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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. Zircon U Pb and Hf isotope constraints on crustal melting associated with the Emeishan mantle plume

    NASA Astrophysics Data System (ADS)

    Xu, Yi-Gang; Luo, Zhen-Yu; Huang, Xiao-Long; He, Bin; Xiao, Long; Xie, Lie-Wen; Shi, Yu-Ruo

    2008-07-01

    SHRIMP zircon U-Pb dates, combined with in-situ Hf isotopic data, provide new constraints on the petrogenesis and protolith of peralkaline, metaluminous and peraluminous intrusions and rhyolitic tuffs in the Emeishan large igneous province, with significant bearing on crustal melting associated with mantle plumes. Syenite and A-type granitic intrusions from Huili, Miyi and Taihe in the center of this large igneous province yield U-Pb dates at ˜260 Ma, consistent with the ages obtained for mafic layered intrusions in the same province. Zircon from these rocks exhibits a wide range of initial Hf isotope ratios ( ɛHf( t) = -1.4 to +13.4), with corresponding TDM1 of 400-900 Ma. The highest ɛHf( t) value is only marginally lower than that of depleted mantle reservoir at 260 Ma, suggesting that their source is primarily juvenile crust added during Emeishan volcanism, with incorporation of variable amounts of Neoproterozoic crust. The trigger of crustal melting is most likely related to advective heating associated with magmatic underplating. In contrast, the 255-251 Ma peraluminous granites from Ailanghe and 238 Ma rhyolitic tuff from Binchuan, have negative initial ɛHf values of -1.3 to -4.4, and of -7.7 to -14, respectively. Hf isotopic model ages and presence of inherited zircons indicate their derivation from Mesoproterozoic and Paleoproterozoic crust, respectively. Given the time lag relative to the plume impact (˜260 Ma) and insignificant mantle contribution to 255-238 Ma magmatism, conductive heating is suggested as the trigger of crustal melting that resulted in formation of delayed felsic magmas. The involvement of older crust in younger felsic magmas is consistent with upward heat transfer to the lithosphere during plume impregnation, if the age of crust is inversely stratified, i.e., changes from Paleoproterozoic to Mesoproterozoic to Neoproterozoic to Permian with increasing depth. Such crust may have resulted from episodic, downward crustal growth

  3. Hf-Nd isotope decoupling in the oceanic lithosphere: constraints from spinel peridotites from Oahu, Hawaii

    NASA Astrophysics Data System (ADS)

    Bizimis, Michael; Sen, Gautam; Salters, Vincent J. M.

    2004-01-01

    We present a detailed geochemical investigation on the Hf, Nd and Sr isotope compositions and trace and major element contents of clinopyroxene mineral separates from spinel lherzolite xenoliths from the island of Oahu, Hawaii. These peridotites are believed to represent the depleted oceanic lithosphere beneath Oahu, which is a residue of a MORB-related melting event some 80-100 Ma ago at a mid-ocean ridge. Clinopyroxenes from peridotites from the Salt Lake Crater (SLC) show a large range of Hf isotopic compositions, from ɛHf=12.2 (similar to the Honolulu volcanics series) to extremely radiogenic, ɛHf=65, at nearly constant 143Nd/ 144Nd ratios ( ɛNd=7-8). None of these samples show any isotopic evidence for interaction with Koolau-type melts. A single xenolith from the Pali vent is the only sample with Hf and Nd isotopic compositions that falls within the MORB field. The Hf isotopes correlate positively with the degree of depletion in the clinopyroxene (e.g. increasing Mg#, Cr#, decreasing Ti and heavy REE contents), but also with increasing Zr and Hf depletions relative to the adjacent REE in a compatibility diagram. The Lu/Hf isotope systematics of the SLC clinopyroxenes define apparent ages of 500 Ma or older and these compositions cannot be explained by mixing between any type of Hawaiian melts and the depleted Pacific lithosphere. Metasomatism of an ancient (e.g. 1 Ga or older) depleted peridotite protolith can, in principle, explain these apparent ages and the Nd-Hf isotope decoupling, but requires that the most depleted samples were subject to the least amount of metasomatism. Alternatively, the combined isotope, trace and major element compositions of these clinopyroxenes are best described by metasomatism of the 80-100 Ma depleted oceanic lithosphere by melts products of extensive mantle-melt interaction between Honolulu Volcanics-type melts and the depleted lithosphere.

  4. 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

  5. Zircon Hf isotopic constraints on the magmatic evolution in Iran: Implications of the Phanerozoic continental growth

    NASA Astrophysics Data System (ADS)

    Chiu, H.; Chung, S.; Zarrinkoub, M. H.; Lee, H.; Pang, K.; Mohammadi, S. S.; Khatib, M. M.

    2013-12-01

    Combined LA-ICPMS analyses of zircon U-Pb and Hf isotope compositions for magmatic rocks from major domains of Iran allow us to better understand the magmatic evolution regarding the development of the Tethys oceans in the regions. In addition to 79 igneous rocks from Iran, 12 others were also collected from Armenia for isotopic studies. Two major episodes of magmatism were identified in the late Neoproterozoic to Cambrian and the Late Triassic. While the former represents the depleted mantle-derived magma and has associated with the magmatic events that produced the peri-Gondwanan terranes and the Arabian-Nubian Shield, the latter shows the continental crust-type zircon Hf isotopic characteristic and is attributed to the subduction and closing of the Paleotethys ocean. The Neotethyan subduction-related magmatism started from the Jurassic period as granitoids that now exposed along the Sanandaj-Sirjan structural zone (SSZ) and in the central part of the Urumieh-Dokhtar magmatic arc (UDMA), and exhibit heterogeneous isotopic affinities of variable zircon ɛHf(T) values between +12 and -5. The igneous activities migrated inland in the southeastern segment of the UDMA from which the Late Cretaceous granitoids occurred in the Jiroft and Bazman areas with zircon ɛHf(T) values from +15 to +11 and from +5 to -9, respectively, implying the remarkable involvement of crustal material in the Bazman magma. Then, the most widespread magmatic activities which took place during the Eocene to Miocene in the UDMA, Armenia, the SSZ and the Alborz yielded mainly positive zircon ɛHf(T) values from +17 to -1. However, the Eocene intrusive rocks from the Central Iran, in the Saghand area have less radiogenic zircon Hf isotopes of ɛHf(T) values between +6 and -7. Magmatic zircons with juvenile signatures, ɛHf(T) values from +17 to 0, were also found during the Oligocene to Quaternary in the southern Sistan suture zone and the Makran region. Significantly, the positive ɛHf(T) values

  6. Isotopic outcomes of N-body accretion simulations: Constraints on equilibration processes during large impacts from Hf/W observations

    NASA Astrophysics Data System (ADS)

    Nimmo, F.; Agnor, C. B.

    2006-03-01

    Most estimates of planetary core formation timescales using hafnium-tungsten (Hf-W) isotopes employ analytical expressions assuming either continuous planetary growth or instantaneous core formation. In contrast, dynamical modelling of planetary accretion suggests that the final stage of terrestrial planet formation is punctuated by multiple large and stochastic impacts. Such giant impacts have significant thermal and isotopic consequences. We present a framework for calculating the Hf-W isotope evolution of individual bodies based on the results of an N-body accretion simulation and assuming constant partition coefficients. The results show that smaller bodies exhibit a larger range in isotopic values than larger bodies, because the latter have suffered more impacts. The analytical core formation timescales calculated using these isotopic values can differ very significantly from the timing of the final giant impact each planet actually experiences. Simulations in which 1) even the largest impactors undergo re-equilibration with the target's mantle, rather than the cores merging directly, and 2) the original planetary embryos possessed radially variable iron : silicate ratios, produce results which are consistent with the observed physical and isotopic characteristics of inner solar system bodies. Varying W partition coefficients (due to changing mantle oxidation state) or initial planetesimal Hf / W ratios might produce similar isotopic outcomes, and potentially permit core mergers without violating the isotopic constraints. The style of re-equilibration required suggests that magma oceans were present on Mars-sized and larger bodies; an alternative for bodies of Mars-size and smaller is that the bulk of the mass was delivered as impactors much smaller than the target. For Mars we conclude that a prolonged (˜10 Myr) accretion process is both dynamically and isotopically plausible. We also predict likely Pd-Ag isotopic anomalies for Vesta-, Mars- and Earth

  7. 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

  8. 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

  9. Detrital zircon U-Pb geochronology, Hf isotopes and geochemistry constraints on crustal growth and Mesozoic tectonics of southeastern China

    NASA Astrophysics Data System (ADS)

    Meng, Lifeng; Li, Zheng-Xiang; Chen, Hanlin; Li, Xian-Hua; Zhu, Chen

    2015-06-01

    In situ U-Pb geochronological, Hf isotopic and REE geochemical analyses of detrital zircons from Mesozoic sandstones in central Jiangxi and east Hunan of the South China Block (SCB) are used to provide not only information about crustal evolution process, but more importantly new constraints on sedimentary provenances as well as Mesozoic basin evolutions in central SCB. A total of 417 concordant zircon analyses define five U-Pb age populations at ca. 2.6-2.3 Ga, ca. 2.0-1.7 Ga, ca. 1.0-0.7 Ga, ca. 500-400 Ma and ca. 300-200 Ma. Integrated analyses of zircon U-Pb ages and Hf isotopes of detrital zircons reveal five episodes of juvenile continental crust growth: ca. 2.5 Ga, ca. 1.7 Ga, ca. 850 Ma, ca. 440 Ma and ca. 230 Ma, with all but the ca. 2.5 Ga episode likely represent that of the SCB. None of the three samples from T3-J1 strata showed strong ca. 1850 Ma and ca. 230 Ma peaks suggesting that the T3-J1 sediments probably sourced from the Yangtze Block. In contrast, stronger peaks of ca. 1850 Ma and ca. 250-230 Ma in post-J1 strata relative to that of T3-J1 strata suggest a dominantly local Cathaysian provenance. In addition, the distinct low εHf(T) values for zircons of ca. 430 Ma from T3-J1 strata in comparison with higher εHf(T) values for that from post-J1 strata also support aforementioned viewpoint. Such sharp changes between the pre-J1 and post-J1 strata coincide with the remarkable change in regional palaeogeography from a broad shallow marine basin in the Late Triassic-Early Jurassic time to a basin-and-range-style province in the Middle Jurassic. The characteristics and time evolution of detrital zircons from the studied area are consistent with the flat-slab subduction model which involves the development of a broad sag basin during the T3-J1 time, and a rapid regional uplift in the Cathaysia Block caused by the reinitiating of normal subduction along the coastal region at ca. 190 Ma.

  10. 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.

  11. 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.

  12. Sr, Nd, Pb and Hf isotopic constraints on mantle sources and crustal contaminants in the Payenia volcanic province, Argentina

    NASA Astrophysics Data System (ADS)

    Søager, Nina; Holm, Paul Martin; Thirlwall, Matthew F.

    2015-01-01

    The presented Sr, Nd, Hf and double-spike Pb-isotopic analyses of Quaternary basalts from the Payenia volcanic province in southern Mendoza, Argentina, confirm the presence of two distinct mantle types feeding the Payenia volcanism. The southern Payenia mantle source feeding the intraplate-type Río Colorado and Payún Matrú volcanism is isotopically distinct from the northern Payenia and arc mantle source with less radiogenic Pb and lower 87Sr/86Sr and 176Hf/177Hf for a given 143Nd/144Nd than the northern Payenia and arc rocks. The basalts from the northern Payenia province (Nevado volcanic field) and the retroarc zone have isotopic compositions overlapping the Andean southern volcanic zone (SVZ) arc rocks and they are probably derived from the same metasomatized South Atlantic N-MORB-like mantle source. A high field strength element melting model suggests addition of ~ 1-2% upper continental crustal material to the mantle source of the Nevado basalts and the transitional SVZ arc and retroarc rocks and similar degrees of melting throughout this arc segment. A gradual depletion of the pre-metasomatic mantle source going from the backarc over the retroarc to the arc is indicated. The depletion is suggested to be caused by step-wise melt extraction due to repeated injections of subduction zone fluids and melts. The lower crustal contamination trends found in the trace element variations of basalt groups from all parts of the Payenia province are also recognized in isotopic space and the dominant isotope assimilation trends indicate lower crustal contaminants with more unradiogenic Pb, lower 143Nd/144Nd and 176Hf/177Hf and higher or similar 87Sr/86Sr as the mantle melts. These characteristics are similar to those found in lower crustal xenoliths from the northern Proterozoic Cuyania terrane, and the Payenia basalts may have been contaminated by this type of crust during passage through the lithosphere or by more recently underplated or intruded material.

  13. Pb, Sr, Nd, and Hf isotopic constraints on the origin of Hawaiian basalts and evidence for a unique mantle source

    NASA Technical Reports Server (NTRS)

    Stille, P.; Unruh, D. M.; Tatsumoto, M.

    1986-01-01

    The isotopic Pb, Sr, Nd, and Hf compositions of rocks from nine Hawaiian volcanos are determined using the analytical procedures described by Tatsumoto and Unruh (1976) and Patchett and Tatsumoto (1980). The results are presented in graphs, tables, and maps and characterized in detail. The mantle plume, the oceanic lithosphere, and the depleted mantle are identified as distinct sources of the Hawaiian basalts, with different mechanisms responsible for the formation of shield-building tholeiites, late-stage alkalic rocks, and posterosional basalts. The uniqueness of the Hawaiian basalts and the possibility that the Koolau end member represents an undepleted 'primitive' mantle reservoir are considered.

  14. 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.

  15. Nd-Hf isotopic mapping of Late Mesozoic granitoids in the East Qinling orogen, central China: Constraint on the basements of terranes and distribution of Mo mineralization

    NASA Astrophysics Data System (ADS)

    Wang, Xiaoxia; Wang, Tao; Ke, Changhui; Yang, Yang; Li, Jinbao; Li, Yinghong; Qi, Qiuju; Lv, Xingqiu

    2015-05-01

    Voluminous Late Mesozoic granitoids and the world's largest Mo deposits occur in the East Qinling. This paper presents the results of Nd-Hf isotopic mapping for the Late Mesozoic granitoids (155-105 Ma) and demonstrates their constraint on the basements and distribution of the Mo deposits in the East Qinling. This isotopic map, made by 98 (21 new and 77 published) whole-rock Nd isotopic and 29 (7 new and 22 published) average zircon Hf isotopic data, shows large variations of whole-rock εNd(t) values from -22.1 to -1.5, and the correspondingly Nd model ages (TDM(Nd)) from 2.83 to 0.79 Ga, and zircon εHf(t) values from -26.3 to +0.1 and two-stage Hf model ages (TDM2(Hf)) from 2.86 to 0.96 Ga. Three regions of variations have been identified from north to south: (a) εNd(t) values range from -22.1 to -10.9 with TDM(Nd) of 2.82-1.47 Ga, and εHf(t) values 26.3 to -13.5 with TDM2(Hf) 2.86-2.04 Ga; (b) εNd(t) values -13.9 to -1.5 with TDM(Nd) 2.02-0.79 Ga, and εHf(t) values -16.2 to +0.1 with TDM2(Hf) 1.96-0.96 Ga; and (c) εNd(t) values -6.3 to -4.5 with TDM(Nd) 1.28-1.12 Ga, and εHf(t) values -1.0 to -0.3 with TDM2(Hf) 1.25-1.22 Ga, respectively. The three regions approximately correspond to the three different terranes, the southern margin of the North China Block (NCB), the North Qinling Belt (NQB) and the South Qinling Belt (SQB), respectively. These demonstrate that the granitoids in the different terranes have distinct sources and their sources change from old to more juvenile from the north (southern margin of the NCB) to the south (SQB). These also reveal the distinct basements for the terranes in Late Mesozoic. The southern margin of the NCB contains widespread Neoarchaean to Paleoproterozoic basement, the NQB comprises Archaean to Neoproterozic basement and the SQB Mesoproterozic to Neoproterozic basement. All these suggest that the three terranes underwent different tectonic evolution and the continental crust of the East Qinling were mainly formed

  16. 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

  17. 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

  18. 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.

  19. Lu-Hf CONSTRAINTS ON THE EVOLUTION OF LUNAR BASALTS.

    USGS Publications Warehouse

    Fujimaki, Hirokazu; Tatsumoto, Mistunobu

    1984-01-01

    The authors show that a cumulate-remelting model best explains the recently acquired data on the Lu-Hf systematics of lunar mare basalts. The authors model is first constructed using the Lu and Hf concentration data and it is then further strengthened by the Hf isotopic evidence. The authors also show that the similarity of MgO/FeO ratios and the Cr//2O//3 contents between high-Ti and low-Ti basalts, which have been given significance by A. E. Ringwood and D. H. Green are not important constraints for lunar basalt petrogenesis. The authors principal aim is to revive the remelting model for further consideration with the powerful constraints of Lu-Hf systematics of lunar basalts.

  20. 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

  1. 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

  2. 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.

  3. Zircon Hf isotopic constraints on the mantle source of felsic magmatic rocks in the Phan Si Pan uplift and Tu Le basin, northern Vietnam

    NASA Astrophysics Data System (ADS)

    Usuki, T.; Lan, C.; Tran, T.; Pham, T.; Wang, K.

    2013-12-01

    Permian plume-related rocks, such as picrites, flood basalts and silicic volcanic rocks occur in northern Vietnam. This area was displaced 600 km southeastward along the Ailao Shan-Red River fault during mid-Tertiary in response to the India-Eurasia collision. The original location of the area was situated at the central Emeishan Large Igneous Province (ELIP) in SW China before Tertiary. The picrites and flood basalts in northern Vietnam have been investigated by many authors and are comparable with the ELIP. While, felsic magmatisms in northern Vietnam has been poorly studied. Zircon U-Pb age and Hf isotopic data are useful to compare the felsic magmatism in northern Vietnam with that in the ELIP, because the magmatisms of the ELIP had a characteristic time period (260-250 Ma) and the Hf isotopes show a remarkable mantle signature. Therefore, this study carried out in-situ U-Pb ages and Hf isotopic compositions for 300 zircon grains in eighteen granitoids and rhyolites in Phan Si Pan uplift and Tu Le basin in northern Vietnam. Zircons from the granitoids and rhyolites occasionally show development of {101} pyramid and {100} prism crystal facies, suggesting typical zircons crystallized from high temperature alkaline granite. 206Pb/238U ages of granitoid and rhyolite yield consistently in a narrow range of 260 to 250 Ma, which coincides with those from peralkaline to metaluminous granites in the ELIP. ɛHf(t) values of zircons in rhyolites and granites of this study dominate in the range of +5 to +10, which is consistent with those from the ELIP. U-Pb ages and Hf isotopic compositions of zircons indicate that felsic magmatic rocks in the Phan Si Pan uplift and Tu La basin have been derived from the same mantle source with the ELIP.

  4. Geochemical and Sr-Nd-Hf-O-C isotopic constraints on the origin of the Neoproterozoic Qieganbulake ultramafic-carbonatite complex from the Tarim Block, Northwest China

    NASA Astrophysics Data System (ADS)

    Ye, Hai-Min; Li, Xian-Hua; Lan, Zhong-Wu

    2013-12-01

    The Qieganbulake ultramafic-carbonatite complex located within the northeastern margin of the Tarim Block of Northwest China hosts the world's second largest vermiculite deposit. Field observations, radiometric dating results and Sr-Nd-Hf isotopes reveal that the parental magmas of the carbonatite and ultramafic rocks are cogenetic and formed synchronously at ~ 810 Ma. They are characterized by unusually enriched Sr-Nd-Hf isotopic compositions (ISr = 0.70570-0.70762, εNd(t) = - 7.7 to - 12.5, and εHf(t) = - 6.7 to - 12.9), indicating that the parent magmas were derived mainly from a subcontinental mantle source that had been metasomatised by subduction processes. Higher δ13C (- 3.65 to - 4.11‰) values compared to primary magmatic carbonate (- 8 to - 4‰) argue for incorporation of recycled inorganic carbon derived from subducted oceanic crusts. The carbonatites and clinopyroxenties define two distinct differentiation trends, which suggest that liquid immiscibility rather than crystal fractionation controlled the petrogenetic process. Pyroxenities have clearly higher apatite ISr and δ18O values than coexisting carbonatites, indicating involvement of crustal components during their emplacement. The Qieganbulake complex is closely associated in time and space with the mid-Neoproterozoic Rodinia breakup event triggered by mantle plume activities in the Tarim Block. Therefore, the mantle plume likely induced partial malting of, and likely mixed with, the metasomatized subcontinental lithospheric mantle, to form the Qieganbulake ultramafic-carbonatite complex.

  5. 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

  6. 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.

  7. 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.

  8. Elemental and Sr-Nd-Hf isotopic constraints on the origin of Late Jurassic adakitic granodiorite in central Fujian province, southeast China

    NASA Astrophysics Data System (ADS)

    Wang, Guo-Chang; Jiang, Yao-Hui; Liu, Zheng; Ni, Chun-Yu; Qing, Long; Zhang, Qiao

    2015-08-01

    This paper presents the first detailed SHRIMP or LA-ICP-MS zircon U-Pb dating, major and trace element geochemical and Sr-Nd-Hf isotopic data of an adakitic pluton (Tangquan pluton) and an I-type granitic pluton (Xiadao pluton) in central Fujian. SHRIMP and LA-ICP-MS zircon U-Pb dating indicates that the Tangquan and Xiadao plutons were emplaced in the Late Jurassic (~160 Ma) and Early Cretaceous (~143 Ma), respectively. The Tangquan pluton is mainly composed of high-K calc-alkaline granodiorite. The rocks show adakitic affinities, characterized by high Sr and low Y and Yb contents, with high Sr/Y ratios, and by high Mg#. They have initial 87Sr/86Sr of 0.7084-0.7087, ɛNd (T) of -8.8 to -9.0 and ɛHf (T) (in-situ zircon) of -11.2. Detailed elemental and isotopic data suggest that the Tangquan adakitic granodiorite was formed by partial melting of Paleoproterozoic metamorphic basement at a depth of ~40 km ( P ~12.5 kbar) plus additional input from coeval basaltic magma. The Xiadao pluton consists of monzogranite, syenogranite and alkali-feldspar granite. These granites are high-K calc-alkaline and belong to I-type. They have relatively low Sr and high Y and Yb contents and show similar Mg# to pure crustal melts. The Xiadao granites have slightly higher initial 87Sr/86Sr (0.7095-0.7102) and lower ɛNd (T) (-9.0 to -9.4) and ɛHf (T) (-14.4; in-situ zircon) than the Tangquan granitoids. Detailed elemental and isotopic data suggest that the Xiadao I-type granites were formed by partial melting of Paleoproterozoic metamorphic basement at a depth of ~30 km ( P ~10 kbar).

  9. Low-degree melting of a metasomatized lithospheric mantle for the origin of Cenozoic Yulong monzogranite-porphyry, east Tibet: Geochemical and Sr Nd Pb Hf isotopic constraints

    NASA Astrophysics Data System (ADS)

    Jiang, Yao-Hui; Jiang, Shao-Yong; Ling, Hong-Fei; Dai, Bao-Zhang

    2006-01-01

    SHRIMP zircon U-Pb dating, mineral chemical, element geochemical and Sr-Nd-Pb-Hf isotopic data have been determined for the Yulong monzogranite-porphyry in the eastern Tibet, China. The Yulong porphyry was emplaced into Triassic strata at about 39 Ma. The rocks are weakly peraluminous and show shoshonitic affinity, i.e., alkalis-rich, high K 2O contents with high K 2O / Na 2O ratios, enrichment in LREE and LILE. They also show some affinities with the adakite, e.g., high SiO 2 and Al 2O 3, and low MgO contents, depleted in Y and Yb, and enrichment in Sr with high Sr / Y and La / Yb ratios, and no Eu anomalies. The Yulong porphyry has radiogenic 87Sr / 86Sr (0.7063-0.7070) and unradiogenic 143Nd / 144Nd ( ɛNd = - 2.0 to - 3.0) ratios. The Pb isotopic compositions of feldspar phenocrysts separated from the Yulong porphyry show a narrow range of 206Pb / 204Pb ratios (18.71-18.82) and unusually radiogenic 207Pb / 204Pb (15.65-15.67) and 208Pb / 204Pb (38.87-39.00) ratios. In situ Hf isotopic composition of zircons that have been SHRIMP U-Pb dated is characterized by clearly positive initial ɛHf values, ranging from + 3.1 to + 5.9, most between + 4 and + 5. Phenocryst clinopyroxene geothermometry of the Yulong porphyry indicates that the primary magmas had anomalously high temperature (> 1200 °C). The source depth for the Yulong porphyry is at least 100 km inferred by the metasomatic volatile phase (phlogopite-carbonate) relations. Detailed geochemical and Sr-Nd-Pb-Hf isotopic compositions not only rule out fractional crystallization or assimilation-fractional crystallization processes, but also deny the possibility of partial melting of subducted oceanic crust or basaltic lower crust. Instead, low degree (1-5%) partial melting of a metasomatized lithosphere (phlogopite-garnet clinopyroxenite) is compatible with the data. This example gives a case study that granite can be derived directly by partial melting of an enriched lithospheric mantle, which is important to

  10. 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.

  11. 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

  12. 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

  13. 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

  14. 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

  15. Geochronology, geochemistry and Sr-Nd-Hf isotopes of mafic dikes in the Huicheng Basin: Constraints on intracontinental extension of the Qinling orogen

    NASA Astrophysics Data System (ADS)

    Li, Wei; Dong, Yunpeng; Guo, Anlin; Liu, Xiaoming; Wang, Yuejun; Liu, Wenhang; Yang, Yuanzhen

    2015-05-01

    The diabase dikes intruded the Lower Cretaceous sandstone in the Huicheng Basin are key to understanding the Mesozoic tectonic evolution of the Qinling orogen. LA-ICP-MS zircon U-Pb dating yields a crystallization age of 107 ± 1 Ma for them. The dikes have low contents of SiO2 (42.46-50.16 wt.%), MgO (3.47-5.59 wt.%) with low Mg# of 49-59, and TiO2 (1.35-1.63 wt.%), high TFe2O3 (8.15-9.36 wt.%), Al2O3 (14.75-17.23 wt.%) and K2O (0.87-3.61 wt.%). Their Ni and Cr contents are in range of 16.8-111 and 45.7-315 ppm, respectively. They are significantly enriched in light rare earth elements and large ion lithophile elements (e.g., Cs, Pb and Ba), and depleted in Rb, K, P, and Ti. The dikes show relatively high whole-rock initial 87Sr/86Sr ratios (0.7071-0.7079) and negative εNd (t) values (-1.5 to -4.8) with single-stage Nd model ages of 941-1186 Ma. In addition, they have low zircon εHf (t) values (-8.6 to +3.5) with single-stage Hf model ages of 674-1117 Ma. Both elemental and isotopic geochemistry suggests that the magma of these dikes has undergone significant crystallization fractionation of olivine and pyroxene but weak crustal contamination during magma evolution. They were derived from partial melting of an enriched lithospheric mantle source. Together with regional geological data, these results suggest that the Qinling orogen experienced an intracontinental extension during the late Early Cretaceous.

  16. 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

    2016-03-01

    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).

  17. 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

  18. 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.

  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. Hawaiian double volcanic chain triggered by an episodic involvement of recycled material: Constraints from temporal Sr-Nd-Hf-Pb isotopic trend of the Loa-type volcanoes

    NASA Astrophysics Data System (ADS)

    Tanaka, Ryoji; Makishima, Akio; Nakamura, Eizo

    2008-01-01

    The two parallel loci of recent Hawaiian volcanoes, Kea and Loa, have been regarded as the best targets to interpret the chemical structure of an upwelling mantle plume derived from the lower mantle. Here we show that the Sr-Nd-Hf-Pb isotopic data of the shield-building lavas along the Loa locus form a systematic trend from the main shield stage of Koolau (> 2.9 Ma) to the active Loihi volcanoes. During the growth of the Koolau volcano, the dominant material in the melting region successively changed from the proposed KEA, DMK (depleted Makapuu), to EMK (enriched Makapuu) components. The proportion of EMK, dominated by a recycled mafic component, is typified by some Koolau Makapuu-stage and some Lanai lavas. Subsequently, the EMK component decreased and LOIHI component increased toward the Loihi lavas. The temporal coincidence between the episodically elevated magma production rate and the abrupt appearance of the typical Loa-type lavas that is restricted to the last 3 Myr should be linked to magma genesis. We suggest that the abrupt appearance of Loa-type magmatism should be attributed to the transient incorporation of the relatively dense recycled material and surrounding less degassed lower mantle material that accumulated near the core-mantle boundary into the upwelling plume. This episodic involvement could have been trigged by episodic thermal pulses and buoyancy increases in the plume. The continuous appearance of Kea-type lavas during the long history of Hawaiian-chain magmatism and the larger magma volume of Kea-type lavas relative to that of the Loa-type lavas in the last 3 Myr indicate that the Kea locus is closer to the thermal centre of the Hawaiian plume relative to that of the Loa locus.

  1. 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

  2. W/Hf Fractionation in Chondrites and the Earth: Constraints on Timing of Core Formation

    NASA Astrophysics Data System (ADS)

    Newsom, H. E.

    1996-03-01

    The recent measurements of the Hf and W isotope systematics in terrestrial and meteorite material has led Lee and Halliday to conclude that terrestrial core formation occurred at least 60 Myr after the formation of the iron meteorites. This calculation depends on assumptions regarding the Hf/W ratio for the bulk silicate Earth, and for the ratio in the chondritic material from which the iron meteorites, and the Earth formed. A new study of the abundance and depletion of W in the Earth, relative to refractory lithophile elements, such as Hf, provides constraints on the terrestrial Hf/W ratio. In the chondrites, the ratio of W to refractory lithophile elements is also variable, because W did not fully participate in the metal-silicate fractionation which occurred in the solar nebula. Therefore, the uncertainties in the Hf/W ratios in the chondrites and in the Earth must be considered to determine the constraints on the timing of core formation in the Earth. In the case of the heterogeneous accretion theory, W is accreted to the Earth's primitive mantle in an additive process, which has important implications for the significance of the Hf-W isotope systematics.

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. Hf isotope and concentration systematics of the Mariana arc

    NASA Astrophysics Data System (ADS)

    Tollstrup, D. L.; Gill, J. B.

    2004-12-01

    Negative Hf concentration anomalies are common but little-discussed geochemical features of island arcs. Because both light rare earth elements (LREE) and Hf may be mobile even in `fluid-dominated' island arcs, it is important to relate their isotopic and elemental ratios to models of slab-mantle mixing. We report new Hf isotope and trace element data for K-rich submarine basalts from the Kasuga seamounts located 10-20 km behind the volcanic front of the southern Northern Seamount Province (NSP) of the Mariana arc. These data, when combined with published data for other Mariana samples, span the full range from low-K tholeiites to high-K shoshonites. Rear-arc Kasuga seamounts seamounts of the NSP have lower 143Nd/144Nd and 176Hf/177Hf ratios than arc-front volcanoes of the Mariana Central Island Province (CIP). Within the CIP, Hf concentration anomalies correlate positively with 176Hf/177Hf ratios. Radiogenic Hf and little or no concentration anomalies characterize samples from fluid-dominated volcanoes (Guguan and Maug), whereas samples from sediment-melt dominated volcanoes (Anatahan and Sarigan) have less radiogenic Hf and larger concentration anomalies. Samples from the Kasuga and Hiyoshi seamounts have even larger negative concentration anomalies and less radiogenic Hf, although the two are not always correlated. These data are consistent with mixing between a depleted mantle and a partial melt of subducted sediment that is saturated with trace accessory phases including zircon, rutile, and monazite. A more volcaniclastic source is needed for the NSP than the CIP. Implications of these findings are three-fold. Partial melts of subducting sediment affect the HFSE and REE budgets of even fluid-dominated island arcs. Slab temperatures must be high enough for a peraluminous melt to be present, even where old, cold slabs are subducting. Refractory accessory phases have the potential to become exotic "nuggets" in the convecting mantle, potentially controlling the

  9. Petrogenesis of late Paleozoic volcanic rocks from the Daheshen Formation in central Jilin Province, NE China, and its tectonic implications: Constraints from geochronology, geochemistry and Sr-Nd-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Yu, Qian; Ge, Wen-Chun; Yang, Hao; Zhao, Guo-Chun; Zhang, Yan-Long; Su, Li

    2014-04-01

    We present geochronological, geochemical, whole-rock Sr-Nd and zircon Hf-isotopic data for late Paleozoic volcanic rocks from the Daheshen Formation in central Jilin Province, northeastern China, and constrain the petrogenesis of the volcanic rocks and late Paleozoic tectonic evolution of the northern margin of the Northern China Craton, which is regarded as the eastern segment of the Central Asian Orogenic Belt (CAOB). Lithologically, the Daheshen Formation is composed mainly of rhyolite, rhyolitic tuff, dacite and andesite, with minor basalt. The zircons from three rhyolites, two dacites, one rhyolitic tuff and one basalt are euhedral-subhedral, display oscillatory zoning and have high Th/U ratios (0.50-2.28), implying a magmatic origin. LA-ICP-MS zircon U-Pb age data indicate that the volcanic rocks from the Daheshen Formation formed during Late Carboniferous-Early Permian time (302-299 Ma). Geochemically, late Paleozoic volcanic rocks have SiO2 = 52.13-81.77 wt.% and K2O = 0.86-6.88 wt.%, belonging to mid-K to high-K calc-alkaline series. These rocks are characterized by enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs, such as Nb, Ta, and Ti) and heavy rare earth elements (HREEs), with affinities to igneous rocks forming in an active continental margin setting. All volcanic rocks have depleted Nd isotopic compositions (ɛNd(t) = + 2.4 to + 2.5 for the basalts and + 5.8 to + 7.1 for the andesites and dacites, respectively). In situ Hf isotopic results of zircon from the rhyolites show that they have ɛHf(t) = - 1.1 to + 10.6. All these geochemical features indicate that the andesites, dacites, and rhyolites likely originated from the partial melting of Meso-Neoproterozoic accreted lower crust (Hf and Nd model ages (TDM2) of 1384-662 Ma and 1061-800 Ma, respectively). In contrast, the basalts were derived from the partial fusion of a depleted lithospheric mantle that

  10. 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

  11. Zircon U-Pb geochronology and Sr-Nd-Pb-Hf isotopic constraints on the timing and origin of Mesozoic granitoids hosting the Mo deposits in northern Xilamulun district, NE China

    NASA Astrophysics Data System (ADS)

    Shu, Qihai; Lai, Yong; Zhou, Yitao; Xu, Jiajia; Wu, Huaying

    2015-12-01

    Located in the east section of the Central Asian orogen in northeastern China, the Xilamulun district comprises several newly discovered molybdenum deposits, primarily of porphyry type and Mesozoic ages. This district is divided by the Xilamulun fault into the southern and the northern parts. In this paper, we present new zircon U-Pb dating, trace elements and Hf isotope, and/or whole rock Sr-Nd-Pb isotopic results for the host granitoids from three Mo deposits (Yangchang, Haisugou and Shabutai) in northern Xilamulun. Our aim is to constrain the age and petrogenesis of these intrusions and their implications for Mo mineralization. Zircon U-Pb LA-ICP-MS dating shows that the monzogranites from the Shabutai and Yangchang deposits formed at 138.4 ± 1.5 and 137.4 ± 2.1 Ma, respectively, which is identical to the molybdenite Re-Os ages and coeval well with the other Mo deposits in this region, thereby indicating an Early Cretaceous magmatism and Mo mineralization event. Zircon Ce/Nd ratios from the mineralized intrusions are significantly higher than the barren granites, implying that the mineralization-related magmas are characterized by higher oxygen fugacity. These mineralized intrusions share similar zircon in-situ Hf and whole rock Sr-Nd isotopic compositions, with slightly negative to positive εHf(t) ranging from - 0.8 to + 10.0, restricted εNd(t) values from - 3.7 to + 1.6 but a little variable (87Sr/86Sr)i ratios between 0.7021 and 0.7074, indicative of formation from primary magmas generated from a dominantly juvenile lower crust source derived from depleted mantle, despite diverse consequent processes (e.g., magma mixing, fractional crystallization and crustal contamination) during their evolution. The Pb isotopes (whole rock) also show a narrow range of initial compositions, with (206Pb/204Pb)i = 18.03-18.88, (207Pb/204Pb)i = 15.48-15.58 and (208Pb/204Pb)i = 37.72-38.28, in agreement with Sr-Nd-Hf isotopes reflecting the dominance of a mantle component

  12. Nd and Hf isotopic analysis of Barberton komatiites

    NASA Astrophysics Data System (ADS)

    Robin, Christophe; Blichert-Toft, Janne; Arndt, Nicholas; Wilson, Allan; Byerly, Gary

    2013-04-01

    In order to constrain the origin of komatiites from the Barberton Belt, particularly the nature of their mantle source and the conditions of partial melting, we analyzed the Nd and Hf isotopic compositions of 33 komatiite samples. Of these 15 were from the ca. 3.5 Ga Komati Formation, 3 were from the 3.47 Ga Hooggenoeg Fm and 15 from the 3.3 Ga Weltevreden Fm. The samples were collected from outcrop and represent the three main types of komatiite found in the Barberton Belt: i.e. Al-depleted, Al-undepleted and Al-enriched. The analyses were carried out at ENS Lyon using the procedure described by Blichert-Toft et al. For each sample suite we obtained a relatively large range of calculated initial isotopic values. In each suite, one or more samples gave an unreasonably high or low value, particularly for the Hf isotopic system. Excluding these outliers, the values are as follows: Komati Fm, epsilon Nd = -0.8 to +2.5, epsilon Hf = +1 to +8; Hooggenoeg Fm, epsilon Nd = -0.1 to +0.2, epsilon Hf = +1 to +2; Weltevreden Fm, epsilon Nd = 0.3 to +2.0, epsilon Hf = +4 to +13. There were no systematic differences between the isotopic compositions of the three different types of komatiite. Within the relatively large variability of the data, the epsilon Hf values tend to become more positive with age while the epsilon Nd values remain essentially constant. These results are broadly in line with those obtained in most of the previous studies of Barberton komatiites. Notably: 1) there is a wide range in initial isotopic compositions that is not compatible with normal magmatic processes. At least part of the range can be attributed to disturbance, particularly of the Lu-Hf system, after eruption of the lavas; 2) notwithstanding this uncertainty, both the Nd and Hf isotopic compositions are slightly radiogenic, indicating formation from a moderately depleted mantle source. To obtain more reliable data, we intend a) to analyse carefully chosen and prepared samples from core

  13. 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

  14. Petrogenesis and tectonic setting of the Late Paleozoic Xing'an complex in the northern Great Xing'an Range, NE China: Constraints from geochronology, geochemistry and zircon Hf isotopes

    NASA Astrophysics Data System (ADS)

    Dong, Yu; Ge, Wenchun; Zhao, Guochun; Yang, Hao; Liu, Xiwen; Zhang, Yanlong

    2016-01-01

    To determine the petrogenesis and tectonic setting of the Late Paleozoic Xing'an complex in the northern Great Xing'an Range (GXR), northeastern China, we undertook zircon U-Pb dating and geochemical analyses (major and trace elements, and Hf isotopic compositions) on samples obtained from the complex. The Xing'an complex is composed mainly of the Xinshali (XSL), Ershihao (ESH), Xinnangou (XNG) and Xing'an Station (XAS) plutons. The U-Pb zircon ages measured by secondary ion mass spectrometry (SIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) indicate that the Xing'an complex was emplaced in three stages, represented by the ∼358 Ma XSL, ∼308 Ma XNG/XAS, and ∼294 Ma ESH plutons. The XSL pluton is composed mainly of gabbro diorites (SiO2 = 53.49-56.81 wt.%; MgO = 4.60-5.52 wt.%) of the mid-K calc-alkaline series. These rocks are weakly enriched in large ion lithophile elements (LILEs) and light rare earth elements (HREEs), and depleted in high field strength elements (HFSEs, e.g., Nb, Ta, and Ti) and heavy rare earth elements (HREEs), with εHf(t) values of +4.07 to +7.59. Based on these geochemical and isotopic features, we propose that the magma of the XSL pluton was derived from partial melting of depleted lithospheric mantle that was metasomatized by subducted slab-derived fluids. The ESH and XSL plutons have similar geochemical compositions and zircon Hf isotopic values, thereby indicating a common petrogenesis. In contrast, the XNG and XAS plutons comprise syenogranites and monzogranites that are geochemically similar to I-type granites based on their high SiO2 (67.93-74.98 wt.%) and Na2O + K2O (7.12-9.20 wt.%) contents, low MgO (0.33-1.14 wt.%) content, enrichment in LILEs (e.g., Rb, Th, U, K) and LREEs, and depletion in Nb, Ta, Ti and P. The positive εHf(t) values (+6.34 to +12.72) of the XNG and XAS plutons and their corresponding Hf two-stage model ages of 1149 Ma to 484 Ma indicate that the parental magma was derived

  15. 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

  16. 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

  17. 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

  18. 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

  19. Petrogenesis and tectonic implications of the early Jurassic Fe-Ti oxide-bearing Xialan mafic intrusion in SE China: Constraints from zircon Hf-O isotopes, mineral compositions and whole-rock geochemistry

    NASA Astrophysics Data System (ADS)

    Bai, Zhong-Jie; Zhu, Wei-Guang; Zhong, Hong; Li, Chusi; Liao, Jing-Qing; Sun, Hui-Si

    2015-01-01

    Abundant Jurassic bimodal igneous rocks are present in the Nanling region, southeastern China. Their relationship with the tectonic evolution of southeastern China in the Jurassic is still a matter of debate. The ~ 194 Ma Xialan gabbroic intrusion is the oldest mafic intrusion in the Jurassic Nanling igneous belt discovered to date. The intrusion also hosts a significant Fe-Ti oxide deposit. Thus, this intrusion is important for the studies of fundamental controls on the early Jurassic basaltic magmatism in the region and on the Fe-Ti oxide mineralization in this type of intrusion. In this paper we report Hf-O isotopic compositions of zircon for the intrusion and the stratigraphic variations of whole-rock and important mineral compositions in the intrusion. Based on variations in mineral assemblages and the compositions of cumulus minerals, the Xialan intrusion is divided into four cyclic units (I to IV from the base to the top). Our results indicate that each unit represents a new input of magma with composition more primitive than the resident magma. The contents of Fo in olivine, which occurs in the base of Units I and II, are ~ 66 mol%. This indicates that the parental magma for the intrusion is highly fractionated. Extensive fractional crystallization and density-driving crystal sorting appear to have played a critical role in the formation of important Fe-Ti oxide layers in the upper parts of Units I and II. Clinopyroxene trace element analyses indicate that the parental magma for the Xialan intrusion is characterized by pronounced negative Nb-Ta anomalies. Some of the zircon crystals from the Xialan intrusion have εHf(t) > 10 and δ18O between 5.2 and 5.8‰. This, together with the depleted Sr-Nd isotopic compositions in whole rocks reported previously by others, indicates a dominant asthenospheric mantle source with significant contribution of the overlying SCLM. The εHf(t) and δ18O values of zircon from the intrusion are negatively correlated

  20. Geochemical, zircon U-Pb dating and Sr-Nd-Hf isotopic constraints on the age and petrogenesis of an Early Cretaceous volcanic-intrusive complex at Xiangshan, Southeast China

    NASA Astrophysics Data System (ADS)

    Yang, Shui-Yuan; Jiang, Shao-Yong; Jiang, Yao-Hui; Zhao, Kui-Dong; Fan, Hong-Hai

    2011-01-01

    The Late Mesozoic geology of Southeast China is characterized by extensive Jurassic to Cretaceous magmatism consisting predominantly of granites and rhyolites and subordinate mafic rocks, forming a belt of volcanic-intrusive complexes. The Xiangshan volcanic-intrusive complex is located in the NW region of the belt and mainly contains the following lithologies: rhyodacite and rhyodacitic porphyry, porphyritic lava, granite porphyry with mafic microgranular enclaves, quartz monzonitic porphyry, and lamprophyre dyke. Major and trace-element compositions, zircon U-Pb dating, and Sr-Nd-Hf isotopic compositions have been investigated for these rocks. The precise SHRIMP and LA-ICP-MS zircon U-Pb dating shows that the emplacement of various magmatic units at Xiangshan took place within a short time period of less than 2 Myrs. The stratigraphically oldest rhyodacite yielded a zircon U-Pb age of 135 ± 1 Ma and the overlying rhyodacitic porphyry has an age of 135 ± 1 Ma. Three porphyritic lava samples yielded zircon U-Pb ages of 136 ± 1 Ma, 132 ± 1 Ma, and 135 ± 1 Ma, respectively. Two subvolcanic rocks (granite porphyry) yielded zircon U-Pb ages of 137 ± 1 Ma and 137 ± 1 Ma. A quartz monzonitic porphyry dyke, which represented the final stage of magmatism at Xiangshan, also yielded a zircon U-Pb age of 136 ± 1 Ma. All these newly obtained precise U-Pb ages demonstrate that the entire magmatic activity at Xiangshan was rapid and possibly took place at the peak of extensional tectonics in SE China. The geochemical data indicate that all these samples from the volcanic-intrusive complex have an A-type affinity. Sr-Nd-Hf isotopic data suggest that the Xiangshan volcanic-intrusive complex derived mainly from remelting of Paleo-Mesoproterozoic crust without significant additions of mantle-derived magma. However, the quartz monzonitic porphyry, which has zircon Hf model ages older than the whole-rock Nd model ages, and which has ɛNd(T) value higher than the other rocks

  1. The anatectic effect on the zircon Hf isotope composition of migmatites and associated granites

    NASA Astrophysics Data System (ADS)

    Chen, Yi-Xiang; Gao, Peng; Zheng, Yong-Fei

    2015-12-01

    Zircon Hf isotope composition is widely used to trace the growth and evolution of continental crust. However, it is controversial whether the Hf isotope composition of magmatic zircons can faithfully reflect that of their sources, especially for S-type granites. In order to provide an insight into this issue, we have revisited the published Lu-Hf isotope data of zircons from well-studied migmatites and associated granites in the Sulu orogen and the Cathaysian terrane, respectively. The results show greatly elevated 176Hf/177Hf ratios (by more than 10ε units) for newly grown zircon domains compared to the relict zircon domains. This indicates considerable contributions from non-zircon Hf to anatectic melts during crustal anatexis and subsequent magmatism. Furthermore, this more radiogenic Hf isotope signature was not erased during magmatic processes such as crystal fractionation during melt ascent and emplacement. The budget of Hf isotopes in source rocks with respect to mineral Lu/Hf ratios suggests the involvement of Hf-bearing major minerals in anatectic reactions by dissolving Hf-bearing major minerals into the anatectic melts. The significant Hf isotope variations in some anatectic and magmatic zircon domains from the migmatites and granites suggest not only the source heterogeneity but also the variable non-zircon Hf contributions. As such, the Hf isotope compositions of anatectic and magmatic zircons are substantially dictated by the mass balance between the non-zircon Hf from anatectic reactions and the zircon-Hf from the dissolution of protolith zircons into the anatectic melts. They are primarily controlled by P-T conditions and mechanism of crustal anatexis, and the magmatic processes during melt evolution. The present study highlights the important contribution of non-zircon Hf to the anatectic and magmatic zircon domains. In this regard, the greatly elevated 176Hf/177Hf ratios for newly grown zircon domains in the migmatites and granites cannot reflect

  2. 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

  3. Geochemical and Sr-Nd-Hf isotopic constraints on the origin of Late Triassic granitoids from the Qinling orogen, central China: Implications for a continental arc to continent-continent collision

    NASA Astrophysics Data System (ADS)

    Jiang, Yao-Hui; Jin, Guo-Dong; Liao, Shi-Yong; Zhou, Qing; Zhao, Peng

    2010-06-01

    The Qinling-Dabie-Sulu orogen marks the junction between the North and South China Blocks. However, the exact timing of the final coalescence of the North and South China Blocks in the Qinling orogen is poorly constrained. This paper presents new SHRIMP zircon U-Pb chronology, major and trace elements, and Sr-Nd-Hf isotope data for five early Mesozoic granitic plutons across the Qinling orogen. SHRIMP zircon U-Pb dating shows that four plutons were emplaced in the Carnian (227-218 Ma) of Late Triassic with a southward-younging trend and one pluton was emplaced in the Norian (˜ 211 Ma) of Late Triassic. The Carnian plutons consist of high-K calc-alkaline granitoids (quartz monzodiorite, quartz monzonite, granodiorite and monzogranite) and calc-alkaline diorite. These rocks are mainly metaluminous and are characterized by high Sr and low Y and Yb contents, with high Sr/Y and La/Yb ratios, and by high Mg#, higher than pure crustal melts. The Norian pluton is composed of high-K calc-alkaline two-mica granites, which are peraluminous. These granites have low Sr and high Y and Yb contents and show similar Mg# to pure crustal melts. Detailed elemental and isotopic data suggest that the Carnian plutons were emplaced in a continental arc setting coupled with the northward subduction of the Paleo-Tethyan oceanic crust. Partial melting of subducted sediments triggered by dehydration of the underlying igneous oceanic crust, with subsequent melts interacting with the overlying mantle wedge, formed the high-K calc-alkaline granitic magmas. Partial melting of the hybridized peridotitic mantle wedge induced by slab melts generated the calc-alkaline dioritic magma. The Norian pluton was emplaced during continental collision between the South Qinling terrane and South China Block, which marks the final integration of the North and South China Blocks. Partial melting of subducted sediments at a shallow depth (< 30 km) in the collision (overthrust) zone could account for the origin

  4. 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.

  5. 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

  6. Diagenesis and metallogenetic mechanisms of the Tuanjiegou gold deposit from the Lesser Xing'an Range, NE China: Zircon U-Pb geochronology and Lu-Hf isotopic constraints

    NASA Astrophysics Data System (ADS)

    Sun, Jing-Gui; Han, Shi-Jiong; Zhang, Yong; Xing, Shu-Wen; Bai, Ling-An

    2013-01-01

    The Tuanjiegou gold deposit is a large-sized breccia-hosted epithermal gold deposit in Heilongjiang Province, eastern China, and was discovered in the late 1950s. The scale and unusual metallogenic setting of this gold deposit have made it the focus of many previous studies. We systematically analyzed ore samples, and volcanic and granitic rocks that are associated with the formation of the deposit by single zircon U-Pb dating, obtaining three distinct concordant groups of U-Pb ages from dacitic pyroclastic rocks and lavas: 119.3 ± 3.3 (n = 2), 113.2 ± 1.1 (n = 13), and 109.1 ± 1.4 Ma (n = 8); four groups of concordant U-Pb ages from an ore-bearing granite porphyry: 112.5 ± 2.2 (n = 2), 107.0 ± 1.2 (n = 4), 103.0 ± 0.70 (n = 10), and 99.9 ± 1.1 Ma (n = 4); two groups of concordant U-Pb ages from a pyrite-rich, quartz vein-cemented, brecciated and silicified granite porphyry: 104.23 ± 0.51 (n = 16) and 101.3 ± 1.3 Ma (n = 2); and five groups of concordant U-Pb ages from pyrite-rich, quartz vein-cemented, brecciated or chunky ore samples: 139 ± 4 to 144 ± 3 (n = 2), 127 ± 4 (n = 1), 113.8 ± 2.1 (n = 8), 101.3 ± 3.4 (n = 3), and 91 ± 3 Ma (n = 1). Combining the whole-rock and Lu-Hf isotope geochemistry with the geology of the mining area and these new U-Pb dates, we have redefined the ore deposit-related temporal history of the area, with formation age of the magma associated with the Ningyuancun Stage volcanics being 119 ± 1.4 Ma and eruption of the volcanics at 113 ± 1.1 Ma. The granodiorite-porphyry and granite porphyry were emplaced at 107.0 ± 1.2 Ma and 103-102 Ma, respectively, with ore deposit formation at 102-100 Ma. The magmato-hydrothermal interaction that formed the deposit was closely related to Late Mesozoic crust-mantle interaction, with the early stage calc-alkaline and high-K calc-alkaline magmatism in this area potentially being derived from interaction between asthenospheric mantle material and the underplated lower crust. However

  7. 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.

  8. Hf-W thermochronometry: Closure temperature and constraints on the accretion and cooling history of the H chondrite parent body

    NASA Astrophysics Data System (ADS)

    Kleine, Thorsten; Touboul, Mathieu; Van Orman, James A.; Bourdon, Bernard; Maden, Colin; Mezger, Klaus; Halliday, Alex N.

    2008-06-01

    We obtained Hf-W metal-silicate isochrons for several H chondrites of petrologic types 4, 5, and 6 to constrain the accretion and high-temperature thermal history of the H chondrite parent body. The silicate fractions have 180Hf/184W ratios up to ˜ 51 and 182W/184W ratios up to ˜ 33 ɛ units higher than the whole-rock. These high 180Hf/184W and radiogenic W isotope ratios result in highly precise Hf-W ages. The Hf-W ages of the H chondrites become younger with increasing metamorphic grade and range from ΔtCAI = 1.7 ± 0.7 Ma for the H4 chondrite Ste. Marguerite to ΔtCAI = 9.6 ± 1.0 Ma for the H6 chondrites Kernouvé and Estacado. Closure temperatures for the Hf-W system in H chondrites were estimated from numerical simulations of W diffusion in high-Ca pyroxene, the major host of radiogenic 182W in H chondrites, and range from 800 ± 50 °C for H4 chondrites to 875 ± 75 °C for H6 chondrites. Owing to these high closure temperatures, the Hf-W system closed early and dates processes associated with the earliest evolution of the H chondrite parent body. Consequently, the high-temperature interval of ˜ 8 Ma as defined by the Hf-W ages is much shorter than intervals obtained from Rb-Sr and Pb-Pb dating. For H4 chondrites, heating on the parent body probably was insufficient to cause W diffusion in high-Ca pyroxene, such that the Hf-W age of ΔtCAI = 1.7 ± 0.7 Ma for Ste. Marguerite was not reset and most likely dates chondrule formation. This is consistent with Al-Mg ages of ˜ 2 Ma for L and LL chondrules and indicates that chondrules from all ordinary chondrites formed contemporaneously. The Hf-W ages for H5 and H6 chondrites of ΔtCAI = 5.9 ± 0.9 Ma and ΔtCAI = 9.6 ± 1.0 Ma correspond closely to the time of the thermal peak within the H chondrite parent body. Combined with previously published chronological data the Hf-W ages reveal an inverse correlation of cooling rate and metamorphic grade: shortly after their thermal peak H6 chondrites cooled at ˜ 10

  9. 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.

  10. Hf-Nd Isotopic and Trace-Element Geochemistry of Global Subducting Sediments

    NASA Astrophysics Data System (ADS)

    Vervoort, J. D.; Plank, T.; Patchett, P. J.

    2001-12-01

    Ferromanganese nodules, crusts, and associated metalliferous clays have long been known to have anomalously high Lu/Hf ratios and highly radiogenic Hf relative to Nd (Patchett et al., 1984; White et al., 1986). These oceanic sediments are some of the few terrestrial materials where Hf and Nd isotopes deviate from the crust-mantle array. This distinctive isotopic signature, therefore, has the potential to trace the fate of oceanic sediments through the subduction zone and into the mantle. It has recently been suggested, for example, that pelagic sediments can be detected in some Hawaiian basalts (Blichert-Toft et al., 1999) and in volcanic rocks from the Luzon arc (Marini et al., 2000) based on their Hf-Nd isotopic compositions. The weak link in this approach, however, is that we do not know, in any quantitative way, how widespread this anomalous signature is in oceanic sediments, what compositions are responsible for this signature, or how volumetrically important these compositions are in terms of the total sediment flux into subduction zones and the mantle. Most marine sediments analyzed thus far have been collected on or near the ocean floor and constitute an incomplete and unrepresentative inventory of the sediment column bound for the subduction zone. There is some reason to suspect that much of the sediment flux is not particularly anomalous, either in terms of Lu/Hf ratios or Hf and Nd isotopic compositions. The most dominant sediment types entering many subduction zones (terrigenous and other continentally derived sediments), have normal Lu/Hf ratios and Hf-Nd isotopic compositions that are indistinguishable from the crust-mantle array. An examination is needed of the Hf-Nd isotopic composition of oceanic sediments, the major and trace-element geochemistry of global sediment flux, how such compositions may relate to Hf-Nd isotopic behavior, and potential Lu/Hf and Nd/Hf fractionation in subduction zones. In addition, the origin of the high Lu/Hf and

  11. 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.

  12. Pb, Sr, Nd and Hf isotopic evidence of multiple sources for Oahu, Hawaii basalts

    NASA Technical Reports Server (NTRS)

    Stille, P.; Unruh, D. M.; Tatsumoto, M.

    1983-01-01

    Pb, Sr, Nd and Hf isotopic compositions of Oahu volcanics indicate that the three principal volcanic series on Oahu Koolau, Honolulu and Waianae - were derived from isotopically distinct sources. Honolulu and Waianae basalts plot on the Nd-Pb-Sr 'mantle plane' whereas Koolau data plot distinctly below the plane.

  13. 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

  14. Building the New England Batholith, eastern Australia-Linking granite petrogenesis with geodynamic setting using Hf isotopes in zircon

    NASA Astrophysics Data System (ADS)

    Phillips, G.; Landenberger, B.; Belousova, E. A.

    2011-02-01

    U-Pb and Hf isotope analysis of zircons from granitoids of the Permian-Triassic New England Batholith (eastern Australia) was carried out to provide constraints on the evolution of an isotopically and compositionally diverse batholith. Incipient plutonism in the early Permian resulted in the formation of isotopically evolved, peraluminous granodiorites of the Hillgrove Suite. Following this, mixing between crustal-derived (+ 5-+ 8 εHf units) and depleted mantle-derived magmas (+ 13-+ 18 εHf units), was responsible for the formation of the c. 282 Ma Bundarra Suite. The strongly metaluminous and isotopically depleted granites of the c. 268 Ma Clarence River Suite (+ 11-+ 16 εHf units) signify an increased role of isotopically depleted magmas during the formation of plutonic rocks in the middle Permian. Interestingly, this isotopic and chemical transience coincides with orogenic extension that was associated with the relocation for the southern New England Orogen (NEO) from a continental margin accretionary setting to that of a back-arc basin. Following attenuation, the NEO was thickened by contraction during the Hunter Bowen Event (265-255 Ma). Forming after the Hunter Bowen Event, the metaluminous rocks of the c. 256 Ma Moonbi Suite were the product of mixing between magmas derived from evolved metaigneous rocks and enriched mantle (+ 3-+ 8 εHf units). We attribute the switch in isotopic character from highly depleted (i.e., Clarence River Suite) to evolved (i.e., Moonbi Suite) to crustal thickness before (i.e., thin) and after (i.e., thick) the Hunter Bowen Event. Evidence of renewed mixing between depleted and evolved magmas characterises the formation of the c. 249 Ma Uralla Suite (+ 7-+ 16 εHf units), which interestingly, was coeval with renewed orogenic extension in the early Triassic. Finally, the melting of deep crustal basalts below the southern New England Orogen in the middle Triassic (c. 233 Ma) resulted in the formation of moderately depleted

  15. Simple Lu-Hf isotope patterns resulting from complex Archean geodynamics: example of the Pietersburg block (South Africa)

    NASA Astrophysics Data System (ADS)

    Laurent, Oscar; Zeh, Armin

    2015-04-01

    The combined use of U-Pb and Lu-Hf isotope data from Hadean and Archean zircons is widely used to constrain the mechanisms of continental crust formation and evolution in the early Earth. Such data generally define ɛHf-time arrays, interpreted as reflecting the closed-system, protracted reworking of single crustal reservoirs episodically extracted from depleted mantle (DM) sources. Many models about early Earth evolution and continental growth rely on this interpretation and its consequences (i.e. determination of Hf model ages and crustal residence times). However, this straightforward interpretation is difficult to reconcile with the complex evolution of Archean terranes, involving progressive crustal maturation and a range of crustal and mantle sources to granitoid magmas. Here we present a database of U-Pb and Lu-Hf isotopes measured in situ by LA-(MC-)ICPMS in zircons from >30 samples, representative of the temporal and spatial record of a single segment of Archean crust, the Pietersburg block (Kaapvaal Craton, South Africa). Coupling of age-Hf data with petrological and geochemical constraints shows that >1 Ga-long crustal evolution in the PB is characterized by (i) crustal nucleation in an intra-oceanic setting between 3.4 and 3.1 Ga; (ii) rapid formation of large volumes of juvenile TTG crust in an accretionary orogen at the northern edge of the proto-Kaapvaal craton between 3.1 and 2.9 Ga; (iii) intracrustal reworking and subduction of TTG-derived sediments along an Andean-type continental margin between 2.9 and 2.75 Ga; (iv) continental collision with the Central Zone of the Limpopo Belt at 2.75-2.69 Ga, resulting in magmatism derived from local crust and metasomatized mantle; (v) a discrete anorogenic event at ~2.05 Ga with the emplacement of SCLM-derived alkaline magmas. Despite the diversity of magmas and geodynamic settings depicted by this evolution, all samples emplaced between 3.0 and 2.0 Ga plot along a single, robust array of decreasing ɛHf

  16. 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

  17. The Hf-Nd isotope record of Archean seawater: potential and pitfalls

    NASA Astrophysics Data System (ADS)

    Viehmann, Sebastian; Bau, Michael; Münker, Carsten; Elis Hoffmann, J.; Marquez, J. Eduardo

    2014-05-01

    Banded Iron Formations (BIFs) are Precambrian marine chemical sediments that may be used as archives of the trace element and isotope compositions of ancient seawater. Comparable to hydrogenetic FeMn crusts which are archives of modern seawater, a recent study [1] successfully used the Neoarchean Temagami BIF to study the (de)coupled Hf-Nd systematics of Late Archean seawater. Here, we evaluate the best approach to discriminate effects of syn- or postdepositional processes (e.g. detrital contamination, metamorphic or hydrothermal overprint) of the pristine seawater signature. To step further back in time we report Hf-Nd isotope and trace element data of pure Si- and Fe-rich layers from the Eoarchean ~3.75 Ga Isua BIF (Greenland) and the Mesoarchean ~3.25 Ga Fig Tree BIF (Barberton Greenstone Belt, South Africa) and compare them to data for the Neoarchean ~2.70 Ga Temagami BIF (Canada). To evaluate the effect of syn- or postdepositional processes on the Nd isotopic budget, shale-normalised REY (rare earths and yttrium) patterns of each particular sample should be compared with those of modern seawater and other Archean marine precipitates. Positive La, Gd and Y anomalies (i.e. super-chondritc Y/Ho ratios) and enrichments of HREE over LREE indicate a pristine seawater-derived REY (including Sm and Nd) composition in a BIF sample. Zr/Hf ratios serve as a perfect tool to distinguish seawater Hf from detrital Hf, because both particle-reactive, geochemical twins behave similar during igneous processes, but show a strong decoupling in aqueous solutions, leading to non-chondritic Zr/Hf [2]. Information about open system behaviour of the Hf-Nd systems during metamorphic events can be evaluated by an isochron approach. In contrast to the lower greenschist facies Temagami BIF with its well-defined Nd and Hf isochrons yielding an accurate depositional age [1], errorchrons derived from the data from the Isua and Barberton BIFs, respectively, yield unrealistically young ages

  18. 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

  19. Nucleosynthetic W isotope anomalies and the Hf-W chronometry of Ca-Al-rich inclusions

    NASA Astrophysics Data System (ADS)

    Kruijer, Thomas S.; Kleine, Thorsten; Fischer-Gödde, Mario; Burkhardt, Christoph; Wieler, Rainer

    2014-10-01

    Ca-Al-rich inclusions (CAI) are the oldest dated objects formed in the solar system and are pivotal reference points in early solar system chronology. Knowledge of their initial 182Hf/180Hf and 182W/184W is essential, not only for obtaining precise Hf-W ages relative to the start of the solar system, but also to assess the distribution of short-lived radionuclides in the early solar nebula. However, the interpretation of Hf-W data for CAI is complicated by nucleosynthetic W isotope variations. To explore their extent and nature, and to better quantify the initial Hf and W isotope compositions of the solar system, we obtained Hf-W data for several fine- and coarse-grained CAI from three CV3 chondrites. The fine-grained CAI exhibit large and variable anomalies in ε183W (εiW equals 0.01% deviation from terrestrial values), extending to much larger anomalies than previously observed in CAI, and reflecting variable abundances of s- and r-process W isotopes. Conversely, the coarse-grained (mostly type B) inclusions show only small (if any) nucleosynthetic W isotope anomalies. The investigated CAI define a precise correlation between initial ε182W and ε183W, providing a direct empirical means to correct the ε182W of any CAI for nucleosynthetic isotope anomalies using their measured ε183W. After correction for nucleosynthetic W isotope variations, the CAI data define an initial 182Hf/180Hf of (1.018±0.043)×10-4 and an initial ε182W of -3.49±0.07. The Hf-W formation intervals of the angrites D'Orbigny and Sahara 99555 relative to this CAI initial is 4.8±0.6 Ma, in good agreement with Al-Mg ages of these two angrites. This renders a grossly heterogeneous distribution of 26Al in the inner solar system unlikely, at least in the region were CAI and angrites formed.

  20. 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.

  1. Strengths and limitations of zircon Lu-Hf and O isotopes in modelling crustal growth

    NASA Astrophysics Data System (ADS)

    Payne, Justin L.; McInerney, David J.; Barovich, Karin M.; Kirkland, Christopher L.; Pearson, Norman J.; Hand, Martin

    2016-04-01

    The robust nature of the mineral zircon, combined with our analytical ability to readily acquire in-situ uranium-lead (U-Pb), lutetium-hafnium (Lu-Hf) and oxygen (O) isotopic data, has resulted in a rapid rise in the use of zircon isotopic datasets for studying both the generation of continental crust and its growth through Earth history. In such studies there has been a strong focus on developing methods to determine the timing and/or proportion of juvenile magmatic addition to the continental crust. One widespread approach to determine the timing of crustal growth has been the construction or fitting of 'reworking arrays' to regional Hf isotopic datasets. Simple stochastic models are presented which highlight that in many cases apparent reworking arrays are much more likely to represent a process of on-going dilution and refertilisation of ancient crust, consistent with "Hot Zone" models of granitoid generation and the need to refertilise lower crustal reservoirs to maintain magmatism. A new compilation of magmatic rock zircon Lu-Hf and O isotope data is used to demonstrate that the use of mantle-like O isotope data as a screening tool for "meaningful" Hf model ages is also unlikely to be reliable, with independently constrained data indicating that as few as 14% of Hf model ages provide a meaningful indicator of the timing of crustal growth. The limitations of Hf model ages are discussed with regard to existing approaches for continental growth and we demonstrate that popular inverse modelling approaches suffer from a bias created by both the use of model ages and numerical artefacts. In an effort to address some of the limitations within existing models, we develop stochastic models based on joint calibration of multiple datasets which allow for more unique solutions.

  2. 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

  3. Extreme Hf-Os Isotope Compositions in Hawaiian Peridotite Xenoliths: Evidence for an Ancient Recycled Lithosphere

    NASA Astrophysics Data System (ADS)

    Bizimis, M.; Lassiter, J. C.; Salters, V. J.; Sen, G.; Griselin, M.

    2004-12-01

    We report on the first combined Hf-Os isotope systematics of spinel peridotite xenoliths from the Salt Lake Crater (SLC), Pali and Kaau (PK) vents from the island of Oahu, Hawaii. These peridotites are thought to represent the Pacific oceanic lithosphere beneath Oahu, as residues of MORB-type melting at a paleo-ridge some 80-100Ma ago. Clinopyroxene mineral separates in these peridotites have very similar Nd and Sr isotope compositions with the post erosional Honolulu Volcanics (HV) lavas that bring these xenoliths to the surface. This and their relatively elevated Na and LREE contents suggest that these peridotites are not simple residues of MORB-type melting but have experience some metasomatic enrichment by the host HV lavas. However, the SLC and PK xenoliths show an extreme range in Hf isotope compositions towards highly radiogenic values (ɛ Hf= 7-80), at nearly constant Nd isotope compositions (ɛ Nd= 7-10), unlike any OIB or MORB basalt. Furthermore, these Oahu peridotites show a bimodal distribution in their bulk rock 187Os/186Os ratios: the PK peridotites have similar ratios to the abyssal peridotites (0.130-0.1238), while the SLC peridotites have highly subchondritic ratios (0.1237-0.1134) that yield 500Ma to 2Ga Re-depletion ages. Hf-Os isotopes show a broad negative correlation whereby the samples with the most radiogenic 176Hf/177Hf have the most unradiogenic 187Os/186Os ratios. Based on their combined Hf-Os-Nd isotope and major element compositions, the PK peridotites can be interpreted as fragments of the Hawaiian lithosphere, residue of MORB melting 80-100Ma ago, that have been variably metasomatized by the host HV lavas. In contrast, the extreme Hf-Os isotope compositions of the SLC peridotites suggest that they cannot be the source nor residue of any kind of Hawaiian lavas, and that Hf and Os isotopes survived the metasomatism or melt-rock reaction that has overprinted the Nd and Sr isotope compositions of these peridotites. The ancient (>1Ga

  4. 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.

  5. The origin of decoupled Hf-Nd isotope compositions in Eoarchean rocks from southern West Greenland

    NASA Astrophysics Data System (ADS)

    Hoffmann, J. Elis; Münker, Carsten; Polat, Ali; Rosing, Minik T.; Schulz, Toni

    2011-11-01

    Radiogenic isotope compositions of Hf and Nd are typically coupled in Phanerozoic and Proterozoic mafic rocks due to a similar behaviour of Lu-Hf and Sm-Nd during mantle melting. Eoarchean rocks, for instance those from southern West Greenland, exhibit an apparent decoupling of Hf and Nd isotope compositions. This apparent decoupling may either indicate metamorphic disturbance or, alternatively, mirror early differentiation processes in the silicate Earth. To evaluate the issue, we performed combined measurements of Hf-Nd isotope compositions together with major and trace element concentrations for well preserved >3720 to >3800 Ma old tholeiitic metabasalts and gabbros from the ˜3700 Ma and ˜3800 Ma old terranes of the Isua Supracrustal Belt, southern West Greenland. In contrast to younger mafic rocks, calculated initial ɛHf-ɛNd values of the Isua tholeiites show similar spreads and are both near chondritic to strongly depleted (-0.7 to +6.3 and -0.8 to +4.4, respectively), also in contrast to previously reported more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit. An evaluation of alteration effects based on preserved major and trace element arrays reveals pristine magmatic trends and therefore the measured isotope compositions indeed in most cases characterize contrasting Eoarchean mantle sources. In accord with this view, compositions of the Isua metabasalts yield Eoarchean regression ages in Sm-Nd and Lu-Hf isochron spaces, overlapping with emplacement ages inferred from crosscutting relationships with tonalites. Lutetium-Hf systematics of the Isua metabasalts studied here, yield clear isochron relationships. For both terranes, there is some scatter in Sm-Nd space, indicating early disturbance of the Sm-Nd system close in time to the extrusion ages, possibly by seafloor alteration. Trace element compositions of the metabasalts indicate an arc setting and a strong source overprint by melt-like subduction components. It is

  6. 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.

  7. 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

  8. 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.

  9. 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

  10. 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.

  11. 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.

  12. New Lu-Hf and Pb-Pb age constraints on the earliest animal fossils

    NASA Astrophysics Data System (ADS)

    Barfod, Gry H.; Albarède, Francis; Knoll, Andrew H.; Xiao, Shuhai; Télouk, Philippe; Frei, Robert; Baker, Joel

    2002-07-01

    The Neoproterozoic Doushantuo Formation, South China, preserves a unique assemblage of early multicellular fossils and overlies rocks, which are thought to have formed during an ice age of global extent. The age of this formation is thus critical for understanding the important biological and climatic events that occurred towards the end of the Proterozoic Eon. Until now, direct dating of sedimentary formations such as the Doushantuo has been difficult and associated with large uncertainties. Here, we show that dating of Doushantuo phosphorites by a novel Lu-Hf dating method and conventional Pb-Pb geochronometry independently yield ages of 584±26 Ma and 599.3±4.2 Ma, respectively. These ages are in agreement with bio- and chemostratigraphical observations and show that the Doushantuo animal remains predate diverse Ediacaran fossil assemblages, making them the oldest unambiguous remains of metazoans currently known. Furthermore, the Pb-Pb age for the post-glacial Doushantuo rocks suggests that the Neoproterozoic glaciation in China might predate glacial rocks in Eastern North America commonly associated with the younger (Marinoan) of two major Neoproterozoic glaciations. The combination of Lu-Hf and Pb-Pb dating shows considerable potential for dating other phosphorite successions and future application of these methods could therefore provide further constraints on Proterozoic biological and environmental history.

  13. Tracing magma mixing in granite genesis: in situ U-Pb dating and Hf-isotope analysis of zircons

    NASA Astrophysics Data System (ADS)

    Yang, Jin-Hui; Wu, Fu-Yuan; Wilde, Simon A.; Xie, Lie-Wen; Yang, Yue-Heng; Liu, Xiao-Ming

    2007-02-01

    In situ zircon U-Pb and Hf-isotopic data have been determined for mafic microgranular enclaves and host granitoids from the Early Cretaceous Gudaoling batholith in the Liaodong Peninsula, NE China, in order to constrain the sources and petrogenesis of granites. The zircon U-Pb age of the enclaves (120 ± 1 Ma) is identical to that of the host monzogranite (120 ± 1 Ma), establishing that the mafic and felsic magmas were coeval. The Hf isotopic composition of the enclaves [ ɛ Hf( t) = +4.5 to -6.2] is distinct from the host monzogranite [ ɛ Hf( t) = -15.1 to -25.4], indicating that both depleted mantle and crustal sources contributed to their origin. The depleted mantle component was not previously revealed by geochemical and Nd and Sr isotopic studies, showing that zircon Hf isotopic data can be a powerful geochemical tracer with the potential to provide unique petrogenetic information. Some wall-rock contamination is indicated by inherited zircons with considerably older U-Pb ages and low initial Hf isotopic compositions. Hafnium isotopic variations in Early Cretaceous zircons rule-out simple crystal-liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks. Instead, mixing of mantle-derived mafic magmas with crustal-derived felsic magmas, coupled with assimilation of wall rocks, is compatible with the data.

  14. 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

  15. 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.

  16. 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.

  17. 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.

  18. 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

  19. 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

  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. Pb, Hf, Nd, and Sr Isotopic Variations of Hualalai Shield Stage Tholeiites from the Submarine North Kona Region

    NASA Astrophysics Data System (ADS)

    Yamasaki, S.; Kani, T.; Hanan, B. B.; Tagami, T.

    2007-12-01

    We present the isotope and trace element compositions of tholeiitic lavas collected from deep submarine portions of North Kona region, the west flank of Hualalai volcano. The samples were collected from the lower section of the North Kona bench (dives K218 and K219), a submarine section at Hualalai volcano's northwest rift zone (dive S690), and an elongate ridge below the central section of the bench (dive S692) during 2001 and 2002 JAMSTEC Hawaii cruises. Hualalai volcano is presently in the post-shield alkalic stage and most of its subaerial surface is covered by alkalic basalt. All analyzed samples of the pillow lavas are tholeiites that erupted during Hualalai shield stage. It is important to identify source materials involved in the volumetrically dominant stage of Hualalai volcano in order to provide constraints for the size and distribution of compositional heterogeneities of Hawaiian plume. The isotopic compositions of the submarine North Kona tholeiites are similar to the data previously reported for Mauna Loa tholeiites. The data trends define clear mixing relationships that require at least three mantle source components. The mixing is dominated by a Koolau-like enriched component and a Kea-like depleted component. The K219 data trend toward higher epsilon Hf and 87Sr/86Sr relative to the K218, S690, and S692 arrays, requiring another component similar to that observed in post-shield lavas of Hualalai volcano. Pb isotopic compositions for samples from dive K218 and K219 form distinct non-overlapping Pb-Pb arrays suggesting further source heterogeneity. Samples from dive S690 and S692 plot on both of these trends. These findings suggest small-scale compositional heterogeneity in the source regions that can be attributed to anomalous irregular 'blobs' involved in late shield stage magmatism of Mauna Loa reported in previous works.

  3. 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

  4. 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

  5. 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.

  6. Clay-sized Hf-Nd-Sr isotopic composition of Mongolian dust as a fingerprint for regional to hemispherical transport

    NASA Astrophysics Data System (ADS)

    Zhao, Wancang; Sun, Youbin; Balsam, William; Zeng, Lin; Lu, Huayu; Otgonbayar, Khureldavaa; Ji, Junfeng

    2015-07-01

    Hf-Nd-Sr isotopic fingerprinting has been employed as a powerful method for distinguishing dust sources. However, the Hf and Sr isotopic compositions of dust are highly dependent on particle size to accurately identify dust provenance. Here we compare the Hf-Nd-Sr isotopic compositions of clay-sized fractions from dust sources (the Mongolian Plateau) to sinks (Beijing and Greenland). Our results document that clay-sized Hf-Nd-Sr isotopic compositions of Mongolian dust are controlled by two isotopic/geochemical provinces: Southern Mongolian Gobi (SMG) and Northern Mongolian Plateau (NMP). Our data indicate that the SMG is potentially an important dust source to the loess in Beijing and Hulun Buir and could be a contributing source of dust to Greenland, whereas the NMP contributes little dust to the Chinese loess and the Greenland. Furthermore, we demonstrate that the deserts of northwest China are one of the main dust contributors to Beijing and Greenland, but not to Hulun Buir.

  7. 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.

  8. Zircon chemistry and magma mixing, SE China: In-situ analysis of Hf isotopes, Tonglu and Pingtan igneous complexes

    NASA Astrophysics Data System (ADS)

    Griffin, W. L.; Wang, Xiang; Jackson, S. E.; Pearson, N. J.; O'Reilly, Suzanne Y.; Xu, Xisheng; Zhou, Xinmin

    2002-04-01

    Field relations and whole-rock geochemistry indicate that magma mixing has been important in the genesis of the late Mesozoic I-type igneous complexes at Pingtan and Tonglu in SE China. Morphological and trace-element studies of zircon populations in rocks from each of these complexes have defined several distinct growth stages [Mineral. Mag. (2001)]. In-situ LAM-MC-ICPMS microanalysis shows large variations in 176Hf/ 177Hf (up to 15 ɛHf units) between zircons of different growth stages within a single rock, and between zones within single zircon grains (up to 9 ɛHf units). These variations suggest that each of the observed magmas in both complexes developed through hybridisation of ≥2 magmas with different sources. Although this mixing has produced similar Sr and Nd isotopic compositions in the different rock types of each complex, the zircons have functioned as "tape recorders" and have preserved details of the assembly of the different magmas. In the Tonglu complex the most primitive magma is a mafic monzonite (preserved as enclaves), whose isotopic composition suggests derivation from the lower crust; rhyodacites, rhyolites and quartz diorites reflect the mixing of the monzonite with ≥2 more felsic magmas, derived from older crustal materials. In the Pingtan complex, zircons in a quartz diorite enclave suggest mixing between a crustal magma and a more primitive mantle-derived component. Zircons from granites and granodiorite enclaves indicate mixing between the quartz diorite and more felsic melts with lower 176Hf/ 177Hf. Major changes in 176Hf/ 177Hf correlate with discontinuous changes in the trace-element composition and morphology of the zircons, in particular the development of sector zoning that suggests rapid disequilibrium crystallisation. We suggest that the magma mixing recorded by the changes in 176Hf/ 177Hf occurred during transport in magma conduits. The in-situ analysis of Hf-isotopic stratigraphy in zircons is a new and powerful tool for

  9. 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

  10. 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.

  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. 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

  13. Hf-Nd-Pb Isotopes in Lavas and Pyroxenites From Kaula Island Reveal a Depleted Component in the Hawaiian Plume

    NASA Astrophysics Data System (ADS)

    Bizimis, M.; Garcia, M. O.; Norman, M. D.; Salters, V. J.

    2008-05-01

    The presence of depleted material within the otherwise isotopically enriched Hawaiian plume has fundamental implications on the source composition and the scales of heterogeneities and mixing in mantle plumes. A depleted plume component has been recognized in the Pb isotope systematics of Oahu rejuvenated stage lavas (the Honolulu Volcanics, or HV) and in the Hf-Nd isotope compositions of garnet pyroxenite xenoliths from Salt Lake Crater (SLC). Also, peridotite xenoliths from SLC with extreme Hf and Os isotope compositions and up to 2 Ga old Re-depletion ages have been interpreted as fragments of an ancient depleted recycled lithosphere that is part of the plume. Here we present the first combined Hf-Nd-Sr-Pd isotope investigation of phonolite and nephelinite lavas and pyroxenite xenoliths from Kaula Island, Hawaii. The location of Kaula some 300km from Oahu and on a cross-trend relation with the main Hawaiian ridge allows to investigate whether this depleted component has been continuously present in the Hawaiian plume the last ~2-3 Ma, or it is only recognized in Oahu. In terms of major and trace element contents, the Kaula nephelinites and pyroxenites generally overlap the compositions of the HV and SLC pyroxenites on Oahu, respectively. The garnet pyroxenites are interpreted as high pressure cumulates near the base of the lithosphere (70-100km), while the spinel pyroxenites possibly come from shallower depths. In Hf-Nd isotope space both Kaula lavas and pyroxenites (data on cpx separates) plot above the Hawaiian tholeiite trend, with relatively radiogenic Hf isotopesHf = 13.5-21) for a given Nd (ɛNd=7.3-8.3). These compositions do not intersect the Pacific MORB field. The Pb isotopic compositions of the Kaula lavas and pyroxenites, and new Pb data on the SLC pyroxenites (all data collected by high precision MC-ICPMS using the Tl addition technique) extend to lower 206Pb/204Pb (18.09-17.80) and 208Pb/204Pb (37.42-37.76) than previously reported in Hawaiian

  14. 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

  15. 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.

  16. 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

  17. ~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

  18. The composition and distribution of the rejuvenated component across the Hawaiian plume: Hf-Nd-Sr-Pb isotope systematics of Kaula lavas and pyroxenite xenoliths

    NASA Astrophysics Data System (ADS)

    Bizimis, Michael; Salters, Vincent J. M.; Garcia, Michael O.; Norman, Marc D.

    2013-10-01

    Rejuvenated volcanism refers to the reemergence of volcanism after a hiatus of 0.5-2 Ma following the voluminous shield building stage of Hawaiian volcanoes. The composition of the rejuvenated source and its distribution relative to the center of the plume provide important constraints on the origin of rejuvenated volcanism. Near-contemporaneous lavas from the Kaula-Niihau-Kauai ridge and the North Arch volcanic field that are aligned approximately orthogonally to the plume track can constrain the lateral geochemical heterogeneity and distribution of the rejuvenated source across the volcanic chain. Nephelinites, phonolites and pyroxenite xenoliths from Kaula Island have radiogenic Hf, Nd and unradiogenic Sr isotope compositions consistent with a time-integrated depleted mantle source. The pyroxenites and nephelinites extend to the lowest 208Pb/204Pb reported in Hawaiian rocks. These data, along with new Pb isotope data from pyroxenites from the Salt Lake Crater (Oahu) redefine the composition of the depleted end-member of the Hawaiian rejuvenated source at 208Pb/204Pb=37.35±0.05, 206Pb/204Pb = 17.75±0.03, ɛNd = 9-10, ɛHf ˜16-17 and 87Sr/88Sr <0.70305. The revised isotope composition also suggests that this depleted component may contribute to LOA and KEA trend shield stage Hawaiian lavas, consistent with the rejuvenated source being part of the Hawaiian plume and not entrained upper mantle. The isotope systematics of rejuvenated magmas along the Kaula-Niihau-Kauai-North Arch transect are consistent with a larger proportion of the rejuvenated depleted component in the periphery of the plume track rather than along its axis.

  19. Isotopic constraints on the pre-industrial oceanic nitrogen budget

    NASA Astrophysics Data System (ADS)

    Somes, C. J.; Oschlies, A.; Schmittner, A.

    2013-09-01

    The size of the bioavailable (i.e., "fixed") nitrogen inventory in the ocean influences global marine productivity and the biological carbon pump. Despite its importance, the pre-industrial rates for the major source and sink terms of the oceanic fixed nitrogen budget, N2 fixation and denitrification, respectively, are not well known. These processes leave distinguishable imprints on the ratio of stable nitrogen isotopes, δ15N, which can therefore help to infer their patterns and rates. Here we use δ15N observations from the water column and a new database of seafloor measurements to constrain rates of N2 fixation and denitrification predicted by a global three-dimensional Model of Ocean Biogeochemistry and Isotopes (MOBI). Sensitivity experiments were performed to quantify uncertainties associated with the isotope effect of denitrification in the water column and sediments. They show that the level of nitrate utilization in suboxic zones, that is the balance between nitrate consumption by denitrification and nitrate replenishment by circulation and mixing (dilution effect), significantly affects the isotope effect of water column denitrification and thus global mean δ15NO3-. Experiments with lower levels of nitrate utilization within the suboxic zone (i.e., higher residual water column nitrate concentrations, ranging from 20 to 32 μM) require higher ratios of benthic to water column denitrification, BD : WCD = 0.75-1.4, to satisfy the global mean NO3- and δ15NO3- constraints in the modern ocean. This suggests that nitrate utilization in suboxic zones plays an important role in global nitrogen isotope cycling. Increasing the net fractionation factor ϵBD for benthic denitrification (ϵBD = 0-4‰) requires even higher ratios, BD : WCD = 1.4-3.5. The model experiments that best reproduce observed seafloor δ15N support the middle to high-end estimates for the net fractionation factor of benthic denitrification (ϵBD = 2-4

  20. Isotopic constraints on the pre-industrial oceanic nitrogen budget

    NASA Astrophysics Data System (ADS)

    Somes, C. J.; Oschlies, A.; Schmittner, A.

    2013-02-01

    The size of the bio-available (i.e. "fixed") nitrogen inventory in the ocean influences global marine productivity and the biological carbon pump. Despite its importance, the pre-industrial rates for the major source and sink terms of the oceanic fixed nitrogen budget, N2 fixation and denitrification, respectively, are not well known. However, these processes leave distinguishable imprints on the ratio of stable nitrogen isotopes, δ15N, which can therefore help to infer their patterns and rates. Here we use δ15N observations from the water column and a new database of seafloor measurements to constrain rates of N2 fixation and denitrification predicted by a global three-dimensional Model of Ocean Biogeochemistry and Isotopes (MOBI). Sensitivity experiments were performed to quantify uncertainties associated with the isotope effect of denitrification in the water column and sediments. They show that the level of nitrate utilization in suboxic zones, that is the balance between nitrate consumption by denitrification and nitrate replenishment by mixing (dilution effect), significantly affects the isotope effect of water column denitrification and thus global mean δ15NO3-. Experiments with lower levels of nitrate utilization within the suboxic zone (i.e. higher residual water column nitrate concentrations, ranging from 20-32 μM) require higher ratios of benthic to water column denitrification (BD:WCD = 0.75-1.4, respectively), to satisfy the global mean NO3- and δ15NO3- constraints in the modern ocean. This suggests that nitrate utilization in suboxic zones play an important role in global nitrogen isotope cycling. Increasing the net fractionation factor for benthic denitrification (ϵBD = 0-4‰) requires even higher ratios of benthic to water column denitrification (BD:WCD = 1.4-3.5, respectively). The model experiments that best reproduce observed seafloor δ15N support the middle to high-end estimates for the net fractionation factor of benthic

  1. Protocrustal evolution of the Nuvvuagittuq Supracrustal Belt as determined by high precision zircon Lu-Hf and U-Pb isotope data

    NASA Astrophysics Data System (ADS)

    Augland, Lars Eivind; David, Jean

    2015-10-01

    The Nuvvuagittuq Supracrustal Belt (NSB) in northern Quebec, Canada, represents one of the oldest known crustal fragments preserved in the Earth's crust. Its age has, however, been disputed and different authors present crustal formation ages varying from ca. 3.8 Ga to 4.4 Ga (e.g. O'Neil et al., 2012; Guitreau et al., 2013). Here we report new high precision U-Pb geochronological and coupled Lu-Hf isotope data from zircons that reveal the age of the NSB and provide new constraints on the source rocks to this piece of early crust. Two rocks have been analysed, a felsic schist, interpreted to represent a volcanite, and a mylonitic tonalite that is intrusive into the NSB. The felsic schist was emplaced at 3771 + 5 / - 3 Ma, dating the formation of the NSB. The Lu-Hf model age indicates that the parental melt to the felsic schist was extracted from the mantle at this age or within a few tens of million years, but no more than 300 m.y., before its extrusion. This mantle extraction age provides an absolute maximum age for the NSB, but the most probable age of its protocrust is ca. 3.8 Ga. The mylonitic tonalite was emplaced at 3667 + 3 / - 1 Ma. Its Lu-Hf isotopic composition reveals the presence of an older crustal component than that of the NSB, requiring that its precursor melt formed from a Hadean source that was older than the NSB. Based on recent models for Archean TTG formation, we propose that this source represents re-melted Hadean hydrated crust that must have been translated below the NSB at ca. 3667 Ma.

  2. Latest Carboniferous closure of the Junggar Ocean constrained by geochemical and zircon U-Pb-Hf isotopic data of granitic gneisses from the Central Tianshan block, 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

    2015-12-01

    Once situated between the Central Tianshan and Junggar terranes during Paleozoic time, the Junggar Ocean was a major southern branch of the Paleo-Asian Ocean. Thus, when and how it was closed are essential in understanding the final assembly of the Central Asian Orogenic Belt. However, the exact closure time of the Junggar Ocean remains unresolved due to the lack of reliable timing of collision-related regional metamorphism. This paper reports whole-rock geochemical and zircon U-Pb and Lu-Hf isotopic data for granitic gneisses from the northern margin of the Central Tianshan block, which can provide crucial constraints on the final closure of the Junggar Ocean. Mineral assemblages and geochemical suggest that the protoliths of the Central Tianshan gneisses are weakly peraluminous high-K calc-alkaline I-type granites, possessing typical subduction-related features such as strong enrichment in LREE and LILE and depletion in HFSE. Negative Eu anomalies (δEu = 0.46-0.81) and highly variable zircon Hf isotope compositions indicate various amounts of residual plagioclase in the source and crustal contamination during magma formation. LA-ICPMS U-Pb dating on magmatic-type zircons (72%), characterized by euhedral to subhedral shapes, concentric oscillatory zoning, high Th/U ratios (0.30-2.05) and large ranges of εHf(t) values (- 3.4 to + 8.7; up to 6 epsilon units in each sample), yields consistent weighted mean 206Pb/238U ages of ca. 325-320 Ma, interpreted as the crystallization ages of the granitic protoliths. Geochemical signatures and dominantly positive zircon εHf(t) values reveal that the protoliths were emplaced in a continental arc setting, pinpointing the development of a late Early to early Late Carboniferous continental arc system on the northern margin of the Central Tianshan block, probably related to the southward subduction of the Junggar oceanic plate. Meanwhile, younger ages at ca. 303-301 Ma were obtained on recrystallized zircon-rims and unzoned

  3. Rapid eclogitisation of the Dabie-Sulu UHP terrane: Constraints from Lu-Hf garnet geochronology

    NASA Astrophysics Data System (ADS)

    Schmidt, Alexander; Weyer, Stefan; Mezger, Klaus; Scherer, Erik E.; Xiao, Yilin; Hoefs, Jochen; Brey, Gerhard P.

    2008-08-01

    The Qinling-Dabie-Sulu orogenic belt in eastern China is one of the largest ultrahigh-pressure (UHP) terranes worldwide. Mineral Sm-Nd- and zircon U-Pb dating has been widely used to reveal the metamorphic history of this collisional orogen. However, the exact timing of the UHP metamorphic event(s) remains controversial and ages ranging from 245 Ma to 220 Ma have been suggested. We present high precision garnet-cpx Lu-Hf ages for six eclogites from the Dabie and Sulu areas. All ages fall in a narrow range between 219.6 and 224.4 Ma. Five samples define a mean age of 223.0 ± 0.9 Ma and one sample yields a slightly younger age of 219.6 ± 1.4 Ma. This very tight age range is particularly remarkable considering the large regional distribution of sample localities (on the order of 100 km at the time of UHP metamorphism) and the wide variety of garnet and eclogite chemical compositions represented. Two samples yield Sm-Nd ages that are indistinguishable from their Lu-Hf ages, albeit with larger uncertainties. The identical ages of eclogites from both the Dabie and the Sulu region emphasize their close genetic relationship and similar metamorphic histories. The Lu-Hf results appear to date a punctuated event of garnet growth. Alternatively, the Lu-Hf garnet ages may represent the onset of rapid, contemporaneous uplift and subsequent cooling. However, trace element zoning of Lu and Hf is still preserved in garnet porphyroblasts, even in those with a homogeneous major element distribution. Thus, complete re-equilibration of the Lu-Hf system during peak-temperature conditions probably did not occur. The garnet forming event can be placed toward the final stage of the UHP metamorphism, in agreement with some published U-Pb zircon ages. A possible trigger for this short-lived and widespread mineral growth episode may have been a fluid that became available at that stage of the metamorphic history. Although HREE-depleted patterns of older zircon grains may indicate the

  4. 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.

  5. Hf Isotopic Variations in Volcanic Rocks From the Caribbean Large Igneous Province and Cocos Ridge (Central East Pacific)

    NASA Astrophysics Data System (ADS)

    Geldmacher, J.; Hanan, B. B.; Blichert-Toft, J.; Hoernle, K. A.; Hauff, F.; Werner, R.; Kerr, A.

    2001-12-01

    The Caribbean Large Igneous Province (CLIP) consists of the Caribbean oceanic plateau and associated magmatic terranes along the Pacific coast of central America and western Colombia and is interpreted to mark the initiation of the Galapagos hotspot 75-95 Ma ago. New 176Hf/177Hf data show: 1) a depleted end-member (176Hf/177Hfin= 0.28324) represented by lavas from DSDP site 152 ( ~75 Ma) in the central Caribbean, lavas from the young Osa Peninsula ( ~62 Ma) and basalts from the Cocos and Carnegie Ridges portions of the Miocene Galapagos hotspot track, and 2) an enriched end-member (176Hf/177Hfin = 0.28298) represented by lavas from DSDP site 151 and by the younger Quepos terrane of Costa Rica ( ~59 Ma). Our results support previous interpretations, based on trace elements and Sr, Nd and Pb isotopic ratios (Hauff et al., 2000) that infer mixing between the regular depleted MORB mantle source, or alternatively a depleted plume component derived from recycled oceanic crust, and an enriched Galapagos plume source. Lavas from the Caribbean Island of Curacao and western Colombia have elevated epsilon Hf for a given epsilon Nd relative to the Nd-Hf mantle array, suggesting influence of a pelagic sediment component. Similar Nd-Hf isotope compositions are observed in lavas from the southern Galapagos island of Floreana (Blichert-Toft and White, 2001). High epsilon Hf relative to the mantle array is also observed in a lava from the Malpelo Ridge, part of the Galapagos hotspot track, ~200 km off the coast of Panama. In addition to the regular depleted MORB and enriched plume source, a crustal source component is required to account for the spatial and temporal isotopic variations in the magmatic products of the Galapagos plume. F. Hauff, K. Hoernle, G. Tilton, D.W. Graham and A.C. Kerr (2000) EPSL 174, p.247-263 J. Blichert-Toft and W.M. White (2001) G-cubed, in press

  6. 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

  7. Os-Hf isotopes of the ultrapotassic rocks in southern Tibet: Significant crustal input into the mantle source region

    NASA Astrophysics Data System (ADS)

    Zhao, Z.; Widom, E.; Meng, Q.; Niu, Y.; Zhu, D.; Mo, X.; Barry, T.

    2010-12-01

    The post-collisional mafic ultrapotassic (ultra-K) magmatism in the western Lhasa terrane, southern Tibet, has been well-studied in the past decade. The model of low-degree partial melting of an enriched subcontinental lithospheric mantle has been proposed to be responsible for the origin of the ultra-K rocks on the basis of trace element and Sr-Nd-Pb-O isotope data (Miller et al., 1999, J. Petrology; Ding et al., 2003, J. Petrology; Gao et al., 2009, J. Petrology; Zhao et al., 2009, Lithos). Here, we report both Os and Hf isotopic data on the same ultra-K samples discussed in Zhao et al. (2009). The rocks have suprachondritic 187Os/188Os (0.1315-0.4828) and sub-chondritic to MORB-like Re/Os ratios (0.019-15.75). In the 187Os/188Os vs. Re/Os plot, the data show a notable trend towards the Himalayan crustal components. The 176Hf/177Hf ratios of 0.282204 to 0.282554 correspond to ɛHf values of -20.1 to -7.9. In the ɛHf vs. ɛNd plot, these mafic ultra-K rocks show strong crustal signatures, similar to, but more enriched than, the lamproites from Spain and Italia (Prelević et al., 2010, Lithos). They also show a linear trend that coincides with the Hf-Nd isotopic array defined by igneous rocks, suggesting significant contributions of some mature continental crustal material in their petrogenesis. The radiogenic Os could be interpreted as resulting from crustal assimilation, but more likely reflects increasing contributions from metasomatic components (Schaefer et al., 2000, Geology). Taking together all the trace element data, Os-Hf-Nd data and petrological data, we suggest that the petrogenesis of the mantle xenolith-bearing ultra-K rocks in southern Tibet involves melting of enriched upper mantle whose enrichments may be genetically associated with metasomatism by a melt component derived from subducted Himalayan continental crustal materials.

  8. 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

  9. Peak Last Glacial weathering intensity on the North American continent recorded by the authigenic Hf isotope composition of North Atlantic deep-sea sediments

    NASA Astrophysics Data System (ADS)

    Gutjahr, Marcus; Frank, Martin; Lippold, Jörg; Halliday, Alex N.

    2014-09-01

    We have retrieved radiogenic hafnium (Hf) isotope compositions (ɛHf) from authigenic Fe-Mn oxyhydroxides of deep northwest Atlantic sediments deposited over the past 26 ka to investigate the oceanic evidence of changes in dissolved weathering inputs from NE America during the last deglaciation. The extraction of seawater-derived Hf isotopic compositions from Fe-Mn oxyhydroxides is not a standard procedure. Comparisons between the Al/Hf ratios and Hf isotopic compositions of the chemically extracted authigenic phase on the one hand, and those of the corresponding detrital fractions on the other, provide evidence that the composition of past seawater has been reliably obtained for most sampled depths with our leaching procedures. This is endorsed most strongly by data for a sediment core from 4250 m water depth at the deeper Blake Ridge, for which consistent replicates were produced throughout. The Hf isotopic composition of the most recent sample in this core also closely matches that of nearby present day central North Atlantic seawater. Comparison with previously published seawater Nd and Pb isotope compositions obtained on the same cores shows that both Hf and Pb were released incongruently during incipient chemical weathering, but responded differently to the deglacial retreat of the Laurentide Ice Sheet. Hafnium was released more congruently during peak glacial conditions of the Last Glacial Maximum (LGM) and changed to typical incongruent interglacial ɛHf signatures either during or shortly after the LGM. This indicates that some zircon-derived Hf was released to seawater during the LGM. Conversely, there is no clear evidence for an increase in the influence of weathering of Lu-rich mineral phases during deglaciation, possibly since relatively unradiogenic Hf contributions from feldspar weathering were superimposed. While the authigenic Pb isotope signal in the same marine sediment samples traced peak chemical weathering rates on continental North America

  10. Fast neutron capture on the Hf isotopes: Cross sections, isomer production, and stellar aspects

    SciTech Connect

    Wisshak, K.; Voss, F.; Kaeppeler, F.; Kazakov, L.; Becvar, F.; Krticka, M.; Gallino, R.; Pignatari, M.

    2006-04-15

    The (n,{gamma}) cross sections of {sup 176}Hf, {sup 177}Hf, {sup 178}Hf, {sup 179}Hf, and {sup 180}Hf have been measured in the energy range from 3 to 225 keV relative to the gold standard. Neutrons were produced via the {sup 7}Li(p,n){sup 7}Be reaction and capture events were registered with the Karlsruhe 4{pi} barium fluoride detector. The overall uncertainties are between 0.9 and 2.6%, about 5 times smaller than in previous experiments. Partial cross sections to ground and isomeric states could be experimentally identified for neutron capture on {sup 176,177,178,179}Hf, indicating a strong population of yet-unknown isomeric states in {sup 177}Hf and {sup 180}Hf. This feature was confirmed by extensive GEANT simulations based on calculated capture cascades. The deduced Maxwellian-averaged (n,{gamma}) cross sections for thermal energies between kT=8 and 100 keV contribute to the analysis of the s-process branchings at A=176 and A=179/180 and have significant consequences for the separation of the solar s- and r-process components.

  11. 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.

  12. 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

  13. 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

  14. Abrupt Change in Zircon Hf Isotopic Compositions at ~420 Ma: Implications for Early Paleozoic Ridge Subduction in the Chinese Altai

    NASA Astrophysics Data System (ADS)

    Sun, M.; Yuan, C.; Long, X.; Cai, K.; Jiang, Y.; Wong, K.; Xiao, W.; Zhao, G.; Wu, F.

    2009-05-01

    Zircon minerals were separated from granitoids, sedimentary rocks, and gneisses from the Chinese Altai. Those with oscillatory zoning and high Th/U ratios are interpreted to have an igneous origin, and were analyzed for their U-Pb and Hf isotopic compositions. These zircons yielded U-Pb ages from 280 to 2800 MaCindicating a long evolutionary history of magmnatic activity in the region. Zircon Hf isotopic compositions show an abrupt change at ~420 Ma, indicating magma sources of both ancient and juvenile materials prior to 420 Ma, but juvenile materials were predominant in the magma sources after 420 Ma. This may imply a large amount of juvenile materials were added to the lithosphere at ~420 Ma and significantly modified the composition of the lithosphere of the Chinese Altai. We use a ridge subduction model to explain such a dramatic change, which can also explain the emplacement of the huge amount of coeval granitic intrusions with depleted isotopic characteristics, the basaltic rocks with complicated chemical compositions, the association of adakite-high Mg andesite-boninite-High Nb basalt, and the high T regional metamorphism. This study was supported by Research Grant Council of Hong Kong (HKU704307P, HKU7040/04P), National Basic Research Program of China (2007CB411308), and the University of Hong Kong.

  15. 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

  16. Petrogenesis of Late Permian sodic metagranitoids in southeastern Korea: SHRIMP zircon geochronology and elemental and Nd-Hf isotope geochemistry

    NASA Astrophysics Data System (ADS)

    Cheong, Chang-sik; Kim, Namhoon; Kim, Jeongmin; Yi, Keewook; Jeong, Youn-Joong; Park, Chan-Soo; Li, Huai-kun; Cho, Moonsup

    2014-12-01

    One of the striking tectonomagmatic features recently found in southeastern Korea is the occurrence of ca. 250 Ma high-silica adakite. Sodic metagranitoids mainly consisting of tonalitic-trondhjemitic-granodioritic gneisses occur in the Andong-Cheongsong area adjacent to the Yeongdeok adakite site. To investigate temporal and petrogenetic relationships of these orthogneisses with the adakite, we conducted SHRIMP zircon U-Pb dating as well as elemental and Nd-Hf isotopic analyses. Zircon core ages of the orthogneisses (ca. 262-251 Ma) confirm the widespread occurrence of arc-related Late Permian magmatism in southeastern Korea. The Late Triassic (ca. 230 Ma) zircon overgrowths reflect a thermal overprint probably related to the initiation of another subduction system. The analyzed orthogneisses have major element compositions comparable to the Phanerozoic adakites and Archean TTG suite, such as high SiO2 (58.7-65.5 wt.%) and Al2O3 (17.1-19.1 wt.%) contents and Na2O/K2O ratios (1.83-4.95). However, their moderate Sr/Y (35-43) and La/Yb (14-53) ratios and negative Eu anomalies (Eu/Eu* = 0.75-0.95) are incompatible with the key features reported from the Yeongdeok adakite. Moreover, initial whole-rock εNd (-7.9 to -3.3) and zircon εHf (-0.3 ± 2.4) values of the orthogneisses negate a direct derivation from the subducted slab. Our elemental and Nd-Hf isotopic data collectively suggest that the protoliths of the tonalitic-trondhjemitic-granodioritic gneisses were generated by partial melting of mafic lower crust at depths shallower than the garnet stability field. Our Nd and Hf model ages of the gneisses, together with those previously reported from the Mesozoic granitoids indicate a selective involvement of young source materials along the margin of the Yeongnam massif. The Hf isotopic compositions of zircons from a trondhjemitic gneiss attest to the involvement of primitive melts during their crystallization. The ridge subduction and consequent development of a slab

  17. 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.

  18. 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.

  19. Isotopic Constraints on the Origin of Lunar Ferroan Anorthosites

    NASA Technical Reports Server (NTRS)

    Borg, L. E.; Shearer, C. K.; Nyquist, L. E.; Norman, M. D.

    2002-01-01

    Ferroan anorthosites have whole rock Nd isotopic compositions that are too radiogenic to have crystallized from a chondritic magma ocean. Mechanisms to disturb the whole rock Sm-Nd isotopic compositions are explored. Additional information is contained in the original extended abstract.

  20. Chemical and isotopic heterogeneity in the Yellowstone magma reservoir revealed through sub-crystal-scale zircon age, trace-element, and Hf-isotopic analyses

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

    The Yellowstone Plateau (USA) hosts one of the largest Quaternary magmatic systems in the world, with caldera forming eruptions at ~2.1 Ma, ~1.3 Ma, and ~0.64 Ma, as well as numerous intracaldera and extracaldera eruptions between caldera-forming events. The most recent eruptive episode at Yellowstone caldera produced the intracaldera Central Plateau Member (CPM) of the Plateau Rhyolite, which erupted intermittently between ~170-70 ka with a cumulative volume ≥600 km3, approaching the volume of rhyolite erupted during the caldera-forming eruptions. Thus, the CPM rhyolites provide snapshots of a large silicic magmatic system though time. In this study we examine the degree of compositional heterogeneity in the Yellowstone magma reservoir by comparing SHRIMP age, SHRIMP trace element, and LA-MC-ICPMS Hf-isotopic data for zircons hosted in five CPM rhyolites. The CPM rhyolites included in this study are: 1) the Solfatara Plateau flow (SPF) and Hayden Valley flow (HVF), which erupted ca. 103 ka from the east side of Yellowstone caldera, 2) the West Yellowstone flow (WYF), which erupted ca. 114 ka from the west side of the caldera, and 3) the Pitchstone Plateau flow (PPF) and Grants Pass flow (GPF), which erupted from the west side of the caldera ca. 75 ka. Linking the age, trace-element, and Hf-isotopic compositions of zones within individual zircons provides a robust method for recognizing distinct crystal populations within the CPM rhyolites and tracking the chemical evolution of the magma reservoir through time. Comparing crystal populations in coeval rhyolites erupted from different parts of the caldera allows for assessment of whether the entire reservoir hosts similar crystal populations at a given time and thus provides insight into the degree of compositional heterogeneity within the magma reservoir. Zircons hosted in the SPF, HVF, and WYF have cores with ages that vary from near eruption age to 200 ka and display a wide range in trace-element and Hf-isotopic

  1. 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.

  2. 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.

  3. 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.

  4. Isotopic constraints on atmospheric moisture transport processes in South America

    NASA Astrophysics Data System (ADS)

    Samuels-Crow, Kimberly E.

    The water cycle in the tropics and subtropics exerts a strong influence on Earth's climate. Isotopic ratios in modern water vapor can provide us with insights into low-latitude moisture-transport processes in the modern climate system, recorded in paleoclimate proxy records, and in general circulation models that we rely on to understand how our climate is changing. Advances in satellite and groundbased instruments in the past decade have improved measurements of isotopes in water vapor. In this study, I focus on satellite (chapters 2 and 3) and ground-based (chapters 2 and 4) measurements of isotopes in atmospheric water vapor to evaluate processes responsible for moisture transport in tropical and subtropical South America. Satellite-measured hydrogen isotope ratios (deltaD), mixing ratios (q), and outgoing longwave radiation (OLR) show that upwind convective intensity controls the seasonal variability in isotopic ratios in tropical Andean water vapor and leads to lower isotopic ratios than predicted by equilibrium isotope fractionation models (i.e. DdeltaD = deltaDmeasured - deltaD Rayleigh < 0 ‰) (chapter 2). Deep convection in the South American Summer Monsoon domain in austral summer and in the Inter-tropical Convergence Zone in austral winter leads to zones where DdeltaD is negative from the Lifted Condensation Level through the mid- to upper-troposphere and possibly above the Level of Neutral Buoyancy (chapter 3). In subtropical South America, nearly continuous measurements of isotopic ratios, mixing ratios, and deuterium-excess (i.e. d-excess = deltaD - (8*delta 18O)) indicate that condensation under ice supersaturated conditions and mixing with moister air play important roles in controlling moisture transport to the hyperarid Chajnantor Plateau.

  5. 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

  6. 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

  7. 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.

  8. Cryptic lower crustal signature in the source of the Ontong Java Plateau revealed by Os and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Tejada, M. L. G.; Suzuki, K.; Hanyu, T.; Mahoney, J. J.; Ishikawa, A.; Tatsumi, Y.; Chang, Q.; Nakai, S.

    2013-09-01

    Previous studies of Ontong Java Plateau (OJP) basalts reveal that they are products of 20-30% degree of melting and possess ocean-island-like Pb, Nd, and Sr isotopic signatures that fall within a limited range yet define two distinct isotopic groups represented by the Kroenke- and Kwaimbaita-type basalts and by the Singgalo-type basalts. The origin of the two groups is not clear, although the Kroenke-Kwaimbaita-type signature has been suggested to represent the plateau's main mantle source, probably originating from the lower mantle. In this study, samples from central Malaita, Solomon Islands and Ocean Drilling Program (ODP) Sites 807, 1185B, and 1187 were analyzed for Os and Hf isotopic composition to further investigate the origin of the bimodal isotopic composition of OJP basalts. The Kroenke-type basalts, which are parental to Kwaimbaita-type basalts, show the least effect of post-emplacement alteration on their Os isotopic composition and have ɛHf(t)=+10.8 to +13.3, Os = 118-174 ppt, Re = 161-1111 ppt, and near-chondritic (=0.1322±0.0029 [γOs(t)=2.7±2.2 and 4.3±2.3 (n=6; 1σ) relative to primitive upper mantle (PUM) and average chondrite values, respectively], suggesting a near-primitive mantle source for the OJP. Up to 25% assimilation of altered Jurassic Pacific MORB crust into fractionating Kroenke-type magma could explain the lower, 11-51 ppt, Os contents and more radiogenic, (=0.1395±0.0020 [γOs(t)=8.4±1.5 and 10.0±1.5 (n=3; 1σ)], composition of some Kwaimbaita-type basalts. These compositions are markedly different from those of the least altered Singgalo-type basalts with ɛHf(t)=+10.2 to +11.5 and (=0.3301±0.0175 [γOs(t)=156±14 and 160±14 (n=8, 1σ)] and low, 21-37 ppt, Os contents that are lower oceanic crust-like. Despite the scatter in the Re-Os isotope data, meaningful isochron ages and initial values were obtained. These ages are 121.4±4.6 Ma for Malaitan Kwaimbaita Fm. basalts and 123±24 Ma for Singgalo-type basalts, with initial

  9. 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

  10. A linear Hf isotope-age array despite different granitoid sources and complex Archean geodynamics: Example from the Pietersburg block (South Africa)

    NASA Astrophysics Data System (ADS)

    Laurent, Oscar; Zeh, Armin

    2015-11-01

    Combined U-Pb and Lu-Hf isotope data from zircon populations are widely used to constrain Hadean-Archean crustal evolution. Linear Hf isotope-age arrays are interpreted to reflect the protracted, internal reworking of crust derived from the (depleted) mantle during a short-lived magmatic event, and related 176Lu/177Hf ratios are used to constrain the composition of the reworked crustal reservoir. Results of this study, however, indicate that Hf isotope-age arrays can also result from complex geodynamic processes and crust-mantle interactions, as shown by U-Pb and Lu-Hf isotope analyses of zircons from well characterized granitoids of the Pietersburg Block (PB), northern Kaapvaal Craton (South Africa). Apart from scarce remnants of Paleoarchean crust, most granitoids of the PB with ages between 2.94 and 2.05 Ga (n = 32) define a straight Hf isotope-age array with low 176Lu/177Hf of 0.0022, although they show a wide compositional range, were derived from various sources and emplaced successively in different geodynamic settings. The crustal evolution occurred in five stages: (I) predominately mafic crust formation in an intra-oceanic environment (3.4-3.0 Ga); (II) voluminous TTG crust formation in an early accretionary orogen (3.0-2.92 Ga); (III) internal TTG crust reworking and subduction of TTG-derived sediments in an Andean-type setting (2.89-2.75 Ga); (IV) (post-)collisional high-K magmatism from both mantle and crustal sources (2.71-2.67 Ga); and (V) alkaline magmatism in an intra-cratonic environment (2.05-2.03 Ga). The inferred array results from voluminous TTG crust formation during stage II, and involvement of this crust during all subsequent stages by two different processes: (i) internal crust reworking through both partial melting and assimilation at 2.89-2.75 Ga, leading to the formation of biotite granites coeval with minor TTGs, and (ii) subduction of TTG-derived sediments underneath the PB, causing enrichment of the mantle that subsequently became

  11. 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.

  12. Isotopic Constraints on the Petrology of Martian Meteorites

    NASA Technical Reports Server (NTRS)

    Jones, J. H.

    2005-01-01

    The SNC (martian) meteorites exhibit complex isotopic characteristics that yield information both about the ages of individual meteorites as well as information about the petrogenetic processes that produced both individual samples and about the origins of suites and sub-suites within the SNC clan. Here I review these data, reiterate earlier interpretations, and offer some new conclusions.

  13. 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

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

    NASA Astrophysics Data System (ADS)

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

    2016-03-01

    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.

  15. 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

  16. 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

  17. 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

  18. Zircon geochronology and Hf-O isotope geochemistry from granites in the Iapetus Suture Zone in Ireland and the Isle of Man

    NASA Astrophysics Data System (ADS)

    Fritschle, Tobias; Daly, J. Stephen; Whitehouse, Martin J.; McConnell, Brian; Buhre, Stephan

    2014-05-01

    Late Caledonian syn- to post-orogenic granites located in the Iapetus Suture Zone (ISZ) in Ireland and Britain have been related to A-type subduction and possible slab breakoff [1] following the Laurentia-Avalonian collision. Lack of reliable age data (especially in Ireland) has inhibited petrogenetic investigations of these rocks. Hence, ion microprobe U-Pb and oxygen isotope analyses as well as LA-MC-ICPMS Lu-Hf isotopic measurements on zircons from Irish and Isle of Man granites have been undertaken to provide better constraints on this enigmatic episode of the Caledonian Orogeny. Four stages of Late Caledonian granitic magmatism (c. 435, 417, 410 and 394 Ma) are indicated by U-Pb dating of oscillatory-zoned magmatic zircons. The Crossdoney, Kentstown, Drogheda and Ballynamuddagh granites together with a rhyolite from Glenamaddy have yielded U-Pb concordia ages, interpreted as intrusion-ages, between 419.9 ± 4.3 Ma (Glenamaddy) and 415.8 ± 2.0 Ma (Crossdoney) with a weighted average of 417.5 ± 0.9 Ma (MSWD = 1.3). The Glenamaddy Granite - which intruded the Glenamaddy Rhyolite - yielded an age of 410 ± 2.1 Ma. In addition, the Rockabill Granite yielded a younger age of 393.9 ± 1.9 Ma, whereas the Carnsore Granite yielded an older age of 434.6 ± 1.9 Ma. Inherited zircons (487 to 453 Ma) occur in several of the granites, and are interpreted to have been derived from Ordovician arc magmatic rocks accreted within the ISZ. A younger group of c. 440 Ma inherited zircons occurs in the c. 417 Ma Crossdoney and Ballynamuddagh granites. These grains could be related to continued or renewed Silurian arc magmatism. Hf-O isotopic measurements on the dated zircon grains range between -2 and +7 ɛHfi units and 5.5 to 8.5 o δ18O. These are interpreted to indicate the contribution of juvenile mantle melts - possibly derived from the Ordovician arc - to some of the granites. Significant heterogeneities in zircon oxygen isotopes in at least four of the granites further

  19. Linking the Indochina block and Gondwana during the Early Paleozoic: Evidence from U-Pb ages and Hf isotopes of detrital zircons

    NASA Astrophysics Data System (ADS)

    Usuki, Tadashi; Lan, Ching-Ying; Wang, Kuo-Lung; Chiu, Han-Yi

    2013-02-01

    To constrain the paleoposition of Indochina within Gondwana during the Early Paleozoic, we performed in-situ U-Pb and Hf isotope analyses on detrital zircons from three river sediment samples in the Truong Son Belt of the Indochina block. The age distributions yield dominant Neoarchean (~ 2.5 Ga), Mesoproterozoic (1.7-1.4 Ga), Grenvillian (~ 0.95 Ga), and Pan-African (0.65-0.5 Ga) age groups and minor Paleo- to Meso-archean zircons. Hf isotope compositions of zircons for each age group exhibit large ranges of ɛHf(T), suggesting that the zircon host rocks have diverse sources. The oldest Hf model ages for zircons of Neoarchean, Grenvillian, and Pan-African age group yield ~ 3.7 Ga or older, while those of Mesoproterozoic age group show ~ 3.3 Ga. The remarkable similarity of age distribution and Hf isotope compositions among detrital zircons of Indochina and those of Tethyan Himalaya, western Cathaysia, and Qiangtang suggests that Indochina was located outboard of Qiangtang and south of South China in the Indian margin of Gondwana during the Early Paleozoic. Our results are consistent with the paleontological correlations of east Gondwana margin during the Early Paleozoic.

  20. Oxygen isotopic constraints on the composition of the moon

    NASA Technical Reports Server (NTRS)

    Grossman, L.; Clayton, R. N.; Mayeda, T. K.

    1974-01-01

    The mean oxygen isotopic composition of 5 Apollo 17 soils, one Apollo 17 breccia and one Apollo 12 soil is delta O-18 = 5.63 + or - .05 and delta O-17 = 3.8 + or - .2%. These values are within several tenths of a part permil of the composition of a large fraction of the lunar interior. High-temperature condensate aggregates from Allende and other C2 and C3 chondrites are vastly enriched in O-16 compared to this composition. The moon cannot be a mixture of ordinary chondrites and Allende inclusions, nor can it be derived from such a mixture by chemical fractionation processes. The moon's isotopic composition is consistent with a mixture of high- and low-temperature condensates but the refractory fraction would have to be free of the O-16-rich component so prevalent in the meteoritic aggregates, a fact which makes such models less attractive than they once seemed.

  1. 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

  2. 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.

  3. 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.

  4. 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.

  5. Constraints on neon and argon isotopic fractionation in solar wind.

    PubMed

    Meshik, Alex; Mabry, Jennifer; Hohenberg, Charles; Marrocchi, Yves; Pravdivtseva, Olga; Burnett, Donald; Olinger, Chad; Wiens, Roger; Reisenfeld, Dan; Allton, Judith; McNamara, Karen; Stansbery, Eileen; Jurewicz, Amy J G

    2007-10-19

    To evaluate the isotopic composition of the solar nebula from which the planets formed, the relation between isotopes measured in the solar wind and on the Sun's surface needs to be known. The Genesis Discovery mission returned independent samples of three types of solar wind produced by different solar processes that provide a check on possible isotopic variations, or fractionation, between the solar-wind and solar-surface material. At a high level of precision, we observed no significant inter-regime differences in 20Ne/22Ne or 36Ar/38Ar values. For 20Ne/22Ne, the difference between low- and high-speed wind components is 0.24 +/- 0.37%; for 36Ar/38Ar, it is 0.11 +/- 0.26%. Our measured 36Ar/38Ar ratio in the solar wind of 5.501 +/- 0.005 is 3.42 +/- 0.09% higher than that of the terrestrial atmosphere, which may reflect atmospheric losses early in Earth's history. PMID:17947578

  6. 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

  7. 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.

  8. 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.

  9. Zircon trace element, and O and Hf isotopic records of magma sources and pluton assembly in the Sierra Crest intrusions (Sierra Nevada batholith, USA)

    NASA Astrophysics Data System (ADS)

    Miller, J. S.; Lackey, J. S.; Davies, G. R.; Sendek, C.

    2014-12-01

    The Sierra Crest Intrusions of the Sierra Nevada Batholith are the last major magmatic pulse associated with the Cretaceous flare-up. They are characterized by long assembly times (several 106 years), and are normally zoned from marginal, horblende-biotite granodiorites to more felsic, K-feldspar megacrystic, biotite granodiorites. Combined trace element and O and Hf isotopes on zircon are presented from the major Sierra Crest Intrusions. Zircon saturation temperatures (TZrc,sat) are similar and low (ca. 700°C) for most of the individual units, but Ti-in-zircon temperatures (TZrn,Ti) and trace element ratios contrast strongly between outer marginal units and inner megacrystic units (low TZrn,Ti ≈ TZrc,sat, high Yb/Gd, low Th/U, high and similar Hf, and high Eu/Eu*). Zircon O and Hf isotopes vary markedly across the suite (ΔɛHf = 15; Δδ18O = 2.5‰). Individual intrusive suites (gabbro to high-silica granite) record variable O-Hf variations; no correlation (John Muir), subtle binary or ternary arrays (e.g., Whitney, Sonora), or bimodal distribution of values (Tuolumne). In some cases single hand samples (small-volume mafic or felsic units), may record the entire variability within a suite. Inner megacrystic units generally have lower ɛHf than outer marginal units. Whole rock geochemical data for the intrusive suites also show an increase in the "garnet signature" with time (higher Sr/Y and Dy/Yb). The isotopic data are consistent with variable mantle sources and progressively cooler, more water-rich magmatism with a simultaneous shift to greater crustal involvement, and deepening of the magma sources. Magmatic underplating and intraplating of mafic arc magmas produced increasing crustal assimilation but under PT conditions that allowed production of more felsic, zircon-saturated, magmas. The isotopic variability requires that plutons are amalgams of many magmas mixed at varying scales before final solidification.

  10. 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

  11. 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

  12. 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.

  13. Aluminum depletion in komatiites and garnet fractionation in the early Archean mantle: Hafnium isotopic constraints

    SciTech Connect

    Gruau, G. Universite de Rennes ); Chauvel, C.; Arndt, N.T. ); Cornichet, J. )

    1990-11-01

    Hafnium isotopic compositions were measured in Al-depleted and Al-enriched komatiites from the 3,450 Ma old Barberton greenstone belt, southern Africa. All samples have initial {var epsilon}{sub Hf} values close to zero. Such values are at variance with the strongly negative or positive values that should be observed if these rocks came from old garnet-depleted or garnet-enriched layers, such as may have formed during the solidification of an ancient terrestrial magma ocean. The garnet fractionation observed in komatiites probably took place during the melting event.

  14. 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.

  15. Isotope Dilution and LA ICPMS Study of Trace Elements in Garnets: Implications for Sm-Nd and Lu-Hf Dating.

    NASA Astrophysics Data System (ADS)

    Anczkiewicz, R.; Platt, J. P.; Thirlwall, M. F.; Alard, O.

    2003-12-01

    One of the main advantages of garnet geochronology is a possibility of establishing a direct link between isotopic ages and PT conditions. However, both Lu-Hf and Sm-Nd garnet dating can strongly be affected by submicroscopic inclusions capable of dominating the Sm-Nd and Lu-Hf budget. We investigated possible effects of various inclusions on Sm-Nd and Lu-Hf systems by combined isotope dilution and LA ICPMS studies. Internal isochrons obtained for 6 high-grade blocks of metabasites from the Franciscan complex yielded highly precise Lu-Hf ages ranging from 114 to 170 Ma, but failed to provide Sm-Nd dates. The main reason for failure of the Sm-Nd dating was a rather large amount of matrix silicate inclusions, which contained > 100 times more Nd than analyzed "impure" garnet fractions. The same inclusions had very limited influence on the Lu-Hf budget. This is mainly due to strong enrichment of garnets in heavy REE and due to much lower Hf concentrations in rock forming silicates. The 176Lu/177Hf ratios obtained for the analyzed garnets are typically between 1.5 and 8, but for two samples with spessartine-rich garnets, values range between 21 and 28 and are the highest yet reported. Small amount of zircon and rutile inclusions did not notably suppress 176Lu/177Hf ratios. This is mainly due to their small size (<10 μ m) and low abundance, but also because of their limited dissolution during sample digestion on a hotplate. Our results demonstrate that even samples with significant amounts of inclusions can yield precise and accurate Lu-Hf dates. For upper amphibolite facies metapelites from Vietnam, we obtained well defined Sm-Nd ages of 52.1+/-2.5 and 31.4+/-1.0 Ma, which are grossly discordant with the corresponding Lu-Hf dates of 84.1+/-1.6 Ma and 77.9+/-1.6 Ma, respectively. We therefore conducted LA ICP MS measurements on 4 selected samples. Sm/Nd ratios show rather flat zonation profiles throughout most of the crystal, with an up to 30% increase in the rims. The

  16. 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

  17. Boron isotopic constraints on the source of Hawaiian shield lavas

    NASA Astrophysics Data System (ADS)

    Tanaka, Ryoji; Nakamura, Eizo

    2005-07-01

    Boron isotopic compositions of lavas from three representative Hawaiian shield volcanoes (Kilauea, Mauna Loa, and Koolau) were analyzed by thermal ionization mass spectrometry. The boron isotopic composition of each sample was analyzed twice, once with and once without acid leaching to evaluate the effect of posteruptive boron contamination. Our acid-leaching procedure dissolved glass, olivine, secondary zeolite, and adsorbed boron; this dissolved boron was completely removed from the residue, which was comprised of plagioclase, pyroxenes, and newly formed amorphous silica. We confirmed that an appropriate acid-leaching process can eliminate adsorbed and incorporated boron contamination from all submarine samples without modifying the original 11B/ 10B ratio. On the other hand, when the sample was weathered, i.e., the olivine had an iddingsite rim, 11B/ 10B of the acid-resistant minerals are also modified, thus it is impossible to get the preeruptive 11B/ 10B value from the weathered samples. Through this elimination and evaluation procedure of posteruptive contamination, preeruptive δ 11B values for the shield lavas are -4.5 to -5.4‰ for Koolau ( N = 8), -3.6 to -4.6‰ for Kilauea ( N = 11), and -3.0 to -3.8‰ for Mauna Loa ( N = 6). Historical Kilauea lavas show a systematic temporal trend for B content and Nb/B coupled with other radiogenic isotopic ratios and trace element ratios, at constant δ 11B, indicating little or no assimilation of crustal materials in these lavas. Uncorrelated B content and δ 11B in Koolau and Mauna Loa lavas may also indicate little or no effect of crustal assimilation in these lavas. The source of KEA-component (identical to the so-called Kea end member in Hawaiian lavas) of the Hawaiian source mantle, represented by Kilauea, should be derived from lower part of subducted oceanic crust or refractory peridotite in the recycled subducted slab. The systematic trend from Kilauea to Koolau—decreasing δ 11B coupled with decreasing

  18. 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

  19. 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.

  20. 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

  1. 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

  2. Boninite petrogenesis: Chemical and Nd-Sr isotopic constraints

    NASA Astrophysics Data System (ADS)

    Cameron, W. E.; McCulloch, M. T.; Walker, D. A.

    1983-10-01

    Major and trace element abundances and Nd and Sr isotopic compositions have been determined for boninites from the Bonin Islands, Cape Vogel (Papua New Guinea) and New Caledonia and for similar lavas from Cyprus and New Zealand. SiO 2, CaO, incompatible element abundances and ɛ Nd values are shown to vary systematically with the degree of light rare earth element (LREE) enrichment. La N/Yb N varies from 5.4 to 0.3. The range of ɛ Nd values is from +8.0 to +1.9. These features are interpreted in terms of mixing of two components: a LREE-depleted magma, derived by partial melting of an already-depleted peridotite source, and a hydrous, LREE-enriched fluid. If La for the fluid is taken as 100 × chondrites, REE abundances observed in boninites can be reproduced by 1-15% addition of the LREE-enriched component. Using a LREE-depleted component inferred from the Cyprus samples (La N/Yb N=0.10), the LREE-enriched component for Tertiary boninites has been calculated to have a La N/Yb N˜20. Th9+-e LREE-depleted component contains most of the CaO, Sc, HREE and TiO 2, and has an ɛ Nd of ⩾ +8. The enriched component contains H 2O and most of the Zr, Nb and LREE, and has an ɛ Nd ranging from +6 to at least -5. Origins for the LREE-enriched fluid include derivation from mantle sources similar to those proposed for ocean islands or, more likely, from subducted oceanic or continental crust in island-arc settings.

  3. Constraints on the formation and diagenesis of phosphorites using carbonate clumped isotopes

    NASA Astrophysics Data System (ADS)

    Stolper, Daniel A.; Eiler, John M.

    2016-05-01

    The isotopic composition of apatites from sedimentary phosphorite deposits has been used previously to reconstruct ancient conditions on the surface of the Earth. However, questions remain as to whether these minerals retain their original isotopic composition or are modified during burial and lithification. To better understand how apatites in phosphorites form and are diagenetically modified, we present new isotopic measurements of δ18O values and clumped-isotope-based (Δ47) temperatures of carbonate groups in apatites from phosphorites from the past 265 million years. We compare these measurements to previously measured δ18O values of phosphate groups from the same apatites. These results indicate that the isotopic composition of many of the apatites do not record environmental conditions during formation but instead diagenetic conditions. To understand these results, we construct a model that describes the consequences of diagenetic modification of phosphorites as functions of the environmental conditions (i.e., temperature and δ18O values of the fluids) during initial precipitation and subsequent diagenesis. This model captures the basic features of the dataset and indicates that clumped-isotope-based temperatures provide additional quantitative constraints on both the formational environment of the apatites and subsequent diagenetic modification. Importantly, the combination of the model with the data indicates that the δ18O values and clumped-isotope temperatures recorded by phosphorites do not record either formation or diagenetic temperatures, but rather represent an integrated history that includes both the formation and diagenetic modification of the apatites.

  4. 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

  5. Pb and Hf isotope variations along the Southeast Indian Ridge and the dynamic distribution of MORB source domains in the upper mantle

    NASA Astrophysics Data System (ADS)

    Hanan, Barry B.; Blichert-Toft, Janne; Hemond, Christophe; Sayit, Kaan; Agranier, Arnaud; Graham, David W.; Albarède, Francis

    2013-08-01

    Lead and Hf isotope data for a suite of closely spaced (5-10 km) basalt glasses, sampled over a distance of ∼1140 km along the Southeast Indian Ridge (SEIR) between 88°E and 100°E, confirm the presence of ancient heterogeneous mantle beneath the Indian Ocean. The Pb isotopes show a non-Gaussian, "fat tail" distribution that is bimodal. The paired Hf and Pb isotope data, combined with previously published data, define two populations that reveal the presence of compositional streaks within the upper mantle. The number and density of the streaks follow a Poisson distribution where the characteristic streak thickness is ∼20 km. The mantle 'unit' giving rise to each MORB sample represents a 'mixture of mixtures' with a multi-stage mixing history. There is no correlation of K/Ti or Na8 with Pb or Hf isotope ratios, and both domains encompass both N- and E-MORB sources, ruling out two different regimes of melting in a homogeneous source. Apparently, mantle convection has folded together distinct composite reservoirs of heterogeneous mantle, and stretched them into streaks that remain coherent and discernable units. The Hf and Pb isotope data reveal the presence of two distinct DUPAL mantle components in the upper mantle. The isotope signature of component 1 resembles a continental lithosphere source having Pb isotope composition similar to Karoo picrites (erupted at ∼180 Ma). This component likely originated by mixing of Kaapvaal cratonic lithosphere and/or lower crust with the asthenosphere through mantle plume-lithosphere interaction during the early rifting stages of Gondwana. Component 2 likely originated from mixing of recycled oceanic lithosphere plus sediment with the asthenosphere (which had been previously contaminated by component 1) during the upwelling of Indian Ocean mantle plumes over the last 180 Ma. The variation in Pb and Hf isotopes along the SEIR reveals that, while the two DUPAL components are spatially distributed in a Poisson fashion, and

  6. 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

  7. 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.

  8. 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.

  9. 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.

  10. Cretaceous seamounts along the continent ocean transition of the Iberian margin: U Pb ages and Pb Sr Hf isotopes

    NASA Astrophysics Data System (ADS)

    Merle, Renaud; Schärer, Urs; Girardeau, Jacques; Cornen, Guy

    2006-10-01

    To elucidate the age and origin of seamounts in the eastern North Atlantic, 54 titanite and 10 zircon fractions were dated by the U-Pb chronometer, and initial Pb, Sr, and Hf isotope ratios were measured in feldspars and zircon, respectively. Rocks analyzed are essentially trachy-andesites and trachytes dredged during the "Tore Madeira" cruise of the Atalante in 2001. The ages reveal different pulses of alkaline magmatism occurring at 104.4 ± 1.4 (2 σ) Ma and 102.8 ± 0.7 Ma on the Sponge Bob seamount, at 96.3 ± 1.0 Ma on Ashton seamount, at 92.3 ± 3.8 Ma on the Gago Coutinho seamount, at 89.3 ± 2.3 Ma and 86.5 ± 3.4 Ma on the Jo Sister volcanic complex, and at 88.3 ± 3.3 Ma, 88.2 ± 3.9, and 80.5 ± 0.9 Ma on the Tore locality. No space-time correlation is observed for alkaline volcanism in the northern section of the Tore-Madeira Rise, which occurred 20-30 m.y. after opening of the eastern North Atlantic. Initial isotope signatures are: 19.139-19.620 for 206Pb/ 204Pb, 15.544-15.828 for 207Pb/ 204Pb, 38.750-39.936 for 208Pb/ 204Pb, 0.70231-0.70340 for 87Sr/ 86Sr, and +6.9 to +12.9 for initial epsilon Hf. These signatures are different from Atlantic MORB, the Madeira Archipelago and the Azores, but they lie in the field of worldwide OIB. The Cretaceous seamounts therefore seem to be generated by melts from a OIB-type source that interact with continental lithospheric mantle lying formerly beneath Iberia and presently within the ocean-continent transition zone. Inheritance in zircon and high 207Pb of initial Pb substantiate the presence of very minor amounts of continental material in the lithospheric mantle. A long-lived thermal anomaly is the most plausible explanation for alkaline magmatism since 104 Ma and it could well be that the same anomaly is still the driving force for tertiary and quaternary alkaline magmatism in the eastern North Atlantic region. This hypothesis is agreement with the plate-tectonic position of the region since Cretaceous time

  11. 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.

  12. Evolution of Mojavian mantle lithosphere influenced by Farallon plate subduction: Evidence from Hf and Nd isotopes in peridotite xenoliths from Dish Hill, CA

    NASA Astrophysics Data System (ADS)

    Armytage, Rosalind M. G.; Brandon, Alan D.; Andreasen, Rasmus; Lapen, Thomas J.

    2015-06-01

    A major issue in the assembly of continents is the role of subduction in building and reworking the continental mantle lithosphere. Spinel lherzolite xenoliths from Dish Hill, CA represent Mojavian sub-continental lithospheric mantle (SCLM) that existed along an off-craton continental edge during late Cretaceous Farallon plate subduction. The Dish Hill locale is well situated for recording any Farallon plate influence, be it as oceanic lithosphere accretion or for its role in providing metasomatic agents to the Mojavian SCLM. The 176Hf/177Hf and 143Nd/144Nd isotopic compositions of clinopyroxenes from these xenoliths are radiogenic with εHf from +12.9 to +134.4 and εNd from +2.2 to +26.1, indicative of ancient Proterozoic melt depletion. Four out of the sixteen samples lie on a 2.1 Ga reference line for melt extraction from primitive mantle for both 176Hf/177Hf and 143Nd/144Nd, confirming their position on the 2.1 Ga 187Os/188Os aluminachron from previous work on these peridotites (Armytage et al., 2014). A second Re-depletion age obtained from an 187Os/188Os aluminachron of 1.3 Ga is also observed in the 176Lu-176Hf and 147Sm-143Nd systematics. The 176Hf/177Hf-143Nd/144Nd data from Dish Hill do not provide strong evidence for the existence of a duplex of oceanic lithosphere and SCLM, or for these peridotites being sourced from modern asthenospheric mantle upwelling after lithospheric removal. However, subchondritic 176Lu/177Hf and 147Sm/144Nd ratios and trace element compositions in some of the peridotites point to the influence of metasomatic processes. In seven of the peridotites 176Hf/177Hf ratios are not complemented by similarly radiogenic 143Nd/144Nd ratios. Such decoupling, relative to the mantle array, indicates that the 176Hf/177Hf record in these peridotites is more robust to resetting by these local metasomatic processes than 143Nd/144Nd. The 87Sr/86Sr ratios measured in these samples fall into two distinct groups based on (Ce/Yb)PM, with the less

  13. The geochemical behaviour of W in subduction zones: constraints from high precision isotope dilution measurements

    NASA Astrophysics Data System (ADS)

    Konig, S.; Munker, C.; Schuth, S.

    2007-12-01

    Assessing the behaviour of W during silicate Earth's differentiation is hampered by low abundances of W in terrestrial reservoirs, making sufficiently precise and accurate measurements difficult. Previous results (e.g., Newsom et al. 1996) indicate a lower W/Th of the mantle (ca. 0.19) compared to the Earth's crust, (ca. 0.26), suggesting that W appears to be more incompatible than Th. New data for MORB (Munker et al. 2007), however, demonstrate that W/Th is not significantly fractionated during dry peridotite melting, tentatively suggesting a fractionation of the two elements during crust formation by subduction related processes. We present high precision W and Nb-Ta, Zr-Hf data obtained by isotope dilution, using a mixed 183W-180Ta- 94Zr-180Hf-176Lu tracer and multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). This enables the direct determination of W and HFSE from one sample digestion. For some samples, a "nugget effect" as previously reported for PGE was observed, reflecting sample heterogeneity. Measured Ta and W abundances determined in basaltic glasses and whole rock powders from various island arc settings yield Ta/W ratios of 0.6 to 1.7, significantly lower than the values reported for MORB (4-6). In contrast, Nb/Ta for the samples overlap with MORB values, suggesting that Nb and Ta were not mobile in the magma sources. These systematic differences indicate that W does not behave as other HFSE (Nb-Ta, Zr-Hf) in subduction zones but rather displays a higher mobility in slab components. Intra-oceanic arc suites involving subducted pelagic sediment in their sources generally display higher W/Th compared to magmas without sediment-derived components in their sources, reflecting the higher initial W abundances in subducted pelagic sediments. A fractionation of W/Th during crust formation could consequently be explained by a selective W enrichment relative to Th during subduction processes.

  14. 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.

  15. Hf Isotopes and Geochemical Evidence Constrain the Nature and Sources of Melting During and After Progressive Accretion of the Wrangellia Composite Terrane to the Southern Alaska Margin

    NASA Astrophysics Data System (ADS)

    Todd, E.; Jones, J. V., III; Kylander-Clark, A. R.

    2015-12-01

    Plutonic rocks in the western Alaska Range were emplaced prior to, during, and after accretion of the Wrangellia Composite Terrane (WCT) to the southern Alaska margin (locally, Farewell terrane, FT). Docking between (mostly) oceanic WCT and (mostly) Paleozoic continental FT was done largely by ca. 80 Ma on the basis of youngest detrital zircon ages from an overlapping flysch basin and the oldest post-deformational plutons. Plutons before and during progressive basin closure and terrane accretion (~100-76 Ma) were emplaced in WCT basement or proximal to the WCT-FT margin, are calcalkaline diorite to granite, and likely products of the migrating arc associated with closure of the intervening ocean basin. Plutons emplaced after 76 Ma are organized axially and cross into both sides of the inferred suture zone, suggesting an association with faults formed during crustal shortening and transcurrent deformation. These Late Cretaceous gabbro to granodiorite plutons have arc to collisional affinity, some with "adakitic" compositions, possibly due to crustal thickening associated with WCT collision. In contrast, younger Paleocene plutons are spatially scattered and widespread fractionated granites. Hf isotopes and U/Pb ages were measured in zircons from ~110 to ~30 Ma plutons by LA-ICPMS using the split-stream configuration. Maximum eHf decreases gradually over time (+15 to +12) suggesting either more enriched mantle or an increasing role of crustal components in the melt source and/or during magma ascent and emplacement. However, most Late Cretaceous and a subset of Paleocene plutons have anomalously low eHf (+6 to -2). Paleocene granite isotopes correlate with location and basement type; plutons emplaced in Paleozoic basement have lower eHf compared with those in Mesozoic basement. This pattern, most extreme in Paleocene plutons, is also seen in Cretaceous to Eocene plutons where similar-aged rocks were emplaced in both domains, suggesting strong basement control on Hf

  16. 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.

  17. 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

  18. Tungsten isotopic compositions of iron meteorites: Chronological constraints vs. cosmogenic effects

    NASA Astrophysics Data System (ADS)

    Markowski, A.; Quitté, G.; Halliday, A. N.; Kleine, T.

    2006-02-01

    High-precision W isotopic compositions are presented for 35 iron meteorites from 7 magmatic groups (IC, IIAB, IID, IIIAB, IIIF, IVA, and IVB) and 3 non-magmatic groups (IAB, IIICD, and IIE). Small but resolvable isotopic variations are present both within and between iron meteorite groups. Variations in the 182W/ 184W ratio reflect either time intervals of metal-silicate differentiation, or result from the burnout of W isotopes caused by a prolonged exposure to galactic cosmic rays. Calculated apparent time spans for some groups of magmatic iron meteorites correspond to 8.5 ± 2.1 My (IID), 5.1 ± 2.3 My (IIAB), and 5.3 ± 1.3 My (IVB). These time intervals are significantly longer than those predicated from models of planetesimal accretion. It is shown that cosmogenic effects can account for a large part of the W isotopic variation. No simple relationship exists with exposure ages, compromising any reliable method of correction. After allowance for maximum possible cosmogenic effects, it is found that there is no evidence that any of the magmatic iron meteorites studied here have initial W isotopic compositions that differ from those of Allende CAIs [ ɛ182W = - 3.47 ± 0.20; [T. Kleine, K. Mezger, H. Palme, E. Scherer and C. Münker, Early core formation in asteroids and late accretion of chondrite parent bodies: evidence from 182Hf- 182W in CAIs, metal-rich chondrites and iron meteorites, Geochim. Cosmochim. Acta (in press)]. Cosmogenic corrections cannot yet be made with sufficient accuracy to obtain highly precise ages for iron meteorites. Some of the corrected ages nevertheless require extremely early metal-silicate segregation no later than 1 My after formation of CAIs. Therefore, magmatic iron meteorites appear to provide the best examples yet identified of material derived from the first planetesimals that grew by runaway growth, as modelled in dynamic simulations. Non-magmatic iron meteorites have a more radiogenic W isotopic composition than magmatic

  19. 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.

  20. A legacy of Hadean silicate differentiation inferred from Hf isotopes in Eoarchean rocks of the Nuvvuagittuq supracrustal belt (Québec, Canada)

    NASA Astrophysics Data System (ADS)

    Guitreau, Martin; Blichert-Toft, Janne; Mojzsis, Stephen J.; Roth, Antoine S. G.; Bourdon, Bernard

    2013-01-01

    New Lu-Hf isotopic data for mafic and felsic rocks from the Nuvvuagittuq supracrustal belt (NSB) in northern Québec (Canada) yield an Eoarchean age of 3864±70 Ma consistent with both zircon U-Pb and whole-rock 147Sm-143Nd chronology, but in disagreement with ca. 4400 Ma ages inferred from the 146Sm-142Nd chronometer (O'Neil et al., 2008, 2012). The Lu-Hf result is interpreted as the mean emplacement age of the different autochthonous units of the NSB. An observed alignment of the data along a Lu-Hf "scatterchron" precludes a Hadean age for the NSB because its isotopic characteristics appear to be controlled by long-term radiogenic ingrowth. Emplacement of the NSB in the Hadean (e.g., 4362-54+35 Ma if the decay constant of 146Sm of Kinoshita et al. (2012) is used with the O'Neil et al., 2008 data) should instead have caused age differences of hundreds of millions of years to manifest as strong deviations from the Lu-Hf scatterchron. Combined Lu-Hf and Sm-Nd data on the same NSB amphibolite samples (Ca-poor cummingtonite- and hornblende-bearing) define a mixing hyperbola at ca. 3800 Ma with end-member compositions representative of the compositional groups identified for these lithologies (O'Neil et al., 2011). Anomalously low 142Nd/144Nd values relative to Bulk Silicate Earth are endemic to a group of rocks in the NSB termed "low-TiO2" amphibolites; this is attributable to an ancient multi-stage history of their mantle source. Modeling shows that the 142Nd/144Nd deficits could have developed in response to a re-fertilization episode within a previously fractionated mantle domain at 4510 Ma.

  1. 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

  2. Re-Os and Lu-Hf isotope evidence for recycled materials in the source of the Ontong Java Plateau lavas

    NASA Astrophysics Data System (ADS)

    Tejada, M. G.; Suzuki, K.; Hanyu, T.; Mahoney, J. J.; Tatsumi, Y.; Nakai, S.; Chang, Q.

    2012-12-01

    Previous studies of Ontong Java Plateau (OJP) lavas reveal ocean-island-like Pb, Nd, and Sr isotopic signatures that fall within a limited range yet define two distinct isotopic types, the Singgalo and Kwaimbaita. Lavas of both groups are products of very large degrees of melting. It is not clear what the origin of the two groups is, although the Kwaimbaita signature has been suggested to represent the plateau's main mantle source, probably originating from the lower mantle. Our work on samples from the central part of the Solomon island of Malaita and Ocean Drilling Program (ODP) Sites 807, 1185, and 1187 shows that the two groups of OJP lavas also possess distinct age-corrected 187Os/188Os signatures, (0.117-0.170 for Kwaimbaita-type lavas and 0.279-0.421 for Singgalo-type lavas) and different from ocean-ridge basalt values (average 0.125). Hf isotope data for the same samples show a slight difference between Singgalo- and Kwaimbaita-type lavas in 176Hf/177Hf, consistent with previous results for ODP lavas. The Re-Os and Lu-Hf results (1) reinforce the interpretation that the Kwaimbaita magma type is derived from the principal OJP mantle source, inferred to be non-chondritic mantle similar to that involved in the source of several other flood basalt provinces; and (2) suggest the influence of old, lower continental crustal material or ancient pelagic sediments in the source of the Singgalo-type lavas. A lower crustal influence may be explained by incorporation of ancient, delaminated mafic continental crust into a thermochemical plume; such incorporation could provisionally involve dense ex-crustal material that had accumulated in the lower mantle or less dense material that had stagnated at mid-mantle depths.

  3. 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

  4. Stable Isotopic Constraints on Climatic and Topographic Development Along the Northern Margin of the Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Kent-Corson, M. L.; Ritts, B. D.; Graham, S. A.; Chamberlain, C. P.

    2007-12-01

    One of the salient features of the greater Himalayan/Tibetan orogen is the large sedimentary basins that sit along the northern margin of the Tibetan Plateau. These basins record the surface uplift of adjacent ranges and the attendant reorganization of climatic patterns. As such, we present a record of oxygen and carbon isotope values of Cenozoic sedimentary units along the northern margin of the Tibetan Plateau. The 15 sedimentary sections that this study encompasses have been measured and sampled at the meter scale, and include strata of fluvial, lacustrine, floodplain, and alluvial fan origin, as well as deposits of one brief marine transgression. Localities studied include sections in the Qaidam Basin (Xiao Qaidam, Lake Mahai, Lao Mangnai, Ganchaigou, Lenghu, and Lulehe), Tarim Basin (Puska, Aertashi, Miran River, and Jianggalsay), Hexi Corridor (Shiyougou, Jiuquan, and Baiyanghe), and the Altun Shan (Subei and Xorkol). These sections were chosen because they are the most temporally complete nonmarine Cenozoic sections in the region and have the most robust age constraints, which include biostratigraphy, magnetostratigraphy, and thermochronology. Many of the same sections have also been previously used to decipher the structural history of nearby ranges. An initial subset of the >3,000 samples collected agrees with the previous findings of Graham et al., 2005, but our increased spatial and temporal resolution reveals how isotopic stratigraphic records locally reflect development of topography and subsequent reorganization of atmospheric circulation. For example, in the Qaidam Basin, two stratigraphic sections in the northern part of the basin show that oxygen isotope values increase steadily by 5-6 permil from the middle Eocene to Pliocene, despite several changes between fluvial and lacustrine depositional environments. This contrasts with the isotope stratigraphies from western Qaidam presented in Graham et al. (2005) and this study, that show a steady oxygen

  5. 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.

  6. Coupled Hf-Nd-Pb isotope co-variations of HIMU oceanic island basalts from Mangaia, Cook-Austral islands, suggest an Archean source component in the mantle transition zone

    NASA Astrophysics Data System (ADS)

    Nebel, Oliver; Arculus, Richard J.; van Westrenen, Wim; Woodhead, Jon D.; Jenner, Frances E.; Nebel-Jacobsen, Yona J.; Wille, Martin; Eggins, Stephen M.

    2013-07-01

    Although it is widely accepted that oceanic island basalts (OIB) sample geochemically distinct mantle reservoirs including recycled oceanic crust, the composition, age, and locus of these reservoirs remain uncertain. OIB with highly radiogenic Pb isotope signatures are grouped as HIMU (high-μ, with μ = 238U/204Pb), and exhibit unique Hf-Nd isotopic characteristics, defined as ΔɛHf, deviant from a terrestrial igneous rock array that includes all other OIB types. Here we combine new Hf isotope data with previous Nd-Pb isotope measurements to assess the coupled, time-integrated Hf-Nd-Pb isotope evolution of the most extreme HIMU location (Mangaia, French Polynesia). In comparison with global MORB and other OIB types, Mangaia samples define a unique trend in coupled Hf-Nd-Pb isotope co-variations (expressed in 207Pb/206Pb vs. ΔɛHf). In a model employing subducted, dehydrated oceanic crust, mixing between present-day depleted MORB mantle (DMM) and small proportions (˜5%) of a HIMU mantle endmember can re-produce the Hf-Nd-Pb isotope systematics of global HIMU basalts (sensu stricto; i.e., without EM-1/EM-2/FOZO components). An age range of 3.5 to <2 Ga is required for HIMU endmember(s) that mix with DMM to account for the observed present-day HIMU isotope compositions, suggesting a range of age distributions rather than a single component in the mantle. Our data suggest that mixing of HIMU mantle endmembers and DMM occurs in the mantle transition zone by entrainment in secondary plumes that rise at the edge of the Pacific Large Low Seismic Velocity Zone (LLSVP). These create either pure HIMU (sensu stricto) or HIMU affected by other enriched mantle endmembers (sensu lato). If correct, this requires isolation of parts of the mantle transition zone for >3 Gyr and implies that OIB chemistry can be used to test geodynamic models.

  7. 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

  8. Timing of Magmatism and Metamorphism in the Lower Crust Beneath the Colorado Plateau From Coupled U-Pb and Lu-Hf Isotopic Analysis of Zircon From Xenoliths

    NASA Astrophysics Data System (ADS)

    Crowley, J. L.; Schmitz, M. D.; Bowring, S. A.; Williams, M. L.; Karlstrom, K. E.

    2003-12-01

    A major uncertainty in models for Proterozoic evolution of the southwestern U.S. is whether lower crust that formed and accreted at 1.8-1.6 Ga was substantially modified ca. 1.4 Ga by mafic underplating that produced voluminous granitic magmatism and associated low P - high T metamorphism (0.35-0.6 GPa, 500-700 C) preserved in middle and upper crustal basement rocks. A regionally extensive high-velocity (7.xx) lower crust has been interpreted as evidence of underplated magmas. We undertook a U-Pb and Lu-Hf isotopic study of zircons from lower crustal xenoliths to constrain the timing of magmatism and metamorphism beneath the Colorado Plateau. Xenoliths of mafic and felsic granulite from the Navajo volcanic field are interpreted as lower crustal samples based on the primary mineralogy of Cpx-Grt-Pl +/- Qtz and P-T estimates of ca. 1.3 GPa and 800 C. CL imaging of the internal zoning in zircon guided our attempts to isolate homogeneous, moderate sized (50-100 microns) domains through grain fragmentation and air-abrasion. Fragments of grains from eight xenoliths were dated by the ID-TIMS U-Pb method and the solutions from many of these fragments were analyzed for Lu-Hf isotopes by MC-ICPMS. Weak, curved, and sector zoning that is typical of metamorphic zircon is prominent in the mafic xenoliths yet sparse in the felsic ones. Fragments from these zoned domains yielded U-Pb dates that define groups at 1420-1414, 1410-1395 (most prevalent dates), 1385, and 1360 Ma. There is substantial variability in the initial Hf isotope compositions of these grains (Epsilon Hf = -0.7 to +13.6; TDM = 1.8-1.3 Ga), from the same xenolith as well as between xenoliths. This dispersion suggests growth of metamorphic zircon from a variety of introduced fluid compositions or from protolith minerals with diverse time-integrated Lu/Hf ratios. Many of the felsic and mafic xenoliths have zircon cores or whole grains with zoning that may indicate magmatic crystallization. Four felsic granulites

  9. Distributions of REE, Nd, Hf and Pb isotopes in the surfaces of Fe-Mn crusts from across the Pacific Ocean

    NASA Astrophysics Data System (ADS)

    Chu, N.; Nesbitt, R. W.; German, C. R.; Halbach, P.

    2001-12-01

    Over the past decade, numerous studies have investigated radiogenic isotopes (i.e. Nd, Pb, Hf, Os) in marine Fe-Mn deposits in an attempt to infer changes in ocean circulation throughout the Cenozoic. The use of radiogenic isotopes as paleoceanographic proxies has been challenged recently by evidence that the isotopic composition of crusts from oceanic domains near major river systems (e.g. Amazon, Congo) or old cratonic areas (i.e. the North Atlantic area) could be influenced significantly by continental inputs. Therefore, for any given crust, it is difficult to deduce the extent to which changes in weathering processes rather than ocean circulation may be responsible for observed isotopic variations. This is partly due to the fact that the oceanic budgets for some of these elements remain poorly constrained. In particular, the influence of both the eastward aeolian transport of Chinese loess and the erosion of the young West-Pacific volcanic belt on the isotopic composition of Pacific water masses has been poorly documented. The deep Pacific Ocean is composed of 4 principal water masses: North Pacific Intermediate Water (NPIW), Antarctic Intermediate Water (AAIW), Pacific Deep Water (PDW) and Antarctic Bottom Water (AABW). AABW, the main source of PDW, flows northward and enters the Central Basin through the Samoan Passage, where it is diverted into two branches: eastward to Line Island Passage and westward through Wake Passage. We will present Nd, Hf and Pb isotopic ratios combined with REE data from the surfaces of 16 Fe-Mn crusts taken at different depths from key areas of the Pacific Ocean. Two crusts were collected from the Izu-Bonin back-arc basin in the western Pacific and, hence, are particularly suitable for monitoring the influence of both continental aeolian and weathering inputs. Two other groups of crusts are from north and south of the equatorial Pacific region. The southern group is situated at the exit of the Samoan Passage, whereas the northern

  10. Geochronology, geochemistry and Hf isotope of Late Triassic magmatic rocks of Qingchengzi district in Liaodong peninsula, Northeast China

    NASA Astrophysics Data System (ADS)

    Duan, Xiaoxia; Zeng, Qingdong; Yang, Jinhui; Liu, Jianming; Wang, Yongbin; Zhou, Lingli

    2014-09-01

    The initiation timing and mechanism of lithospheric thinning of the North China Craton (NCC) was still controversial. Late Triassic igneous rocks especially mantle derived mafic rocks would provide constrains on Early Mesozoic lithospheric mantle geodynamics and initiation of lithospheric thinning. This paper reports Late Triassic magmatic rocks, including lamprophyre, diorite dykes and biotite monzogranite cropped out in Qingchengzi district of Liaodong peninsula, northeastern NCC. LA-ICPMS zircon U-Pb dating yield ages of 210-227 Ma and 224 Ma for lamprophyres and biotite monzogranite respectively. Lamprophyre is ultrapotassic, strongly enriched in REE and LILEs, depleted in HFSEs, and negative Hf isotopes, which are discriminating signatures of crustal source, but distinguishingly high compatible element contents indicate the primary magma originated from mantle source-a fertile one. Lamprophyre derived from partial melting of an enriched lithospheric mantle, which was modified by slab-derived hydrous fluids/melts associated with deep subduction between the Yangtze Craton and the NCC. The diorite displays distinct features with relatively enriched Nb, Ta, HREE and depleted Th, U, which suggest it derived from a relatively depleted source. The depletion was caused by break-off of the Yangtze slab during deep subduction introducing asthenospheric mantle into the source. The biotite monzogranite shows adakitic affinity, and originated from partial melting of the thickened lower crust with addition of small proportion of mantle material. The recognition of Late Triassic magmatism implies extensional tectonic settings in Liaodong peninsula and suggests initiation of lithospheric thinning of North China Craton in eastern segment might begin early in Late Triassic.

  11. 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.

  12. Uncoupled O and Hf isotopic systems in zircon from the contrasting granite suites of the New England Orogen, eastern Australia: Implications for studies of Phanerozoic magma genesis

    NASA Astrophysics Data System (ADS)

    Jeon, Heejin; Williams, Ian S.; Bennett, Vickie C.

    2014-12-01

    The Permo-Triassic granites of the New England Orogen, eastern Australia, were emplaced into a volcanic arc complex accreted to the eastern Gondwana margin in the Late Devonian or Early Carboniferous. Zircon U-Pb dating shows that the S-type Hillgrove (∼297 Ma) and Bundarra (∼287 Ma) Supersuites predated intrusion of the I-type Moonbi Supersuite (∼250 Ma) by up to 50 Ma. The high δ18Ozrn of the S-type granites (10.0-11.5‰), and range of U-Pb ages (∼370-300 Ma) and δ18Ozrn (∼5-10‰) of their inherited zircon cores, show that their source rocks were predominantly weathered Carboniferous volcaniclastics, the youngest deposited < 25 Ma before the granites were emplaced. In contrast, the lower δ18Ozrn (6.9-7.8‰) and lack of inheritance in the I-type granites is consistent with a zircon poor, more juvenile source, probably a mafic igneous underplate mixed with a small amount of volcanogenic and/or oceanic sediment. Despite the differences in source materials, the εHf(t) values of all granites, both S- and I-type, are similar (+5.0 ± 0.5 cf. +5.9 ± 0.5), consistent with both the mafic and sedimentary components in the granite sources being relatively young and similar in Hf isotopic composition at the time of granite genesis. In young, isotopically juvenile orogens, the O isotopic composition of well-dated igneous and inherited zircon can be a much more sensitive indicator of petrogenetic processes than the zircon Hf isotopic compositions alone.

  13. 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

  14. 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

  15. 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

  16. 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.

  17. 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

  18. Trace Element and Pb Isotope Constraints on Dynamic Evolution of Earth Reservoirs

    NASA Astrophysics Data System (ADS)

    Collerson, K.; Kamber, B.

    2001-12-01

    Advances in interpretation of Pb isotope systematics provide constraints for modelling Earth evolution. Such improved understanding of Pb isotope systematics has coincided with advances in techniques for accurate Pb isotope ratio measurement by MC-ICPMS. Continental growth since 3.75 Ga has occurred at convergent margins via dehydration of subducted slabs and supra-subduction zone melting. Nb is preferentially retained in slabs relative to U and Th, which are lost to escaping fluids. Over time, the depleted upper mantle (DM) lost U and Th relative to Nb. Thus Nb/Th and Nb/U of UM mirror amount of continental crust present. Because Nb, Th and U are similarly incompatible during MORB melting, temporal Nb-Th-U systematics of mantle can be reconstructed from uncontaminated, depleted-mantle derived rocks1. Excellent agreement exists between crustal growth curve based on Nb/Th and those based on Pb isotope systematics2 and geophysics 3. Temporal variation of Nb/U reflects crustal extraction until 2 Ga. It then reflects preferential U recycling into DM, constraining timing of preservation of a pandemic oxygenated atmosphere. Increase in atmospheric O2 explains the second Pb paradox and refines understanding of DM evolution. Key to understanding mantle Pb isotope evolution is the realization that DM has highly dynamic U/Pb and Th/U ratios relative to undegassed lower mantle (LM). Thus, so-called OIB EM-1 reservoir could reflect LM4. Pb data for Phanerozoic and Proterozoic Gp 2 kimberlites from South Africa plot in thorogenic and uranogenic Pb space consistent with a LM source [4]. Mineralogically, chemically and isotopically different Gp 1 kimberlites, which are readily discernable in plots of PM normalized Ta/U and Nb/Th have very radiogenic 206Pb/204Pb and 208Pb/204Pb but relatively unradiogenic 207Pb/204Pb, compositions identical to HIMU OIB's. We have suggested in [4] that the HIMU isotopic composition can be derived from EM-1 during a transient <100 Ma stage of strong

  19. 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.

  20. Subduction zone Hf-anomalies: Mantle messenger, melting artefact or crustal process?

    NASA Astrophysics Data System (ADS)

    Woodhead, Jon; Hergt, Janet; Greig, Alan; Edwards, Louise

    2011-04-01

    The origin of Hf elemental depletions in subduction zone magmas is investigated using new major- and trace-element data for cumulate xenoliths from the Mariana arc, and deep sea sediments recovered by the DSDP and ODP drilling programmes. Results indicate that most of the rare earth element (REE) and Hf inventory in the xenoliths is contained within two minerals—clinopyroxene and titanomagnetite—and that removal of a typical gabbroic fractionating assemblage reduces the depletion in Hf relative to neighbouring REE on a mantle normalised trace element diagram (commonly denoted Hf/Hf*) in the evolving magmas. Confirmation of this observation is provided by a variety of literature data from different subduction zones in which bulk-rock samples also define a positive correlation between Hf/Hf* and the silica content of the magmas. In agreement with experimental studies on REE-HFSE partitioning, we observe that the ability of clinopyroxene to influence the Hf/Hf* of fractionating magmas is associated with its aluminium content. This decoupling of Hf from the REE in differentiating arc magmas suggests that bulk rock Hf/Hf* values, when used in isolation, are unlikely to provide a robust measure of source REE-Hf characteristics, even when suites are filtered to exclude all but the most mafic samples. It may be possible to normalise data to a constant degree of fractionation, and in this way distinguish subtle changes in source Hf/Hf* but most existing datasets are of neither the size nor quality to attempt such calculations. Modification of Hf/Hf* is also seen when modelling mantle melting processes and there are strong suggestions that source variations are influenced by not only subducted sediment, which exhibits a remarkably wide range in Hf/Hf*, but also subduction zone fluids. These observations remove some of the constraints imposed on recent models that attempt to reconcile Hf isotope data with Hf-REE abundance data in some arc suites. Although a case may be

  1. 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

  2. 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".

  3. 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

  4. Depositional Provenance of the Himalayan Metamorphic Core of Garhwal Region, India: Constrained by U-Pb and Hf isotopes in zircons

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    U-Pb and Hf isotopes in zircon from the highly metamorphosed core of the Himalaya in the Garhwal Region of India indicate that the ortho- and paragneiss units of the Greater Himalayan Sequence are geochemical distinct from the older Lesser Himalayan Crystalline Sequence. Zircon grains from the Paleoproterozoic metasediments of the Lesser Himalayan Crystalline Sequence yield U-Pb crystallization ages from 1614±14 Ma to, 2679±14 Ma with a single distinct peak in the age spectra at 1870 Ma. ɛHf values from zircon in the Lesser Himalayan Crystalline Sequence range from 1 to -8. In contrast, zircon grains from the Neoproterozoic paragneiss of the Greater Himalayan Sequence yield U-Pb crystallization ages from 481±10 Ma to, 2560±8 Ma with a single distinct peak at 860 Ma. ɛHf values from these zircons range from 13 to -21. The orthogneiss unit of Greater Himalayan Sequence yields zircon grians with U-Pb crystallization ages from 416±2 Ma to, 2740±2 Ma with a single distinct peak at 472 Ma. These grains often exhibited older inherited cores ranging from ~800 Ma to ~2740 Ma with Cambro-Ordovician rims (416 Ma to 510 Ma). The ɛHf values of zircon grains in orthogneiss have two distinct modes: the ~860 Ma inherited zircon cores have a similar range to the ~860 Ma zircons from the GHS paragneiss (13 to -21) while the ~480 Ma rims range from -7 to -11. The detrital zircon U-Pb age spectra from the Greater Himalayan Sequence paragneiss in the Garhwal region differs significantly from the U-Pb age spectra in other parts of the range (Nepal and Bhutan). These differences indicate wide lateral variations in provenance for the various source regions of the Greater Himalayan Sequence paragneiss. Comparisons of the U-Pb and Hf data from the Garhwal region with other regions with zircon grains of ~500 Ma show overlap with deposits of the Aravalli shield of India. In paragneiss of the Greater Himalayan Sequence the most juvenile ɛHf values overlap with those of the northern

  5. Decoupling of the Lu-Hf, Sm-Nd, and Rb-Sr isotope systems in eclogites and a garnetite from the Sulu ultra-high pressure metamorphic terrane: Causes and implications

    NASA Astrophysics Data System (ADS)

    Liu, Yung-Hsin; Yang, Huai-Jen; Takazawa, Eiichi; Satish-Kumar, Madhusoodhan; You, Chen-Feng

    2015-10-01

    The whole-rock Hf, Sr and Nd isotope data of five high-Fe-Ti eclogites, nine high-Al eclogites, and a garnetite from the Sulu ultrahigh pressure (UHP) metamorphic terrane at eastern China were analyzed to resolve the causes for the decoupling of the Lu-Hf, Sm-Nd, and Rb-Sr isotope systems in these UHP rocks and to infer their protolith characteristics. Seven of the nine high-Al eclogites define an 87Rb/86Sr-87Sr/86Sr errorchron age of 192 ± 43 Ma (MSWD = 2.8), which is within the time span of retrograde metamorphism despite the large uncertainty. The high-Fe-Ti eclogites and garnetite, however, have low 87Rb/86Sr ratios of < 0.031 with scattered 87Sr/86Sr ratios of 0.7042-0.7058. Accordingly, it is inferred that the Rb-Sr isotope system in the samples reflects the effects of processes postdating the UHP metamorphism. Despite having different constituent mineral assemblages and whole rock geochemistry, the samples, however, define a 147Sm/144Nd-143Nd/144Nd errorchron age of 232 ± 36 Ma (MSWD = 4.6). Although the uncertainty of ± 36 Ma implies incomplete Nd isotope equilibrium among the samples, the large overlap between this errorchron age span and the mineral isochron ages of 245-210 Ma for the UHP metamorphism indicates the control of peak metamorphism on the Sm-Nd isotope system. The incomplete Nd isotope re-equilibration was accompanied by metamorphic modification on the Sm/Nd ratios as indicated by the U-shaped LREE patterns. The initial εNd(780) values of the protolith rocks calculated from the Sm/Nd ratios of the samples deviate from the igneous initial εNd(t)-εHf(t) trend to significantly lower values, consistent with the metamorphic increase in the Sm/Nd ratios. In contrast, the Lu/Hf ratios are generally within the range for basalts and do not vary systematically with the 176Hf/177Hf ratios. The protolith εHf(780) values calculated from the Lu/Hf ratios of the samples are nearly identical to the initial εHf(t) values of the ~ 780 Ma magmatic zircon

  6. 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.

  7. 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

  8. 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).

  9. In situ zircon Hf-O isotopic analyses of late Mesozoic magmatic rocks in the Lower Yangtze River Belt, central eastern China: Implications for petrogenesis and geodynamic evolution

    NASA Astrophysics Data System (ADS)

    Yan, Jun; Liu, Jianmin; Li, Quanzhong; Xing, Guangfu; Liu, Xiaoqiang; Xie, Jiancheng; Chu, Xiaoqiang; Chen, Zhihong

    2015-06-01

    A combined study of whole-rock major and trace elements, Sr-Nd isotopes, zircon U-Pb dating, and in situ zircon Hf-O isotopes has been carried out for late Mesozoic magmatic rocks in the Lower Yangtze River Belt. The results provide insights into the origin of mantle sources of magma above a subduction zone, and thus into the petrogenesis of high-K calc-alkaline rocks, shoshonites, and A-type granites on continental margins, and the associated tectonic transformation from a continental arc to a back-arc extensional setting. The late Mesozoic magmatism can be subdivided into three stages: high-K calc-alkaline intrusions (148-133 Ma), shoshonitic rocks (133-127 Ma), and A-type granitoids (127-123 Ma). All the rocks have consistent arc-like trace element characteristics with positive anomalies of Rb, Th, U, Pb, and LREE, negative anomalies of Nb, Ta, and Ti, and enriched Sr-Nd-Hf isotopic signatures. The first-stage intrusions in the Tongling area usually host dark enclaves of diorite, have high Sr/Y ratios, and low Y contents, and contain zircons with relatively low εHf(t) values (- 38.6 to - 6.6) and high δ18O values (5.7‰ to 10.1‰). A few inherited zircons with Neoarchean to Paleoproterozoic ages and highly enriched Hf isotopic compositions were detected in both the host intrusive rocks and the enclaves. The second-stage Ningwu volcanics contain zircons with moderate εHf(t) values (- 13.3 to - 3.8) and elevated δ18O values (5.4‰ to 7.6‰). The third-stage intrusions can be divided into A1- and A2-type granitoids, and their zircons have relatively high δ18O values of 6.7‰ to 10.3‰ and high εHf(t) values of 0 to - 7.9. Based on these geochemical data we drew the following conclusions. Before 148 Ma, following metasomatism by slab-derived fluid/melts, partial melting of the lithospheric mantle produced basaltic magma in the context of a subducting paleo-Pacific plate. This basaltic magma mixed with magma derived from the Archean lower crust, and the

  10. 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.

  11. 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.

  12. 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.

  13. Magma mixing origin for the post-collisional adakitic monzogranite of the Triassic Yangba pluton, Northwestern margin of the South China block: geochemistry, Sr-Nd isotopic, zircon U-Pb dating and Hf isotopic evidences

    NASA Astrophysics Data System (ADS)

    Qin, Jiang-Feng; Lai, Shao-Cong; Diwu, Chun-Rong; Ju, Yin-Juan; Li, Yong-Fei

    2010-03-01

    Petrogenesis of high Mg# adakitic rocks in intracontinental settings is still a matter of debate. This paper reports major and trace element, whole-rock Sr-Nd isotope, zircon U-Pb and Hf isotope data for a suite of adakitic monzogranite and its mafic microgranular enclaves (MMEs) at Yangba in the northwestern margin of the South China Block. These geochemical data suggest that magma mixing between felsic adakitic magma derived from thickened lower continental crust and mafic magma derived from subcontinental lithospheric mantle (SCLM) may account for the origin of high Mg# adakitic rocks in the intracontinental setting. The host monzogranite and MMEs from the Yangba pluton have zircon U-Pb ages of 207 ± 2 and 208 ± 2 Ma, respectively. The MMEs show igneous textures and contain abundant acicular apatite that suggests quenching process. Their trace element and evolved Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.707069-0.707138, and ɛNd( t) = -6.5] indicate an origin from SCLM. Some zircon grains from the MMEs have positive ɛHf( t) values of 2.3-8.2 with single-stage Hf model ages of 531-764 Ma. Thus, the MMEs would be derived from partial melts of the Neoproterozoic SCLM that formed during rift magmatism in response to breakup of supercontinent Rodinia, and experience subsequent fractional crystallization and magma mixing process. The host monzogranite exhibits typical geochemical characteristics of adakite, i.e., high La/Yb and Sr/Y ratios, low contents of Y (9.5-14.5 ppm) and Yb, no significant Eu anomalies (Eu/Eu* = 0.81-0.90), suggesting that garnet was stable in their source during partial melting. Its evolved Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.7041-0.7061, and ɛNd( t) = -3.1 to -4.3] and high contents of K2O (3.22-3.84%) and Th (13.7-19.0 ppm) clearly indicate an origin from the continental crust. In addition, its high Mg# (51-55), Cr and Ni contents may result from mixing with the SCLM-derived mafic magma. Most of the zircon grains from the

  14. 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

  15. 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

  16. 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

  17. 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.

  18. 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

  19. 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

  20. 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

  1. 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.

  2. Boron content and isotopic composition of tektites and impact glasses: Constraints on source regions

    NASA Astrophysics Data System (ADS)

    Chaussidon, Marc; Koeberl, Christian

    1995-02-01

    Abundances of Li, Be, and B, as well as boron isotopic compositions, were determined in twenty-seven tektite and impact glass samples, using an ion microprobe. Samples included tektites from the Australasian, North American, and Ivory Coast strewn fields, and Aouelloul and Darwin impact glasses. Variations of B abundance and isotopic composition in a flanged australite were also studied. δ 11B variations of only a few permil were found within the australite flange. The isotopic composition shows no correlation with the B contents or with the distance from the rim of the flange. The mean δ 11B value for the flanged australite is very similar to that of Muong-Nong type tektites (-1.9 ± 1.9‰). Thus, vapor fractionation has been unimportant during tektite formation. This is supported by the observation that B contents and the δ 11B values of the different samples from the Australasian tektite strewn field are not correlated with each other. Most tektites show a rather limited range of δ 11B values (-9.3 ± 1.5 to +2.7 ± 1.5%o), which is small compared to the range observed for common terrestrial rocks (-30 to +40‰). The B abundance and isotopic data can be used to place constraints on the tektite source rocks. Australasian tektites have high B and Li abundances; only clay-rich sediments, such as pelagic and neritic sediments, as well as river and deltaic sediments have B contents (up to 100 ppm) and δ 11B values that are in agreement with the range shown by Australasian tektites (-4.9 to + 1.4‰). 10Be and RbSr data indicate continental crustal source rocks and exclude pelagic and neritic sediments. However, deltaic sediments, e.g., from the Mekong river, which are of continental crustal origin, agree with 10Be, RbSr, and B data, and support a possible source locality close to the coast of SE Indochina in the South China Sea. On the other hand, one bediasite sample has a very high δ 11B value of +15.1 ± 2.1‰, requiring the presence of marine

  3. 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

  4. U-Pb zircon geochronology, geochemical and Sr-Nd-Hf isotopic compositions of the Early Indosinian Tongren Pluton in West Qinling: Petrogenesis and geodynamic implications

    NASA Astrophysics Data System (ADS)

    Li, Xiaowei; Mo, Xuanxue; Huang, Xiongfei; Dong, Guochen; Yu, Xuehui; Luo, Mingfei; Liu, Yanbin

    2015-01-01

    The West Qinling Orogenic Belt (WQOB), an important part of the Qinling-Dabie-Sulu Orogen (Central Orogen), is essential to understand the prolonged evolution of the northeastern branch of the Paleo-Tethys in East Asia. Zircon LA-ICP-MS U-Pb age and Lu-Hf isotopes, bulk-rock major and trace element, and Sr-Nd isotope data for the granodiorite intrusion from the Tongren area, West Qinling are presented. LA-ICP-MS zircon U-Pb dating analyses from two samples constrain the time of crystallization of the granodiorite to ca. 241 Ma. Combined with newly published age data from other coeval intrusive rocks in West Qinling, it is concluded that an Early Indosinian magmatic event occurred in West Qinling. In-situ zircon Hf isotope data on the two samples dated exhibit εHf(t) values of -0.6 to -5.8 and -0.8 to -3.9, with corresponding TDMC ages of 1312 to 1637 Ma and 1321 to 1516 Ma, respectively. Granodiorites are characterized by relatively high Mg# values (51.4-58.0), intermediate SiO2 (63.75-67.04 wt.%), high Al2O3 (14.24-15.58 wt.%) and total alkalies (6.60-8.00 wt.%) and relatively low A/CNK values (0.93-1.10). Granodiorites have I-type, high-K to shoshonitic, and metaluminous to weakly peraluminous character, showing bulk-rock geochemical features of arc-related granitoids. They are enriched in large-ion lithophile and light rare-earth elements, and depleted in some high-field-strength elements. The rocks show uniformly high (87Sr/86Sr)t (0.707616-0.708237) and low εNd(t) values (-7.9 to -7.5). Estimated crystallization temperatures for the rocks range from 720 ± 48 °C (Ti-in-zircon thermometer) to 759 ± 25 °C (zircon saturation thermometer). The data suggest that magmas were generated by dehydration melting of a mafic lower crustal component with additional input of a mafic component derived from the subcontinental lithospheric mantle. We conclude that the Tongren granodiorite formed in a subduction-related regime in response to slab roll-back of the northward

  5. 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

  6. Sr, Nd, Pb and Hf Isotopic Compositions of Late Cenozoic Alkali Basalts in South Korea: Evidence for Mixing Between the Two Dominant Asthenospheric Mantle Domains beneath East Asia

    NASA Astrophysics Data System (ADS)

    Choi, S.; Mukasa, S. B.; Kwon, S.; Andronikov, A. V.

    2004-12-01

    We determined the Sr, Nd, Pb and Hf isotopic compositions of late Cenozoic basaltic rocks from six lava-field provinces in South Korea, including Baengnyeong Island, Jogokni, Ganseong area, Jeju Island, Ulleung Island and Dog Island, in order to understand the nature of the mantle source. The basalts have OIB-like trace element abundance patterns, and also contain mantle-derived xenoliths. Available isotope data of late Cenozoic basalts from East Asia, along with ours, show that the mantle source has a DMM-EM1 array for northeast China and a DMM-EM2 array for Southeast Asia. We note that the basalts falling on an array between DMM and an intermediate end member between EM1 and EM2, are located between the two large-scale isotopic provinces, i.e., around the eastern part of South Korea. The most intriguing observation on the isotopic correlation diagrams is spatial variation from predominantly EM2 signatures in the basaltic lavas toward increasingly important addition of EM1, starting from Jeju Island to Ulleung and Dog Islands to Ganseong area, and to Baengnyeong Island. This is without any corresponding changes in the basement and the lithospheric mantle beneath the region. These observations suggest that the asthenospheric mantle source is dominant for the Cenozoic intraplate volcanism in East Asia, which is characterized by two distinct, large-scale domains. Previous studies on East Asian Cenozoic volcanic rocks have invoked origins by either plume activity or decompressional melting in a rift environment. On the basis of our new trace element and isotopic compositions which have OIB-like characteristics, we prefer a plume origin for these lavas. However, because tomographic images do not show distinct thermal anomaly that would be interpreted as a plume, we suggest that the magmatism might be the product of small, difficult to image multiple plumes that tapped the shallow part of the asthenosphere (probably the transition zone in the upper mantle).

  7. 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.

  8. 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.

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. Implications of U-Pb and Lu-Hf isotopic analysis of detrital zircons for the depositional age, provenance and tectonic setting of the Permian-Triassic Palaeotethyan Karakaya Complex, NW Turkey

    NASA Astrophysics Data System (ADS)

    Ustaömer, Timur; Ustaömer, Petek Ayda; Robertson, Alastair H. F.; Gerdes, Axel

    2016-01-01

    New zircon U-Pb age data, combined with Lu-Hf isotopic data, are presented here for sandstones of mainly arkosic composition from the Permian-Triassic Karakaya Complex. Predominantly, Carboniferous, Triassic and Devonian zircon age groups are recognised, most of which have a Late Triassic (Carnian-Norian) maximum depositional age. Carboniferous- and Devonian-aged zircon populations exhibit intermediate ɛ Hf( t) values (-11 to +2), consistent with formation in a continental margin arc setting where juvenile mantle-derived magma mixed with (recycled) old crust of Palaeoproterozoic Hf model age. In contrast, the Triassic-aged zircon population exhibits higher ɛ Hf(t) values (-5 to +4), consistent with mixing of juvenile mantle-derived melts with (recycled) old crust of Neoproterozoic Hf model age. Potential igneous source rocks for the sandstones of the Karakaya Complex exist in the Devonian and Carboniferous granitic rocks of the Sakarya continental basement to the north. Their ɛ Hf( t) and corresponding model ages are nearly identical to the age-equivalent zircon populations within the Karakaya Complex sandstones. However, the Triassic granitic rocks of the Sakarya continental crust differ significantly in ɛ Hf( t) and corresponding model age from the sandstones of the Karakaya Complex. Late Triassic sandstones from the Tauride continental unit to the south lack the dominant Late Palaeozoic and Triassic zircon populations of the Karakaya Complex sandstones. Triassic granitic bodies and intermediate-composition extrusive rocks in the Tauride continental unit also differ in ɛ Hf( t) and corresponding Hf model ages from the Karakaya Complex sandstones. In addition, Late Triassic sandstones of the Kocaeli Triassic unit (İstanbul Terrane) in the north differ strongly from the Karakaya Complex sandstones in zircon population ages and ɛ Hf( t). In the regional context, the new zircon age and lutetium-hafnium isotopic data are consistent with derivation of the Late

  14. 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.

  15. 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.

  16. Petrogenesis and geodynamic setting of Neoproterozoic and Late Paleozoic magmatism in the Manzhouli-Erguna area of Inner Mongolia, China: Geochronological, geochemical and Hf isotopic evidence

    NASA Astrophysics Data System (ADS)

    Gou, Jun; Sun, De-You; Ren, Yun-Sheng; Liu, Yong-Jiang; Zhang, Shu-Yi; Fu, Chang-Liang; Wang, Tian-Hao; Wu, Peng-Fei; Liu, Xiao-Ming

    2013-05-01

    U-Pb dating and Hf isotopic analyses of zircons from various granitoids, combined with major and trace element analyses, were undertaken to determine the petrogenesis and geodynamic setting of Neoproterozoic and Late Paleozoic magmatism in the Manzhouli-Erguna area of Inner Mongolia, China. The Neoproterozoic granitoids are mainly biotite monzogranites with zircon U-Pb ages of 894 ± 13 Ma and 880 ± 10 Ma, and they are characterised by enrichment in large ion lithophile elements (LILEs; e.g., Rb, Ba, K) and light rare earth elements (LREEs), depletion in high field strength elements (HFSEs; e.g., Nb, Ta, Ti) and heavy rare earth elements (HREEs). The Late Devonian granitoids are dominantly syenogranites and mylonitised syenogranites with zircon U-Pb ages of 360 ± 4 Ma, and they form a bimodal magmatic association with subordinate gabbroic rocks of the same age. The Late Devonian syenogranites have A-type characteristics including high total alkalis, Zr, Nb, Ce and Y contents, and high FeOt/MgO, Ga/Al and Rb/Sr ratios. The Carboniferous granitoids are mainly tonalites, granodiorites and monzogranites with U-Pb ages varying from 319 to 306 Ma, and they show very strong adakitic characteristics such as high La/Yb and Sr/Y ratios but low Y and Yb contents. The Late Permian granitoids are dominated by monzogranites and syenogranites with zircon U-Pb ages ranging between 257 and 251 Ma. Isotopically, the ɛHf(t) values of the Neoproterozoic granitoids range from +4.3 to +8.3, and the two-stage model ages (TDM2) from 1.2 to 1.5 Ga. The Late Devonian granitoids are less radiogenic [ɛHf(t) from +12.0 to +12.8 and TDM2 from 545 to 598 Ma] than the Carboniferous [ɛHf(t) from +6.8 to +9.5 and TDM2 from 722 to 894 Ma] and Late Permian granitoids [ɛHf(t) from +6.1 to +9.4 and TDM2 in the range of 680-895 Ma]. These data indicate (1) the Neoproterozoic granitoids may have been generated by melting of a juvenile crust extracted from the mantle during the Mesoproterozoic

  17. Geodynamic controls on the contamination of Cenozoic arc magmas in the southern Central Andes: Insights from the O and Hf isotopic composition of zircon

    NASA Astrophysics Data System (ADS)

    Jones, Rosemary E.; Kirstein, Linda A.; Kasemann, Simone A.; Dhuime, Bruno; Elliott, Tim; Litvak, Vanesa D.; Alonso, Ricardo; Hinton, Richard

    2015-09-01

    Subduction zones, such as the Andean convergent margin of South America, are sites of active continental growth and crustal recycling. The composition of arc magmas, and therefore new continental crust, reflects variable contributions from mantle, crustal and subducted reservoirs. Temporal (Ma) and spatial (km) variations in these contributions to southern Central Andean arc magmas are investigated in relation to the changing plate geometry and geodynamic setting of the southern Central Andes (28-32° S) during the Cenozoic. The in-situ analysis of O and Hf isotopes in zircon, from both intrusive (granitoids) and extrusive (basaltic andesites to rhyolites) Late Cretaceous - Late Miocene arc magmatic rocks, combined with high resolution U-Pb dating, demonstrates distinct across-arc variations. Mantle-like δ18O(zircon) values (+5.4‰ to +5.7‰ (±0.4 (2σ))) and juvenile initial εHf(zircon) values (+8.3 (±0.8 (2σ)) to +10.0 (±0.9 (2σ))), combined with a lack of zircon inheritance suggests that the Late Cretaceous (∼73 Ma) to Eocene (∼39 Ma) granitoids emplaced in the Principal Cordillera of Chile formed from mantle-derived melts with very limited interaction with continental crustal material, therefore representing a sustained period of upper crustal growth. Late Eocene (∼36 Ma) to Early Miocene (∼17 Ma) volcanic arc rocks present in the Frontal Cordillera have 'mantle-like' δ18O(zircon) values (+4.8‰ (±0.2 (2σ) to +5.8‰ (±0.5 (2σ))), but less radiogenic initial εHf(zircon) values (+1.0 (±1.1 (2σ)) to +4.0 (±0.6 (2σ))) providing evidence for mixing of mantle-derived melts with the Late Paleozoic - Early Mesozoic basement (up to ∼20%). The assimilation of both Late Paleozoic - Early Mesozoic Andean crust and a Grenville-aged basement is required to produce the higher than 'mantle-like' δ18O(zircon) values (+5.5‰ (±0.6 (2σ) to +7.2‰ (±0.4 (2σ))) and unradiogenic, initial εHf(zircon) values (-3.9 (±1.0 (2σ)) to +1.6 (±4.4 (2

  18. 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.

  19. Boron Isotope Constraints on Fluid-Rock Interactions in the Shallow Megathrust at the Japan Trench

    NASA Astrophysics Data System (ADS)

    Ishikawa, T.; Matsuoka, J.; Kameda, J.; Sample, J. C.; Mori, J. J.; Chester, F. M.

    2014-12-01

    IODP Expedition 343 (JFAST) drilled three holes through the plate boundary near the Japan Trench to investigate the cause of very large fault slip during the 2011 Tohoku-Oki earthquake. Interstitial fluids and rocks within and around the plate-boundary fault were recovered from Hole C0019E. Chemical characteristics of these fluids and rocks provide useful information for understanding fluid-related processes that occurred in the shallow megathrust fault zone at the Japan Trench. In this paper, we report concentrations and isotope ratios of boron determined for JFAST fluids and rocks, as well as for sediments from DSDP site 436, which is a nearby input site. Depth profiles of B content and B isotope ratio (δ11B value) for the interstitial fluids show a clear minimum and a maximum, respectively, around the plate boundary fault. Fluids from the vicinity of the fault are characterized by lower B content and higher δ11B value compared with seawater. The B contents and δ11B values of the plate-boundary fault rocks are indistinguishable from those of smectite-rich sediments from DPDP site 436, which is consistent with observations for other trace element compositions. In the systems composed of seawater-like fluid and sediment, both B concentrations and δ11B values in the fluid and solid phases are temperature-sensitive, and higher temperatures result in higher B and lower δ11B in the fluid phase and lower B and lower δ11B in the solid phase. Actually, interstitial fluids from ODP site 808 (Nankai Trough) showed a clear increase and a decrease of B and δ11B, respectively, with increasing depth at temperatures higher than 50 deg. C (You et al., 1995). The ODP site 808 rocks also showed distinct decreases of B and δ11B at the depths with temperatures higher than 100 deg. C (You et al., 1995). The observed B and δ11B characteristics of the JFAST fluids and rocks thus indicate that fluids and rocks within and around the plate-boundary fault have no clear record for

  20. 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

  1. 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.

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

    NASA Astrophysics Data System (ADS)

    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.

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

    NASA Technical Reports Server (NTRS)

    Mcsween, H. Y., Jr.

    1985-01-01

    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.

  4. Re-Os molybdenite ages and zircon Hf isotopes of the Gangjiang porphyry Cu-Mo deposit in the Tibetan Orogen

    NASA Astrophysics Data System (ADS)

    Leng, Cheng-Biao; Zhang, Xing-Chun; Zhong, Hong; Hu, Rui-Zhong; Zhou, Wei-De; Li, Chao

    2013-06-01

    The Miocene porphyry Cu-(Mo) deposits in the Gangdese orogenic belt in southern Tibet were formed in a post-subduction collisional setting. They are closely related to the Miocene adakite-like porphyries which were probably derived from a thickened basaltic lower crust. Furthermore, mantle components have been considered to have played a crucial role in formation of these porphyry deposits (Hou et al. Ore Geol Rev 36: 25-51, 2009; Miner Deposita doi:10.1007/s00126-012-0415-6, 2012). In this study, we present zircon Hf isotopes and molybdenite Re-Os ages on the newly discovered Gangjiang porphyry Cu-Mo deposit in southern Tibet to constrain the magma source of the intrusions and the timing of mineralization. The Gangjiang porphyry Cu-Mo deposit is located in the Nimu ore field in the central Gangdese porphyry deposits belt, southern Tibet. The copper and molybdenum mineralization occur mainly as disseminations and veins in the overlapped part of the potassic and phyllic alteration zones, and are predominantly hosted in the quartz monzonite stock and in contact with the rhyodacite porphyry stock. SIMS zircon U-Pb dating of the pre-mineral quartz monzonite stock and late intra-mineral rhyodacite porphyry yielded ages of 14.73 ± 0.13 Ma (2 σ) and 12.01 ± 0.29 Ma (2 σ), respectively. These results indicate that the magmatism could have lasted as long as about 2.7 Ma for the Gangjiang deposit. The newly obtained Re-Os model ages vary from 12.51 ± 0.19 Ma (2 σ) to 12.85 ± 0.18 Ma (2 σ) for four molybdenite samples. These Re-Os ages are roughly coincident with the rhyodacite porphyry U-Pb zircon age, and indicate a relatively short-lived episode of ore deposition (ca. 0.3 Ma). In situ Hf isotopic analyses on zircons by using LA-MC-ICP-MS indicate that the ɛ Hf( t) values of zircons from a quartz monzonite sample vary from +2.25 to +4.57 with an average of +3.33, while zircons from a rhyodacite porphyry sample vary from +5.53 to +7.81 with an average of +6.64. The

  5. 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

  6. 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.

  7. 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

  8. 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

  9. 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

  10. 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.

  11. Hf-W, Sm-Nd, and Rb-Sr isotopic evidence of late impact fractionation and mixing of silicates on iron meteorite parent bodies

    NASA Astrophysics Data System (ADS)

    Snyder, Gregory A.; Lee, Der-Chuen; Ruzicka, Alex M.; Prinz, Martin; Taylor, Lawrence A.; Halliday, Alex N.

    2001-03-01

    We report the first Sm-Nd and Rb-Sr isotopic analyses of silicate inclusions in four IIE iron meteorites: Miles, Weekeroo Station A and B, and Watson. We also report the Hf-W isotopic composition of a silicate inclusion from Watson and 182W/ 184W of the host FeNi metal in all four IIEs. The host metal in Watson has a negative ɛW value (-2.21±0.24), similar to or higher than other iron meteorites [1,35] and consistent with segregation of metal from silicate early in solar system history. However, the large silicate inclusion in the Watson IIE iron yielded a chondritic ɛW value (-0.50±0.55), thus indicating a lack of equilibration with the FeNi host within the practical lifetime of activity of the parent 182Hf (˜50 Ma). One of the silicate inclusions in Miles is roughly chondritic in major-element composition, has a present-day ɛNd of +10.3, relatively non-radiogenic 87Sr/ 86Sr (0.714177±13), and a TCHUR age of 4270 Ma. Two silicate inclusions from Weekeroo Station and one from Watson exhibit fractionated Sm/Nd and Rb/Sr ratios, and more radiogenic 87Sr/ 86Sr (0.731639±12 to 0.791852±11) and non-radiogenic ɛNd values (-5.9 to -13.4). The silicate inclusion in Watson has a TCHUR age of 3040 Ma, in agreement with previously determined 4He and 40Ar gas retention ages, indicative of a late thermal event. A later event is implied for the two silicate inclusions in Weekeroo Station, which yield indistinguishable TCHUR ages of 698 and 705 Ma. Silicate inclusions in IIE iron meteorites formed over a period of 3 billion yr by impacts, involving an H-chondrite parent body and an FeNi metal parent body. The LILE-enriched nature of some of these silicates suggests several stages of melting, mixing, and processing. However, there is little evidence to suggest that the silicates in the IIE irons were ever in equilibrium with the host FeNi metal.

  12. 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

  13. 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

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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

  19. 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

  20. 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.

  1. Radiogenic isotopic constraints from the Project Hotspot Kimama core: implications for Hotspot-controlled lithosphere interactions beneath the Snake River Plain

    NASA Astrophysics Data System (ADS)

    Potter, K. E.; Hanan, B. B.; Shervais, J. W.

    2013-12-01

    Project Hotspot, the Snake River Scientific Drilling Project, seeks to understand the evolution of Snake River Plain -Yellowstone Plateau volcanism through time. Radiogenic isotope chemistry, paleomagnetic stratigraphy, and 40Ar/39Ar geochronology from the Kimama core temporally constrain the mass proportions and flux of magma source components in Snake River Plain-Yellowstone Plateau (SRP-YP) basaltic volcanism. We present new radiogenic isotope data for the Kimama core of the central Snake River Plain that support the regional model of plume-continental lithosphere interaction and westward source variation over the past ~ 12 Ma. The 1912 m Kimama core provides a nearly continuous depositional record of basaltic lava flows on the central Snake River Plain from the late Miocene through Pleistocene. Most of the basalt flows are Snake River olivine tholeiites with MgO 6-10%, Fe2O3 < 16%, and K2O <0.9%. Compositionally evolved basalts similar to those erupted at Craters of the Moon (high K2O, Fe2O3, and Zr) were identified at various depths throughout the core. We analyzed 15 basalt samples from a range of geochemical compositions and depths within the Kimama core for Nd, Sr, Hf, and Pb. Radiogenic Pb isotope values for Kimama basalts ranged from 206Pb/204Pb ~18.0--18.5, 207Pb/204Pb ~15.6--15.7, and 208Pb/204Pb ~38.5--39.0. Radiogenic Hf isotopes range from 0.282683--0.282745. Evolved basalts span the same range of 177Hf/176Hf, 207Pb/204Pb, and 208Pb/204Pb as the more primitive basalt compositions (high MgO, Cr, and Ni). Ar/Ar and paleomagnetic dating establish a relatively linear basalt accumulation rate of 305 m/m.y. and a projected bottom hole age of 6.2 Ma.

  2. 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

  3. 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.

  4. 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

  5. Decoupling of U-Pb and Lu-Hf isotopes and trace elements in zircon from the UHP North Qaidam orogen, NE Tibet (China): Tracing the deep subduction of continental blocks

    NASA Astrophysics Data System (ADS)

    Xiong, Qing; Zheng, Jianping; Griffin, William L.; O'Reilly, Suzanne Y.; Pearson, Norman J.

    2012-12-01

    Eclogites and gneisses from the Yuka and Xitieshan areas in the North Qaidam orogen, NE Tibet, experienced ultrahigh-pressure (UHP) metamorphism in the early Paleozoic, when the Qaidam block subducted northwards beneath the Qilian block. U-Pb ages, trace-element patterns and Hf-isotope compositions of zircon in the two rock types were used to trace the isotopic and elemental changes within zircon during continental subduction and exhumation, and to examine the links between the igneous protolith and tectonic behavior of the subducting block. Zircon data suggest that the eclogites had igneous protoliths with ages of ca. 785 ± 20 Ma (2σ, MSWD = 3.4, n = 5), and most zircons metamorphically recrystallized during eclogitization at ca. 441 ± 2 Ma (2σ, MSWD = 1.15, n = 41); minor changes were caused by retrogression during ~ 417-409 Ma. Trace elements of these zircons were incompletely reset in the prograde and retrograde processes; however, their Lu-Hf isotopic compositions retained the primary igneous 176Hf/177Hf ratios, reflecting a large extent of initial heterogeneity (ɛHf(t) = + 14.0 to - 19.5), even within single grains. Lu/Hf ratios decreased as a result of the eclogitization, and then increased again during retrogression. The behaviors of the U-Pb and Lu-Hf isotopic systems as well as the trace elements of the eclogitic zircons were decoupled during the early Paleozoic orogeny. Zircons from the gneisses mostly preserved their primary diverse signatures (ɛHf(t) = + 15.8 to - 17.9; crustal model ages = 2.54-0.80 Ga), but some ~ 440 Ma metamorphic grains show lower trace-element concentrations than the Proterozoic zircons. U-Pb and Lu-Hf isotopic data in this study and from the literature reveal the existence of Neoarchean (ca. 2.8-2.5 Ga) components in the Qaidam block as the initial protoliths of the gneisses, which were intruded by depleted-mantle (DM) magmas at ~ 2.1-2.0 Ga and 1.1-0.8 Ga. The igneous protoliths of the eclogites were emplaced into the

  6. 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.

  7. 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

  8. Ca Cycle Constraints from the Ca Isotope Composition of Precambrian Sedimentary Carbonates

    NASA Astrophysics Data System (ADS)

    Blättler, C. L.; Higgins, J. A.

    2014-12-01

    The geochemical cycle of Ca in seawater is relatively straightforward - Ca is ultimately sourced from the weathering of silicates and largely removed as carbonates. Most of these carbonates are then recycled through subsequent uplift and weathering, but some are metamorphosed or returned to the mantle via subduction. Ca isotopes in sedimentary marine carbonates can provide new insights into the global Ca (and by corollary, C) cycle by constraining the flux and isotopic composition of the recycled and subducted Ca sinks on billion-year timescales. Precambrian applications of Ca isotopes have so far been limited to relatively small datasets which cover unusual, dynamic intervals of the Proterozoic. In order to address questions about long-timescale Ca cycling, Ca isotopes were measured on an extensive suite of Precambrian carbonates (n > 300) which represent environments, textures, and morphologies that are typical for their age, and whose carbon isotope values have been interpreted as reflecting precipitation from seawater. Because marine carbonates constitute the dominant geological sink for Ca, this representative sample set places specific limits on the evolution of the sedimentary and crustal Ca reservoirs and suggests further applications for Ca isotopes in Precambrian time.

  9. 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.

  10. 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

  11. 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

  12. Srsbnd Ndsbnd Pb isotopic compositions of Early Cretaceous granitoids from the Dabie orogen: Constraints on the recycled lower continental crust

    NASA Astrophysics Data System (ADS)

    He, Yongsheng; Li, Shuguang; Hoefs, Jochen; Kleinhanns, Ilka C.

    2013-01-01

    subduction. Earth and Planetary Science Letters 308, 229-244]. Combining the Srsbnd Ndsbnd Pbsbnd Hf and Osbnd C isotopic data, two types of commonly present magma components have been identified in the PCMI, reflecting crust-mantle interaction during the Triassic subduction and the Early Cretaceous orogen collapse, respectively.

  13. 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.

  14. 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

  15. 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.

  16. 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.

  17. 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

  18. 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

  19. 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.

  20. Lithium and strontium isotopic systematics in playas in Nevada, USA: constraints on the origin of lithium

    NASA Astrophysics Data System (ADS)

    Araoka, Daisuke; Kawahata, Hodaka; Takagi, Tetsuichi; Watanabe, Yasushi; Nishimura, Koshi; Nishio, Yoshiro

    2014-03-01

    Lithium-rich brine in playas is a major raw material for lithium production. Recently, lithium isotopic ratios (δ7Li) have been identified as a tool for investigating water-rock interactions. Thus, to constrain the origin of lithium in playas by the use of its isotopes, we conducted leaching experiments on various lacustrine sediment and evaporite deposit samples collected from playas in Nevada, USA. We determined lithium and strontium isotopic ratios and contents and trace element contents of the leachate, estimated the initial δ7Li values in the water flowing into the playas, and examined the origin of lithium in playas by comparison with δ7Li values of the possible sources. In samples from the playas, δ7Li values show some variation, reflecting differences both in isotopic fractionation during mineral formation and in initial δ7Li value in water flowing into each playa. However, all δ7Li values in this study are much lower than those in river water and groundwater samples from around the world, but they are close to those of volcanic rocks. Considering the temperature dependence of lithium isotopic fractionation between solid and fluid, these results indicate that the lithium concentrated in playas in Nevada was supplied mainly through high-temperature water-rock interaction associated with local hydrothermal activity and not directly by low-temperature weathering of surface materials. This study, which is the first to report lithium isotopic compositions in playas, demonstrates that δ7Li may be a useful tracer for determining the origin of lithium and evaluating its accumulation processes in playas.

  1. 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.

  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. U-Pb zircon dating, geochemical and Sr-Nd-Hf isotopic compositions of Early Indosinian intrusive rocks in West Qinling, central China: petrogenesis and tectonic implications

    NASA Astrophysics Data System (ADS)

    Luo, Biji; Zhang, Hongfei; Lü, Xinbiao

    2012-10-01

    The Qinling-Dabie-Sulu orogenic belt is the junction between the North and South China blocks, which resulted from the final amalgamation of China continents during the Indosinian. Indosinian granitoids are widespread in the Qinling orogen, and their geneses can thus constrain the evolution of China continent. We carried out a combined U-Pb zircon dating and geochemical study for the Shuangpengxi granodiorite pluton and the Xiekeng diorite-granodiorite pluton in the middle part of the West Qinling orogen. U-Pb zircon dating shows that the magma crystallization ages of 242 ± 3 Ma for the Shuangpengxi pluton and ~244-242 Ma for the Xiekeng pluton. Geochemical and Sr-Nd-Hf isotopic compositions reveal that the magma of the Shuangpengxi granodiorite was derived from partial melting of crustal materials. The Xiekeng diorites can be divided into high-Al diorite and high-Mg diorite. Both of them resulted from partial melting of enriched lithospheric mantle, but their mantle source had been modified by previous slab-derived melt. The high-Al diorite was formed by fractional crystallization of olivine, pyroxene and/or preferential accumulation of plagioclase, and the high-Mg diorite was formed by fractional crystallization of olivine and/or preferential accumulation of pyroxene. The Xiekeng granodioritic porphyry was formed by mixing of crust-derived and mantle-derived melts. We propose that the Early Indosinian magmatism resulted from break-off of subducted oceanic slab after collision. The slab break-off model can well explain the linear distribution of the Early Indosinian plutons and rapid crustal uplift during the Middle Triassic in the West Qinling.

  4. Iron isotope fractionation in sulfides: constraints on mechanisms of sulfide formations in hydrothermal and magmatic systems.

    NASA Astrophysics Data System (ADS)

    Polyakov, Veniamin; Soultanov, Dilshod

    2010-05-01

    Data on non-traditional stable isotope fractionations (e.g., Fe, Cu) provide further insight into mechanisms of sulfide mineralization. Correct interpretation of these data is impossible without knowledge on equilibrium isotopic fractionation factors of sulfides. We present data on iron isotope fractionation factors (β-factors) of chalcopyrite (CuFeS2) and mackinawite (FeS). Iron β-factors for chalcopyrite were derived from synchrotron experimental data on inelastic nuclear resonant x-ray scattering (INRXS) [1] using the method described elsewhere [2,3]. The β-factors for mackinawite were found from the Moessbauer second-order Doppler shift data [4] by the method presented in [5]. The temperature dependence of the iron β-factors are fitted by following third-order polynomials: 103lnβcpy = 0.82560x - 0.01298x2 + 0.0005246x3 103lnβmcw = 0.2542x - 0.0001847x2 + 2.072×103x3 where x=106/T2 Using these data along with β-factors for pyrite and troilite [3,6], we compared iron isotope fractionation between pyrite and chalcopyrite in hydrothermal and magmatic conditions. Rouxel et al. [7] studied iron isotope of seafloor of hydrothermal vents in detail. They found that pyrite is enriched in light iron isotope relative to chalcopyrite in the case of black smoker Bio 9. This result evidences absence of iron isotope equilibrium between pyrite and chalcopyrite, because in equilibrium pyrite is enriched in heavy iron isotope (βpy > βcpy). Quantitatively, iron isotope fractionation between chalcolpyrite and pyrite is very close to equilibrium iron isotope fractionation between chalcolpyrite and FeS phase (mackinawite or troilite). This agrees the mechanism of pyrite formation through intermidient FeS phase if to assume isotopic equilibrium between the FeS phase and dissolved iron and no isotopic effect in the final stage of conversion FeS to FeS2 (pyrite). Another iron isotope fractionation was observed between pyrite and chalcopyrite in the case of the Cu-Au porphyry

  5. 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

  6. Sr, Nd, Hf and Pb isotope systematics of basaltic samples from IODP Site U1349A, Ori Massif of the Shatsky Rise Oceanic Plateau, Northwest Pacific

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

    Shatsky Rise is located on the northwest Pacific plate, and has an elongated structure from southeast to northwest. It represents a large oceanic plateau (up to 500 km wide) that was formed between 140-150 Ma in an active spreading ridge at relatively high emplacement rates (1.2-4.6 km3/y), similar to those of some flood basalt provinces [1]. Whether the Shatsky Rise formed by the impact of a mantle plume head or by shallow mantle processes at a rapidly extending triple junction is a matter of ongoing debate. Geochemical research will provide vital information on mantle sources and melting conditions contributing to LIP magmatism [1]. The Ori Massif is one of the three elevated massifs within the Shatsky Rise, comprising a volume of 0.7 × 106 km3. It is situated in the middle of the plateau with the largest edifice Tamu Massif to the southeast and the smaller Shirshov Massif to the northwest. Within plume theory Ori Massif formed at an intermediate to early stage of a plume volcanism. In shallow mantle theory it formed from a shallow lithospheric source during oceanic spreading at a triple junction [1]. During IODP Expedition 324 two sites were drilled on Ori Massif, one of which, U1349A, targeted the summit. The Hole U1349A penetrated 250.4m beneath the seafloor, and includes 165.1m of sediments and 85.3m of igneous basement. The uppermost volcanic flows (~55m) are highly vesicular, stongly altered to reddish brown clays and contain abundant pseudomorphs after olivine. These flows are underlain by highly altered volcaniclastic breccia (>40m) [1]. Here we present Sr, Nd, Hf and Pb isotope characterisation of the U1349A basaltic basement samples. Our results on the Ori Massif will be combined with isotope data from the Tamu Massif and Shirshov Massif. Thus, some preliminary conclusions about the source of mantle material that formed the Shatsky Rise oceanic plateau and its variation in space and time can be made. [1] Expedition 324 Scientists (2010) Expedition

  7. An Integrated Analytical Approach to Obtaining Reliable U-Pb and Hf Isotopic Data from Complex (>3.9 to 3.3 Ga) Zircon from the Acasta Gneiss Complex

    NASA Astrophysics Data System (ADS)

    Bauer, A.; Bowring, S. A.; Vervoort, J. D.; Fisher, C. M.

    2014-12-01

    The Acasta Gneiss Complex (AGC) of northwestern Canada preserves some of Earth's oldest granitic crust (>4.03 Ga) and thereby contains important insight into crust forming processes on the early Earth. In general, rocks of the AGC have undergone a complex history of metamorphism and deformation (Archean and Paleoproterozoic)1,2, and, as a consequence, the zircons retain a complex history including inheritance, magmatic and metamorphic overgrowths, recrystallization, and multi-stage Pb loss. Previously published Hf isotopic data on zircons show within sample variability in excess of analytical uncertainty2,3,4. In order to assess the meaning and significance of this apparent isotopic variability, we are using two different methods to obtain coupled U-Pb and Lu-Hf isotopic data in zircon from a suite of rocks ranging in age from ca. > 3.9 Ga to 3.3 Ga. To obtain these data from the same volume of zircon, our approach involves: 1) split stream LA-ICPMS for U-Pb and Lu-Hf; 2) mechanical isolation of zircon domains for chemical abrasion and ID-TIMS U-Pb analyses and solution ICPMS for Lu-Hf recovered from U-Pb ion exchange chromatography. The deconvolution of complex histories requires this integrated approach and permits us to take advantage of both high spatial resolution and highest precision measurements to ultimately decipher the age and isotopic composition of discrete domains of multi-phase zircon. We demonstrate our approach with both relatively simple and complex grain populations in an attempt to understand within and between grain heterogeneity. The samples with the simplest zircon systematics have increasingly negative ɛHf from oldest to youngest, consistent with involvement of 4.0 Ga or older crust in later generations; also, none of our samples have been derived solely from strongly depleted sources. The presence of intra-zircon variability within samples from the AGC reflects a complex history of magmatic additions requiring melting/assimilation of older

  8. 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

  9. Constraints on ocean circulation at the Paleocene-Eocene Thermal Maximum from neodymium isotopes

    NASA Astrophysics Data System (ADS)

    Abbott, A. N.; Haley, B. A.; Tripati, A. K.; Frank, M.

    2015-06-01

    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 for 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 and comparing data with published data from fossil fish debris 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 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 global recovery of the ocean-atmosphere system after the PETM.

  10. 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

  11. 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

  12. Shear heating and solid state diffusion: Constraints from clumped isotope thermometry in carbonate faults

    NASA Astrophysics Data System (ADS)

    Siman-Tov, S.; Affek, H. P.; Matthews, A.; Aharonov, E.; Reches, Z.

    2015-12-01

    Natural faults are expected to heat rapidly during seismic slip and to cool quite quickly after the event. Here we examine clumped isotope thermometry for its ability to identify short duration elevated temperature events along frictionally heated carbonate faults. This method is based on measured Δ47 values that indicate the relative atomic order of oxygen and carbon stable isotopes in the calcite lattice, which is affected by heat and thus can serve as a thermometer. We examine three types of calcite rock samples: (1) samples that were rapidly heated and then cooled in static laboratory experiments, simulating the temperature cycle experienced by fault rock during earthquake slip; (2) limestone samples that were experimentally sheared to simulate earthquake slip events; and (3) samples taken from principle slip zones of natural carbonate faults that likely experienced earthquake slip. Experimental results show that Δ47 values decrease rapidly (in the course of seconds) and systematically both with increasing temperature and shear velocity. On the other hand, carbonate shear zone from natural faults do not show such Δ47 reduction. We propose that the experimental Δ47 response is controlled by the presence of high-stressed nano-grains within the fault zone that can reduce the activation energy for diffusion by up to 60%, and thus lead to an increased rate of solid-state diffusion in the experiments. However, the lowering of activation energy is a double-edged sword in terms of clumped isotopes: In laboratory experiments, it allows for rapid disordering so that isotopic signal appears after very short heating, but in natural faults it also leads to relatively fast isotopic re-ordering after the cessation of frictional heating, thus erasing the high temperature signature in Δ47 values within relatively short geological times (<1 Ma).

  13. 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.

  14. 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

  15. New Constraints on K-Pg boundary Environmental Changes with Lithium Isotopes

    NASA Astrophysics Data System (ADS)

    Vigier, N.; Ravizza, G. E.; Nagashima, K.; Norris, R. D.; Petit, S.; Beaufort, D.; Karpoff, A. M.

    2014-12-01

    Climatic and oceanic perturbations associated with the Deccan Traps eruption and with the Chicxulub impact are still strongly debated. Because lithium isotopes significantly fractionate during low temperature processes, the composition of mineral phases formed across the K-Pg boundary can provide quantitative information concerning environmental changes at that time. In this study, we investigate in parallel the Li isotope composition of both marine authigenic smectite and benthic foraminifera over a time window of 3 Ma. Comparing both phases is important because the d7Li of foraminifera may be subject to vital effects during calcification, while authigenic clays are not. The objectives are: 1/ To determine the long-term variation of seawater d7Li due to changes in continental flux and alteration rate and 2/ To detect short-term variations of the ocean carbon chemistry, recorded by Li isotope fractionation during foraminifera growth. Li isotopes are measured in clays by MC-ICP-MS, and in benthic foraminifera tests using the ims 1280 ion microprobe, at University of Hawaii. Initial results show a limited variation (<2‰) of clay d7Li across the Ir-rich layer, indicating little disturbance of the ocean Li at that time. Compared with 0-6 Ma seawater, K-Pg seawater d7Li was 5.5‰ lower. In contast, contemporaneous benthic foraminifera tests vary significantly on time scales much shorter than the oceanic residence time of Li (≈1.5 Ma), too short to attribute to changes in the d7Li of the global ocean Also, these variations precede deposition of the Ir-rich layer. These results strongly suggest changes in Li isotope fractionation through time. The d7Li of cultured foraminifera indicate a strong dependency on the DIC concentration, but no impact of pH and T on Li isotope fractionation during foraminifera growth. If these data are representative of natural conditions, then our results indicate significant short-term oscillations of ocean DIC before Chicxulub, possibly

  16. 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

  17. 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

  18. U-Pb zircon dating, geochemical and Sr-Nd-Hf isotopic compositions of mafic intrusive rocks in the Motuo, SE Tibet constrain on their petrogenesis and tectonic implication

    NASA Astrophysics Data System (ADS)

    Pan, Fa-Bin; Zhang, Hong-Fei; Xu, Wang-Chun; Guo, Liang; Luo, Bi-Ji; Wang, Shuai

    2016-02-01

    Mafic intrusive rocks are widely exposed in the Motuo tectono-magmatic belt, southeast Lhasa terrane. LA-ICP-MS U-Pb zircon dating shows that they have magma crystallization ages of 69 and ca. 50 Ma. These mafic intrusive rocks are characterized by variable SiO2 (44.60-57.60 wt.%), high Al2O3 (17.19-20.86 wt.%), and low MgO (1.85-5.38 wt.%) with Mg# of 31-55. Their chemical composition is comparable with low-MgO high-Al basalts to basaltic andesites. They are enriched in LILEs (Rb, Ba, K) and LREE and depleted in HFSEs (Nb, Ta, Zr, Hf, Ti), with weakly evolved Sr-Nd-Hf compositions (whole-rock (87Sr/86Sr)0 = 0.7064 to 0.7086, εNd(t) = - 3.41 to + 1.22, and zircon εHf(t) = - 3.8 to + 6.4). The mafic rocks were derived from partial melting of metasomatized lithospheric mantle. Geochemical and Sr-Nd-Hf isotopic data show that they were insignificantly modified during magma emplacement. We provide a different secular evolution of the lithospheric mantle beneath the eastern part relative to the central part of the southern Lhasa terrane. Sr-Nd isotopic composition of the Motuo Late Cretaceous-Eocene mafic rocks argues that they were derived from partial melting of a relatively homogeneous and depleted lithospheric mantle. We propose that the Late Cretaceous delamination resulted in the replacement of ancient lithospheric mantle by the juvenile homogeneous lithospheric mantle in the eastern Lhasa terrane. The foundered ancient materials may subsequently re-fertilize the lithospheric mantle not only in the eastern Lhasa terrane but also in the surrounding areas.

  19. Trace element and isotopic constraints on magmatic evolution at Lassen Volcanic Center

    NASA Astrophysics Data System (ADS)

    Bullen, Thomas D.; Clynne, Michael A.

    1990-11-01

    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 andesitic, or "Brokeoff phase" of volcanism range in composition from basaltic andesite io dacite, whereas those of silicic, or "Lassen phase" range in composition from basaltic andesite to rhyolite. The compositions of magmas from each phase define well organized but distinct variation trends. Compared with Brokeoff-phase magmas of similar SiO2 content, most Lassen-phase magmas contain lower concentrations of most incompatible minor and trace elements. Based on the behavior of both incompatible and compatible trace elements, the geochemical trends defined by the Brokeoff-phase magmas cannot be ascribed merely to fractional crystallization from a single or multiple mafic parental magmas, Moreover, the Lassen-phase magmas cannot be derived from the Brokeoff-phase magmas by fractional crystallization. Rather, the geochemical trends that characterize each volcanic phase define arrays that primarily indicate mixing between well-homogenized silicic and heterogeneous mafic magmas. 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 isotopic 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 KSamath and Sierra Nevada provinces ("KSN" component). The isotopic compositions of the most silicic LVC magmas lie within the ranges defined by the more mafic LVC magmas, which in turn lie within broad ranges defined by primitive mafic lavas sampled from the Lassen region. The lack of a correlation between the major element and isotopic compositions of LVC

  20. 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

  1. Stable isotopic constraints on the nature of the syntectonic fluid regime of the Canadian cordillera

    SciTech Connect

    Nesbitt, B.E.; Muehlenbacks, K. )

    1991-05-01

    Oxygen, hydrogen and carbon isotopic analyses of rocks from the Canadian Cordillera document regional variations in the origins and evolution of crustal fluids. High temperature systems in unmetamorphosed, felsic volcanic rocks are characterized by low {delta}{sup 18}O values. Veins in regional metamorphic rocks are characterized by {delta}{sup 18}O values that reflect approximate oxygen isotopic equilibration of the fluids with regional rock units. The {delta} D studies indicate that both systems were dominated by meteoric water. The {delta}{sup 13}C values for vein carbonate indicate a regional lithologic control on {delta}{sup 13}C values. The data indicate that convection of surface fluids to depth of 10 or more kilometers during orogenesis was common. The existence of deep convection cells involving meteoric water further indicates that fluid pressures to these depths must have been approximately hydrostatic. Regional structural style appears to exert an important control on the depth of penetration of surface fluids.

  2. 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.

  3. 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.

  4. 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.

  5. 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

  6. 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

  7. 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.

  8. 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

  9. Zircon SHRIMP U Pb ages and in-situ Hf isotopic analysis for the Mesozoic intrusions in South Taihang, North China craton: Evidence for hybridization between mantle-derived magmas and crustal components

    NASA Astrophysics Data System (ADS)

    Chen, B.; Tian, W.; Jahn, B. M.; Chen, Z. C.

    2008-04-01

    The North China craton (NCC) was stabilized in Paleo-Proterozoic times, and the eastern part of it was remobilized in the Mesozoic, as is indicated by the presence of voluminous Mesozoic magmas in eastern NCC. In particular, the lithospheric mantle beneath eastern NCC was significantly thinned during the intense Mesozoic tectono-magmatic activity, which makes the NCC different from most other ancient cratons. The Mesozoic magmas may thus provide crucial evidence regarding the processes that transformed the NCC from craton to a tectonically active region. The South Taihang magmatic complex lies in the western part of the Mesozoic magmatic zone in the NCC. The complex falls into three main plutonic rocks: the Hongshan syenite, the Xishu gabbroic diorite-monzonite and the Wu'an monzonitic rocks. Zircon U-Pb dating reveals that the three plutonic rocks were emplaced contemporaneously at ˜ 132 Ma. In-situ Hf isotopic analyses for the dated zircons indicate that the Hongshan syenites show homogeneous ɛHf( t) values (- 10 to - 13) which are slightly lower than those (- 9.0) for the enriched subcontinental lithospheric mantle (SCLM), as revealed by zircons from mafic rocks that originated from melting of SCLM. This suggests that the parental magma to the syenites was likely derived from melting of the enriched SCLM and contaminated slightly by old lower continental crustal material during ascent. The Xishu and Wu'an gabbroic diorites to monzonitic rocks, however, show significantly varied Hf isotopic compositions, with ɛHf( t) = - 9.2 to - 16 and - 15.5 to - 24, respectively. The wide range of ɛHf( t) values for a single sample suggests that the Xishu and Wu'an plutons formed through a process of hybridization between enriched mantle-derived mafic magma and crustal components in variable proportions. This is supported by the textural and compositional disequilibrium of plagioclase phenocrysts from the two plutons. The Wu'an monzonitic rocks show lower ɛHf( t) values

  10. 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

  11. 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.

  12. 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.

  13. A trio of laser ablation in concert with two ICP-MSs: Simultaneous, pulse-by-pulse determination of U-Pb discordant ages and a single spot Hf isotope ratio analysis in complex zircons from petrographic thin sections

    NASA Astrophysics Data System (ADS)

    Tollstrup, Darren L.; Xie, Lie-Wen; Wimpenny, Josh B.; Chin, Emily; Lee, Cin-Ty; Yin, Qing-Zhu

    2012-03-01

    We have developed a technique for the simultaneous in situ determination of U-Pb ages and Hf isotope ratios from a single spot in complex,discordantzircons by combining both a single-collector and a multicollector sector field inductively coupled plasma-mass spectrometry (ICP-MS) with a 193 nm excimer laser ablation system. With a suite of zircon standards of various ages, we first show that U-Pb ages can be determined accurately to within 0.3-2.5% (2σ) compared to the nominal value, while the internal errors are better than 0.4-0.7%; hafnium isotope ratios are accurate, relative to solution analyses, within one epsilon unit, and internal errors are typically <0.008%. We then apply the technique to complex, discordant zircons with variable 206Pb/238U and 207Pb/235U ratios, commonly discarded previously as "un-reducible data," to construct a Discordia in U-Pb Concordia plot, using every scan, every laser pulse as individual data points from a single laser ablation spot (typically > 200-250 data points). We show that the upper and lower intercept ages from the Discordia, augmented by high precision Hf isotope data obtained on the same spot, reveal invaluable information that permit unique insight to geological processes not available by other means. We demonstrate that our technique is useful for provenance studies of small, complex detrital zircons in sedimentary and high-grade metamorphic rocks, in relation to crustal growth and evolution.

  14. 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

  15. 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

  16. 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

  17. The U-Pb age and Hf isotope record of detrital zircons from Paleozoic sedimentary rocks of the proto-Andean accretionary orogen

    NASA Astrophysics Data System (ADS)

    Bahlburg, Heinrich

    2010-05-01

    -Pb detrital zircon ages (n= c. 5900) from Paleozoic sedimentary rocks representing the proto-Andes from Ecuador to Patagonia shows that age peaks coincide very well with the timing of assembly and dispersal of supercontinents. The coincidence with the respective record including collisional orogenic belts indicates that the preservational bias is not necessarily as pronounced a factor in shaping this record as previously thought. A similar compilation of Hf isotope data from proto-Andean detrital zircons (n= c. 620) shows a major divergence from the global patterns presented by Hawkesworth et al. (2009). The global data indicate peaks of juvenile crust production at the Archean-Paleoproterozoic transition and between 1.9 and 1.6 Ga. Subsequent orogenic cycles are not strongly reflected by juvenile data. This dichotomy is interpreted as due to the preferential preservation of evolved late stage orogenic crust in the rock record. In contrast, the detrital zircon Hf data from Paleozoic proto-Andean sedimentary rocks located in the the Andes from northern Peru to Patagonia reveal two important patterns, (i) production of juvenile crust occurred in the SW Amazonia Orogenic System and the proto-Andes throughout the Late Archean and Proterozoic; and (ii) a maximum seems to have been attained in the Rondonia-San Ignacio and Sunsas orogenic cycles between 1.55 and 0.9 Ga. The production of juvenile crust appears to have become a minor factor in this region at the Precambrian-Phanerozoic boundary. References Campbell, I.H. & Allen, C.M., 2008, Nature Geoscience 1, 554-558. Evans, D.A.D., 2009, Geological Society, London, Special Publication 327, 371-404. Hawkesworth, C. et al., 2009. Science 323, 49-50.

  18. 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

  19. 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

  20. 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

  1. 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

  2. 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.

  3. Crust-mantle interaction in the central North China Craton during the Mesozoic: Evidence from zircon U-Pb chronology, Hf isotope and geochemistry of syenitic-monzonitic intrusions from Shanxi province

    NASA Astrophysics Data System (ADS)

    Ying, Ji-Feng; Zhang, Hong-Fu; Tang, Yan-Jie

    2011-07-01

    In-situ zircon U-Pb ages, Hf isotopic compositions and whole rock geochemical and Sr-Nd isotopic compositions are presented for three Mesozoic syenitic-monzonitic intrusions from Shanxi province, central North China Craton. Zircons from these intrusions all show core-rim structures in that the oscillatory rims recorded their intrusive ages, whereas the cores are interpreted as xenocrysts. The U-Pb age data reveal that the northernmost Dishuiyan monzonite was emplaced at 241 Ma, while the Huyanshan and Erfengshan syenitic-monzonitic complexes were emplaced at 130 Ma and 128-134 Ma, respectively. The Dishuiyan monzonite is petrologically and geochemically uniform, it shows LREE enrichment and HFSE depletion and exhibits enriched Sr and Nd isotopic compositions with (87Sr/86Sr)i = 0.7065 and εNd(t) = - 8.3--7.9. The zircon rims in the Dishuiyan monzonite show highly varied Hf isotopic compositions with εHf(t) = - 27.8--6.4. The varied Hf isotopic compositions and enriched Sr-Nd isotopic compositions, together with the ubiquitous xenocrystic zircon cores, suggest the Dishuiyan monzonite was produced by the mixing of melts from enriched lithospheric mantle and lower crust. The monzonite and syenite from the Huyanshan complexes exhibit different geochemical features. The εNd(t) values of syenite, which are higher than those of monzonite resemble the enriched lithospheric mantle, and together with the absence of zircon in the syenite, we propose that it was originated by partial melting of enriched lithospheric mantle. Monzonites from Huyanshan and Erfengshan share similar petrological and geochemical characteristics, being enriched in LREE and depleted in HFSE, and they show low εNd(t) values between - 18.2 and - 13.9. Similar to zircon rims in the Dishuiyan monzonite, those in the Huyanshan and Erfengshan monzonites also exhibit highly varied Hf isotopic compositions. The consistent ages between the xenocrystic zircons in monzonites and the lower crustal basement

  4. 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.

  5. Isotopic constraints on the origin of ultramafic and mafic dikes in the Balmuccia peridotite (Ivrea Zone)

    NASA Astrophysics Data System (ADS)

    Voshage, H.; Sinigoi, S.; Mazzucchelli, M.; Demarchi, G.; Rivalenti, G.; Hofmann, A. W.

    1988-11-01

    The Balmuccia peridotite massif in the central Ivrea Zone constitutes an upper mantle slice which has been tectonically emplaced into the crust. It represents the residue from partial melting of undepleted mantle material and varies in composition from lherzolite to harzburgite and subordinate dunite. Dikes of websterite and gabbroic pods within the peridotite can be subdivided into an older Crdiopside suite and a younger Al-augite suite. Nd isotopic data on whole rocks of these lithotypes in combination with independent observations suggest that the dikes formed during a Hercynian event about 270 Ma ago. The rocks of the Cr-diopside dikes, in particular, display isotopic signatures similar to those of the lherzolite and represent fractionates from partial melts derived from the lherzolite wall rock. The Sm-Nd data of the pyroxenites and gabbros of the Al-augite suite, in contrast, scatter widely and suggest that partial melting of lherzolite was triggered or at least accompanied by introduction of fluids and/or liquid phases. These fluids or liquids carried exotic isotopic components from elsewhere in the crust-mantle complex, and deposited them within the rocks by metasomatic reactions. Two distinct types of metasomatism must have operated not only within the Balmuccia body, but also in the complex of Finero: The first type of metasomatism introduced mantle-derived volatiles and is responsible for formation of amphibole. The other type has a crustal source and led to formation of phlogopite, which occurs mainly within mantle rocks of Finero, but occasionally, within the Balmuccia body also.

  6. 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.

  7. 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.

  8. Re Os isotope constraints on subcontinental lithospheric mantle evolution of southern South America

    NASA Astrophysics Data System (ADS)

    Schilling, Manuel Enrique; Carlson, Richard Walter; Conceição, Rommulo Vieira; Dantas, Celine; Bertotto, Gustavo Walter; Koester, Edinei

    2008-04-01

    We present Re-Os isotopic data for widely dispersed mantle xenoliths carried to the surface of southern South America (36°-52° S) by Eocene to recent alkaline magmatism. Our hypothesis is that the lithospheric mantle sections formed as the roots of southern South America reflect the history of crust formation and amalgamation at different periods of time and so present a complimentary picture of continent growth in South America by sampling deeper sections of continental lithosphere than provided by crustal rocks from the area. The Re-Os isotopic system gives unique chronological information about the time of mantle depletion that is associated with lithosphere formation. Our data show coherent model ages for the lithospheric mantle that can be correlated with some hypotheses for crustal evolution of this region. Most samples show Os isotopic values similar to the present suboceanic mantle, suggesting a relatively recent lithospheric mantle formation from the convecting mantle. Xenoliths from Agua Poca and Prahuaniyeu represent fragments of an ancient depleted lithosphere, probably corresponding to the roots of the Cuyania terrane inferred to be a fragment derived from Laurentia and formed during the Mesoproterozoic. Alternatively, all or parts of the recognized ancient lithosphere are relicts of other known ancient continental blocks, such as the Pampia terrane or the Río de la Plata craton. Samples erupted in the southwest corner of the Deseado Massif give Proterozoic depletion ages (1.34 to 2.11 Ga) that are considerably older than previous radiogenic formation ages obtained for the very few basements rocks of this continental block. We propose that Deseado Massif is Proterozoic in age, probably related to the Malvinas/Falkland Islands and plateau and so should be considered for the reconstruction of the supercontinent of Rodinia.

  9. 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.

  10. Geochronological and geochemical constraints on the formation and evolution of the mantle underneath the Kaapvaal craton: Lu-Hf and Sm-Nd systematics of subcalcic garnets from highly depleted peridotites

    NASA Astrophysics Data System (ADS)

    Shu, Qiao; Brey, Gerhard P.; Gerdes, Axel; Hoefer, Heidi E.

    2013-07-01

    Subcalcic garnets carry the major inventory of most trace elements of their host harzburgites and are thus proxies of the bulk composition. We used single garnet grains from heavy mineral concentrates from the Kaapvaal craton (Roberts Victor and Lace mine) to determine the major and trace elements and the Sm-Nd and Lu-Hf isotope systematics of these highly depleted members of the peridotitic suite. The combination of the results with previous work from the Finsch mine (Lazarov et al., 2009a) allowed us to reconstruct the formation and evolution of the mantle underneath the Kaapvaal craton. Several lines of evidence from major and trace elements suggest that mantle melting was mainly at shallow pressures followed by subduction into the garnet stability field. A 3.22 Ga metasomatic event underneath the East block occurred in a previously depleted mantle (ɛHf = +16) which was sufficiently stabilized by that time to hold a crust with tonalite-trondhjemite-granodiorite (TTG's) and greenstone belts. Such high ɛHf values can be produced within a few hundred million years by 25% non-modal fractional melting in the spinel stability field. This is the first prove of a mantle underneath the East block with an age similar to a 3.65 Ga crustal age. Before the amalgamation around 2.88 Ga, oceanic lithosphere was created between the W- and E-block around 2.95 Ga (group RV1 samples from the Roberts Victor) and subducted underneath the W-block. Another mantle portion (group RV2 garnets) from Roberts Victor yielded a seeming age of 3.27 ± 0.15 Ga with ɛHf = +17.6. It actually results from an enrichment process in a highly depleted mantle about 2.8-2.9 Ga ago. This may have been the depleted mantle wedge above the subduction and final collision between the West and the East block. The creation of a cratonic nucleus for the West-block is unknown until 3.2 Ga when the oldest mantle TRD and oldest crustal zircon ages are reported. Such ages were not directly obtained from the study

  11. Oxygen isotope constraints on the sulfur cycle over the past 10 million years.

    PubMed

    Turchyn, Alexandra V; Schrag, Daniel P

    2004-03-26

    Oxygen isotopes in marine sulfate (delta18O(SO4)) measured in marine barite show variability over the past 10 million years, including a 5 per mil decrease during the Plio-Pleistocene, with near-constant values during the Miocene that are slightly enriched over the modern ocean. A numerical model suggests that sea level fluctuations during Plio-Pleistocene glacial cycles affected the sulfur cycle by reducing the area of continental shelves and increasing the oxidative weathering of pyrite. The data also require that sulfate concentrations were 10 to 20% lower in the late Miocene than today. PMID:15044800

  12. Neodymium and strontium isotopic constraints on soil sources in Barbados, West Indies

    NASA Astrophysics Data System (ADS)

    Borg, Lars E.; Banner, Jay L.

    1996-11-01

    Neodymium and strontium isotopic compositions and Sm/Nd ratios are used to constrain the sources of silicate-rich soils developed on uplifted Pleistocene coral-reef limestones on Barbados, West Indies. The geographic and geologic setting of Barbados facilitates the application of these tracers to the evaluation of the following soil sources: (1) Pleistocene reef limestone regolith, (2) Tertiary carbonate rocks, sandstones, and mudstones that are exposed in northeastern Barbados, (3) volcanic ash erupted from the Lesser Antilles arc, (4) Saharan dust transported by trade winds, and (5) fertilizer. The soils have ɛNd values that range from -6.6 to -1.9, 87Sr /86Sr values of 0.70890 to 0.71067, and Sm/Nd ratios of 0.223-0.260. The Pleistocene limestone component is the most significant source of Sr in the soils and a negligible source of Nd. Comparison of Sm and Nd concentrations and neodymium isotopic compositions of soil samples that are weathered to varying extents indicates that Sm and Nd are relatively unfractionated and retained in the soils during weathering. ɛNd and Sm/Nd variations in the soils, therefore, primarily reflect the compositions and proportions of the silicate sources. Mass balance calculations based on SmNd systematics require that the silicate soil components contain between 30-85% volcanic ash, with the remaining silicate fraction comprised of old, continentally-derived sediment. In contrast to Sm and Nd, Sr is mobilized and removed from the soils during weathering. Strontium from volcanic and carbonate sources is preferentially removed relative to continental silicate sources. The strontium isotopic compositions of the soils, therefore, reflect the combined effects of the degree of weathering and the compositions and proportions of the soil sources. Mass balance calculations indicate that at least 35-60% of the initial Sr in the soils has been removed by weathering. These results illustrate (1) the utility of radiogenic isotopes in

  13. Stable isotopic constraints on formation of continental lithospheric mantle: a case study from the Colorado Plateau

    NASA Astrophysics Data System (ADS)

    Marshall, E. W., IV; Barnes, J.; Lassiter, J. C.

    2014-12-01

    Mantle melt depletion is fundamental to stabilization of thick lithospheric mantle and to craton formation and survival. Three potential mechanisms to grow lithospheric mantle are: partial melting of upwelling mantle plumes; imbrication and accretion of oceanic lithosphere at subduction zones; and flux melting of the mantle wedge at volcanic arcs. Stable isotopes can distinguish these models because stable isotopes fractionate substantially at low temperature, making the mantle sensitive to the incorporation of subducted material. Correlations between stable isotopes and trace elements can then distinguish seafloor weathering processes from flux melting processes, allowing subducted oceanic lithosphere to be discerned from flux-melted lithosphere. We find that indices of melt depletion (e.g. cpx Cr#, Yb concentrations and Mg#) in xenoliths from the Colorado Plateau correlate with δ18O values of olivine. Xenoliths from The Thumb volcanic neck have δ18Ool values ranging from +5.05 to +5.50‰ (n=12) that correlate positively with indices of melt depletion, and xenoliths from the Green Knobs diatreme have δ18Ool values that range from +4.85 to +5.36‰ (n=9) and correlate negatively with indices of melt depletion. These trends may reflect coupled fluid input and melt depletion, typical of flux melting. The different trends at each locality may be due to fluxing of different fluids- either high δ18O fluids derived from altered oceanic crust or low δ18O fluids derived from recycled serpentinite. Canil and Lee (2009) interpreted correlations between whole rock δ18O values and MgO/SiO2 in xenoliths from The Thumb to be due to seafloor weathering and Mg loss of abyssal peridotites followed by later accretion of the rocks to the Colorado Plateau lithosphere. However, chrome and alumina are immobile during seafloor weathering, so correlations between δ18O values and Cr# also present in The Thumb xenoliths are inconsistent with seafloor weathering. Evidence for flux

  14. 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

  15. Lithospheric origin for Neogene-Quaternary Middle Atlas lavas (Morocco): Clues from trace elements and Sr-Nd-Pb-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Bosch, Delphine; Maury, René C.; El Azzouzi, M'hammed; Bollinger, Claire; Bellon, Hervé; Verdoux, Patrick

    2014-09-01

    This study presents new geochemical data on 26 mafic lavas from the Middle Atlas and Central Morocco volcanic provinces, including Miocene nephelinites and Pliocene-Quaternary (3.9-0.6 Ma) nephelinites, basanites, alkali and subalkaline basalts. Most of them represent near-primary magmas, although some alkali basalts were derived from the minor fractionation of olivine and diopside phenocrysts. These evolved samples and the subalkaline basalt display higher 207Pb/204Pb and Zr/Nb ratios and lower εNd consistent with their contamination by lower crustal granulites during an open fractionation process. The progressive enrichment in incompatible elements observed from alkali basalts to nephelinites suggests their derivation from decreasing partial melting degrees of an enriched mantle source located at the garnet-spinel transition zone. The strong negative spikes observed for K in multielement patterns indicate that this source contained a residual pargasitic amphibole. We propose that partial melting occurred at around 2 GPa, i.e. near the lithosphere-asthenosphere boundary beneath the Middle Atlas (60-80 km). The trace element and isotopic Sr-Nd-Pb-Hf signature of the uncontaminated lavas displays a geochemical flavour intermediate between those of high μ (HIMU), “C”, and enriched mantle components. It is very similar to that of abundant metasomatic amphibole- and clinopyroxene-rich lithospheric peridotites and pyroxenites carried by Middle Atlas lavas, which likely represent an analog of the source of these lavas. It is therefore not necessary to postulate the contribution of a “fresh” asthenospheric mantle to their genesis. We propose that they resulted from the partial melting of the base of a veined lithospheric mantle metasomatised during the late Cretaceous by alkaline melts from the Central Atlantic plume, the ancestor of the Canary plume. Melting was probably triggered by the flux of a hot mantle within a regional SW-NE sub-lithospheric channel, in

  16. Traditional applications and novel approaches in Lu-Hf geochronology

    NASA Astrophysics Data System (ADS)

    Herwartz, D.; Nagel, T. J.; Sandmann, S.; Vitale Brovarone, A.; Rexroth, S.; Rojas-Agramonte, Y.; Froitzheim, N.; Kröner, A.; Skublov, S. G.; Münker, C.

    2012-04-01

    Lutetium-Hf geochronology is currently becoming a routine method for dating metamorphism of garnet bearing rocks, such as eclogites. Prograde garnet growth ages are mostly preserved because blocking temperatures exceed 630 °C [1] and prograde Lu zoning patterns have even been observed in samples that were exposed to temperatures above 800 °C [2]. Here we discuss Lu-Hf ages from various eclogite localities, such as the Northern Tianshan, Kyrgyzstan (~ 470 Ma), the Kola Peninsula, Russia (~ 1900 Ma) [3], Cuba (~70 Ma and ~124 Ma), Alpine Corsica (~ 34 Ma) and the Tauern Window (~32.7 Ma). Age precisions are in the order of 0.1 to 1 % and all ages can be safely attributed to the timing of garnet growth. Some samples, however, contain two garnet populations which complicates Lu-Hf geochronology. In the Adula Nappe (Central Alps) relict garnet has survived a second orogenic cycle, including subduction to mantle depth. By carefully separating the two garnet populations present within the same eclogite sample we obtained a minimum Variscan age of 333 Ma and a maximum Alpine age of 38 Ma [4]. A similar relationship is now evident in samples from the Tauern Window (Eastern Alps), where only one population of garnet generation is visible macroscopically. However, few relics of Variscan garnet inside Alpine garnet are observed in electron microprobe element maps and are also evident from isotopic heterogeneity in 176Lu/177Hf vs. 176Hf/177Hf space. Garnet relics stemming from previous metamorphic events are frequently observed in HP units around the world and the Lu-Hf system is a promising tool to resolve the respective growth ages. Apart from garnet, lawsonite Lu-Hf geochronology was recently identified as a new tool to investigate subduction processes [5]. Here we present a lawsonite Lu-Hf isochron 37,6 ± 1.4 Ma (MSWD = 0.30; n =5) from a lawsonite blueschist from Alpine Corsica. The lawsonite slightly predates the timing of garnet growth (~34 Ma) in three eclogite

  17. 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

  18. 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.

  19. Water on Mars: isotopic constraints on exchange between the atmosphere and surface.

    PubMed

    Kass, D M; Yung, Y L

    1999-12-15

    Using a new measurement of the D/H fractionation efficiency and new estimates of the water loss, we calculate that Mars has the equivalent of a approximately 9 m global water layer in a reservoir that exchanges with the atmosphere. The measured D/H enrichment is about 5 times the terrestrial value, but without exchange, the atmosphere converges on an enrichment of 50 in about 0.5 Ma. Due to the large buffering reservoir and the rapid loss rate (10(-3) pr-micrometers yr-1), the small atmospheric reservoir, averaging 10 pr-micrometers, is unlikely to be in continuous isotopic equilibrium with the full 9 m exchangeable reservoir. Instead, it presumably equilibrates during periods of high obliquity; the atmospheric D/H ratio is expected to be enriched in between such periods. If isotopic exchange with a small (4 mm global layer) reservoir occurs under current conditions, it possible for the atmospheric D/H ratio to be within 10% of its long term equilibrium. PMID:11543401

  20. 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.

  1. Sr-isotopic constraints on the petrogenesis of Apollo 12 mare basalts

    NASA Technical Reports Server (NTRS)

    Nyquist, L. E.; Bansal, B. M.; Wooden, J. L.; Wiesmann, H.

    1977-01-01

    As part of a study of Apollo 12 basalts, crystallization ages and initial Sr-87/Sr-86 (I) were determined for pigeonite basalts 12039 and 12055 and ilmenite basalt 12051. Sr-86/Sr-87 measurements for a larger suite of whole rock and plagioclase samples were obtained, and it is found that I-values for olivine and pigeonite basalts are indistinguishable while I-values for ilmenite basalts are distinctly lower. A self-consistent model for the evolution of the Sr-isotopic composition and REE abundances suggests that basalt was formed by small (2-10%) degrees of melting of cumulate sources composed of varying proportions of olivine, orthopyroxene, and clinopyroxene. This model requires nonchondritic relative abundances of the REE in the parental liquid at the time of formation of the cumulate sources. The model also describes Sr-isotopic composition and REE abundances of Apollo 17 high-Ti basalts and Apollo 15 low-Ti basalts. A lunar 'scenario' involving open system crystallization of a magma ocean from the crust downward is proposed.

  2. 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.

  3. 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

  4. 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

  5. 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

  6. 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

  7. Constraints in calculations of evaporative losses in arid climates using the stable isotope composition of water

    NASA Astrophysics Data System (ADS)

    Skrzypek, G.; Mydlowski, A.; Dogramaci, S.; Hedley, P.; Gibson, J. J.; Grierson, P. F.

    2014-12-01

    Accurate quantification of evaporative losses to the atmosphere from surface water bodies is essential for calibration and validation of hydrological models, particularly in remote arid and semi-arid regions, where rivers and lakes are generally minimally gauged. In this study, we reviewed and combined the most recent equations for estimation of evaporative losses based on the revised Craig-Gordon model. We designed new software, called Hydrocalculator, which allows quick and robust estimation of evaporative losses based on the isotopic composition of water. We validated Hydrocalculator by testing the range of uncertainty in the estimation of evaporative losses in arid climates by cross-validating a simplified stable isotope model with field pan evaporation experiments. The use of standardized pans (1.2 m diameter, volume 300 dm3) in hot and dry climates (temperature 29°C and relative humidity between 19 and 26%) allowed simulation of fast evaporation from shallow water bodies. Several factors may contribute to the uncertainty in the evaporative loss calculations. The analytical uncertainty in the determination of the stable isotope composition of water may contribute to ~0.6% for δ18O and ~1.4% for δ2H. The model is less sensitive to uncertainty in climatic variables and an uncertainty of 1°C in air temperature will result only in the ~0.1% uncertainty in δ18O and δ2H. However, uncertainty in relative humidity of 10% will result in an uncertainty in the final outcome of 0.4% (δ18O) and 1.0% (δ2H). Significantly higher uncertainty in evaporative loss estimation is thus associated with uncertainty in ambient air moisture estimation or analysis. An error of 20‰ in δ2H and 5.0‰ in δ18O will result in a maximum difference of 2.4% (δ2H) and 1.7% (δ18O) in the final calculations. Hydrocalculator can thus provide accurate, rapid and cost-effective insight into the water balance of surface water pools. We used the new software to determine the origin of

  8. 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

  9. 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.

  10. 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

  11. Carbon isotope constraints on the deglacial CO₂ rise from ice cores.

    PubMed

    Schmitt, Jochen; Schneider, Robert; Elsig, Joachim; Leuenberger, Daiana; Lourantou, Anna; Chappellaz, Jérôme; Köhler, Peter; Joos, Fortunat; Stocker, Thomas F; Leuenberger, Markus; Fischer, Hubertus

    2012-05-11

    The stable carbon isotope ratio of atmospheric CO(2) (δ(13)C(atm)) is a key parameter in deciphering past carbon cycle changes. Here we present δ(13)C(atm) data for the past 24,000 years derived from three independent records from two Antarctic ice cores. We conclude that a pronounced 0.3 per mil decrease in δ(13)C(atm) during the early deglaciation can be best explained by upwelling of old, carbon-enriched waters in the Southern Ocean. Later in the deglaciation, regrowth of the terrestrial biosphere, changes in sea surface temperature, and ocean circulation governed the δ(13)C(atm) evolution. During the Last Glacial Maximum, δ(13)C(atm) and atmospheric CO(2) concentration were essentially constant, which suggests that the carbon cycle was in dynamic equilibrium and that the net transfer of carbon to the deep ocean had occurred before then. PMID:22461496

  12. 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

  13. Extraterrestrial water of possible Martian origin in SNC meteorites: Constraints from oxygen isotopes

    NASA Technical Reports Server (NTRS)

    Karlsson, H. R.; Clayton, R. N.; Gibson, E. K.; Mayeda, T. K.; Socki, R. A.

    1991-01-01

    Many lines of evidence suggest that the SNC group of meteorites (Shergotty-Nakhla-Chassigny) are derived from the planet Mars. Because SNC meteorites have water, they may contain information on the origin and fate of water on the planet Mars. The first measurements are presented of delta O-18 and delta O-17 of water extracted from SNC's. The water analyses are displayed with each meteorite designated. The isotopic composition is shown of the SNC waters relative to the SNC whole rocks and the terrestrial line. The O-17 excess is shown of the SNC water as a function of pyrolysis temperature. And delta O-18 values are displayed of the water as a function of pyrolysis temperature for the four SNC meteorites. The analyses are briefly examined.

  14. 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

  15. Weathering and transport of sediments in the Bolivian Andes: Time constraints from uranium-series isotopes

    NASA Astrophysics Data System (ADS)

    Dosseto, A.; Bourdon, B.; Gaillardet, J.; Maurice-Bourgoin, L.; Allègre, C. J.

    2006-08-01

    Rivers from the upper Rio Madeira basin (Bolivia) have been studied with uranium-series isotopes in order to constrain the timescales of weathering and sediment transfer from the Andes through the Amazon tropical plain. Uranium (U), thorium (Th) and radium (Ra) isotopes ( 238U- 234U- 230Th- 226Ra and 232Th) have been analyzed in the suspended load (> 0.2 μm) of rivers. Increasing 230Th excesses relative to 238U in suspended particles from the Andes to the tropical plain is interpreted as an increasing duration of weathering during sediment transport and storage in the foreland basin. Model calculations for ( 230Th/ 238U) and ( 226Ra/ 230Th) activity ratios in suspended particles using a continuous weathering model indicates that: (i) the timescale for production, storage and transport of sediments in the Andean Cordillera is only a few kyr, (ii) the storage time of suspended sediments in the foreland basin is 5 ± 1 kyr and (iii) the average transfer time of suspended sediments from the Andes to the confluence of Rio Madeira with the Amazon River is 17 ± 3 kyr. An implication of these short timescales is that the bedrock eroded must have lost part of its uranium during one or several past erosion cycles. This demonstrates the recycling of sediments through several erosion cycles before transfer to the oceans. The calculation of long-term (> 1 kyr), steady-state erosion rates indicates that they are much lower than present-day rates. This increase in denudation rates must be recent and could be explained by an increase in precipitation ˜ 4 ka ago, as suggested by palaeoclimatic evidences and the draining of transient sedimentary basins encountered on the Altiplano and easily eroded. This suggests that climatic variability rather than tectonics alone produces high erosion rates.

  16. 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

  17. 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.

  18. 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.

  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. 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.

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. Constraints on the origin of the HIMU reservoir from He-Ne-Ar isotope systematics

    NASA Astrophysics Data System (ADS)

    Hanyu, Takeshi; Tatsumi, Yoshiyuki; Kimura, Jun-Ichi

    2011-07-01

    We present a new set of He-Ne-Ar isotopic compositions for HIMU and EM1 lavas from the Cook-Austral Islands in the south Pacific. 3He/ 4He of the HIMU lavas are lower than MORB values, as previously demonstrated, but a coherent variation in 3He/ 4He with Pb isotopes indicates two-component mixing to form the lavas. One component is the HIMU reservoir with 3He/ 4He of 6 Ra or lower, and the other is the local lithosphere, which is commonly involved in EM1 lavas. Relative abundances of radiogenic and nucleogenic 4He, 21Ne and 40Ar show systematic variations. In 4He/ 40Ar*- 4He/ 21Ne* space, the HIMU lavas define a trend that is parallel to, but offset from the trend previously observed for other OIBs. Using 4He/ 21Ne* as a monitor of elemental fractionation of noble gasses, fractionation-corrected 4He/ 40Ar* is higher than the 4He/ 40Ar* production ratio in the mantle, reflecting a HIMU reservoir with a lower K/U (approximately 3000) than canonical mantle value (13000). Radiogenic 3He/ 4He and the low K/U are best explained by a model where the HIMU reservoir was formed by direct accumulation of, or deep mantle metasomatism with, ancient subducted oceanic crust modified by hydrothermal alteration and dehydration during its subduction. If the subducted oceanic crust with fractionated K/U forms a large isolated reservoir in the mantle, previous estimates of K/U and K concentrations for the bulk silicate Earth, that did not take this reservoir into consideration, will be too high. Moreover, the mass balance calculation indicates that the subducted oceanic crust may make a significant contribution to the U mantle budget, requiring there to be less in the primitive mantle. However, a contribution from the primitive mantle is not ruled out entirely, unless the subducted oceanic crust had a relatively high U concentration and low K/U over geologic time and was totally preserved in a convecting mantle.

  9. 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

  10. 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).

  11. 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.

  12. 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).

  13. 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