δ18O(WR) >~7.5‰ are partially to pervasively altered. Zircon preserves magmatic δ18O and is used here to quantitatively evaluate the measured δ18O(WR) values. If δ18O(Zrc) and wt.% SiO2 are known, a comparison of measured vs. predicted δ18O(WR) can be made (Valley et al., 2005, CMP). The measured δ18O(WR) values record variable amounts of alteration, ranging from virtually undetectable, to WR δ18O elevations of ~4‰, indicative of low-T subsolidus alteration. The Δ18O (WR-Zrc) values using calculated δ18O(WR) yield the following fractionations: -1.57 to 1.00‰ for granodiorites (wt.% SiO2=66 to 57); -0.85‰ for diorite (wt.% SiO2=55); and -0.56‰ for the only gabbro analyzed (wt.% SiO2=50). Mafic xenoliths (53-57 wt.% SiO2) from 4 granitoids yield

  1. Age of Alpine Corsica ophiolites revisited: Insights from in situ zircon U-Pb age and O-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Li, Xian-Hua; Faure, Michel; Rossi, Philippe; Lin, Wei; Lahondère, Didier

    2015-04-01

    Knowledge of the age and timing of ophiolite sequences is essential for understanding the mechanisms of plate tectonics. The ophiolites in the Schistes Lustrés and the Upper nappes of Alpine Corsica represent remnants of the Liguria-Piemonte ocean basin that formed as a branch of the Central Atlantic basin during the opening of the Mesozoic Western Alpine Tethys. Despite numerous isotopic and paleontological studies, the age and timing of the ophiolites in the Schistes Lustrés nappe are still controversial. This study presents integrated in situ analyses of zircon U-Pb age and O-Hf isotopic data for ophiolitic gabbros and plagiogranites from three localities in the Schistes Lustrés nappe of Eastern Corsica. Our new results demonstrate that these rocks crystallized synchronously at ~ 159 Ma, approximately 10 m.y. younger than the ophiolites in the Balagne Upper nappe. Zircons from the gabbros and plagiogranites are characterized by highly positive εHf(t) (+ 15.0 to + 15.9) and mantle-like δ18O (5.2-5.4‰) values. Thus, these ophiolitic rocks were cogenetic, and crystallized from magmas produced by partial melting of a depleted, N-MORB type mantle. By contrast, in the Balagne Upper nappe, the ~ 169 Ma ophiolites contain numerous xenocrystic zircons inherited from a continental crust. Our current knowledge of isotopic geochronology and geochemistry supports a paleogeographic reconstruction, in which the earliest ophiolites in the Balagne nappe were emplaced close to a continental margin at ~ 169 Ma, while the N-MORB type ophiolites in the Schistes Lustrés nappe were likely formed approximately 10 m.y. later in the central part of the Liguria-Piemonte oceanic basin. The relative location of the Schistes Lustrés and Balagne Upper nappes with respect to continental margins is discussed.

  2. Zircon and baddeleyite U-Pb geochronology and Hf isotopes from the Central Atlantic Magmatic Province (CAMP)

    NASA Astrophysics Data System (ADS)

    Davies, Joshua; Marzoli, Andrea; Bertrand, Herve; Youbi, Nasrrddine; Schaltegger, Urs

    2016-04-01

    Large Igneous Provinces (LIPs) are anomalously large volumes of dominantly mafic magma that erupted and intruded into the upper crust over short time scales. The origin of these volcanic provinces is very likely specific for each case, partly explained by plate tectonic processes or mantle plumes. Despite an ambivalent plate tectonic connection, there is a striking temporal correlation between the timing of LIPs and periods of mass extinction on Earth. However, establishing the relationship between these two is quite complicated since mass extinctions are typically recognised in the marine record, and LIPs are usually terrestrially emplaced. High precision geochronology of LIPs is essential to (i) establish the synchrony and infer the causal relationship with mass extinctions, and (ii) to understand how LIPs form. In this study, we apply high-precision zircon and baddeleyite U-Pb geochronology to rocks from the ~200 Ma Central Atlantic Magmatic Province (CAMP), in an attempt to reconstruct the overall timing of the event, its spatial distribution in time, and determine its relationship with the end-Triassic mass extinction. We also present Hf isotope data from the separated zircon and baddeleyite to both elucidate the origin of the LIP and also to determine if the magmas all originate from the same source. Our data suggest that the majority of the CAMP magmas were emplaced over a 0.5 Ma period from ~201.5 Ma to ~201.0 Ma with a possible small secondary event occurring much later at ~199 Ma. Spatially, it appears that CAMP magmatism occurred roughly simultaneously over the entire province (i.e. ~8000 Km North to South). However, the Hf isotopic composition varies over this length with the highest values (~5.5 ɛHf) occurring in a small area to the south of the province in Brazil and Sierra Leone. Towards the north, the ɛHf values become negative, indicating the presence of an older or more enriched component in the magmas. Our geochronology also indicates that CAMP

  3. Continental growth through accreted oceanic arc: Zircon Hf-O isotope evidence for granitoids from the Qinling orogen

    NASA Astrophysics Data System (ADS)

    Wang, Hao; Wu, Yuan-Bao; Gao, Shan; Qin, Zheng-Wei; Hu, Zhao-Chu; Zheng, Jian-Ping; Yang, Sai-Hong

    2016-06-01

    The continental crust is commonly viewed as being formed in subduction zones, but there is no consensus on the relative roles of oceanic or continental arcs in the formation of the continental crust. The main difficulties of the oceanic arc model are how the oceanic arcs can be preserved from being subducted, how we can trace the former oceanic arcs through their high-Si products, and how the oceanic arcs can generate the high-Si, K-rich granitoid composition similar to the upper continental crust. The eastern Qinling orogen provides an optimal place to address these issues as it preserves the well-exposed Erlangping oceanic arc with large amounts of granitoids. In this study, we present an integrated investigation of zircon U-Pb ages and Hf-O isotopes for four representative granitoid plutons in the Erlangping unit. In situ zircon SIMS U-Pb dating indicated that the Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons formed at 472 ± 7, 458 ± 6 and 443 ± 5 Ma, respectively, all of which postdated the deep subduction of the Qinling microcontinent under the Erlangping oceanic arc. The Zhangjiadazhuang, Xizhuanghe, and Taoyuan plutons are sodic granitoid and have highly positive εHf(t) (+7.6 to +12.9) and relatively low δ18O (4.7-5.0‰) values, which were suggested to result from prompt remelting of hydrothermally altered lower oceanic crust of the accreted Erlangping oceanic arc. The zircon grains from the Manziying monzogranitic pluton show similar Hf-O isotopic compositions to those of the Xizhuanghe pluton, and thus the Manziying monzogranitic pluton was likely derived from the dehydration melting of previous tonalites as exemplified by the Xizhuanghe pluton. The deep subduction of Qinling microcontinent resulted in the accretion of the Erlangping oceanic arc, which implies that arc-continent collision provides an effective way for preventing oceanic arcs from being completely subducted. The highly positive εHf(t) and relatively low δ18O values of zircon

  4. Isotopic constraints on the origin of meteoritic organic matter

    NASA Technical Reports Server (NTRS)

    Kerridge, J. F.

    1991-01-01

    Salient features of the isotopic distribution of H, C and N in the organic material found in carbonaceous meteorites are noted. Most organic fractions are strongly enriched in D with respect to the D/H ratio characteristic of H2 in the protosolar system; substantial variations in C-13/C-12 ratio are found among different molecular species, with oxidised species tending to be C-13 enriched relative to reduced species; some homologous series reveal systematic decrease in C-13/C-12 with increasing C number; considerable variation in N-15/N-14 ratio is observed within organic matter, though no systematic pattern to its distribution has yet emerged; no interelement correlations have been observed between isotope enrichments for the different biogenic elements. The isotopic complexity echoes the molecular diversity observed in meteoritic organic matter and suggests that the organic matter was formed by multiple processes and/or from multiple sources. However, existence of a few systematic patterns points towards survival of isotopic signatures characteristic of one or more specific processes. The widespread D enrichment implies either survival of many species of interstellar molecule or synthesis from a reservoir containing a significant interstellar component. Several of the questions raised above can be addressed by more detailed determination of the distribution of the H, C and N isotopes among different well-characterized molecular fractions. Thus, the present study is aimed at discovering whether the different amino acids have comparable D enrichments, which would imply local synthesis from a D-enriched reservoir, or very viable D enrichments, which would imply survival of some interstellar amino acids. The same approach is also being applied to polycyclic aromatic hydrocarbons. Because the analytical technique employed (secondary ion mass spectrometry) can acquire data for all three isotopic systems from each molecular fraction, any presently obscured interelement

  5. Laboratory constraints on the stability of oxygen isotopes in sulfate

    NASA Astrophysics Data System (ADS)

    Turchyn, A. V.; Rennie, V.

    2012-12-01

    The oxygen isotopic composition of sulfate (δ18OSO4) is a powerful new tool for exploring the transformations of sulfur in laboratory experiments and the natural environment. The δ18OSO4 is central to our understanding of bacterial sulfate reduction because variations in δ18OSO4 reflect differences in the rates of various metabolic steps. Additionally, the δ18OSO4 provides key information on the pathways of both biotic and abiotic sulfide oxidation. If preserved in the geological record, the δ18OSO4, along with the sulfur isotope composition of sulfate, provides unique insight into the long-term variability of the sulfur cycle. Use of the δ18OSO4 rests on the assumption that there is minimal abiotic exchange between sulfate-oxygen and water-oxygen during the variety of conditions imposed by sample storage and laboratory processing. Previous work has shown that oxygen isotope exchange between sulfate and water does occur at high temperatures (>100°C) and/or at low pH values (<1). Estimates of the timescales for exchange under laboratory temperatures and pH values have been extrapolated from highly dissimilar conditions. The controls on abiotic oxygen isotope exchange between sulfate and water under laboratory conditions thus remain enigmatic. We explore two mechanisms that could allow for isotope exchange and thus interfere with the interpretation of δ18OSO4: oxygen isotope exchange under low pH and oxygen isotope exchange in the presence of sulfide by the formation and subsequent reaction of thiosulfate complexes. Our results definitively rule out short and medium-term exchange of sulfate-oxygen with water-oxygen over a range of acidic conditions. Furthermore, although it has been suggested that thiosulfate complexes will form from coexisting aqueous sulfide and sulfate, we demonstrate that this mechanism does not facilitate sulfate-oxygen isotope exchange at low temperatures. Our results confirm that the δ18OSO4 is robust to standard laboratory conditions.

  6. Implications for the evolution of continental crust from Hf isotope systematics of Archean detrital zircons

    NASA Technical Reports Server (NTRS)

    Stevenson, Ross K.; Patchett, P. Jonathan

    1990-01-01

    Results from the fractionation of zircon by sedimentary processes into continental margin sandstone yield information on the preservation of preexisting continental crust in the form of zircon, making it possible to distinguish between the contrasting theories of gradual growth versus constant volume of continental crust over geologic time. In this work, Hf-176/Hf-177 ratios were determined for detrital zircon fractions from 2.0-2.5, 2.6-3.0, and pre-3.0 Gyr old sandstones from the Canadian-Shield, the North-Atlantic, the Wyoming, and the Kaapvaal Cratons. Results pointed to small amounts of continental crust prior to 3.0 Gyr ago and a rapid addition of continental crust between 2.5 and 3.0 Gyr ago, consistent with the gradual growth of continental crust, and giving evidence against no-growth histories.

  7. Magma mixing and crystal exchange at Yellowstone caldera revealed though sub-crystal-scale age, trace-element, and Hf-isotopic analyses of zircons

    NASA Astrophysics Data System (ADS)

    Stelten, M. E.; Cooper, K. M.; Vazquez, J. A.; Wimpenny, J.; Yin, Q.

    2011-12-01

    We examine magma mixing and crystal exchange in a young magma reservoir by correlating sub-crystal-scale SIMS age, SIMS trace element, and LA-MC-ICPMS Hf-isotopic data from zircons in the coeval ca. 100ka, yet compositionally distinct rhyolites of the Solfatara Plateau flow (SPF) and Hayden Valley flow (HVF) at Yellowstone Caldera. The SPF and HVF lavas are part of the Central Plateau Member (CPM) of the Plateau Rhyolite that is composed of the youngest intracaldera rhyolite flows at Yellowstone, erupted between ca. 170-70ka. We compare these data to age and trace element data from zircons in 1) the Pitchstone Plateau Flow, West Yellowstone Flow, and Dry Creek Flow of the CPM as representative of main reservoir zircons, 2) the ca. 118ka extracaldera Gibbon River Flow rhyolite (GRF), and 3) the ca. 260ka Scaup Lake Flow of the Upper Basin Member rhyolites. Additionally, we compare the zircon data to new MC-ICPMS Hf-isotopic data from CPM glasses. Correlating age, trace element, and Hf-isotopic data from zircons in the HVF and SPF reveals the presence of four zircon populations. Main reservoir-like (MR-like) zircons have trace element compositions similar to main CPM reservoir zircons, young ages (<200ka), a range in ɛHf (0.2 to -7.2), and are commonly zoned with high ɛHf cores and rims with ɛHf values within error of CPM glasses (-6.5 to -7.2 ɛHf). Extracaldera-like (EC-like) zircons are indistinguishable in age, trace element, and Hf-isotopic composition (-5.1 to -9.2 ɛHf) from zircons in the GRF. Mixed zircons have cores with either MR-like or EC-like compositions but rims of intermediate composition. Lastly, a population of zircons (which we interpret to be inherited) have cores with older ages (>350ka), a range in trace element compositions, and high ɛHf (-5.8 to -3.6) whereas the rims have restricted MR-like trace element compositions and ɛHf within error of CPM glasses. The sense of core to rim zoning specific to each population suggests that each

  8. Magma mixing and the generation of isotopically juvenile silicic magma at Yellowstone caldera inferred from coupling 238U–230Th ages with trace elements and Hf and O isotopes in zircon and Pb isotopes in sanidine

    USGS Publications Warehouse

    Stelten, Mark E.; Cooper, Kari M.; Vazquez, Jorge A.; Reid, Mary R.; Barfod, Gry H.; Wimpenny, Josh; Yin, Qing-Zhu

    2013-01-01

    The nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time. We examine compositional heterogeneity in the youngest (~170 to 70 ka) post-caldera volcanism at Yellowstone caldera, the Central Plateau Member (CPM) rhyolites, as a case study. We compare 238U–230Th age, trace-element, and Hf isotopic data from zircons, and major-element, Ba, and Pb isotopic data from sanidines hosted in two CPM rhyolites (Hayden Valley and Solfatara Plateau flows) and one extracaldera rhyolite (Gibbon River flow), all of which erupted near the caldera margin ca. 100 ka. The Hayden Valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the CPM reservoir. The Gibbon River flow hosts one zircon population that is compositionally distinct from Hayden Valley flow zircons. The Solfatara Plateau flow contains multiple sanidine populations and all three zircon populations found in the Hayden Valley and Gibbon River flows, demonstrating that the Solfatara Plateau flow formed by mixing extracaldera magma with the margin of the CPM reservoir. This process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs. More generally, Hf isotopic data from the CPM zircons provide the first direct evidence for isotopically juvenile magmas contributing mass to the youngest post-caldera magmatic system and demonstrate that the sources contributing magma to the CPM reservoir were heterogeneous in 176Hf/177Hf at ca. 100 ka. Thus, the limited compositional variability of CPM glasses reflects homogenization occurring within the CPM reservoir, not a homogeneous source.

  9. Magma mixing and the generation of isotopically juvenile silicic magma at Yellowstone caldera inferred from coupling 238U-230Th ages with trace elements and Hf and O isotopes in zircon and Pb isotopes in sanidine

    NASA Astrophysics Data System (ADS)

    Stelten, Mark E.; Cooper, Kari M.; Vazquez, Jorge A.; Reid, Mary R.; Barfod, Gry H.; Wimpenny, Josh; Yin, Qing-zhu

    2013-08-01

    The nature of compositional heterogeneity within large silicic magma bodies has important implications for how silicic reservoirs are assembled and evolve through time. We examine compositional heterogeneity in the youngest (~170 to 70 ka) post-caldera volcanism at Yellowstone caldera, the Central Plateau Member (CPM) rhyolites, as a case study. We compare 238U-230Th age, trace-element, and Hf isotopic data from zircons, and major-element, Ba, and Pb isotopic data from sanidines hosted in two CPM rhyolites (Hayden Valley and Solfatara Plateau flows) and one extracaldera rhyolite (Gibbon River flow), all of which erupted near the caldera margin ca. 100 ka. The Hayden Valley flow hosts two zircon populations and one sanidine population that are consistent with residence in the CPM reservoir. The Gibbon River flow hosts one zircon population that is compositionally distinct from Hayden Valley flow zircons. The Solfatara Plateau flow contains multiple sanidine populations and all three zircon populations found in the Hayden Valley and Gibbon River flows, demonstrating that the Solfatara Plateau flow formed by mixing extracaldera magma with the margin of the CPM reservoir. This process highlights the dynamic nature of magmatic interactions at the margins of large silicic reservoirs. More generally, Hf isotopic data from the CPM zircons provide the first direct evidence for isotopically juvenile magmas contributing mass to the youngest post-caldera magmatic system and demonstrate that the sources contributing magma to the CPM reservoir were heterogeneous in 176Hf/177Hf at ca. 100 ka. Thus, the limited compositional variability of CPM glasses reflects homogenization occurring within the CPM reservoir, not a homogeneous source.

  10. U-Pb age and Hf-O isotopes of detrital zircons from Hainan Island: Implications for Mesozoic subduction models

    NASA Astrophysics Data System (ADS)

    Jiang, Xiao-Yan; Li, Xian-Hua; Collins, W. J.; Huang, Hui-Qing

    2015-12-01

    A compilation of magmatic ages from the Mesozoic South China Block suggests a number of "magmatic quiescence" periods at ca. 205-195 Ma, ca. 150-140 Ma and ca. 125-115 Ma, casting doubt on tectonic models that suggest ongoing Andean-type subduction along the South China continental margin. However, SIMS U-Pb analyses on two detrital zircon samples from the Cretaceous Lumuwan Formation on Hainan Island, southeast China, reveal three major age peaks at ca. 120 Ma, ca. 155 Ma and ca. 235 Ma. Zircons of these ages are mostly euhedral and show typical magmatic oscillatory zoning, suggesting short-distance transport from nearby magmatic sources. The extremely rare occurrence of ca. 120 Ma magmatic records onshore suggests that detrital zircons of this age population may be derived from a source proximal to Hainan Island but presently missing. Therefore, our data provide new evidence for ongoing magmatic activity in late Mesozoic South China. In situ Hf and O isotope analyses of the Mesozoic detrital zircons reveal large variations in both εHf(t) (- 21.2 to 10.5) and δ18O (4.4‰ to 13.6‰) values. A general negative correlation between them suggests the reworking of old supracrustal materials (average crustal residence age of ca. 2.0 Ga) by juvenile mantle-derived magmas. The progression of increasing εHf(t) and decreasing δ18O values of zircons from the Triassic to the Cretaceous suggests progressive crustal growth during the Mesozoic. The results are consistent with hybridization at an active continental margin. We briefly review tectonic models for the Indosinian orogeny and suggest that the petrologic evidence indicates that Mesozoic magmatism was part of the circum-Pacific accretionary orogens that formed along the continental margin of East Asia no later than ca. 250 Ma and continued at least to the late Cretaceous.

  11. Isotopic constraints on the rise in atmospheric methane.

    NASA Astrophysics Data System (ADS)

    Nisbet, E. G.; Manning, M. R.; Lowry, D.; Fisher, R. E.; France, J.; Brownlow, R.

    2015-12-01

    The rise in atmospheric methane that began in 2007 and continued strongly through 2014 has been accompanied by a global decrease in methane's stable carbon isotopic ratio. The δ13CH4 isotopic shifts and loci of growth observed globally imply that a major driver of recent growth in atmospheric methane has been the response of microbial methane sources, particularly tropical wetlands, to meteorological changes over the past 8 years. In the southern tropics, summer (Dec-March) wetland or ruminant emissions can lead to isotopically depleted excursions, while winter (June-Sept) biomass burning of C4 grasslands will produce isotopically relatively enriched methane. At Ascension (8oS) in marine boundary air a trend (>0.2‰) to more 13C-depleted values began in 2009, becoming more marked with excursions to much more negative values in early 2011 and 2012. Values have since recovered slightly but Ascension δ13CH4 values in early 2015 remained markedly more negative than in 2007-8. At Cape Point (34oS), the methane record (2011-2014) also shows a similar isotopic shift. To identify the causes of the large changes in the global methane budget, we perform a budget analysis of methane mole fraction and δ13CH4 data from NOAA and RHUL sites. The increase in tropical methane from 2011 fits a shift to more isotopically negative (lighter) δ13CH4 values, most likely a consequence of the exceptional water transfer to wetland by rainfall in the major La Niña event. Strong tropical wetland emissions may have continued in warm subsequent years. High precipitation and floods east of the Andes, and exceptional global warmth (emissions are exponentially temperature-dependent provided enough water is present) may have combined to give strong emission in 2014. In contrast to the methane increase in the 1980s, which was probably mainly driven by rising anthropogenic emissions, the recent isotopic shift suggests the present growth is more likely to have been a consequence of

  12. Tungsten isotopic constraints on the age and origin of chondrules

    PubMed Central

    Kleine, Thorsten; Kruijer, Thomas S.; Burkhardt, Christoph; Metzler, Knut

    2016-01-01

    Chondrules may have played a critical role in the earliest stages of planet formation by mediating the accumulation of dust into planetesimals. However, the origin of chondrules and their significance for planetesimal accretion remain enigmatic. Here, we show that chondrules and matrix in the carbonaceous chondrite Allende have complementary 183W anomalies resulting from the uneven distribution of presolar, stellar-derived dust. These data refute an origin of chondrules in protoplanetary collisions and, instead, indicate that chondrules and matrix formed together from a common reservoir of solar nebula dust. Because bulk Allende exhibits no 183W anomaly, chondrules and matrix must have accreted rapidly to their parent body, implying that the majority of chondrules from a given chondrite group formed in a narrow time interval. Based on Hf-W chronometry on Allende chondrules and matrix, this event occurred ∼2 million years after formation of the first solids, about coeval to chondrule formation in ordinary chondrites. PMID:26929340

  13. Oxygen Isotope Constraints on the Origin of Georgia Tektites

    NASA Astrophysics Data System (ADS)

    Albin, E. F.

    1997-07-01

    Georgia tektites ("georgiaites") are North American tektites that occur in east-central Georgia. In this investigation small chips of tektite material was separated from 24 individual specimens for oxygen isotope analysis. Results have an analytical precision of 0.2% based on duplicate analysis of a NIST silicate standard (NBS-28) and tektites. Oxygen isotope ratios (i.e., delta (18) O - SMOW) range from + 6.9 to + 10.7 parts per mil. The mean delta (18) O for Georgia tektites is + 9.1 parts per mil. These results are consistent with a delta (18) O value reported previously on a single georgiaite [1]. It appears that the tektites have oxygen isotope ratios similar to igneous and/or metamorphic rocks. Blum and Chamberlain [2] argue that since sea water has a relatively low delta (18) O value, its addition to isotopically heavier sedimentary rocks could account for the relatively low oxygen isotope ratios determined for the tektites. Such a model would require vaporization of the target material in order to exchange oxygen between the water and silicates. However, vaporization is not a popular theory of tektite petrogenesis, and since tektites contain very little water, it is difficult to reconcile the low delta (18) O values due to mixing with sea water. An alternative explanation may be that the low delta (18) O values are derived from a crystalline basement or it may be that the sedimentary target rocks at the proposed source crater (i.e., Chesapeake Bay crater) have delta (18) O values similar to that of the tektites. To resolve the issue, it will be necessary to melt samples of the proposed target material and make delta (18) O measurements on the resulting glass and then compare the results to the tektites. References: [1] Taylor, H.P. and Epstein, S. (1969) J. Geophys. Res., 74, 6834-6844. [2] Blum J.D. and Chamberlain C.D. (1992) Science, 257, 1104-1107.

  14. Age and origin of post collision Baltoro granites, south Karakoram, North Pakistan: Insights from in-situ U-Pb, Hf and oxygen isotopic record of zircons

    NASA Astrophysics Data System (ADS)

    Mahar, Munazzam Ali; Mahéo, Gweltaz; Goodell, Philip C.; Pavlis, Terry L.

    2014-09-01

    Origin of post collision plutonism is critical to understand the tectonothermal evolution of the over thickened continental crust in collision zones. This has proven difficult to reconcile with the conventional whole rock geochemical and field based studies alone. We report in-situ study of zircon U-Pb, Hf and O isotopes from five samples of the Baltoro Plutonic Unit (BPU) in south Karakoram. The plutonic unit is the western part of the southern Asian margin of the India-Asia convergent zone. Baltoro granites and a biotite-rich enclave yielded similar and overlapping U-Pb ages ranging from 26 to 15 Ma. Hafnium isotopic composition (εHf (0)) is very heterogeneous ranging from - 17.1 to + 4.4 while the oxygen isotopic composition of the granites is homogeneous with mean δ18O ranging from 7.2 to 9.4‰. Based on U-Pb geochronology and Hf-O isotopic composition, the involvement of two main sources is suggested (1) Cretaceous calc-alkaline Karakoram crust and (2) Karakoram gneisses. Moreover, possible involvement of metasomatized Asian lithospheric mantle is supported by elevated oxygen composition of granites and identical Hf composition of biotite-rich enclave to the mantle derived Baltoro lamprophyre. However, direct contribution from juvenile pristine mantle is unlikely as no juvenile mantle type Hf and oxygen values were obtained. This also precludes the involvement of southward juvenile arc related component of Kohistan-Ladakh batholith. Our new U-Pb and Hf data are comparable to the Mesozoic Karakoram batholith, Miocene two-mica leucogranites in the Pangong Range and magmatism from the Lhasa terrane in south Tibet, suggesting a genetic link between the Karakoram and the rocks to the east. This magmatic event is best explained by lower crust partial melting promoted by both thermal equilibration following crustal thickening and heat advection by ultrapotassic magmas associated with the breakoff of the Indian continental margin.

  15. Arabian Shield magmatic cycles and their relationship with Gondwana assembly: Insights from zircon U-Pb and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Robinson, F. A.; Foden, J. D.; Collins, A. S.; Payne, J. L.

    2014-12-01

    The Arabian Shield preserves a protracted magmatic record of amalgamated juvenile terranes that host a diverse range of early Neoproterozoic to Cambrian granitoids intruding volcanosedimentary basin assemblages that have corollaries in other parts of the East African Orogen. New zircon U-Pb geochronology of 19 granitoids intruding eight Arabian Shield terranes, define four discrete magmatic events: island arc (∼845 Ma), syncollisional (∼710 Ma), post-tectonic (∼620 Ma) and anorogenic (∼525 Ma). Zircon Lu-Hf isotopic analyses indicate that all studied granitoids are juvenile with typical εHf values of >+5 to +10 and Stenian-Tonian (∼1100-900 Ma) model ages, regardless of their precise intrusive ages or spatial relationship. Subtle changes in isotopic signatures between ∼850 and 600 Ma, suggest the result from changes in granite source materials brought about by; basaltic underplating, limited crustal interaction with Palaeoproterozoic basement and a change to lithospheric delamination/subduction roll-back processes driving juvenile ANS crustal growth. The cycle of granite intrusion reflects accretionary cycles initiated during Mozambique Ocean closure and during Gondwana amalgamation and final assembly. Post-tectonic magmatism is divided into a ∼636-600 Ma phase and post 600 Ma event that reflects first subduction and then within-plate related processes. The identification of magmatism at ∼525 Ma is now the youngest granitoid identified so far in the Saudi Arabian Shield and may change the identified age of the regional, basal Palaeozoic unconformity. This late magmatism may be generated by the Najd Fault reactivation correlating with the Malagasy/Kunnga Orogeny that marked the final stages of Gondwana assembly.

  16. Experimental Constraints on Fe Isotope Fractionation in Carbonatite Melt Systems

    NASA Astrophysics Data System (ADS)

    Stuff, M.; Schuessler, J. A.; Wilke, M.

    2015-12-01

    Iron isotope data from carbonatite rocks show the largest variability found in igneous rocks to date [1]. Thus, stable Fe isotopes are promising tracers for the interaction of carbonate and silicate magmas in the mantle, particularly because their fractionation is controlled by oxidation state and bonding environment. The interpretation of Fe isotope data from carbonatite rocks remains hampered, since Fe isotope fractionation factors between silicate and carbonate melts are unknown and inter-mineral fractionation can currently only be assessed by theoretical calculations [1;2]. We present results from equilibration experiments in three natrocarbonatite systems between immiscible silicate and carbonate melts, performed at 1200°C and 0.7 GPa in an internally heated gas pressure vessel at intrinsic redox conditions. The Fe isotope compositions of the silicate melt (sil.m.), quenched to a glass, and the carbonate melt (carb.m.), forming fine-grained quench crystals, were analysed by solution MC-ICP-MS. Our first data indicate a remarkable fractionation of Δ56Fesil.m.‒carb.m.= 0.29 ±0.07 ‰ near equilibrium. At short run durations, even stronger fractionation up to Δ56Fesil.m.‒carb.m. = 0.41 ±0.07 ‰ occurs, due to kinetic effects. Additionally, Δ56Fesil.m.‒carb.m. changes with bulk chemical composition, likely reflecting considerable differences between the studied systems in terms of the Fe3+/Fe2+-ratios in the two immiscible liquids. Our findings provide experimental support for a carbonatite genesis model, in which extremely negative δ56Fe values in carbonatites result from differentiation processes, such as liquid immiscibility [1]. This effect can be enhanced by disequilibrium during fast ascent of carbonatite magmas. Their sensitivity to chemical and redox composition makes Fe isotopes a potential tool for constraining the original compositions of carbonatite magmas. [1] Johnson et al. (2010) Miner. Petrol. 98, 91-110. [2] Polyakov & Mineev (2000

  17. Stable Isotopic Constraints of the Turpan Basin in Northwestern China

    NASA Astrophysics Data System (ADS)

    Schaen, A. J.

    2010-12-01

    Stable isotopic analysis of sedimentary rocks can be used to reconstruct past geologic changes in the elevation and climate of topographic features such as mountain ranges and plateaus. The Tibetan Plateau is an ideal field laboratory for conducting this type of study because of the Plateau’s extreme topographic relief and relatively recent geologic growth. Here we present oxygen and carbon isotope compositions from a suite of sedimentary rock samples taken from the western Turpan Basin in northwestern China. This area of the basin collects sediment from weathering and erosion of the Bogda Shan located to the north. The goal of this study is to analyze changes in the stable isotope composition as a function of stratigraphic position to reconstruct paleoelevations and paleoclimates in this part of the Tibetan Plateau. The sedimentary rock samples analyzed in this study are Late Jurassic to Neogene age and are primarily mudstone, siltstone, fine sandstone along with lesser limestone. Samples were powered and then dissolved with phosphoric acid at 72οC. The liberated CO2 gas was then analyzed using a Finnigan Delta Plus XL mass spectrometer with a gasbench inlet system. Oxygen isotope values range from -13.72 to -1.62‰ (PDB) and exhibit a large scale trend to more negative values toward the top of the stratigraphic sequence. Superimposed on this large scale trend are systematic variations in isotopic composition as a function of age. The most positive δ18O values occur at approximately 160, 115, 60, and 5 ma. Conversely, δ18O minima are observed at 150, 90, and 40 ma. δ13C values range from -10.69‰ to 1.40‰ (PDB). The most positive δ13C values (-4.3 to 1.4) occur from 120-160 ma. Younger samples display small scale variations with age with notable δ13C minima of -10.7, -14.7, and -7.6‰ at 108, 80, and 17 ma, respectively. The variable δ18O and positive δ13C values from the Jurassic (145-160 ma) are consistent with an arid climate and high atmospheric

  18. Accretionary history of the Altai-Mongolian terrane: perspectives from granitic zircon U-Pb and Hf-isotope data

    NASA Astrophysics Data System (ADS)

    Cai, Keda; Sun, Min; Xiao, Wenjiao

    2014-05-01

    The Central Asian Orogenic Belt (CAOB) consists of many tectonic terranes with distinct origin and complicated evolutionary history. Understanding of individual block is crucial to reconstruct the geodynamic history of the gigantic accetionary collage. This study presents zircon U-Pb ages and Hf isotopes for the granitoid rocks in the Russian Altai mountain range (including Gorny Altai, Altai-Mongolian terrane and CTUS suture zone between them), in order to clarify the timing of granitic magmatism, source nature, continental crustal growth and tectonic evolution. Our dating results suggest that granitic magmatism of the Russian Altai mountain range occurred in three major episodes including 445~429 Ma, 410~360 Ma and ~241 Ma. Most of the zircons within the Paleozoic granitoids present comparable positive ɛHf(t) values and Neoproterozoic crustal model ages, which favor the interpretation that the juvenile crustal materials produced in the early stage of CAOB were probably dominant sources for the Paleozoic magmatism in the region. The inference is also supported by widespread occurrence of short-lived juvenile materials including ophiolites, seamount relics and arc assemblages in the north CAOB. Consequently, the Paleozoic massive granitic rocks maybe not represent continental crustal growth at the time when they were emplaced, but rather record reworking of relatively juvenile Proterozoic crustal rocks although mantle-derived mafic magma was possibly involved to sever as heat engine during granitic magma generation. The Early Triassic granitic intrusion may be product in an intra-plate environment, as the case of same type rocks in the adjacent areas. The positive ɛHf(t) values (1.81~7.47) and corresponding Hf model ages (0.80~1.16 Ga) together with evidence of petrology are consistent with the interpretation that the parental magma of the Triassic granitic intrusion was produced from enriched mantle-derived sources under an usually high temperature condition

  19. Evolution of the mantle source in an evolving arc-backarc system (Torres del Paine, Patagonia): Evidence from Hf isotopes in zircon

    NASA Astrophysics Data System (ADS)

    Ewing, T. A.; Muntener, O.; Leuthold, J.; Baumgartner, L. P.; Putlitz, B.; d'Abzac, F. X.; Chiaradia, M.

    2015-12-01

    The Miocene Torres del Paine intrusive complex (TPIC) in Patagonia is a transitional alkaline backarc intrusion1 emplaced on short timescales of 162 ± 11 ka2. It is subdivided into two units with distinct ages of ~12.6 Ma and ~12.45 Ma1. Smaller intrusive bodies in the area record a change in chemistry from calc-alkaline at ~16 Ma, to transitional alkaline at ~12.5 Ma. Zircons from ~16 Ma intrusives and the 12.6 Ma part of the TPIC have remarkably consistent, slightly enriched Hf isotope compositions with ɛHf(i) of -1 to +2. An abrupt shift towards more juvenile Hf isotope compositions is observed in the ~12.45 Ma part of the TPIC, with ɛHf(i) of +3 to +6. Bulk rock Nd and Sr isotopes for the TPIC show the same shift towards more juvenile compositions at this time1. The long-term consistency of ɛHf(i) from 16 to 12.6 Ma is surprising, given that in the same period the bulk rock chemistry changes from calc-alkaline to transitional alkaline. Conversely, the major shift in ɛHf(i) is not correlated with any change in bulk rock chemistry, which remains transitional alkaline from 12.6 to 12.45 Ma. The decoupling of major element chemical evolution and Hf isotope signatures suggests that the subsequent rapid influx of juvenile material recorded by our Hf isotope data must have occurred by renewed mantle melting. Subduction of the Chile ridge at ~12.5 Ma in this area caused arc magmatism to move westwards and back-arc extension to initiate. We propose that the first TPIC magmas (12.6 Ma) came from a mantle wedge with a residual subduction signature. Subsequent melting of more juvenile mantle, less contaminated by a subduction component, generated the 12.45 Ma TPIC magmas. These results demonstrate that magmatic complexes such as the TPIC may tap distinct mantle sources even on very short timescales, fingerprinting arc-backarc transition processes. 1Leuthold et al., 2013, JPET, 54: 273-303 2Leuthold et al., 2012, EPSL, 325: 85-92

  20. Decoupling of whole-rock Nd-Hf and zircon Hf-O isotopic compositions of a 284 Ma mafic-ultramafic intrusion in the Beishan Terrane, NW China

    NASA Astrophysics Data System (ADS)

    Su, Ben-Xun; Qin, Ke-Zhang; Lu, Ying-Huai; Sun, He; Sakyi, Patrick Asamoah

    2015-10-01

    Abundant Permian mafic-ultramafic intrusions in the Beishan Terrane, NW China, are parts of the Tarim large igneous province. Among these intrusions, Luodong intrusion is composed of dunite, wehrlite and gabbro. These rocks have whole-rock Sr-Nd-Hf and zircon Hf-O isotope compositions that display significant decoupling. The decoupling of these generally well-correlated systems demonstrates contrasting evolving trends. Systematic compositional and mineralogical controls on decoupling have been investigated. Wehrlites and gabbros show MORB-like trace element patterns with negligible crustal contamination. They have high initial 143Nd/144Nd [ɛNd(t) = +6.6 to +11.2] and 176Hf/177HfHf(t) = +12.2 to +16.9] and low initial 87Sr/86Sr [(87Sr/86Sr)i = 0.702949-0.704098] ratios and plot within the MORB field, indicating that their parental magmas were derived from a depleted mantle source. The enrichment features are present in the zircon crystals separated from the gabbro. These homogeneously unzoned zircon crystals have high U concentrations, low Th/U ratios and a U-Pb age of 284.0 ± 2.3 Ma. They are therefore interpreted as having formed rapidly in a highly fractionated/evolved magma. Their ɛHf(t) and δ18O values range from -9.3 to -6.7 and +10.25 ‰ to +11.42 ‰, respectively. The decoupling is linked with crustal contamination by ancient crust (probably Proterozoic schist in the Beishan Terrane) that occurred during zircon crystallization. However, the contamination signature in the whole-rock composition was soon overprinted by magma mixing process. The inference is evidenced by zoning textures preserved in plagioclase, clinopyroxene, spinel and particularly olivine, and the presence of coeval diabase dykes cutting through the intrusion. The isotope decoupling observed in this mantle-plume-related mafic-ultramafic intrusion supports the idea that Nd-Hf decoupling and Lu/Hf and Sm/Nd parent/daughter variations exist only on a small hand-size scale in a

  1. Evolution of the African continental crust as recorded by U-Pb, Lu-Hf and O isotopes in detrital zircons from modern rivers

    NASA Astrophysics Data System (ADS)

    Iizuka, Tsuyoshi; Campbell, Ian H.; Allen, Charlotte M.; Gill, James B.; Maruyama, Shigenori; Makoka, Frédéric

    2013-04-01

    To better understand the evolutionary history of the African continental crust, a combined U-Pb, Lu-Hf and O isotopic study has been carried out by in situ analyses of approximately 450 detrital zircon grains from the Niger, Nile, Congo, Zambezi and Orange Rivers. The U-Pb isotopic data show age peaks at ca. 2.7, 2.1-1.8, 1.2-1.0, ca. 0.8, 0.7-0.5 and ca. 0.3 Ga. These peaks, with the exception of the one at ca. 0.8 Ga, correspond with the assembly of supercontinents. Furthermore, the detrital zircons that crystallized during these periods of supercontinent assembly have dominantly non-mantle-like O and Hf isotopic signatures, in contrast to the ca. 0.8 Ga detrital zircons which have juvenile characteristics. These data can be interpreted as showing that continental collisions during supercontinent assembly resulted in supermountain building accompanied by remelting of older continental crust, which in turn led to significant erosion of young igneous rocks with non-mantle-like isotopic signatures. Alternatively, the data may indicate that the major mode of crustal development changed during the supercontinent cycle: the generation of juvenile crust in extensional settings was dominant during supercontinent fragmentation, whereas the stabilization of the generated crust via crustal accretion and reworking was important during supercontinent assembly. The Lu-Hf and O isotope systematics indicate that terreigneous sediments could attain elevated 18O/16O via prolonged sediment-sediment recycling over long crustal residence time, and also that reworking of carbonate and chert which generally have elevated 18O/16O and low Hf contents is minor in granitoid magmatism. The highest 18O/16O in detrital zircon abruptly increased at ca. 2.1 Ga and became nearly constant thereafter. This indicates that reworking of mature sediments increased abruptly at that time, probably as a result of a transition in the dynamics of either granitoid crust formation or sedimentary evolution

  2. Isotopic Constraints on Processes of Mantle Recycling (Subduction?) in the Hadean and Archean

    NASA Astrophysics Data System (ADS)

    Shirey, S. B.; Basu, A. R.; Kamber, B.; Mueller, P. A.; Whitehouse, M.

    2006-12-01

    The earliest evidence for subduction and its effects on the Hadean and Archean Earth can be found in the isotopic signatures of recycling in Archean igneous and metasedimentary rocks. The separation of an early terrestrial reservoir from the upper mantle shown by ^{142}Nd isotope data (Boyet & Carlson, Science, 2005) obviates the need for massive extraction and recycling of early continental crust (Armstrong, Phil Trans A, 1981) to explain the depleted mantle. Without extensive continents, an intra-oceanic-arc type of recycling process dominated the evolution of the early mantle. When continental crustal recycling did occur, it appears limited to the margins of emergent and juvenile continental nuclei. Intra-oceanic arc recycling has long been supported by geochemical studies of present mantle heterogeneity that show it represents ancient (>3 Ga) recycled components and geodynamic studies of mantle convection that show the mantle can remain poorly mixed at these time scales. Such studies cannot readily distinguish Hadean to Eoarchean foundering from true plate subduction. However, ^{142}Nd excesses (Caro et al, GCA 2006) and Pb isotopic variability in Eoarchean rocks (Kamber et al, CMP 2003), Hf isotopic variability in Hadean zircon (Harrison et al, Science 2005) and a solar component in mantle rare gases (Tolsthikin and Hofmann, PEPI 2005) require long term isolation of a mafic Hadean crust incompatible with a dynamic process of plate destruction accompanied by efficient return. In contrast to this oldest record, minerals and rocks from <3.6 Ga no longer show extreme heterogeneity in Hf and Pb isotopes and the ^{142}Nd excess in highly depleted mantle had apparently disappeared. Subduction can be strongly inferred from surficial isotopic signatures in crustal and mantle rocks and minerals preserved in the Paleo- to Meso-Archean portions of various continents and their lithospheric mantle keels: 3.5 Ga old diamonds from the Slave craton mantle lithosphere that

  3. Osmium isotope constraints on ore metal recycling in subduction zones

    PubMed

    McInnes; McBride; Evans; Lambert; Andrew

    1999-10-15

    Veined peridotite xenoliths from the mantle beneath the giant Ladolam gold deposit on Lihir Island, Papua New Guinea, are 2 to 800 times more enriched in copper, gold, platinum, and palladium than surrounding depleted arc mantle. Gold ores have osmium isotope compositions similar to those of the underlying subduction-modified mantle peridotite source region, indicating that the primary origin of the metals was the mantle. Because the mantle is relatively depleted in gold, copper, and palladium, tectonic processes that enhance the advective transport and concentration of these fluid soluble metals may be a prerequisite for generating porphyry-epithermal copper-gold deposits. PMID:10521343

  4. Osmium isotope constraints on ore metal recycling in subduction zones

    PubMed

    McInnes; McBride; Evans; Lambert; Andrew

    1999-10-15

    Veined peridotite xenoliths from the mantle beneath the giant Ladolam gold deposit on Lihir Island, Papua New Guinea, are 2 to 800 times more enriched in copper, gold, platinum, and palladium than surrounding depleted arc mantle. Gold ores have osmium isotope compositions similar to those of the underlying subduction-modified mantle peridotite source region, indicating that the primary origin of the metals was the mantle. Because the mantle is relatively depleted in gold, copper, and palladium, tectonic processes that enhance the advective transport and concentration of these fluid soluble metals may be a prerequisite for generating porphyry-epithermal copper-gold deposits.

  5. Interaction of Sublithospheric Mantle with a Complex Continental Lithosphere: Radiogenic Isotope Constraints

    NASA Astrophysics Data System (ADS)

    Hanan, B. B.; Jean, M. M.; Shervais, J. W.; Graham, D. W.; Vetter, S.

    2012-12-01

    The Yellowstone-Snake River Plain (YSRP) consists of an 800 km swath of bimodal volcanic centers in southern Idaho and western Wyoming formed as the North American continent overrode the Yellowstone hotspot since ˜17 Ma. The rhyolitic centers show a time transgressive relationship with plate motion, but basalt volcanism persisted long after the locus of rhyolitic volcanism moved to the NE. The hotspot track is underlain by a 10-km-thick mafic sill complex that contains much of the basaltic melt produced. Seismic tomography, the age progressive nature, its relationship the Columbia River Basalts, and the isotopic signature of 3He/4He in the basalts suggest presence of a mantle hotspot originating in the sublithospheric mantle. Basalt major and trace element, and He isotope systematics are consistent with a deep mantle source, similar to ocean island basalt (OIB). In contrast, the Pb, Sr, and Nd isotopes are indistinguishable from xenoliths and melts from sub-continental lithospheric mantle (SCLM) underlying the YSRP. The SCLM stabilized in the Late Archean to Early Proterozoic, and was subsequently rejuvenated/enriched during subduction related metasomatism. Initial Pb and Sr isotope ratios are higher, and Nd lower than expected for a depleted upper mantle source of Late Archean age. Incompatible element concentrations in OIB-plume sources are more than 10X lower than found in the SCLM. Assimilation of small percentage partial melts of continental lithosphere into larger degree partial melts derived from the sublithospheric mantle source produces hybrid magmas whose Pb (Nd,Sr,Hf) isotopic compositions are controlled by the isotopic composition of the continental component, while the deeper mantle source dominates the 3He/4He signature. We tested this prediction with analyses of 75 basalts from the YSRP. The Pb isotope results are consistent with mixing between an OIB-like plume component with 1% to 4% melt derived from an enriched SCLM source and show that the

  6. Isotopic constraints on the petrogenesis of jurassic plutons, Southeastern California

    USGS Publications Warehouse

    Mayo, D.P.; Anderson, J.L.; Wooden, J.L.

    1998-01-01

    The 165 Ma Eagle Mountain intrusion is a heterogeneous, enclave-bearing, metaluminous remnant of the Cordilleran Jurassic arc that cuts regionally metamorphosed pre-Mesozoic rocks in the southeastern Mojave Desert of California. The main phase of the intrusion consists of granodiorite to tonalite host facies, diorite mixed facies, and homogeneous monzogranite facies. The host facies contains microdiorite enclaves interpreted as intermingled masses of mafic magma. Late-phase leucogranite stocks cut the main phase. Mineral equilibria indicate emplacement at ???6.5 km depth, with solidus temperatures ranging from 760??C for diorite to 700??C for felsic granodiorite. Although uniform radiogenic-isotope compositions (Sri = 0.7085, ???Ndi = -9.4) suggest derivation from a single source, no known source has the composition required. A hybrid source is proposed, consisting of various proportions of juvenile mantle and recycled lower crust. Calculations indicate that the source of the Eagle Mountain intrusion comprised >60% juvenile mantle and <40% recycled crust. On the basis of their isotopic compositions, other mafic Jurassic plutons in the region were derived from sources containing different proportions of mantle and crustal components.

  7. U Pb, Hf and O isotope evidence for two episodes of fluid-assisted zircon growth in marble-hosted eclogites from the Dabie orogen

    NASA Astrophysics Data System (ADS)

    Wu, Yuan-Bao; Zheng, Yong-Fei; Zhao, Zi-Fu; Gong, Bing; Liu, Xiaoming; Wu, Fu-Yuan

    2006-07-01

    A combined study of internal structure, U-Pb age, and Hf and O isotopes was carried out for metamorphic zircons from ultrahigh-pressure eclogite boudins enclosed in marbles from the Dabie orogen in China. CL imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain, respectively. The metamorphic zircons have low Th and U contents with low Th/U ratios, yielding two groups of 206Pb/ 238U age at 245 ± 3 to 240 ± 2 Ma and 226 ± 4 to 223 ± 2 Ma, respectively. Anomalously high δ 18O values were obtained for refractory minerals, with 9.9 to 21.4‰ for garnet and 16.9‰ for zircon. This indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction-zone metamorphism. Most of the zircons are characterized by very low 176Lu/ 177Hf ratios of 0.000001-0.000028, indicating their growth in association with garnet recrystallization. A few of them falling within the older age group have comparatively high 176Lu/ 177Hf ratios of 0.000192-0.000383, suggesting their growth prior to the formation of garnet in the late stage of subduction. The variations in the Lu/Hf ratios for zircons can thus be used to correlate with garnet growth during eclogite-facies metamorphism. In either case, the zircons have variable ɛHf ( t) values for individual samples, suggesting that their protolith is heterogeneous in Hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle. Nevertheless, a positive correlation exists between 206Pb/ 238U ages and Lu-Hf isotope ratios for the metamorphically recrystallized zircons, suggesting that eclogite-facies metamorphism in the presence of fluid has the identical effect on zircon Lu-Hf and U-Th-Pb isotopic systems. We conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh-pressure metamorphism, whereas the younger zircons

  8. Hf sbnd Nd sbnd Sr isotopes and incompatible element abundances in island arcs: implications for magma origins and crust-mantle evolution

    NASA Astrophysics Data System (ADS)

    White, William M.; Patchett, Jonathan

    1984-02-01

    We present Hf, Nd and Sr isotopic data and abundances of K, Rb, Cs, Ba, Sr, Hf and REE for 32 samples from seven intra-oceanic island arcs. Samples from the Marianas, Izu, Aleutian and New Britain arcs have tightly grouped 176Hf/ 177Hf˜ 0.28320, 143Nd/ 144Nd˜ 0.51303 and 87Sr/ 86Sr˜ 0.7035 close to, but distinct from, mid-ocean ridge basalts (MORB) for 143Nd/ 144Nd and 87Sr/ 86Sr . In contrast, samples from the Sunda, Banda and Lesser Antilles arcs are much more variable towards lower 176Hf/ 177Hf and 143Nd/ 144Nd , and higher 87Sr/ 86Sr . Isotopically, island arcs on the whole are closely similar to ocean islands. Some commonly-occurring features of the trace element geochemistry of island arcs are apparent in our data: alkali and alkaline-earth elements, particularly Cs, have high abundance relative to LREE compared to oceanic basalts; negative Ce anomalies occur in six out of seven arcs. However, Hf does not appear underabundant relative to REE. The isotopic data require a continental component in all island arcs, in addition to probable mantle and oceanic crust contributions, even for the arcs with isotope ratios close to MORB. In the absence of continental crust, we can best explain this component by subducted pelagic sediment in the arc magma source region. The involvement of sediments in all arcs implies that there is an inherent recycling of older continent to island arcs, and potentially to new continent, of at least 1%. Conservative calculations show that the upper subducted slab (basalt + sediment) passes beyond the arc magma genesis zone and enters the deep mantle with a minimum of 500-1000 ppm K, and corresponding amounts of other incompatible elements. If this material is not completely homogenized with the mantle and later becomes part of the source of ocean island magmas, then the ocean island—island arc isotopic similarity is a result of their similar mix of source materials—mantle peridotite with trace element signatures from oceanic crust

  9. Osmium isotope constraints on Earth's late accretionary history

    USGS Publications Warehouse

    Morgan, J.W.

    1985-01-01

    Osmium isotope measurements reported by Alle??gre and Luck 1,2 indicate that terrestrial osmiridiums evolved in a mantle source region in which the osmium/rhenium ratio falls strictly within the range found in chondrites. This suggests that the highly siderophile elements in the Earth's mantle were introduced by a late influx of chondritic material and are not a result of endogenous processes. I have now examined the available data in more detail and conclude that the inferred Os/Re ratio of the Earth's mantle matches the E group and C3 chondrites, but that C1 and probably C2 chondrites were not major components of the material accreted in the late stages of mantle formation. ?? 1985 Nature Publishing Group.

  10. A hybrid origin for two Cretaceous monzonitic plutons in eastern Zhejiang Province, Southeast China: Geochronological, geochemical, and Sr-Nd-Hf isotopic evidence

    NASA Astrophysics Data System (ADS)

    Liu, Liang; Qiu, Jian-Sheng; Zhao, Jiao-Long

    2016-01-01

    Monzonites can provide important information about the nature of the mantle sources and the mechanism of crust-mantle interactions. However, details on the origin of Late Mesozoic monzonites in the Southeastern China remain poorly constrained. This paper presents whole-rock geochemical, Sr-Nd isotopic and zircon U-Pb and Hf isotopic data for two monzonitic plutons (Huangtanyang and Kanggu) in eastern Zhejiang Province, with the aim of elucidating their petrogenesis, and providing important insights into the process of crust-mantle interaction. LA-ICP-MS zircon U-Pb dating results imply that the Huangtanyang and Kanggu quartz monzonites were emplaced in Cretaceous (104-109 Ma). All quartz monzonites are intermediate to acidic, metaluminous to weakly peraluminous, subalkaline, and K-rich in composition. They are enriched in large ion lithophile (e.g., Rb, Ba and Pb) and light rare earth elements, depleted in high-field strength elements (e.g., Nb, Ta, and Ti), and show weakly negative or no Eu anomalies (δEu = 0.78-1.02). All quartz monzonites have homogeneous initial ISr values (0.7084-0.7090) and εNd(t) values (-7.50 to -6.84). They are characterised by highly variable zircon Hf isotopic compositions, with εHf(t) values ranging from -13.3 to -5.7. The combined geochemical evidences (such as high Mg# values, low Nb/U and Ta/U ratios, and variable zircon Hf isotopic compositions) suggests that both depleted asthenospheric and metasomatically enriched mantle components were involved in the formation of the monzonites. The existence of some zircons with unusually low εHf(t) values (low to -13.3) and Palaeoproterozoic two-stage Hf model ages from the Huangtanyang and Kanggu quartz monzonites also argues strongly for Palaeoproterozoic crustal involvement. Magma mixing played a dominated role in the genesis of these monzonites, as indicated by their wide range in zircon Hf isotopic compositions and the occurrence of mafic microgranular enclaves (MMEs). The MMEs show

  11. Zircon-apatite U-Pb geochronology, zircon Hf isotope composition and geochemistry of granite batholith in the northern Mexico: Implications for Tectonomagmatic evolution of southern Cordillera.

    NASA Astrophysics Data System (ADS)

    Mahar, M. A.; Goodell, P.

    2015-12-01

    We present the zircon-apatite U-Pb ages and zircon Hf isotope composition of the granite batholith exposed at the western boundary of Chihuahua. Granidiorite samples were analyzed from both, north and south of the Rio El Fuerte and Sinforosa Lineament. Based on previous studies, the WWN-EES trending Sinforosa Lineament is proposed as the manifestation of a terrane boundary between Seri in the north and Tahue terrane in the south. Zircon U-Pb data indicate that the magmatism spans a time period of 36 Ma from 89 to 53 Ma to the north of the Sinforosa Lineament while granodiorites in the south of the Sinforosa Lineament are dated at 59 Ma. The U-Pb apatite ages are variable in the north of the Sinforosa Lineament and range from 86-51 Ma. These apatite dates are 1-28 Ma younger than the corresponding zircon U-Pb crystallization ages. This indicates variable cooling rates and moderate to shallow emplacement. In contrast, in the south of the Sinforosa Lineament, the U-Pb apatite ages (64-59 Ma) are indistinguishable from the zircon U-Pb age (59 Ma), indicating rapid cooling and shallow emplacement. Zircon morphology and U-Pb dating revealed the absence of inherited component in the zircon ages, as no inheritance of any age has been observed. Most of the northwestern Mexico is underlain by Precambrian-Paleozoic-Jurassic basement. However, in the study area, U-Pb dating does not support the involvement of the older basement in generating the granite magmas. The weighted mean initial ɛHf (t) isotope composition of granodiorites on both sides of the Sinforosa Lineament varies from +2 to +5. However, Hf isotope composition in the south of the Sinforosa Lineament is more heterogeneous and relatively evolved with weighted Mean ɛHf (t) = +1.45. The Hf isotope composition is consistent with the previously reported near bulk silicate Sr-Nd isotope values. We suggest that the magmatic rocks in this region are not derived from melting of a felsic older crust beneath the batholith

  12. Tracing the history of submarine hydrothermal inputs and the significance of hydrothermal hafnium for the seawater budget - A combined Pb-Hf-Nd isotope approach

    USGS Publications Warehouse

    van de Flierdt, T.; Frank, M.; Halliday, A.N.; Hein, J.R.; Hattendorf, B.; Gunther, D.; Kubik, P.W.

    2004-01-01

    Secular variations in the Pb isotopic composition of a mixed hydrogenous-hydrothermal ferromanganese crust from the Bauer Basin in the eastern Equatorial Pacific provide clear evidence for changes in hydrothermal contributions during the past 7 Myr. The nearby Galapagos Rise spreading center provided a strong hydrothermal flux prior to 6.5 Ma. After 6.5 Ma, the Pb became stepwise more radiogenic and more similar to Equatorial Pacific seawater, reflecting the westward shift of spreading to the presently active East Pacific Rise (EPR). A second, previously unrecognized enhanced hydrothermal period occurred between 4.4 and 2.9 Ma, which reflects either off-axis hydrothermal activity in the Bauer Basin or a late-stage pulse of hydrothermal Pb from the then active, but waning Galapagos Rise spreading center. Hafnium isotope time-series of the same mixed hydrogenous-hydrothermal crust show invariant values over the past 7 Myr. Hafnium isotope ratios, as well as Nd isotope ratios obtained for this crust, are identical to that of hydrogenous Equatorial Pacific deep water crusts and clearly indicate that hydrothermal Hf, similar to Nd, does not travel far from submarine vents. Therefore, we suggest that hydrothermal Hf fluxes do not contribute significantly to the global marine Hf budget. ?? 2004 Elsevier B.V. All rights reserved.

  13. Bulk chemical and Hf-W isotopic consequences of incomplete accretion during planet formation

    NASA Astrophysics Data System (ADS)

    Dwyer, Christina A.; Nimmo, Francis; Chambers, John E.

    2015-01-01

    Late-stage accretion involves collisions which may result in complete or incomplete merging of the two objects, hit-and-run encounters, or mass loss from the target. We use a recent N-body study incorporating these different collision styles (Chambers, J.E. [2013]. Icarus 224, 43-56) to investigate how collision style affects the bulk chemical and isotopic outcomes of terrestrial planet formation. Compared with simulations in which all collisions result in perfect mergers, the variability in modeled silicate mass fraction and tungsten isotope anomaly is larger, especially for lower-mass planets. The final tungsten anomaly also shows a systematic reduction, because the timescale to finish planet growth is longer when incomplete mergers are included. Simulations including incomplete merging can reproduce the observed scatter in both tungsten anomaly and silicate mass fraction of the terrestrial planets.

  14. Isotope geochemistry of recent magmatism in the Aegean arc: Sr, Nd, Hf, and O isotopic ratios in the lavas of Milos and Santorini-geodynamic implications

    USGS Publications Warehouse

    Briqueu, L.; Javoy, M.; Lancelot, J.R.; Tatsumoto, M.

    1986-01-01

    In this comparative study of variations in the isotopic compositions (Sr, Nd, O and Hf) of the calc-alkaline magmas of the largest two volcanoes, Milos and Santorini, of the Aegean arc (eastern Mediterranean) we demonstrate the complexity of the processes governing the evolution of the magmas on the scale both of the arc and of each volcano. On Santorini, the crustal contamination processes have been limited, effecting the magma gradually during its differentiation. The most differentiated lavas (rhyodacite and pumice) are also the most contaminated. On Milos, by contrast, these processes are very extensive. They are expressed in the 143Nd/144Nd vs. 87Sr/86Sr diagram as a continuous mixing curve between a mantle and a crustal end member pole defined by schists and metavolcanic rocks outcropping on these volcanoes. In contrast with Santorini, the least differentiated lavas on Milos are the most contaminated. These isotopic singularities can be correlated with the geodynamic evolution of the Aegean subduction zone, consisting of alternating tectonic phases of distension and compression. The genesis of rhyolitic magmas can be linked to the two phases of distension, and the contamination of the calc-alkaline mantle-derived magmas with the intermediate compressive phase. The isotopic characteristics of uncontaminated calc-alkaline primitive magmas of Milos and Santorini are directly comparable to those of magmas generated in subduction zones for which a contribution of subducted sediments to partial melts from the mantle is suggested, such as in the Aleutian, Sunda, and lesser Antilles island arcs. However, in spite of the importance of the sediment pile in the eastern Mediterranen oceanic crust (6-10 km), the contribution of the subducted terrigenous materials remains of limited amplitude. ?? 1986.

  15. Neutron capture cross sections of /sup 178/,/sup 179/,/sup 180/Hf and the origin of nature's rarest stable isotope /sup 180/Ta

    SciTech Connect

    Beer, H.; Macklin, R.L.

    1982-01-01

    The neutron capture cross sections of /sup 178/,/sup 179/,/sup 180/Hf were measured in the energy range 2.6 keV to 2 MeV. The average capture cross sections were derived and fitted in terms of strength functions. Resonance parameters for the observed resonances below 10 keV were determined by shape analysis. Maxwellian-averaged capture cross sections were computed for thermal energies with kT between 5 and 100 keV. The cross sections for kT = 30 keV were used to determine the population probability of the 8- isomeric level in /sup 180/Hf by neutron capture as (1.24 +- 0.06)% and the r-process abundance of /sup 180/Hf as 0.0290 (Si = 10/sup 6/). These quantities served to analyze s- and r-process nucleosynthesis of /sup 180/Ta, nature's rarest stable isotope.

  16. Zircon from East Antarctica: evidence for Archean intracrustal recycling in the Kaapvaal-Grunehogna Craton from O and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Marschall, H. R.; Hawkesworth, C. J.; Storey, C.; Leat, P. T.; Dhuime, B.

    2010-12-01

    The Grunehogna Craton (GC, East Antarctica) is interpreted as part of the Archean Kaapvaal Craton of southern Africa prior to Gondwana breakup. The basement of the GC is only exposed within a small area comprising the dominantly leucocratic Annandagstoppane (ADT) granite. The granite (and hence the craton) has been dated previously only by Rb-Sr and Pb-Pb mica and whole-rock methods. Here, the crystallisation age of the granite was determined to 3,067 ± 8 Ma by U-Pb dating of zircon. This age is coeval with granitoids and volcanics in the Swaziland and Witwatersrand blocks of the Kaapvaal Craton. Inherited grains in the ADT granite were discovered with ages of up to 3,433 ±7 Ma, and are the first evidence of Palaeoarchean basement in Dronning-Maud Land. The age spectrum of the inherited grains reflects well-known tectono-magmatic events in the Kaapvaal Craton and form important pieces of evidence for the connection of the GC to the Kaapvaal Craton for at least three billion years and probably longer. Whole-rock chemistry and zircon O isotopes demonstrate a supracrustal sedimentary source for the granite, and Hf model ages show that at least two or three different crustal sources were contributing to the magma with model ages of ~3.50, ~3.75 and possibly ~3.90 Ga, respectively. 3.1 Ga granites covering ~60 % of the outcrop area of the Kaapvaal-Grunehogna Craton played a major role in the mechanical stabilisation of the continental crust during the establishment of the craton in the Mesoarchean. Combined zircon Hf-O isotope data and the lack of juvenile additions to the crust in the Mesoarchean strongly suggest that crustal melting and granite formation was caused by the deep burial of clastic sediments and subsequent incubational heating of the crust. Intracrustal recycling of this type may be an important process during cratonisation and the long-term stabilisation of continental crust.

  17. Stable Isotope Constraints on the Ocean from Hydrothermally-altered Igneous Rocks

    NASA Astrophysics Data System (ADS)

    Gregory, R. T.

    2007-12-01

    The 18O/16O ratio of the ocean provides an important constraint on the global geochemical cycles in the Precambrian Earth. The oxygen isotope ratio of the ocean is most likely buffered near its present day value as long as plate tectonics is operative. A quasi-steady state value for oxygen isotopes is reached on a 100 Myr timescale after the onset of plate tectonics. Hydrothermally-altered igneous rocks constrain the oxygen and hydrogen isotope value of the hydrosphere back through time. Whereas, the oxygen isotope composition of seawater owes its value to the competition between low temperature chemical weathering and mid-ocean ridge hydrothermal exchange, there is no such process for hydrogen isotopes. Changes in the oxygen isotope ratio of seawater should be reflected in hydrothermally altered rocks by the presence of low or high 18O exchanged igneous rocks with normal δD values. The distribution of D and 18O in hydrothermally rocks is used to infer the position of the meteoric water line back through time. Results from the Phanerozoic, the Proterozoic, and the Archean fail to confirm the hypothesis that the global oceans were ever strongly 18O-depleted. The meteoric water line is anchored to the isotopic composition of seawater, the isotope standard for both oxygen and hydrogen isotopes. The ability to use sedimentary rocks or other proxies for climate depend upon the variation in the stable isotopic composition of seawater. Thus far, the hydrothermal record does not support the existence of low 18O oceans. This suggests that low 18O values observed in carbonates and cherts result from either precipitation from oceans with higher temperature or from bodies of water isolated from the open ocean.

  18. Isotopic and Experimental Constraints on Subsurface U Transport and Fate

    NASA Astrophysics Data System (ADS)

    Porcelli, D.; Strekopytov, S.; Shaw, S.; Baskaran, M.; Hilton, D.; Kulongoski, J.

    2009-04-01

    U- and Th- series nuclides have provided essential tools for studying weathering and subsurface element transport processes. The radionuclides U, Th, Ra, Rn, and Pb have a range of half-lives and contrasting chemical behaviours, and the isotopic decay systematics that connect the different isotopes can be used to quantify the rates of trace element transport and the extents of interaction with the aquifer rocks. This can often be linked to rates of soil formation, chemical and mechanical weathering of watersheds, and potentially, subsurface water flow rates. However, this requires a comprehensive understanding of the mechanisms controlling the behaviour of each element. Data from the unconfined Mojave River Basin aquifer, with a reasonably well-defined flow pattern and groundwater ages of up to ~40,000 years illustrates the response of the U/Th series nuclides to extended water- rock interaction. Measurements of 222Rn find relatively uniform rates of Rn recoil ejection by decay of parent 226Ra throughout the aquifer. If the ratio of recoil rates 222Rn/234Th=1, then the rate of release of 234Th, which rapidly decays to 234U, can be obtained. This therefore defines the input of 234U, which is expected to increase with groundwater age, regardless of the extent of reversible adsorption. Although 234U is also released by weathering, the amount of recoil-derived ‘excess' 234U is obtained directly from 234U/238U ratios. Measured excess 234U concentrations are actually relatively constant and up to 104 times lower than calculated. The simplest explanations are that one of the basic assumptions widely used to interpret U behavior is invalid, and either the release of 222Rn is by mechanisms that do not apply to other nuclides and so the 234Th recoil rate is up to 104 times less, or U is irreversibly removed even from oxic groundwaters. To address the first possibility, we have made the first direct laboratory measurements of recoil of both 222Rn and 234Th from mineral

  19. Hf isotope study of Palaeozoic metaigneous rocks of La pampa province and implications for the occurrence of juvenile early Neoproterozoic (Tonian) magmatism in south-central Argentina

    NASA Astrophysics Data System (ADS)

    Chernicoff, C. J.; Zappettini, E. O.; Santos, J. O. S.; Belousova, E.; McNaughton, N. J.

    2011-12-01

    On a global scale, juvenile Tonian (Early Neoproterozoic) magmatic rocks are associated with the extensional events that lead to the breakup of the Rodinia supercontinent. In Argentina, no geological record is available for this time interval, lasting from 1000 to 850 Ma. We present indirect evidence for the existence of Tonian extension in Argentina, as supported by Hf and Nd isotope determinations on Phanerozoic magmatic and sedimentary rocks. We mainly focus on our own Hf isotope determinations carried out on U-Pb SHRIMP dated zircons from Palaeozoic metaigneous rocks of La Pampa province, south-central Argentina, i.e. metagabbros of Valle Daza, dioritic orthogneiss of Estancia Lote 8, and metadiorite of Estancia El Carancho, having found that these rocks were derived from sources of ca. 920 to ca 880 Ma, with ɛHf values between +6.83 and + 9.59. Inherited zircons of this age and character identified in these rocks also point to the same source. We also compile additional Hf and Nd studies from previous work on Phanerozoic magmatic and sedimentary rocks. We preliminarily compare the age of the juvenile Tonian sources referred to in our work with that of two extensional events identified in the São Francisco craton, Brazil.

  20. U-Pb age and Hf isotope data of detrital zircons of exotic Devonian sandstones from the southeastern Rheinisches Schiefergebirge near Giessen, Germany

    NASA Astrophysics Data System (ADS)

    Nawrat, Joscha; Bahlburg, Heinrich; Axel, Gerdes

    2013-04-01

    In the Rhenohercynian zone, southeastern Rheinisches Schiefergebirge, of Germany early and late Devonian sedimentary successions of suspect provenance occur in the allochthonous assemblage of the Lindener Mark south of Giessen and the Giessen nappe. Both tectonic units were emplaced over the autothonous Rhenohercynian units as northward moving nappes during the Variscan orogeny. A combination of faunal and sedimentological data suggested that the allochthonous sedimentary successions do not belong to the Rhenohercynian zone as part of the southern margin of the Old Red Continent and most likely derive originally from northern Gondwana. In order to test the interpretation of a NW African origin of the Hercynian units we applied U-Pb detrital zircon age determination and Hf isotope analysis by laser-ablation-sector field-inductively coupled plasma mass spectrometry (LA-SF-ICP-MS). We studied the Dalmanitensandstein in the Lindener Mark south of Giessen, the Giessener Grauwacke, Kalkige Grauwacke and the Erbslochgrauwacke near Marburg and Densberg. We analyzed around 150 detrital zircons of each sample and considered only ages which are less than 10% discordant. Viewed synoptically the age distributions of all 5 samples are very similar. The U-Pb ages range between 3300 Ma and 372 Ma. The age distributions show two major peaks in each of the samples. The older peak is around 2000 Ma with ~27% of ages, the younger one is around 600 Ma with ~71% of ages. Ages between 1650 and 1200 Ma are very scarce (~2%). These essentially bimodal zircon age spectra are similar to typical NW African zircon age spectra with the main abundances connected to the Eburnean and Cadomian orogenies at c. 2000 Ma and c. 600 Ma, respectively. Contrastingly, siliciclastic units derived from the Old Red Continent to the north include abundant zircon ages between 2000 and 1000 Ma. The Hf isotope patterns of the samples of all studied formations are also very similar. The Hf isotopic compositions of

  1. Hf and Nd Isotope Evidence for Production of an Incompatible Trace Element Enriched Crustal Reservoir in Early Earth (Invited)

    NASA Astrophysics Data System (ADS)

    Brandon, A. D.; Debaille, V.; Lapen, T. J.

    2010-12-01

    The final significant stage of accretion of the Earth was likely a collision between proto-Earth and a Mars sized impactor that formed the Moon. This event is thought to have produced enough thermal energy to melt all or most of the Earth, with a consequent magma ocean (MO). During subsequent cooling, the Earth would have formed its protocrust and corresponding mantle lithosphere, consisting of solidified basalt-komatiitic melt, in combination with buoyant cumulates and late stage residual melts from the MO. Relative to the convecting mantle, portions of this protolithosphere are likely to have been enriched in incompatible trace elements (ITE) in sufficient quantities to contain a significant amount of the bulk Earth’s budget for rare earth elements, U, Th, and Hf. If the protolithosphere was negatively buoyant, it may have overturned at or near the final stages of MO crystallization and a significant portion of that material may have been transported into the deep mantle where it resided and remixed into the convecting mantle over Earth history [1,2]. If the protolithosphere remained positively buoyant, its crust would have likely begun to erode from surface processes, and subsequently recycled back into the mantle over time as sediment and altered crust, once a subduction mechanism arose. The Nd and Hf isotopic compositions of Earth’s earliest rocks support the idea that an early-formed ITE-enriched reservoir was produced. The maxima in 142Nd/144Nd for 3.85 to 3.64 Ga rocks from Isua, Greenland decreases from +20 ppm to +12 ppm relative to the present day mantle value, respectively [3]. This indicates mixing of an early-formed ITE enriched reservoir back into the convecting mantle. In addition, zircons from the 3.1 Ga Jack Hills conglomerate indicate that material with an enriched 176Lu/177Hf of ~0.02 and an age of 4.4 Ga or greater was present at the Earth’s surface over the first 2 Ga of Earth history, supporting the scenario of a positively buoyant

  2. Protracted diagenetic alteration of REE contents in fossil bioapatites: Direct evidence from Lu-Hf isotope systematics

    NASA Astrophysics Data System (ADS)

    Kocsis, László; Trueman, Clive N.; Palmer, Martin R.

    2010-11-01

    Fossil bones and teeth are potentially important repository for geochemical proxy data and a target for radiometric dating. The concentration of many trace elements in bones and teeth increases by orders of magnitude after death and it is this diagenetic incorporation that forms the basis for several areas of geochemical study. The use of bones and teeth in this context relies on two assumptions: first, that target metal ions are incorporated rapidly after death, reflecting a known environmental signal, and second, that after early incorporation, the bone or tooth remains as an essentially closed system, resistant to later diagenetic change. A wide literature has developed exploring these assumptions, but relatively little direct evidence has been used to assess the long-term diagenetic stability of trace elements within bones and teeth. In this study, we use the Lu-Hf isotope system to show that bones and teeth of Cretaceous and Triassic age from both terrestrial and marine settings experience continued, long-term diagenetic change, most likely through gradual addition of trace elements. Modelling suggests that diagenetic addition after initial recrystallisation may account for >50% of the total REE content in the sampled bones, the extent depending on initial uptake conditions. Tooth enamel and enameloid may be more resistant to late diagenetic changes, but dentine is probably altered to the same extent as bone. These results have significant implications for the use of bones and teeth as hosts of chronological, palaeoceanographic, palaeoenvironmental and taphonomic information, particularly in Mesozoic and Palaeozoic contexts.

  3. Insights into the metasomatic history of Kaapvaal SCLM from a Hf isotope study of the ~2.06 Ga Bushveld Large Igneous Province

    NASA Astrophysics Data System (ADS)

    Zirakparvar, N. A.; Mathez, E. A.; Choe, S.

    2015-12-01

    The Bushveld Large Igneous Province (B-LIP) comprises a diverse array of >30 magma bodies that intruded the Kaapvaal Craton (KC) at ~2.06 Ga. To determine whether the B-LIP formed in response to the arrival of a plume(s) from the deep mantle or from melting of the depleted upper mantle during foundering of an eclogitized residue at the base of the lithosphere, we have measured zircon Hf isotope compositions for many of the bodies in the B-LIP. Most of the intrusions have relatively unradiogenic and internally homogeneous ɛHf (2.06 Ga) values (intrusion-specific average ɛHf (2.06 Ga) range from -21.2 ± 5.2 to -2.7 ± 2.8), consistent with published values for the Bushveld and Phalaborwa complexes (two prominent intrusions in the B-LIP with ɛHf (2.06 Ga) = -8.6 ± 2.6 and -7.5 ± 2.4, respectively). Because the most radiogenic Hf isotope compositions in the B-LIP are within error of ɛHf (2.06 Ga) = 0, it is likely that the heat source was a plume(s) from the deep mantle, as opposed to delamination-driven decompression melting of the depleted upper mantle. Many of the more unradiogenic values in the B-LIP can be reconciled with melt generation in subduction modified and metasomatically refertilized subcontinental lithospheric mantle (SCLM). Support for this model comes from B-LIP aged zircons in a metasomatically altered xenolithic fragment of the SCLM associated with a basement inlier near the geographic center of the B-LIP. Some domains in these zircons grew in the presence of a medium with a highly unradiogenic Hf isotope signature. This signature suggests that ancient (>3.8 Ga) crustal material residing in the KC-SCLM at the time of B-LIP magmatism became mobilized during arrival of the plume at ~2.06 Ga. Pervasive metasomatic alteration leading to weakening of the SCLM beneath parts of the KC is known to have occurred in the Mesozoic, but these results suggest that the KC has also withstood plume related metasomatic weakening during the Paleoproterozoic.

  4. Hf isotope compositions and chronology of magmatic zircons from Tarim continental flood basalts: implications for magmatic evolution of the Early Permian Tarim Large Igneous Province in NW China

    NASA Astrophysics Data System (ADS)

    Li, Y.; Li, Z.; Yu, X.; Langmuir, C. H.; Yang, S.; Chen, H.

    2013-12-01

    The Early Permian Tarim Large Igneous Province (TLIP) in the Tarim cratonic block of northwestern China has been largely regarded to be genetically linked with a mantle plume. Recently, some euhedral zircon crystals with magmatic growth zoning have been obtained from the Tarim continental flood basalts (TCFB) for detailed U-Pb chronological and genetic study. The zircons have the concordant 206Pb/238U ages of 297~283 Ma, coinciding with the previously reported whole-rock 40K/39Ar and 40Ar/39Ar ages (292~283 Ma) of their host basalts. In-situ LA-MC-ICPMS Lu-Hf isotopic analyses of Early Permian zircons from the Keping area of the TCFB reveal that the zircons from two basalt sub-groups (Groups 1a, 1b) have a narrow range of 176Hf/177Hf ratios between 0.282422 and 0.282568. Their corresponding ɛHf(t) (t = 290 Ma) values (-6.8~-1.4) are generally lower than their host basalts (-2.3~2.1), and distinctively different from the intrusive rocks (3.0~7.1) and their zircons (4.9~8.8) from the TLIP and the Precambrian crustal rocks (<-18) in the Tarim block. Combined with their embayed margins produced by magmatic corrosion, these zircons may have crystallized in a concealed pluton shortly prior to the extrusion of basalts and been captured as xenocrysts by the rapidly erupted basaltic lavas. Almost the same ɛHf(t) values between the corroded and uncorroded zircons suggest that the zircons have preserved the initial Hf isotopic compositions from their original source region. Moreover, the very close but relatively higher ɛHf(t) values from the zircons than the inferred sub-continental lithospheric mantle (SCLM) beneath Tarim in the Early Permian [ɛHf(t) = -8.7~-5.2; t = 290 Ma] indicate that the zircons were probably originated from the SCLM with minor addition of depleted mantle magmas during the mantle source partial melting. Both the zircons and their host basalts have almost the same formation ages (~290 Ma) and Hf TDM model ages (ca. 1300~1000 Ma), suggesting that

  5. Zircon U-Pb geochronology, Sr-Nd-Hf isotopic composition and geological significance of the Late Triassic Baijiazhuang and Lvjing granitic plutons in West Qinling Orogen

    NASA Astrophysics Data System (ADS)

    Duan, Meng; Niu, Yaoling; Kong, Juanjuan; Sun, Pu; Hu, Yan; Zhang, Yu; Chen, Shuo; Li, Jiyong

    2016-09-01

    The Qinling Orogen was a consequence of continental collision of the South China Craton with the North China Craton in the Triassic and caused widespread granitoid magmatism. However, the petrogenesis of these granitoids remains controversial. In this paper, we choose the Baijiazhuang (BJZ) and Lvjing (LJ) plutons in the West Qinling Orogen for a combined study of the zircon U-Pb geochronology, whole-rock major and trace element compositions and Sr-Nd-Hf isotopic characteristics. We obtained zircon crystallization ages of ~ 216 Ma and ~ 212 Ma for the BJZ and the LJ plutons, respectively. The granitoid samples from both plutons have high K2O metaluminous to peraluminous compositions. They are enriched in large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depleted in high field-strength elements (HFSEs) with significant negative Eu anomalies. The BJZ samples have initial Sr isotopic ratios of 0.7032 to 0.7078, εNd(t) of - 10.99 to - 8.54 and εHf (t) of - 10.22 to - 6.41. The LJ granitoids have initial Sr isotopic ratios of 0.7070 to 0.7080, εNd(t) of - 5.37 to - 4.58 and εHf(t) of - 3.64 to - 1.78. The enriched isotopic characteristics of the two plutons are consistent with their source being dominated by ancient continental crust. However, two BJZ samples show depleted Sr isotope compositions, which may infer possible involvement of mantle materials. Mantle-derived melt, which formed from partial melting of mantle wedge peridotite facilitated by dehydration of the subducted/subducting Mianlue ocean crust, provide the required heat for the crustal melting while also contributing to the compositions of these granitoids. That is, the two granitic plutons are magmatic responses to the closure of the Mianlue ocean basin and the continental collision between the Yangtze and South Qinling crustal terranes.

  6. Ultra-depleted isotopic compositions in fertile asthenosphere-derived peridotites: constraints on the composition of the upper mantle

    NASA Astrophysics Data System (ADS)

    Byerly, B. L.; Lassiter, J. C.

    2012-12-01

    Recent studies of abyssal peridotites (AP) and OIB xenoliths have reported refractory, isotopically ultra-depleted domains within the convecting upper mantle with Nd- and Hf-isotope compositions that extend far beyond the MORB field. These results have important implications regarding the average composition of the depleted upper mantle and the genetic relationship between MORB and AP. However, the abundance of ultra-depleted domains in the mantle is unclear. In addition, recent melt extraction processes at mid-ocean ridges make it difficult to evaluate the compositions of ultra-depleted domains prior to exhumation and thus evaluate their role in melt generation. To better constrain the abundance and composition of typical convecting upper mantle, we examined a suite of spinel peridotite xenoliths from the central Rio Grande Rift (RGR) where most of the preexisting lithosphere has been convectively removed and replaced with depleted upper mantle. Seismic tomography indicates that the lithosphere beneath the RGR has been substantially removed (Gao, 2004), and geochemical evidence supports this. Two distinct populations of xenoliths are observed from Elephant Butte, central RGR. One population, interpreted to derive from residual Proterozoic lithospheric mantle, is refractory (bulk Al2O3 <2.3 wt.%), LREE- and LILE-enriched, has enriched Sr, Nd, and Pb isotopic compositions and along with xenoliths from the Eastern Colorado Plateau define a strong Lu/Hf-176Hf/177Hf "pseudo-isochron" with an apparent age of ~1.6 Ga. In contrast, the majority of the RGR xenoliths have fertile major element compositions (bulk Al2O3 ~ 4.0 wt %), low spinel Cr# (~10), and LREE-depleted trace element patterns, and overlap with composition estimates for the depleted mantle (Workman & Hart, 2005). We interpret these xenoliths to reflect recent replacement of the pre-existing lithosphere with material from the convecting upper mantle. The fertile xenoliths have cpx Sr-, Nd-, and Hf-isotope

  7. Rapid Rejuvenation of the Source of a Backarc Sheeted Magmatic Complex (Torres del Paine, Patagonia): Evidence From Hf isotopes in Zircon

    NASA Astrophysics Data System (ADS)

    Ewing, T. A.; Muntener, O.; Leuthold, J.; Chiaradia, M.; Baumgartner, L. P.; Putlitz, B.

    2014-12-01

    The Miocene Torres del Paine intrusive complex (TPIC) in Patagonia is a spectacularly exposed example of a bimodal shallow crustal laccolith, made up of a sill complex and a subvertical feeder system. The TPIC was emplaced in a back-arc setting, but slightly older arc-related intrusive units in this area testify to an earlier shift from an arc to a backarc setting. The entire ~88 km3 main complex was emplaced over short time scales of 162 ± 11 ka between ~12.4 and 12.6 Ma, with mafic units from the feeder zone found to be older than mafic units from the sill complex1,2. We aim to assess whether successive pulses of mafic magmatism can tap different geochemical reservoirs in sheeted magmatic complexes emplaced on such short timescales. Hf isotope compositions of individual zircons from mafic units from both the feeder zone and the sill complex were determined by solution MC-ICPMS. Zircons from all units have Hf isotope compositions that indicate a slightly enriched mantle source. Zircons from the mafic sill complex units have higher (more juvenile) initial ɛHf than zircons from feeder zone mafic units. The shift towards more depleted Hf isotope compositions in the sill complex units, which are younger, demonstrates the rapid input of new juvenile material into the source region between ~12.6 Ma and ~12.5 Ma. A similar shift is also seen in bulk rock Nd and Sr isotope data for related samples3. The Hf isotope data demonstrate that significant variability in source geochemistry is possible for sheeted magmatic complexes built up on very short timescales. Analysis of zircons from a range of dikes and intrusive bodies external to the main Torres del Paine complex, with ages that span ~12-29 Ma, will provide a more complete picture in time and space of the geochemical evolution of this magmatic system as it switches between an arc and backarc setting. 1Leuthold et al., 2012, EPSL, 325: 85-92 2Michel et al., 2008, Geology, 36: 459-462 3Leuthold et al., 2013, JPET, 54

  8. Rubidium isotopes in primitive chondrites: Constraints on Earth's volatile element depletion and lead isotope evolution

    NASA Astrophysics Data System (ADS)

    Nebel, O.; Mezger, K.; van Westrenen, W.

    2011-05-01

    The bulk silicate Earth (BSE) shows substantial deficits in volatile elements compared to CI-chondrites and solar abundances. These deficits could be caused by pre-accretionary depletion in the solar nebula during condensation of solids, or by later heat-driven evaporation during collision of small bodies that later accreted to form the Earth. The latter is considered to result in isotope fractionation for elements with low condensation temperatures that correlates with the degree of depletion. Here, we report first high-precision isotope ratio measurements of the moderately volatile and lithophile trace element Rb. Data from seventeen chondrite meteorites show that their Rb isotope abundances are nearly indistinguishable from Earth, not deviating more than 1 per mil in their 87Rb/85Rb. The almost uniform solar system Rb isotope pool suggests incomplete condensation or evaporation in a single stage is unlikely to be the cause of the volatile element deficit of the Earth. As Rb and Pb have similar condensation temperatures, we use their different degrees of depletion in the BSE to address the mechanisms and timing of terrestrial volatile depletion. The Rb isotope data are consistent with a scenario in which the volatile budget of the Earth was generated by a mixture of a highly volatile-element depleted early Proto-Earth with undepleted material in the course of terrestrial accretion. Observed Pb and Rb abundances and U-Pb and Rb-Sr isotope systematics suggest that volatile addition occurred at approximately the same time at which last core-mantle equilibration was achieved. In line with previous suggestions, this last equilibration involved a second stage of Pb (but not Rb) depletion from the BSE. The timing of this second Pb loss event can be constrained to ~ 110 Ma after the start of the solar system. This model supports a scenario with core storage of Pb in the aftermath of a putative Moon forming giant impact that also delivered the bulk of the volatile

  9. Geochemistry and zircon U-Pb-Hf isotopes of the granitoids of Baolidao and Halatu plutons in Sonidzuoqi area, Inner Mongolia: Implications for petrogenesis and geodynamic setting

    NASA Astrophysics Data System (ADS)

    Hu, Chuansheng; Li, Wenbo; Xu, Cheng; Zhong, Richen; Zhu, Feng

    2015-01-01

    The Baolidao and Halatu plutons are located in the Northern Orogenic Belt (NOB) in Sonidzuoqi area of Inner Mongolia, which has an important significance for the tectonic evolution of Xing-Meng Orogenic Belt (XMOB). The two plutons have been intensively studied but the conclusions are still controversial. Combined with the previous study, this paper gives new geological data about the two correlative plutons for gaining a better understanding of their petrogenesis and the geodynamic setting. The Baolidao granitoids contain two different series, calc-alkaline series mainly formed in the Ordovician and high K calc-alkaline series mainly formed in the Carboniferous. The Halatu granites are formed in the Triassic and belong to high-K calc-alkaline series. This study got the zircon U-Pb ages of 316-322 Ma for the Baolidao granitoids and 233 ± 2 Ma for the Halatu syenogranites, respectively. In the tectonic discrimination diagrams, they mainly fall into the area of post-orogenic granites (POG). Hf isotopic analyses for the Baolidao granitoids (Sample BLD-1 and 3) shows εHf (t) = 3.0-14.0, with two-stage Hf model age (TDM2) of 436-1138 Ma. The Halatu syenogranite (Sample HLT-1) also shows a depleted εHf (t) = 3.8-8.2, with TDM2 of 741-1024 Ma, suggesting the major involvement of juvenile crustal components. The various εHf values of the Carboniferous Baolidao and Triassic Halatu granitoids indicates a hybrid magma source of juvenile material with old crustal component, and the εHf (t) values decrease from the Carboniferous to Triassic, suggesting the increasing proportion of old continental material during this period. Combined with the regional geology, the Carboniferous Baolidao granitoids are possibly not arc rocks, but originated from the post-collisional setting. The Triassic Halatu granites were formed in the subsequently extensional environment.

  10. Zircon Lu-Hf isotopes and granite geochemistry of the Murchison Domain of the Yilgarn Craton: Evidence for reworking of Eoarchean crust during Meso-Neoarchean plume-driven magmatism

    NASA Astrophysics Data System (ADS)

    Ivanic, Timothy J.; Van Kranendonk, Martin J.; Kirkland, Christopher L.; Wyche, Stephen; Wingate, Michael T. D.; Belousova, Elena A.

    2012-09-01

    New in situ Lu-Hf data on zircons from GSWA geochronology samples has provided a unique isotopic dataset with a high temporal resolution for the Murchison Domain of the Yilgarn Craton in Western Australia. These data identify extended periods of juvenile mantle input (positive ɛHf values) into the crust firstly at c. 2980 Ma and then from c. 2820 Ma to c. 2640 Ma with significant pulses of crustal recycling at c. 2750 Ma and c. 2620 Ma (highly negative ɛHf values). Geochemical data from well-characterised granitic suites of the Murchison Domain provide additional constraints on the crustal evolution of the area and indicate a prolonged period of crustal melting and remelting at progressively shallower depths from c. 2750 to c. 2600 Ma. At c. 2760-2753 Ma, widespread calc-alkaline, intermediate to silicic volcanic rocks of the Polelle Group were erupted, accompanied by intrusion of felsic to intermediate melts derived from a variety of crustal sources that likely formed by partial mixing with basaltic melts. The intrusive rocks include a wide geochemical array of rocks in the Cullculli and Eelya suites that were sourced over a wide range of crustal depths. At this time a major departure to negative ɛHf values (<-5) occurred, indicating sampling of c. 3.80 Ga model aged source rocks as well as continued juvenile input. Post-volcanic granitic rocks emplaced between c. 2710 and c. 2600 Ma show geochemical evidence for progressive fractionation through time and derivation from an evolving crustal source. We interpret the driving force for this protracted history of mantle and crustal melting to be two mantle plumes at 2.81 and 2.72 Ga. These data document the process of cratonization through progressive melt depletion of the lower crust, progressively fractionating and shallower melts, culminating with a final phase of crustal recycling (ɛHf < - 5) and the cessation of juvenile input at c. 2630-2600 Ma during intrusion of the Bald Rock Supersuite, resulting in

  11. Evolution of Seawater Nd and Hf Isotope in the South China Sea over the last 30 Ma from Fe-Mn oxyhydroxide

    NASA Astrophysics Data System (ADS)

    Chen, C.; You, C.; Huang, K.; Wang, B.; Liu, H.

    2009-12-01

    South China Sea (SCS) is the largest marginal sea in the western Pacific and plays an important role in regional evolution and ocean circulation. The sedimentary Fe-Mn oxyhydroxides record faithfully of ocean water mass variations and deep seawater circulation. Nd-Hf isotopes and REEs in Fe-Mn oxyhydroxides are powerful proxies for climatic and environmental reconstruction in the past ocean. In this study, Fe-Mn oxyhydroxides were leached and separated from Leg 184 ODP site 1148 sediments. Subsequently these Nd and Hf isotopes were analyzed using MC-ICP-MS. Nod P-1 (USGS) standard was selected for examining the chemical procedure and isotope technique. Our new results show Ce anomaly and low HREE in the Fe-Mn oxyhydroxides. The seawater Nd isotope (ɛNd) in the SCS varies between -7.0ɛ to -5.5ɛ with an increase from the present-day to ca. 3-4 Ma ago and is followed by a general decrease back to ca. 30 Ma. This is consistent with the scenarios of gradually increases in the North Atlantic Deep Water (NADW) component presented in the Pacific from 3-5 Ma to present, and the Panamanian gateway restriction from ~10 Ma to 3-5 Ma.

  12. Eocene Kashmar granitoids (NE Iran): Petrogenetic constraints from U-Pb zircon geochronology and isotope geochemistry

    NASA Astrophysics Data System (ADS)

    Shafaii Moghadam, Hadi; Li, Xian-Hua; Ling, Xiao-Xiao; Santos, Jose F.; Stern, Robert J.; Li, Qiu-Li; Ghorbani, Ghasem

    2015-02-01

    Kashmar granitoids outcrop for ~ 100 km along the south flank of the Sabzevar ophiolite (NE Iran) and consist of granodiorite and monzogranite along with subordinate quartz monzonite, syenogranite and aplitic dikes. These granitoids intruded Early to Middle Eocene high-K volcanic rocks and can spatially be grouped into eastern and western granitoids. Five samples of granite have identical zircon U-Pb ages of ca. 40-41 Ma. The granitoids have quite high K2O (~ 1.3-5.3 wt.%) and Na2O (~ 1.1-4.6 wt.%) with SiO2 ranging between ~ 62 and 77 wt.%. They are metaluminous to peraluminous, calc-alkaline and I-type in composition. Their chondrite-normalized REE patterns are characterized by LREE enrichment and show slight negative Eu anomalies. Kashmar granitoids have low whole rock εNd (- 0.43 to - 2.3), zircon εHf values (- 1.9 to + 7.2), and somewhat elevated δ18O (+ 6.1 to + 8.7‰) in the range of I-type granites. The Kashmar granitoids show Early Neoproterozoic zircon second-stage Hf and bulk rock Nd model ages at ca. 500-1000 Ma (associated with ca. 640 Ma old inherited zircons). Bulk rock Nd-Sr isotopic modeling suggests that 10-20% assimilation of Cadomian lower crust by juvenile mantle melts and then fractional crystallization (AFC process) can explain the Sr-Nd isotopic compositions of Kashmar granitoids. Kashmar granitoids are products of crustal assimilation by mantle melts associated with extension above the subducting Neotethyan Ocean slab beneath SW Eurasia. Similar subduction-related extension was responsible for the flare-up of Eocene-Oligocene magmatism across Iran, associated with core complex formation in central Iran.

  13. Timing and origin of migmatitic gneisses in south Karakoram: Insights from U-Pb, Hf and O isotopic record of zircons

    NASA Astrophysics Data System (ADS)

    Mahar, Munazzam Ali; Mahéo, Gweltaz; Goodell, Philip C.; Pavlis, Terry L.

    2016-04-01

    The timing and origin of partial melting in collision belts is crucial to understand the thermotectonic evolution and the relationship between HT metamorphism and magmatism in over-thickened crust. In the present study, we used the in-situ isotopic (Hf, O and U-Pb) record of zircons to investigate the timing and origin of migmatitic gneisses exposed in the core of the Dassu dome in south Karakoram. The new U-Pb zircon dating identified the Proterozoic inherited cores (1.8-1.9 Ga and 2.3-2.5 Ga) surrounded by a Neogene overgrowth with ages ranging from ∼6 to ∼20 Ma. These ages imply that the partial melting in the Karakoram Metamorphic Complex lasted from >20 Ma to ∼6 Ma and can be correlated with the Miocene magmatism in the adjacent Baltoro region. Oxygen isotopic data from Proterozoic inherited cores (1.8-1.9 Ga) and Neogene overgrowths are indistinguishable and generally vary from 8‰ to 9.5‰. These values are slightly higher than the most igneous zircons (6.5-8‰, Valley et al., 2005) indicating an igneous precursor with heavy initial O composition that later might have equilibrated with low temperature environment or some involvement of supracrustal material is likely. However, a few low U/Th, relatively old inherited cores (2.3-2.5 Ga) showed mantle-like (δ18O = 5.3 ± 0.6‰, Valley et al., 2005) values of δ18O = 5.5 ± 2.7‰. The present-day weighted mean εHf (0) of the Proterozoic inherited cores ranges from -50 ± 1.0 to -44.3 ± 1.2. In contrast, the Neogene rims are 15-20 ε-units higher than the inherited core with present-day εHf (0) = -30.6 ± 0.9. This implies that the Hf composition of the Neogene overgrowth is not controlled exclusively by the dissolution of the inherited cores and that contamination by external melts is likely. We suggest a contribution from the Neogene, less-evolved magmatism in the Baltoro region (εHf (0) = ∼-4 to -10). The elevated oxygen composition is not consistent with the contribution from pristine

  14. Constraints from Li isotope systematics on subduction recycling, arc magmatism, and continent growth: An overview

    NASA Astrophysics Data System (ADS)

    Leeman, W. P.; Lee, C. A.; Chan, L. H.

    2008-12-01

    Great expectations that Li isotopic systematics can uniquely constrain many fluid-mitigated geologic processes have met with mixed success for a variety of reasons. On a local scale (some volcanic arc segments) Li composition can be highly correlated with other geochemical tracers of subduction fluids whereas, globally, such correlations tend to be disappointingly poor. The utility of Li isotopes as a tracer is limited in part by extensive overlap between mantle and subduction inputs, by limited understanding of equilibrium isotopic fractionation effects, and by apparent departures from equilibrium behavior. On the other hand, Li elemental systematics provide important constraints on global recycling processes because major litho-tectonic reservoirs have distinctive enrichments or depletions with respect to Nb or other HFSEs. Such chemical fractionations can be understood in terms of differential solubility of these elements in aqueous fluids vs. silicate melts, as well as the roles of weathering, dehydration, metamorphic or melting processes. For example, arc lavas are are systematically enriched in Li compared to those from other settings and typically have Li/Nb greater than BSE (consistent with addition of Li-rich fluids to their sources). In contrast, bulk continental crust and orogenic granitoids tend to have lower Li/Nb than BSE or arc lavas. Moreover, mass balance implies that the residual mantle (DM) produced by segregation of crust has higher Li/Nb than BSE. However, if continental crust is ultimately derived by subduction related magmatism, high Li/Nb would be expected for the crust and low Li/Nb for the upper mantle. This interesting conundrum is easiest explained in terms of selective Li removal from crustal protolith rocks via chemical weathering and erosion, which also is consistent with Li isotopic compositions of crust, mantle and seawater reservoirs. Thus, Li elemental and isotopic systematics (and relevant proxies) provide complementary

  15. Cryogenian alkaline magmatism in the Southern Granulite Terrane, India: Petrology, geochemistry, zircon U-Pb ages and Lu-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Santosh, M.; Yang, Qiong-Yan; Ram Mohan, M.; Tsunogae, T.; Shaji, E.; Satyanarayanan, M.

    2014-11-01

    22 Ma correlating with the ages of the basement rocks from these areas. The initial 176Hf/177Hf isotope ratios of the zircon grains from the AM syenite fall in the range between 0.281771 and 0.282284, with moderately negative εHf(t) values between - 5.9 and 0.1. Similarly, the initial 176Hf/177Hf isotope ratios for the zircon grains of PM ultrapotassic granite range between 0.281197 and 0.281970, albeit with more negative εHf(t) values in the range between - 22.7 and - 0.3 (average εHf (t) value - 18.8). The Lu-Hf data suggest the involvement of variable extent of older crust with distinct crustal residence times, either in the form of assimilation during magma emplacement, or crustal recycling during magma genesis. Based on the geochemical and isotopic systematics, a possible petrogenetic model would be asthenospheric upwelling in an extensional setting, melting of enriched lithosphere, and interaction of the magmas with lower crustal domains with subduction-related components of various ages. The disposition of these alkali plutons along two paleo sutures that weld the Meso-Neoarchean crustal blocks in the northern periphery of SGT suggests that the zones of emplacement might represent an aborted rift. The paleo-sutures probably served as a weak zone along which extension occurred broadly coeval with the Cryogenian subduction further south.

  16. U-Pb zircon geochronology and Nd-Hf-O isotopic systematics of the Neoproterozoic Hadb adh Dayheen ring complex, Central Arabian Shield, Saudi Arabia

    NASA Astrophysics Data System (ADS)

    Ali, Kamal A.; Jeon, Heejin; Andresen, Arild; Li, Shuang-Qing; Harbi, Hesham M.; Hegner, Ernst

    2014-10-01

    A combined study of single zircon U-Pb dating, Hf-O zircon isotopic analyses and whole-rock Nd isotopic compositions was carried out to infer the magma sources of Neoproterozoic post-collisional A-type granitoids in Saudi Arabia. U-Pb zircon dating of magmatic zircons of two samples from the Hadb adh Dayheen ring complex yielded ages of 625 ± 11 Ma for a hornblende-biotite granite sample, and 613 ± 4 Ma for a monzogranite sample. The granitic rocks show initial εNd values of + 4.1 to + 5.3 and εHf of + 4.5 to + 8.4 that are lower than those of a model depleted mantle (εHf ~+ 14 and εNd ~+ 6.5) and consistent with melting of subduction-related crustal protoliths that were formed during the Neoproterozoic assembly of the Arabian-Nubian Shield (ANS). Crustal-model ages (Hf-tNC) of 0.81 to 1.1 Ga are inconsistent with depleted-mantle Nd model ages of 0.71 to 0.81 Ga and indicate that the post-collisional Hadb adh Dayheen granites were derived mostly from juvenile crust formed in Neoproterozoic time. Single zircons data show a wide range in δ18O values from + 3.2‰ to + 6.4‰, possibly indicating crystallization of zircon from magma derived from magmatic rocks altered by meteoric water in a magma chamber-caldera system.

  17. Adding geochemical and isotope tracers to models of hillslope evolution: valuable constraints or monumental headache?

    NASA Astrophysics Data System (ADS)

    Mudd, S. M.; Yoo, K.; Hurst, M. D.; Weinman, B. A.; Maher, K.

    2011-12-01

    Landscapes evolve through time, both in terms of their geomorphology and their geochemistry. Past studies have highlighted that topography suffers from the problem of equifinality: the topographic configuration of landscapes can be the result of many different, yet equally plausible, erosion histories. In hillslope soils the properties and chemistry of the soils themselves could provide additional constraints on landscape evolution. Here we present results from a combination of modelling and field studies that seek to quantify the co-evolution of hillslope morphology and the solid state chemistry of hillslope soils. The models follow large numbers of individual particles as they are entrained into a physically mobile soil layer, weathered, and accumulate isotopes such as 10Be and 21Ne. We demonstrate that multiple hillslope properties mitigate (but do not eliminate) the problem of equifinality and demonstrate the importance of accounting for individual particle residence times and ages in interpretation of both isotope and weathering data.

  18. Re-Os geochronology and coupled Os-Sr isotope constraints on the Sturtian snowball Earth.

    PubMed

    Rooney, Alan D; Macdonald, Francis A; Strauss, Justin V; Dudás, Francis Ö; Hallmann, Christian; Selby, David

    2014-01-01

    After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitudes at least twice between 717 and 635 Mya. Although the initiation mechanism of these Neoproterozoic Snowball Earth events has remained a mystery, the broad synchronicity of rifting of the supercontinent Rodinia, the emplacement of large igneous provinces at low latitude, and the onset of the Sturtian glaciation has suggested a tectonic forcing. We present unique Re-Os geochronology and high-resolution Os and Sr isotope profiles bracketing Sturtian-age glacial deposits of the Rapitan Group in northwest Canada. Coupled with existing U-Pb dates, the postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological constraints for both the onset and demise of a Cryogenian glaciation from the same continental margin and suggests a 55-My duration of the Sturtian glacial epoch. The Os and Sr isotope data allow us to assess the relative weathering input of old radiogenic crust and more juvenile, mantle-derived substrate. The preglacial isotopic signals are consistent with an enhanced contribution of juvenile material to the oceans and glacial initiation through enhanced global weatherability. In contrast, postglacial strata feature radiogenic Os and Sr isotope compositions indicative of extensive glacial scouring of the continents and intense silicate weathering in a post-Snowball Earth hothouse.

  19. Re-Os geochronology and coupled Os-Sr isotope constraints on the Sturtian snowball Earth

    PubMed Central

    Rooney, Alan D.; Macdonald, Francis A.; Strauss, Justin V.; Dudás, Francis Ö.; Hallmann, Christian; Selby, David

    2014-01-01

    After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitudes at least twice between 717 and 635 Mya. Although the initiation mechanism of these Neoproterozoic Snowball Earth events has remained a mystery, the broad synchronicity of rifting of the supercontinent Rodinia, the emplacement of large igneous provinces at low latitude, and the onset of the Sturtian glaciation has suggested a tectonic forcing. We present unique Re-Os geochronology and high-resolution Os and Sr isotope profiles bracketing Sturtian-age glacial deposits of the Rapitan Group in northwest Canada. Coupled with existing U-Pb dates, the postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological constraints for both the onset and demise of a Cryogenian glaciation from the same continental margin and suggests a 55-My duration of the Sturtian glacial epoch. The Os and Sr isotope data allow us to assess the relative weathering input of old radiogenic crust and more juvenile, mantle-derived substrate. The preglacial isotopic signals are consistent with an enhanced contribution of juvenile material to the oceans and glacial initiation through enhanced global weatherability. In contrast, postglacial strata feature radiogenic Os and Sr isotope compositions indicative of extensive glacial scouring of the continents and intense silicate weathering in a post–Snowball Earth hothouse. PMID:24344274

  20. Re-Os geochronology and coupled Os-Sr isotope constraints on the Sturtian snowball Earth.

    PubMed

    Rooney, Alan D; Macdonald, Francis A; Strauss, Justin V; Dudás, Francis Ö; Hallmann, Christian; Selby, David

    2014-01-01

    After nearly a billion years with no evidence for glaciation, ice advanced to equatorial latitudes at least twice between 717 and 635 Mya. Although the initiation mechanism of these Neoproterozoic Snowball Earth events has remained a mystery, the broad synchronicity of rifting of the supercontinent Rodinia, the emplacement of large igneous provinces at low latitude, and the onset of the Sturtian glaciation has suggested a tectonic forcing. We present unique Re-Os geochronology and high-resolution Os and Sr isotope profiles bracketing Sturtian-age glacial deposits of the Rapitan Group in northwest Canada. Coupled with existing U-Pb dates, the postglacial Re-Os date of 662.4 ± 3.9 Mya represents direct geochronological constraints for both the onset and demise of a Cryogenian glaciation from the same continental margin and suggests a 55-My duration of the Sturtian glacial epoch. The Os and Sr isotope data allow us to assess the relative weathering input of old radiogenic crust and more juvenile, mantle-derived substrate. The preglacial isotopic signals are consistent with an enhanced contribution of juvenile material to the oceans and glacial initiation through enhanced global weatherability. In contrast, postglacial strata feature radiogenic Os and Sr isotope compositions indicative of extensive glacial scouring of the continents and intense silicate weathering in a post-Snowball Earth hothouse. PMID:24344274

  1. Geochemistry, zircon U-Pb dating and Hf isotopies composition of Paleozoic granitoids in Jinchuan, NW China: Constraints on their petrogenesis, source characteristics and tectonic implication

    NASA Astrophysics Data System (ADS)

    Zeng, Renyu; Lai, Jianqing; Mao, Xiancheng; Li, Bin; Ju, Peijiao; Tao, Shilong

    2016-05-01

    Granitoids are widely distributed in Jinchuan at the southwestern margin of the North China plate, which is also an important area of mineral deposits. The research subject of this article are two Paleozoic granitoids, a cataclastic syenogranite and a granodiorite porphyry. This study presents whole rock geochemistry and zircon U-Pb-Hf isotope data for the two granitoids to determine their petrogenesis, source characteristics and tectonic significance. The cataclastic syenogranite is characterized by metaluminous composition with high potassium, and LaN/YbN from 39 to 48. The composition with strong negative Eu anomalies and Zircon saturation temperatures (TZr) from 947 to 1072 °C classify this intrusion as an A-type granite. The granodiorite porphyry is metaluminous with high sodium, sub-alkaline, LaN/YbN ratios from 27 to 32. These I-type intrusions have no Eu anomalies and TZr ranges from 818 to 845 °C. Both the cataclastic syenogranite and granodiorite porphyry show enrichment of LREE and LILE and depletion of HREE and HFSE, except Hf and Zr. Using single zircon LA-ICP-MS U-Pb dating, the emplacement age of the cataclastic syenogranite and granodiorite porphyry are determined at 433.4 ± 3.7 Ma and 361.7 ± 4.6 Ma, respectively. Zircons from the cataclastic syenogranits have uniform negative εHf(t) values (-11 ± 0.5 to -9 ± 0.5), implying the involvement of an old Palaeoproterozoic crustal source in magma genesis. The zircons from the granodiorite porphyry have εHf(t) values that range from -8 ± 1.0 to +10 ± 0.6, suggesting heterogeneous source materials involving both juvenile and ancient crust reworked crustal components. Based on the geological significance of granites at the southwestern margin of the North China plate, the closure of the North Qilian Ocean occurred at ∼444 Ma. Geochemical features suggest that the cataclastic syenogranite and granodiorite porphyry formed in an intraplate extensional and compressional setting, respectively. Hence

  2. Temperature and Oxygen Isotope Composition of The Ediacaran Ocean: Constraints From Clumped Isotope Carbonate Thermometry

    NASA Astrophysics Data System (ADS)

    Bonifacie, M.; Eiler, J. M.; Fike, D. A.

    2008-12-01

    The temperature and chemical variations of the early oceans on Earth are highly debated, particularly for periods associated with significant evolutionary change and/or extinction. The temperature of past oceans has been estimated based on conventional carbonate-water and/or silicate-water stable oxygen isotope thermometry. Precambrian carbonates and silicates both exhibit a long-term secular trend of increasing δ18O values with decreasing age. This trend has been used to support two opposite - though related - interpretations: the Earth's oceans gradually cooled over the course of the Proterozoic eon, from a maximum of ~ 60-90°C at ~ 2.5Ga (and were, on average, relatively warm during much of the Paleozoic era) [1]. This interpretation has been supported by Si-isotope proxies and the thermal tolerances of proteins in various classes of microbial organisms [2-3]. Alternatively, the δ18O value of the oceans has gradually increased through time [4-5], and mean Earth surface temperatures varied over a narrow range similar to modern conditions. In other terms, one either assumes an ocean of constant δ18O and infers that climate varied dramatically, or vise versa. Finally, it is possible that post- depositional processes (e.g., diagenesis, burial metamorphism, weathering) has modified the δ18O values of all or most Precambrian sedimentary carbonates and silicates, overprinting any paleoclimatic variations. Carbonate 'clumped isotope' thermometry provides a new way to independently test these hypotheses because it allows one to determine the apparent growth temperatures of carbonate minerals based on their abundances of 13C-18O bonds, as reflected by the 'Δ47' value of CO2 extracted by phosphoric acid digestion [6]. This method is thermodynamically based and independent of the δ18O of water from which the carbonate grew. We will report the initial results of measurements of 'Δ47 for a suite of carbonates from the Sultanate of Oman. This Ediacaran age (~ 635 to

  3. Oxygen isotope variations in granulite-grade iron formations: constraints on oxygen diffusion and retrograde isotopic exchange

    USGS Publications Warehouse

    Sharp, Z.D.; O'Neil, J.R.; Essene, E.J.

    1988-01-01

    The oxygen isotope ratios of various minerals were measured in a granulite-grade iron formation in the Wind River Range, Wyoming. Estimates of temperature and pressure for the terrane using well calibrated geothermometers and geobarometers are 730??50?? C and 5.5??0.5 kbar. The mineral constraints on fluid compositions in the iron formation during retrogression require either very CO2-rich fluids or no fluid at all. In the iron formation, isotopic temperature estimates from quartz-magnetite fractionations are controlled by the proximity to the enclosing granitic gneiss, and range from 500?? C (??qz - mt=10.0???) within 2-3 meters of the orthogneiss contact to 600?? C (??qz - mt=8.0???) farther from the contact. Temperature estimates from other isotopic thermometers are in good agreement with those derived from the quartz-magnetite fractionations. During prograde metamorphism, the isotopic composition of the iron formation was lowered by the infiltration of an external fluid. Equilibrium was achieved over tens of meters. Closed-system retrograde exchange is consistent with the nearly constant whole-rock ??18Owr value of 8.0??0.6???. The greater ??qz-mt values in the iron formation near the orthogneiss contact are most likely due to a lower oxygen blocking temperature related to greater exchange-ability of deformed minerals at the contact. Cooling rates required to preserve the quartz-magnetite fractionations in the central portion of the iron formation are unreasonably high (???800?? C/Ma). In order to preserve the 600?? C isotopic temperature, the diffusion coefficient D (for ??-quartz) should be two orders of magnitude lower than the experimentally determined value of 2.5??10-16 cm2/s at 833 K. There are no values for the activation energy (Q) and pre-exponential diffusion coefficient (D0), consistent with the experimentally determined values, that will result in reasonable cooling rates for the Wind River iron formation. The discrepancy between the diffusion

  4. Complexity of In-situ zircon U-Pb-Hf isotope systematics during arc magma genesis at the roots of a Cretaceous arc, Fiordland, New Zealand

    NASA Astrophysics Data System (ADS)

    Milan, L. A.; Daczko, N. R.; Clarke, G. L.; Allibone, A. H.

    2016-11-01

    Zircons from seventeen samples of Western Fiordland Orthogneiss (WFO) diorites and three samples of country rock (two schists and one Darran Suite diorite) from the lowermost exposed sections of the Median Batholith, Fiordland, New Zealand, were analysed for in-situ U-Pb and Hf-isotopes. The WFO represents the deeper levels of Early Cretaceous continental arc magmatism on the Pacific margin of Gondwana, marking the final stage of long-lived arc magmatism on the margin spanning the Palaeozoic. The WFO plutons were emplaced at high-P (mid to deep crust at c. 8-12 kbar) between 124 and 114 Ma. Minor very high-P (c. 18 kbar) WFO eclogite and omphacite granulite facies orthogneiss (Breaksea Orthogneiss) are inferred to have crystallised in the base of thickened crust at c. 124 Ma. Zircons from the Breaksea Orthogneiss are considered to be variably affected by Pb-loss due to emplacement of the adjacent (Malaspina) Pluton at c. 114 Ma. By identifying Pb-loss, magmatic ages were able to be inferred in respect to apparent Pb-loss ages. Hf isotope data for the WFO define an excursion to less radiogenic Hf isotope ratios with time, reflecting increased recycling of an old source component. Peaks at c. 555, 770 and 2480 Ma, determine the age spectra of inherited populations of zircons within the WFO. This contrasts with detrital zircon patterns in country rocks of the Takaka terrane, which include peaks at c. 465 Ma, and 1250-900 Ma that are absent in the WFO inheritance pattern. These results indicate a previously unrecognised Precambrian lower crustal component of New Zealand. Recycling of this lower crust became increasingly important as a source for the final stage or Mesozoic arc magmatism along this segment of the palaeo-Pacific margin of Gondwana.

  5. Laser Ablation Split Stream (LASS) U-Pb & Lu-Hf Isotope Analysis of Detrital Zircons from the Old Red Sandstone, NW Svalbard: Implications for Northern Caledonian Paleogeography

    NASA Astrophysics Data System (ADS)

    Beranek, L. P.; Gee, D. G.; Fisher, C. M.

    2015-12-01

    The Svalbard archipelago consists of three Caledonian provinces that were assembled by thrusting and transcurrent faulting during the Silurian and Devonian in a location directly northeast of the Greenland Caledonides. Syn- to post-orogenic alluvial strata, referred to as the Old Red Sandstones, filled pull-apart basins adjacent to the transcurrent faults and comprise cover assemblages that help constrain the timing of the Caledonian orogeny. To further investigate the tectonic history and paleogeography of the Raudfjorden-Liefdefjorden-Woodfjorden area of Spitsbergen, NW Svalbard, we analyzed rock samples of the Old Red Sandstones and underlying Precambrian basement complexes for detrital zircon analysis. Laboratory studies of the Old Red Sandstones include the novel Laser Ablation Split Stream (LASS) technique, which allows for simultaneous U-Pb & Lu-Hf isotope analysis of zircon crystals. Lower Devonian Red Bay Group strata contain a range of early Neoproterozoic to Neoarchean detrital zircons with prominent age peaks c. 960, 1050, 1370, 1450, 1650, and 2700 Ma; subordinate Ordovician (c. 460-490 Ma) and Cryogenian (c. 650 Ma) detrital zircons occur in a subset of the samples. Underlying Precambrian metasedimentary rocks are composed of similar earliest Neoproterozoic to Neoarchean age populations, which argues for much of the Red Bay Group to be derived from local basement rocks during thrusting and other faulting. The U-Pb ages and Hf isotope compositions of Paleozoic to Neoarchean detrital zircons are consistent with Arctic crustal evolution, and support the hypothesis that northwestern and northeastern provinces of the Svalbard Caledonides are extruded fragments of the northeast Greenland allochthons. The new Hf isotope results further allow paleogeographic and stratigraphic comparisons with rock assemblages proximal to the North Atlantic Caledonides during the Silurian-Devonian, including the Pearya terrane of Ellesmere Island, Alexander terrane of NW

  6. Geochronology, geochemistry, and Sr-Nd-Hf isotopes of the early Paleozoic igneous rocks in the Duobaoshan area, NE China, and their geological significance

    NASA Astrophysics Data System (ADS)

    Wu, Guang; Chen, Yuchuan; Sun, Fengyue; Liu, Jun; Wang, Guorui; Xu, Bei

    2015-01-01

    The Duobaoshan area of northwestern Heilongjiang Province is the most important copper resource concentration region in NE China. To date, the Duobaoshan superlarge Cu-Mo deposit and the Tongshan large Cu-Mo deposit have been discovered in the Duobaoshan area. Both the deposits are hosted by granodiorites and volcanic rocks. Zircon LA-ICP-MS U-Pb dating indicates that these granodiorites emplaced approximately 479 Ma ago and that those volcanic rocks erupted between 447 and 450 Ma. The early Ordovicain granodiorites belong to the high-K to medium-K calc-alkaline series and are characterized by high Al2O3 and Sr contents, low Yb and Y contents, and relatively low Mg# values and Na2O/K2O ratios, with positive Eu or slight negative Eu anomalies (averaging 1.18). All of these geochemical characters are similar to those of the adakites generated by partial melting of a thickened lower crust in the world. Moreover, the granodiorites have low initial 87Sr/86Sr ratios (varying from 0.703474 to 0.704436), very high zircon εHf(t) and whole-rock εNd(t) values (varying from 13.0 to 16.8 and 5.27 to 5.46, respectively), and young zircon Hf and whole-rock Nd single-stage and two-stage model ages. Taking these geochemical characteristics and Sr-Nd-Hf isotope compositions together, we suggest that the early Ordovician granodiorites in the Duobaoshan area occurred in a post-collision environment and were formed by partial melting of a juvenile thickened lower crust dominated by depleted mantle-derived material. These late Ordovician volcanic rocks, which are composed of basalt, basaltic andesite, and andesite, belong to the tholeiitic or calc-alkaline series. They are generally enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs; e.g., Nb, Ta, Zr, Hf, P, and Ti), consistent with the geochemistry of igneous rocks from island arcs or active continental margins. Compared with the early Ordovician granodiorites, these volcanic rocks

  7. Coupled Iron and Sulfur Isotope Constraints on the Archean and Paleoproterozoic Ocean Redox State

    NASA Astrophysics Data System (ADS)

    Rouxel, O. J.; Bekker, A.

    2009-05-01

    The rise of atmospheric oxygen level by ca. 2.3 Ga have led to dramatic shifts in the iron and sulfur oceanic cycles. Past studies of non-mass dependent and mass dependent sulfur isotope record in sedimentary sulfides over geological time have placed important constraints on biogeochemical cycle of sulfur and evolution of Precambrian ocean chemistry. Recently, we applied a similar time-record approach to explore potential changes in Fe isotope composition of pyrite in black shales. Although the underlying mechanisms for Fe isotope fractionation in organic-rich sediments are debated, we identified direct link between the rise of atmospheric oxygen and changes in the Fe ocean cycle suggesting that Fe isotopes are useful proxies to the past ocean redox state. Since biogeochemical cycles of Fe and S are closely coupled in marine systems, Fe-limitation and S-limitation for pyrite formation in black shales should leave imprint on the isotopic record of both elements. Coupled Fe and S isotope systematics of Devonian pyrite display a range of 50‰ in δ34S values whereas δ56Fe values vary between - 1.0 and +0.1‰ consistent with Fe isotope variations in modern marine sediments. Similarly, pyrite in the 1.88 Ga Gunflint Formation has δ34S values ranging from - 32‰ to +10‰ and displays a range of δ56Fe values between 0 to - 0.4‰. In contrast, Archean black shales (e.g. Manjeri Fm., Belingwe Belt and Jeerinah Fm., Hamersley Basin) display a smaller range of δ34S values between together with ubiquitous non-mass dependent S-isotope fractionation but a larger range of δ56Fe values from - 3.5 to +0.2‰. A transitional period between ca. 2.3 and ca. 1.8 Ga is marked by a larger spread of δ34S values from - 34 to +28‰, disappearance of MIF and a larger range of δ56Fe values from - 1.7 to +1.1‰. These results confirm that after the rise of atmospheric oxygen by ca. 2.3 Ga, Paleoproterozoic ocean became stratified and gradually affected by an increase of seawater

  8. Peri-Amazonian provenance of the Proto-Pelagonian basement (Greece), from zircon U-Pb geochronology and Lu-Hf isotopic geochemistry

    NASA Astrophysics Data System (ADS)

    Zlatkin, Olga; Avigad, Dov; Gerdes, Axel

    2014-01-01

    The basement of the Pelagonian zone of the Hellenides, in the Eastern Mediterranean realm, has been shaped by mid-Neoproterozoic (700 Ma) and Variscan (300 Ma) igneous activities. In the present study, detrital zircon U-Pb geochronology and Lu-Hf isotope geochemistry of ca. 700 Ma-aged granites and of pre-700 Ma metasediments from the Pelagonian zone allow a genuine perspective into the provenance and origin of this terrane which hosts one of the oldest sedimentary sequences known in SE Europe. Pelagonian crustal vestiges comprising 700 Ma granitoids and their hosting metasediments are termed here "Proto-Pelagonian".

  9. The U-Pb, Hf and O isotopic record of ancient detrital zircons in Zimbabwean sediments - formation, reworking and nature of early Archaean crust

    NASA Astrophysics Data System (ADS)

    Bolhar, Robert; Hofmann, Axel; Kemp, Anthony I. S.; Whitehouse, Martin J.; Wind, Sandra; Feng, Yuexing

    2014-05-01

    Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon from different Archaean sedimentary successions belonging to the 2.9-2.8 Ga Belingwean/Bulawayan Groups and undated Sebakwian Group are presented to better define the crustal evolution of the Zimbabwe Craton prior to 3.0 Ga. Textural and compositional criteria were employed to minimize effects arising from Pb loss, metamorphic overprinting and interaction with low temperature fluids. 207Pb/206Pb age spectra (concordance > 90%) reveal prominent peaks at 3.8, 3.6, 3.5, and 3.35 Ga, corresponding to documented geological events both globally and within the Zimbabwe craton. O isotope compositions of ~ 4 - 10 opoint to both derivation from magmas in equilibrium with mantle and the assimilation of supracrustal material or interaction with metamorphic fluids. In ɛHf-time space, 3.8-3.6 Ga grains define an array consistent with derivation from a mafic to intermediate source reservoir (Lu/Hf ~0.015) that separated from chondritic mantle at ~ 3.9 Ga. Crustal domains formed after 3.6 Ga depict a more complex evolution, involving contribution from juvenile mantle sources and reworking of pre-existing crust. Importantly, initial Hf isotopic compositions document a protracted history of remelting, without evidence for significant mantle depletion prior to 3.35 Ga. This suggests that production of earliest crust in the Zimbabwe Craton did not cause complementary enriched and depleted reservoirs, possibly because heterogeneous mantle was effectively remixed by rapid convection due to higher temperatures in the early Archaean or the volume of crust was too small in volume to influence the isotopic mantle evolution. Similar Hf-O-time relationships observed in southern West Greenland were used as a basis to propose a transition in geodynamics 3.2 Ga ago. The absence of detrital zircons with crystallization ages > 3.8 Ga, along with a simple ɛHf-time array consistent with reworking of a mafic protolith

  10. The Palaeocene-Eocene carbon isotope excursion: constraints from individual shell planktonic foraminifer records.

    PubMed

    Zachos, James C; Bohaty, Steven M; John, Cedric M; McCarren, Heather; Kelly, Daniel C; Nielsen, Tina

    2007-07-15

    The Palaeocene-Eocene thermal maximum (PETM) is characterized by a global negative carbon isotope excursion (CIE) and widespread dissolution of seafloor carbonate sediments. The latter feature supports the hypothesis that the PETM and CIE were caused by the rapid release of a large mass (greater than 2000Gt C) of 12C-enriched carbon. The source of this carbon, however, remains a mystery. Possible sources include volcanically driven thermal combustion of organic-rich sediment, dissociation of seafloor methane hydrates and desiccation and oxidation of soil/sediment organics. A key constraint on the source(s) is the rate at which the carbon was released. Fast rates would be consistent with a catastrophic event, e.g. massive methane hydrate dissociation, whereas slower rates might implicate other processes. The PETM carbon flux is currently constrained by high-resolution marine and terrestrial records of the CIE. In pelagic bulk carbonate records, the onset of the CIE is often expressed as a single- or multiple-step excursion extending over 10(4) years. Individual planktonic shell records, in contrast, always show a single-step CIE, with either pre-excursion or excursion isotope values, but no transition values. Benthic foraminifera records, which are less complete owing to extinction and diminutive assemblages, show a delayed excursion. Here, we compile and evaluate the individual planktonic shell isotope data from several localities. We find that the most expanded records consistently show a bimodal isotope distribution pattern regardless of location, water depth or depositional facies. This suggests one of several possibilities: (i) the isotopic composition of the surface ocean/atmosphere declined in a geologic instant (<500yr), (ii) that during the onset of the CIE, most shells of mixed-layer planktonic foraminifera were dissolved, or (iii) the abundances or shell production of these species temporarily declined, possibly due to initial pH changes.

  11. The Palaeocene-Eocene carbon isotope excursion: constraints from individual shell planktonic foraminifer records.

    PubMed

    Zachos, James C; Bohaty, Steven M; John, Cedric M; McCarren, Heather; Kelly, Daniel C; Nielsen, Tina

    2007-07-15

    The Palaeocene-Eocene thermal maximum (PETM) is characterized by a global negative carbon isotope excursion (CIE) and widespread dissolution of seafloor carbonate sediments. The latter feature supports the hypothesis that the PETM and CIE were caused by the rapid release of a large mass (greater than 2000Gt C) of 12C-enriched carbon. The source of this carbon, however, remains a mystery. Possible sources include volcanically driven thermal combustion of organic-rich sediment, dissociation of seafloor methane hydrates and desiccation and oxidation of soil/sediment organics. A key constraint on the source(s) is the rate at which the carbon was released. Fast rates would be consistent with a catastrophic event, e.g. massive methane hydrate dissociation, whereas slower rates might implicate other processes. The PETM carbon flux is currently constrained by high-resolution marine and terrestrial records of the CIE. In pelagic bulk carbonate records, the onset of the CIE is often expressed as a single- or multiple-step excursion extending over 10(4) years. Individual planktonic shell records, in contrast, always show a single-step CIE, with either pre-excursion or excursion isotope values, but no transition values. Benthic foraminifera records, which are less complete owing to extinction and diminutive assemblages, show a delayed excursion. Here, we compile and evaluate the individual planktonic shell isotope data from several localities. We find that the most expanded records consistently show a bimodal isotope distribution pattern regardless of location, water depth or depositional facies. This suggests one of several possibilities: (i) the isotopic composition of the surface ocean/atmosphere declined in a geologic instant (<500yr), (ii) that during the onset of the CIE, most shells of mixed-layer planktonic foraminifera were dissolved, or (iii) the abundances or shell production of these species temporarily declined, possibly due to initial pH changes. PMID:17513259

  12. "Taconic" arc magmatism in the central Brooks Range, Alaska: New U-Pb zircon geochronology and Hf isotopic data from the lower Paleozoic Apoon assemblage of the Doonerak fenster

    NASA Astrophysics Data System (ADS)

    Strauss, J. V.; Hoiland, C. W.; Ward, W.; Johnson, B.; McClelland, W.

    2015-12-01

    The Doonerak fenster in the central Brooks Range, AK, exposes an important package of early Paleozoic volcanic and sedimentary rocks called the Apoon assemblage, which are generally interpreted as para-autochthonous basement to the Mesozoic-Cenozoic Brookian fold-thrust belt. Recognition in the 1970's of a major pre-Mississippian unconformity within the window led to correlations between Doonerak and the North Slope (sub-) terrane of the Arctic Alaska Chukotka microplate (AACM); however, the presence of arc-affinity volcanism and the apparent lack of pre-Mississippian deformation in the Apoon assemblage makes this link tenuous and complicates Paleozoic tectonic reconstructions of the AACM. Previous age constraints on the Apoon assemblage are limited to a handful of Middle Cambrian-Silurian paleontological collections and five K-Ar and 40Ar/39Ar hornblende ages from mafic dikes ranging from ~380-520 Ma. We conducted U-Pb geochronologic and Hf isotopic analyses on igneous and sedimentary zircon from the Apoon assemblage to test Paleozoic links with the North Slope and to assess the tectonic and paleogeographic setting of the Doonerak region. U-Pb analyses on detrital zircon from Apoon rocks yield a spectrum of unimodal and polymodal age populations, including prominent age groups of ca. 420-490, 960-1250, 1380­-1500, 1750-1945, and 2650-2830 Ma. Hf isotopic data from the ca. 410-490 Ma age population are generally juvenile (~7-10 ɛHf), implying a distinct lack of crustal assimilation during Ordovician-Silurian Doonerak arc magmatism despite its proximity to a cratonic source terrane as indicated by an abundance of Archean and Proterozoic zircon in the interbedded siliciclastic strata. These data are in stark contrast to geochronological data from the non-Laurentian portions of the AACM, highlighting a prominent tectonic boundary between Laurentian- and Baltic-affinity rocks at the Doonerak window and implying a link to "Taconic"-age arc magmatism documented along

  13. Zircon U-Pb and K-feldspar megacryst Rb-Sr isotopic ages and Sr-Hf isotopic composition of the Mesozoic Heyu pluton, eastern Qingling orogen, China

    NASA Astrophysics Data System (ADS)

    Zhu, Xi-Yan; Chen, Fukun; Liu, Bing-Xiang; Siebel, Wolfgang

    2013-01-01

    Mesozoic granitoids are ubiquitous in China and have attracted the interests of many geologists not only because of their mineralization potential but also because they carry information on source material, emplacement mechanisms and crustal architecture. Most of the plutons were created by multi-stage magma emplacement and offer particular challenge for unraveling the emplacement and cooling history. A multi-isotope approach combined with single grain dating analyses provides a means to understand magma geneses even in complex magmatic settings. This study presents Rb-Sr isotope data for K-feldspar megacrysts and U-Pb ages and Hf isotopic composition for zircons from the composite Heyu pluton exposed in eastern Qinling orogen, central China. Zircon U-Pb concordia ages of 150 and 140 Ma and single K-feldspar Rb-Sr isochron ages between 133 and 121 Ma were obtained from early and late magmatic stages, respectively, consistent with slow cooling or isotopic re-equilibration of the Rb-Sr K-feldspar system. The distinct K-feldspar morphology of early and late intrusive phases is controlled largely by the emplacement depth, likely to be associated with a gravity-driven sinking model. Corresponding zircon ɛHf(t) values are - 21.8 to - 18.0 and - 17.2 to - 10.3 for early and late intrusive phases, respectively, indicating different magma sources.

  14. Stages of late Paleozoic to early Mesozoic magmatism in the Song Ma belt, NW Vietnam: evidence from zircon U-Pb geochronology and Hf isotope composition

    NASA Astrophysics Data System (ADS)

    Hieu, Pham Trung; Li, Shuang-Qing; Yu, Yang; Thanh, Ngo Xuan; Dung, Le Tien; Tu, Vu Le; Siebel, Wolfgang; Chen, Fukun

    2016-05-01

    The Song Ma zone in NW Vietnam bears important tectonic implications as a potential subduction corridor between the Indochina and South China blocks. On the basis of U-Pb ages, the Hf isotopic characteristics of zircons and the geochemical composition of granitoids, a two-stage magmatic evolution process of the Song Ma zone at ~290-260 and ~245-230 Ma can be proposed. Isotopic analyses indicate magmatic contributions from Neoproterozoic oceanic island basalt, Proterozoic continental crust, and depleted mantle or juvenile lithosphere. By combining geochronological and geochemical data from the granitoid rocks, we suggest that the staged magmatic processes of Song Ma zone may be related to a long-lasting period of ocean subduction (ca. 290-260 Ma) and subsequent syn-/post-collisional evolution (ca. 245-230 Ma).

  15. Geochemistry and zircon U-Pb ages and Hf isotopic composition of Permian alkali granitoids of the Phan Si Pan zone in northwestern Vietnam

    NASA Astrophysics Data System (ADS)

    Hiếu, Phạm Trung; Chen, Fu-kun; Thủy, Nguyễn Thị Bích; Cu'ò'ng, Nguyễn Quốc; Li, Shuang-quing

    2013-09-01

    The late Permian granitoids exposed in the Phan Si Pan zone of northwestern Vietnam consist mainly of the Ye Yen Sun metaluminous granites and the Nam Xe-Tam Duong peralkaline granites. Laser ablation inductively coupled plasma mass spectrometry U-Pb zircon analysis reveals that both the granite suites were emplaced from 253 Ma to 251 Ma. They have a distinctive A-type geochemistry of high 10,000 × Ga/Al ratios of 3.0-5.7 and are also characterized by elevated contents of high field strength elements, A/CNK values of 0.85-1.58, negative Eu-anomalies. Magmatic zircons from the granitoids exhibit positive initial ɛHf values ranging from 6.4 to 15.9 and yield single-stage depleted mantle Hf model ages of 257-663 Ma. This Hf isotopic feature implies significant contribution of juvenile mantle material to the magmas of the spatially and temporally associated Ye Yen Sun metaluminous and Nam Xe-Tam Duong peralkaline granites.

  16. Isotopic and trace element constraints on the origin and evolution of saline groundwaters from central Missouri

    SciTech Connect

    Banner, J.L. Louisiana State Univ., Baton Rouge ); Wasserburg, G.J.; Dobson, P.F. ); Carpenter, A.B. ); Moore, C.H. )

    1989-02-01

    Na-Ca-Cl groundwaters with salinities of 1 to 30{per thousand} discharge from natural springs and artesian wells in Mississippian carbonates and Ordovician sandstones and carbonates in central Missouri. Carbonate saturation and quartz supersaturation are maintained throughout the salinity range. Major and trace element and isotopic variations in the waters are used to place constraints on models for rock-water interaction and regional hydrology. The integration of geochemical, isotopic and hydrologic data on a local and regional scale suggests a history for the central Missouri groundwaters involving: (1) meteoric recharge in the Front Range of Colorado; (2) dissolution of Permian halite in the subsurface of Kansas; (3) interaction with predominantly silicate mineral assemblages in Paleozoic strata (and possibly Precambrian basement), with aquisition of crustal Sr and REE signatures; (4) dilution and migration to shallow aquifer levels in central Missouri; and (5) mixing with local meteoric recharge and discharge through Mississippian carbonates with no significant change of the isotopic signatures acquired in stage (3).

  17. Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field, Washington

    USGS Publications Warehouse

    Jicha, B.R.; Hart, G.L.; Johnson, C.M.; Hildreth, W.; Beard, B.L.; Shirey, S.B.; Valley, J.W.

    2009-01-01

    Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P)n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. ??18O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal

  18. Isotopic and trace element constraints on the petrogenesis of lavas from the Mount Adams volcanic field, Washington

    NASA Astrophysics Data System (ADS)

    Jicha, Brian R.; Hart, Garret L.; Johnson, Clark M.; Hildreth, Wes; Beard, Brian L.; Shirey, Steven B.; Valley, John W.

    2009-02-01

    Strontium, Nd, Pb, Hf, Os, and O isotope compositions for 30 Quaternary lava flows from the Mount Adams stratovolcano and its basaltic periphery in the Cascade arc, southern Washington, USA indicate a major component from intraplate mantle sources, a relatively small subduction component, and interaction with young mafic crust at depth. Major- and trace-element patterns for Mount Adams lavas are distinct from the rear-arc Simcoe volcanic field and other nearby volcanic centers in the Cascade arc such as Mount St. Helens. Radiogenic isotope (Sr, Nd, Pb, and Hf) compositions do not correlate with geochemical indicators of slab-fluids such as (Sr/P) n and Ba/Nb. Mass-balance modeling calculations, coupled with trace-element and isotopic data, indicate that although the mantle source for the calc-alkaline Adams basalts has been modified with a fluid derived from subducted sediment, the extent of modification is significantly less than what is documented in the southern Cascades. The isotopic and trace-element compositions of most Mount Adams lavas require the presence of enriched and depleted mantle sources, and based on volume-weighted chemical and isotopic compositions for Mount Adams lavas through time, an intraplate mantle source contributed the major magmatic mass of the system. Generation of basaltic andesites to dacites at Mount Adams occurred by assimilation and fractional crystallization in the lower crust, but wholesale crustal melting did not occur. Most lavas have Tb/Yb ratios that are significantly higher than those of MORB, which is consistent with partial melting of the mantle in the presence of residual garnet. δ 18O values for olivine phenocrysts in Mount Adams lavas are within the range of typical upper mantle peridotites, precluding involvement of upper crustal sedimentary material or accreted terrane during magma ascent. The restricted Nd and Hf isotope compositions of Mount Adams lavas indicate that these isotope systems are insensitive to crustal

  19. Similar crustal evolution in the western units of the Adrar Souttouf Massif (Moroccan Sahara) and the Avalonian terranes: Insights from Hf isotope data

    NASA Astrophysics Data System (ADS)

    Gärtner, Andreas; Villeneuve, Michel; Linnemann, Ulf; Gerdes, Axel; Youbi, Nasrrddine; Hofmann, Mandy

    2016-06-01

    The Adrar Souttouf Massif is located at the western margin of the West African Craton and consists of several NNE-SSW trending units. Of them, the two westernmost have been interpreted to be linked with the Avalonian terrane assemblage and Meguma, respectively. New Hf isotopic data corroborates the Avalon correlation but has no impact one way or another on the possible Meguma connection, as there is no Hf data available from the latter. The obtained pattern of εHf(t) values vs. zircon age of the likely Avalonia related Oued Togba unit is similar to published data from Avalonia. Zircons of this unit show characteristic patterns of crustal mixing at 0.7 to 1.3 Ga and 1.75 to 2.25 Ga, while juvenile crust was likely formed around 0.6 to 0.75 Ga, from 1.2 to 2.2 Ga, and between 2.5 and 3.2 Ga. The zircons of the Sebkha Gezmayet unit reveal crustal mixing for the entire Palaeozoic and Neoproterozoic, from 2.05 to 2.11 Ga, and 2.8 to 2.9 Ga. Juvenile crust formation is interpreted to have occurred from 0.5 to 0.7 Ga, at around 2.1 Ga, and at ca. 2.9 Ga. As Mesoproterozoic zircons are abundant in the likely Avalonia-like Oued Togba unit, but uncommon at the West African Craton, their origin has to be found elsewhere. A comparison of available Hf data from Amazonia and Baltica, the two potential source cratons of Avalonia, shows similarities but is hampered by the lack of available data from Amazonia. Finally, a few grains from both units have Eoarchaean model ages. Among similar grains from other peri-Gondwanan terranes, they give indication of partial recycling of Eoarchaean crust in the vicinity of the northwestern West African Craton.

  20. Isotopic and trace element constraints on the genesis of the Faeroe lava pile

    NASA Astrophysics Data System (ADS)

    Gariépy, Clément; Ludden, John; Brooks, Christopher

    1983-05-01

    Basaltic lavas from the Faeroe Islands form three stratigraphic series which define two geochemical groups. Both the lower and middle series are LREE enriched ((La/Yb) e.f.: 2-3) and are characterized by convex LREE profiles; in contrast, the upper series comprises both depleted ((La/Yb) e.f.: 0.45-0.6) and enriched lavas. This twofold geochemical division is also evident from the incompatible trace elements such as Zr, Nb, Hf and Ta and the compatible trace elements Cr, Ni, Sr and Y. Nd-Sr-Pb isotopic measurements show that the basalts are contaminated by crustal materials, implying the presence of Precambrian sialic basement underneath the Faeroes block, a conclusion supported by geophysical data [35,36]. The uncontaminated end-members, for the LREE-depleted basalts ( 87Sr/ 86Sr) 0 ˜ 0.7026 and ɛNd 0 + 10 and for the LREE-enriched basalts ( 87Sr/ 86Sr) 0 ˜ 0.7034 and ɛNd 0 + 9, require two different mantle source regions thus posing serious problems for petrogenetic models such as dynamic partial melting which have been proposed for the Faeroes. We interpret the LREE-depleted basalts as partial melts of the oceanic asthenosphere whilst the LREE-enriched basalts may result either from the partial melting of deep mantle blobs or of the subcontinental lithosphere during upwelling of the asthenosphere.

  1. Isotopic constraints on the age and early differentiation of the Earth.

    PubMed

    McCulloch, M T

    1996-03-01

    The Earth's age and early differentiation history are re-evaluated using updated isotopic constraints. From the most primitive terrestrial Pb isotopic compositions found at Isua Greenland, and the Pilbara of Western Australia, combined with precise geochronology of these localities, an age 4.49 +/- 0.02 Ga is obtained. This is interpreted as the mean age of core formation as U/Pb is fractionated due to sequestering of Pb into the Earth's core. The long-lived Rb-Sr isotopic system provides constraints on the time interval for the accretion of the Earth as Rb underwent significant depletion by volatile loss during accretion of the Earth or its precursor planetesimals. A primitive measured 87Sr/86Sr initial ratio of 0.700502 +/- 10 has been obtained for an early Archean (3.46 Ga) barite from the Pilbara Block of Western Australia. Using conservative models for the evolution of Rb/Sr in the early Archean mantle allows an estimate to be placed on the Earth's initial Sr ratio at approximately 4.50 Ga, of 0.69940 +/- 10. This is significantly higher than that measured for the Moon (0.69900 +/- 2) or in the achondrite, Angra dos Reis (0.69894 +/- 2) and for a Rb/Sr ratio of approximately 1/2 of chondrites corresponds to a mean age for accretion of the Earth of 4.48 + /- 0.04 Ga. The now extinct 146Sm-142Nd (T1/2(146)=103 l0(6)yrs) combined with the long-lived 147Sm-143Nd isotopic systematics can also be used to provide limits on the time of early differentiation of the Earth. High precision analyses of the oldest (3.8-3.9 Ga) Archean gneisses from Greenland (Amitsoq and Akilia gneisses), and Canada (Acasta gneiss) do not show measurable (> +/- l0ppm) variations of 142Nd, in contrast to the 33 ppm 142Nd excess reported for an Archean sample. The general lack of 142Nd variations, combined with the presence of highly positive epsilon 143 values (+4.0) at 3.9 Ga, indicates that the record of large-scale Sm/Nd fractionation events was not preserved in the early-Earth from 4

  2. Archaean fluid-assisted crustal cannibalism recorded by low δ18O and negative ɛHf(T) isotopic signatures of West Greenland granite zircon

    NASA Astrophysics Data System (ADS)

    Hiess, Joe; Bennett, Vickie C.; Nutman, Allen P.; Williams, Ian S.

    2011-06-01

    The role of fluids during Archaean intra-crustal magmatism has been investigated via integrated SHRIMP U-Pb, δ18O and LA-MC-ICPMS 176Hf isotopic zircon analysis. Six rock samples studied are all from the Nuuk region (southern West Greenland) including two ~3.69 Ga granitic and trondhjemitic gneisses, a 3.64 Ga granitic augen gneiss, a 2.82 Ga granodioritic Ikkattoq gneiss, a migmatite with late Neoarchaean neosome and a homogeneous granite of the 2.56 Ga Qôrqut Granite Complex (QGC). All zircon grains were thoroughly imaged to facilitate analysis of magmatic growth domains. Within the zircon analysed, there is no evidence for metamictization. Initial ɛHf zircon values ( n = 63) are largely sub-chondritic, indicating the granitic host magmas were generated by the remelting of older, un-radiogenic crustal components. Zircon from some granite samples displays more than one 207Pb/206Pb age, and correlated with 176Hf/177Hf compositions can trace multiple phases of remelting or recrystallization during the Archaean. Model ages calculated using Lu/Hf arrays for each sample indicate that the crustal parental rocks to the granites, granodiorites and trondhjemites segregated from a chondrite-like reservoir at an earlier time during the Archaean, corresponding to known formation periods of more primitive tonalite-trondhjemite-granodiorite (TTG) gneisses. Zircon from the ~3.69 Ga granite, the migmatite and QGC granite contains Eoarchaean cores with chondritic 176Hf/177Hf and mantle-like δ18O compositions. The age and geochemical signatures from these inherited components are identical to those of surrounding tonalitic gneisses, further suggesting genesis of these granites by remelting of broadly tonalitic protoliths. Zircon oxygen isotopic compositions ( n = 62) over nine age populations (six igneous and three inherited) have weighted mean or mean δ18O values ranging from 5.8 ± 0.6 to 3.7 ± 0.5‰. The 3.64 Ga granitic augen gneiss sample displays the highest δ18O with

  3. Rhenium-osmium-isotope constraints on the age of iron meteorites

    NASA Technical Reports Server (NTRS)

    Horan, M. F.; Morgan, J. W.; Walker, R. J.; Grossman, J. N.

    1992-01-01

    Rhenium and osmium concentrations and the osmium isotopic compositions of iron meteorites were determined by negative thermal ionization mass spectrometry. Data for the IIA iron meteorites define an isochron with an uncertainty of approximately +/-31 million years for meteorites about 4500 million years old. Although an absolute rhenium-osmium closure age for this iron group cannot be as precisely constrained because of uncertainty in the decay constant of Re-187, an age of 4460 million years ago is the minimum permitted by combined uncertainties. These age constraints imply that the parent body of the IIAB magmatic irons melted and subsequently cooled within 100 million years after the formation of the oldest portions of chondrites. Other iron meteorites plot above the IIA isochron, indicating that the planetary bodies represented by these iron groups may have cooled significantly later than the parent body of the IIA irons.

  4. Rhenium-osmium isotope constraints on the age of iron meteorites

    USGS Publications Warehouse

    Horan, M.F.; Morgan, J.W.; Walker, R.J.; Grossman, J.N.

    1992-01-01

    Rhenium and osmium concentrations and the osmium isotopic compositions of iron meteorites were determined by negative thermal ionization mass spectrometry. Data for the IIA iron meteorites define an isochron with an uncertainty of approximately ??31 million years for meteorites ???4500 million years old. Although an absolute rhenium-osmium closure age for this iron group cannot be as precisely constrained because of uncertainty in the decay constant of 187Re, an age of 4460 million years ago is the minimum permitted by combined uncertainties. These age constraints imply that the parent body of the IIAB magmatic irons melted and subsequently cooled within 100 million years after the formation of the oldest portions of chondrites. Other iron meteorites plot above the IIA isochron, indicating that the planetary bodies represented by these iron groups may have cooled significantly later than the parent body of the IIA irons.

  5. Isotopic and ion analysis of erupting Lusi water for constraints on numerical models

    NASA Astrophysics Data System (ADS)

    Faubert, Maïté; Sohrabi, Reza; Mauri, Guillaume; Mazzini, Adriano; Miller, Stephen

    2016-04-01

    The LUSI mud eruption, in the Sidoarjo district, East Java, Indonesia, has been continuously erupting great amounts of material for ten years. From a hydrogeological point of view, the hypothesis that this is a newly born deep hydrothermal system is supported by geochemistry, thermal properties, and its geyser-like behavior. The present work investigates the configuration of this hydrogeological system through hydro-chemical analysis of the erupting fluids, and to establish constraints on numerical model parameters. We used two different radioactive isotope dating methods (δ14C and δ3H) to constrain travel time from inflow to outflow, and major ion analyses to determine water-type from LUSI. We also measured δ2H and δ18O to determine the source of the water. Additionally, it has been reported that significant amounts of Li is found in the erupting fluid. Result of δ14C provides ages in the range of 16ka, and ion analyses show the water is of the Na-Cl type, typical for hydrothermal volcanic fluids. However, typical volcanic fluids have high K, and the low K that we measured in the LUSI erupting waters could result from K-consumption associated with smectite-illite metamorphism (e.g. dehydration) of the Upper Kalibeng formation. The quantity of Li reinforces the volcanic source hypothesis, while the stable isotope results show that the water feeding the erupting system is a combination of formation dehydration, magmatic origin, and mixed with some meteoric water. We propose that the erupting water originates from deep strata, likely below the carbonate formation at a depth of > 4 km deep. The carbonate formation provides the necessary permeability to feed the substantial outflow observed at the surface. The Arjuno-Welirang volcanic complex, situated at ~20 km from LUSI, offers the necessary hydraulic gradient to drive the eruption. These parameters provide constraints on numerical models that we are developing to understand LUSI's deep hydrodynamic

  6. Molybdenite Re-Os, zircon U-Pb dating and Lu-Hf isotopic analysis of the Xiaerchulu Au deposit, Inner Mongolia Province, China

    NASA Astrophysics Data System (ADS)

    Wang, Jia-xin; Nie, Feng-Jun; Zhang, Xue-ni; Jiang, Si-hong

    2016-09-01

    The Xiaerchulu Au deposit, located in the Southern Orogenic Belt (SOB) of Western Inner Mongolia (WIM), is hosted in an Early Permian (271-261 Ma) volcanic-plutonic sequence. Mineralization took place in silicified biotite granites or along the contact zone between the Neoproterozoic Baiyinbaolage Group and the biotite granite. In order to constrain the timing of the Xiaerchulu mineralization and discuss the petrogenesis of the hosting granites, molybdenite Re-Os, and zircon U-Pb and, Lu-Hf, and REE, geochemical, and Sr-Nd isotopic studies were completed in this study. We measured Re-Os isotopes of six molybdenite samples from the main ore body, which yielded a weighted average model age of 261.7 ± 1.5 Ma with a MSWD of 0.55, indicating that the time of mineralization was at ca. 262 Ma. High precision U-Pb dating for the studied granites yields Permian 206Pb/238U ages ranging from 271 to 269 Ma. These age data confirm that both the intrusion and related mineralization were initiated in Early Permian period. These granites are strongly peraluminous with A/CNK = 1.11-1.12, high SiO2-K2O contents, as well as containing biotite and muscovite, indicating a petrogenesis of typical S-type granites, the above consideration is also consistent with the result of discrimination diagrams. The Re contents of molybdenite, εNd(t), and zircon εHf(t), as well as the 176Hf/177Hf values of the granites, fall into the ranges from 1.153 to 2.740 μg/g, - 11.1 to - 9.3, - 8.8 to - 0.9, and 0.282358 to 0.282688, respectively. All of this evidence suggests that the metals were derived from a predominantly crustal source, the granites originated from crust in an extensional setting, and the rejuvenation of the continent may have play an important role during the ore-forming processes of the Early Permian epoch.

  7. Multiple S isotopes, zircon Hf isotopes, whole-rock Sr-Nd isotopes, and spatial variations of PGE tenors in the Jinchuan Ni-Cu-PGE deposit, NW China

    NASA Astrophysics Data System (ADS)

    Duan, Jun; Li, Chusi; Qian, Zhuangzhi; Jiao, Jiangang; Ripley, Edward M.; Feng, Yanqing

    2016-04-01

    Previous geochemical data for the Jinchuan Ni-Cu-(platinum-group elements, PGE) deposit, the single largest magmatic sulfide deposit in the world, are derived primarily from the upper parts of the deposit. This paper reports new PGE and S-Hf-Sr-Nd isotope data for the lower parts of the deposit that have become accessible for sampling by ongoing underground mining activity. New PGE data from this study, together with previous results, indicate that PGE tenors in the bulk sulfide ores of the deposit increase eastward, except for two fault-offset ore zones which occur together within the western part of the deposit. Generally, these two ore zones show depletions in IPGE (Ir, Ru, Rh) but not in PPGE (Pt, Pd) and Cu, and more fractionated olivine and Cr-spinel compositions than the rest of the deposit. These differences can be explained by a more evolved parental magma for the IPGE-depleted ore zones. The eastward increase of PGE tenors in the rest of the deposit can be explained by upgrading of preexisting sulfide liquid in a subhorizontal conduit by a new surge of magma moving through the conduit from west to east, which took place before the formation of the IPGE-depleted ore zones. The Jinchuan ultramafic rocks are characterized by elevated initial 87Sr/86Sr ratios from 0.7077 to 0.7093, negative ɛ Nd values from -9.2 to -10.5, and zircon ɛ Hf values from -4 to -7. These data indicate up to 20 % of crustal contamination in the Jinchuan magma. Four of nine multiple sulfur isotope analyses for the Jinchuan deposit show anomalous ∆33S values varying from 0.12 to 2.67 ‰. These results, together with elevated δ34S values (>2 ‰) for some of the samples analyzed previously by other researchers, indicate the involvement of external sulfur from Archean and Proterozoic sedimentary rocks. Modeling results based on our olivine data and magma compositions estimated previously by other researchers indicate that fractional crystallization did not play a major role in

  8. Radiogenic Isotope Constraints on Fluid Sources in the Yellowstone Hydrothermal System

    NASA Astrophysics Data System (ADS)

    Scott, S. R.; Sims, K. W. W.; Role, A.; Shock, E.; Boyd, E. S.

    2015-12-01

    For decades, researchers in Yellowstone National Park (YNP) have used major and trace element and light stable isotope geochemistry to evaluate fluid sources and geochemical reactions in the Yellowstone hydrothermal system. However, the results can be affected by mixing, boiling and vapor-phase separation. We present new strontium (Sr), neodymium (Nd), and lead (Pb) isotopic data from hydrothermal waters and fumarole condensates that allow us to evaluate fluid sources independent of near-surface mixing and boiling. Our sample set was selected to explore the range of fluid compositions found in the Yellowstone hydrothermal system, including waters/fluids that are thought to be exclusively meteoric, exclusively from the deep hydrothermal system, and those which are a mixture of these end members and/or that have been influenced by various hydrothermal processes such as boiling or gas/water interaction. We have identified at least three isotopic endmembers that persist in various features throughout the YNP hydrothermal system. The first endmember has relatively unradiogenic Pb with Sr, Nd, and Pb isotopic compositions that are consistent with Yellowstone basalts and rhyolites. This endmember is typified by low pH features. We interpret this fluid as surface water and shallow groundwater that has interacted with volcanic rocks associated with the YNP magmatic system, with the acidity derived from oxidation of volcanic gases. The second endmember has relatively radiogenic Pb, radiogenic Sr, and unradiogenic Nd. This endmember is typified by neutral pH features and near neutral fumarole condensates. We interpret this endmember to represent the hypothesized deep hydrothermal reservoir that interacts with and reflects the isotopic composition of the host rock. The third endmember contains radiogenic Pb, unradiogenic Nd, and unradiogenic Sr. We observe this endmember in neutral features, which are interpreted as hydrothermal waters (shallow, deep, or mixtures) that have

  9. Isotopic constraints on open system evolution of the Laacher See magma chamber (Eifel, West Germany)

    NASA Astrophysics Data System (ADS)

    Wörner, G.; Staudigel, H.; Zindler, A.

    1985-09-01

    The Laacher See phonolite tephra sequence (11,000 years B.P.) of the Quaternary East Eifel volcanic field (West Germany) represents an inverted, chemically zoned magma column. Mafic and differentiated phonolites, respectively, represent the lowermost and uppermost erupted portion of the Laacher See magma chamber. Sr and Nd isotopic compositions of whole rocks, matrices and phenocrysts have been analyzed in order to provide constraints for open versus closed system evolution of the Laacher See magma chamber. 87Sr/ 86Sr isotope ratios of mafic phonolites and their phenocrysts are slightly more radiogenic than parental East Eifel basanite magmas. Bulk rock samples show a drastic increase in 87Sr/ 86Sr from mafic towards the most differentiated compositions that were erupted from the top of the magma chamber. Glass matrix separates show a parallel, but less pronounced, increase in 87Sr/ 86Sr . Phenocrysts, in contrast, show a narrow range in 87Sr/ 86Sr with a slight, but significant, increase towards the top of the magma chamber. Phenocrysts from the uppermost portion of the magma column were not in isotopic (or chemical) equilibrium with their host matrices. 143Nd/ 144Nd isotope ratios for whole rocks, matrices, and phenocrysts fall within a restricted range similar to that of East Eifel mafic magmas. A representative suite of crustal rocks (lower crustal granulites, quartzo-feldspathic gneisses, mica schists, Devonian slates and graywacke) was also analyzed in order to permit an evaluation of possible assimilation models. Our results are consistent with chemical evolution of the zoned Laacher See magma chamber mainly through crystal fractionation accompanied by minor amounts of assimilation. Slight contamination of the magma system may have involved (a) the assimilation of gneisses (?) and mica schists during the initial stage of magma chamber evolution (basanite-mafic phonolite), (b) combined assimilation-fractional crystallization (AFC) concurrent with the second

  10. Chemical and Isotopic Constraints on the Origin of Cenozoic Pacific Northwest Volcanism

    NASA Astrophysics Data System (ADS)

    Carlson, R. W.; Hart, W. K.; Grove, T. L.; Donnelly-Nolan, J. M.; Barr, J. A.; Till, C. B.

    2009-12-01

    variation and overlapping Pb isotope composition of most of the flood basalts and HLP volcanism, both the Saddle Mountains unit of the Columbia River basalts and most Snake River Plain basalts have Pb isotope compositions strongly displaced from values seen in oceanic basalts plotting instead along a circa 2.5 Ga trend. The shift in Pb (and Sr, Nd, and Hf) isotope composition occurs precisely at the boundary between young accreted terranes to the west and Precambrian North America to the east, suggesting that ancient continental lithospheric mantle is an important source component of Snake River basaltic volcanism. At precisely the same location, however, 4He/3He shifts to low values in Snake River Plain basalts (Graham et al., JVGR, 2009), which is the only chemical and isotopic characteristic of Snake River Plain basalts that suggests input from the deep mantle.

  11. When the dust settles: stable xenon isotope constraints on the formation of nuclear fallout.

    PubMed

    Cassata, W S; Prussin, S G; Knight, K B; Hutcheon, I D; Isselhardt, B H; Renne, P R

    2014-11-01

    Nuclear weapons represent one of the most immediate threats of mass destruction. In the event that a procured or developed nuclear weapon is detonated in a populated metropolitan area, timely and accurate nuclear forensic analysis and fallout modeling would be needed to support attribution efforts and hazard assessments. Here we demonstrate that fissiogenic xenon isotopes retained in radioactive fallout generated by a nuclear explosion provide unique constraints on (1) the timescale of fallout formation, (2) chemical fractionation that occurs when fission products and nuclear fuel are incorporated into fallout, and (3) the speciation of fission products in the fireball. Our data suggest that, in near surface nuclear tests, the presence of a significant quantity of metal in a device assembly, combined with a short time allowed for mixing with the ambient atmosphere (seconds), may prevent complete oxidation of fission products prior to their incorporation into fallout. Xenon isotopes thus provide a window into the chemical composition of the fireball in the seconds that follow a nuclear explosion, thereby improving our understanding of the physical and thermo-chemical conditions under which fallout forms. PMID:25014883

  12. When the dust settles: stable xenon isotope constraints on the formation of nuclear fallout.

    PubMed

    Cassata, W S; Prussin, S G; Knight, K B; Hutcheon, I D; Isselhardt, B H; Renne, P R

    2014-11-01

    Nuclear weapons represent one of the most immediate threats of mass destruction. In the event that a procured or developed nuclear weapon is detonated in a populated metropolitan area, timely and accurate nuclear forensic analysis and fallout modeling would be needed to support attribution efforts and hazard assessments. Here we demonstrate that fissiogenic xenon isotopes retained in radioactive fallout generated by a nuclear explosion provide unique constraints on (1) the timescale of fallout formation, (2) chemical fractionation that occurs when fission products and nuclear fuel are incorporated into fallout, and (3) the speciation of fission products in the fireball. Our data suggest that, in near surface nuclear tests, the presence of a significant quantity of metal in a device assembly, combined with a short time allowed for mixing with the ambient atmosphere (seconds), may prevent complete oxidation of fission products prior to their incorporation into fallout. Xenon isotopes thus provide a window into the chemical composition of the fireball in the seconds that follow a nuclear explosion, thereby improving our understanding of the physical and thermo-chemical conditions under which fallout forms.

  13. Growth of continental crust and its episodic reworking over >800 Ma: evidence from Hf-Nd isotope data on the Pietersburg block (South Africa)

    NASA Astrophysics Data System (ADS)

    Laurent, Oscar; Zeh, Armin; Moyen, Jean-François; Doucelance, Régis; Martin, Hervé

    2014-05-01

    The formation and evolution of the continental crust during the Precambrian, and in particular during the Archaean eon (4.0-2.5 Ga), is still a matter of debate. In particular, it is not yet clear in which tectonic environment the genesis of crust took place and how the large volume of granitoid rocks that form ~70% of the Archaean crust were extracted from the mantle. Many studies highlighted that radiogenic isotope systems, especially Lu-Hf and Sm-Nd, are powerful tools to unravel the respective extent of crustal growth and recycling in Archaean terranes. This work presents coupled Hf and Nd isotope data (analyzed both in situ in accessory minerals and in whole rock samples) of Meso- to Neoarchaean granitoids, applied to unravel the processes of crust formation and evolution of the Pietersburg crustal block in South Africa. This crustal segment, the northermost one of the Archaean Kaapvaal Craton, is separated from older crust (3.65-3.10 Ga) by a large-scale suture zone, and the processes related to amalgamation of both blocks and their subsequent evolution are still unclear. The Pietersburg block is made up of a wide range of Archaean granitoid rocks, including tonalite-trondhjemite-granodiorite (TTG) series, high-K monzogranites as well as (grano)diorites belonging to the so-called "sanukitoid" group [1], all intruded by late Paleoproterozoic alkaline complexes. Age determinations highlighted two stages of granitoid formation: (1) TTG magmatism took place episodically over >400 Ma between 3.34 and 2.89 Ga, with a major pulse at 2.97-2.90 Ga; while (2) all the other (high-K) granitoid types emplaced subsequently between 2.84 and 2.69 Ga before a long magmatic shutdown until the intrusion of alkaline complexes at ~2.00 Ga [2-3]. Isotope systematics reveal that these two stages are related to juvenile crust formation and crust reworking, respectively. Indeed, all Hf-Nd isotope data from TTG gneisses are suprachondritic, pointing to a juvenile origin and precluding

  14. Re-Assessment of Cascade Arc Mantle Heterogeneity and Slab Inputs using High-Precision Pb-Hf-Sr-Nd Isotopic Data

    NASA Astrophysics Data System (ADS)

    Mullen, E.; Weis, D.; Martindale, M.

    2015-12-01

    In the Cascade Arc of western North America, several primitive magma lineages are distinguished by major and trace elements: calc-alkaline basalt (CAB), high-alumina olivine tholeiite (HAOT) and relatively minor intraplate basalt (IPB). Previous studies have concluded that these basalt groups represent distinct mantle sources 1. However, new high precision Sr-Nd-Hf-Pb isotope data for primitive magmas from 7 High Cascades volcanic centers show that CAB and HAOT are derived from the same isotopically depleted mantle, with the exception of Mt. Adams-Simcoe backarc basalts. In isotope space, High Cascades CAB and HAOT have similar compositional ranges, forming a single mixing array between two end members that coincide with Juan de Fuca (JdF) MORB and bulk average northern Cascadia sediment2. The High Cascades array is consistent with a depleted sub-arc mantle similar to JdF MORB-source, modified by a homogenized subducted sediment component. The High Cascades array does not intersect Astoria Fan compositions, consistent with the young depositional age of this sediment3. Trace element data for CAB also indicate contributions from a third end member that is a match to fluid derived from subducting JdF MORB. Glacier Peak, the southernmost Garibaldi Belt center, also plots on the High Cascades array. More northerly Garibaldi Belt basalts have lower 208Pb*/206Pb* and ɛHf, reflecting influx of enriched mantle at the northern slab edge that generates IPB4. Mt. Adams-Simcoe HAOT and IPB tap a second enriched mantle component that is consistent with a slab tear in this backarc region. The most isotopically 'enriched' High Cascades CAB and HAOT overlap in isotope and trace element compositions with the Imnaha (C2) component of the Columbia River basalts5, indicating that this mantle is a widespread and long-lived feature in the Pacific Northwest. 1Schmidt et al. 2008, EPSL 266, 166. 2Carpentier et al. 2014, Chem. Geol. 382, 67. 3Prytulak et al. 2006, Chem. Geol. 233, 276. 4

  15. Hybrid genesis of Jurassic fayalite-bearing felsic subvolcanic rocks in South China: Inspired by petrography, geochronology, and Sr-Nd-O-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Guo, Chunli; Zeng, Lingsen; Li, Qiuli; Fu, Jianming; Ding, Tiping

    2016-11-01

    Fayalite-bearing felsic (FBF) magmatic rocks are a special type of granitic rocks with controversial origins. A suite of fayalite- and ferrosilite-bearing subvolcanic rocks, namely, the Xishan FBF rocks in South China, is investigated in this study. The Xishan FBF rocks have high SiO2 contents of 69-70 wt.%, high K2O/Na2O ratios of 1.71-1.95, and high FeOt/(FeOt + MgO) ratios of 0.88-0.89. Fayalite (Fo = 7.3-9.6) and ferrosilite (Fs = 74.1-76.5) minerals are found in the Xishan FBF rocks. According to the Unmix function of Isoplot, the zircon U-Pb ages and initial Hf isotope compositions are categorized into two groups with ages of 156.6 Ma and 151.5 Ma and εHf(t) values of - 7.1 and - 5.2, respectively. The minerals show δ18O values of 8.8-9.8‰ for zircon, 6.0-8.1‰ for fayalite, and 7.0-8.2‰ for ferrosilite. The oxygen isotope fractionations between ferrosilite and fayalite (ΔOpx-Ol) vary from - 0.8‰ to + 1.5‰, which indicates disequilibrium crystallization. Whole-rock analyses show high initial 87Sr/86Sr ratios of 0.7169 to 0.7180 and negative εNd(t) values of - 7.3 to - 6.8; zircon analyses show εHf(t) values of - 9.1 to - 3.8 and δ18O values of 8.8-9.8‰. So whole rock and zircon isotopes indicate a crustal signature. Based on these mineralogical and geochemical data, the Xishan FBF rocks were attributed to A-type granites and derived from the mixing of two batches of crustal magmas, which were all derived from the partial melting of ancient igneous protolith under the conditions of high temperature (683-893 °C), moderate water (3-5 wt.%), and low oxygen fugacity (lg fO2 = - 1.21). Such rigorous physical conditions may be common for the FBF igneous rocks all over the world, which may be the primary factors controlling occurrence of the FBF rocks in limited volume and quantity.

  16. Late Neoproterozoic magmatism in South Qinling, Central China: Geochemistry, zircon U-Pb-Lu-Hf isotopes and tectonic implications

    NASA Astrophysics Data System (ADS)

    Wang, Ruirui; Xu, Zhiqin; Santosh, M.; Yao, Yuan; Gao, Li'e.; Liu, Chunhua

    2016-06-01

    The Neoproterozoic tectonic evolution of the northern margin of the Yangtze Block in South China remains debated. In this study, we present results from LA-ICP-MS zircon U-Pb geochronology on a suite of intermediate-felsic rocks in South Qinling, Central China which show a mean age of ca. 630 Ma. The zircon εHf(t) values of these rocks mostly range from + 0.44 to + 14.78. Geochemically, the granites and syenite show high total alkali contents, with enrichment in LREE, LILE (Rb, Ba, and K), and HFSE (Th, U, Nb, Ta, Zr, and Hf), and depletion in Sr, P, and Ti, similar to the features of A-type granites. The meta-diorite shows high Na2O, with depletion in Eu, Ti, and LILE (Sr, Rb, Ba, and K), and enrichment in HFSE (Th, U, Nb, Ta, Zr, and Hf). The geochemical features are consistent with formation of the intermediate-felsic suite through fractionation from underplated basaltic magma that originated from sub-continental lithospheric mantle metasomatized by asthenosphere-derived oceanic-island-basalt-like (OIB-like) melts, coupled with minor crustal contamination. We correlate the ca. 630 Ma magmatism with a back-arc rift setting that probably developed in relation to slab tearing during continued slab rollback.

  17. Double-layer structure of the crust beneath the Zhongdian arc, SW China: U-Pb geochronology and Hf isotope evidence

    NASA Astrophysics Data System (ADS)

    Cao, Kang; Xu, Ji-Feng; Chen, Jian-Lin; Huang, Xiao-Xiao; Ren, Jiang-Bo; Zhao, Xiang-Dong; Liu, Zhen-Xing

    2016-01-01

    U-Pb ages and Hf isotopes of zircons in Late Triassic and Cretaceous intrusive rocks from the Zhongdian arc, SW China, are used to decipher the tectonic, magmatic, and metallogenic processes that occurred during this period. New U-Pb dating of zircons from Late Triassic porphyries yielded ages of ca. 216 Ma and εHf(t) values of -2.1 to +6.1. Combined with previous results, the data indicate that these Late Triassic rocks were most likely derived from a juvenile mafic lower-crust with minor old crust material. However, the Cretaceous granites (∼80 Ma) have lower εHf(t) values (-7.6 to -2.4) than the Late Triassic rocks, indicating that the former originated from old crust. Based on the new data and previous studies of Mesozoic magmatic activity, a plausible model for the tectono-magmatism and metallogenesis of the Zhongdian arc is proposed. The westwards subduction of the Ganzi-Litang oceanic crust began before ∼230 Ma, resulting in the formation of a juvenile lower crust beneath the Zhongdian arc due to the underplating of mafic arc magmas during ca. 230-216 Ma. At ca. 216 Ma, break-off or slab-tearing of the west-dipping Ganzi-Litang oceanic slab led to partial melting of the juvenile lower crust, which gave rise to Cu-bearing porphyries. In the Late Cretaceous, the Zhongdian arc probably underwent post-collision extension, triggering the partial melting of the old middle-upper crustal materials and producing various granites and related Mo-Cu deposits. According to this model, the crust beneath the Zhongdian arc probably has a double-layer structure, with older crust at shallow levels and juvenile crust at deeper levels.

  18. Lithium isotope constraints on crust-mantle interactions and surface processes on Mars

    NASA Astrophysics Data System (ADS)

    Magna, Tomáš; Day, James M. D.; Mezger, Klaus; Fehr, Manuela A.; Dohmen, Ralf; Aoudjehane, Hasnaa Chennaoui; Agee, Carl B.

    2015-08-01

    Lithium abundances and isotope compositions are reported for a suite of martian meteorites that span the range of petrological and geochemical types recognized to date for Mars. Samples include twenty-one bulk-rock enriched, intermediate and depleted shergottites, six nakhlites, two chassignites, the orthopyroxenite Allan Hills (ALH) 84001 and the polymict breccia Northwest Africa (NWA) 7034. Shergottites unaffected by terrestrial weathering exhibit a range in δ7Li from 2.1 to 6.2‰, similar to that reported for pristine terrestrial peridotites and unaltered mid-ocean ridge and ocean island basalts. Two chassignites have δ7Li values (4.0‰) intermediate to the shergottite range, and combined, these meteorites provide the most robust current constraints on δ7Li of the martian mantle. The polymict breccia NWA 7034 has the lowest δ7Li (-0.2‰) of all terrestrially unaltered martian meteorites measured to date and may represent an isotopically light surface end-member. The new data for NWA 7034 imply that martian crustal surface materials had both a lighter Li isotope composition and elevated Li abundance compared with their associated mantle. These findings are supported by Li data for olivine-phyric shergotitte NWA 1068, a black glass phase isolated from the Tissint meteorite fall, and some nakhlites, which all show evidence for assimilation of a low-δ7Li crustal component. The range in δ7Li for nakhlites (1.8 to 5.2‰), and co-variations with chlorine abundance, suggests crustal contamination by Cl-rich brines. The differences in Li isotope composition and abundance between the martian mantle and estimated crust are not as large as the fractionations observed for terrestrial continental crust and mantle, suggesting a difference in the styles of alteration and weathering between water-dominated processes on Earth versus possibly Cl-S-rich brines on Mars. Using high-MgO shergottites (>15 wt.% MgO) it is possible to estimate the δ7Li of Bulk Silicate Mars

  19. Geochemical Fingerprinting of Trans-Atlantic African Dust Based on Radiogenic Sr-Nd-Hf Isotopes and Rare Earth Element Anomalies

    NASA Astrophysics Data System (ADS)

    Pourmand, Ali; Prospero, Joseph; Sharifi, Arash

    2015-04-01

    Mineral dust is an important component of Earth's climate system and biogeochemical cycles on a global scale. In order to understand the relationship between climate processes in the source areas and the properties of aerosols at distant receptor sites, we must be able to identify the source provenance of dust. Here we present a multiproxy study that characterizes the temporal variability in the geochemical composition of long-range African dust (LRAD) collected between 2003 and 2011 in the trade winds on the Caribbean island of Barbados. We find systematic differences between Sr-Nd-Hf isotopic composition and rare earth element anomalies of individual dust events and evidence of seasonal shifts in dust source activity and transport. These results indicate that coherent geochemical source signatures of LRAD can be preserved even after transport across thousands of kilometers. We investigated the possibility of identifying the potential source areas through comparisons with literature data. However, these data are almost entirely based on measurements of soil and sediment samples; this could lead to biases because of soil-aerosol particle size and composition differences. Nonetheless, our data suggest that many samples are linked to sources in Mali and sub-Saharan regions. Radiogenic Nd-Hf composition of aerosols can potentially be a useful proxy to study the proximity of mineral dust sources to depositional sites. In order to establish firmer links between LRAD and dust source areas, however, we require much more data on the geochemical composition of aerosols from potential source areas in North Africa.

  20. Geochemical Fingerprinting of Trans-Atlantic African Dust Based on Radiogenic Sr-Nd-Hf Isotopes and Rare Earth Element Anomalies

    NASA Astrophysics Data System (ADS)

    Pourmand, A.; Prospero, J. M.; Sharifi, A.

    2014-12-01

    Mineral dust is an important component of Earth's climate system and biogeochemical cycles on a global scale. In order to understand the relationship between climate processes in the source areas and the properties of aerosols at distant receptor sites, we must be able to identify the source provenance of dust. Here we present a multiproxy study that characterizes the temporal variability in the geochemical composition of long-range African dust (LRAD) collected between 2003 and 2011 in the trade winds on the Caribbean island of Barbados. We find systematic differences between Sr-Nd-Hf isotopic composition and rare earth element anomalies of individual dust events and evidence of seasonal shifts in dust source activity and transport. These results indicate that coherent geochemical source signatures of LRAD can be preserved even after transport across thousands of kilometers. We investigated the possibility of identifying the potential source areas through comparisons with literature data. However, these data are almost entirely based on measurements of soil and sediment samples; this could lead to biases because of soil-aerosol particle size and composition differences. Nonetheless, our data suggest that many samples are linked to sources in Mali and sub-Saharan regions. Radiogenic Nd-Hf composition of aerosols can potentially be a useful proxy to study the proximity of mineral dust sources to depositional sites. In order to establish firmer links between LRAD and dust source areas, however, we require much more data on the geochemical composition of aerosols from potential source areas in North Africa.

  1. Characterization of Backbending in Even-Even Isotopes of 164-174Hf and 154-164Dy Nuclei by a Modified Phenomenological Model

    NASA Astrophysics Data System (ADS)

    Najim, L. A.; Kheder, Malek. H.

    2013-07-01

    A modified phenomenological model is used to calculate nuclear energy levels and describe successfully the backbending of the moment of inertia for the ground state bands in even-even isotopes of Hf and Dy nuclei. The model is a combination of the Myers and Swiatecki model with variable moment inertia (VMI) model. Since the Myers and Swiatecki model has a deviation from experimental energies in which it takes into account pairing effect with constant moment of inertia, in the rotation of nuclei, the Coriolis force acts to de-pair the nucleons pair and align their angular momentum with nuclei total angular momentum, thus Coriolis force increasing and decrease the rotational energy. So, the moment of inertia varies with the angular momentum. Therefore, we modified this model by adding a term to make the moment of inertia vary with angular momentum in the same manner of the VMI model which has a term added to the rotational energy equation. The modified model fits remarkably with the experimental observation and other models in many cases with the use of few parameters especially in rotational nuclei regions similar to Hf and Dy nuclei.

  2. New Insights Into the Genesis and Compositional Evolution of I-type Granitic magmas in the Lachlan Fold Belt (SE Australia) by in situ Hf Isotopic Analysis of Zircon

    NASA Astrophysics Data System (ADS)

    Kemp, T. I.; Hawkesworth, C. J.; Hergt, J. M.; Woodhead, J.

    2004-05-01

    Isotope studies have proved of enormous benefit in fingerprinting the source rocks of silicic magmas and tracing open system petrogenetic processes, such as crustal assimilation or magma mixing. Quantification of these processes, especially the role of mantle-derived magmas, is essential to formulating realistic models for the thermal regime and compositional evolution of the continental crust. However, this remains problematic, since whole-rock isotopic data registers the final state of the magmatic system but gives no information on the pathways by which this state was attained. For example, the eNd - initial 87Sr/86Sr isotopic array defined by the classic I- and S-type granites of the Lachlan Fold Belt has been variously interpreted to reflect (1) mixing between two end-member magmas, one depleted mantle-like, the other evolved and continental crust-like, (2) mixing between a juvenile magma and a magma sourced from mafic lower crust, accompanied by sediment assimilation, (3) derivation of the granites from mixed source rocks and (4) derivation from a sequence of protoliths of various ages and sedimentary maturity. The implications of these possibilities for crustal architecture, and whether granitic magmatism was associated with the recycling or growth of new continental crust are drastically different. One way to now resolve such ambiguities is by unravelling the isotopic information encoded in the fine-scale growth zoning of minerals such as zircon, which potentially tracks the processes operative during crystallisation. To this end we report the first laser-ablation ICP-MS study into the Hf isotope stratigraphy of zircons hosted by LFB I-type granites and their mafic enclaves. This is integrated with a prior U-Pb isotope study and trace element concentrations measured on the same zircons. Two suites were investigated, the Cobargo and Why Worry Suites of the Bega Batholith. Although the bulk rock isotopic variation within these suites is restricted, this study

  3. Geochemical and Lu/Hf isotopic (LA-ICP-MS) signature of detrital zircons from sandstones of the basal levels of the Riphean stratotype

    NASA Astrophysics Data System (ADS)

    Romanyuk, T. V.; Kuznetsov, N. B.; Maslov, A. V.; Belousova, E. A.; Krupenin, M. T.; Ronkin, Yu. L.; Gorozhanin, V. M.; Gorozhanina, E. N.

    2014-11-01

    This paper presents the results of selective study of trace elements (29 analyses) and the Lu/Hf isotopic system (41 analyses) in preliminarily dated (U-Pb) detrital zircons (dZrs) from sandstones of the Ai Formation of the Burzyan Group of the Bashkirian Anticlinorium, which compose the basal horizons of the typical Riphean section of the Southern Urals. The statistically processed trace-element patterns of dZrs showed that "diorites" were dominant over "syenites" among the source rocks of dZrs. The rock types estimated by trace-element patterns for the cores and rims of two large grains ("diorite" and "syenite") coincided. The analysis of the Lu/Hf isotopic system of dZrs revealed a wide dispersion of the ɛHf value from +7.1 to -20.1 at the T {DM/C} model age of the substrate from 2.25 to 3.95 Ga. Four grains (in one case with the core and rim studied) from the population of the large transparent cherry zircons (TCZ) are characterized by the "syenitic" rock type and extremely ancient T {DM/C} values of 3.22, 3.45, 3.64, 3.66, and 3.75 Ga at ages of zircons of 2486, 2784, 2873, 1977, and 1984 Ma, respectively. Two "dioritic" grains from the TCZ population have significantly distinct parameters: 2.37 and 2.51 Ga at 2049 and 2057 Ma, respectively. It is evident that this specific population of dZrs was formed with a significant contribution of very ancient crustal material, which became active under "syenitic" magmatism and provided the T {DM/C} value of >3.5 Ga. Numerous juvenile dZrs form a compact cluster, which correspond to the rocks of the southern part of the Volga-Sarmatian orogen (age of 2.1-2.0 Ga, T {DM/C} = 2.1-2.4 Ga). The complexes of the entire the Volga-Uralia, the Volga-Sarmatian orogen, and adjacent areas could be the provenance areas for the Ai sandstones in contrast to the northeastern areas of the East-European Platform with dominant "granitic" source rocks and T {DM/C} values lower than 3.5 Ga.

  4. Zircon U-Pb geochronology, geochemistry, and Sr-Nd-Hf isotopes of granitoids in the Yulekenhalasu copper ore district, northern Junggar, China: Petrogenesis and tectonic implications

    NASA Astrophysics Data System (ADS)

    Yang, Fuquan; Chai, Fengmei; Zhang, Zhixin; Geng, Xinxia; Li, Qiang

    2014-03-01

    The Yulekenhalasu porphyry copper deposit is located in the Kalaxiange'er metallogenic belt in northern Junggar, China. We present the results from zircon U-Pb geochronology, and geochemical and Sr-Nd-Hf isotope analyses of the granitoids associated with the ore deposits with a view to constrain their petrogenesis and tectonic setting. The granitoids consist of quartz diorite, diorite porphyry, porphyritic monzonite, and quartz porphyry, emplaced at 382, 379, 375-374, and 348 Ma, respectively, which span Late Devonian to early Carboniferous ages. The ore-bearing intrusion is mainly diorite porphyry, with subordinate porphyritic monzonite. The Late Devonian intrusions are characterized by SiO2 contents of 54.5-64.79 wt.%, Na2O contents of 3.82-8.24 wt.%, enrichment in Na, light rare-earth elements (LREEs), and large ion lithophile elements. They also display relative depletion in Y, Ba, P, Nb, Ta, and Ti, and weak negative Eu anomalies (δEu = 0.6-0.87). The early Carboniferous quartz porphyry is characterized by high SiO2 content (72.26-73.35 wt.%), enrichment in LREEs, K, and Sr, and relative depletion in Y (10.82-12.52 ppm) and Yb (1.06-1.15 ppm). The Late Devonian and early Carboniferous granitoids are characterized by positive ɛNd(t) values (5.2-10.1, one sample at - 1.9), positive ɛHf(t) values (7.46-18.45), low (87Sr/86Sr)i values (0.70363-0.70476), and young crustal residence ages. These data indicate that the sources of the granitoids were mainly mantle-derived juvenile rocks. Geochemical and Nd-Sr-Hf isotopic data demonstrate that the Late Devonian granitoids formed in an oceanic island arc, and they were formed from different sources, among which the mineralized diorite porphyry might have originated from a mixed slab-derived and mantle wedge melt source. The early Carboniferous quartz porphyry was likely emplaced in a mature island arc environment, and was probably derived from juvenile crust.

  5. Provenance of Permian-Triassic Gondwana Sequence Units Accreted to the Banda Arc: Constraints from U/Pb and Hf Analysis of Zircons and Igneous Geochemistry

    NASA Astrophysics Data System (ADS)

    Flores, J. A.; Spencer, C. J.; Harris, R. A.; Hoiland, C.

    2011-12-01

    Analysis of zircons from Australian affinity Permo-Triassic units of the Timor region yield age distributions with large peaks at 230-400 Ma and 1750-1900 Ma (n=435). Similar zircon age peaks are also found in rocks from NE Australia and the eastern Cimmerian block. It is likely that these terranes, which are now widely separated, were once part of the northern edge of Gondwana near what is now the NW margin of Australia. The Cimmerian Block was removed from Gondwana during Early Permian rifting and initiation of the Neo-Tethys Ocean. Hf analysis of zircon from the Aileu Complex in Timor and Kisar shows bimodal (juvenial and evolved) magmatism in the Gondwana Sequence of NW Australia at ~300 Ma. The magmatic event produced basalt with rift valley and ocean floor geochemical affinities, and rhyolite. Similar rock types and isotopic signatures are also found in Permo-Triassic igneous units throughout the Cimmerian continental block. The part of the Cimmerian Block with zircon distributions most like the Gondwana Sequence of NW Australia is the terranes of northern Tibet and Malaysia. The large 1750-1900 Ma zircon peak is much more wide spread, and appears in terranes from Baoshan (SW China) to Borneo. The Permo-Triassic rocks of the Timor region fill syn-rift intracratonic basins that successfully rifted in the Jurassic to form the NW margin of Australia. This passive continental margin first entered the Sunda Trench in the Timor region at around 8 Ma causing the Permo-Triassic rocks to accrete to the edge of the Asian Plate and emerge as a series of mountainous islands in the young collision zone. Eventually, the Australian continental margin will collide with the southern edge of the Asian plate and these Gondwana terranes will rejoin. However, it may be difficult to reconstruct the various ventures of they made over the past 300 Ma.

  6. Zircon Geochemical and Isotopic Constraints on the Evolution of the Mount Givens Pluton, Central Sierra Nevada Batholith

    NASA Astrophysics Data System (ADS)

    Sendek, C.; Lackey, J. S.; Miller, J. S.; Davies, G. R.; Valley, J. W.; Kitajima, K.

    2015-12-01

    The Late Cretaceous Mt. Givens pluton (central Sierra Nevada batholith, CA) is noteworthy for its large size (≈1400 km2) and relative compositional and textural homogeneity. It has been proposed as a plutonic analog for "monotonous intermediate" ignimbrites. The pluton is characterized by a 30 km wide ellipse shaped northern lobe that connects with a long mass about 15 km wide and extending 50 km SE. The northern lobe was constructed over 7 m.y. (from 98 to 91 Ma) with progressively younger ages toward the interior. This inward younging is accompanied by transitions to more felsic compositions and from equigranular to K-spar porphyritic textures. The large elongated mass extending to the SE (ca. 95-91 Ma) is more homogeneous, mostly equigranular granodiorite with subordinate K-spar-phyric granodiorite. Small diorite intrusions (10's to 100's m2) are also present and locally mingle and hybridize with the host granodiorite. Unlike other Late Cretaceous zoned intrusions (the Sierra Crest intrusions), the equigranular, and K-spar porphyritic phases of the Givens have similar trace element characteristics. All zircons have high Ti-in-zircon model temperatures (850-1000 °C), pronounced negative Eu anomalies, and curved MREE and HREE patterns. These characteristics indicate that zircon grew early and that initial magmas were likely undersaturated in zircon. Significant within sample variations in δ18O (up to 1.5‰) and eHf (up to 8 units) require mixing of isotopically distinct magmas in the Givens magma system after they had begun crystallizing zircon, but well before solidification. O and Hf isotopic variation within the granodiorites shows distinct geographic variation, with higher δ18O and more negative eHf values along the western margin of the pluton. This trend is consistent with earlier work suggesting that the Givens intruded across the Panthalassan-North American lithospheric boundary.

  7. Late Cretaceous granites from the giant Dulong Sn-polymetallic ore district in Yunnan Province, South China: Geochronology, geochemistry, mineral chemistry and Nd-Hf isotopic compositions

    NASA Astrophysics Data System (ADS)

    Xu, Bin; Jiang, Shao-Yong; Wang, Rong; Ma, Liang; Zhao, Kui-dong; Yan, Xiong

    2015-03-01

    As a world-class tin-tungsten province, South China is well known for its extensive Mesozoic granitic magmatism. The Dulong district, located in the western Cathaysia Block of the South China tin-tungsten province, is characterized by widespread Mesozoic granitoids and accompanying Sn-polymetallic ore deposit (~ 30 Mt of Sn). It is one of the most important polymetallic tin ore districts in China. In this study, three mineralization-related granite types were identified in the Dulong district, including the Dulong coarse-grained granite (DCG), the Dulong fine-grained granite (DFG), and the Dulong porphyritic granite (DPG). Detailed studies are presented on zircon U-Pb ages, major and trace elements, mineral chemical and Nd-Hf isotopic compositions of the tin-bearing granites from the Dulong district. LA-ICP-MS U-Pb dating of zircon grains from these three granite bodies yields ages of 90.1 ± 0.7 Ma, 89.7 ± 0.8 Ma and 86.0 ± 0.5 Ma, respectively. Geochemically, the granites are strongly peraluminous, with high contents of alkalis, enrichment in P, Li, Rb, Cs, Ta, Sn, W and U, depletion in Ti, Mg, Co, Ni, Sr, Ba, Zr, Hf, Th and rare earth elements. Fractional crystallization of plagioclase and K-feldspar was the principal process of magmatic differentiation that controlled Rb, Sr, Ba and Eu concentrations, whereas rare earth elements were fractionated by accessory minerals, such as apatite and monazite. The geochemical data suggest that the rocks are highly fractionated S-type granites. The granites show bulk rock εNd(t) values in the range of - 12.2 to - 10.8 and zircon εHf(t) values from - 15.5 to - 2.5, with Meso-Paleoproterozoic TDMC ages for both Nd and Hf isotopes. Geochemical and isotopic data suggest that these highly fractionated S-type granites DCG, DFG and DPG were originated from the same episode of partial melting of the protolith, which have analogous components of metamorphosed pelitic rocks from the Meso-Paleoproterozoic continental crust

  8. Source and mode of the Permian Panjal Trap magmatism: Evidence from zircon U-Pb and Hf isotopes and trace element data from the Himalayan ultrahigh-pressure rocks

    NASA Astrophysics Data System (ADS)

    Rehman, Hafiz Ur; Lee, Hao-Yang; Chung, Sun-Lin; Khan, Tahseenullah; O'Brien, Patrick J.; Yamamoto, Hiroshi

    2016-09-01

    We present an integrated study of LA-ICP-MS U-Pb age, Hf isotopes, and trace element geochemistry of zircons from the Himalayan eclogites (mafic rocks) and their host gneisses (felsic rocks) from the Kaghan Valley in Pakistan in order to understand the source and mode of their magmatic protoliths and the effect of metamorphism. Zircons from the so-called Group I (high-pressure) eclogites yielded U-Pb mean ages of 259 ± 10 Ma (MSWD = 0.74), whereas those of Group II (ultrahigh-pressure) eclogites yielded 48 ± 3 Ma (MSWD = 0.71). In felsic gneisses the central or core domains of zircons yielded ages similar to those from Group I eclogites but zircon overgrowth domains yielded 47 ± 1 Ma (MSWD = 1.9). Trace element data suggest a magmatic origin for Group I-derived (having Th/U ratios: > 0.5) and metamorphic origin for Group II-derived (Th/U < 0.07) zircons, respectively. Zircon Hf isotope data, obtained from the same dated spots, show positive initial 176Hf/177Hf isotopic ratios referred to as "ƐHf(t)" of around + 10 in Group I eclogites; + 7 in Group II eclogites; and + 8 in felsic gneisses zircons, respectively, thus indicate a juvenile mantle source for the protolith rocks (Panjal Traps) with almost no contribution from the ancient crustal material. The similar ƐHf(t) values, identical protolith ages and trace element compositions of zircons in felsic (granites or rhyolites) and mafic (basalt and dolerite) rocks attest to a bimodal magmatism accounting for the Panjal Traps during the Permian. Later, during India-Asia collision in Eocene times, both the felsic and mafic lithologies were subducted to mantle-depths (> 90 km: coesite-stable) and experienced ultrahigh-pressure metamorphism before their final exhumation.

  9. Late Hadean-Eoarchean transitions in crustal evolution from Hf isotopic evidence in the Jack Hills zircons

    NASA Astrophysics Data System (ADS)

    Bell, E. A.; Harrison, M.; Kohl, I. E.; Young, E. D.

    2013-12-01

    The evolution of the Earth's earliest crust remains largely unknown due to the dearth of Hadean (>4 Ga) rocks, with most observational evidence of the planet's first few hundred million years deriving from geochemical studies of 4.4-4.0 Ga detrital zircons from Jack Hills (Narryer Gneiss Complex, Yilgarn craton). Previous Lu-Hf investigations of the zircons have suggested that continental-like (low Lu/Hf) crust formation began by ~4.4-4.5 Ga and may have continued for several hundred million years. The most ancient crust represented in the Jack Hills population was preserved until at least ~4 Ga. However, evidence for the involvement of Hadean materials in later crustal evolution is sparse, and even at Jack Hills the most unradiogenic, ancient materials represented by some Hadean zircons have not been identified in the younger rock and zircon record. We present new Lu-Hf results from <4 Ga Jack Hills zircons that indicate an important transition in Yilgarn crustal evolution between 4.0 and 3.6 Ga. Although Hadean samples are permissive of crustal extraction from the mantle up to ~4 Ga, crust in the Jack Hills source evolves dominantly by internal reworking 4.0-3.8 Ga, and both the most ancient and most juvenile components of the crust are lost from the zircon record after ~4 Ga. New juvenile additions to the crust at ~3.8-3.7 Ga are accompanied by the disappearance of crust with model ages >4.3 Ga. These new data indicate a tectonic regime in the Eoarchean (4.0-3.6 Ga) Yilgarn characterized by internal crustal reworking punctuated by one relatively short juvenile crust extraction event. The coupling in time of juvenile crust formation with the loss of ancient crust is best explained by a mechanism similar to subduction, which accomplishes both processes on the modern Earth. We interpret these data as consistent with the action of destructive plate boundary interactions by Eoarchean times.

  10. Zircon U-Pb ages, Hf isotope data, and tectonic implications of Early-Middle Triassic granitoids in the Ailaoshan high-grade metamorphic belt of Southeast Tibet

    NASA Astrophysics Data System (ADS)

    Wu, Wenbin; Liu, Junlai; Chen, Xiaoyu; Zhang, Lisheng

    2016-05-01

    The Ailaoshan tectonic belt, where the effects of the Paleo-Tethyan ocean evolution and Indian-Eurasian plate collision are superimposed, is one of the most significant geological discontinuities in western Yunnan province of southeast Tibet. An Ailaoshan micro-block within the belt is bounded by the Ailaoshan suture zone to the west and the Red River Fault to the east, and consists of low- and high-grade metamorphic belts. Late Permian-Middle Triassic granitoids that are widely distributed to the west of the Ailaoshan suture zone and within the Ailaoshan micro-block may yield significant information on the Tethyan tectonic evolution of the Ailaoshan tectonic belt. This study reports new LA-ICP-MS zircon U-Pb geochronology and Hf isotope data of four granitoids from the Ailaoshan high-grade metamorphic belt. Zircon grains from the Yinjie granitoid do not have inherited cores and yield a weighted mean U-Pb age of 247.1 ± 2.0 Ma. The zircon ɛ Hf(t) values range from 7.8 to 12.1, and Hf model ages from 775 to 546 Ma, indicating that the granitoid was derived from juvenile crust. The rims of zircons from the Majie and Yuanjiang granitoids yield weighted mean U-Pb ages of 239.5 ± 1.8 and 237.9 ± 2.6 Ma, respectively, whereas the cores yield ages of 1608-352 Ma. The ɛ Hf(t) values of zircon rims range from -20.4 to -5.3, yielding Hf model ages from 2557 to 1606 Ma and suggesting that the source magma of the Majie and Yuanjiang granitoids was derived from ancient crust. An additional granitoid located near the Majie Village yields a zircon U-Pb age of 241.2 ± 1.0 Ma. Based on our geochronological and geochemical data, combined with geological observations, we propose that the Ailaoshan micro-block was derived from the western margin of the Yangtze block, and is comparable to the Zhongzan and Nam Co micro-blocks. The presence of late Permian mafic rocks with rift-related geochemical characteristics within the Ailaoshan micro-block, together with granitoids derived

  11. Geochemistry and detrital zircon U-Pb and Hf isotopes of the paragneiss suite from the Quanji massif, SE Tarim Craton: Implications for Paleoproterozoic tectonics in NW China

    NASA Astrophysics Data System (ADS)

    Zhang, Lu; Wang, Qinyan; Chen, Nengsong; Sun, Min; Santosh, M.; Ba, Jin

    2014-12-01

    The Delingha paragneiss suite in the Quanji massif, southeastern Tarim Craton, is composed of mica schist, paragneiss, leptynite and quartzite, similar to the 'khondalite suites' described from elsewhere in the world. The mica schist is rich in Al2O3 (up to ∼26 wt%) and contains graphite and diagnostic minerals including sillimanite and garnet, with metamorphism under amphibolite-facies to locally granulite-facies conditions as manifested by association with amphibolite and granulite. The detrital zircon U-Pb ages and geochemical data indicate that the protolith materials of the Delingha paragneiss suite were mainly sourced from 2.20 to 2.45 Ga granites, felsic volcanic rocks and TTG, and were deposited at 2.17-1.92 Ga. The detrital zircon Hf and whole-rock Nd isotopes document important crustal growth at ∼2.5-2.7 Ga. The detrital zircon age spectra, the whole rock Nd and zircon Hf model ages, the low-maturity of the protolith, and short-distance transportation suggest that the detritus were derived from the underlying Delingha Complex and the lower Dakendaban sub-Group. The timing of magmatic activities in the source region, the depositional age and metamorphic histories of the Delingha paragneiss suite are all comparable to those recorded in the khondalite belt along northern margin of the Ordos Block in the North China Craton. Our study shows that the 2.2-2.45 Ga magmatic rocks were generated in arc or active continental margin settings, suggesting a prolonged subduction and accretion history prior to final amalgamation (∼2.5-1.8 Ga) to form the unified North China Craton and the assembly of the Tarim Craton in NW China.

  12. Metasedimentary melting in the formation of charnockite: Petrological and zircon U-Pb-Hf-O isotope evidence from the Darongshan S-type granitic complex in southern China

    NASA Astrophysics Data System (ADS)

    Jiao, Shu-Juan; Li, Xian-Hua; Huang, Hui-Qing; Deng, Xi-Guang

    2015-12-01

    Charnockites are Opx-bearing igneous rocks commonly found in high-grade metamorphic terranes. Despite being volumetrically minor, they show a wide range in both bulk geochemistry and intensive parameters. They form a characteristic component of the AMCG (anorthosite-mangerite-charnockite-granite) suite, but their association with typical S-type granites is less well-known. The Darongshan S-type granitic complex (DSGC) in Guangxi Province, southern China, contains granites varying in mafic silicate mineral assemblages from Bt + Crd (Darongshan suite) to Opx + Grt + Bt + Crd (Jiuzhou suite) and Opx + Crd ± Bt (Taima suite), corresponding to a geochemical transition from magnesian calc-alkalic to ferroan calc-alkalic. However, its genesis, even the accurate age of intrusion, remains highly contentious despite intensive research. In order to understand the genesis of charnockite and its genetic relationship with S-type granite; here, we first determined zircon U-Pb ages of each suite using a SIMS on the basis of a detailed petrological study. Zircon U-Pb ages show that all suites of the complex were emplaced contemporaneously at ca. 249 Ma. Monazite apparent U-Pb ages are indistinguishable from zircon U-Pb ages within analytical error. Further in situ zircon Hf-O isotope analyses reveal that the granitic complex was dominantly derived from reduced melting metasedimentary rocks (δ18Ozircon = ca. 11‰; εHf(t)zircon = ca. - 10; Δlog FMQ ≤ 0; Mn in apatite oxybarometer) with rare material input from the mantle. The variation in δ18O (7.8‰-12.9‰) is more likely a result of hybridization, whereas that in εHf(t) (- 31.9 to - 1.8) is a result of both hybridization and disequilibrium melting. The variation in mineralogy and geochemistry may be interpreted as a result of entrainment of peritectic garnets from biotite-dehydration melting. Nevertheless, heat input from mantle through basaltic intrusion/underplating is considered to play a major role in high

  13. Zircon U-Pb and Lu-Hf isotope study of the Neoproterozoic Haizhou Group in the Sulu orogen: Provenance and tectonic implications

    NASA Astrophysics Data System (ADS)

    Zhou, Jian-Bo; Wilde, Simon A.; Liu, Fu-Lai; Han, Jie

    2012-04-01

    The Neoproterozoic Haizhou Group crops out sporadically in the Sulu orogen in east-central China. It is divided into the Jinping and Yuntai formations and consists of quartzite, quartz schist, marble and graphite- and apatite-bearing sequences. Major and trace element data for quartz schist from the two formations indicate that these rocks have a greywacke protolith and have been deposited during strong tectonic activity. LA-ICPMS U-Pb dating of detrital zircon yields ages of 635 to 1074 Ma for three samples from the Jinping Formation and 611 to 943 Ma for two samples from the Yuntai Formation. More than 78% of the detrital zircons from the two formations have U-Pb ages grouped between 700 and 890 Ma, with two clusters peaking at 758 Ma and 828 Ma, respectively. This indicates that their provenance is magmatic rocks of Neoproterozoic age that have a tectonic affinity to the South China Block (SCB). A few older zircon populations with peak U-Pb ages at 943 and 1074 Ma are also present. A younger population shows peaks at 661 and 611 Ma. This suggests that deposition of the Haizhou Group was later than ~ 611 Ma rather than during the Mesoproterozoic as previously thought. Zircon Lu-Hf isotope data collected from the same U-Pb sites show negative ɛHf(t) values of - 22.8 to - 7.4 and Hf model ages of 2341 to 3100 Ma. This indicates that the Neoproterozoic magmatic rocks were derived from reworking of ancient Paleoproterozoic to Archean crust. The results support the contention that the Haizhou Group is similar to the Wulian Group at the northwestern edge of the Sulu orogen, both having a SCB affinity, but that the Penglai Group does not belong to the SCB because of the absence of Neoproterozoic ages. This lends support to the conclusion that the Triassic suture between the North China and South China blocks is located along the Baichihe-Yantai Fault, which lies north of the Wulian Complex and south of the Jiaobei Terrane; thus the Wulian-Yantai Fault is not the suture

  14. Tracking the multiple origins of salinity in three different karstic aquifers (southern France): Sr isotopes constraints.

    NASA Astrophysics Data System (ADS)

    Le Gal La Salle, Corinne; Khaska, Mahmoud; Lancelot, Joël

    2014-05-01

    Groundwater resources of the Mediterranean area are submitted to a high anthropic pressure and face a set of major climatic and geological constraints. The potential exploitation of karst aquifers is still unclear and probably underestimated, but their vulnerability to pollution is high and potential for salinization in coastal aquifer increases with over exploitation and the rise of sea level. In order to trace the origin of salinity in karst aquifers in a Mediterranean coastal environment, a multi-tracer approach coupling major, specific trace elements and stable (δ18O, δ2H) and radiogenic (87Sr/86Sr) isotopes was held. Three close sites in southern France have been studied to investigate a different origin of the salinity. In the coastal karst aquifer of la Clape (Aude), salinity originated from deep salt water due to a paleoseawater intrusion followed by water-rock interaction with the carbonate host rock. On land and off-shore, powerful tertiary sedimentary deep deposits limit the karst network communications with the seawater. The presence of many faults could be a contributing factor to the mixing of salt water within the karst water. There it was shown that the paleoseawater proportions in the aquifer ranged from 0 to 16 %. Slightly further inland, in another similar karstic aquifer, the source of Oeillal (Aude) displayed a high salinity. Salinity most surely originated from deep horizons that come to the surface by a major normal fault where it mixes with karst waters. Deep brines from ancient meteoric water evolved by water-rock interaction with evaporites in the underlying Keuper formation. There calculated proportions of salt water into the mixture with karst water varied between 30 and 40%. In the third site located on the edge of the seawater shoreline, the simple limestone karst aquifer of Pliocene in Frontignan (Hérault) was under increasing salinity intrusion of seawater, which proportions of mixing between seawater and karst water varied from 2

  15. New Isotopic Constraints on the Sources of Methane at Sites of Active Continental Serpentinization

    NASA Astrophysics Data System (ADS)

    Wang, D. T.; Gruen, D.; Morrill, P. L.; Rietze, A.; Nealson, K. H.; Kubo, M. D.; Cardace, D.; Schrenk, M. O.; Hoehler, T. M.; McCollom, T. M.; Etiope, G.; Hosgormez, H.; Schoell, M.; Ono, S.

    2014-12-01

    At continental sites of serpentinization, high concentrations of reduced gases (e.g., H2, CH4) are frequently found in association with highly-alkaline groundwater. Identification of the process(es) responsible for the generation of methane—as well as the source(s) of C & H—in these environments has been challenging. The difficulty is due to both the wide range of processes (microbial, thermal, abiotic) that could be involved, and the limited number of parameters that are accessible to currently-available analytical technologies (e.g., δ13C, δD). The recent development of a new technique based on tunable infrared laser spectroscopy [1] has enabled the fully-resolved quantification of four isotopologues of methane: 12CH4, 13CH4, 12CH3D, and 13CH3D, a doubly-substituted ("clumped") isotopologue. We used this technique to measure 13CH3D in gases sampled from continental sites of serpentinization, in order to provide independent constraints on C-H bond-forming processes involved in the generation of the methane found in these systems. Our study sites are hosted in ultramafic units that are presently undergoing serpentinization. These include The Cedars peridotite body (Calif., USA) [2], the Coast Range Ophiolite Microbial Observatory (Calif., USA) [3], and the Chimaera seep (Tekirova Ophiolite, Turkey) [4]. Preliminary measurements indicate that Δ13CH3D (the deviation of the abundance of 13CH3D from the stochastic distribution) in methane sampled from these sites spans nearly the entire range of thermodynamically-predicted values, from >+5‰ (13CH3D-based apparent equilibrium temperature < 45 °C) to ~0‰ (Tapparent → ∞). The new 13CH3D data is complemented by conventional geochemical analyses (e.g., dissolved ions/organics, δ13C, δD) on samples collected during the same field campaigns. Our study demonstrates that the measurement of 13CH3D provides a new dimension of isotopic constraints for unraveling the complex processes controlling the distribution

  16. Thermal Evolution of Juvenile Subduction Zones ' New Constraints from Lu-Hf Geochronology on HP oceanic rocks (Halilbaǧi, Central Anatolia)

    NASA Astrophysics Data System (ADS)

    Pourteau, Amaury; Scherer, Erik; Schmidt, Alexander; Bast, Rebecca

    2015-04-01

    The thermal structure of subduction zones plays a key role on mechanical and chemical processes taking place along the slab-mantle interface. Until now, changes through time of this thermal structure have been explored mostly by the means of numerical simulations. However, both "warm" (i.e., epidote-bearing), and "cold" (i.e., lawsonite-bearing) HP oceanic rocks have been reported in some fossil subduction complexes exposed at the Earth's surface (e.g., Franciscan Complex, California; Rio San Juan Complex, Hispañola; Halilbağı Unit, Central Anatolia). These a-priori "incompatible" rocks witness different thermal stages of ancient subduction zones and their study might provide complementary constraints to numerical models. To decipher the meaning of these contrasting metamorphic rocks in the Halilbağı Unit, we are carrying out Lu-Hf geochronology on garnet (grt) and lws from a variety of HP oceanic rocks, as well as the metamorphic sole of the overlying ophiolite. We selected five samples that are representative of the variety of metamorphic evolutions (i.e. peak conditions and P-T paths) encountered in this area. Preliminary analyses yielded 110 Ma (grt-hbl isochron) for a sub-ophiolitic grt amphibolite; 92 Ma (grt-omp) for an eclogite with prograde and retrograde ep; 90 Ma (grt-omp) for an eclogitic metabasite with prograde ep and retrograde ep+lws; 87 Ma (grt-gln) for a lws eclogite with prograde ep; and 86 Ma (grt-gln) for a blueschist with prograde and retrograde lws. These ages are mainly two-point isochrons. Further-refined data will be presented at the EGU General Assembly 2015, in Vienna. The consistent younging trend from "warm" to "cold" metamorphic rocks revealed by these first-order results points to metamorphic-sole formation during the initiation of intra-oceanic subduction at ~110 Ma, and subsequent cooling of the slab-mantle interface between 92 and 86 Ma. Therefore, the contrasting metamorphic evolutions encountered in the Halilbağı Unit

  17. Zircon U-Pb Geochronology, Hf Isotopic Composition and Geological Implications of the Neoproterozoic Huashan Group in the Jingshan Area, Northern Yangtze Block, China

    NASA Astrophysics Data System (ADS)

    Yang, Z.; Yang, K.

    2015-12-01

    In the northern Yangtze Block, a clear angular unconformity between the Mesoproterozoic sequences (e.g. Dagushi Group) and the overlying Neoproterozoic strata (e.g. Huashan Group) marks the the Jinning orogeny. A combined study of Lu-Hf isotopes and U-Pb ages for detrital zircons from Huashan Group can provide information on the crustal evolution of sedimentary provenances and the timing of the Jinning orogeny. Detrital zircons from Huashan Group have two major U-Pb age populations of about 2.0Ga, 2.65Ga, and three subordinate age groups of about 0.82Ga, 2.5Ga, 2.9Ga with minor >3.0Ga ages. The youngest five analyses yield a weighted average age of 816±9Ma, which is consistent with that of interlayered basalt (824±9Ma, Deng et al., 2013) and roughly defines the minimum depositional age of Huashan Group. Detrital zircons of Huashan Group mostly have two stage Hf isotope model ages (TDM2) between 3.0 to 3.3Ga, indicating that the northern Yangtze Block experienced significant continental crustal growth during the Paleo- to Meso-archean. Similar U-Pb ages of detrital zircons have been obtained from Precambrian sedimentary rocks in the northern Yangtze Block from previous studies (Liu et al., 2008; Guo et al., 2014 and references therein). Recently, ca. 2.65Ga A-type granites had been reported from the Kongling and Huji area, which likely record the thermally stable lithosphere (Chen et al., 2013; Zhou et al., 2015). In combination with this study, it documents the widespread 2.6-2.7Ga magmatic rocks in the northern Yangtze Block. Zhao et al. (2013) demonstrated both the ca. 850Ma tonalite and trondhjemite of the Huangling igneous complex were formed in a continental arc setting. This suggests the Miaowan-Huashan oceanic basin proposed by Bader et al. (2013) has not been closed at ca. 850Ma. This evidence, together with the depositional age of the Huashan Group, indicates the Jinning orogeny took place at 850-820 Ma. [1] Bader et al., 2013 Tectonics [2] Deng et al

  18. U-Pb-Nd-Hf isotope geochemistry of the Mesoproterozoic A-type granites in Mannefallknausane, western Dronning Maud Land, Antarctica

    NASA Astrophysics Data System (ADS)

    Ramo, O. T.; Kurhila, M.; Luttinen, A. V.; Andersen, T.

    2009-12-01

    The bedrock of western Dronning Maud Land, Antarctica records several stages of anorogenic magmatism. The Grenvillean-age metamorphic basement gneisses of Heimefrontfjella and Mannefallknausane were intruded by mafic dikes (Bauer et al., 2003) and A-type granite plutons (Jacobs, 1991) at circa 1 Ga. A 590 Ma suite of mafic dikes manifests a subsequent episode of Proterozoic anorogenic magmatism (Bauer et al., 2003). Jurassic (180 Ma) continental flood basalts (CFBs), their intrusive equivalents, and associated alkaline mafic rocks represent the third and youngest episode of anorogenic magmatism (Luttinen et al., 1998; Romu and Luttinen, 2007). The crystalline bedrock in western Dronning Maud Land is composed of the Archean Grunehogna craton and the Mesoproterozoic Maud mobile belt. About 100 km south of Archean-Proterozoic transition, in the Proterozoic realm, nunataks of Mannefallknausane (74.5oS, 15oW) are dominated by Precambrian granitoid rocks and rare paragneisses. Three principal granites can be identified: a white, garnet-bearing K-feldspar-megacrystic biotite granite; a red biotite-hornblende±clinopyroxene granite with or without plagioclase-mantled K-feldspar-megacrysts (rapakivi texture); and a dark green porphyritic charnockite with orthopyroxene and hornblende. The presence of rapakivi texture, the mode of occurrence, and geochemical composition of the granites of Mannefallknausane imply A typology. For two varieties of the red granite (wiborgite and pyterlite), our new U-Pb data imply crystallization ages of 1073 ± 6 Ma and 1084 ± 8 Ma, respectively. These are compatible with a U-Pb zircon upper intercept age of 1073 ± 8 Ma of the charnockite (Arndt et al., 1991). The initial Nd isotope composition of these rocks is relatively radiogenic [epsilon-Nd (1075 Ma) value of the biotite granite -0.5; red granite +0.3, +0.5; charnockite +1.4], as is that of a country-rock gneiss from the surrounding bedrock (+1.0). Initial zircon epsilon-Hf values of the

  19. In-situ chemical, U-Pb dating, and Hf isotope investigation of megacrystic zircons, Malaita (Solomon Islands): Evidence for multi-stage alkaline magmatic activity beneath the Ontong Java Plateau

    NASA Astrophysics Data System (ADS)

    Simonetti, Antonio; Neal, Clive R.

    2010-06-01

    Previous investigations of pipe-like intrusions of alnöite within northern Malaita (Solomon Islands) have detailed the chemical and isotopic nature of the alnöite and entrained megacrysts/xenoliths. Alnöite emplacement is poorly constrained since available ages include an Ar-Ar date of 34 Ma (phlogopite) from a mantle xenolith, and a 206Pb/238U date of 33.9 Ma for a zircon megacryst. Hence, we report chemical data, in-situ U-Pb age determinations and Hf isotope compositions for megacrystic zircons recovered from alnöite-derived, ilmenite-rich gravels in the Auluta, Kwainale, and Faufaumela rivers of Malaita. The Zr/Hf ratio (39 to 50) is variable for zircons from Auluta and Faufaumela, whereas it is relatively uniform (40 to 42) in most zircons from Kwainale. Chemical imaging reveals the homogeneous nature for all of the 16 grains analyzed. Trace element compositions obtained by LA-ICP-MS indicate parallel chondrite-normalized REE patterns at variable levels of enrichment; these patterns combined with their low abundances (< 1 to 10 ppm) of U, Th, and Pb confirm their mantle origin. In-situ U-Pb dating conducted by LA-ICP-MS (n = 94 analyses) define a total range in weighted mean (WM) 206Pb/238U ages between ∼ 35 and ∼ 52 Ma. The zircons from Auluta define a range of WM 206Pb/238U ages between 34.9 ± 2.0 Ma and 45.1 ± 2.5 Ma (2σ) that correlate negatively with Zr/Hf ratios and total REE contents. Conversely, the chemically homogeneous zircons from Kwainale define a uniform age spectrum yielding a WM 206Pb/238U age of 36.7 ± 0.5 Ma (2σ). In-situ Hf isotope analyses (n = 30) are uniform and define a WM 176Hf/177Hf value of 0.282933 ± 0.000013 (2σ), which is identical to the previously reported whole rock value for the Malaitan alnöite (0.282939 ± 0.000007). Correlations between ages and chemical compositions (i.e., Auluta zircons), and the uniform Hf isotope compositions are consistent with zircon formation from a common Ontong Java Plateau (OJP

  20. Juvenile accretion (2360-2330 Ma) in the São Francisco Craton, and implications for the Columbia supercontinent: evidence from U-Pb zircon ages, Sr-Nd-Hf and geochemical constraints

    NASA Astrophysics Data System (ADS)

    Teixeira, W.; Ávila, C.

    2012-12-01

    suite is coeval with the Resende Costa pluton. This suite consists of tonalites and trondhjemites with U-Pb crystallization ages of 2356 +3/-2 Ma and 2350 ± 4 Ma, and ɛNd(t) values from +1.0 to +2.1 (TDM = 2.4-2.5 Ga). Chemically, these rocks are metaluminous to slightly peraluminous, with low-Mg#, low-K2O and high-CaO, and varied SiO2 contents (˜62 to 73wt.%). The Lagoa Dourada suite also shows high-Sr/Y ratios (≥41 up to 81), high-(La/Yb)N ratios (≥12 up to 46), and positive Eu/Eu* anomalies, likewise the Resende Costa rocks. From the above, a juvenile accretionary event (2.36-2.33 Ga) is first time defined in the Southern São Francisco craton, correlating well with the early stage of the Itabuna-Salvador-Curaçá belt, according to geologic evidence and isotopic inferences. Roughly contemporary provinces resulted from Early Proterozoic crustal growth are similarly documented in South America and West Africa, Laurentia, Baltica, Siberia, Central Australia, North China and India. From a geodynamic perspective, they are part of a global assembly that took place mainly between 2.0-1.9 Ga - the Columbia Supercontinent - as suggested by geologic correlations, age constraints and paleomagnetic evidences.

  1. Evaluating the provenance of Permian-Triassic and Palaeocene-Eocene ash beds by high precision U-Pb and Lu-Hf isotopic analyses of zircons: linking local sedimentary records to global events

    NASA Astrophysics Data System (ADS)

    Eivind Augland, Lars; Jones, Morgan; Planke, Sverre; Svensen, Henrik; Tegner, Christian

    2016-04-01

    Zircons are a powerful tool in geochronology and isotope geochemistry, as their affinity for U and Hf in the crystal structure and the low initial Pb and Lu allow for precise and accurate dating by U-Pb ID-TIMS and precise and accurate determination of initial Hf isotopic composition by solution MC-ICP-MS analysis. The U-Pb analyses provide accurate chronostratigraphic controls on the sedimentary successions and absolute age frames for the biotic evolution across geological boundaries. Moreover, the analyses of Lu-Hf by solution MC-ICP-MS after Hf-purification column chemistry provide a powerful and robust fingerprinting tool to test the provenance of individual ash beds. Here we focus on ash beds from Permian-Triassic and Palaeocene successions in Svalbard and from the Palaeocene-Eocene Thermal Maximum (PETM) in Fur, Denmark. Used in combination with whole rock geochemistry from the ash layers and the available geochemical and isotopic data from potential source volcanoes, these data are used to evaluate the provenance of the Permian-Triassic and Palaeocene ashes preserved in Svalbard and PETM ashes in Denmark. If explosive eruptions from volcanic centres such as the Siberian Traps and the North Atlantic Igneous Province (NAIP) can be traced to distal basins as ash layers, they provide robust tests of hypotheses of global synchronicity of environmental changes and biotic crises. In addition, the potential correlation of ash layers with source volcanoes will aid in constraining the extent of explosive volcanism in the respective volcanic centres. The new integrated data sets will also contribute to establish new reference sections for the study of these boundary events when combined with stable isotope data and biostratigraphy.

  2. Exotic crustal components at the northern margin of the Bohemian Massif-Implications from Usbnd Thsbnd Pb and Hf isotopes of zircon from the Saxonian Granulite Massif

    NASA Astrophysics Data System (ADS)

    Sagawe, Anja; Gärtner, Andreas; Linnemann, Ulf; Hofmann, Mandy; Gerdes, Axel

    2016-06-01

    The Saxonian Granulite Massif is located at the northern margin of the Saxo-Thuringian Zone of the peri-Gondwana Bohemian Massif. Eight felsic and mafic granulites were studied with respect to their geochemistry and Usbnd Pb zircon geochronology. The felsic granulites are interpreted to be derived from continental crust of possible granitoid composition. An origin from depleted mantle sources with IAT to MORB composition can be assumed for the mafic granulites. The peak of metamorphism is thought to be timed at about 340 Ma, while several earlier metamorphic events are supposed to have occurred at about 355-360, 370-375, 405, and 450 Ma. They reveal a complex and polyphased geologic evolution of the Saxonian Granulite Massif. Protolith emplacement likely took place at c. 450 and 494 Ma. Hf isotopic data suggest Mesoproterozoic crustal ages at least for parts of the massif. As these crustal ages are exotic for the Bohemian Massif, their origin has to be searched elsewhere. Potential source areas could be Amazonia and Baltica, of which the latter is the one preferred. Furthermore, a composite architecture with at least two components-the felsic granulites with Mesoproterozoic crustal model ages, and the mafic granulites of potential island arc origin-is hypothesised. Their amalgamation to the recent appearance of the Saxonian Granulite Massif is likely bracketed between 375 and 340 Ma.

  3. Tracing source terranes using U-Pb-Hf isotopic analysis of detrital zircons: provenance of the Orhanlar Unit of the Palaeotethyan Karakaya subduction-accretion complex, NW Turkey

    NASA Astrophysics Data System (ADS)

    Ustaömer, Timur; Ayda Ustaömer, Petek; Robertson, Alastair; Gerdes, Axel

    2016-04-01

    Sandstones of the Late Palaeozoic-Early Mesozoic Karakaya Complex are interpreted to have accumulated along an active continental margin related to northward subduction of Palaeotethys. The age of deposition and provenance of the sandstones are currently being determined using radiometric dating of detrital zircons, coupled with dating of potential source terranes. Our previous work shows that the U-Pb-Hf isotopic characteristics of the sandstones of all but one of the main tectonostratigraphic units of the Karakaya Complex are compatible with a provenance that was dominated by Triassic and Permo-Carboniferous magmatic arc-type rocks, together with a minor contribution from Lower to Mid-Devonian igneous rocks (Ustaömer et al. 2015). However, one of the tectono-stratigraphic units, the Orhanlar Unit, which occurs in a structurally high position, differs in sedimentary facies and composition from the other units of the Karakaya Complex. Here, we report new isotopic age data for the sandstones of the Orhanlar Unit and also from an extensive, associated tectonic slice of continental metamorphic rocks (part of the regional Sakarya Terrane). Our main aim is to assess the provenance of the Orhanlar Unit sandstones in relation to the tectonic development of the Karakaya Complex as a whole. The Orhanlar Unit is composed of shales, sandstone turbidites and debris-flow deposits, which include blocks of Devonian radiolarian chert and Carboniferous and Permian neritic limestones. The sandstones are dominated by rock fragments, principally volcanic and plutonic rocks of basic-to-intermediate composition, metamorphic rocks and chert, together with common quartz, feldspar and mica. This modal composition contrasts significantly with the dominantly arkosic composition of the other Karakaya Complex sandstones. The detrital zircons were dated by the U-Pb method, coupled with determination of Lu-Hf isotopic compositions using a laser ablation microprobe attached to a multicollector

  4. Detrital zircon U-Pb geochronology, Lu-Hf isotopes and REE geochemistry constrains on the provenance and tectonic setting of Indochina Block in the Paleozoic

    NASA Astrophysics Data System (ADS)

    Wang, Ce; Liang, Xinquan; Foster, David A.; Fu, Jiangang; Jiang, Ying; Dong, Chaoge; Zhou, Yun; Wen, Shunv; Van Quynh, Phan

    2016-05-01

    In situ U-Pb geochronology, Lu-Hf isotopes and REE geochemical analyses of detrital zircons from Cambrian-Devonian sandstones in the Truong Son Belt, central Vietnam, are used to provide the information of provenance and tectonic evolution of the Indochina Block. The combined detrital zircon age spectra of all of the samples ranges from 3699 Ma to 443 Ma and shows with dominant age peaks at ca. 445 Ma and 964 Ma, along with a number of age populations at 618-532 Ma, 1160-1076 Ma, 1454 Ma, 1728 Ma and 2516 Ma. The zircon age populations are similar to those from time equivalent sedimentary sequences in continental blocks disintegrated from the East Gondwana during the Phanerozoic. The younger zircon grains with age peaks at ca. 445 Ma were apparently derived from middle Ordovician-Silurian igneous and metamorphic rocks in Indochina. Zircons with ages older than about 600 Ma were derived from other Gondwana terrains or recycled from the Precambrian basement of the Indochina Block. Similarities in the detrital zircon U-Pb ages suggest that Paleozoic strata in the Indochina, Yangtze, Cathaysia and Tethyan Himalayas has similar provenance. This is consistent with other geological constrains indicating that the Indochina Block was located close to Tethyan Himalaya, northern margin of the India, and northwestern Australia in Gondwana.

  5. Tracing source terranes using U-Pb-Hf isotopic analysis of detrital zircons: provenance of the Orhanlar Unit of the Palaeotethyan Karakaya subduction-accretion complex, NW Turkey

    NASA Astrophysics Data System (ADS)

    Ustaömer, Timur; Ayda Ustaömer, Petek; Robertson, Alastair; Gerdes, Axel

    2016-04-01

    Sandstones of the Late Palaeozoic-Early Mesozoic Karakaya Complex are interpreted to have accumulated along an active continental margin related to northward subduction of Palaeotethys. The age of deposition and provenance of the sandstones are currently being determined using radiometric dating of detrital zircons, coupled with dating of potential source terranes. Our previous work shows that the U-Pb-Hf isotopic characteristics of the sandstones of all but one of the main tectonostratigraphic units of the Karakaya Complex are compatible with a provenance that was dominated by Triassic and Permo-Carboniferous magmatic arc-type rocks, together with a minor contribution from Lower to Mid-Devonian igneous rocks (Ustaömer et al. 2015). However, one of the tectono-stratigraphic units, the Orhanlar Unit, which occurs in a structurally high position, differs in sedimentary facies and composition from the other units of the Karakaya Complex. Here, we report new isotopic age data for the sandstones of the Orhanlar Unit and also from an extensive, associated tectonic slice of continental metamorphic rocks (part of the regional Sakarya Terrane). Our main aim is to assess the provenance of the Orhanlar Unit sandstones in relation to the tectonic development of the Karakaya Complex as a whole. The Orhanlar Unit is composed of shales, sandstone turbidites and debris-flow deposits, which include blocks of Devonian radiolarian chert and Carboniferous and Permian neritic limestones. The sandstones are dominated by rock fragments, principally volcanic and plutonic rocks of basic-to-intermediate composition, metamorphic rocks and chert, together with common quartz, feldspar and mica. This modal composition contrasts significantly with the dominantly arkosic composition of the other Karakaya Complex sandstones. The detrital zircons were dated by the U-Pb method, coupled with determination of Lu-Hf isotopic compositions using a laser ablation microprobe attached to a multicollector

  6. Isotopic constraints on the origin of the Atlantis II, Suakin and Valdivia brines, Red Sea

    USGS Publications Warehouse

    Zierenberg, R.A.; Shanks, Wayne C.

    1986-01-01

    The origin of three Red Sea submarine brine pools was investigated by analysis of the S and O isotope ratios of dissolved sulfate and Sr isotope ratios of dissolved Sr in the brines. Sulfur and O isotope ratios of sulfate and Sr isotope ratios of evaporitic source rocks for the brines were measured for comparison. The S, O and Sr isotope ratios of evaporites recovered from DSDP site 227 are consistent with an upper Miocene evaporites age. The Valdivia Deep brine formed by karstic dissolution of Miocene evaporites by overlying seawater and shows no signs of hydrothermal input. The Suakin Deep brines are derived from, or have isotopically exchanged with Miocene or older evaporites. There has been only minor dilution of the brine by overlying seawater. Strontium isotope ratios of Suakin brine may indicate addition of a minor (15%) amount of volcanic Sr to the brine, but there is no evidence of high temperature brine-rock interaction. The sulfate in the Atlantis II brine was apparently derived from seawater. The O isotope ratio of sulfate in the present Atlantis II brine could reflect isotopic exchange between seawater sulfate and the brine at approximately 255??C. Approximately 30% of the Sr in the Atlantis II brine is derived from the underlying basalt, probably by hydrothermal leaching. Atlantis II brine is the only known example from the Red Sea which has a significant high-temperature hydrothermal history. ?? 1986.

  7. Petrogenesis of Mt. Baker basalts (Cascade arc): Constraints from thermobarometry, phase equilibria, trace elements and isotopes

    NASA Astrophysics Data System (ADS)

    Mullen, E. K.; McCallum, I. S.

    2010-12-01

    Primitive arc basalts provide information on sub-arc mantle compositions and processes. The relative abundance of basalts in the Cascade arc decreases northward, and basalts are rare in the most northerly segment of the arc (Garibaldi belt) where the Mt. Baker volcanic field (MBVF) is located. Following reconstruction of the compositions of the primary basalts at MBVF (olivine addition ± plag subtraction), we have applied phase equilibria and forward-modeled trace element abundances and isotope ratios to obtain petrogenetic constraints. The most primitive lavas are the Sulphur Cr, Lk Shannon, and Park Butte basalts and the Hogback, Tarn Plateau, and Cathedral Crag basaltic andesites, ranging from 716 to 10 ka. Most erupted peripheral to the major centers. Spinel/olivine and Fe-Ti oxide oxybarometry indicate redox states of ~QFM + 1 corresponding to Fe3+/ΣFe = 0.20. Mg# ranges from 51 to 71. The lavas are medium-K and similar to calc-alkaline basalts and high-Mg basaltic andesites from the High Cascades. MBVF basalts have higher MgO and lower CaO and Al2O3 than typical CAB and HAOT, grading to alkalic compositions with TiO2 and Na2O of up to 1.65 and 5.4 wt%, respectively (Sulphur Cr). Phenocryst contents are 5 to 33% (plag + olivine ± cpx) and the lavas are holo- or hypocrystalline with glass contents of up to 15%. The whole rocks are close to equilibrium with olivine cores (range Fo 87-68). Plagioclase cores range from An 88-68. Reconstructed primary basalt compositions give liquidus T and P values (from olivine-liquid equilibria and silica activities) ranging from 1280°C and 1 GPa (Tarn Plateau) to 1350°C and 1.4 GPa (Sulphur Cr), corresponding to the upper mantle above the core of the mantle wedge. These estimates take into account the 1 to 3 wt% initial H2O contents of the basalts calculated using plagioclase cores. Phase equilibria of the primary basalts indicate a similar pressure range of 1-2 GPa and indicate a residual mantle assemblage of harzburgite

  8. Hf-Nd Isotopes in West Philippine Basin Basalts: Results from International Ocean Discovery Program (IODP) Site U1438 and Implications for the Early History of the Izu-Bonin-Mariana (IBM) Subduction System

    NASA Astrophysics Data System (ADS)

    Yogodzinski, G. M.; Hocking, B.; Bizimis, M.; Hickey-Vargas, R.; Ishizuka, O.; Bogus, K.; Arculus, R. J.

    2015-12-01

    Drilling at IODP Site U1438, located immediately west of Kyushu-Palau Ridge (KPR), the site of IBM subduction initiation, penetrated 1460 m of volcaniclastic sedimentary rock and 150 m of underlying basement. Biostratigraphic controls indicate a probable age for the oldest sedimentary rocks at around 55 Ma (51-64 Ma - Arculus et al., Nat Geosci in-press). This is close to the 48-52 Ma time period of IBM subduction initiation, based on studies in the forearc. There, the first products of volcanism are tholeiitic basalts termed FAB (forearc basalt), which are more depleted than average MORB and show subtle indicators of subduction geochemical enrichment (Reagan et al., 2010 - Geochem Geophy Geosy). Shipboard data indicate that Site U1438 basement basalts share many characteristics with FABs, including primitive major elements (high MgO/FeO*) and strongly depleted incompatible element patterns (Ti, Zr, Ti/V and Zr/Y below those of average MORB). Initial results thus indicate that FAB geochemistry may have been produced not only in the forearc, but also in backarc locations (west of the KPR) at the time of subduction initiation. Hf-Nd isotopes for Site 1438 basement basalts show a significant range of compositions from ɛNd of 7.0 to 9.5 and ɛHf of 14.5 to 19.8 (present-day values). The data define a well-correlated and steep array in Hf-Nd isotope space. Relatively radiogenic Hf compared to Nd indicates an Indian Ocean-type MORB source, but the dominant signature, with ɛHf >16.5, is more radiogenic than most Indian MORB. The pattern through time is from more-to-less radiogenic and more variable Hf-Nd isotopes within the basement section. This pattern culminates in basaltic andesite sills, which intrude the lower parts of the sedimentary section. The sills have the least radiogenic compositions measured so far (ɛNd ~6.6, ɛHf ~13.8), and are similar to those of boninites of the IBM forearc and modern IBM arc and reararc rocks. The pattern within the basement

  9. U-series Isotope Constraints on Rhyolite Generation at South Sister and Newberry Volcanoes, Central Oregon

    NASA Astrophysics Data System (ADS)

    Mitchell, E. C.; Asmerom, Y.

    2011-12-01

    fractionated. In the same area the most primitive mafic lavas have 230Th excesses of ˜ 20-40% and 226Ra excesses of ˜ 50-125%. If the South Sister rhyolites are crustal melts, then the Sr and Nd data preclude any significant involvement of old continental crust, and require the source to be isotopically similar to recent mafic lavas. Such crustal melts have been implicated in the genesis of lavas in central Oregon and elsewhere along the arc. The lack of 238U or 230Th excess suggests either i) the lack of any restite phase able to fractionate U from Th; or ii) a high degree of melting; or iii) melt crustal residence of > ˜ 350 ky. The small 230Th excesses observed in the Newberry obsidians are consistent with their being small degree melts of a garnet-bearing crustal source. The AFC model for the South Sister rhyolites requires apatite to be present in the fractionating assemblage, which will typically generate 238U excesses, inconsistent with the U-series data. Trace element modeling will place further constraints on the processes that generated these rocks and the observed (230Th/238U) ratios.

  10. Re - Os isotopic constraints on the origin of volcanic rocks, Gorgona Island, Colombia: Os isotopic evidence for ancient heterogeneities in the mantle

    USGS Publications Warehouse

    Walker, R.J.; Echeverria, L.M.; Shirey, S.B.; Horan, M.F.

    1991-01-01

    The Re - Os isotopic systematics of komatiites and spatially associated basalts from Gorgona Island, Colombia, indicate that they were produced at 155??43 Ma. Subsequent episodes of volcanism produced basalts at 88.1??3.8 Ma and picritic and basaltic lavas at ca. 58 Ma. The age for the ultramafic rocks is important because it coincides with the late-Jurassic, early-Cretaceous disassembly of Pangea, when the North- and South-American plates began to pull apart. Deep-seated mantle upwelling possibly precipitated the break-up of these continental plates and caused a tear in the subducting slab west of Gorgona, providing a rare, late-Phanerozoic conduit for the komatiitic melts. Mantle sources for the komatiites were heterogeneous with respect to Os and Pb isotopic compositions, but had homogeneous Nd isotopic compositions (??Nd+9??1). Initial 187Os/186Os normalized to carbonaceous chondrites at 155 Ma (??Os) ranged from 0 to +22, and model-initial ?? values ranged from 8.17 to 8.39. The excess radiogenic Os, compared with an assumed bulk-mantle evolution similar to carbonaceous chondrites, was likely produced in portions of the mantle with long-term elevated Re concentrations. The Os, Pb and Nd isotopic compositions, together with major-element constraints, suggest that the sources of the komatiites were enriched more than 1 Ga ago by low (<20%) and variable amounts of a basalt or komatiite component. This component was added as either subducted oceanic crust or melt derived from greater depths in the mantle. These results suggest that the Re - Os isotope system may be a highly sensitive indicator of the presence of ancient subducted oceanic crust in mantle-source regions. ?? 1991 Springer-Verlag.

  11. Constraints on chondrule origin from petrology of isotopically characterized chondrules in the Allende meteorite

    SciTech Connect

    Mcsween, H.Y. Jr.

    1985-09-01

    The petrologic and chemical properties of the ferromagnesian chondrules in the Allende carbonaceous chondrite were examined in terms of the isotopic composition and the correlations between isotopic patterns. Areas of thin sections were studied with a SEM and bulk chemical fractions of 12 constituents were quantified to calculate correlations with petrologic features. A possible correlation between (CaO + Al2O2)/MgO and oxygen isotopes imply the formation of oxygen isotopic compositions in the chondrules by exchanges between isotopically heavy nebular gases and O-16 enriched solids. Different rates of gaseous exchange occurred with the various types of chondrules. Factors which may have controlled the exchanges are discussed. 21 references.

  12. Zircon U-Pb age and Sr-Nd-Hf isotope geochemistry of the Ganluogou dioritic complex in the northern Triassic Yidun arc belt, Eastern Tibetan Plateau: Implications for the closure of the Garzê-Litang Ocean

    NASA Astrophysics Data System (ADS)

    Wu, Tao; Xiao, Long; Wilde, Simon A.; Ma, Chang-Qian; Li, Zi-Long; Sun, Yi; Zhan, Qiong-Yao

    2016-04-01

    The Triassic Yidun arc belt (YAB) lies between the Jinshajiang suture zone to the west and the Garzê-Litang suture zone to the east, Eastern Tibetan Plateau. To study the YAB can not only help us to better understand the evolutionary history of the Garzê-Litang Ocean but can also provide some important information to constrain the evolution of the eastern Paleo-Tethys. In this paper, the geochronological and geochemical data of the Ganluogou dioritic complex were systematically investigated in order to decipher the geodynamic setting of the complex and to further determine the final closure time of the Garzê-Litang Ocean. The Ganluogou dioritic complex is located in the northern part of the YAB. It consists of ferrodiorite, diorite and a mixing zone between them and is the largest intermediate-mafic pluton in the YAB. The ferrodiorites were emplaced at 213 ± 2 Ma have low SiO2 and high Fe2O3* contents, whereas the diorites formed at 209 ± 2 Ma and have relatively higher SiO2, Na2O + K2O, Th, U, Zr, and Hf contents, but lower Al2O3, MgO, CaO, Co, and Sr contents than the ferrodiorites. Relative to the primitive mantle both the ferrodiorites and diorites are depleted in Nb and Ta. However, the ferrodiorites exhibit strong depletion in Zr and Hf, whereas the diorites contain relatively higher Th and U contents without negative Zr and Hf anomalies. Both rock-types have similar chondrite-normalized rare earth element patterns with (La/Yb)N ratios = 4.4 to 18.2, and show weak Eu anomalies, with Eu/Eu* of 0.47 to 1. They both show narrow ranges in Sr-Nd-Hf isotopic compositions. However, the ferrodiorites contain lower initial 87Sr/86Sr ratios (0.7052-0.7057) and relatively higher εNd(t) values (- 3.8 to - 2.4) than the diorites, which record values of 0.7062-0.7066 and - 5.5 to - 5.7, respectively. For the zircon Hf isotopic composition, the ferrodiorites also exhibit higher 176Hf/177Hf ratios (0.282738-0.282804) and more depleted εHf(t) values (3.4-5.6) than

  13. Triple oxygen and multiple sulfur isotope constraints on the evolution of the post-Marinoan sulfur cycle

    NASA Astrophysics Data System (ADS)

    Crockford, Peter W.; Cowie, Benjamin R.; Johnston, David T.; Hoffman, Paul F.; Sugiyama, Ichiko; Pellerin, Andre; Bui, Thi Hao; Hayles, Justin; Halverson, Galen P.; Macdonald, Francis A.; Wing, Boswell A.

    2016-02-01

    Triple oxygen isotopes within post-Marinoan barites have played an integral role in our understanding of Cryogenian glaciations. Reports of anomalous Δ17O values within cap carbonate hosted barites however have remained restricted to South China and Mauritania. Here we extend the Δ17O anomaly to northwest Canada with our new measurements of barites from the Ravensthroat cap dolostone with a minimum Δ17O value of - 0.75 ‰. For the first time we pair triple oxygen with multiple sulfur isotopic data as a tool to identify the key processes that controlled the post-Marinoan sulfur cycle. We argue using a dynamic 1-box model that the observed isotopic trends both in northwest Canada and South China can be explained through the interplay between sulfide weathering, microbial sulfur cycling and pyrite burial. An important outcome of this study is a new constraint placed on the size of the post-Marinoan sulfate reservoir (≈0.1% modern), with a maximum concentration of less than 10% modern. Through conservative estimates of sulfate fluxes from sulfide weathering and under a small initial sulfate reservoir, we suggest that observed isotopic trends are the product of a dynamic sulfur cycle that saw both the addition and removal of the Δ17O anomaly over four to five turnovers of the post-Marinoan marine sulfate reservoir.

  14. Zircon U-Pb dating, geochemistry and Sr-Nd-Pb-Hf isotopes of the Wajilitag alkali mafic dikes, and associated diorite and syenitic rocks: Implications for magmatic evolution of the Tarim large igneous province

    NASA Astrophysics Data System (ADS)

    Zou, Si-Yuan; Li, Zi-Long; Song, Biao; Ernst, Richard E.; Li, Yin-Qi; Ren, Zhong-Yuan; Yang, Shu-Feng; Chen, Han-Lin; Xu, Yi-Gang; Song, Xie-Yan

    2015-01-01

    The Early Permian Tarim large igneous province (Tarim LIP) consists mainly of basaltic lavas, mafic-ultramafic intrusions including dikes and, syenite bodies in the Tarim Basin, NW China. A major unit of the Tarim LIP, the Wajilitag intrusive complex, consists of olivine pyroxenite, clinopyroxenite and gabbro units (from bottom to top), diorite and syenite rocks occurred in the upper part of the complex and alkali mafic dikes intrude the clinopyroxenite phase. Here we report the zircon U-Pb age and Hf isotopes, geochemical characteristics and Sr-Nd-Pb isotopic data of the alkali mafic dikes, and diorite, aegirine-nepheline syenite and syenite porphyry units in the Wajilitag intrusive complex. Zircons from the diorite and alkali mafic rocks yield concordant crystallization ages of 275.2 ± 1.2 Ma and 281.4 ± 1.7 Ma, respectively. The diorite and syenitic rocks in Wajilitag area have a narrow range of SiO2 contents (51.9-57.3 wt.%), and are enriched in total alkalis (Na2O + K2O = 8.3-14.3 wt.%), among which the aegirine-nepheline syenite and syenite porphyry have the geochemical affinity of A-type granites. The alkali mafic rocks and syenitic rocks have high Al2O3 (19.4-21.1 wt.%), Zr, Hf, Ba contents, total rare earth element abundances and LREE/HREE ratios and low Mg# value, K, P and Ti contents. Diorites have lower Al2O3 contents, total REE abundances and LREE/HREE ratios and higher Mg# values than the alkali mafic rocks and syenitic rocks. The diorites and syenitic rocks have low initial 87Sr/86Sr ratios (0.7034-0.7046), and high εNd(t) values (0.1-4.1) and zircon εHf(t) values (- 0.9-4.4). All the diorites and syenitic rocks show the 206Pb/204Pb ratios ranging of 18.0-19.5, 207Pb/204Pb of 15.4-15.6 and 208Pb/204Pb of 38.0-39.9. Sr-Nd isotopic ratios indicate a FOZO-like mantle source for the diorite and syenitic rocks, similar to that of the mafic-ultramafic rocks in the Wajilitag complex. In contrast, zircon Hf isotopes of basalt and syenite elsewhere in the

  15. The Schwartzwalder uranium deposit, II: Age of uranium mineralization and lead isotope constraints on genesis.

    USGS Publications Warehouse

    Ludwig, K. R.; Wallace, A.R.; Simmons, K.R.

    1985-01-01

    Schwartzwalder ores have high amounts of initial (common) Pb that was both variable and relatively radiogenic in its isotope ratios. Assuming the common Pb in these ores to have sources of similar age and similar Th/U, samples with initial Pb isotope ratios are identified - and others with variable initial ratios are normalized - to obtain U-Pb isochrons yielding an early Laramide age of 69.3 + or - 1.1 m.y. for the ores. The initial Pb-isotope systematics indicate local sources of U, dispersed in concentrations <100 ppm, in rocks of 1730 + or - 130 m.y. age. -G.J.N.

  16. Constraints on the vital effect in coccolithophore and dinoflagellate calcite by oxygen isotopic modification of seawater

    NASA Astrophysics Data System (ADS)

    Hermoso, Michaël; Horner, Tristan J.; Minoletti, Fabrice; Rickaby, Rosalind E. M.

    2014-09-01

    In this study, we show that there are independent controls of 18O/16O and 13C/12C fractionation in coccolithophore and dinoflagellate calcite due to the contrasting kinetics of each isotope system. We demonstrate that the direction and magnitude of the oxygen isotope fractionation with respect to equilibrium is related to the balance between calcification rate and the replenishment of the internal pool of dissolved inorganic carbon (DIC). As such, in fast growing cells, such as those of Emiliania huxleyi and Gephyrocapsa oceanica (forming the so-called “heavy group”), calcification of the internal carbon pool occurs faster than complete isotopic re-adjustment of the internal DIC pool with H2O molecules. Hence, coccoliths reflect the heavy oxygen isotope signature of the CO2 overprinting the whole DIC pool. Conversely, in large and slow growing cells, such as Coccolithus pelagicus ssp. braarudii, complete re-equilibration is achieved due to limited influx of CO2 leading to coccoliths that are precipitated in conditions close to isotopic equilibrium (“equilibrium group”). Species exhibiting the most negative oxygen isotope composition, such as Calcidiscus leptoporus (“light group”), precipitate coccolith under increased pH in the coccolith vesicle, as previously documented by the “carbonate ion effect”. We suggest that, for the carbon isotope system, any observed deviation from isotopic equilibrium is only “apparent”, as the carbon isotopic composition in coccolith calcite is controlled by a Rayleigh fractionation originating from preferential incorporation of 12C into organic matter. Therefore, species with low PIC/POC ratios as E. huxleyi and G. oceanica are shifted towards positive carbon isotope values as a result of predominant carbon fixation into the organic matter. By contrast, cells with higher PIC/POC as C. braarudii and C. leptoporus maintain, to some extent, the original negative isotopic composition of the CO2. The calcareous

  17. Zircon U-Pb dating, geochemical and Sr-Nd-Hf isotopic characteristics of the Jintonghu monzonitic rocks in western Fujian Province, South China: Implication for Cretaceous crust-mantle interactions and lithospheric extension

    NASA Astrophysics Data System (ADS)

    Li, Bin; Jiang, Shao-Yong; Lu, An-Huai; Zhao, Hai-Xiang; Yang, Tang-Li; Hou, Ming-Lan

    2016-09-01

    Comprehensive petrological, in situ zircon U-Pb dating, Ti-in-zircon temperature and Hf isotopic compositions, whole rock geochemical and Sr-Nd isotopic data are reported for the Jintonghu monzonitic intrusions in the western Fujian Province (Interior Cathaysia Block), South China. The Jintonghu monzonitic intrusions were intruded at 95-96 Ma. Their Sr-Nd-Hf isotopic compositions are similar to the coeval and nearby enriched lithospheric mantle-derived mafic and syenitic rocks, indicating that the Jintonghu monzonitic rocks were likely derived from partial melting of the enriched mantle sources. Their high Nb/Ta ratios (average 21.6) suggest that the metasomatically enriched mantle components were involved, which was attributed to the modification of slab-derived fluid and melt by the subduction of the paleo-Pacific Plate. The presence of mafic xenoliths, together with geochemical and isotopic features indicates a mafic-felsic magma mixing. Furthermore, the Jintonghu intrusions may have experienced orthopyroxene-, biotite- and plagioclase-dominated crystallization. Crust-mantle interaction can be identified as two stages, including that the Early Cretaceous mantle metasomatism and lithospheric extension resulted from the paleo-Pacific slab subduction coupled with slab rollback, and the Late Cretaceous crustal activation and enhanced extension induced by dip-angle subduction and the underplating of mantle-derived mafic magma.

  18. Complementary constraints from carbon (13C) and nitrogen (15N) isotopes on the glacial ocean's soft-tissue biological pump

    NASA Astrophysics Data System (ADS)

    Schmittner, A.; Somes, C. J.

    2016-06-01

    during the LGM. Both circulation and biological nutrient utilization could contribute. However, these conclusions are preliminary given our idealized experiments, which do not consider changes in benthic denitrification and spatially inhomogenous changes in aeolian iron fluxes. The analysis illustrates interactions between the carbon and nitrogen cycles as well as the complementary constraints provided by their isotopes. Whereas carbon isotopes are sensitive to circulation changes and indicate well the three-dimensional Corg distribution, nitrogen isotopes are more sensitive to biological nutrient utilization.

  19. Mg isotope constraints on soil pore-fluid chemistry: Evidence from Santa Cruz, California

    NASA Astrophysics Data System (ADS)

    Tipper, Edward T.; Gaillardet, Jérôme; Louvat, Pascale; Capmas, Françoise; White, Art F.

    2010-07-01

    Mg isotope ratios ( 26Mg/ 24Mg) are reported in soil pore-fluids, rain and seawater, grass and smectite from a 90 kyr old soil, developed on an uplifted marine terrace from Santa Cruz, California. Rain water has an invariant 26Mg/ 24Mg ratio (expressed as δ26Mg) at -0.79 ± 0.05‰, identical to seawater δ26Mg. Detrital smectite (from the base of the soil profile, and therefore unweathered) has a δ26Mg value of 0.11‰, potentially enriched in 26Mg by up to 0.3‰ compared to the bulk silicate Earth Mg isotope composition (although within the range of all terrestrial silicates). The soil pore-waters show a continuous profile with depth for δ26Mg, ranging from -0.99‰ near the surface to -0.43‰ at the base of the profile. Shallow pore-waters (<1 m) have δ26Mg values that are similar to, or slightly lower than the rain waters. This implies that the degree of biological cycling of Mg in the pore-waters is relatively small and is quantified as <32%, calculated using the average Mg isotope enrichment factor between grass and rain ( δ26Mg-δ26Mg) of 0.21‰. The deep pore-waters (1-15 m deep) have δ26Mg values that are intermediate between the smectite and rain, ranging from -0.76‰ to -0.43‰, and show a similar trend with depth compared to Sr isotope ratios. The similarity between Sr and Mg isotope ratios confirms that the Mg in the pore-waters can be explained by a mixture between rain and smectite derived Mg, despite the fact that Mg and Sr concentrations may be buffered by the exchangeable reservoir. However, whilst Sr isotope ratios in the pore-waters span almost the complete range between mineral and rain inputs, Mg isotopes compositions are much closer to the rain inputs. If Mg and Sr isotope ratios are controlled uniquely by a mixture, the data can be used to estimate the mineral weathering inputs to the pore-waters, by correcting for the rain inputs. This isotopic correction is compared to the commonly used chloride correction for precipitation

  20. Geochemistry, zircon U-Pb and Lu-Hf isotopes of an Early Cretaceous intrusive suite in northeastern Jiangxi Province, South China Block: Implications for petrogenesis, crust/mantle interactions and geodynamic processes

    NASA Astrophysics Data System (ADS)

    Deng, Zhengbin; Liu, Shuwen; Zhang, Lifei; Wang, Zongqi; Wang, Wei; Yang, Pengtao; Luo, Ping; Guo, Boran

    2014-07-01

    The Early Cretaceous Tieshan intrusive suite, in northeastern Jiangxi Province along the northern margin of the Eastern Cathaysia Block, is composed of diabase porphyrites, monzodiorites, syenite porphyries, quartz monzonites, monzogranites and granite porphyries. LA-ICPMS zircon U-Pb isotopic analyses reveal that this intrusive complex was emplaced between 142 Ma and 117 Ma. The ~ 135 Ma diabase porphyrites, monzodiorites, and syenite porphyries are characterized by low to moderate SiO2 and MgO contents, with high K2O and total alkaline contents. These rocks exhibit slightly to strongly fractionated REE patterns and upper crust-like multi-element patterns with depletions of Nb, Ta and Ti, and show strongly negative εHf (t) values of - 9.0 to - 11.8. All these patterns are identical to those of the Caiyuan syenites, Huangtuling gabbros in the east, and Lengshuikeng trachyandesites and quartz syenites in the west. These geochemical and zircon Lu-Hf isotopic features indicate that their magmatic precursors were generated by 0.2%-2% partial melting of a phlogopite-bearing enriched subcontinental lithospheric mantle source that was metasomatized by sediments. The ~ 117 Ma quartz monzonite has slightly higher εHf (t) values (- 5.6 to - 8.7) like those of the Honggong syenites, indicating an interaction between the asthenosphere and the lithosphere. The ~ 142-134 Ma granite porphyries and monzogranites are characterized by high SiO2 levels but low concentrations of refractory elements, and show enrichment of LREEs and LILEs, with variable negative anomalies of Nb, Ta, Ti, Sr, P and Ba in multi-element diagrams normalized by primitive mantle. The monzogranite exhibits strongly negative εHf (t) values of - 10.5 to - 13.3 and TDM2 (Hf) values of 1849-2023 Ma, and the granite porphyries display relatively wide εHf (t) values of - 7.2 to - 13.4 and TDM2 (Hf) values of 1645-2043 Ma, indicating that these monzogranites and granite porphyries are highly fractionated granites

  1. Trace element and isotopic constraints on magmatic evolution at Lassen volcanic center

    SciTech Connect

    Bullen, T.D.; Clynne, M.A. )

    1990-11-10

    Magmatic evolution at the Lassen volcanic center (LVC) is characterized by a transition from predominantly andesitic to predominantly silicic volcanism with time. Magmas of the adesitic, or Brokeoff phase of volcanism range in composition from basaltic andesite to dacite, whereas those of silicic, or Lassen phase range in composition from basaltic andesite to rhyolite. The distinctive mixing-dominated arrays for each volcanic phase manifest the generation and evolution of two physically distinct, but genetically related magma systems. The LVC magmas have Sr, Nd, and Pb isotope characteristics that approximate two-component mixing arrays. One isotopic component is similar in composition to that of NE Pacific Ocean ridge and seamount basalts (MORB component), the other to mafic Mesozoic granitoids sampled from the neighboring Klamath and Sierra Nevada provinces (KSN component). The lack of a correlation between the major element and isotopic compositions of LVC magmas seriously limits any model for magmatic evolution that relies on assimilation of old middle to upper crust by isotopically homogeneous mafic magmas during their ascent through the crust. Alternatively, the isotopic and geochemical uniformity of the most silicic magmas of the Brokeoff and Lassen phases suggests that they are well-homogenized partial melts. The likely source region for these silicic melts is the lower crust, which the authors envision to consist primarily of mafic igneous rocks that are similar in geochemical and isotopic diversity to the regional mafic lavas.

  2. Paleocene and Early Eocene volcanic ash layers in the Schlieren Flysch, Switzerland: U-Pb dating and Hf-isotopes of zircons, pumice geochemistry and origin

    NASA Astrophysics Data System (ADS)

    Koch, Simone; Winkler, Wilfried; Von Quadt, Albrecht; Ulmer, Peter

    2015-11-01

    Thin mm to cm thick bentonite layers of Paleocene to Early Eocene age in the Tonsteinschichten of the Schlieren Flysch represent volcanic ash layers. Heavy mineral analysis of the layers indicates basic to acidic volcanic sources. U/Pb dating of single zircon crystals of a Paleocene layer (WW1948) by LA-ICP-MS points to an eruption at 59.87 ± 0.41 Ma, whereas ID-TIMS shows an eruption age of 60.96 ± 0.07 Ma. Taking into account the external precision of LA-ICP-MS analyses of 1-2% both ages are overlapping and indicate an apparent minimal durations of zircon crystallization of 350 ka. Hf-isotope analysis of the same zircon crystals reveals the hybrid character of the source magma. The geochemical composition of the pumice grains of all bentonite layers is strongly affected by alteration. Nevertheless, the original character of the volcanic source can be evaluated. The Paleocene ashes (Lower Tonsteinschichten, LT) show a more fractionated multi-element pattern than the ashes of Early Eocene (Upper Tonsteinschichten, UT). The LT ash series are of rhyodacite to dacite character whereas the UT ashes fall in the field of alkali basalts. Both ash series seem to originate from a within-plate volcanic setting according to their trace element concentrations. Geochemical and temporary counterparts can be found in ash layers from Anthering (Austria) and the Danish Basin. As proposed for those ashes, volcanism connected to the opening of the North Atlantic might be the source as well for the ashes in the Schlieren Flysch. By comparison of the composition of rocks from the British Paleogene Igneous Province BPIP and the Schlieren Flysch ashes many correlations can be drawn which supports the suggestion of a North Atlantic origin of the Alpine ashes.

  3. New constraints on the sources and behavior of neodymium and hafnium in seawater from Pacific Ocean ferromanganese crusts

    USGS Publications Warehouse

    van de Flierdt, T.; Frank, M.; Lee, D.-C.; Halliday, A.N.; Reynolds, B.C.; Hein, J.R.

    2004-01-01

    The behavior of dissolved Hf in the marine environment is not well understood due to the lack of direct seawater measurements of Hf isotopes and the limited number of Hf isotope time-series obtained from ferromanganese crusts. In order to place better constraints on input sources and develop further applications, a combined Nd-Hf isotope time-series study of five Pacific ferromanganese crusts was carried out. The samples cover the past 38 Myr and their locations range from sites at the margin of the ocean to remote areas, sites from previously unstudied North and South Pacific areas, and water depths corresponding to deep and bottom waters. For most of the samples a broad coupling of Nd and Hf isotopes is observed. In the Equatorial Pacific ENd and EHf both decrease with water depth. Similarly, ENd and EHf both increase from the South to the North Pacific. These data indicate that the Hf isotopic composition is, in general terms, a suitable tracer for ocean circulation, since inflow and progressive admixture of bottom water is clearly identifiable. The time-series data indicate that inputs and outputs have been balanced throughout much of the late Cenozoic. A simple box model can constrain the relative importance of potential input sources to the North Pacific. Assuming steady state, the model implies significant contributions of radiogenic Nd and Hf from young circum-Pacific arcs and a subordinate role of dust inputs from the Asian continent for the dissolved Nd and Hf budget of the North Pacific. Some changes in ocean circulation that are clearly recognizable in Nd isotopes do not appear to be reflected by Hf isotopic compositions. At two locations within the Pacific Ocean a decoupling of Nd and Hf isotopes is found, indicating limited potential for Hf isotopes as a stand-alone oceanographic tracer and providing evidence of additional local processes that govern the Hf isotopic composition of deep water masses. In the case of the Southwest Pacific there is

  4. A lead isotopic study of the Stillwater Complex, Montana: constraints on crustal contamination and source regions

    USGS Publications Warehouse

    Wooden, J.L.; Czamanske, G.K.; Zientek, M.L.

    1991-01-01

    Analyses of the Pb isotopic compositions of plagioclase from 23 samples covering the stratigraphic thickness of the Stillwater Complex indicate a narrow range of apparent initial isotopic compositions (206Pb/ 204Pb=13.95; 207Pb/204Pb=14.95-15.01; 208Pb/204Pb=33.6). The uniformity of our data is in contrast to, but not necessarily contradictory to, other recent investigations which give indications that the complex formed by repeated injection of magmas with at least two distinct compositions that were presumably derived from different source regions. Samples from the Basal series of the complex have consistently higher 207Pb/204Pb ratios, suggesting either minor contamination from adjacent country rocks or a slight distinction between parental magmas. Apparent initial Pb isotopic compositions of the complex are very radiogenic compared to Late Archean model-mantle values, but are nearly identical to initial Pb isotopic compositions found for the the adjacent, slightly older (2.73-2.79 Ga), Late Archean crustal suite in the Beartooth Mountains. Contamination of magmas parental to the Stillwater Complex by the Late Archean crustal suite is rejected for two reasons: (1) Th and U concentrations in Stillwater rocks and plagioclase are very low (about 0.08 and 0.02 ppm respectively), yet Th/U ratios are uniform at about 4, in contrast to the highly variable (2-26) but often high Th/U ratios found for the Late Archean crustal complex; (2) it seems improbable that any contamination process would have adjusted the isotopic compositions of the diverse magmas entering the Stillwater chamber to near-identical values. The preferred hypothesis to explain the Pb isotopic data for the Stillwater Complex and the associated Late Archean crustal suite involves a major Late Archean crust-forming event that resulted in a compositionally complex crust/mantle system with relatively homogeneous and unusual Pb isotopic compositions. The parental magmas of the Stillwater Complex were

  5. Os isotopic composition of steels: Constraints on sources of Os in steel & crustal isotopic evolution of iron ores

    NASA Astrophysics Data System (ADS)

    Chatterjee, R. N.; Lassiter, J. C.

    2013-12-01

    Metal contamination during sample processing is a potential concern in Os-isotope studies. We examined Os concentrations and Os isotopes in industrial steels. Samples include high Cr stainless steels (>10.5% Cr), low alloy steels (>=92% Fe) and high alloy steels (<92% Fe). The chief components used to make steel are iron ore, chromites and coke. Coke is derived from coals that have low Os concentration (~36 ppt) [1]. Chromites in steels are mined from chromitites, which have high average Os concentrations and mantle-like 187Os/188Os ratios (~88 ppb Os, 187Os/188Os ≈ 0.127×24) [2]. Iron ores used in US steel manufacturing derive chiefly from magnetites mined from iron-bearing formations such as Banded Iron Formations (BIF), which have median Os concentration of ~4.8 ppb and radiogenic 187Os/188Os ≈ 0.358×388 [3]. Os concentrations in the measured steels span a wide range, from 0.03 to 22 ppb. The 187Os/188Os ratios vary from 0.144-4.12. Such high Os concentrations and radiogenic isotopic compositions confirm that metal contamination can affect Os-isotope compositions during sample processing, particularly for low-[Os] samples. There is no correlation between C and Os concentration in steel, indicating that coke is not a major Os source in steels. Os concentrations in steels are positively correlated with Cr content, suggesting that chromite-derived Os dominates the Os budget in stainless steels. 187Os/188Os is negatively correlated with Cr content, ranging from 0.144-0.195 in high-Cr (>10.5 % Cr) steels but from 0.279-4.12 in low-Cr steels. In addition, there is a positive correlation between 1/Os and 187Os/188Os, consistent with two-component mixing of Os derived from magnetite ore and chromites. Lower Os concentrations in steels than expected from simple mixing of magnetite and chromitite suggest some volatile Os loss during smelting. Although the current data is limited, the 186Os-187Os trend defined by the steel analyses can be utilized to extrapolate

  6. Zirconium isotope constraints on the composition of Theia and current Moon-forming theories

    NASA Astrophysics Data System (ADS)

    Akram, W.; Schönbächler, M.

    2016-09-01

    The giant impact theory is the most widely recognized formation scenario of the Earth's Moon. Giant impact models based on dynamical simulations predict that the Moon acquired a significant amount of impactor (Theia) material, which is challenging to reconcile with geochemical data for O, Si, Cr, Ti and W isotopes in the Earth and Moon. Three new giant impact scenarios have been proposed to account for this discrepancy - hit-and-run impact, impact with a fast-spinning protoEarth and massive impactors - each one reducing the proportion of the impactor in the Moon compared to the original canonical giant impact model. The validity of each theory and their different dynamical varieties are evaluated here using an integrated approach that considers new high-precision Zr isotope measurements of lunar rocks, and quantitative geochemical modelling of the isotopic composition of the impactor Theia. All analysed lunar samples (whole-rock, ilmenite and pyroxene separates) display identical Zr isotope compositions to that of the Earth within the uncertainty of 13 ppm for 96Zr/90Zr (2σ weighted average). This 13 ppm upper limit is used to infer the most extreme isotopic composition that Theia could have possessed, relative to the Earth, for each of the proposed giant impact theories. The calculated Theian composition is compared with the Zr isotope compositions of different solar system materials in order to constrain the source region of the impactor. As a first order approximation, we show that all considered models (including the canonical) are plausible, alleviating the initial requirement for the new giant impact models. Albeit, the canonical and hit-and-run models are the most restrictive, suggesting that the impactor originated from a region close to the Earth. The fast-spinning protoEarth and massive impactor models are more relaxed and increase the allowed impactor distance from the Earth. Similar calculations carried out for O, Cr, Ti and Si isotope data support

  7. Constraints on ocean circulation at the Paleocene-Eocene Thermal Maximum from neodymium isotopes

    NASA Astrophysics Data System (ADS)

    Abbott, April N.; Haley, Brian A.; Tripati, Aradhna K.; Frank, Martin

    2016-04-01

    Global warming during the Paleocene-Eocene Thermal Maximum (PETM) ˜ 55 million years ago (Ma) coincided with a massive release of carbon to the ocean-atmosphere system, as indicated by carbon isotopic data. Previous studies have argued for a role of changing ocean circulation, possibly as a trigger or response to climatic changes. We use neodymium (Nd) isotopic data to reconstruct short high-resolution records of deep-water circulation across the PETM. These records are derived by reductively leaching sediments from seven globally distributed sites to reconstruct past deep-ocean circulation across the PETM. The Nd data for the leachates are interpreted to be consistent with previous studies that have used fish teeth Nd isotopes and benthic foraminiferal δ13C to constrain regions of convection. There is some evidence from combining Nd isotope and δ13C records that the three major ocean basins may not have had substantial exchanges of deep waters. If the isotopic data are interpreted within this framework, then the observed pattern may be explained if the strength of overturning in each basin varied distinctly over the PETM, resulting in differences in deep-water aging gradients between basins. Results are consistent with published interpretations from proxy data and model simulations that suggest modulation of overturning circulation had an important role for initiation and recovery of the ocean-atmosphere system associated with the PETM.

  8. Isotopic Constraints on the Chemical Evolution of Geothermal Fluids, Long Valley, CA

    SciTech Connect

    Brown, Shaun; Kennedy, Burton; DePaolo, Donald; Evans, William

    2008-08-01

    A spatial survey of the chemical and isotopic composition of fluids from the Long Valley hydrothermal system was conducted. Starting at the presumed hydrothermal upwelling zone in the west moat of the caldera, samples were collected from the Casa Diablo geothermal field and a series of monitoring wells defining a nearly linear, ~;;14 km long, west-to-east trend along the proposed fluid flow path (Sorey et al., 1991). Samples were analyzed for the isotopes of water, Sr, Ca, and noble gases, the concentrations of major cations and anions and total CO2. Our data confirm earlier models in which the variations in water isotopes along the flow path reflect mixing of a single hydrothermal fluid with local groundwater. Variations in Sr data are poorly constrained and reflect fluid mixing, multiple fluid-pathways or water-rock exchange along the flow path as suggested by Goff et al. (1991). Correlated variations among total CO2, noble gases and the concentration and isotopic composition of Ca suggest progressive fluid degassing (loss of CO2, noble gases) driving calcite precipitation as the fluid flows west-to-east across the caldera. This is the first evidence that Ca isotopes may trace and provide definitive evidence of calcite precipitation along fluid flow paths in geothermal systems.

  9. Iron isotope constraints on the Archean and Paleoproterozoic ocean redox state.

    PubMed

    Rouxel, Olivier J; Bekker, Andrey; Edwards, Katrina J

    2005-02-18

    The response of the ocean redox state to the rise of atmospheric oxygen about 2.3 billion years ago (Ga) is a matter of controversy. Here we provide iron isotope evidence that the change in the ocean iron cycle occurred at the same time as the change in the atmospheric redox state. Variable and negative iron isotope values in pyrites older than about 2.3 Ga suggest that an iron-rich global ocean was strongly affected by the deposition of iron oxides. Between 2.3 and 1.8 Ga, positive iron isotope values of pyrite likely reflect an increase in the precipitation of iron sulfides relative to iron oxides in a redox stratified ocean. PMID:15718467

  10. Constraints on Galactic Cosmic-Ray Origins from Elemental and Isotopic Composition Measurements

    NASA Technical Reports Server (NTRS)

    Binns, W. R.; Christian, E. R.; Cummings, A. C.; deNolfo, G. A.; Israel, M. H.; Leske, R. A.; Mewaldt, R. A,; Stone, E. C.; vonRosevinge, T. T.; Wiedenbeck, M. E.

    2013-01-01

    The most recent measurements by the Cosmic Ray Isotope Spectrometer (CRIS) aboard the Advanced Composition Explorer (ACE) satellite of ultra-heavy cosmic ray isotopic and elemental abundances will be presented. A range of isotope and element ratios, most importantly Ne-22/Ne-20, Fe-58/Fe-56, and Ga-31/Ge -32 show that the composition is consistent with source material that is a mix of approx 80% ISM (with Solar System abundances) and 20% outflow/ejecta from massive stars. In addition, our data show that the ordering of refractory and volatile elements with atomic mass is greatly improved when compared to an approx 80%/20% mix rather than pure ISM, that the refractory and volatile elements have similar slopes, and that refractory elements are preferentially accelerated by a factor of approx 4. We conclude that these data are consistent with an OB association origin of GCRs.

  11. Iron isotope constraints on the Archean and Paleoproterozoic ocean redox state.

    PubMed

    Rouxel, Olivier J; Bekker, Andrey; Edwards, Katrina J

    2005-02-18

    The response of the ocean redox state to the rise of atmospheric oxygen about 2.3 billion years ago (Ga) is a matter of controversy. Here we provide iron isotope evidence that the change in the ocean iron cycle occurred at the same time as the change in the atmospheric redox state. Variable and negative iron isotope values in pyrites older than about 2.3 Ga suggest that an iron-rich global ocean was strongly affected by the deposition of iron oxides. Between 2.3 and 1.8 Ga, positive iron isotope values of pyrite likely reflect an increase in the precipitation of iron sulfides relative to iron oxides in a redox stratified ocean.

  12. Oxygen isotope constraints on the origin of impact glasses from the cretaceous-tertiary boundary

    SciTech Connect

    Blum, J.D.; Chamberlain, C.P. )

    1992-08-21

    Laser-extraction oxygen isotope and major element analyses of individual glass spherules from Haitian Cretaceous-Tertiary boundary sediments demonstrate that the glasses fall on a mixing line between an isotopically heavy ({delta}{sup 18}O = 14 per mil) high-calcium composition and an isotopically light ({delta}{sup 18}O = 6 per mil) high-silicon composition. This trend can be explained by melting of heterogeneous source rocks during the impact of an asteroid (or comet) {approximately}65 million years ago. The data indicate that the glasses are a mixture of carbonate and silicate rocks and exclude derivation of the glasses either by volcanic processes or as mixtures of sulfate-high evaporate and silicate rocks.

  13. Oxygen isotope constraints on metamorphic fluid flow, Townshend Dam, Vermont, USA

    SciTech Connect

    Kohn, M.J.; Valley, J.W.

    1994-12-01

    Fluid-rock interaction during amphibolite-facies metamorphism has been investigated for rocks exposed in a single 400 m long, lithologically heterogeneous outcrop near Townshend, Vermont, USA. Oxygen isotopic compositions have been measured in profiles across single garnet crystals from thirteen samples, and from hornblende and garnet separates from thirty-three fine-grained samples. This outcrop was previously studied, who measured large oxygen isotope gradients (3{per_thousand}) in garnets from one sample, and inferred large amounts of pervasive fluid flow. All of the garnets that we have analyzed show less isotopic zonation, {le}1{per_thousand}, twelve of them have zonation <0.5{per_thousand}, and the mineral separate data imply both a strong correlation of isotopic composition with rock type and large gradients in peak metamorphic fluid isotopic compositions. Although devolatilization reactions in these rocks must have produced metamorphic fluids, the data preclude cross-foliation time-integrated fluid fluxes greater than 300-600 cm{sup 3}/cm{sup 2} during prograde amphibolite-facies metamorphism. The isotopic trends can all be interpreted in terms of closed-system behavior, channeled fluid flow, or diffusive exchange of oxygen in an interconnected grain-boundary fluid, and within uncertainty cross-strike advective fluxes could have been zero. Any significant flow at this locality was dominantly either layer parallel or channeled out of the system in veins. If all fluid flow occurred in veins, then their spacing must have been less than 300-600 m. The data are inconsistent with massive and pervasive metamorphic fluid flow across strike but do not address layer parallel flow.

  14. Where Did the Ureilite Parent Body Accrete? Constraints from Chemical and Isotopic Compositions

    NASA Astrophysics Data System (ADS)

    Goodrich, Cyrena; O'Brien, David P.

    2014-11-01

    Almahata Sitta and other polymict ureilites contain a remarkable diversity of materials, including EH, EL, OC, R- and CB chondrites, in addition to the dominant ureilitic material [1]. These materials represent at least 6 different parent asteroids and a wide range of chemical and isotopic environments in the early Solar System. To understand the origin of this diversity it is critical to know where (heliocentric distance) the ureilite parent body (UPB) accreted. The chemical and isotopic compositions of ureilite precursors (inferred from the compositions of ureilites) can provide clues. Lithophile element ratios such as Si/Mg and Mn/Mg [2,3], and deficits in neutron-rich Cr, Ti and Ni isotopes [3], indicate that ureilite precursors were similar to ordinary or enstatite chondrites (OC or EC), not carbonaceous chondrites (CC). In contrast, high carbon contents, carbon isotopes and oxygen isotopes suggest a genetic link to CC. This poses a conundrum considering the variation of asteroid types, which suggests that EC and OC dominate the inner asteroid belt and CC the outer belt. However, the CC-like oxygen isotopes of ureilites strongly suggest the effects of parent-body aqueous alteration [4,5], which clearly implies that the UPB accreted beyond the ice line. Lithophile element properties of ureilites compared with chondrites may not be a reliable indicator of location of accretion, because lithophile elements in chondrites are sited mainly in chondrules and the UPB accreted before most chondrules formed [6]. Ureilite Cr, Ti and Ni isotopes may indicate late introduction of the neutron rich isotopes of these elements to the CC-formation region [7]. We conclude that the UPB accreted in the outer belt, like CC. The UPB or one of its offspring must have migrated to the inner belt to acquire OC, EC and R-chondrite materials.[1] Horstmann M. & Bischoff A. [2014] Chemie der Erde 74, 149.[2] Goodrich C. [1999] MAPS 34, 109.[3] Warren P. [2011] GCA 46, 53.[4] Young E. [1999

  15. Abiotic Organic Chemistry of the Terrestrial Deep Subsurface: Isotopic Constraints on Hydrocarbon Formation

    NASA Astrophysics Data System (ADS)

    Sherwood Lollar, B.; McCollom, T. M.; Seewald, J. S.; Lacrampe-Couloume, G.

    2008-12-01

    In serpentinized terrains in both marine and terrestrial subsurface, recent attention has focused on H2 and hydrocarbon gases - on their potential production by abiogenic processes of water-rock interaction; the possibility of their use by deep microbial communities as substrates for life; and on the relevance of such subsurface analogs for the origin of life on earth or elsewhere in the solar system. In deep subsurface Precambrian Shield rocks in South Africa, Canada and Finland, H2, methane and higher hydrocarbon gases have been identified at depths of 1-4 km. While some sites are dominated by gases produced by microbial methanogenesis, the deepest, most ancient fracture waters with residence times on the order of tens of millions of years contain hydrocarbon gases with a pattern of carbon isotope depletion in 13C and hydrogen isotope enrichment in 2H between methane and ethane consistent with abiogenic polymerization1. More recently, the carbon and hydrogen isotope variation between the higher hydrocarbon homologues have also been demonstrated to fit a simple mass balance model consistent with abiogenic polymerization reactions2. In this study, a series of experiments were performed by heating aqueous solutions at 250°C and 170Mpa under reducing conditions using powdered native Fe as a source of H2 and catalyst, and CO as a carbon source in a flexible cell hydrothermal apparatus. Experiments resulted in rapid generation of methane and higher hydrocarbon products typical of Fischer- Tropsch abiotic organic synthesis. These gases were analyzed for carbon and hydrogen isotopes to verify the polymerization model. Unlike the field samples, the experiments showed a carbon isotope enrichment between methane and ethane - suggesting that the extent of fractionation in the first, most highly fractionating step may vary as a function of different reaction mechanisms or parameters such as catalysts or conversion ratios. For the higher hydrocarbons however, carbon isotope

  16. Refined separation of combined Fe–Hf from rock matrices for isotope analyses using AG-MP-1M and Ln-Spec chromatographic extraction resins

    PubMed Central

    Cheng, Ting; Nebel, Oliver; Sossi, Paolo A.; Chen, Fukun

    2014-01-01

    A combined procedure for separating Fe and Hf from a single rock digestion is presented. In a two-stage chromatographic extraction process, a purified Fe fraction is first quantitatively separated from the rock matrix using AG-MP-1M resin in HCl. Hafnium is subsequently isolated using a modified version of a commonly applied method using Eichrom LN-Spec resin. Our combined method includes:•Purification of Fe from the rock matrix using HCl, ready for mass spectrometric analysis.•Direct loading of the matrix onto the resin that is used for Hf purification.•Collection of a Fe-free Hf fraction. PMID:26150946

  17. Zircon U-Pb ages and Hf isotopes of the Askot klippe, Kumaun, northwest India: Implications for Paleoproterozoic tectonics, basin evolution and associated metallogeny of the northern Indian cratonic margin

    NASA Astrophysics Data System (ADS)

    Mandal, Subhadip; Robinson, Delores M.; Kohn, Matthew J.; Khanal, Subodha; Das, Oindrila; Bose, Sukhanjan

    2016-04-01

    Throughout the Himalayan thrust belt, klippen of questionable tectonostratigraphic affinity occur atop Lesser Himalayan rocks. Integrated U-Pb ages, Hf isotopic, and whole rock trace element data establish that the Askot klippe, in northwest India, is composed of Paleoproterozoic lower Lesser Himalayan rocks, not Greater Himalayan rocks, as previously interpreted. The Askot klippe consists of 1857 ± 19 Ma granite-granodiorite gneiss, coeval 1878 ± 19 Ma felsic volcanic rock, and circa 1800 Ma Berinag quartzite, representing a small vestige of a Paleoproterozoic (circa 1850 Ma) continental arc, formed on northern margin of the north Indian cratonic block. Detrital zircon from Berinag quartzite shows ɛHf1850 Ma values between -9.6 and -1.1 (an average of -4.5) and overlaps with ɛHf1850 Ma values of the Askot klippe granite-granodiorite gneiss (-5.5 to -1.2, with an average of -2.7) and other Paleoproterozoic arc-related Lesser Himalayan granite gneisses ( -4.8 to -2.2, with an average of -4.0). These overlapping data suggest a proximal arc source for the metasedimentary rocks. Subchondritic ɛHf1850 Ma values (-5.5 to -1.2) of granite-granodiorite gneiss indicate existence of a preexisting older crust that underwent crustal reworking at circa 1850 Ma. A wide range of ɛHf1850 Ma values in detrital zircon (-15.0 to -1.1) suggests that a heterogeneous crustal source supplied detritus to the northern margin of India. These data, as well as the presence of a volcanogenic massive sulphide deposit within the Askot klippe, are consistent with a circa 1800 Ma intra-arc extensional environment.

  18. Constraints on Early Triassic carbon cycle dynamics from paired organic and inorganic carbon isotope records

    NASA Astrophysics Data System (ADS)

    Meyer, K. M.; Yu, M.; Payne, J.

    2010-12-01

    Marine anoxia and euxinia are widely cited as a leading cause of the end-Permian mass extinction and a factor limiting recovery during the Early Triassic. Middle Triassic diversification coincided with the waning of anoxia and stabilization of the global carbon cycle, suggesting that environment-ecosystem linkages were important to biological recovery. However, the mechanisms responsible for these phenomena remain poorly constrained. Here we employ a carbon isotope approach to examine the nature of the carbon cycle from Late Permian to Middle Triassic time. We measured the carbon isotopic composition of carbonates (δ13Ccarb) and organic matter (δ13Corg) from an exceptionally preserved carbonate platform in the Nanpanjiang Basin of south China. The δ13Ccarb of limestones from 5 stratigraphic sections spanning a paleoenvironmental gradient in south China records multiple large isotope excursions characteristic of the Lower Triassic. Previous modeling suggests that the carbon isotope record is best explained by multiple pulses carbon release to the ocean-atmosphere system. Addition of Δ13C values (δ13Ccarb - δ13Corg) for this interval allows us to evaluate whether the carbon cycle perturbations are indeed due to changes in atmospheric CO2 or from changing sources of organic matter input or fluctuating redox state of the oceans during this interval.

  19. Isotopic constraints on the genesis of base-metal ores in southern and central Sardinia

    USGS Publications Warehouse

    Ludwig, K. R.; Vollmer, R.; Turi, B.; Simmons, K.R.; Perna, G.

    1989-01-01

    The Pb-Zn-Ag deposits of southwestern and central Sardinia occur within a restricted region, but comprise a variety of mineralization ages and styles. New Pb-isotope data, together with published data provide a reasonably coherent picture of possible sources for the various types of deposits. -from Authors

  20. Derivation of Apollo 14 High-Al Basalts at Discrete Times: Rb-Sr Isotopic Constraints

    NASA Astrophysics Data System (ADS)

    Hui, H.; Neal, C. R.; Shih, C.-Y.; Nyquist, L. E.

    2012-03-01

    Four eruption episodes were identified for A-14 high-Al basalts. Rb-Sr isotopic data and ITE ratios show that their parental melt compositions of are correlated through mixing of evolved components with a relatively primitive magma ocean cumulate.

  1. Isotopic Constraints on Sources and Benthic Turnover at Mound 12, Western Costa Rican Margin

    NASA Astrophysics Data System (ADS)

    Rehder, G.; Mau, S.; Linke, P.; Stange, K.

    2004-12-01

    During several expeditions, we investigated the emission and isotopic signature of methane at several mounds of the western continental margin off Costa Rica and Nicaragua. All of the mounds investigated, either created by mud volcanism or mud diapirism, show indications of fluid venting, including authigenic carbonates, chemoautotrophic consortia, salt depleted pore waters, and methane plumes in the water column. However, the amount of methane released as well as the stable carbon isotopic ratio (del C-13) vary considerably. Here we report on results from Mound 12, a mound with a very weak morphological expression; that is only 30 m high and elongated in northeast-southwest direction with diameters of about 1 to 1.6 km. Data were gathered using standard CTD/rosette equipment, a bottom water sampler enabling to resolve the methane distribution within the lowermost meter of the water column, a benthic chamber lander (BCL), multicorer and piston corer deployments. Data show a very light biogenic methane source (del C-13 < -90 permil within the sediments), -76 permil in the lowermost water samples with concentrations up to 100 nmo/L, and a methane background of - 45 permil 20 m above the vent site. High oxygen demand immediately at a site with bacterial mats in connection to lower carbon stable isotopic ratios with increasing sediment depth is in contrast to low oxygen demand and heavier stable isotopic ratios with increasing sediment depth only one meter apart. Moreover, the relation of methane concentration vs. isotopic signature above the vent sites implies considerable oxidation and fractionation in the benthic boundary layer (BBL) above the vent site, which is supported by some biomarker investigations at the same site. Significant oxidation of methane above vent sites within the BBL has not been reported so far. An alternative explanation, which is the existence of an additional methane source with an isotopic signature similar to the background ( del C-13 =-45

  2. Laboratory and Natural Constraints on the Temperature Limit for Preservation of the Dolomite Clumped Isotope Thermometer

    NASA Astrophysics Data System (ADS)

    Lloyd, M. K.; Eiler, J. M.

    2014-12-01

    Kinetic barriers generally inhibit intercrystalline equilibration of cations and isotopic compositions at temperatures below ~350˚C, greatly limiting the geothermometers available to study the upper 10-15 km of the crust. Calcite 'clumped' isotopes commonly appear to record homogeneous equilibrium during crystallization at surface temperatures, but kinetic models predict that reordering due to solid-state exchange among nearby carbonate groups modifies primary compositions at temperatures above ~115˚C on timescales of 10^6 - 10^8 years and fully re-equilibrates above 200˚C in most geological environments1. Slowly cooled dolomitic marbles commonly preserve apparent temperatures of ~300˚C, indicating that dolomite may have slower reordering kinetics and thus greater preservation of primary crystallization temperatures. If so, dolomite clumped isotope thermometry may be a useful geothermometer in much of the the shallow crust. We measured the kinetics of clumped isotope reordering in dolomite with heating experiments at 400-800˚C in a TZM cold seal apparatus pressurized with CO2. Results predict that no detectable reordering occurs in dolomite held at temperatures less than ~250˚C over timescales of up to 10^8 years, demonstrating the viability of the system as a shallow crustal geothermometer. The non-first order behavior observed in calcite1,2,3is exhibited by dolomite as well, albeit at higher temperatures. To test these predictions, we measured the clumped isotopic compositions of coexisting calcite and dolomite in marbles from the Notch Peak aureole, UT. Dolomite clumped isotope temperatures in the outer aureole match peak conditions predicted by thermal models up to ~275˚C, indicating that the system resisted reordering below this grade. Calcite clumped isotope temperatures are never greater than ~150˚C at all grades in the aureole; this is consistent with the ambient burial temperature in the section and indicates that all metamorphic calcite was fully

  3. Carbonate clumped isotope constraints on Silurian ocean temperature and seawater δ18O

    NASA Astrophysics Data System (ADS)

    Cummins, Renata C.; Finnegan, Seth; Fike, David A.; Eiler, John M.; Fischer, Woodward W.

    2014-09-01

    Much of what we know about the history of Earth’s climate derives from the chemistry of carbonate minerals in the sedimentary record. The oxygen isotopic compositions (δ18O) of calcitic marine fossils and cements have been widely used as a proxy for past seawater temperatures, but application of this proxy to deep geologic time is complicated by diagenetic alteration and uncertainties in the δ18O of seawater in the past. Carbonate clumped isotope thermometry provides an independent estimate of the temperature of the water from which a calcite phase precipitated, and allows direct calculation of the δ18O of the water. The clumped isotope composition of calcites is also highly sensitive to recrystallization and can help diagnose different modes of diagenetic alteration, enabling evaluation of preservation states and identification of the most pristine materials from within a sample set-critical information for assessing the quality of paleoproxy data generated from carbonates. We measured the clumped isotope composition of a large suite of calcitic fossils (primarily brachiopods and corals), sedimentary grains, and cements from Silurian (ca. 433 Ma) stratigraphic sections on the island of Gotland, Sweden. Substantial variability in clumped isotope temperatures suggests differential preservation with alteration largely tied to rock-buffered diagenesis, complicating the generation of a stratigraphically resolved climate history through these sections. Despite the generally high preservation quality of samples from these sections, micro-scale observations of calcite fabric and trace metal composition using electron backscatter diffraction and electron microprobe analysis suggest that only a subset of relatively pristine samples retain primary clumped isotope signatures. These samples indicate that Silurian tropical oceans were likely warm (33 ± 7 °C) and similar in oxygen isotopic composition to that estimated for a “modern” ice-free world (δ18OVSMOW of -1

  4. Geochemistry, geochronology and zircon Hf isotopic study of peralkaline-alkaline intrusions along the northern margin of the North China Craton and its tectonic implication for the southeastern Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Zhao, Pan; Jahn, Bor-ming; Xu, Bei; Liao, Wen; Wang, Yanyang

    2016-09-01

    A giant Permian alkaline magmatic belt has recently been identified in southern Inner Mongolia, along the northern margin of the North China Craton (NCC). This belt is mainly composed of syenite, quartz syenite, alkaline granite and mafic microgranular enclaves (MME)-bearing granodiorite. In order to study the petrogenesis and tectonic implications of these rocks, we undertook zircon U-Pb dating and geochemical analysis of two Permian alkaline plutons. The first Guangxingyuan Pluton occurs in the Hexigten area and is composed of MME-bearing tonalite, K-feldspar granite and syenite. The second Durenwuliji Pluton, located in the Xianghuangqi area, comprises syenite, quartz syenite and K-feldspar granite. Zircon U-Pb dating on tonalite, K-feldspar granite, syenite and quartz syenite from the two plutons yielded a tight range of ages from 259 to 267 Ma. The peralkaline-alkaline rocks show high abundance of total alkalis (K2O + Na2O = 7.9-12.9%) and K2O contents (3.9-8.0%), enrichment in large ion lithophile elements (LILE) and light rare earth element (LREE), and depletion of high field strength elements (HFSE). The associated tonalite and MMEs display I-type granitic geochemical affinity, with less total abundance of trace elements than the peralkaline-alkaline rocks. Zircon Hf isotopic analysis of the Guangxingyuan pluton yielded a large range of εHf(t) values from - 15.5 to + 6.7 and model ages (TDMC) from 781 to 2012 Ma. By contrast, the Hf isotopic data of the Durenwuliji pluton shows a small range of εHf(t) from + 6.2 to + 8.9 and TDMC from 667 to 816 Ma. The geochemical and Hf isotopic characteristics indicate that the parental magma was derived from a mixing of metasomatic mantle-derived mafic magma with different amount of crust-derived felsic magma, and followed by fractional crystallization. Considering previous tectonic studies in Inner Mongolia, a Permian post-orogenic extension was proposed to account for these peralkaline-alkaline intrusions following

  5. Abiotic Formation of Hydrocarbons Under Hydrothermal Conditions: Constraints from Chemical and Isotope Data

    SciTech Connect

    Fu, Q.; Lollar, Barbara Sherwood; Horita, Juske; Lacrampe-Couloume, Georges; Seyfried, W. E.

    2007-01-01

    To understand reaction pathways and isotope systematics during mineral-catalyzed abiotic synthesis of hydrocarbons under hydrothermal conditions, experiments involving magnetite and CO{sub 2} and H{sub 2}-bearing aqueous fluids were conducted at 400 C and 500 bars. A robust technique for sample storage and transfer from experimental apparatus to stable isotope mass spectrometer provides a methodology for integration of both carbon and hydrogen isotope characterization of reactants and products generated during abiogenic synthesis experiments. Experiments were performed with and without pretreatment of magnetite to remove background carbon associated with the mineral catalyst. Prior to experiments, the abundance and carbon isotope composition of all carbon-bearing components were determined. Time-series samples of the fluid from all experiments indicated significant concentrations of dissolved CO and C{sub 1}-C{sub 3} hydrocarbons and relatively large changes in dissolved CO{sub 2} and H{sub 2} concentrations, consistent with formation of additional hydrocarbon components beyond C{sub 3}. The existence of relatively high dissolved alkanes in the experiment involving non-pretreated magnetite in particular, suggests a complex catalytic process, likely involving reinforcing effects of mineral-derived carbon with newly synthesized hydrocarbons at the magnetite surface. Similar reactions may be important mechanisms for carbon reduction in chemically complex natural hydrothermal systems. In spite of evidence supporting abiotic hydrocarbon formation in all experiments, an 'isotopic reversal' trend was not observed for {sup 13}C values of dissolved alkanes with increasing carbon number. This may relate to the specific mechanism of carbon reduction and hydrocarbon chain growth under hydrothermal conditions at elevated temperatures and pressures. Over time, significant {sup 13}C depletion in CH{sub 4} suggests either depolymerization reactions occurring in addition to

  6. Zinc mobility in an infiltration basin (Lyon city, France): constraints from Zn stable isotope ratios in the plant and sediment

    NASA Astrophysics Data System (ADS)

    Queyron, M.; Aucour, A.-M.; Pichat, S.; Saulais, M.; Bedell, J.-P.

    2012-04-01

    heavy isotopes (0.16‰) versus the aerial parts support the export of Zn to the roots before leaf senescence and/or the Zn transfer from the sediment to the litter matrix. We conclude that stable Zn isotope ratios bring relevant constraints for Zn cycling model between the plant and urban sediment.

  7. Pb Nd isotopic constraints on sedimentary input into the Lesser Antilles arc system

    NASA Astrophysics Data System (ADS)

    Carpentier, Marion; Chauvel, Catherine; Mattielli, Nadine

    2008-07-01

    The Lesser Antilles arc is a particularly interesting island arc because it is presently very active, it is located perpendicular to the South American continent and its chemical and isotopic compositions display a strong north-south gradient. While the presence in the south of a thick pile of sedimentary material coming from the old South American continent has long been suspected to explain the geochemical gradient, previous studies failed to demonstrate unambiguously a direct link between the arc lava compositions and the subducted sediment compositions. Here, we present new Nd, Sm, Th, U and Pb concentrations and Nd-Pb isotopic data for over 60 sediments from three sites located in the fore arc region of the Lesser Antilles arc. New data for DSDP Site 543 drill core located east of Dominica Island complement the data published by White et al. [White, W.M., Dupré, B. and Vidal, P., 1985. Isotope and trace element geochemistry of sediments from the Barbados Ridge-Demerara Plain region, Atlantic Ocean. Geochimica et Cosmochimica Acta, 49: 1875-1886.] and confirm their relatively uniform isotopic compositions (i.e., 206Pb/ 204Pb between 19.13 and 19.53). In contrast, data obtained on DSDP Site 144 located further south, on the edge of the South American Rise and on sediments from Barbados Island are much more variable ( 206Pb/ 204Pb ranges from 18.81 to 27.69). The very radiogenic Pb isotopic compositions are found in a 60 m thick black shale unit, which has no age equivalent in the Site 543 drill core. We interpret the peculiar composition of the southern sediments as being due to two factors, (a) the proximity of the South American craton, which contributes coarse grain old detrital material that does not travel far from the continental shelf, and (b) the presence of older sediments including the thick black shale unit formed during Oceanic Anoxic events 2 and 3. The north-south isotopic change known along the Lesser Antilles arc can be explained by the observed

  8. From Gene Expression to the Earth System: Isotopic Constraints on Nitrogen Cycling Across Scales

    NASA Astrophysics Data System (ADS)

    Houlton, B. Z.

    2015-12-01

    A central motivation of the Biogeosciences is to understand the cycling of biologically essential elements over multiple scales of space and time. This charge is vital to basic knowledge of Earth system functioning. It is also relevant to many of the global challenges we face, such as climate change, biodiversity conservation, and the multifaceted role of global fertilizer use in maximizing human health and well-being. Nitrogen is connected to all of these; yet it has been one of the more vexing elements to quantitatively appraise across systems and scales. Here I discuss how research in my group has been exploring the use of natural nitrogen isotope abundance (15N/14N) as a biogeochemical tracer - from the level of gene expression to nitrogen's role in global climate change. First, I present evidence for a positive correlation between the bacterial genes that encode for gaseous nitrogen production (i.e., nirS) and the 15N/14N of soil extractable nitrate pools across an array of terrestrial ecosystems. Second, I demonstrate how these local-scale results fit with our work on ecosystem-scale nitrogen isotope budgets, where we quantify a uniformly small isotope effect (i.e., < 1 per mil) of nitrogen leaching losses from tropical rainforest to highly disturbed arid sites. Third, I present results from our global isotope model, which is based on results from our field investigations, providing a new nitrogen "benchmarking" scheme for global computational models and climate change forecasts. Finally, I move to a new research frontier where we have been developing a technique to measure the nitrogen isotope composition of ancient terrestrial plant compounds (i.e., chlorins) buried in the soil. This research aims to address the response of the nitrogen cycle to glacial-interglacial transitions over millennia, which is beyond the window of experimental testing. Together, this research highlights the utility of nitrogen isotope composition in addressing the myriad scales of

  9. Ca and Mg isotope constraints on the origin of Earth's deepest δ13 C excursion

    NASA Astrophysics Data System (ADS)

    Husson, Jon M.; Higgins, John A.; Maloof, Adam C.; Schoene, Blair

    2015-07-01

    Understanding the extreme carbon isotope excursions found in carbonate rocks of the Ediacaran Period (635-541 Ma), where δ13 C of marine carbonates (δ13 Ccarb) reach their minimum (- 12 ‰) for Earth history, is one of the most vexing problems in Precambrian geology. Known colloquially as the 'Shuram' excursion, the event has been interpreted by many as a product of a profoundly different Ediacaran carbon cycle. More recently, diagenetic processes have been invoked, with the very negative δ13 C values of Ediacaran carbonates explained via meteoric alteration, late-stage burial diagenesis or growth of authigenic carbonates in the sediment column, thus challenging models which rely upon a dramatically changing redox state of the Ediacaran oceans. Here we present 257 δ 44 / 40 Ca and 131 δ26 Mg measurements, along with [Mg], [Mn] and [Sr] data, from carbonates of the Ediacaran-aged Wonoka Formation (Fm.) of South Australia to bring new isotope systems to bear on understanding the 'Shuram' excursion. Data from four measured sections spanning the basin reveal stratigraphically coherent trends, with variability of ∼1.5‰ in δ26 Mg and ∼1.2‰ in δ 44 / 40 Ca. This Ca isotope variability dwarfs the 0.2-0.3 ‰ change seen coeval with the Permian-Triassic mass extinction, the largest recorded in the rock record, and is on par with putative changes in the δ 44 / 40 Ca value of seawater seen over the Phanerozoic Eon. Changes in both isotopic systems are too large to explain with changes in the isotopic composition of Ca and Mg in global seawater given modern budgets and residence times, and thus must be products of alternative processes. Relationships between δ 44 / 40 Ca and [Sr] and δ26 Mg and [Mg] are consistent with mineralogical control (e.g., aragonite vs. calcite, limestone vs. dolostone) on calcium and magnesium isotope variability. The most pristine samples in the Wonoka dataset, preserving Sr concentrations (in the 1000s of ppm range) and δ 44 / 40

  10. Sedimentologic, Chemical, and Isotopic Constraints on the Anthropogenic Influence on Chilika Lake, India

    NASA Astrophysics Data System (ADS)

    Vennemann, T. W.; Decrouy, L.; Ecuyer, M.; Delavy, K.; Lange, P.; Rastogi, G.; Pattnaik, A.; Suar, M.

    2014-12-01

    Chilika Lake, the largest Asian lagoon on the east coast of India, has a surface area of 1160 km2 or about 900 km2, respectively for the wet, monsoon vs. dry winter-spring season. The average depth is only about 1.2 m. It is separated from the Bay of Bengal by a 100 m to 1.5 km wide sand bar of about 30 km length, separating the outer channel that connects the lagoon naturally to the sea. Long-shore development of this sand bar as of the Late Holocene increasingly isolated the lagoon from the sea, until final closure in 1992. Given the population increase in the catchment and according changes in land use policies, agricultural practices, and water resource management, Chilika Lake has been subjected to increasing anthropogenic influence. As a consequence the unique biodiversity and also primary production within the lagoon decreased, while eutrophication and siltation increased. As a counter-initiative it was decided to artificially open the lagoon to the sea by dredging. To help trace and quantify the anthropologic effects on Chilika Lake, a combined sedimentologic, chemical, and isotopic study of the lagoon and its sediments was is in progress. First results from a campaign during the monsoon season suggest that the large gradients in salinity, sediment and nutrient input, as well as primary productivity within the lagoon are controlled by variable fluxes of water, sediment, and nutrients from the three separate catchments to the lagoon. Trends in changes of salinity, H- and O-isotope compositions of waters, but also of concentrations and C- and/or N-isotope compositions of the dissolved inorganic carbon (DIC), particulate organic matter (POM), and aquatic plants indicate that mixing in the lagoon occurs between new freshwater inputs and evaporated water within the basin itself. Except for the outer channel, mixing with seawater is limited. In contrast, the C-isotope composition of the organic matter in the sediments either suggests a higher overall proportion

  11. Cadomian basement and Paleozoic to Triassic siliciclastics of the Taurides (Karacahisar dome, south-central Turkey): Paleogeographic constraints from U-Pb-Hf in zircons

    NASA Astrophysics Data System (ADS)

    Abbo, Avishai; Avigad, Dov; Gerdes, Axel; Güngör, Talip

    2015-06-01

    The Tauride block in Turkey is a peri-Gondwana, Cadomian-type terrane that rifted from the Afro-Arabian margin of Gondwana in the Permo-Triassic and re-accreted to Arabia in the Neogene. In the Karacahisar dome in the southern-central Taurides, Neoproterozoic basement metasediments and intrusive rocks are overlain by Cambro-Ordovician, Carboniferous and Triassic sediments. We studied U-Pb-Hf in zircons from major rock units exposed in Karacahisar in order to constrain the Cadomian crustal evolution of the Taurides, to evaluate the provenance of the Neoproterozoic and overlying sediments, to constrain the paleogeography of the Taurides, and to assess their linkage to Gondwana. The Neoproterozoic metasediments are low-grade metamorphic wacke-type turbidites that evolved in a broad back-arc basin peripheral to Afro-Arabia. Their detrital zircon U-Pb signal comprises a preponderance (40-68%) of Neoproterozoic-aged zircons (peak ages defined at 635 and 830 Ma), indicating that the sedimentary pile was built mainly from the erosion of Pan-African terranes from Afro-Arabia. The εHf values of the younger population (635 Ma) are mostly positive, indicating derivation from a juvenile arc, whereas Cryogenian-Tonian detrital zircons spread vertically (- 25 < εHf < 15), indicating a different provenance where mixing of juvenile magmas with Paleoproterozoic to Neoarchean crust was widespread. An unusually high proportion of pre-Neoproterozoic zircons is found in all Cadomian metasediments, including up to 31% Grenvillian-aged (ca. 1.0 Ga) and up to 35% of ca. 2.5 Ga zircons; about a third of the latter possess positive εHf values. Because only minor exposures of 1.0 and 2.5 Ga crustal vestiges are currently known in North Africa and Arabia, we infer that pre-Neoproterozoic terranes were dispersed within the Cadomian realm itself. The youngest detrital zircons in all Cadomian metasediments concentrate at 0.58 Ga, indicating that the proto-Cadomian back-arc basin was formed

  12. Constraints on the mantle mineralogy of an ultra-slow ridge: Hafnium isotopes in abyssal peridotites and basalts from the 9-25°E Southwest Indian Ridge

    NASA Astrophysics Data System (ADS)

    Mallick, Soumen; Standish, Jared J.; Bizimis, Michael

    2015-01-01

    We report on the Hf isotopic compositions of clinopyroxene mineral separates from eleven abyssal peridotites and Nd and Hf-isotopic compositions of twenty-seven co-located basalts from 9-25°E South West Indian Ridge (SWIR). In Nd-Hf isotope space the SWIR peridotites plot within the global MORB field (εNd = 4.5- 12.5, εHf = 9.6- 18.7), with the 15.23°E peridotites being the most radiogenic. The lack of correlation between Hf isotopes and trace or major element systematics including Lu/Hf ratios suggests that the 15.23°E peridotites were recently processed beneath the ridge and therefore participated in the production of the SWIR lavas. The Hf isotopic compositions of 15.23°E peridotites are more radiogenic than all basalts from the 9-25°E ridge, whereas the 9.98°E and 16.64°E peridotites partially overlap with the Hf isotope ratios of the spatially co-located basalts. This indicates the upwelling mantle beneath the SWIR contains material with enriched isotope signatures in addition to an isotopically depleted peridotitic mantle, which is consistent with the SWIR peridotites and basalt Nd isotope systematics from previously published studies. As the enriched isotope signatures are not observed in the peridotites we assume that they are sourced from material with lower solidus temperature than a typical peridotite. This enriched material was consumed during melting, and therefore may be mineralogically distinct (e.g. pyroxenite). Moreover, the variable spatial distribution of the enriched isotope signatures requires preferential sampling of the enriched component at distinct along-axis locations. The Hf-Nd isotope variability of the 9-25°E basalts can be entirely explained by mixing between a depleted peridotitic mantle end-member with the isotope composition of the 15.23°E peridotites and an enriched end-member with the isotope composition of the Narrowgate Segment lavas at 14.6°E. We estimate a maximum of 5% modal abundance of the enriched material in a

  13. Geochronology, petrology and Hf-S isotope geochemistry of the newly-discovered Xiarihamu magmatic Ni-Cu sulfide deposit in the Qinghai-Tibet plateau, western China

    NASA Astrophysics Data System (ADS)

    Li, Chusi; Zhang, Zhaowei; Li, Wenyuan; Wang, Yalei; Sun, Tao; Ripley, Edward M.

    2015-02-01

    This paper reports the first set of data for the newly-discovered Xiarihamu magmatic Ni-Cu sulfide deposit in the Eastern Kunlun Paleozoic arc terrane which is located in the northern part of the Qinghai-Tibet plateau. An on-going drilling campaign reveals ~ 100 million tons of sulfide mineralization with the average grade of 0.8 wt.% Ni and 0.1 wt.% Cu for the deposit. This makes the Xiarihamu deposit one of the 20 largest magmatic Ni-Cu sulfide deposits in the world and the largest ever found in arc settings. The deposit is hosted in a small ultramafic body intruding older gabbroic and metamorphic rocks. New zircon U-Pb isotope age data reveal that the ultramafic body (411.6 ± 2.4 Ma) is ~ 20 Ma younger than the host gabbroic intrusion (431.3 ± 2.1 Ma). The ultramafic body is composed predominantly of lherzolite and olivine websterite, with minor dunite, websterite and orthopyroxenite. Mineralization mainly occurs as sub-horizontal to gently dipping (< 30°) disseminated sulfide zones that are generally concordant with the lithological structure of the ultramafic intrusion. The lateral extension and thickness of individual mineralized zones are up to ~ 200 m and ~ 100 m, respectively. Sulfide mineral assemblages are composed of pyrrhotite, pentlandite and chalcopyrite. The Xiarihamu ultramafic rocks show light REE enrichments and pronounced negative Nb anomalies, plus significant Ca-depletion in olivine (< 700 ppm Ca), which are characteristic of many arc basalts in the world. Olivine crystals in the Xiarihamu ultramafic rocks have relatively primitive compositions, with Fo contents up to 90 mol%, close to the mantle value. The contrasting Ni contents of olivine crystals with similar Fo contents from different sulfide-mineralized zones in a single drill core indicate that at least two pulses of sulfide-laden magma with different Ni compositions were involved in the development of the deposit. Estimated parental magma for the Xiarihamu lherzolites contains 52

  14. K = 6+ Isomers in Hf, yb and W Nuclei

    NASA Astrophysics Data System (ADS)

    Rath, Aswini Kumar; Walker, P. M.; Praharaj, C. R.; Xu, F. R.

    Using deformed Hartree-Fock and angular momentum projection (PHF) technique we try to understand the intrinsic structure and the systematics in the life times of K = 6+ isomers in the Hf isotopes (in 172-178Hf nuclei) and N = 104 Yb, Hf and W isotones. The band structure in 172Hf is reasonably well reproduced. The variation in the B(E2;2+ → 0+) values in the Hf isotopes as well as N = 104 isotones are well reproduced. The calculated K-forbidden E2 transition probabilities from the isomer bandheads to the 4+ yrast states qualitatively explain the variation of the lifetimes with N and Z.

  15. Observations of the Li, Be, and B Isotopes and Constraints on Cosmic-ray Propagation

    NASA Technical Reports Server (NTRS)

    deNolfo, G. A.; Moskalenko, I. V.; Binns, W. R.; Christian, E. R.; Cummings, A. C.; Davis, A. J.; George, J. S.; Hink, P. L.; Israel, M. H.; Leske, R. A.; Lijowski, M.; Mewaldt, R. A.; Stone, E. C.; Strong, A. W.; vonRosenvinge, T. T.; Wiedenbeck, M. E.; Yanasak, N. E.

    2007-01-01

    The abundance of Li, Be, and B isotopes in galactic cosmic rays (GCR) between E=50-200 MeV/nucleon has been observed by the Cosmic Ray Isotope Spectrometer (CRIS) on NASA's ACE mission since 1997 with high statistical accuracy. Precise observations of Li, Be, B can be used to constrain GCR propagation models. We find that a diffusive reacceleration model with parameters that best match CRIS results (e.g. B/C, Li/C, etc) are also consistent with other GCR observations. A approx. 15-20% overproduction of Li and Be in the model predictions is attributed to uncertainties in the production cross-section data. The latter becomes a significant limitation to the study of rare GCR species that are generated predominantly via spallation.

  16. Carbon isotopic constraints on the contribution of plant material to the natural precursors of trihalomethanes

    USGS Publications Warehouse

    Bergamaschi, B.A.; Fram, M.S.; Kendall, C.; Silva, S.R.; Aiken, G.R.; Fujii, R.

    1999-01-01

    The ??13C values of individual trihalomethanes (THM) formed on reaction of chlorine with dissolved organic carbon (DOC) leached from maize (corn, Zea maize L) and Scirpus acutus (an aquatic bulrush), and with DOC extracted from agricultural drainage waters were determined using purge and trap introduction into a gas chromatograph-combustion-isotope ratio monitoring mass spectrometer. We observed a 1-6.8??? difference between the ??13C values of THM produced from the maize and Scirpus leachates, similar to the isotopic difference between the whole plant materials. Both maize and Scirpus formed THM 12??? lower in 13C than whole plant material. We suggest that the low value of the THM relative to the whole plant material is evidence of distinct pools of THM-forming DOC, representing different biochemical types or chemical structures, and possessing different environmental reactivity Humic extracts of waters draining an agricultural field containing Scirpus peat soils and planted with maize formed THM with isotopic values intermediate between those of maize and Scirpus leachates, indicating maize may contribute significantly to the THM-forming DOC. The difference between the ??13C values of the whole isolate and that of the THM it yielded was 3 9???, however, suggesting diagenesis plays a role in determining the ??13C value of THM-forming DOC in the drainage waters, and precluding the direct use of isotopic mixing models to quantitatively attribute sources.The ??13C values of individual trihalomethanes (THM) formed on reaction of chlorine with dissolved organic carbon (DOC) leached from maize (corn; Zea maize L.) and Scirpus acutus (an aquatic bulrush), and with DOC extracted from agricultural drainage waters were determined using purge and trap introduction into a gas chromatograph-combustion-isotope ratio monitoring mass spectrometer. We observed a 16.8qq difference between the ??13C values of THM produced from the maize and Scirpus leachates, similar to the isotopic

  17. Oxygen isotopic constraints on the genesis of carbonates from Martian meteorite ALH84001

    NASA Astrophysics Data System (ADS)

    Leshin, Laurie A.; McKeegan, Kevin D.; Harvey, Ralph P.

    1997-03-01

    With a crystallization age of 4.5 Ga, ALH84001 is unique among the Martian meteorites. It is also the only Martian meteorite that contains an appreciable amount of carbonate, and significantly, this carbonate occurs without associated secondary hydrated minerals. Moreover, McKay et al. (1996) have suggested that ALH84001 contains evidence of past Martian life in the form of nanofossils, biogenic minerals, and polycyclic aromatic hydrocarbons. The presence of carbonate in ALH84001 is especially significant. The early Martian environment is thought to have been more hospitable to life than todays cold, dry climate. In order to better assess the true delta-O-18 values, as well as the isotopic diversity and complexity of the ALH84001 carbonates, direct measurements of the oxygen and carbon isotopic compositions of individual carbonate phases are needed. Here we report in situ analyses of delta-O-18 values in carbonates from two polished thin sections of ALH84001.

  18. Observations of the Li, Be, and B isotopes and Constraints on Cosmic-ray Propagation

    SciTech Connect

    de Nolfo, Georgia A.; Moskalenko, I.V.; Binns, W.R.; Christian, E.R.; Cummings, A.C.; Davis, A.J.; George, J.S.; Hink, P.L.; Israel, M.H.; Leske, R.A.; Lijowski, M.; Mewaldt, R.A.; Stone, E.C.; Strong, A.W.; von Rosenvinge, T.T.; Wiedenbeck, M.E.; Yanasak, N.E.; /NASA, Goddard /Stanford U., HEPL /Washington U., St. Louis /NASA, Headquarters /Caltech, SRL /Aerospace Corp. /Garching, Max Planck Inst., MPE /Caltech, JPL

    2006-11-15

    The abundance of Li, Be, and B isotopes in galactic cosmic rays (GCR) between E=50-200 MeV/nucleon has been observed by the Cosmic Ray Isotope Spectrometer (CRIS) on NASA's ACE mission since 1997 with high statistical accuracy. Precise observations of Li, Be, B can be used to constrain GCR propagation models. We find that a diffusive reacceleration model with parameters that best match CRIS results (e.g. B/C, Li/C, etc) are also consistent with other GCR observations. A {approx}15-20% overproduction of Li and Be in the model predictions is attributed to uncertainties in the production cross-section data. The latter becomes a significant limitation to the study of rare GCR species that are generated predominantly via spallation.

  19. Strontium isotope constraints on fluid flow in the upper oceanic crust at the East Pacific Rise

    NASA Astrophysics Data System (ADS)

    Gillis, Kathryn M.; Coogan, Laurence A.; Pedersen, Rolf

    2005-03-01

    Strontium isotopes are useful tracers of fluid-rock interaction in marine hydrothermal systems and provide a potential way to quantify the amount of seawater that passes through these systems. We have determined the whole-rock Sr-isotopic compositions of a section of upper oceanic crust that formed at the fast-spreading East Pacific Rise, now exposed at Hess Deep. This dataset provides the first detailed comparison for the much-studied Ocean Drilling Program (ODP) drill core from Site 504B. Whole-rock and mineral Sr concentrations indicate that Sr-exchange between hydrothermal fluids and the oceanic crust is complex, being dependent on the mineralogical reactions occurring; in particular, epidote formation takes up Sr from the fluid increasing the 87Sr/ 86Sr of the bulk-rock. Calculating the fluid-flux required to shift the Sr-isotopic composition of the Hess Deep sheeted-dike complex, using the approach of Bickle and Teagle [1] [M.J. Bickle, D.A.H. Teagle, Strontium alteration in the Troodos ophiolite: implications for fluid fluxes and geochemical transport in mid-ocean ridge hydrothermal systems. Earth Planet. Sci. Lett. 113 (1992) 219-237] gives a fluid-flux similar to that determined for ODP Hole 504B. This suggests that the level of isotopic exchange observed in these two regions is probably typical for modern oceanic crust. Unfortunately, uncertainties in the modeling approach do not allow us to determine a fluid-flux that is directly comparable to fluxes calculated by other methods.

  20. Hydrogen and oxygen isotope constraints on hydrothermal alteration of the Trinity peridotite, Klamath Mountains, California

    USGS Publications Warehouse

    Liakhovitch, V.; Quick, J.E.; Gregory, R.T.

    2005-01-01

    The Trinity peridotite represents a rare opportunity to examine a relatively fertile plagioclase peridotite that was exhumed and later subjected to intrusive events in a seafloor environment, followed by its emplacement and incorporation into a continent. Over 250 stable isotopic determinations on whole rocks and minerals elucidate the hydrothermal evolution of the Trinity complex. All three serpentine polymorphs are present in the Trinity peridotite; these separate on the basis of their ??D values: antigorite, -46 < ??D < -82??? and lizardite and chrysotile, -90 < ??D < -106 and -110 < ??D < -136???, respectively. Antigorite coexists with chlorite, talc, and tremolite in contact aureole assemblages associated with Silurian/Devonian gabbroic plutons. Lizardite and chrysotile alteration carries a meteoric signature, which suggests association with post-emplacement serpentinization, or overprinting of earlier low-temperature seafloor serpentinization. Regionally, contours of ??D values exhibit bull's-eye patterns associated with the gabbroic plutons, with ??D maxima coinciding with the blackwall alteration at the margins on the plutons. In contrast to the hydrogen isotope behavior, oxygen isotope values of the three polymorphs are indistinguishable, spanning the range 5.3 < ??18O< 7.5, and suggesting low integrated fluid fluxes and strongly 18O-shifted fluids. Inferred primary ?? 18O values for peridotite, gabbro, and late Mesozoic granodiorite indicate a progressive 18O enrichment with time for the source regions of the rocks. These isotopic signatures are consistent with the geology, petrochemistry, and geochronology of the Trinity massif, which indicate the following history: (1) lithospheric emplacement and cooling of the peridotite in an oceanic environment ??? 472 Ma; (2) intrusion of gabbroic plutons into cold peridotite in an arc environment between 435 and 404 Ma; and finally (3) intrusion of felsic plutons between 171 and 127 Ma, long after the peridotite

  1. Isotopic constraints on anorthosite genesis and implications for crust-mantle evolution

    SciTech Connect

    Ashwal, L.D.

    1985-01-01

    Crystallization ages of anorthosite massifs, determined from whole-rock and internal Sm-Nd and Rb-Sr isochrons range between about 1.1 and 1.6 Ga, arguing against a discrete anorthosite event. Metamorphic ages of some massifs are as much as 200-300 Ma younger, indicating that the Grenville orogeny was not a causative factor in anorthosite genesis. Variable crustal contamination effects are evident in many massifs, particularly in border zones. In some late-stage ferrogabbros, mafic silicates and/or Fe-Ti oxides are not in isotopic equilibrium with plagioclase, suggesting that crystallization took place both before and after contamination. The most isotopically primitive materials are Al-rich opx megacrysts. Isotopic data to date are compatible with a two-stage model involving (1) emplacement of basaltic magma into lower crustal chambers where fractionation and accumulation of olivine and Al-rich opx, and eventually plagioclase took place, and (2) detachment and ascent of buoyant anorthositic mushes to upper crustal emplacement sites. Besides being useful as indicators of Proterozoic mantle evolution, anorthosites can be used as tracers to map our basement types through which they were emplaced.

  2. Anomalous biogeochemical behavior of cadmium in subantarctic surface waters: Mechanistic constraints from cadmium isotopes

    NASA Astrophysics Data System (ADS)

    Gault-Ringold, Melanie; Adu, Toyin; Stirling, Claudine H.; Frew, Russell D.; Hunter, Keith A.

    2012-08-01

    Cadmium, a highly toxic metal, exhibits a nutrient-type profile in the oceans that is closely correlated to that of the major nutrients phosphate and nitrate. Despite its complexity, the relationship between cadmium and phosphate has been used to infer historic phosphate utilization and biological controls on oceanic CO2 concentrations. Cadmium isotopes offer the potential to constrain the mechanisms controlling cadmium cycling in the oceans, reducing uncertainty associated with the cadmium paleonutrient proxy. Using techniques in double spiking and MC-ICPMS, we report seasonal Cd isotopic and concentration data along with major nutrients and other essential trace metal (Fe, Zn, and Co) concentrations from subantarctic surface waters. We show, for the first time, a 50-fold seasonal decrease in dissolved cadmium concentrations in subantarctic waters that is due to biological uptake. However, this drawdown in Cd is decoupled from phosphate and shows no coincident shift in cadmium isotopic composition. These data, along with the preferential removal of Cd from surface waters relative to Zn, imply that cadmium is supply-limited to phytoplankton and may have a more significant biological role in these low Zn subantarctic surface waters than in regions with higher Zn concentrations.

  3. Osmium and neodymium isotopic constraints on the temporal and spatial evolution of Siberian flood basalt sources

    USGS Publications Warehouse

    Horan, M.F.; Walker, R.J.; Fedorenko, V.A.; Czamanske, G.K.

    1995-01-01

    Picrites from the Gudchikhinsky suite, the oldest rocks examined, have ??Os of +5.3 to +6.1 and ??Nd of +3.7 to +4.0. The osmium and neodymium isotopic compositions of these rocks are similar to some modern ocean-island basalts (OIB), consistent with their derivation from an mantle plume. Picrites from the stratigraphically higher Tuklonsky suite have similar ??Os of +3.4 to +6.5, but ??Nd of -0.9 to -2.6. The similar ??Os, but lower ??Nd , suggest that some magmas from the same OIB-type, mantle source were contaminated by lithospheric components. A differentiated ankaramite flow, associated with the top of the stratigraphically higher Morongovsky suite, has ??Os of +9.8 to +10.2 and ??Nd of +1.3 to +1.4. The higher ??Os may indicate that the plume source was heterogeneous with respect to osmium isotopic composition, consistent with osmium isotopic measurements in rocks from other plume sources. Mg-rich, alkaline rocks (meymechites) from the Guli area that erupted much nearer the end of the flood-basalt event have ??Os of -1.2 to -2.6 and ??Nd of +3.7 to +4.9. These rocks were probably produced by low degrees of partial melting of mantle after the main stages of flood-basalt production. -from Authors

  4. Petrogenesis of metaluminous A-type granitoids in the Tengchong-Lianghe tin belt of southwestern China: Evidences from zircon U-Pb ages and Hf-O isotopes, and whole-rock Sr-Nd isotopes

    NASA Astrophysics Data System (ADS)

    Chen, Xiao-Cui; Hu, Rui-zhong; Bi, Xian-Wu; Zhong, Hong; Lan, Jiang-Bo; Zhao, Cheng-Hai; Zhu, Jing-Jing

    2015-01-01

    The Tengchong-Lianghe tin belt is the most importantpart of the tin metallogenic belt in southwest China's Sanjiang Metallogenic province. In this district, two A-type granite plutons that are spatially associated with the tin deposits have recently been discovered. These granitoids are dominated by biotite granites with weakly peraluminous to metaluminous compositions, high SiO2 contents (73.3-76.2 wt.%), and high alkali contents (K2O + Na2O = 8.3-9.17 wt.%). Trace element spider diagrams show that these granitoids are also clearly enriched with the large-ion lithophile elements Rb, Th, U, and K, but are markedly depleted in Ba and Sr. They are significantly depleted in the high field-strength elements Nb and Ti, but are enriched with Zr and Hf. These rocks have relatively high zircon saturation temperatures (774-833 °C), high Zr + Nb + Ce + Y contents (272-416 ppm), and 10,000 × Ga/Al ratios (almost > 2.7) that are typical of A-type granites. They also have high total rare earth element (REE) contents (174-404 ppm) and relatively flat chondrite-normalized REE patterns with significantly negative Eu anomalies. LA-ICP-MS zircon U-Pb dating results indicate that the emplacement of these two granite plutons occurred during the early Tertiary (52.7 ± 0.3-53.0 ± 0.4 Ma) and the Late Cretaceous (73.3 ± 0.5-73.3 ± 0.5 Ma) periods, respectively. Isotopic compositions show that the granitoids have highly variable (87Sr/86Sr)i ratios (0.7182-0.7457), relatively constant low εNd(t) values (- 11.2 to - 12.4), and ancient Nd and Hf model ages (1.56-1.88 Ga), suggesting that they were derived from the partial melting of the Paleoproterozoic continental crust. Their zircon δ18O values (6.6-8.5‰) and εHf values (from - 8.6 to - 11.3) also suggest that these granitoids mainly originated from the middle-lower continental crust consisting of mafic and metasedimentary rocks. Such geochemical characteristics indicate that these two A-type granite plutons were generated by

  5. Petrogenesis of Tertiary continental intra-plate lavas between Siebengebirge and Westerwald, Germany: Constraints from trace element systematics and Nd, Sr and Pb isotopes

    NASA Astrophysics Data System (ADS)

    Schubert, S.; Jung, S.; Pfänder, J. A.; Hauff, F.; Garbe-Schönberg, D.

    2015-10-01

    New 39Ar/40Ar ages and major- and trace-element and radiogenic isotope data are presented for basanites and alkali basalts from the transition area between the Westerwald and Siebengebirge volcanic fields (Germany) that belongs to the Central European Volcanic Province (CEVP). The 39Ar/40Ar ages indicate ages of c. 24 and c. 5 Ma which are fully compatible with previous K/Ar ages indicating that the evolution of this volcanic field belongs to the Westerwald area (28-22 Ma and 5 Ma) rather than to the Siebengebirge area (26-23 Ma). Based on the occurrence of > 30 isolated volcanic plugs with a simple igneous history, this volcanic field can be viewed as a monogenetic volcanic field. Compositions of some basanites are primitive, whereas others and the alkali basalts show decreasing Cr and Ni contents and CaO/Al2O3 ratios. However, increasing TiO2, Al2O3 and incompatible elements (Sr, Zr, Y, Hf, Ta) concentrations with decreasing MgO indicating fractionation of mainly olivine with minor amounts of clinopyroxene and spinel can be noticed. Rare earth element systematics suggest that most of the alkaline rocks are generated by different degrees of melting (5%-10%) of a garnet-bearing peridotite containing some residual amphibole. Negative anomalies of Rb and K in primitive mantle-normalized diagrams and a lack of Ba/Rb fractionation suggest that amphibole was the major OH-bearing mineral phase in the mantle. The alkaline rocks have a restricted range in 87Sr/86Sr and 143Nd/144Nd ratios ranging from 0.7033 to 0.7044 and from 0.51275 to 0.51285, respectively. Lead isotope compositions (206Pb/204Pb: 19.21-19.65; 207Pb/204Pb: 15.62-15.67; 208Pb/204Pb: 39.10-39.46) of the alkaline rocks are within the range of most OIB in which the higher values approach the composition of the European Asthenospheric Reservoir (EAR). The correlation between Sr and Nd isotopes and trace element constraints (Ce/Pb; Nb/U) indicates that for some samples interaction with crustal rocks during

  6. Mapping magma sources in the east Sunda-Banda arcs, Indonesia: Constraints from helium isotopes

    NASA Astrophysics Data System (ADS)

    Hilton, D. R.; Hoogewerff, J. A.; van Bergen, M. J.; Hammerschmidt, K.

    1992-02-01

    We report new helium isotope analyses of olivine and clinopyroxene separates from recent lavas for eleven volcanoes from Flores in the east Sunda arc through the inactive segment between the arcs to Banda Island at the extreme of the contiguous Banda arc. In the east Sunda arc, 3He /4He ratios ( R) vary between 4.5 RA ( R A = air 3He /4He ) for the leucitic Batu Tara volcano to a remarkable low of pure radiogenic helium (0.0075 RA) for Werung at the southern tip of Lomblen Island. Lavas from the inactive zone, which represents the locus of collision of the Australian continent with the arc, have a narrower range in R/R A - from 3.9 for Kisu in the straits of Pantar to 1.0 for Romang Island. Our one locality (Banda Island) in the Banda arc gives the highest R/R A ratio (3.1) observed along this arc to date. The results are consistent with the involvement of crustal material in magma genesis throughout the east Sunda/Banda arcs, as far west as Iya in central Flores. We combine these helium isotope results with published and on-going strontium isotope studies, and show that the source of the helium in the crustal component is unlikely to be terrigenous sediments derived from the Australian continent; rather, degassing of Australian continental crust appears to be the dominant process controlling the helium budget. The He-Sr isotope systematics also provide a framework to account for the areal pattern of 3He /4He in this part of the arcs: the radiogenic crustal component is diluted with mantle helium both in a down-dip direction and with increasing lateral distance from the collision zone. These factors result in an excellent first-order relationship between the 3He /4He ratio, degree of He/Sr enrichment (relative to the postulated mantle endmember), and alkalinity of the erupted lavas. Such a relationship has a direct bearing on models of the tectonic evolution of the collision zone, and on the observation that helium isotopes are decoupled from strontium and other

  7. Iron isotope constraints on the mineralization processes of the Sandaowanzi telluride gold deposit, NE China

    NASA Astrophysics Data System (ADS)

    Li, Xingxing; Liu, Junlai; Lu, Di; Ren, Shunli; Liu, Zhengyang

    2016-04-01

    Iron isotopes have been widely applied to interpret the fluid evolution, supergene alteration and the metallogenic material sources of the hydrothermal deposit. It may also have significant potentials on the research of the deposit. The Sandaowanzi telluride gold deposit, located in the Great Hinggan Range metallogenic Belt in NE China, is a large epithermal gold deposit of low-sulphidation type. It has a total reserve of ≥25t of Au and an average of 15 g/t. Gold-bearing quartz veins or gold lodes strike to the NW and dip 50-80°northeastward. Ore bodies, including low-grade ores along margins and high-grade ores in the central parts, principally occur in quartz veins. More than the 95 percent Au budgets are hosted in gold-silver tellurides. A six-stage paragenetic sequence of mineralization is revealed according to the compositions and microstructures of the mineral assemblages. Although sulfide minerals in the bonanza quartz veins are rare, pyrite are widespread in quartz veins and altered host rocks. Meanwhile there are always chalcopyrite veins within bonanza quartz veins. Pyrite Fe isotope compositions from different levels (from +50m to +210m) of the main ore body of the Sandaowanzi gold ore deposit are investigated. There is an overall variation in δ57Fe values from -0.09 to +0.99 (av. 0.33). Among them, twenty three samples from different mining levels give positiveδ57Fe values, with the maximum positive value at the economic bonanza ores (level +130m). Four samples, however, possess negative values, one at level 170m, one at level 130m, and two at level 50m, respectively. The two negative values from the levels 170m and 130m are near the cores of the high grade ore body. The two negative values from the level 50m occur at one end of the lode ore body. The above data set shows that the δ57Fe values are not homogeneous at different levels of the ore body. On the other hand, a general trend for the positive values is that the highest δ57Fe value is

  8. Chlorine isotope geochemistry of hydrothermally altered oceanic crust: Mineralogical controls and experimental constraints

    NASA Astrophysics Data System (ADS)

    Cisneros, M.; Barnes, J.; Jenkins, D. M.; Gardner, J. E.

    2012-12-01

    Chlorine stable isotopes (37Cl and 35Cl) can provide an important fingerprint for geochemical recycling of subducted oceanic lithosphere and fluid-rock interaction due to chlorine's high solubility in aqueous phases. To implement Cl isotopes as a tracer of volatile element recycling, we must constrain the δ37Cl value of potential Cl reservoirs and determine fractionation factors between Cl-bearing phases. δ37Cl and Cl concentrations of hydrothermally altered oceanic crust (AOC) samples from seven IODP/ODP/DSDP drill sites have been measured on bulk rock samples (n = 50). For ease of comparing results, samples are categorized into three lithologies: 1) extrusive lavas, 2) sheeted dikes, and 3) gabbros. Extrusive lava Cl concentrations vary from <0.01 wt% to 0.03 wt% (avg = 95 ppm Cl; n= 20) and δ37Cl values range from -1.4 to +1.0‰ (avg = 0.0 ± 0.6‰). Chlorine concentrations of the sheeted dikes range from < 0.01 wt% to 0.05 wt% (avg = 163 ppm Cl; n = 11) and δ37Cl values of dikes range from - 0.4 to + 1.4‰ (avg = 0.1 ± 0.3‰). Bulk chlorine concentrations of the gabbros range from < 0.01 wt% to 0.09 wt% (avg = 244 ppm Cl; n = 19). δ37Cl values of gabbros range from - 0.6 to + 1.8‰ (avg = 0.6 ± 0.6‰). Three general conclusions can be derived from these AOC bulk rock results: 1) δ37Cl values and Cl concentrations increase with increasing total amphibole content. 2) Based on re-calculations of mass balance equations using updated AOC Cl concentrations (~3 times higher than previous estimates, this study), the total amount of Cl recycled into the mantle is higher than previously estimated. 3) [Cl] and δ37Cl values can provide a crude estimate of metamorphic grade in AOC samples. Amphibole-water Cl isotope fractionation experiments are necessary for quantifying the magnitude of Cl fractionation and to aid in interpreting the range of natural Cl isotope variation. Determination of equilibrium fractionation factors between hydrous minerals and co

  9. GEOCHEMICAL AND ISOTOPIC CONSTRAINTS ON GROUND-WATER FLOW DIRECTIONS, MIXING AND RECHARGE AT YUCCA MOUNTAIN, NEVADA

    SciTech Connect

    A. Meijer; E. Kwicklis

    2000-08-17

    This analysis is governed by the Office of Civilian Radioactive Waste Management (OCRWM) Analysis and Modeling Report Development Plan entitled ''Geochemical and Isotopic Constraints on Groundwater Flow Directions, Mixing and Recharge at Yucca Mountain'' (CRWMS M&O 1999a). As stated in this Development Plan, the purpose of the work is to provide an analysis of groundwater recharge rates, flow directions and velocities, and mixing proportions of water from different source areas based on groundwater geochemical and isotopic data. The analysis of hydrochemical and isotopic data is intended to provide a basis for evaluating the hydrologic system at Yucca Mountain independently of analyses based purely on hydraulic arguments. Where more than one conceptual model for flow is possible, based on existing hydraulic data, hydrochemical and isotopic data may be useful in eliminating some of these conceptual models. This report documents the use of geochemical and isotopic data to constrain rates and directions of groundwater flow near Yucca Mountain and the timing and magnitude of recharge in the Yucca Mountain vicinity. The geochemical and isotopic data are also examined with regard to the possible dilution of groundwater recharge from Yucca Mountain by mixing with groundwater downgradient from the potential repository site. Specifically, the primary tasks of this report, as listed in the AMR Development Plan (CRWMS M&O 1999a), consist of the following: (1) Compare geochemical and isotopic data for perched and pore water in the unsaturated zone with similar data from the saturated zone to determine if local recharge is present in the regional groundwater system; (2) Determine the timing of the recharge from stable isotopes such as deuterium ({sup 2}H) and oxygen-18 ({sup 18}O), which are known to vary over time as a function of climate, and from radioisotopes such as carbon-14 ({sup 14}C) and chlorine-36 ({sup 36}Cl); (3) Determine the magnitude of recharge from relatively

  10. Detrital zircon U-Pb ages and Hf isotopes of Permo-Carboniferous sandstones in central Inner Mongolia, China: Implications for provenance and tectonic evolution of the southeastern Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    Chen, Yan; Zhang, Zhicheng; Li, Ke; Yu, Haifei; Wu, Tairan

    2016-03-01

    The tectonic setting of the southeastern Central Asian Orogenic Belt (CAOB) during the Late Paleozoic has been debated for many years. Provenance analysis of Permo-Carboniferous sedimentary rocks can effectively address this issue. In this study, eight sandstone samples were collected for zircon U-Pb and Lu-Hf isotopic analyses combined with petrographic analysis. Framework petrography and zircon morphology suggest that the samples were from recycled orogen of an igneous origin. Carboniferous rocks, with a significant age peak at 432 Ma and εHf (t) values of - 9.0 to 13.6, were mainly derived from Early to Mid-Paleozoic magmatic rocks and deposited in a piedmont zone, namely, the margin of an inland sea. Permian rocks, mostly with age peaks at 445 Ma and/or 280 Ma and εHf (t) values of - 25.2 to 11.4, dominantly originated from a pre-existing Early to Mid-Paleozoic magmatic arc and Late Paleozoic igneous rocks. These rocks formed in restricted basins of the piedmont and intermountain zones. Based on zircon spectral discrimination, sedimentary environmental analysis, and previous studies, this study supports the interpretation that the southeastern CAOB entered stages of extension and rifting during the Late Paleozoic. In the end, this study proposes a tectonic-paleogeographic reconstruction to explain the tectonic evolution of the southeastern CAOB and the exhumation-transportation-deposition processes between the basins and ranges developed in this orogen.

  11. Barium Isotopic Composition of Mainstream Silicon Carbides from Murchison: Constraints for s-process Nucleosynthesis in Asymptotic Giant Branch Stars

    NASA Astrophysics Data System (ADS)

    Liu, Nan; Savina, Michael R.; Davis, Andrew M.; Gallino, Roberto; Straniero, Oscar; Gyngard, Frank; Pellin, Michael J.; Willingham, David G.; Dauphas, Nicolas; Pignatari, Marco; Bisterzo, Sara; Cristallo, Sergio; Herwig, Falk

    2014-05-01

    We present barium, carbon, and silicon isotopic compositions of 38 acid-cleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing barium contamination. Strong depletions in δ(138Ba/136Ba) values are found, down to -400‰, which can only be modeled with a flatter 13C profile within the 13C pocket than is normally used. The dependence of δ(138Ba/136Ba) predictions on the distribution of 13C within the pocket in asymptotic giant branch (AGB) models allows us to probe the 13C profile within the 13C pocket and the pocket mass in AGB stars. In addition, we provide constraints on the 22Ne(α, n)25Mg rate in the stellar temperature regime relevant to AGB stars, based on δ(134Ba/136Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative δ(134Ba/136Ba) values that cannot be explained by any of the current AGB model calculations. Instead, such negative values are consistent with the intermediate neutron capture process (i process), which is activated by the very late thermal pulse during the post-AGB phase and characterized by a neutron density much higher than the s process. These two grains may have condensed around post-AGB stars. Finally, we report abundances of two p-process isotopes, 130Ba and 132Ba, in single SiC grains. These isotopes are destroyed in the s process in AGB stars. By comparing their abundances with respect to that of 135Ba, we conclude that there is no measurable decay of 135Cs (t 1/2 = 2.3 Ma) to 135Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before 135Cs decayed.

  12. Constraints on continental crustal mass loss via chemical weathering using lithium and its isotopes

    NASA Astrophysics Data System (ADS)

    Rudnick, R. L.; Liu, X. M.

    2012-04-01

    The continental crust has an "intermediate" bulk composition that is distinct from primary melts of peridotitic mantle (basalt or picrite). This mismatch between the "building blocks" and the "edifice" that is the continental crust points to the operation of processes that preferentially remove mafic to ultramafic material from the continents. Such processes include lower crustal recycling (via density foundering or lower crustal subduction - e.g., relamination, Hacker et al., 2011, EPSL), generation of evolved melts via slab melting, and/or chemical weathering. Stable isotope systems point to the influence of chemical weathering on the bulk crust composition: the oxygen isotope composition of the bulk crust is distinctly heavier than that of primary, mantle-derived melts (Simon and Lecuyer, 2005, G-cubed) and the Li isotopic composition of the bulk crust is distinctly lighter than that of mantle-derive melts (Teng et al., 2004, GCA; 2008, Chem. Geol.). Both signatures mark the imprint of chemical weathering on the bulk crust composition. Here, we use a simple mass balance model for lithium inputs and outputs from the continental crust to quantify the mass lost due to chemical weathering. We find that a minimum of 15%, a maximum of 60%, and a best estimate of ~40% of the original juvenile rock mass may have been lost via chemical weathering. The accumulated percentage of mass loss due to chemical weathering leads to an average global chemical weathering rate (CWR) of ~ 1×10^10 to 2×10^10 t/yr since 3.5 Ga, which is about an order of magnitude higher than the minimum estimates based on modern rivers (Gaillardet et al., 1999, Chem. Geol.). While we cannot constrain the exact portion of crustal mass loss via chemical weathering, given the uncertainties of the calculation, we can demonstrate that the weathering flux is non-zero. Therefore, chemical weathering must play a role in the evolution of the composition and mass of the continental crust.

  13. Oxygen Isotope Mass-Balance Constraints on Pliocene Sea Level and East Antarctic Ice Sheet Stability

    NASA Astrophysics Data System (ADS)

    Winnick, M. J.; Caves, J. K.

    2015-12-01

    The mid-Pliocene Warm Period (MPWP, 3.3-2.9 Ma), with reconstructed atmospheric pCO2 of 350-450 ppm, represents a potential analogue for climate change in the near future. Current highly cited estimates place MPWP maximum global mean sea level (GMSL) at 21 ± 10 m above modern, requiring total loss of the Greenland (GIS) and marine West Antarctic Ice Sheets (WAIS) and a substantial loss of the East Antarctic Ice Sheet (EAIS), with only a concurrent 2-3 ºC rise in global temperature. Many estimates of Pliocene GMSL are based on the partitioning of oxygen isotope records from benthic foraminifera (δ18Ob) into changes in deep-sea temperatures and terrestrial ice sheets. These isotopic budgets are underpinned by the assumption that the δ18O of Antarctic ice (δ18Oi) was the same in the Pliocene as it is today, and while the sensitivity of δ18Ob to changing meltwater δ18O has been previously considered, these analyses neglect conservation of 18O/16O in the ocean-ice system. Using well-calibrated δ18O-temperature relationships for Antarctic precipitation along with estimates of Pliocene Antarctic surface temperatures, we argue that the δ18Oi of the Pliocene Antarctic ice sheet was at minimum 1‰-4‰ higher than present. Assuming conservation of 18O/16O in the ocean-ice system, this requires lower Pliocene seawater δ18O (δ18Osw) without a corresponding change in ice sheet mass. This effect alone accounts for 5%-20% of the δ18Ob difference between the MPWP interglacials and the modern. With this amended isotope budget, we suggest that Pliocene GMSL was likely 9-13.5 m and very likely 5-17 m above modern, which suggests the EAIS is less sensitive to radiative forcing than previously inferred from the geologic record.

  14. Derivation of Apollo 14 High-Al Basalts at Discrete Times: Rb-Sr Isotopic Constraints

    NASA Technical Reports Server (NTRS)

    Hui. Hejiu; Neal, Clive, R.; Shih, Chi-Yu; Nyquist, Laurence E.

    2012-01-01

    Pristine Apollo 14 (A-14) high-Al basalts represent the oldest volcanic deposits returned from the Moon [1,2] and are relatively enriched in Al2O3 (>11 wt%) compared to other mare basalts (7-11 wt%). Literature Rb-Sr isotopic data suggest there are at least three different eruption episodes for the A-14 high-Al basalts spanning the age range approx.4.3 Ga to approx.3.95 Ga [1,3]. Therefore, the high-Al basalts may record lunar mantle evolution between the formation of lunar crust (approx.4.4 Ga) and the main basin-filling mare volcanism (<3.85 Ga) [4]. The high-Al basalts were originally classified into five compositional groups [5,6], and then regrouped into three with a possible fourth comprising 14072 based on the whole-rock incompatible trace element (ITE) ratios and Rb-Sr radiometric ages [7]. However, Rb-Sr ages of these basalts from different laboratories may not be consistent with each other because of the use of different 87Rb decay constants [8] and different isochron derivation methods over the last four decades. This study involved a literature search for Rb-Sr isotopic data previously reported for the high-Al basalts. With the re-calculated Rb-Sr radiometric ages, eruption episodes of A-14 high-Al basalts were determined, and their petrogenesis was investigated in light of the "new" Rb-Sr isotopic data and published trace element abundances of these basalts.

  15. Carbon isotope constraints on degassing of carbon dioxide from Kilauea Volcano

    USGS Publications Warehouse

    Gerlach, T.M.; Taylor, B.E.

    1990-01-01

    We examine models for batch-equilibrium and fractional-equilibrium degassing of CO2 from magma at Kilauea Volcano. The models are based on 1. (1) the concept of two-stage degassing of CO2 from magma supplied to the summit chamber, 2. (2) C isotope data for CO2 in eruptive and noneruptive (quiescent) gases from Kilauea and 3. (3) data for the isotopic fractionation of C between CO2 and C dissolved in tholeiitic basalt melt. The results of our study indicate that 1. (1) both eruptive and noneruptive degassing of CO2 most closely approach a batch equilibrium process, 2. (2) the ??13C of parental magma supplied to the summit chamber is in the range -4.1 to-3.4??? and 3. (3) the ??13C of melt after summit chamber degassing is in the range -7 to -8???, depending upon the depth of equilibration. We also present ??13C data for CO2 in eruptive gases from the current East Rift Zone eruption. These are the first C isotope data for CO2 in high-temperature (>900??C) eruptive gases from Kilauea; they have a mean ??13C value of -7.82 ?? 0.24??? and are similar to those predicted for the melt after summit chamber degassing. The minor role played by fractional degassing of ascending magma at Kilauea means that exsolved CO2 tends to remain entrained in and coherent with its host melt during ascent from both mantle source regions and crustal magma reservoirs. This has important implications for magma dynamics at Kilauea. ?? 1990.

  16. Evaporative isotope enrichment as a constraint on reach water balance along a dryland river.

    PubMed

    Gibson, John J; Sadek, Mostafa A; Stone, D J M; Hughes, Catherine E; Hankin, S; Cendon, Dioni I; Hollins, Suzanne E

    2008-03-01

    Deuterium and oxygen-18 enrichment in river water during its transit across dryland region is found to occur systematically along evaporation lines with slopes of close to 4 in (2)H-(18)O space, largely consistent with trends predicted by the Craig-Gordon model for an open-water dominated evaporating system. This, in combination with reach balance assessments and derived runoff ratios, strongly suggests that the enrichment signal and its variability in the Barwon-Darling river, Southeastern Australia is acquired during the process of evaporation from the river channel itself, as enhanced by the presence of abundant weirs, dams and other storages, rather than reflecting inherited enrichment signals from soil water evaporation in the watershed. Using a steady-state isotope mass balance analysis based on monthly (18)O and (2)H, we use the isotopic evolution of river water to re-construct a perspective of net exchange between the river and its contributing area along eight reaches of the river during a drought period from July 2002 to December 2003, including the duration of a minor flow event. The resulting scenario, which uses a combination of climatological averages and available real-time meteorological data, should be viewed as a preliminary test of the application rather than as a definitive inventory of reach water balance. As expected for a flood-driven dryland system, considerable temporal variability in exchange is predicted. While requiring additional real-time isotopic data for operational use, the method demonstrates potential as a non-invasive tool for detecting and quantifying water diversions, one that can be easily incorporated within existing water quality monitoring activities.

  17. Sulphur isotope constraints on formation conditions of the Luiswishi ore deposit, Democratic Republic of Congo (DRC)

    NASA Astrophysics Data System (ADS)

    Lerouge, C.; Cailteux, J.; Kampunzu, A. B.; Milesi, J. P.; Fléhoc, C.

    2005-07-01

    Luiswishi is a Congo-type Neoproterozoic sediment-hosted stratiform Cu-Co ore deposit of the Central Africa Copperbelt, located northwest of Lubumbashi (DRC). The ores form two main Cu-Co orebodies hosted by the Mines Subgroup, one in the lower part of the Kamoto Formation and the other at the base of the Dolomitic Shales Formation. Sulphides occur essentially as early parallel layers of chalcopyrite and carrolite, and secondarily as late stockwork sulphides cross-cutting the bedding and the early sulphide generation. Both types of stratiform and stockwork chalcopyrite and carrolite were systematically analyzed for sulphur isotopes, along the lithostratigraphic succession of the Mine Series. The quite similar δ 34S values of stratiform sulphides and late stockwork sulphides suggest an in situ recrystallization or a slight remobilization of stockwork sulphides without attainment of isotopic equilibrium between different sulphide phases (chalcopyrite and carrolite). The distribution of δ 34S values (-14.4‰ to +17.5‰) combined with the lithology indicates a strong stratigraphic control of the sulphur isotope signature, supporting bacterial sulphate reduction during early diagenesis of the host sediments, in a shallow marine to lacustrine environment. Petrological features combined with sulphur isotopic data of sulphides at Luiswishi and previous results on nodules of anhydrite in the Mine Series indicate a dominant seawater/lacustrine origin for sulphates, precluding a possible hydrothermal participation. The high positive δ 34S values of sulphides in the lower orebody at Luiswishi, hosted in massive chloritic-dolomitic siltite (known as Grey R.A.T.), fine-grained stratified dolostone (D.Strat.) and silicified-stromatolitic dolomites alternating with chloritic-dolomitic silty beds (R.S.F.), suggest that they were probably deposited during a period of regression in a basin cut off from seawater. The variations of δ 34S values (i.e. the decrease of δ 34S values

  18. Constraints on Earth degassing history from the argon isotope composition of Devonian atmosphere

    NASA Astrophysics Data System (ADS)

    Stuart, F. M.; Mark, D.

    2012-04-01

    The primordial and radiogenic isotopes of the noble gases combine to make them a powerful tool for determining the time and tempo of the outgassing of the Earth's interior. The outgassing history of the Earth is largely constrained from measurements of the isotopic composition of He, Ne, Ar and Xe in samples of modern mantle, crust and atmosphere. There have been few unequivocal measurement of the isotopic composition of noble gases in ancient atmosphere. We have re-visited whether ancient Ar is trapped in the ~400 Ma Rhynie chert [1]. We have analysed samples of pristine Rhynie chert using the ARGUS multi-collector mass spectrometer calibrated against the new determination of atmospheric Ar isotope ratios [2]. 40Ar/36Ar ratios are low, with many lower than the modern air value (298.8). Importantly these are accompanied by atmospheric 38Ar/36Ar ratios indicating that the low 40Ar/36Ar are not due to mass fractionation. We conclude that the Rhynie chert has captured Devonian atmosphere-derived Ar. The data indicate that the Devonian atmosphere 40Ar/36Ar was at least 3 % lower than the modern air value. Thus the Earth's atmosphere has accumulated at least 5 ± 0.2 x 1016 moles of 40Ar in the last 400 million years, at an average rate of 1.24 ± 0.06 x 108 mol 40Ar/year. This overlaps the rate determined from ice cores for the last 800,000 years [3] and implies that there has been no resolvable temporal change in Earth outgassing rate since mid-Palaeozoic times. The new data require the Earth outgassed early, and suggests that pristine samples of Archaean and Proterozoic chert may prove useful as palaeo-atmosphere tracers. [1] G. Turner, J. Geol. Soc. London 146, 147-154 (1989) [2] D. Mark, F.M. Stuart, M. de Podesta, Geochim. Cosmochim. Acta 75, 7494-7501 [3] M. Bender et al., Proc. Nat. Acad. Sci. 105, 8232-8237 (2008)

  19. Geochronology, geochemistry, and Hf isotopes of Jurassic intermediate-acidic intrusions in the Xing'an Block, northeastern China: Petrogenesis and implications for subduction of the Paleo-Pacific oceanic plate

    NASA Astrophysics Data System (ADS)

    Dong, Yu; Ge, Wen-chun; Yang, Hao; Xu, Wen-liang; Zhang, Yan-long; Bi, Jun-hui; Liu, Xi-wen

    2016-03-01

    Zircon U-Pb dating, whole-rock geochemistry, Hf isotopic compositions, and regional geological observations of Jurassic intermediate-acidic intrusions in the Xing'an Block, northeastern China, are presented to constrain their petrogenesis and the tectonic evolution of the Paleo-Pacific Ocean. Zircon U-Pb age dating indicates that the intrusions were emplaced in three stages: during the Early Jurassic (180-177 Ma), Middle Jurassic (171-170 Ma), and Late Jurassic (∼151 Ma). Despite the wide range in ages of the intrusions, the magmas of Jurassic acidic intrusions were likely derived from a similar or common source and experienced different degrees of magmatic differentiation, as inferred from their geochemical and Hf isotopic characteristics. The Jurassic acidic intrusions are characterized by high SiO2 and total Na2O + K2O, low MgO, and I-type affinities, suggesting that the primary magmas were derived from partial melting of lower crustal material. These findings, combined with their εHf(t) values and two-stage model ages, indicate the primary magmas originated from partial melting of juvenile crustal material accreted during the Neoproterozoic to Phanerozoic. The Middle Jurassic intermediate-acidic rocks (diorites and granodiorites of the TJ pluton) have SiO2 contents of 57.96-69.10 wt.%, MgO contents of 4.48-1.81 wt.%, and high Mg numbers (45-54). They are enriched in large ion lithophile elements (e.g., Rb, Ba, Th, U, and K) and light rare earth elements, depleted in high field strength elements (e.g., Nb, Ta, Zr, Hf, and Ti) and heavy rare earth elements, and have εHf(t) values of +6.5 to +9.1. These data suggest that the magma was derived from partial melting of a depleted mantle wedge that had been metasomatized by subduction-related fluids. According to these findings and previous studies that focused on contemporaneous magmatic-tectonic activity in northeastern China, we conclude that the generation of Jurassic intermediate-acidic intrusions in the Xing

  20. Late Triassic intrusive complex in the Jidong region, Jiamusi-Khanka Block, NE China: Geochemistry, zircon U-Pb ages, Lu-Hf isotopes, and implications for magma mingling and mixing

    NASA Astrophysics Data System (ADS)

    Yang, Hao; Ge, Wen-chun; Zhao, Guo-chun; Dong, Yu; Xu, Wen-liang; Ji, Zheng; Yu, Jie-jiang

    2015-05-01

    Whole-rock major and trace element geochemistry together with zircon U-Pb ages and Lu-Hf isotope compositions are reported for a Late Triassic intrusive complex in the Jidong region, Jiamusi-Khanka Block, NE China. Zircon U-Pb dating yields ages between 211 and 208 Ma for enclaves of microgranular diorite and quartz diorite, and between 211 and 209 Ma for the host granitoids. These ages correlate with a previously established intensive Late Triassic magmatic event along the eastern Asian margin. Field observations, together with petrographic features, geochemistry, and zircon Hf isotope data, preclude simple crystal fractionation or restite unmixing as a genetic link for the various rock types within the intrusive complex. The syenogranite suite has high SiO2 (75.5-76.3 wt.%) and low MgO (0.15-0.19 wt.%), and yields enriched LILE and LREE patterns. Most of the zircons in the syenogranites have two-stage model ages of 766 and 1461 Ma, together with positive εHf(t) values of + 0.6 to + 9.1. These results indicate that the granitoid magmas were generated by partial melting of Meso- to Neoproterozoic lower crust. The gabbro suite has a restricted range of SiO2 (46.1-51.9 wt.%) together with high Mg# values (49-70) and high concentrations of Ni, Co, and Cr. Zircons from two diorite samples have single-stage Hf model ages of 557-787 Ma and εHf(t) values of + 1.9 to + 8.3 that are consistent with the coeval gabbros previously studied in the Jidong region. These features, together with the observation that all the gabbros are enriched in LREE and LILE, suggest that the mafic magmas were derived from melting of depleted Neoproterozoic lithospheric mantle that had been metasomatized by slab-derived fluids. It is concluded that the dominant igneous suites within the Late Triassic intrusive complex formed by mingling/mixing of felsic and mafic magmas. The geochemical data, combined with regional geological investigations, indicate that the Late Triassic intrusive complex

  1. Geochemical and isotopic constraints on the tectonic setting of Serra dos Carajas belt, eastern Para, Brazil

    NASA Technical Reports Server (NTRS)

    Olszewski, W. J., Jr.; Gibbs, A. K.; Wirth, K. R.

    1986-01-01

    The lower part of the Serra dos Carajas belt is the metavolcanic and metasedimentary Grao para Group (GPG). The GPG is thought to unconformably overlie the older (but undated) Xingu Complex, composed of medium and high-grade gneisses and amphibolite and greenstone belts. The geochemical data indicate that the GPG has many features in common with ancient and modern volcanic suites erupted through continental crust. The mafic rocks clearly differ from those of most Archean greenstone belts, and modern MORB, IAB, and hot-spot basalts. The geological, geochemical, and isotopic data are all consistent with deposition on continental crust, presumably in a marine basin formed by crustal extension. The isotopic data also suggest the existence of depleted mantle as a source for the parent magmas of the GPG. The overall results suggest a tectonic environment, igneous sources, and petrogenesis similar to many modern continental extensional basins, in contrast to most Archean greenstone belts. The Hammersley basin in Australia and the circum-Superior belts in Canada may be suitable Archean and Proterozoic analogues, respectively.

  2. Isotope constraints on the involvement of fluids in the San Andreas Fault System, California

    SciTech Connect

    Pili, E.; Kennedy, B.M.; Conrad, S.M.; Gratier, J.-P.; Poitrasson, F.

    1998-07-01

    Fluids are suspected to play a major role in earthquake mechanics, especially in the case of the weak San Andreas Fault (SAF). Models developed to explain the weakness of the fault are similar but rely on different fluid sources. A recent study of groundwaters associated with the SAF has provided evidence for a geopressured mantle fluid source (Kennedy et al., 1997). We present here an isotope study comparing deformation zones (gouges, breccias, fault veins, slickensides, cataclasites), and vein fillings with their hosts and the fluids associated with these materials, as sampled by fluid inclusions. We are investigating ca. 250 samples from over 20 localities along the San Andreas and adjacent faults from South San Francisco to East Los Angeles. Samples from the exhumed San Gabriel Fault, a deeper equivalent of the SAF, are included as well as samples from the Santa Ynez Fault, another former strand of the SAF embedded in Miocene limestones. All the major lithologies (granites, gneisses, sandstones, limestones, marbles and serpentinites) have been sampled for isotope analyses of C, O, H, He, Ne, Ar, Sr, Nd, and Pb.

  3. Isotopic constraints on the source of Pluto's nitrogen and the history of atmospheric escape

    NASA Astrophysics Data System (ADS)

    Mandt, Kathleen E.; Mousis, Olivier; Luspay-Kuti, Adrienn

    2016-10-01

    The origin and evolution of nitrogen in solar system bodies is an important question for understanding processes that took place during the formation of the planets and solar system bodies. Pluto has an atmosphere that is 99% molecular nitrogen, but it is unclear if this nitrogen is primordial or derived from ammonia in the protosolar nebula. The nitrogen isotope ratio is an important tracer of the origin of nitrogen on solar system bodies, and can be used at Pluto to determine the origin of its nitrogen. After evaluating the potential impact of escape and photochemistry on Pluto's nitrogen isotope ratio (14N/15N), we find that if Pluto's nitrogen originated as N2 the current ratio in Pluto's atmosphere would be greater than 324 while it would be less than 157 if the source of Pluto's nitrogen were NH3. The New Horizons spacecraft successfully visited the Pluto system in July 2015 providing a potential opportunity to measure 14N/15N in N2.

  4. Isotopic constraints on sources of methane in Los Angeles, California, USA (Invited)

    NASA Astrophysics Data System (ADS)

    Townsend-Small, A.; Tyler, S. C.; Christensen, L.; Xu, X.; Pataki, D. E.

    2009-12-01

    Methane is a powerful greenhouse gas and an important contributor to global warming. Recent studies have suggested that methane emissions in large cities are underestimated with several models even indicating that substantial emissions attributed to cities are in part from regional and/or encroaching agricultural sources rather than from urban fossil fuel sources. We have found that stable isotopes (13-C and D) and radiocarbon (C-14) are excellent tracers of various sources of methane in Los Angeles, California. Measurements of the d13C and dD of methane from discrete sources show excellent separation between urban sources, such as vehicle emissions, power plants, oil refineries, landfills, and sewage treatment plants and agricultural sources like cows, biogas, and cattle feedlots. In addition, radiocarbon is an excellent tracer of modern versus fossil fuel contributions to methane emissions in the region. Preliminary measurements of background air in Los Angeles indicate that the major source of excess methane is vehicle emissions with most additional CH4 likely contributed from among other fossil fuel sources such as oil refining or power plants. We are currently confirming these results with broader field campaigns and additional measurements, including continuous measurements of atmospheric methane concentration using tunable laser spectroscopy. The combination of high-resolution tunable laser concentration measurements and precise isotope measurements using mass spectrometry is a very promising and powerful tool for methane source monitoring.

  5. Geochemical, isotopic, and mineralogical constraints on atmospheric deposition in the hyper-arid Atacama Desert, Chile

    NASA Astrophysics Data System (ADS)

    Wang, Fan; Michalski, Greg; Seo, Ji-hye; Ge, Wensheng

    2014-06-01

    Modern atmospheric deposition across the Atacama was collected by an array of dust traps that stretched from the Pacific coast to the Andean altiplano, and the material was analyzed for its geochemical, mass and isotopic composition. The coastal trap had the second-highest insoluble mineral particle and highest soluble salt deposition rates due to significant inputs from the Morro Mejillones Range and the Pacific Ocean, respectively. The Andean trap had the highest insoluble mineral particle deposition owing to transport of weathered material, but the lowest deposition rate of soluble salts due to its distance from the ocean and anthropogenic sources. The removal of oceanic material was effective by the coastal mountains, while the westward transport of the Andean material was determined to be minimal. The atmospheric deposition in the inland traps was mainly from the local entrainment of surface material, inland anthropogenic emissions, and transport of marine aerosols. The nitrate isotopes (δ15N and Δ17O) suggested that NOx sources and NO3- chemistry shifted along the west-east transect, and were greatly impacted by anthropogenic emissions with soil NO3- being a minor source of deposited nitrogen.

  6. Processes controlling the chromium isotopic composition of river water: Constraints from basaltic river catchments

    NASA Astrophysics Data System (ADS)

    D'Arcy, Joan; Babechuk, Michael G.; Døssing, Lasse Nørbye; Gaucher, Claudio; Frei, Robert

    2016-08-01

    We report chromium (Cr) isotope compositions and concentrations (and additional geochemical and physicochemical data) of bedrock, soils and river waters from two geographically distinct basaltic river catchments, the Uruguay River catchment (Uruguay) and the Glenariff River catchment (Northern Ireland, United Kingdom), to investigate the processes that control Cr mobilisation and fractionation during weathering and riverine transport to the sea. Our results show that the Cr isotope compositions of soils are a function of the modal abundance and weathering rates of Cr-bearing minerals. The accumulation of weathering resistant Cr-spinels in the soils of Northern Ireland results in soils which are enriched in Cr and have δ53Cr values within the range of local bedrock (δ53Cr value of -0.21 ± 0.12‰, 2σ, n = 4). By contrast, the more easily weathered Cr-silicates in the bedrock of Uruguay results in greater Cr loss from the soil and a depletion in the heavy isotopes of Cr (with average δ53Cr value of -0.32 ± 0.04‰, 2σ, n = 4) relative to the local bedrock (δ53Cr value of -0.22 ± 0.08‰, 2σ, n = 4). The river waters in both catchments are predominantly enriched in the heavy 53Cr isotope relative to bedrock, although the range and average river water δ53Cr values differ significantly between each. The Uruguay rivers exhibit a restricted range in δ53Cr values, with a mean of +0.08 ± 0.06‰ (2σ, n = 5) that represents a positive fractionation of +0.2‰ relative to bedrock, and is best explained by the unidirectional formation of Cr(VI) during weathering that has not been significantly modified by back-reduction to Cr(III). By contrast, the Glenariff stream and river waters (Northern Ireland) exhibit a wide range in δ53Cr values from -0.17 ± 0.3‰ (2σ, n = 4) to +1.68 ± 0.3‰ (n = 1) that appears to reflect the variable redox conditions of the catchment. In general, the values with the lowest 53Cr enrichment have higher Cr concentrations, the lowest

  7. Tracking selenium behaviour in chalk aquifer (northern France): Sr and 34S-sulphates isotopes constraints.

    NASA Astrophysics Data System (ADS)

    Cary, Lise; Benabderraziq, Hind; Elkhattabi, Jamal; Parmentier, Marc; Gourcy, Laurence; Négrel, Philippe

    2014-05-01

    Groundwaters in parts of the Paris Basin (France) are facing increasing selenium (Se) contents that can exceed the drinking water limit of 10 μg/L according to the European Framework Directive in the field of water policy (2000/60/EC). To better understand the groundwater origins and the selenium dynamics, the water chemistry of the Chalk aquifer supplying drinkable water to Lille city was studied. This area is submitted to quantitative and qualitative pressure from industrial, urban and agriculture origins. An integrated study was settled to determine the water sources and dynamics of elements, with a focus on Se. After a large chemical characterisation of the groundwater chemistry in the four field wells, a monthly monitoring was held in four wells and in the Deûle channel. Chemical analysis of major and trace elements, stable isotopes (δ18O, δ2H), strontium isotopes, and δ34S and δ18O of sulphates were realised. The chemical composition of solids sampled at various depths at vicinity of the four wells was also analysed. The specific geochemical signature of groundwater as revealed by Sr isotopes, in addition to element concentrations ratios like Mg/Sr and Se/Sr, highlighted mixture of three main groundwaters bodies: (1) the upstream groundwaters in the recharge area with the most radiogenic 87Sr/86Sr isotopic signature; (2) the confined groundwaters with high Sr concentrations due to water-rock interactions and the lowest 87Sr/86Sr isotopic signature close to the one of the chalk in Paris and London basins; (3) the Se-rich formations of Tertiary and Quaternary. The contents of Se, mainly present as SeV I (and locally as SeIV ), displayed spatial and temporal disparities that can be explained by geological and hydrogeological conditions. Se-rich clayed sediments originating from the dismantling of Se-rich tertiary formations (i.e. Ypresian) overlay the chalk formation and can be found in saturated conditions depending of the water table level. Oxidation of

  8. Zircon U-Pb ages, geochemistry, and Nd-Hf isotopes of the TTG gneisses from the Jiaobei terrane: Implications for Neoarchean crustal evolution in the North China Craton

    NASA Astrophysics Data System (ADS)

    Shan, Houxiang; Zhai, Mingguo; Wang, Fang; Zhou, Yanyan; Santosh, M.; Zhu, Xiyan; Zhang, Huafeng; Wang, Wei

    2015-02-01

    The Precambrian basement in the Jiaobei terrane is largely composed of Tonalite-Trondhjemite-Granodiorite (TTG) suite of rocks and offers important insights into the crustal evolution history of the North China Craton (NCC). The LA-ICP-MS zircon U-Pb age data presented in this study show that the magmatic protoliths of the TTG gneisses formed during 2508-2547 Ma and recorded the Paleoproterozoic metamorphism (∼1905 Ma). The rocks are enriched in LILE (Rb, Ba and Sr) and depleted in HFSE (Nb, Ta, Zr and Hf). They are characterized by high Sr contents (406-2906 ppm), Sr/Y ratios (31.3-355) and subchondritic Nb/Ta ratios (18.5-68.9). The TTGs show relatively high ΣREE contents (72.0-266 ppm) with strongly enriched LREE ((La/Yb)N = 11.5-121) and positive or negligible negative Eu anomalies (Eu/Eu∗ = 0.84-1.89). These geochemical features suggest that the magma source might have been rutile-bearing amphibole eclogite. Their high Mg# numbers (42-56) and high Cr (153-285 ppm) and Ni contents (22.2-74.5 ppm) indicate interaction with the mantle wedge during magma ascent. The whole rock εNd (t) values (+2.6 to +3.8) and most of the magmatic zircon εHf (t) values (+1.3 to +7.6) suggest juvenile to evolved isotopic signatures. All these lines of evidence suggest that the TTG rocks in this study formed through partial melting of subducted oceanic slab in a continental arc environment. The drill holes in the Jiaobei terrane are dominated by ∼2.5 Ga TTG gneisses, suggesting that the TTG magma at ∼2.5 Ga is more widely distributed deep underground than that of ∼2.7-2.9 Ga, at least within the approachable depth range of our research. Some zircon grains from Jiaobei TTGs give high εHf (t) values plotting above the curve of 0.75 ∗ εHf of DM, and their TCDM ages are very close to the time of the zircon crystallization. However, the majority of the εHf (t) values fall below the curve of 0.75 ∗ εHf of DM and their TCDM ages are concentrated between ∼2.7-2.9 Ga

  9. Permian-Carboniferous arc magmatism in southern Mexico: U-Pb dating, trace element and Hf isotopic evidence on zircons of earliest subduction beneath the western margin of Gondwana

    NASA Astrophysics Data System (ADS)

    Ortega-Obregón, C.; Solari, L.; Gómez-Tuena, A.; Elías-Herrera, M.; Ortega-Gutiérrez, F.; Macías-Romo, C.

    2014-07-01

    Undeformed felsic to mafic igneous rocks, dated by U-Pb zircon geochronology between 311 and 255 Ma, intrude different units of the Oaxacan and Acatlán metamorphic complexes in southwestern Mexico. Rare earth element concentrations on zircons from most of these magmatic rocks have a typical igneous character, with fractionated heavy rare earths and negative Eu anomalies. Only inherited Precambrian zircons are depleted in heavy rare earth elements, which suggest contemporaneous crystallization in equilibrium with metamorphic garnet during granulite facies metamorphism. Hf isotopic signatures are, however, different among these magmatic units. For example, zircons from two of these magmatic units (Cuanana pluton and Honduras batholith) have positive ɛHf values (+3.8-+8.5) and depleted mantle model ages (using a mean crustal value of 176Lu/177Hf = 0.015) ( T DMC) ranging between 756 and 1,057 Ma, whereas zircons from the rest of the magmatic units (Etla granite, Zaniza batholith, Carbonera stock and Sosola rhyolite) have negative ɛHf values (-1 to -14) and model ages between 1,330 and 2,160 Ma. This suggests either recycling of different crustal sources or, more likely, different extents of crustal contamination of arc-related mafic magmas in which the Oaxacan Complex acted as the main contaminant. These plutons thus represent the magmatic expression of the initial stages of eastward subduction of the Pacific plate beneath the western margin of Gondwana, and confirm the existence of a Late Carboniferous-Permian magmatic arc that extended from southern North America to Central America.

  10. Rb-Sr, Sm-Nd and Lu-Hf isotope systematics of the lunar Mg-suite: the age of the lunar crust and its relation to the time of Moon formation.

    PubMed

    Carlson, Richard W; Borg, Lars E; Gaffney, Amy M; Boyet, Maud

    2014-09-13

    New Rb-Sr, (146,147)Sm-(142,143)Nd and Lu-Hf isotopic analyses of Mg-suite lunar crustal rocks 67667, 76335, 77215 and 78238, including an internal isochron for norite 77215, were undertaken to better define the time and duration of lunar crust formation and the history of the source materials of the Mg-suite. Isochron ages determined in this study for 77215 are: Rb-Sr=4450±270 Ma, (147)Sm-(143)Nd=4283±23 Ma and Lu-Hf=4421±68 Ma. The data define an initial (146)Sm/(144)Sm ratio of 0.00193±0.00092 corresponding to ages between 4348 and 4413 Ma depending on the half-life and initial abundance used for (146)Sm. The initial Nd and Hf isotopic compositions of all samples indicate a source region with slight enrichment in the incompatible elements in accord with previous suggestions that the Mg-suite crustal rocks contain a component of KREEP. The Sm/Nd-(142)Nd/(144)Nd correlation shown by both ferroan anorthosite and Mg-suite rocks is coincident with the trend defined by mare and KREEP basalts, the slope of which corresponds to ages between 4.35 and 4.45 Ga. These data, along with similar ages for various early Earth differentiation events, are in accord with the model of lunar formation via giant impact into Earth at ca 4.4 Ga. PMID:25114305

  11. Rb-Sr, Sm-Nd and Lu-Hf isotope systematics of the lunar Mg-suite: the age of the lunar crust and its relation to the time of Moon formation

    PubMed Central

    Carlson, Richard W.; Borg, Lars E.; Gaffney, Amy M.; Boyet, Maud

    2014-01-01

    New Rb-Sr, 146,147Sm-142,143Nd and Lu-Hf isotopic analyses of Mg-suite lunar crustal rocks 67667, 76335, 77215 and 78238, including an internal isochron for norite 77215, were undertaken to better define the time and duration of lunar crust formation and the history of the source materials of the Mg-suite. Isochron ages determined in this study for 77215 are: Rb-Sr=4450±270 Ma, 147Sm-143Nd=4283±23 Ma and Lu-Hf=4421±68 Ma. The data define an initial 146Sm/144Sm ratio of 0.00193±0.00092 corresponding to ages between 4348 and 4413 Ma depending on the half-life and initial abundance used for 146Sm. The initial Nd and Hf isotopic compositions of all samples indicate a source region with slight enrichment in the incompatible elements in accord with previous suggestions that the Mg-suite crustal rocks contain a component of KREEP. The Sm/Nd—142Nd/144Nd correlation shown by both ferroan anorthosite and Mg-suite rocks is coincident with the trend defined by mare and KREEP basalts, the slope of which corresponds to ages between 4.35 and 4.45 Ga. These data, along with similar ages for various early Earth differentiation events, are in accord with the model of lunar formation via giant impact into Earth at ca 4.4 Ga. PMID:25114305

  12. Rb-Sr, Sm-Nd and Lu-Hf isotope systematics of the lunar Mg-suite: the age of the lunar crust and its relation to the time of Moon formation.

    PubMed

    Carlson, Richard W; Borg, Lars E; Gaffney, Amy M; Boyet, Maud

    2014-09-13

    New Rb-Sr, (146,147)Sm-(142,143)Nd and Lu-Hf isotopic analyses of Mg-suite lunar crustal rocks 67667, 76335, 77215 and 78238, including an internal isochron for norite 77215, were undertaken to better define the time and duration of lunar crust formation and the history of the source materials of the Mg-suite. Isochron ages determined in this study for 77215 are: Rb-Sr=4450±270 Ma, (147)Sm-(143)Nd=4283±23 Ma and Lu-Hf=4421±68 Ma. The data define an initial (146)Sm/(144)Sm ratio of 0.00193±0.00092 corresponding to ages between 4348 and 4413 Ma depending on the half-life and initial abundance used for (146)Sm. The initial Nd and Hf isotopic compositions of all samples indicate a source region with slight enrichment in the incompatible elements in accord with previous suggestions that the Mg-suite crustal rocks contain a component of KREEP. The Sm/Nd-(142)Nd/(144)Nd correlation shown by both ferroan anorthosite and Mg-suite rocks is coincident with the trend defined by mare and KREEP basalts, the slope of which corresponds to ages between 4.35 and 4.45 Ga. These data, along with similar ages for various early Earth differentiation events, are in accord with the model of lunar formation via giant impact into Earth at ca 4.4 Ga.

  13. Influx of Different Galapagos Plume Components to the Galapagos Spreading Center: Evidence From Sr-Nd-Pb-Hf Isotope Variations in Axial Lavas Between 86W and 92.5W

    NASA Astrophysics Data System (ADS)

    Hauff, F.; Hanan, B.; Hoernle, K. A.; Kokfelt, T. F.; Christie, D.; Werner, R.

    2006-12-01

    We present new Sr-Nd-Pb-Hf isotope data of basaltic glasses from the GSC between 86W and 92.5W. In this part of the ridge the main structural inventory includes an overlapping spreading center (OSC) at 87.3W, a transform fault (TF) at 91W and a series of seamount chains intersecting the GSC West of the 91W TF. The systematic transition from an axial-valley and rift morphology at axial depths of 2450 m.b.s.l. in the 86W area to an axial-high morphology at progressively shallower depths of 1500 m.b.s.l in the 90.5W area together with gradational changes in major and trace element chemistry of the axial lavas is believed to reflect increasing mantle temperature and compositional changes related to the Galapagos plume [1]. Previous work from 83W to 105W revealed a broad symmetric gradational pattern at around 91W and lead to the conclusion that this region is the main point of plume influx on the GSC [2]. Based on published isotope data Christie et al. (2005) inferred the probable existence of two enrichment peaks immediately East and West of the 91TF. Indeed, along axis variations of our new Sr-Nd-Pb-Hf isotope data map two distinct peaks of enrichment at 92W and 90.5W, suggesting that mantle from the Galapagos hotspot enters the GSC melting zone on both sides of the 91W transform fault. Lavas from within the 91W TF are isotopically intermediate and show a distinct depletion in incompatible trace elements, which may reflect repeated melting at shallow depth of passively upwelling mantle in this unique extensional regime. At least three different components are required to generate the observed isotope correlations. When compared to the geographically distinct isotopic domains of the Galapagos islands; GSC lavas East of the 91W TF form tight correlations from the rim of the Central Galapagos domain through the Eastern Galapagos domain towards DMM. Most axial lavas and corresponding off axis seamounts West of the 91W TF have higher 87Sr/86Sr, 207Pb/204Pb, 208Pb/204Pb

  14. Stable Isotopic Constraints on the Geographic Sources of Marijuana in Alaska

    NASA Astrophysics Data System (ADS)

    Booth, A. L.; Wooller, M. J.; Haubenstock, N. A.; Howe, T. A.

    2007-12-01

    Marijuana in Alaska can have numerous sources. Confiscated plants are known to originate either from within the state (e.g., Fairbanks and the Matanuska-Susitna Valley) or from numerous areas outside the state (e.g., Latin America, Canada and the contiguous United States). Latin America reportedly supplies a large percentage of the marijuana currently distributed in the lower 48 states of the U.S.A. However, in more remote areas of the country such as Fairbanks, Alaska, the supply proportions from different geographic areas are not well known. This is due to an insufficient ability to trace source regions from which confiscated marijuana was originally grown. As such, we have analyzed multiple stable isotopes (C, N, O and H) preserved in marijuana samples to identify the likely geographic source from which the marijuana originated (Drug Enforcement Agency license # RW0324551). These samples were confiscated in Fairbanks, Alaska and supplied to us by the University of Alaska Fairbanks (UAF) Police Department. Among 36 marijuana plant samples, we found an unexpectedly large range in the stable carbon isotope compositions (‰13C = -62.2‰ to -24.4‰), with twelve of the 36 samples exhibiting exceedingly low δ13C (-36.1‰ to -62.2‰) relative to typical δ13C of other C3 plants. Interior growing conditions (e.g., hydroponics and/or greenhouses) and a variety of CO2 sources (e.g., CO2 from tanks and fermentation CO2 generators) frequently supplied to growing marijuana to improve yields may account for these exceptionally low δ13C values. Stable oxygen and hydrogen isotope compositions (δ18O and δD vs. V-SMOW) of the marijuana samples were found to range from 10.0‰ to 27.6‰ and -197.1‰ to -134.9‰ respectively. The large range of values suggests that the samples originated from multiple sources ranging from low to high latitudes. δ15N of the marijuana samples also exhibited a large range (-7.0‰ to 14.8‰). This project has implications for the

  15. Isotopic and geophysical constraints on the structure andevolution of the Clear Lake volcanic system

    SciTech Connect

    Hammersley, L.; DePaolo, D.

    2005-03-09

    New Sr and Nd isotopic data are combined with availableinformation on the composition and petrology of lavas and the thermal andseismic structure of the underlying crust to develop a detailed model forthe deep structure and magmatic processes of the Clear Lake volcanicsystem in northern California. The isotopic data require a two-stagemodel for magmatic evolution. In stage I, basaltic magma (eNd=+6 to +8;87Sr/86Sr=0.703 to 0.7035; SiO2V50 percent) is fed from the mantle intothe lower and middle crust and evolves through combined crustalassimilation and fractional crystallization to basaltic andesite (eNd=+5to +0.4; 87Sr/86Sr=0.70328 to 0.70485; SiO2655 percent to 57 percent). Instage II, the basaltic andesite magmas are transported upward and areeither erupted at the surface or stored in shallow magma chambers wherethey evolve by fractional crystallization to form dacitic and rhyoliticmagmas (SiO2665 percent to 70 percent). High-silica rhyolites (SiO2675percent; high 87Sr/86Sr) show evidence that further crustal assimilationcan occur where upper crustal temperatures are elevated. Calculateddensities of Clear Lake lavas indicate that basalt should pond at a depthof 12-18 km where seismic data show a pronounced density boundary withinthe crust. Thermodynamic models of assimilation require that mid-crustaltemperatures are at least 600-800 8C to allow for enough assimilation toexplain the isotopic data. Both surface heat flow and thermobarometry ofcrustal xenoliths in andesites are consistent with these inferred hightemperatures. The Clear Lake volcanic system provides an opportunity tocross-calibrate petrological, geochemical and geophysical approaches. Theresults confirm that magma supply, magma buoyancy, and crustaltemperatures control magmatic evolution. A temporal trend of increasingeNd over the past 2 million years suggests that magma supply in the ClearLake volcanic field has been increasing and is still high. This isconsistent with high heat flow in the area

  16. Mixing the mantle marble-cake: timescale constraints from Os isotopes

    NASA Astrophysics Data System (ADS)

    Parman, S.; Pearson, G.; Nowell, G.; van Hunen, J.

    2007-12-01

    In their seminal paper, Allegre and Turcotte (1986) presented a model in which the upper mantle is a mixture of depleted, harzburgitic mantle and subducted basalt that has been mechanically mixed together, the mantle marble-cake. Since their publication, most studies of mantle heterogeneity have focused on the enriched components, which are equated with subducted basalt and/or sediments, and successfully explain OIB Sr-Nd-Pb isotopic systematics. In this talk, we will focus on a different part of the marble-cake, depleted (harzburgitic) heterogeneities. Though abundant in abyssal peridotites and ophiolites, these have been difficult to study geochemically because they have very low concentrations of typical trace elements and radiogenic isotopes, and are overprinted by any mixing with enriched mantle or melts. However, Os is compatible during mantle melting, is enriched in depleted mantle and thus is robust with respect to mixing with enriched components or metasomatism. Somewhat surprisingly, Os isotope studies of the convecting mantle show clear evidence for depleted heterogeities up to 2 billion years old, but the relative paucity of data (less than 100 analyses), makes it difficult to extract meaningful mixing information. Rapid analysis of osmiridium grains by laser-ablation inductively coupled multi-collector plasma mass spectrometry now allows large Os datasets to be acquired (100s of datapoints), which are suitable for statistical analyses (Meibom, 2002). Here we present new and published laser-ablation analyses of osmiridiums from a global collection. The data generally show an exponential decrease in heterogeneities with age, such that over 90% of heterogeneities are destroyed within 2 billion years, though rare heterogeneities as old as 2.7 Ga survive. The exponential decrease in survivorship is generally consistent with the mechanical mixing model of Allegre & Turcotte (1986). Subsequent 2-dimensional mixing models suggest that high-viscosity blobs can

  17. Lead isotope constraints on the origin of andesite and dacite magmas at Tungurahua volcano (Ecuador)

    NASA Astrophysics Data System (ADS)

    Nauret, Francois; Ancellin, Marie-Anne; Vlastelic, Ivan; Tournigand, Pierre-Yves; Samaniego, Pablo; Le Pennec, Jean Luc; Gannoun, Mouhcine; Hidalgo, Silvana; Schiano, Pierre

    2016-04-01

    Understanding the occurrence of large explosive eruptions involving silica-rich magmas at mostly andesitic volcanoes is crucial for volcanic hazard assessment Here we focus on the well-known active Tungurahua volcano (Ecuador), specifically its eruptive sequence for the last 3000 years BP, which are characterized by VEI 3 explosive events involving mostly homogeneous andesitic compositions (56-59 wt.% SiO2). However, some large eruptions (VEI ≥ 4) involving andesitic and dacitic magmas (up to 66 wt.% SiO2) also occur at 3000 BP, 1250 BP and 1886 AD. An additional outburst of siliceous magmas occurred during the last eruptive eruption of this volcano in 2006 [1]. Volcanic products at Tungurahua are described as been generated by a binary mixing between a silica-rich and a silica-poor end-member, but the origin of these components was not discussed [2]. Major, trace elements and Sr-Nd-Pb isotopes were used to investigate the genesis of the andesites and dacites. Andesites are heterogeneous in terms of Pb isotopes (206Pb/204Pb: 18.189-19.154, 207Pb/204Pb:15.658-15.696, 208Pb/204Pb: 38.752-38.918, 207Pb/206Pb: 0.8240-0.8275) but homogeneous in terms of major-trace element. Dacite are characterized by homogenous and low 207Pb/206Pb (0.8235±0.0001), very low Nb/U (1.97 to 4.49) and Ce/Pb (2.52-2.99) and high Th/La ratios (0.24 to 0.49). Triangular distribution of data in major element or trace element ratio vs. Pb isotopes plots suggests that at least three components control geochemical variability at Tungurahua. We interpret andesite compositions as reflecting mainly a deep mixture of two mantle components, with small addition of crustal material. We suggest that dacite results from a mixing between various andesite compositions and a larger amount of a contaminant derived from the volcanic basement of the Tungurahua made of late Cretaceous to Palaeogene oceanic plateau basalts and volcano-sedimentary rocks volcanic. Since andesite and dacite occur during the same

  18. Chlorine Isotope Constraints on the Origin and Distribution of Earth's Chlorine

    NASA Astrophysics Data System (ADS)

    Bonifacie, M.; Agrinier, P.; Jendrzejewski, N.; Coleman, M.; Javoy, M.

    2006-12-01

    Volatile elements exert a strong influence over the chemical and physical properties of the Earth's mantle. Due to its incompatible, soluble and volatile element chemical characteristics, chlorine is especially valuable in understanding the current and past evolution of the Earth (e.g., melting, recycling, degassing, differentiation). Comprehensive understanding of the exchanges of chlorine among Earth's reservoirs may help constrain the origin and the budget of chlorine and other volatile elements on Earth. Due to the large uncertainties in the estimated range of mantle Cl flux inputs and outputs, we present here the chlorine isotopic compositions (δ37Cl) of mantle and subducted materials as well as those in chondrites with the aim of better understanding the global Cl cycle. Mantle, subduction and chondrites δ37Cl. Based on fresh N- and E- MORB samples affected by various degrees of assimilation of seawater-derived materials (e.g., intergranular brines; Bonifacie et al., Chem Geol, 2005), we estimate that the δ37Cl value of the mean upper mantle is inferior or equal to - 1.9‰. Analyses on HP metaperidotites from the Alps suggest that no Cl-isotopes fractionation occurs during the Cl loss associated with the dehydration of serpentines throughout prograde subduction. Considering HP metaperidotites as suitable candidates for Cl transfer to the mantle, and excluding the possible contribution of sediments, we estimate that the subducted material has δ37Cl values superior or equal to -1.4‰. Various types of chondrites show relatively homogeneous δ37Cl values (~ - 1.7\\mp‰). The global chlorine cycle and implications on the origin of Earth's Cl. The slight but significant difference between the Cl-isotopic signature of recycled Cl and upper mantle implies that the δ37Cl value of the mantle increased while that of exogenous reservoirs decreased over geological time. Box modeling predicts for early Earth: i/ a large amount of Cl in exogenous reservoirs, and ii

  19. Contrasting zircon Hf-O isotopes and trace elements between ore-bearing and ore-barren adakitic rocks in central-eastern China: Implications for genetic relation to Cu-Au mineralization

    NASA Astrophysics Data System (ADS)

    Wang, Fangyue; Liu, Sheng-Ao; Li, Shuguang; He, Yongsheng

    2013-01-01

    The petrogenesis of Early Cretaceous adakitic intrusions in the Lower Yangtze River belt (LYRB), central-eastern China, and their genetic association with Cu-Au mineralization have recently been debated. This study presented integrated in-situ zircon U-Pb-Hf-O isotopic and trace elemental data for the LYRB adakites, and a comparison with ore-barren adakites from the south Tan-Lu fault (STLF) adjacent to the LYRB. Magmatic zircons from these two series of intrusions have U-Pb ages of 145-132 Ma and 136-132 Ma respectively. The STLF zircons have δ18O ranging from 5.6 to 6.7‰ and ɛHf(t) from - 28.8 to - 16.4, plotted within the range of global lower crustal metabasaltic xenoliths, consistent with low-radiogenic Pb of the host adakitic rocks. In contrast, both Hf and O isotopic compositions of zircons from the LYRB are greatly variable with heavier δ18O (4.7 to 9.6‰) and higher ɛHf(t) values (- 25.5 to + 2.0) compared with the STLF series. The co-variations of Hf-O isotopes in the LYRB series reflect source heterogeneity as a result of mixing of basaltic oceanic crust with sediments (10-20%), consistent with high-radiogenic Pb and enriched Sr-Nd isotopic compositions of the host adakites. The high La, U and low Ti concentrations in the LYRB zircons also imply a volatile (perhaps, CO32 --rich, carbonatite-like) source. Combined with whole-rock geochemical data, the new results further suggest contrasting origins of the LYRB and STLF adakites from subducted oceanic crust and foundering lower continental crust, respectively. The LYRB zircons have much higher ratios of Ce4 +/Ce3 + (avg.417) and Eu/Eu* (avg. 0.67) than the STLF zircons (avg. 84 and 0.44). This difference confirms that the ore-bearing adakitic magmas are more oxidized relative to the ore-barren ones. There is roughly a positive correlation between zircon Ce4 +/Ce3 + and δ18O in the LYRB series, probably indicating that the elevated fO2 was related to components enriched in heavy oxygen isotopes. A

  20. Groundwater processes and landscape evolution in Saharan Africa: Remote sensing, isotopic and geophysical constraints

    NASA Astrophysics Data System (ADS)

    Farag, A. Z.; Sultan, M.; El Kadiri, R.; Mohamed, L.

    2013-12-01

    Paleoclimatic regimes of the North African Sahara Desert alternated between dry and wet periods throughout the Pleistocene Epoch and it is during these wet periods that the fossil aquifers in North Africa were recharged. The largest of these aquifer systems is the Nubian Sandstone Aquifer System (NSAS; area: 2.2 million km2) in Egypt, Libya, Sudan and Chad and the North Western Sahara Aquifer (NWSA; area: 1 million km2) in Algeria, Tunisia and Libya. These aquifers have similar stratigraphic and hydrogeologic settings: (1) the main aquifer is composed largely of older clastic sediments (NAS: Nubian Sandstone; CI: Continental Intercalaire Aquifer) that is overlain by non-clastic carbonates with intercalations of clays and marls ( PNAS: Post Nubian Aquifer System, CT: Complexe Terminal) (2) unconfined conditions in the south that give way to confined conditions in the north, and (3) during wet periods, the NAS and the CI were recharged, groundwater levels rose, and groundwater flowed from the south to the north. In this study we present evidences (remote sensing, field, geophysical, isotopic) to support the hypothesis that in wet periods: (1) groundwater under high hydrostatic pressures access deep seated deep structures and discharge at the near surface causing sapping features and in the overlying carbonate sequences causing karstic features, and (2) many of the present topographic features including natural depressions across the NSAS and the NWSA were largely controlled by the groundwater system processes in previous wet climatic features. Evidences include: (1) Stubby-looking channels with U- shaped valley floors and theater-like valley heads indicative of sapping processes were mapped (using high spatial resolution IKONOS images, ASTER Digital Elevation Model (DEM), slope, hill shade and Landsat mosaics) along scarps in Egypt and Libya (scarp length: 2190 km) and in Algeria (scarp length: 400 km), (2) many of the mapped channel networks (length up to 50 km

  1. Isotopic and Climate Model Constraints on Paleo-CO2 in the Late Paleozoic

    NASA Astrophysics Data System (ADS)

    Grossman, E. L.; Hyde, W. T.; Pollard, D.; Scotese, C. R.

    2003-12-01

    Atmospheric CO2 is one of the most important drivers controlling ancient climate and one of the hardest to quantify. We have combined three methods for quantifying paleoclimate, a coupled energy balance-ice sheet model (EB/ISM), an atmospheric general circulation model (AGCM), and oxygen isotope analyses of fossils, to constrain late Paleozoic pCO2 levels. Our estimated pCO2 is that which yields the same ice volume determined using two independent approaches, a δ 18O-AGCM method and an EB/ISM. We calculate ice volume from the δ 18O of brachiopod shells and AGCM temperatures (δ 18O-AGCM method). Brachiopod shell δ 18O values depend on two variables, ambient temperature and seawater δ 18O. Using the oxygen isotope paleotemperature equation and ambient temperatures derived from AGCM results, we calculate seawater δ 18O. From this seawater δ 18O we use 18O mass balance to calculate ice volume. We run the AGCM with various values of pCO2, which produce different temperatures and different δ 18O-derived ice volumes. Ice volumes deduced from brachiopod δ 18O increase with pCO2. Ice volumes as a function of pCO2 are also determined from the ice sheet model in the EB/ISM, and those ice volumes decrease with increasing pCO2. Our estimated pCO2 is the intersection of the two ice volume-pCO2 curves. Three different time slices and paleogeographies have been investigated in detail: 360, 320, and 280 Ma. GENESIS 2 AGCM simulations were performed at 1x and 4x modern preindustrial levels (280 ppm) for all time slices, and at 8x pCO2 for 360 Ma. EB/ISM simulations were run with and without topography, with lapse rates of 5 and 7 ° C/km, and with outgoing infrared radiation (OIR) ranging from 187.3 to 205.3 W/m2, equivalent to pCO2 levels of 1x to 16x. EB/ISM simulations yielded ice volumes ranging from 0 to greater than 129 x 106 km3, depending on lapse rate, topography, and outgoing IR radiation. The highest ice volumes were obtained with topography, 7 ° C/km lapse rate

  2. Stable Carbon Isotope Constraints on the Timing and Magnitude of Phytoplankton Blooms in San Francisco Bay

    NASA Astrophysics Data System (ADS)

    Goodwin, D.; Roopnarine, P. D.

    2010-12-01

    Recent work on phytoplankton dynamics in San Francisco Bay (SFB) revealed new seasonal blooms. Historic observations (1978-1998) of chlorophyll a (Chl a) showed an annual pattern of short-lived spring blooms. In 1999, this pattern changed with the appearance of autumnal blooms in addition to the characteristic large vernal blooms. This change was attributed to decreases in bivalve mollusk populations concurrent with increases in macro invertebrate and vertebrate mollusk predators. Previous work, however, suggests that inter-annual variation in phytoplankton biomass is a function of river discharge. These observations suggest phytoplankton abundances in SFB reflect multiple forcing mechanisms and underscore the importance of understanding prehistoric variations in bloom dynamics. Here, we present stable isotope data from the exotic oyster Crassostrea gigas, which record the timing and magnitude of past phytoplankton blooms. These data may be useful for identifying patterns of phytoplankton bloom dynamics prior to instrumental observations. Stable oxygen (δ18O) and carbon (δ13C) isotope profiles from recent live-collected (2006) specimens of the oyster C. gigas collected in southern SFB were analyzed in conjunction with in situ records of environmental variability (water temperature and δ18Owater calculated from salinity). Their observed δ18Ocarb profiles are characterized by several unique features that correlate with predicted δ18Ocarb values calculated from water temperature and δ18Owater measurements indicating that these oysters were recruited at the end of 2001 or early in 2002. A prominent 1-2 ‰ spike characterizes the carbon isotope profile from each of these specimens. These positive excursions most likely reflect a large phytoplankton bloom, during which algae preferentially assimilated 12C, resulting in the enrichment of 13C in the dissolved inorganic carbon (DIC) of seawater. Furthermore, phytoplankton blooms may appear as positive spikes in a

  3. Oxygen isotopic constraints on the origin of Mg-rich olivines from chondritic meteorites

    NASA Astrophysics Data System (ADS)

    Libourel, Guy; Chaussidon, Marc

    2011-01-01

    Chondrules are the major high temperature components of chondritic meteorites which accreted a few millions years after the oldest solids of the solar system, the calcium-aluminum-rich inclusions, were condensed from the nebula gas. Chondrules formed during brief heating events by incomplete melting of solid dust precursors in the protoplanetary disk. Petrographic, compositional and isotopic arguments allowed the identification of metal-bearing Mg-rich olivine aggregates among the precursors of magnesian type I chondrules. Two very different settings can be considered for the formation of these Mg-rich olivines: either a nebular setting corresponding mostly to condensation-evaporation processes in the nebular gas or a planetary setting corresponding mostly to differentiation processes in a planetesimal. An ion microprobe survey of Mg-rich olivines of a set of type I chondrules and isolated olivines from unequilibrated ordinary chondrites and carbonaceous chondrites revealed the existence of several modes in the distribution of the ∆17O values and the presence of a large range of mass fractionation (several ‰) within each mode. The chemistry and the oxygen isotopic compositions indicate that Mg-rich olivines are unlikely to be of nebular origin (i.e., solar nebula condensates) but are more likely debris of broken differentiated planetesimals (each of them being characterized by a given ∆17O). Mg-rich olivines could have crystallized from magma ocean-like environments on partially molten planetesimals undergoing metal-silicate differentiation processes. Considering the very old age of chondrules, Mg-rich olivine grains or aggregates might be considered as millimeter-sized fragments from disrupted first-generation differentiated planetesimals. Finally, the finding of only a small number of discrete ∆17O modes for Mg-rich olivines grains or aggregates in a given chondrite suggests that these shattered fragments have not been efficiently mixed in the disk and

  4. Isotopic constraints on marine and terrestrial N2O emissions during the last deglaciation.

    PubMed

    Schilt, Adrian; Brook, Edward J; Bauska, Thomas K; Baggenstos, Daniel; Fischer, Hubertus; Joos, Fortunat; Petrenko, Vasilii V; Schaefer, Hinrich; Schmitt, Jochen; Severinghaus, Jeffrey P; Spahni, Renato; Stocker, Thomas F

    2014-12-11

    Nitrous oxide (N2O) is an important greenhouse gas and ozone-depleting substance that has anthropogenic as well as natural marine and terrestrial sources. The tropospheric N2O concentrations have varied substantially in the past in concert with changing climate on glacial-interglacial and millennial timescales. It is not well understood, however, how N2O emissions from marine and terrestrial sources change in response to varying environmental conditions. The distinct isotopic compositions of marine and terrestrial N2O sources can help disentangle the relative changes in marine and terrestrial N2O emissions during past climate variations. Here we present N2O concentration and isotopic data for the last deglaciation, from 16,000 to 10,000 years before present, retrieved from air bubbles trapped in polar ice at Taylor Glacier, Antarctica. With the help of our data and a box model of the N2O cycle, we find a 30 per cent increase in total N2O emissions from the late glacial to the interglacial, with terrestrial and marine emissions contributing equally to the overall increase and generally evolving in parallel over the last deglaciation, even though there is no a priori connection between the drivers of the two sources. However, we find that terrestrial emissions dominated on centennial timescales, consistent with a state-of-the-art dynamic global vegetation and land surface process model that suggests that during the last deglaciation emission changes were strongly influenced by temperature and precipitation patterns over land surfaces. The results improve our understanding of the drivers of natural N2O emissions and are consistent with the idea that natural N2O emissions will probably increase in response to anthropogenic warming. PMID:25503236

  5. Age and isotopic constraints on pleistocene pluvial episodes in the Western Desert, Egypt.

    SciTech Connect

    Crombie, M. K.; Arvidson, R. E.; Sturchio, N. C.; El Alfy, Z.; Abu Zeid, K.; Environmental Research; Washington Univ.; Egyptian Geological Survey and Mining Authority

    1997-01-01

    North Africa has undergone drastic climatic changes over the past several hundred thousand years. The timing of humid intervals called pluvials was investigated by uranium-series disequilibrium dating of travertines from the Kurkur Oasis, Western Desert, Egypt. The youngest and best dated travertines (70-160 ka) are found in Wadi Kurkur and include spring and lacustrine units exposed as 2 to 3 m high terraces. Travertines having an age of approximately 191-220 ka are exposed by differential erosion as linear mounds produced by spring systems over fracture zones in ancient wadis. The oldest travertines, having ages >260 ka, are extensive, cap limestone units above the oasis, and were deposited in paludal and lacustrine environments. Oxygen isotope ratios were measured for the wadi travertines ({delta}{sup 18}O values ranging from 16.7 to 19.1{per_thousand} SMOW) and for spring mound travertines (18.5-20.5{per_thousand}). Equilibrium oxygen isotope fractionation calculations indicate that the Kurkur travertines were deposited from waters having {delta}{sup 18}O values similar to ancient Western Desert groundwaters ({approx} -11{per_thousand}). The ages of the travertines correspond to times of monsoonal maxima, eustatic sea level high stands and interglacial maxima. Rainfall producing these groundwaters (and travertines) was significantly fractionated during atmospheric transport, in contrast to modern meteoric waters (-2.09{per_thousand}), implying a distant source for the pluvial waters. Increased precipitation, recharge of Western Desert groundwaters, and resultant travertine deposition are interpreted to be consequences of Milankovitch insolation cycle forcing, through enhanced Atlantic and Indian Ocean monsoons during interglacial time periods.

  6. Recycling of Oceanic Lithosphere: Water, fO2 and Fe-isotope Constraints

    NASA Technical Reports Server (NTRS)

    Bizmis, M.; Peslier, A. H.; McCammon, C. A.; Keshav, S.; Williams, H. M.

    2014-01-01

    Spinel peridotite and garnet pyroxenite xenoliths from Hawaii provide important clues about the composition of the oceanic lithosphere, and can be used to assess its contribution to mantle heterogeneity upon recycling. The peridotites have lower bulk H2O (approximately 70-114 ppm) than the MORB source, qualitatively consistent with melt depletion. The garnet pyroxenites (high pressure cumulates) have higher H2O (200-460 ppm, up to 550 ppm accounting for phlogopite) and low H2O/Ce ratios (less than 100). The peridotites have relatively light Fe-isotopes (delta Fe -57 = -0.34 to 0.13) that decrease with increasing depletion, while the pyroxenites are significantly heavier (delta Fe-57 up to 0.3). The observed xenolith, as well as MORB and OIB total Fe-isotope variability is larger that can be explained by existing melting models. The high H2O and low H2O/Ce ratios of pyroxenites are similar to estimates of EM-type OIB sources, while their heavy delta Fe-57 are similar to some Society and Cook-Austral basalts. Therefore, recycling of mineralogically enriched oceanic lithosphere (i.e. pyroxenites) may contribute to OIB sources and mantle heterogeneity. The Fe(3+)/Sigma? systematics of these xenoliths also suggest that there might be lateral redox gradients within the lithosphere, between juxtaposed oxidized spinel peridotites (deltaFMQ = -0.7 to 1.6, at 15 kb) and more reduced pyroxenites (deltaFMQ = -2 to -0.4, at 20-25kb). Such mineralogically and compositionally imposed fO2 gradients may generate local redox melting due to changes in fluid speciation (e.g. reduced fluids from pyroxenite encountering more oxidized peridotite). Formation of such incipient, small degree melts could further contribute to metasomatic features seen in peridotites, mantle heterogeneity, as well as the low velocity and high electrical conductivity structures near the base of the lithosphere and upper mantle.

  7. Constraints on the timing of the Moon-forming giant impact from MORB Xe isotopes

    NASA Astrophysics Data System (ADS)

    Parai, R.; Mukhopadhyay, S.

    2014-12-01

    As Earth accreted, volatiles were delivered by accreting material and lost by degassing and impact-driven ejection to space. The Moon-forming giant impact initiated the final catastrophic outgassing and bulk volatile ejection event on the early Earth. I-Pu-U-Xe systematics provide a powerful tool to probe degassing of the early Earth. Radiogenic 129Xe was produced by β-decay of the extinct nuclide 129I (t1/2 = 15.7 Ma) in the first ~90 Myr of Earth history. Fissiogenic 131Xe, 132Xe, 134Xe, 136Xe were produced in distinct, characteristic proportions by the fission of extinct short-lived 244Pu (t1/2 = 80.0 Myr) and extant long-lived 238U (t1/2 = 4.468 Gyr). Here we present radiogenic and fission Xe data in basalts from the Southwest Indian Ridge, and discuss them with other mantle-derived samples to shed light on early Earth volatile accretion and loss. Based on the ratio of radiogenic 129Xe to plutogenic 136Xe determined for the MORB source, we calculate an I-Pu-Xe closure age for the upper mantle of ~44-70 Myr after the start of the Solar System. The closure age should correspond to the end of catastrophic mantle outgassing during accretion, and thus constrains the age of the last giant impact (LGI). Our closure age is significantly older than previous Xe closure age determinations of ~100 Myr, and is also older than some direct radiometric ages of lunar crustal samples. In order to explore the effects of accretion timescales, partial early retention of Xe, and degassing associated with long-term mantle processing on Xe closure age, we develop a new model of I-Pu-U-Xe systematics. We find that for LGI's between ~35 and 70 Myr after the start of the Solar System, we are able to satisfy constraints on I-Pu-U-Xe systematics simultaneously without invoking partial retention of Xe prior to the last giant impact. For LGI's after ~80 Myr, partial retention of Xe prior to the LGI is required. Non-zero early retention of Xe is necessary to explain the budgets of primordial

  8. LU-HF Age of Martian Meteorite Larkman Nunatek 06319

    NASA Technical Reports Server (NTRS)

    Shafer, J. T.; Brandon, A. D.; Lapen, T. J.; Righter, M.; Beard, B.; Peslier, A. H.

    2009-01-01

    Lu-Hf isotopic data were collected on mineral separates and bulk rock powders of LAR 06319, yielding an age of 197+/- 29 Ma. Sm-Nd isotopic data and in-situ LA-ICP-MS data from a thin section of LAR 06319 are currently being collected and will be presented at the 2009 LPSC. These new data for LAR 06319 extend the existing data set for the enriched shergottite group. Martian meteorites represent the only opportunity for ground truth investigation of the geochemistry of Mars [1]. At present, approximately 80 meteorites have been classified as Martian based on young ages and distinctive isotopic signatures [2]. LAR 06319 is a newly discovered (as part of the 2006 ANSMET field season) martian meteorite that represents an important opportunity to further our understanding of the geochemical and petrological constraints on the origin of Martian magmas. Martian meteorites are traditionally categorized into the shergottite, nakhlite, and chassignite groups. The shergottites are further classified into three distinct isotopic groups designated depleted, intermediate, and enriched [3,4] based on the isotope systematics and compositions of their source(s).

  9. Paleoproterozoic magmatism across the Archean-Proterozoic boundary in central Fennoscandia: Geochronology, geochemistry and isotopic data (Sm-Nd, Lu-Hf, O)

    NASA Astrophysics Data System (ADS)

    Lahtinen, Raimo; Huhma, Hannu; Lahaye, Yann; Lode, Stefanie; Heinonen, Suvi; Sayab, Mohammad; Whitehouse, Martin J.

    2016-10-01

    The central Fennoscandia is characterized by the Archean-Proterozoic (AP) boundary and the Central Finland Granitoid Complex (CFGC), a roundish area of approximately 40,000 km2 surrounded by supracrustal belts. Deep seismic reflection profile FIRE 3A runs across these units, and we have re-interpreted the profile and crustal evolution along the profile using 1.92-1.85 Ga plutonic rocks as lithospheric probes. The surface part of the profile has been divided into five subareas: Archean continent (AC) in the east, AP, CFGC, boundary zone (BZ) and the Bothnian Belt (BB) in the west. There are 12 key samples from which zircons were studied for inclusions and analyzed (core-rim) by ion probe for U-Pb dating and oxygen isotopes, followed by analyzes for Lu-Hf by LA-MC-ICP-MS. The AC plutonic rocks (1.87-1.85 Ga) form a bimodal suite, where the proposed mantle source for the mafic rocks is 2.1-2.0 Ga metasomatized lower part of the Archean subcontinental lithospheric mantle (SCLM) and the source for the felsic melts is related plume-derived underplated mafic material in the lower crust. Variable degrees of contamination of the Archean lower crust have produced "subduction-like" Nb-Ta anomalies in spidergrams and negative εNd (T) values in the mafic-intermediate rocks. The felsic AC granitoids originate from a low degree melting of eclogitic or garnet-bearing amphibolites with titanite ± rutile partly prevailing in the residue (Nb-Ta fractionation) followed by variable degree of assimilation/melting of the Archean lower crust. The AP plutonic rocks (ca. 1.88 Ga) can be divided into I-type and A-type granitoids (AP/A), where the latter follow the sediment assimilation trend in ASI diagram, have high δ18O values (up to 8‰) in zircons and exhibit negative Ba anomalies (Rb-Ba-Th in spidergram), as found in sedimentary rocks. A mixing/assimilation of enriched mantle-derived melts with melts from already migmatized sedimentary rocks ± amphibolites is proposed. The CFGC is

  10. Late Eocene-Miocene tectono-magmatic response to the Indian- Eurasian plate collision: constraints from structural analysis, and Sr-Nd and Hf geochemistry of leucocratic intrusions along the Ailao Shan Red-River shear zone, SE Tibet

    NASA Astrophysics Data System (ADS)

    Liu, J.; Tang, Y.; Cao, S.; Ngyuen, Q.; Song, Z.; Tran, M.; Chen, Y.; Ji, M.; Zhang, Z.; Zhao, Z.

    2010-12-01

    The over 1000 km Ailao Shan-Red River (ASRR) shear zone is one of the most important geological discontinuities in Southeast Asia. Great controversies remain on the nature of the shear zone and its role in shaping the tectonic framework of Southeast Asia. Our observation reveals the existence of the Paleogene high potassic alkaline rocks and calc-alkaline intrusions (>30Ma) and the late Oligocene to early Miocene calc-alkaline granitic rocks (28-21Ma). The former are concordant dykes and are generally strongly sheared into mylonitic rocks. The latter are either concordant and show weak strain fabric, or discordant and show no strain fabric. Meanwhile, they have distinct REE, Sr-Nd, Hf isotope signatures and are different in mineralizing features. The Paleogene intrusions are characterized by enriched LREE and depleted HREE without any Eu anomalies (whole rock). Whole rock Sr-Nd (87Sr/86Sr(i): 0.7069 to 0.7098; ɛNd(t): -7.98 to -3.31) and in situ Zircon Hf isotope (-0.79 to +6.2) analyses yield a binary mixing trend between the mantle- and supracrustal-derived melts for the Paleogene magma. Here our new data suggest that most of the Paleogene magmatic rocks are either sheared high potassium alkaline rocks or deformed calc-alkaline intrusions. They are identical to and are the deformed counterparts of rocks from the two Paleogene mineralizing magmatic provinces on both sides of the ASRR shear zone, i.e. the Jinping-Fan Si Pan province and the Dali-Beiya province. These two types of leucocratic rocks are formed as the result of post-collisional delamination of a thickened crust, and deformed and offset by the left lateral shearing along the ASSR shear zone. The late Oligo-Miocene calc-alkaline granitic rocks are localized within the ASRR shear zone. They are in overall concordant to the mylonitic foliation in the shear zone and preserve microstructures typical of syn- to late kinematic emplacement. They have negative Eu anomalies, variable but mostly higher Sr ratios

  11. Isotopic and chemical constraints on the petrogenesis of Blackburn Hills volcanic field, western Alaska

    USGS Publications Warehouse

    Moll-Stalcup, E. J.; Arth, Joseph G.

    1991-01-01

    The Blackburn Hills volcanic field is one of several Late Cretaceous and early Tertiary (75-50 Ma) volcanic fields in western Alaska that comprise a vast magmatic province extending from the Arctic Circle to Bristol Bay. It consists of andesite flows, rhyolite domes, a central granodiorite to quartz monzonite pluton, and small intrusive rhyolite porphyries, overlain by basalt and alkali-rhyolites. Most of the field consists of andesite flows which can be divided into two groups on the basis of elemental and isotopic composition: a group having lower ( 87Sr 86Sr)i, higher ( 143Nd 144Nd)i, and moderate LREE and HREE contents (group 1), and a group having higher ( 87Sr 86Sr)i, lower ( 143Sr 144Sr)i, and lower HREE contents. Basalts are restricted to the top of the stratigraphic section, comprise the most primitive part of group 1 [( 87Sr 86Sr)i = 0.7033; ( 143Nd 144Nd)i = 0.5129], and have trace-element ratios that are similar to those of oceanic island basalts (OIBs). In contrast to the basalts, group 1 andesites have higher ( 87Sr 86Sr)i and lower ( 143Nd 144Nd)i, and represent interaction of mantle-derived magmas with the lower crust of Koyukuk terrane. Group 2 andesites have ( 87Sr 86Sr)i and ( 143Nd 144Nd)i that are near bulk-earth values and probably formed by partial melting of the lower crust of Koyukuk terrane. The central pluton and rhyolite porphyries are isotopically uniform ( 87Sr 86Sr)i ??? 0.704, ( 143Nd 144Nd)i ??? 0.51275, and are interpreted to have formed by melting of young mafic to intermediate crustal rocks or by fractionation of group 1 andesites. The rhyolite domes have an isotopic range similar to that of the basalts and andesites [( 87Sr 86Sr)i = 0.70355-0.70499; ( 143Nd 144Nd)i = 0.51263-0.51292], which suggests they formed by fractionation of the and site and basalt magmas. Although some workers have suggested that the volcanic field is underlain by old continental crust, none of the data require the presence of Paleozoic or Precambrian

  12. Geochemical and Isotopic Constraints on the Source of Groundwater to Lower Kane Cave, Wyoming

    NASA Astrophysics Data System (ADS)

    Edwards, M. C.; Bennett, P. C.; Engel, A. S.

    2003-12-01

    Most karst features occur due to the dissolution of limestone by carbonic acid charged phreatic or meteoric water. However, an important subset of caves forms when anaerobic groundwater transports hydrogen sulfide into an oxidizing environment, resulting in speleogenesis via sulfuric rather than carbonic acid. The actively forming Lower Kane Cave in the Mississippian Madison Limestone of the Bighorn Basin near Lovell, Wyoming, is an accessible example of this alternative method of cave development. Located along the fold axis of the Little Sheep Mountain anticline of the Bighorn Basin, this system hosts a diverse range of microbial organisms, including acid-producing and sulfide and sulfate utilizing species, whose role in speleogenesis is currently under investigation. Water samples were collected from cave springs, nearby springs, freshwater wells and produced water from oil wells in the local area. Samples were analyzed for major and trace elements, stable isotopes and Sr isotopes by multi-collector ICP-MS, as well as dissolved gas and organic acid analyses. These data were used to examine the regional flow of groundwater to the cave and potential oil-field sources of hydrogen sulfide. The Madison Aquifer in this area is characterized by relatively fresh water, and in the cave vicinity is the source of municipal water supplies for the towns of Cowley and Greybull. The Madison water samples collected in the area are Ca-HCO3 to Mg-SO4 type, with relatively little Na and Cl. Overall the cave water chemistries are characterized as Ca-Mg-HCO3-SO4 waters; Ca = 70 ppm, Mg = 25 ppm, HCO3 = 205 ppm and SO4 = 110 ppm. However, when compared to other Madison water samples, the waters of Lower Kane Cave are slightly higher in TDS (around 400 ppm), significantly warmer (22£ C versus between 6-12£ C), and contain much higher dissolved sulfide (up to 2ppm). Additionally, Sr isotope signatures for the cave waters are significantly more radiogenic than that of other Madison

  13. Potential leakage between aquifers in a deeply anthropized coastal sedimentary basin (Recife, Brazil): Strontium isotope constraints

    NASA Astrophysics Data System (ADS)

    Petelet-Giraud, Emmanuelle; Cary, Lise; Hirata, Ricardo; Martins, Veridiana; Bertrand, Guillaume; Montenegro, Suzana; Pauwels, Helene; Kloppmann, Wolfram; Aquilina, Luc

    2013-04-01

    Due to an increasing demographic pressure, the Metropolitan Region of Recife (RMR) went through remarkable changes of water and land uses over the last decades. These evolutions gave rise to numerous environmental consequences, such as a dramatic decline of the piezometric levels, groundwater salinization and contamination. This degradation of natural resources is linked to the increase of water demand, punctually amplified by drought periods which induced the construction of thousands of private wells. Recife was built on the estuarine area of the Capibaribe River and other small rivers. The Recife coastal plain is located in the geographic limits of the sedimentary basins of Cabo and Pernambuco-Paraíba which consist out of fluvial and marine geologic formations. It is composed of three main aquifers: the two semi-confined Cabo and Beberibe aquifers, both underlining the superficial Boa Viagem unconfined aquifer, which is the most directly exposed to contamination, since it is connected to mangroves, rivers, estuaries and highly urbanized areas. The Boa Viagem aquifer is made of marine terraces of sand, silt and clay has an average thickness of 40 m. The Cabo aquifer occurs in the south of Recife and comprises sandstones, siltstones and mudstones, with an average thickness of 90 m. The Beberibe aquifer occurs in the north and central area of Recife with an average thickness of 100 m of sandstones with intercalations of mudstone; it is the most important one, with the highest amount of good quality water. Both the Beberibe and Cabo aquifers contain large clay levels. The hydraulic connections between the three aquifers are not well known but isotopic studies have shown that the recharge processes are similar, suggesting that exchanges may occur and may be modified or amplified by overexploitation especially between the Cabo and Boa Viagem aquifers. Two other aquifers can be found west of the city: the Barreiras aquifer, characterized by alternating well stratified

  14. Pb-Pb Isotopic and X-ray Tomographic Constraints on the Origin of Chondrules

    NASA Astrophysics Data System (ADS)

    Charles, Christopher R. J.

    207Pb*/206Pb* chronometry was used to obtain the ages of Ca,Al-rich inclusions (CAIs) and chondrules found in ancient meteorites. Assuming a 238U/235U=137.88, Pb/Pb ages of chondrules in NWA801 (a CR2 meteorite) are 4564.6+/-1.0 Ma, chondrules in Mokoia (a CV3 chondrite) are 4564.2+/-1.1Ma, and CAIs in Mokoia are 4567.9+/-5.4 Ma. The Pb/Pb age of NWA801 chondrules is concordant with 26Al/ 26Mg ages of CR chondrules. However if a 238U/ 235U<137.88 is used, the age for NWA801 chondrules becomes younger by ˜1Ma and discordant with26Al/26Mg ages of CR chondrules. This suggests either a discrepancy with the U compositions or the initial Mg isotopic compositions of NWA801 chondrules. The shapes of NWA801 chondrules, and blebs of FeNi metal in the meteorite matrix, were further studied by 3D X-ray micro-computed tomography (CT). Most chondrules (92%) were 'armoured' with one discontinuous layer of FeNi metal. Two layers of FeNi metal (one on the exterior and one concentric through the interior separated by silicate) were rare <8%. Chondrules and matrix blebs occur as oblates, prolate, spheres and triaxial spheroids. It is proposed that the shapes were made free-floating in the nebula likely by ash-melting precursors into molten droplets that were vibrating as harmonic oscillators that 'froze-in' their shapes during cooling. Parent-body metamorphism and shock are not likely processes affecting the matrix-bleb and chondrule shapes. Chondrules with≥2 FeNi metal layers were likely formed by mergers and not by successive deposition and annealing of metal in multiple ash-melting events. Attempts to obtain 207Pb*/206Pb* ages from chondrules and CAIs by thermal extraction (TE)-TIMS were unsuccessful. However LA-ICP-MS was shown to be useful for rapidly determining Pb isotopic trends in meteorites and unknown objects. In particular, it was shown that 137La (T1/2=60ky) should be detectable in recently fallen meteorites using LaF-4 to suppress the 137Ba isobar during tandem

  15. Foraminiferal stable isotope constraints on salinity changes in the deglacial and early Holocene Baltic Sea region

    NASA Astrophysics Data System (ADS)

    Quintana Krupinski, Nadine; Filipsson, Helena; Bokhari-Friberg, Yasmin; Knudsen, Karen-Luise; Mackensen, Andreas; Groeneveld, Jeroen; Austin, William

    2015-04-01

    The northern European Baltic Sea shows evidence of strong coupling with North Atlantic climate over recent glacial-interglacial cycles, but existing climate proxy evidence from regional sediment records suggest that the coupling may occur through non-linear processes. High-resolution regional climate records in Europe and from the Baltic Sea are critical for evaluating this coupling and the regional sensitivity to North Atlantic and global climate signals. However, evaluating the drivers and mechanisms of proposed links between the North Atlantic and Baltic Sea climate has often been hampered by a lack of long, continuous, high-resolution climate records from this area. New high-resolution sediment cores collected by IODP/ECORD Expedition 347 (Baltic Sea Paleoenvironment) allow such records to be generated, including foraminiferal geochemistry records of Baltic Sea hydrographic conditions during the most recent deglaciation and early Holocene (~19-7 cal. ka). The dramatic changes in salinity, sea level, circulation, temperature, and oxygenation during this period, e.g. through massive meltwater release from proglacial lakes and the early Holocene inundation of the Baltic by seawater highlight these non-linear links between the Baltic and North Atlantic. This work uses benthic foraminiferal stable isotope records (δ18O and δ13C) from sites in the western Baltic (M0059, Lillebælt, early Holocene marine stage (Littorina Sea)) and Kattegat (M0060, Anholt, deglaciation) to constrain salinity changes during these intervals. Because of the dramatic changes in salinity this region experiences today and during the study periods, oxygen isotope records (δ18O) here primarily reflect a signal of changing salinity, with a reduced temperature effect. Early δ18O results from the western Baltic (M0059) show a trend of declining δ18O/salinity during the first several kyr of the Littorina Sea stage, in agreement with previous work indicating declining salinity due to gradual

  16. Constraints from sulfur isotopes on the origin of gypsum at concrete/claystone interfaces

    NASA Astrophysics Data System (ADS)

    Lerouge, Catherine; Claret, Francis; Tournassat, Christophe; Grangeon, Sylvain; Gaboreau, Stéphane; Boyer, Bernard; Borschnek, Daniel; Linard, Yannick

    Two in situ concrete/claystone interfaces were sampled at the laboratory level in the Andra Meuse/Haute Marne (France) Underground Research Laboratory (URL) in order to study five years of interactions between Callovian-Oxfordian (COx) claystone and two cementitious materials (concrete bottom slab and shotcrete on the walls of the main gallery), with a specific focus on sulfur. Combined mineralogical, chemical and sulfur isotopic investigations were carried out to define the degree of the perturbation of the sulfur system in the claystone and in both the cementitious materials. At both interfaces, results show that the main perturbation on the claystone side is the formation of scarce μm-sized gypsum, the sulfur content of which is essentially derived from pyrite oxidation. The distribution of gypsum is highly correlated with the fissure network of the damaged zone due to excavation of the gallery. Its presence is also often associated with a loss of cohesion of the concrete/claystone interface. Due to the small amounts of gypsum and its μm-size, measurements were performed by ion microprobe. Adaptations were needed on account of the reactivity of gypsum and sulfates in general under the beam. The use of ion microprobe analysis provided evidence of high local isotopic heterogeneity that could be attributed to kinetic fractionation effects. Some analyses suggest a minor contribution of dissolved sulfates in pore water of claystone and possibly of concrete. The perturbation on the concrete side is marked by a significant increase in the bulk sulfur content within three millimeters of the interface with the claystone, showing a sulfur gradient from claystone to concrete. The main objective of this work was to define the extent of the chemical and mineralogical perturbations, taking into account in situ URL conditions, i.e. hydrodynamic conditions (shotcrete sprayed on the gallery walls and subjected to ventilation of the galleries), damaged zone of claystone induced

  17. Lead Isotope Constraints on the Sources of Ore Metals in SW Mexican Deposits

    NASA Astrophysics Data System (ADS)

    Potra, A.; Macfarlane, A. W.

    2007-12-01

    Lead isotope ratios from mineral deposits in southern Mexico increase with distance from the trench from 206Pb/204Pb values between 18.597 and 18.650 in the coastal area to values between 18.712 and 19.069 approximately 800 km east from the trench. This variation has been attributed to increasing assimilation of radiogenic lead from the crust with increasing distance from the trench. New sampling was undertaken in this area to provide a clearer picture of the potential sources of ore metals in this arc system, and also, if possible, to examine whether ore metal sources differ among the proposed tectonostratigraphic exotic terranes of southern Mexico. New TIMS lead isotope analyses are presented for samples from the metamorphic basement rocks of the Guerrero Terrane, the Late Cretaceous clastic sedimentary rocks from the Upper Mesozoic Assemblage, and for mid-Cretaceous igneous rocks, as well as for samples from the Oligocene La Verde, Esmeralda, and El Malacate copper prospects. Whole rock samples of schist from the Jurassic-Cretaceous Arteaga Complex and phyllite and slate from the Tierra Caliente Complex contain radiogenic lead relative to bulk earth models, with 206Pb/204Pb ranging from 18.981-19.256. These values are substantially more radiogenic than published values of analyses of metagabbro and charnockite from the Grenvillian-age Oaxaca Terrane. Sedimentary rocks (sandstones, siltstones, and marls) belonging to the Huetamo Sequence have 206Pb/204Pb values ranging between 18.630 to 18.998, close to the published data for the sediments from IPOD-DSDP Sites 487 and 488, Cocos Plate. Whole rock analyses of igneous rocks (granodiorite) collected from La Verde and El Malacate have 206Pb/204Pb ranging from 18.764 to 18.989, clustering between the fields represented by the sedimentary and the metamorphic rocks, suggesting assimilation of lead from these components. Ore samples from La Verde and Esmeralda have 206Pb/204Pb between 18.685 and 18.731 and plot within

  18. U Pb and Lu Hf isotope record of detrital zircon grains from the Limpopo Belt Evidence for crustal recycling at the Hadean to early-Archean transition

    NASA Astrophysics Data System (ADS)

    Zeh, Armin; Gerdes, Axel; Klemd, Reiner; Barton, J. M., Jr.

    2008-11-01

    Detrital zircon grains from Beit Bridge Group quartzite from the Central Zone of the Limpopo Belt near Musina yield mostly ages of 3.35-3.15 Ga, minor 3.15-2.51 Ga components, and numerous older grains grouped at approximately 3.4, 3.5 and 3.6 Ga. Two grains yielded concordant Late Hadean U-Pb ages of 3881 ± 11 Ma and 3909 ± 26 Ma, which are the oldest zircon grains so far found in Africa. The combined U-Pb and Lu-Hf datasets and field relationships provide evidence that the sedimentary protolith of the Beit Bridge Group quartzite was deposited after the emplacement of the Sand River Gneisses (3.35-3.15 Ga), but prior to the Neoarchean magmatic-metamorphic events at 2.65-2.60 Ga. The finding of abundant magmatic zircon detritus with concordant U-Pb ages of 3.35-3.15 Ga, and 176Hf/ 177Hf of 0.28066 ± 0.00004 indicate that the Sand River Gneiss-type rocks were a predominant source. In contrast, detrital zircon grains older than approximately 3.35 Ga were derived from the hinterland of the Limpopo Belt; either from a so far unknown crustal source in southern Africa, possibly from the Zimbabwe Craton and/or a source, which was similar but not necessarily identical to the one that supplied the Hadean zircons to Jack Hills, Western Australia. The Beit Bridge Group zircon population at >3.35 Ga shows a general ɛHf t increase with decreasing age from ɛHf 3.9Ga = -6.3 to ɛHf 3.3-3.1Ga = -0.2, indicating that Hadean crust older than 4.0 Ga ( TDM = 4.45-4.36 Ga) was rejuvenated during magmatic events between >3.9 and 3.1 Ga, due to a successive mixing of crustal rocks with mantle derived magmas. The existence of a depleted mantle reservoir in the Limpopo's hinterland is reflected by the ˜3.6 Ga zircon population, which shows ɛHf 3.6Ga between -4.6 and +3.2. In a global context, our data suggest that a long-lived, mafic Hadean protocrust with some tonalite-trondhjemite-granodiorite constituents was destroyed and partly recycled at the Hadean/Archean transition, perhaps

  19. Hafnium isotope variations in oceanic basalts.

    USGS Publications Warehouse

    Patchett, P.J.; Tatsumoto, M.

    1980-01-01

    Routine low-blank chemistry and 0.01-0.04% precision on the ratio 176Hf/177Hf allows study of Hf isotopic variations, generated by beta --decay of 176Lu, in volcanic rocks derived from the suboceanic mantle. Normalized to 176Hf/177Hf = 0.7325, 176Hf/177Hf ranges 0.2828-0.2835, based on 24 basalt samples. 176Hf/177Hf is positively correlated with 143Nd/144Nd, and negatively correlated with 87Sr/86Sr and 206Pb/204Pb. Along the Iceland-Reykjanes ridge traverse, 176Hf/177Hf increases southwards. The coherence of Hf, Nd and Sr isotopes in the oceanic mantle allows an approximate bulk Earth 176Hf/177Hf of 0.28295 to be inferred from the bulk Earth 143Nd/144Nd. This requires the bulk Earth Lu/Hf to be 0.25, similar to that of the Juvinas eucrite. 60% of the Hf isotopic variation in oceanic basalts occurs among mid-ocean ridge samples. Lu-Hf fractionation probably decouples from Sm-Nd and Rb-Sr fractionation in very depleted source regions, with high Lu/Hf, and consequent high 176Hf/177Hf ratios developing in mantle residual from partial melting. (Authors' abstract) -T.R.

  20. Oxygen isotope geochemistry of the silicic volcanic rocks of the Etendeka-Parana province: Source constraints

    SciTech Connect

    Harris, C.; Milner, S.C.; Armstrong, R.A. ); Whittingham, A.M. )

    1990-11-01

    Oxygen isotope ratios of pyroxene phenocrysts in the silicic volcanic rocks from the Cretaceous Etendeka-Parana flood basalt province (Namibia, South America) are believed to reflect the {delta}{sup 18}O values of the original magmas. The authors recognize a high {delta}{sup 18}O value type ({delta}{sup 18}O pyroxene {approximately} +10{per thousand}) found in the south of both regions, and a low {delta}{sup 18}O value type ({delta}{sup 18}O pyroxene {approximately} +6.5{per thousand}) found in the north. Other differences between thee two rhyolite types include higher concentrations of incompatible elements and lower initial {sup 87}Sr/{sup 86}Sr ratios in the low {delta}{sup 18}O value type. The authors suggest that the regional distribution of rhyolite types reflects differences in source composition, which can best be explained if the sources are lower crustal, Late Proterozoic mobile belt material (high {delta}{sup 18}O) and Archean lower crust (low {delta}{sup 18}O).

  1. The growth of the continental crust: Constraints from radiogenic isotope geochemistry

    NASA Technical Reports Server (NTRS)

    Taylor, Paul N.

    1988-01-01

    Most models for evolution of continental crust are expressed in the form of a diagram illustrating the cumulative crustal mass (normalized relative to the present crustal mass) as a function of time. Thus, geochronological data inevitably play a major role in either constructing or testing crustal growth models. For all models, determining the start-time for effective crustal accretion is of vital importance. To this end, the continuing search for, and reliable characterization of, the most ancient crustal rock-units remains a worthy enterprise. Another important role for geochronology and radiogenic isotope geochemistry is to assess the status of major geological events as period either of new crust generation or of reworking of earlier formed continental crust. For age characterization of major geological provinces, using the critieria outined, the mass (or volume) of crust surviving to the present day should be determinable as a function of crust formation age. More recent developments, however, appear to set severe limitations on recycling of crust, at least by the process of sediment subduction. In modeling crustal growth without recycling, valuable constaints on growth rate variations through time can be provided if variations in the average age of the continental crust can be monitored through geological history. The question of the average age of the exposed continental crust was addressed by determining Sm-Nd crustal residence model ages (T-CR) for fine-grained sediment loads of many of the world's major rivers.

  2. Stable isotope constraints on Holocene carbon cycle changes from an Antarctic ice core.

    PubMed

    Elsig, Joachim; Schmitt, Jochen; Leuenberger, Daiana; Schneider, Robert; Eyer, Marc; Leuenberger, Markus; Joos, Fortunat; Fischer, Hubertus; Stocker, Thomas F

    2009-09-24

    Reconstructions of atmospheric CO(2) concentrations based on Antarctic ice cores reveal significant changes during the Holocene epoch, but the processes responsible for these changes in CO(2) concentrations have not been unambiguously identified. Distinct characteristics in the carbon isotope signatures of the major carbon reservoirs (ocean, biosphere, sediments and atmosphere) constrain variations in the CO(2) fluxes between those reservoirs. Here we present a highly resolved atmospheric delta(13)C record for the past 11,000 years from measurements on atmospheric CO(2) trapped in an Antarctic ice core. From mass-balance inverse model calculations performed with a simplified carbon cycle model, we show that the decrease in atmospheric CO(2) of about 5 parts per million by volume (p.p.m.v.). The increase in delta(13)C of about 0.25 per thousand during the early Holocene is most probably the result of a combination of carbon uptake of about 290 gigatonnes of carbon by the land biosphere and carbon release from the ocean in response to carbonate compensation of the terrestrial uptake during the termination of the last ice age. The 20 p.p.m.v. increase of atmospheric CO(2) and the small decrease in delta(13)C of about 0.05 per thousand during the later Holocene can mostly be explained by contributions from carbonate compensation of earlier land-biosphere uptake and coral reef formation, with only a minor contribution from a small decrease of the land-biosphere carbon inventory.

  3. Zircon U-Pb age, Hf isotopic compositions and geochemistry of the Silurian Fengdingshan I-type granite Pluton and Taoyuan mafic-felsic Complex at the southeastern margin of the Yangtze Block

    NASA Astrophysics Data System (ADS)

    Zhong, Yufang; Ma, Changqian; Zhang, Chao; Wang, Shiming; She, Zhenbing; Liu, Lei; Xu, Haijin

    2013-09-01

    This work presents an integrated study of zircon U-Pb ages and Hf isotope along with whole-rock geochemistry on Silurian Fengdingshan I-type granites and Taoyuan mafic-felsic intrusive Complex located at the southeastern margin of the Yangtze Block, filling in a gap in understanding of Paleozoic I-type granites and mafic-intermediate igneous rocks in the eastern South China Craton (SCC). The Fengdingshan granite and Taoyuan hornblende gabbro are dated at 436 ± 5 Ma and 409 ± 2 Ma, respectively. The Fengdingshan granites display characteristics of calc-alkaline I-type granite with high initial 87Sr/86Sr ratios of 0.7093-0.7127, low ɛNd(t) values ranging from -5.6 to -5.4 and corresponding Nd model ages (T2DM) of 1.6 Ga. Their zircon grains have ɛHf(t) values ranging from -2.7 to 2.6 and model ages of 951-1164 Ma. The Taoyuan mafic rocks exhibit typical arc-like geochemistry, with enrichment in Rb, Th, U and Pb and depletion in Nb, Ta. They have initial 87Sr/86Sr ratios of 0.7053-0.7058, ɛNd(t) values of 0.2-1.6 and corresponding T2DM of 1.0-1.1 Ga. Their zircon grains have ɛHf(t) values ranging from 3.2 to 6.1 and model ages of 774-911 Ma. Diorite and granodiorite from the Taoyuan Complex have initial 87Sr/86Sr ratios of 0.7065-0.7117, ɛNd(t) values from -5.7 to -1.9 and Nd model ages of 1.3-1.6 Ga. The petrographic and geochemical characteristics indicate that the Fengdingshan granites probably formed by reworking of Neoproterozoic basalts with very little of juvenile mantle-derived magma. The Taoyuan Complex formed by magma mixing and mingling, in which the mafic member originated from a metasomatized lithospheric mantle. Both the Fengdingshan and Taoyuan Plutons formed in a post-orogenic collapse stage in an intracontinental tectonic regime. Besides the Paleozoic Fengdingshan granites and Taoyuan hornblende gabbro, other Neoproterozoic and Indosinian igneous rocks located along the southeastern and western margin of the Yangtze Block also exhibit decoupled

  4. U-Pb zircon dating, geochemical and Sr-Nd-Hf isotopic compositions of Motuo quartz-monzonite: Implication for the genesis and diversity of the high Ba-Sr granitoids in orogenic belt

    NASA Astrophysics Data System (ADS)

    Pan, Fa-Bin; Zhang, Hong-Fei; Xu, Wang-Chun; Guo, Liang; Luo, Bi-Ji; Wang, Shuai

    2016-02-01

    Early Paleogene granitoids in Southern Lhasa subterrane have been widely investigated and many petrogenesis and geodynamic models have been proposed in the past few years. However, contemporaneous granitoids in the Motuo tectono-magmatic belt, southeast Lhasa terrane, are still limitedly studied. Here we present the petrology, zircon U-Pb geochronology, whole-rock geochemistry, and Sr-Nd-Hf isotope data of the Damu and 52 K quartz-monzonite in the Motuo area. LA-ICP-MS U-Pb zircon dating shows that they have magma crystallization ages of 49 and 69 Ma, respectively. The Damu quartz-monzonite (SiO2 = 63.76-68.33 wt.%) is high-K calc-alkaline (K2O = 2.54-4.02 wt.% with K2O/Na2O = 0.59-1.09) and metaluminous to weakly peraluminous (A/CNK = 0.99-1.07). The 52 K quartz-monzonite (SiO2 = 61.12-66.12 wt.%) shows slightly higher K2O contents (3.80-5.28 wt.% with K2O/Na2O = 1.03-1.45) and metaluminous series (A/CNK = 0.96-1.00). The analyzed samples are characterized by high Ba (850-2573 ppm), Sr (534-986 ppm) contents, and fractionated REE patterns ((La/Yb)N = 22-72 and (Sm/Yb)N = 4.55-8.24). These geochemical features are comparable with those of high Ba-Sr granite. They display weakly evolved Sr-Nd-Hf compositions (whole-rock (87Sr/86Sr)0 = 0.7068 to 0.7086, εNd(t) = - 4.20 to - 3.41, and zircon εHf(t) = - 5.2 to - 0.9). Geochemical and Sr-Nd-Hf isotopic data reflect that the Damu and 52 K quartz-monzonite represent residual magma from AFC processes of lithospheric mantle-derived mafic melts. The over-thickened lower crust in the eastern Lhasa terrane had been delaminated during ca. 83-70 Ma, which led to the replacement of ancient lithospheric mantle by the juvenile lithospheric mantle. The juvenile mantle wedge in the study area was suspected to be metasomatized by melts that were derived from the foundering arc root, rather than the subducted sediments. Thus, the early Paleogene high Ba-Sr magmas from the SE Lhasa terrane may provide evidence for recycling of

  5. Paleozoic magmatism and metamorphism in the Central Tianshan block revealed by U-Pb and Lu-Hf isotope studies of detrital zircons from the South Tianshan belt, NW China

    NASA Astrophysics Data System (ADS)

    Zhang, Xiaoran; Zhao, Guochun; Eizenhöfer, Paul R.; Sun, Min; Han, Yigui; Hou, Wenzhu; Liu, Dongxing; Wang, Bo; Liu, Qian; Xu, Bing

    2015-09-01

    As a major Precambrian microcontinent in the southernmost Central Asian Orogenic Belt (CAOB), the Central Tianshan block (CTS) in the Chinese Tianshan is essential for understanding the final assembly of the southern CAOB. It experienced multistage Paleozoic magmatism and metamorphism, but the detailed processes are still controversial and far from being completely understood. This paper reports coupled U-Pb and Lu-Hf isotopic data of detrital zircons from late Paleozoic (meta-)sedimentary strata in the South Tianshan belt, which can provide new insight into deciphering the Paleozoic evolution of the eastern segment of the CTS block. Characterized by typical oscillatory zoning and high Th/U ratios (> 0.2), detrital zircons in the Permian sedimentary samples yield dominant age populations at ca. 505-490 Ma, 475-440 Ma, 430-400 Ma and 340-250 Ma, pinpointing the development of four episodes of magmatism in the eastern CTS block. Particularly, Ordovician-Silurian (475-440 Ma) zircons, possessing low negative εHf(t) values, predominate in sedimentary strata in and surrounding the CTS block, indicating that the 475-440 Ma magmatic rocks probably constitute the main body of the CTS block. The origin of this (early Paleozoic) episode of magmatism was most likely related to the southward subduction of the Junggar Ocean beneath the CTS block. Carboniferous-Triassic (340-250 Ma) zircons have dominantly positive εHf(t) values, probably derived from the post-collisional juvenile rocks in the CTS block. Combined with previous studies, our data suggest that the single source terrane for the sampled strata was the CTS block, which had been a topographic high providing substantial detritus to the surrounding areas at least since the Early Permian. In the metasedimentary sample, detrital zircons mostly show partially/fully recrystallized internal textures and low Th/U ratios (< 0.2), probably sourced from the amphibolite- to granulite-facies metamorphosed rocks in the eastern CTS

  6. New constraints on Tibetan plateau uplift from carbonate clumped isotope thermometry (Invited)

    NASA Astrophysics Data System (ADS)

    Huntington, K. W.; Saylor, J. E.; Quade, J.

    2013-12-01

    The timing and pattern of Tibetan plateau rise provide a critical test of the possible mechanisms for the development and support of high topography, yet there is divergence on whether the modern high (~4.5 km) elevations developed recently or are largely a continuation of high elevations developed prior to Indo-Asian collision in the Eocene. To address this issue we present clumped isotope thermometry (T(Δ47)) data from two well-studied basins in central and southwestern Tibet, for which previous stable isotopic data have been used to reconstruct high paleoelevations from late Oligocene to Pliocene time. Although δ18Occ and δ13Ccc values for Oligocene-age marls from the Nima basin in central Tibet are thought to reflect original depositional conditions, T(Δ47) values are above Earth-surface temperatures, indicating that the samples have experienced some degree of diffusive 13C-18O bond reordering. Two samples record different T(Δ47) values (31 and 57 °C) indicating different susceptibilities to C-O bond reordering, but the remaining five samples record temperatures within error of 45 °C. Although the controls on susceptibility to C-O bond reordering are not yet well understood, the similarity in T(Δ47) values for these samples suggests there may be hope for predicting reordering kinetics based on class or textures of natural carbonates for use in cooling rate studies. In contrast, Miocene-Pliocene aragonitic gastropod shells from the Zhada basin in southwestern Tibet do record primary environmental temperatures, which we interpret in the context of modern shell T(Δ47) values and lake water temperatures. While tufa T(Δ47) values agree with modern summer (JJA) water temperatures, modern shell T(Δ47) values are on average 9×2°C warmer than JJA water temperatures. For the ~9 to 3 Ma shell samples, the mean difference between T(Δ47) and expected JJA water temperature (assuming modern climate and elevation) is 0×1°C. Assuming the modern and ancient

  7. Stable Isotope and Isotopomeric Constraints on Nitrous Oxide Production in a Wastewater Treatment Plant

    NASA Astrophysics Data System (ADS)

    Bellucci, F.; Gonzalez-Meler, M. A.; Sturchio, N. C.; Bohlke, J. K.; Ostrom, N. E.; Kozak, J. A.

    2011-12-01

    Estimates of US anthropogenic greenhouse gas emissions by USEPA (Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2009; 2011) indicate that wastewater treatment plants are the 7th highest contributor to atmospheric nitrous oxide. This unregulated gas has an estimated global warming potential (GWP) 310 times that of carbon dioxide on a per mol basis. There is general agreement that, within wastewater treatment plants, the vast majority of the nitrous oxide emissions occur in the aerobic zones for biological ammonia oxidation and/or downstream from anoxic zones used in biological nitrogen removal. However, the exact mechanism of production is not well understood, as both incomplete nitrification and denitrification might contribute to the overall nitrous oxide emissions. Determining the dominant biological pathways responsible for these emissions is important for the development of improved treatment systems that can reduce nitrous oxide greenhouse gas emissions to the atmosphere. In this study, we determined the total nitrous oxide flux from a single tank of one of the aeration basins from a large metropolitan wastewater treatment plant in Stickney, Illinois. Furthermore, we analyzed the changes in nitrogen and oxigen stable isotopic composition for ammonium, nitrate, and nitrous oxide, as well as the intramolecular site preference (SP) for δ15N within the linear N-N-O molecule, along the 520 meter wastewater flow path within the tank. Assuming the measured tank was representative of the 32 tanks constituting the 4 aeration basins of the plant, we estimate the combined annual nitrous oxide flux from this source to be approximately 230 metric ton/y. The δ15N values for ammonium ranged between +19.9% and +6.4%, those for nitrate ranged between +20.4% and +5.3%, and those for nitrous oxide ranged between -34.4% and 0.4%. The nitrous oxide SP ranged between +11.7% and -4.5%. The concentrations and δ15N values of ammonium and nitrate showed trends along the

  8. Barium isotopic composition of mainstream silicon carbides from Murchison: Constraints for s-process nucleosynthesis in asymptotic giant branch stars

    SciTech Connect

    Liu, Nan; Davis, Andrew M.; Pellin, Michael J.; Dauphas, Nicolas; Savina, Michael R.; Gallino, Roberto; Bisterzo, Sara; Straniero, Oscar; Cristallo, Sergio; Gyngard, Frank; Willingham, David G.; Pignatari, Marco; Herwig, Falk

    2014-05-01

    We present barium, carbon, and silicon isotopic compositions of 38 acid-cleaned presolar SiC grains from Murchison. Comparison with previous data shows that acid washing is highly effective in removing barium contamination. Strong depletions in δ({sup 138}Ba/{sup 136}Ba) values are found, down to –400‰, which can only be modeled with a flatter {sup 13}C profile within the {sup 13}C pocket than is normally used. The dependence of δ({sup 138}Ba/{sup 136}Ba) predictions on the distribution of {sup 13}C within the pocket in asymptotic giant branch (AGB) models allows us to probe the {sup 13}C profile within the {sup 13}C pocket and the pocket mass in AGB stars. In addition, we provide constraints on the {sup 22}Ne(α, n){sup 25}Mg rate in the stellar temperature regime relevant to AGB stars, based on δ({sup 134}Ba/{sup 136}Ba) values of mainstream grains. We found two nominally mainstream grains with strongly negative δ({sup 134}Ba/{sup 136}Ba) values that cannot be explained by any of the current AGB model calculations. Instead, such negative values are consistent with the intermediate neutron capture process (i process), which is activated by the very late thermal pulse during the post-AGB phase and characterized by a neutron density much higher than the s process. These two grains may have condensed around post-AGB stars. Finally, we report abundances of two p-process isotopes, {sup 130}Ba and {sup 132}Ba, in single SiC grains. These isotopes are destroyed in the s process in AGB stars. By comparing their abundances with respect to that of {sup 135}Ba, we conclude that there is no measurable decay of {sup 135}Cs (t {sub 1/2} = 2.3 Ma) to {sup 135}Ba in individual SiC grains, indicating condensation of barium, but not cesium into SiC grains before {sup 135}Cs decayed.

  9. Isotopic and chemical constraints on the petrogenesis of Blackburn Hills volcanic field, western Alaska

    SciTech Connect

    Moll-Stalcup, E.J.; Arth, J.G. )

    1991-12-01

    The Blackburn Hills volcanic field is one of several Late Cretaceous and early Tertiary (75-50 Ma) volcanic fields in western Alaska that comprise a vast magmatic province extending from the Arctic Circle to Bristol Bay. It consists of andesite flows, rhyolite domes, a central granodiorite to quartz monzonite pluton, and small intrusive rhyolite porphyries, overlain by basalt and alkali-rhyolites. Most of the field consists of andesite flows which can be divided into two groups on the basis of elemental and isotopic composition: a group having lower ({sup 87}Sr/{sup 86}Sr){sub i}, higher ({sup 143}Nd/{sup 144}Nd){sub i}, and moderate LREE and HREE contents (group 1), and a group having higher ({sup 87}Sr/{sup 86}Sr){sub i}, lower ({sup 143}Nd/{sup 144}Nd){sub i}, and lower HREE contents. Basalts are restricted to the top of the stratigraphic section, comprise the most primitive part of group 1 (({sup 87}Sr/{sup 86}Sr){sub i} = 0.7033; ({sup 143}Nd/{sup 144}Nd){sub i} = 0.5129), and have trace-element ratios that are similar to those of oceanic island basalts (OIBs). Although some workers have suggested that the volcanic field is underlain by old continental crust, none of the data require the presence of Paleozoic or Precambrian continental middle or upper crust under this part of the volcanic field. However, the ultimate source of some of the rocks in the Yukon-Koyukuk province that have high {sup 87}Sr/{sup 86}Sr and low {sup 143}Nd/{sup 144}Nd ratios may be old sub-continental mantle and/or lower crust, which was previously subducted beneath the Yukon-Koyukuk province during Early Cretaceous arc-continent collision.

  10. Mantle evolution in the Variscides of SW England: Geochemical and isotopic constraints from mafic rocks

    NASA Astrophysics Data System (ADS)

    Dupuis, Nicolle E.; Murphy, J. Brendan; Braid, James A.; Shail, Robin K.; Nance, R. Damian

    2016-06-01

    The geology of SW England has long been interpreted to reflect Variscan collisional processes associated with the closure of the Rhenohercynian Ocean and the formation of Pangea. The Cornish peninsula is composed largely of Early Devonian to Late Carboniferous volcanosedimentary successions that were deposited in pre- and syn-collisional basins and were subsequently metamorphosed and deformed during the Variscan orogeny. Voluminous Early Permian granitic magmatism (Cornubian Batholith) is broadly coeval with the emplacement of ca. 280-295 Ma lamprophyric dykes and flows. Although these lamprophyres are well mapped and documented, the processes responsible for their genesis and their relationship with regional Variscan tectonic events are less understood. Pre- to syn-collisional basalts have intra-continental alkalic affinities, and have REE profiles consistent with derivation from the spinel-garnet lherzolite boundary. εNd values for the basalts range from + 0.37 to + 5.2 and TDM ages from 595 Ma to 705 Ma. The lamprophyres are extremely enriched in light rare earth elements, large iron lithophile elements, and are depleted in heavy rare earth elements, suggesting a deep, garnet lherzolite source that was previously metasomatised. They display εNd values ranging from - 1.4 to + 1.4, initial Sr values of ca. 0.706, and TDM ages from 671 Ma to 1031 Ma, suggesting that metasomatism occurred in the Neoproterozoic. Lamprophyres and coeval granite batholiths of similar chemistry to those in Cornwall occur in other regions of the Variscan orogen, including Iberia and Bohemia. By using new geochemical and isotopic data to constrain the evolution of the mantle beneath SW England and the processes associated with the formation of these post-collisional rocks, we may be able to gain a more complete understanding of mantle processes during the waning stages of supercontinent formation.

  11. Alunite in the Pascua-Lama high-sulfidation deposit: Constraints on alteration and ore deposition using stable isotope geochemistry

    USGS Publications Warehouse

    Deyell, C.L.; Leonardson, R.; Rye, R.O.; Thompson, J.F.H.; Bissig, T.; Cooke, D.R.

    2005-01-01

    The Pascua-Lama high-sulfidation system, located in the El Indio-Pascua belt of Chile and Argentina, contains over 16 million ounces (Moz) Au and 585 Moz Ag. The deposit is hosted primarily in granite rocks of Triassic age with mineralization occurring in several discrete Miocene-age phreatomagmatic breccias and related fracture networks. The largest of these areas is Brecha Central, which is dominated by a mineralizing assemblage of alunite-pyrite-enargite and precious metals. Several stages of hydrothermal alteration related to mineralization are recognized, including all types of alunite-bearing advanced argillic assemblages (magmatic-hydrothermal, steam-heated, magmatic steam, and supergene). The occurrence of alunite throughout the paragenesis of this epithermal system is unusual and detailed radiometric, mineralogical, and stable isotope studies provide constraints on the timing and nature of alteration and mineralization of the alunite-pyiite-enargite assemblage in the deposit. Early (preore) alteration occurred prior to ca. 9 Ma and consists of intense silicic and advanced argillic assemblages with peripheral argillic and widespread propylitic zones. Alunite of this stage occurs as fine intergrowths of alunite-quartz ?? kaolinite, dickite, and pyrophyllite that selectively replaced feldspars in the host rock. Stable isotope systematics suggest a magmatic-hydrothermal origin with a dominantly magmatic fluid source. Alunite is coeval with the main stage of Au-Ag-Cu mineralization (alunite-pyrite-enargite assemblage ore), which has been dated at approximately 8.8 Ma. Ore-stage alunite has an isotopic signature similar to preore alunite, and ?? 34Salun-py data indicate depositional temperatures of 245?? to 305??C. The ??D and ?? 18O data exclude significant involvement of meteoric water during mineralization and indicate that the assemblage formed from H2S-dominated magmatic fluids. Thick steam-heated alteration zones are preserved at the highest elevations in

  12. Zircon U-Pb age, Hf isotope and geochemistry of Carboniferous intrusions from the Langshan area, Inner Mongolia: Petrogenesis and tectonic implications

    NASA Astrophysics Data System (ADS)

    Liu, Min; Zhang, Da; Xiong, Guangqiang; Zhao, Hongtao; Di, Yongjun; Wang, Zhong; Zhou, Zhiguang

    2016-04-01

    Late Paleozoic was a critical period for the tectonic evolution of the northern margin of the Alxa-North China craton, but the evolutionary history is not well constrained. The Carboniferous intrusions in the Langshan area in the western part of the northern margin of the Alxa-North China craton are mainly composed of tonalite, quartz diorite, olivine gabbro and pyroxene peridotite. Zircon LA-ICP-MS U-Pb dating indicates that the Langshan Carboniferous intrusions were emplaced at ca. 338-324 Ma. The quartz diorites are characterized by high amounts of compatible trace elements (Cr, Ni and V) and high Mg# values, which may suggest a significant mantle source. The positive Pb and negative Nb-Ta-Ti anomalies, the variable εHf(t) (-6.9 to 2.0) values and the old Hf model ages (1218-1783 Ma) imply some involvement of ancient continental materials in its petrogenesis. The tonalite has relatively high Sr/Y ratios, low Mg#, Yb and Y contents, features of adakite-like rocks, negative εHf(t) values (-9.8 to -0.1) and older Hf model ages (1344-1953 Ma), which suggest significant involvement of ancient crust materials and mantle-derived basaltic component in its petrogenesis. The high Mg# values, high Cr and Ni contents, and low Zr and Hf contents of the mafic-ultramafic rocks show evidence of a mantle source, and the relatively low zircon εHf(t) values (-5.9 to 3.2) might point to an enriched mantle. The trace element characteristics indicate the influence of subducted sediments and slab-derived fluids. In the tectonic discrimination diagrams, all the rocks plot in subduction-related environment, such as volcanic arc and continental arc. Considering the regional geology, we suggest that the Carboniferous intrusions in the Langshan area were likely emplaced during the late stage of the southward subduction of the Paleo-Asian Ocean plate, which formed a continental arc along the northern margin of the Alxa-North China craton.

  13. Zircon U-Pb and Hf isotopic studies of the Xingxingxia Complex from Eastern Tianshan (NW China): Significance to the reconstruction and tectonics of the southern Central Asian Orogenic Belt

    NASA Astrophysics Data System (ADS)

    He, Zhen-Yu; Zhang, Ze-Ming; Zong, Ke-Qing; Xiang, Hua; Chen, Xi-Jie; Xia, Ming-Jun

    2014-03-01

    The Chinese Tianshan occupies the southernmost part of the Central Asian Orogenic Belt (CAOB). High-grade metamorphic rocks are widely distributed in its central uplift zones, composing the crustal basement of the Central Tianshan Block of the Chinese Tianshan. However, the origin of the crustal basement and whether the high-grade metamorphism was associated with the Paleozoic orogeny are still unresolved. Here, we present precise LA-ICP-(MC)-MS zircon U-Pb dating and Hf isotopic data for three meta-sedimentary rocks and one orthogneiss from the Xingxingxia Complex, which represents the Precambrian basement of the Central Tianshan Block. Zircon U-Pb dating results show that the protolith age of the orthogneiss is ca. 880 Ma and that the meta-sedimentary rocks consist dominantly of 0.8-1.0 Ga and 1.3-2.0 Ga materials with minor early Paleoproterozoic components. In combination with the zircon Hf isotopic data, it is suggested that the crustal basement of the Central Tianshan Block was formed in the early Mesoproterozoic, which is evidently different from the Archean basement formation of the Tarim Craton. We suggest that the Central Tianshan Block may have formed through a magmatic arc accretionary orogen along the continental margin of Baltica during the Mesoproterozoic. Moreover, both the meta-sedimentary rocks and the orthogneiss of the Xingxingxia Complex simultaneously suffered a metamorphic overprint at ca. 380 Ma. Zircon REE and Hf isotopic data indicate that the metamorphic zircon rims were formed by new zircon overgrowths in partial melts. Geothermobarometry and average P-T calculations using THERMOCALC on the Grt-Bt gneiss yielded ca. 720-730 °C and 4-6 kbar, suggesting amphibolite- to granulite-facies conditions. The Late Devonian metamorphism of the Xingxingxia Complex is possibly related to orogenic low-pressure/high-temperature metamorphism in the middle or upper crust during the closure of the eastern segment of South Tianshan Ocean. Our new data

  14. Connectivity of the Tisza River System: trace element and isotopic constraints

    NASA Astrophysics Data System (ADS)

    Cherry, W.; Forray, F. L.; Lefticariu, L.

    2013-12-01

    At the watershed scale, a number of complex biogeochemical processes govern riverine geochemistry, and the use of multiple isotopic and trace element analyses has the potential to elucidate these dynamic processes. Such a study was undertaken within the Tisza River Basin (TRB) which is the largest river basin within the Carpathian Basin of Central Europe. The TRB stretches 157,186 km2 and encompasses Romania, Hungary, Serbia, Slovakia, and Ukraine. Geologically, the TRB covers the Pannonian Basin, the Eastern, Western and Southern Carpathians and the Apuseni Mountains. These units have a very complex geology and lithology. The formations range from igneous and metamorphic rocks to sediments, covering a time span from Paleozoic to Quaternary. Our study aimed to determine the influence of various bedrock lithologies on the water chemistry of tributaries and the impact of the biogeochemical and anthropogenic factors on the downstream chemical evolution of the rivers. Twenty-three sampling locations were chosen within the TRB based on the dominant bedrock lithology of the drainage area. In June of 2013, water and rock samples were collected from the Apuseni Mountains and the Southern Carpathians, sampling two main tributary systems - the Cris (Körös) and Mure (Maros) Rivers - and the Lower Tisa (Tisza) River. At each sampling location, field parameters (temperature, pH, dissolved oxygen, and specific conductivity) were measured and water samples were retrieved. Water samples were analyzed for alkalinity, major and minor cations (Fe, Al, Si, Mn, Cu, Sr, Cd, Ni, Zn, Ca, Mg, Na, Se, Pb, As, K) and anions (SO42-, NO3-, Cl-, F-, PO43-), as well as 87Sr/86Sr ratios, and δ2D and δ18O values. Tributaries drained primarily by carbonates tended to exhibit more of an effect on the downstream water chemistry compared to areas drained by silicate rocks, which had little effect on mainstem rivers. Additionally, δ2D and δ18O values (ranging from -80.53 to -41.97‰ for δ2D and

  15. High-spatial-resolution isotope geochemistry of monazite (U-Pb & Sm-Nd) and zircon (U-Pb & Lu-Hf) in the Old Woman and North Piute Mountains, Mojave Desert, California

    NASA Astrophysics Data System (ADS)

    Phillips, Stacy E.; Hanchar, John M.; Miller, Calvin F.; Fisher, Christopher M.; Lancaster, Penny J.; Darling, James R.

    2014-05-01

    Recent improvements in analytical capabilities allow us to reveal details of magmatic processes at an increasingly finer spatial and temporal scale. In situ analyses of the isotopic and trace element composition of accessory minerals at the sub-grain scale have proven to be effective tools for solving a wide range of geological problems. This study presents new data on accessory minerals including monazite & zircon, examined by in situ LA-ICP-MS and Laser Ablation Split Stream (LASS) techniques, analyzing multiple isotopic systems (U-Pb + Sm-Nd, and U-Pb + Lu-Hf in monazite and zircon, respectively) in order to track geochemical changes over time through a magmatic system. The late Cretaceous granitoids of the Old Woman Mountains in the Mojave Desert, California, provide an excellent opportunity to apply these analytical techniques. The peraluminous granites of the Sweetwater Wash, Painted Rock, and North Piute plutons represent different depths of the magmatic system, and are well understood in terms of field relations and whole-rock geochemistry. A preliminary study on the Sweetwater Wash monazites (Fisher et al., in preparation) has revealed significant inter-grain isotopic heterogeneity in the ɛNd composition of the source region (~1700 Ma); however, the U-Pb ages show an isotopic resetting during emplacement at ~75 Ma. This decoupling of U-Pb and Sm-Nd isotopic systems is suggested by Fisher et al. to be due to recrystallisation and/or dissolution-reprecipitation of monazite. If grain boundary diffusion of Pb overrides the more kinetically limited volume diffusion, then the U-Pb systematics will be reset while Sm and Nd remain immobile in the monazite structure as essential structural components of the lattice. This new data will allow the further investigation of these preliminary results, providing new insights into the observed isotopic disequilibrium, with the LASS technique accurately linking the multiple isotopic systems. This will provide important

  16. 1.57 Ga protolith age of the Neoproterozoic Forquilha eclogites, Borborema Province, NE-Brazil, constrained by U-Pb, Hf and Nd isotopes

    NASA Astrophysics Data System (ADS)

    Amaral, Wagner; Santos, Ticiano José; Ancelmi, Matheus Fernando; Fuck, Reinhardt Adolfo; Dantas, Elton Luiz; Matteini, Massimo; Moreto, Carolina Penteado

    2015-03-01

    The 30 km-long, N-S-trending Forquilha eclogite zone, occurs within a Paleoproterozoic block mainly composed of gneisses and migmatites, in the Ceará Central domain, Borborema Province, NE Brazil. The Forquilha eclogite zone contains lenses of high to ultra-high pressure metamafic rocks, found as granulites and amphibolites associated with kyanite-sillimanite gneisses. Three samples of clinopyroxene-garnet amphibolite yielded the U-Pb zircon ages of 1566 ± 9 Ma, 1547 ± 37 Ma and 1532 ± 24 Ma, interpreted as the timing of igneous crystallization of the mafic protolith. Additionally, zircon grains of a leucocratic layer of a metamafic rock and a retrograded eclogite provided the less precise U-Pb ages of 1613 ± 40 Ma and 1454 ± 120 Ma, respectively. Lu-Hf and Sm-Nd model ages provided TDM (Hf) between 1.55 and 1.81 Ga with positive ɛHf values of +7.50 to +10.48, and TDM (Nd) ranging between 1.57 and 1.92 Ga with positive ɛNd values of +1.84 to +4.36. It is believed that part of the rocks of the Forquilha eclogite zone were emplaced as mafic dikes in an extensional setting at ca. 1.57 Ga.

  17. Fluid and gas migration in the North German Basin: fluid inclusion and stable isotope constraints

    NASA Astrophysics Data System (ADS)

    Lüders, Volker; Reutel, Christian; Hoth, Peer; Banks, David A.; Mingram, Birgit; Pettke, Thomas

    2005-12-01

    inclusions and reservoirs is Corg-rich Carboniferous shales with high nitrogen content. Intensive interaction of brines with Carboniferous or even older shales is proposed from fluid inclusion data (enrichment in Li, Ba, Pb, Zn, Mg) and sulfur isotopic compositions of abundant anhydrite from fissures. The mainly light δ34S values of the fissure anhydrites suggest that sulfate is either derived through oxidation and re-deposition of biogenic sulfur or through mixing of SO{4/2-}-rich formation waters with variable amounts of dissolved biogenic sulfide. An igneous source for nitrogen seems to be unlikely since these rocks have low total nitrogen content and, furthermore, even extremely altered volcanic rocks from the study area do not show a decrease in total nitrogen content.

  18. Tracing magma sources of three different S-type peraluminous granitoid series by in situ U-Pb geochronology and Hf isotope zircon composition: The Variscan Montes de Toledo batholith (central Spain)

    NASA Astrophysics Data System (ADS)

    Merino Martínez, E.; Villaseca, C.; Orejana, D.; Pérez-Soba, C.; Belousova, E.; Andersen, T.

    2014-07-01

    Three distinct S-type peraluminous granitoid types have been identified within the Variscan Montes de Toledo batholith, located in the Central Iberian Zone (SW European Variscides): type-1, extremely high peraluminous restite-rich granitoids; type-2, highly peraluminous restite-bearing granitoids; and type-3, moderately peraluminous granitoids with mafic microgranular enclaves. Type-1 and type-2 granitoids are restricted to the western part of the batholith, whereas type-3 granites are mostly restricted to the eastern segment. There is a sequential youngering of emplacement age from type-1 (late-tectonic) to type-2 and type-3 granitoids (post-tectonic), extending the timing of the batholith formation for about 19 Ma between 316 and 297 Ma. Although the degree of peraluminousity of the different series could be related to different partial melting conditions or to the variable entrainment of restitic components (including the peritectic mineral assemblage of the melting reactions), whole-rock geochemical signatures and isotope zircon composition of the peraluminous granitoid types suggest contribution of different crustal sources. There is no evidence for the direct mantle-derived material contribution in the genesis of these peraluminous melts. Type-1 and type-2 granitoids contain mostly Archean to Neoproterozoic inherited zircons, whereas type-3 granites show preferentially Neoproterozoic (up to late Cryogenian) and Ordovician inheritance. The wide range of initial Hf isotope composition, ranging to highly radiogenic values (ƐHf up to + 10), of Neoproterozoic zircon inheritances in type-1 and type-2 granitoids suggests derivation from heterogeneous Neoproterozoic metasedimentary sources composed of both juveline and recycled crustal materials, similar in composition to the host Schist-Greywacke Complex metasediments. Trace-element modelling clearly suggests the involvement of metasediments similar to those mentioned from the southern part of the Central Iberian

  19. The evolution of the Bangong-Nujiang Neo-Tethys ocean: Evidence from zircon U-Pb a