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Sample records for hf isotopic constraints

  1. Hf-Nd isotope and trace element constraints on subduction inputs at island arcs: limitations of Hf anomalies as sediment input indicators

    NASA Astrophysics Data System (ADS)

    Handley, H. K.; Turner, S.; MacPherson, C.; Davidson, J. P.; Gertisser, R.

    2010-12-01

    New Nd-Hf isotope and trace element data for Javanese volcanoes are combined with recently published data to place constraints on subduction inputs at the Sunda arc in Indonesia and assess the value of Hf anomalies (expressed as Hf/Hf* and Sm/Hf ratios) as tracers of such inputs. Hf anomaly does not correlate with Hf isotope ratio in Javanese lavas however, Hf/Hf* and Sm/Hf ratios do correlate with SiO2. Contrary to previous work, we show that Hf anomaly variation may be controlled by fractionation of clinopyroxene and/or amphibole during magmatic differentiation and does not represent the magnitude or type of subduction input in some arcs. Correlation of Sm/Hf with indices of differentiation for other arcs (e.g. Vanuatu, New Britain, Mariana) suggests that differentiation control on Sm/Hf ratios of volcanic rocks may be a relatively common phenomenon. This study corroborates the use of Nd-Hf isotope co-variations in arc volcanic rocks to ascertain subduction input characteristics. The trajectories of regional volcano groups (East, Central and West Java) in Nd-Hf isotope space reveal heterogeneity in the subducted sediment input along Java, which reflects present-day spatial variations in sediment compositions on the down-going plate in the Java Trench.

  2. Hf-Nd isotope and trace element constraints on subduction inputs at island arcs: Limitations of Hf anomalies as sediment input indicators

    NASA Astrophysics Data System (ADS)

    Handley, Heather K.; Turner, Simon; Macpherson, Colin G.; Gertisser, Ralf; Davidson, Jon P.

    2011-04-01

    New Nd-Hf isotope and trace element data for Javanese volcanoes are combined with recently published data to place constraints on subduction inputs at the Sunda arc in Indonesia and assess the value of Hf anomalies (expressed as Hf/Hf* and Sm/Hf ratios) as tracers of such inputs. Hf anomaly does not correlate with Hf isotope ratio in Javanese lavas, however, Hf/Hf* and Sm/Hf ratios do correlate with SiO 2. Contrary to previous work, we show that Hf anomaly variation may be controlled by fractionation of clinopyroxene and/or amphibole during magmatic differentiation and does not represent the magnitude or type of subduction input in some arcs. Correlation of Sm/Hf with indices of differentiation for other arcs (e.g., Vanuatu, New Britain, and Mariana) suggests that differentiation control on Sm/Hf ratios in volcanic arc rocks may be a relatively common phenomenon. This study corroborates the use of Nd-Hf isotope co-variations in arc volcanic rocks to ascertain subduction input characteristics. The trajectories of regional volcano groups (East, Central and West Java) in Nd-Hf isotope space reveal heterogeneity in the subducted sediment input along Java, which reflects present-day spatial variations in sediment compositions on the down-going plate in the Java Trench. The high Sm/Hf ratio required in the sediment end-member for some Javanese basalts suggests that partial melting of subducted sediment occurs in the presence of residual zircon, and is inconsistent with residual monazite or allanite.

  3. 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.

  4. Constraints from loess on the Hf-Nd isotopic composition of the upper continental crust

    NASA Astrophysics Data System (ADS)

    Chauvel, Catherine; Garçon, Marion; Bureau, Sarah; Besnault, Adeline; Jahn, Bor-ming; Ding, Zhongli

    2014-02-01

    Knowledge of the average composition of the upper continental crust is crucial to establish not only how it formed but also when. While well constrained averages have been suggested for its major and trace element composition, no values exist for its Nd and Hf isotopic compositions even though radiogenic isotopic systems provide valuable information on its average model age. Here we present Nd and Hf isotopic data determined on a large number of loess deposits from several continents. We demonstrate that these deposits have very uniform Nd and Hf isotopic compositions. We obtain an average Nd isotopic composition that is similar to previous estimates for the upper continental crust (εNd=-10.3±1.2(1σ)) and we establish a new Hf average value at εHf=-13.2±2(1σ). This average falls on the “Terrestrial Array”, demonstrating that the two parent-daughter ratios are not decoupled during crust formation. Trace element data acquired on the same set of samples allow us to calculate an average 147Sm/144Nd ratio for the upper continental crust: 0.1193±0.0026, a value slightly higher than previous estimates. Based on the relationship between Sm/Nd and Nd isotopes, we estimate the average Nd extraction age of upper continental crust from the depleted mantle at TDM(Nd)=1.82±0.07 Ga. This model age is entirely consistent with previous suggestions made for example by Goldstein et al. (1984). Assuming that for each individual sample, the Hf model age cannot be younger than the Nd model age, our new Nd-Hf isotopic data provide a value for the very poorly known 176Lu/177Hf ratio of the upper crust. Our estimate is 176Lu/177Hf = 0.0125 ± 0.0018, a value significantly lower than commonly used values (0.0150-0.0159; Griffin et al., 2002; Goodge and Vervoort, 2006; Hawkesworth et al., 2010) but higher than Rudnick and Gao's (2003) estimate of 0.0083. The impact of our new 176Lu/177Hf ratio on crustal model ages of zircon populations is not simple to evaluate but the Hf model

  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. The Lu-Hf and Sm-Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets

    NASA Astrophysics Data System (ADS)

    Bouvier, Audrey; Vervoort, Jeffrey D.; Patchett, P. Jonathan

    2008-08-01

    The Lutetium-Hafnium radiogenic isotopic system is widely used as a chronometer and tracer of planetary evolution. In order for this isotopic system to fulfill its potential in planetary studies, the Lu-Hf system parameters need to be more tightly constrained, in particular the Lu-Hf isotopic composition of the chondritic uniform reservoir (CHUR) and, by extension, the bulk silicate Earth (BSE). We present new Lu-Hf and Sm-Nd isotopic compositions of unequilibrated carbonaceous, ordinary, and enstatite chondrites of petrologic types 1, 2, and 3 which define a narrow range of Lu/Hf ratios (3%) identical with that of Sm/Nd. This contrasts with previously published data from mostly equilibrated ordinary chondrites of petrologic types 4, 5, and 6 which have a much larger range in Lu/Hf (28%). This heterogeneity has hampered an unambiguous choice for the Lu-Hf isotopic composition of CHUR. Our new determinations of Lu-Hf CHUR parameters are 176Lu/177Hf = 0.0336 ± 1 and 176Hf/177Hf = 0.282785 ± 11 (2σm), which are higher than previous estimates and, together with average Sm-Nd chondrite compositions of unequilibrated chondrites of 147Sm/144Nd = 0.1960 ± 4 and 143Nd/144Nd = 0.512630 ± 11 (2σm), now provide firm constraints on the chondritic parameters for both Lu-Hf and Sm-Nd isotopic systems. A comparison of Lu-Hf and Sm-Nd data show that terrestrial planets, as well as early differentiated planetesimals, converge toward a common initial Hf and Nd isotope composition corresponding to the average of chondrites. Finally, a compilation of Lu-Hf isotopic data of unequilibrated and equilibrated chondrites demonstrates that the 176Lu decay decay-constant value cannot be resolved by age comparison on metamorphosed or shocked planetary materials which have a complex history.

  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.

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  10. 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.

  11. Early Archean crust in the Ukrainian Shield - U-Pb and Hf isotopic constraints

    NASA Astrophysics Data System (ADS)

    Claesson, S.; Bibikova, E.; Shumlyanskyy, L.; Hawkesworth, C. J.

    2012-04-01

    The Ukrainian Shield includes early Archean components which have been strongly reworked in both Archean and Proterozoic time. In the Podolian Domain in the west, the oldest rocks occur in the Dniestr-Bug formation composed of dominating granulite facies granitoids (enderbites), mafic and supracrustal rocks. Zircon from these rocks commonly have complex internal structures, reflecting multiple episodes of growth and recrystallisation in both Archean and Paleoproterozoic time. U-Pb SIMS dating of enderbite zircon has identified a group with 207Pb/206Pb ages of c. 3.65 Ga, and also older zircon up to 3.75 Ga. Dniestr-Bug rocks interpreted as metasediments also have yielded ages up to c 3.75 Ga. In Hf-time space, most enderbite zircon analyses form a well defined array with a slope corresponding to 176Lu/177Hf close to zero, which intersects the CHUR composition at 3.75 Ga and an assumed DM evolution curve at c. 3.9 Ga. Some analyses plot on the young side of this array. We suggest that zircon along this Hf-age array crystallized during a magmatic event at c. 3.6 Ga, or earlier, and that individual zircon crystals since then have been individual small closed Lu-Hf reservoirs. At the same time, the crystals have been open to Pb loss during the metamorphic reworking which has caused recrystallisation and alteration of primary magmatic zircon, and growth of new zircon. The crustal provenance age of the material which was melted to produce the enderbite magma may be constrained by assumed compositions of this crustal precursor and of the mantle from which it was derived. For a mafic primary crust with 176Lu/177Hf=0.022, derived from a depleted mantle source, the crustal provenance age is c. 4.2 Ga. A more differentiated primary crust, less depleted mantle source, or older age of the enderbite result in younger model ages for this primary crust. In the Azov Domain in the east, ages for detrital zircon from the Soroki and Fedorovka greenstone belts vary from c. 3.6 to 2

  12. U Th Pb and Lu Hf isotopic constraints on the evolution of sub-continental lithospheric mantle, French Massif Central

    NASA Astrophysics Data System (ADS)

    Wittig, Nadine; Baker, Joel A.; Downes, Hilary

    2007-03-01

    We have carried out a Pb double-spike and Lu-Hf isotope study of clinopyroxenes from spinel-facies mantle xenoliths entrained in Cenozoic intraplate continental volcanism of the French Massif Central (FMC). U-Th-Pb and Lu-Hf isotope systematics verify the existence of different lithospheric domains beneath the northern and southern FMC. Northern FMC clinopyroxenes have extreme Lu/Hf ratios and ultra-radiogenic Hf ( ɛHf = +39.6 to +2586) that reflect ˜15-25% partial melting in Variscan times (depleted mantle model ages ˜360 Ma). Zr, Hf and Th abundances in these clinopyroxenes are low and unaffected by hydrous/carbonatitic metasomatism that overprinted LILE and light REE abundances and caused decoupling of Lu/Hf-Sm/Nd ratios and Nd-Hf isotopes ( ɛNd = +2.1 to +91.2). Pb isotopes of northern FMC clinopyroxenes are radiogenic ( 206Pb/ 204Pb > 19), and typically more so than the host intraplate volcanic rocks. 238U/ 204Pb ratios range from 17 to 68, and most samples have distinctively low 232Th/ 238U (<1) and 232Th/ 204Pb (3-22). Clinopyroxenes from southern FMC lherzolites are generally marked by overall incompatible trace element enrichment including Zr, Hf and Th abundances, and have Pb isotopes that are similar to or less radiogenic than the host volcanic rocks. Hf isotope ratios are less radiogenic ( ɛHf = +5.4 to +41.5) than northern FMC mantle and have been overprinted by silicate-melt-dominated metasomatism that affected this part of FMC mantle. Major element and Lu concentrations of clinopyroxenes from southern FMC harzburgites are broadly similar to northern FMC clinopyroxenes and suggest they experienced similar degrees of melt extraction as northern FMC mantle. 238U/ 204Pb (53-111) and 232Th/ 204Pb ratios (157-355) of enriched clinopyroxenes from the southern FMC are extreme and significantly higher than the intraplate volcanic rocks. In summary, mantle peridotites from different parts of the FMC record depletion at ˜360 Ma during Variscan subduction

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  14. 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.

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

    USGS Publications Warehouse

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

    1986-01-01

    Pb, Sr, Nd, and Hf isotopic relationships among basalts from the Hawaiian Islands suggest that these basalts were derived from three sources; the oceanic lithosphere (Kea end member), the depleted asthenosphere (posterosional end member) and a deep-mantle plume (Koolau end member). Hawaiian tholeiites are derived within the lithosphere and the isotopic trends collectively defined by the tholeiite data are interpreted as a plume-lithosphere mixing trend. The isotopic characteristics of late-stage basalts are derived from the tholeiite source (lithosphere + plume) with additional input from the lithosphere, asthenosphere, or both. These basalts probably originate from near the asthenosphere-lithosphere boundary. Posterosional basalts are derived from the depleted asthenosphere, but their isotopic characteristics have been slightly modified by either the plume or the source of previously erupted volcanics. The isotopic data require little or no mixing of asthenospheric material into the plume during tholeiite production and thus are consistent with the concept of a rapidly ascending, fluid-rich plume. In addition to providing a source of heat, the plume may supply volatiles to both the sources of tholeiites and posterosional basalts. The isotopic characteristics of the Koolau (plume) component are unique among OIB sources. If undifferentiated or "primitive" mantle material still exists, then the radiogenic-isotope data for Koolau in combination with rare gas data for Hawaiian basalts in general suggest that the Hawaiian plume may be derived from such material. In any case, the Hawaiian Islands data, when compared to those of other OIB, serve to illustrate the isotopically diverse nature of mantle sources. ?? 1986.

  16. 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.

  17. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  19. 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.

  20. 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

  1. 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

  2. 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

  3. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

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

  5. 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

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

    NASA Astrophysics Data System (ADS)

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

    2013-12-01

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

  7. 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.

  8. Precisely dating Paleozoic kimberlites in the North China Craton and Hf isotopic constraints on the evolution of the subcontinental lithospheric mantle

    NASA Astrophysics Data System (ADS)

    Li, Qiu-Li; Wu, Fu-Yuan; Li, Xian-Hua; Qiu, Zhi-Li; Liu, Yu; Yang, Yue-Heng; Tang, Guo-Qiang

    2011-09-01

    Kimberlite, a deep-sourced ultramafic potassic rock, carries not only diamond, but also invaluable mantle xenoliths and/or xenocrysts, which are important for tracking the evolution of subcontinental lithospheric mantle (SCLM). However, it is challenging to accurately determine the emplacement age of kimberlite and its compositions of primary magma because of modifications by crustal and/or mantle contamination and post-emplacement alteration. This paper reports emplacement ages of diamondiferous kimberlites in Mengyin and Fuxian of the North China Craton (NCC) using three different dating methods. For Mengyin kimberlite, single-grain phlogopite Rb-Sr dating yields an isochron age of 485 ± 4 Ma, U-Th-Pb analyses on perovskite give a 238U- 206Pb age of 480.6 ± 2.9 Ma and a 232Th- 208Pb age of 478.9 ± 3.9 Ma, and baddeleyite yields a 207Pb- 206Pb age of 480.4 ± 3.9 Ma. For Fuxian kimberlite, baddeleyite gives a 207Pb- 206Pb age of 479.6 ± 3.9 Ma, indicating that the Paleozoic kimberlites in the NCC were emplaced at ~ 480 Ma. Numerous lines of evidence indicate that the studied baddeleyites are xenocrysts from the SCLM, and can be used to constrain Hf isotope compositions ( ɛHf(t) ~ - 6) of the SCLM when kimberlite erupted. Combined with data from Mesozoic-Cenozoic mantle-derived rocks and xenoliths, the Hf isotope evolution trend of the SCLM beneath NCC before craton destruction was tentatively constructed, which suggested that the Archean SLCM was enriched by metasomatism at ~ 1.3 Ga. Further Hf isotope investigations on additional SCLM-derived materials could be used to compare with the constructed Hf isotope evolution trend before craton destruction to determine when lithospheric thinning occurred.

  9. 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.

  10. Genesis and open-system evolution of Quaternary magmas beneath southeastern margin of Tibet: Constraints from Sr-Nd-Pb-Hf isotope systematics

    NASA Astrophysics Data System (ADS)

    Zou, Haibo; Ma, Mingjia; Fan, Qicheng; Xu, Bei; Li, Shuang-Qing; Zhao, Yongwei; King, David T.

    2017-02-01

    Post-collisional volcanic rocks on the Tibetan Plateau and its margins contain valuable information about the geodynamic processes associated with this Cenozoic continent-continent collision. The Quaternary Tengchong volcanic field at the southeastern margin of the Tibetan Plateau formed high-potassium calc-alkaline volcanic rocks. Herein, we present comprehensive Nd-Sr-Pb-Hf isotopic and elemental data for trachybasalts, basaltic trachyandesites, and trachyandesites from four Quaternary Tengchong volcanoes (Maanshan, Dayingshan, Heikongshan, and Laoguipo) in order to understand their magma genesis and evolution as well as tectonic significance. Good correlations between SiO2 content and the ratios 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, and 177Hf/176Hf for these Quaternary volcanics strongly suggest that the combined assimilation and fractional crystallization (AFC) was an important process in the origin of basaltic trachyandesites and trachyandesites. High Y and Yb contents and low Sr/Y ratios of these basaltic trachyandesites and trachyandesites are uncharacteristic of adakites that formed by partial melting of eclogitic lower crust or partial melting of basaltic oceanic crust with eclogite as a restite. A combined assimilation-fractional crystallization model is proposed for these basaltic trachyandesites and trachyandesites. Nd-Sr-Pb-Hf isotopes for the uncontaminated Tengchong magma (trachybasalts with SiO2 < 52.5 wt.% and MgO > 5.5% wt.%) reflect a heterogeneous enriched mantle source. High Th/U, Th/Ta, and Rb/Nb ratios and Nd-Sr-Pb-Hf isotope characteristics of the uncontaminated magmas suggest that the enriched mantle beneath Tengchong formed as a result of subduction of clay-rich sediments, which probably came from the Indian continental plate. Partial melting of the enriched mantle was generated by deep continental subduction coupled with recent regional extension in the Tengchong area.

  11. A convergent continent marginal volcanism source of ash beds near the Permian-Triassic boundary, South China: Constraints from trace elements and Hf-isotopes

    NASA Astrophysics Data System (ADS)

    Wang, X.; Zhao, L.; Chen, Z. Q.; Ma, D.; Yan, P.; Guo, F.; Wang, F.; Wan, Q.; Han, X.

    2015-12-01

    Growing evidence shows that volcanism near the Permian-Triassic boundary (PTB) may have been crucial in triggering the PTB biocrisis. However, whether this trigger is the Siberian traps or arc island volcanisms has long been debating. Meanwhile, multiple claystone beds are prominent near the PTB, South China. The nature and origin of the volcanic ashes therefore provide clue to find out the trigger of the PTB mass extinction. Following previous studies (Gao et al., 2013), 21 PTB ash beds from three additional PTB sections, namely the Shangsi, Jianshi and Meishan, all from South China have been systematically sampled. The U-Pb ages, trace elements, and Hf-isotope compositions of zircon grains from these ash beds were analyzed using LA-ICPMS and LA-MC-ICPMS. Volcanic ash geochemistry shows presence of Rhyolite or Dacite and reveal a collision-tectonic setting. Zircons from these ash layers yield comparatively low Nb/Hf and high Th/Nb ratios, dropping into the range of arc/orogenic-related settings. Zircon Hf-isotope compositions show that ɛHf(t) values vary from -11.7 to 1.8, indicating that at least two kinds of crustal component have been involved: juvenile lower crust and ancient middle-upper crust. The ash beds (Ss27a, Js129, Js130, Ms25, Ms26) near biotic extinction horizon have significant larger variation range of ɛHf(t) and relatively positive averages, implying that more juvenile lower crustal material had contributed to the volcanisms. This means that these volcanisms may have originated deeper depth or the volcanisms erupted so rapidly that there was no enough time for the mixing of different components. The volcanisms associated with biotic extinction should be the most intense and have greatest heat put. Spatial and temporal distributions of ash beds from thirty PTB sections worldwide reveal that the PTB volcanic ashes occurred only in the Paleo-Tethys region, suggesting that the volcanisms may be likely limited to the Paleo-Tethys continental

  12. Evolution of the lithospheric mantle beneath Mt. Baekdu (Changbaishan): Constraints from geochemical and Sr-Nd-Hf isotopic studies on peridotite xenoliths in trachybasalt

    NASA Astrophysics Data System (ADS)

    Park, Keunsu; Choi, Sung Hi; Cho, Moonsup; Lee, Der-Chuen

    2017-08-01

    Major and trace element compositions of minerals as well as Sr-Nd-Hf isotopic compositions of clinopyroxenes from spinel peridotite xenoliths entrained in Late Cenozoic trachybasalt from Mt. Baekdu (Changbaishan) were used to elucidate lithospheric mantle formation and evolution in the eastern North China Craton (NCC). The analyzed peridotites were mainly spinel lherzolites with rare harzburgites. They consisted of olivine (Fo89.3-91.0), enstatite (Wo1-2En88-90Fs8-11), diopside (Wo45-50En45-51Fs4-6), and spinel (Cr# = 8.8-54.7). The peridotite residues underwent up to 25% partial melting in fertile mid-ocean-ridge basalt (MORB) mantle. Plots of the Cr# in spinel against the Mg# in coexisting olivine or spinel suggested an affinity with abyssal peridotites. Comparisons of Cr# and TiO2 in spinel were also compatible with an abyssal peridotite-like composition; however, harzburgites were slightly enriched in TiO2 because of the reaction with MORB-like melt. Temperatures estimated using two-pyroxene thermometry ranged from 750 to 1010 °C, reflecting their lithospheric mantle origin. The rare earth element (REE) patterns in clinopyroxenes of the peridotites varied from light REE (LREE) depleted to spoon shaped to LREE enriched, reflecting secondary overprinting effects of metasomatic melts or fluids on the residues from primordial melting. The calculated trace element pattern of metasomatic melt equilibrated with clinopyroxene in Mt. Baekdu peridotite showed strong enrichment in large-ion lithophile elements, Th and U together with slight fractionation in heavy REEs (HREEs) and considerable depletion in Nb and Ti. The Sr-Nd-Hf isotopic compositions of clinopyroxenes separated from the peridotites varied from more depleted than present-day MORB to bulk Earth values. However, some clinopyroxene showed a decoupling between Nd and Sr isotopes, deviating from the mantle array with a high 87Sr/86Sr ratio. This sample also showed a significant Nd-Hf isotope decoupling lying

  13. 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.

  14. 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.

  15. 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.

  16. Petrogenesis of the igneous Mucajaí AMG complex, northern Amazonian craton — Geochemical, U-Pb geochronological, and Nd-Hf-O isotopic constraints

    NASA Astrophysics Data System (ADS)

    Heinonen, A. P.; Fraga, L. M.; Rämö, O. T.; Dall'Agnol, R.; Mänttäri, I.; Andersen, T.

    2012-10-01

    The ca. 1525 Ma igneous Mucajaí anorthosite-monzonite-granite (AMG) complex in northern Brazil is a rare manifestation of Mesoproterozoic intraplate magmatism in the northern Amazonian Craton. The complex comprises a two-phase rapakivi granite batholith with subordinate quartz-fayalite monzonites and syenites and the closely associated Repartimento anorthosite. Zircon U-Pb (ID-TIMS) geochronology reveals that the anorthosite (1526 ± 2 Ma), monzonite (1526 ± 2 Ma), and the main-phase biotite-hornblende granite (1527 ± 2 Ma) of the complex intruded the Paleoproterozoic (~ 1.94 Ga) country rocks simultaneously at ~ 1526 Ma and that the more evolved biotite granite is marginally younger at 1519 ± 2 Ma. Intraplate magmatism in the Mucajaí region was relatively short-lived and lasted 12 million years (1529-1517 Ma) at maximum. The Nd (whole-rock, ID-TIMS; ɛNd from - 1.9 to - 2.8), Hf (zircon, LAM-ICP-MS; ɛHf from - 2.0 to - 3.1), and O (zircon, SIMS; δ18O from 6.1 to 7.0‰) isotopic compositions of the studied rocks are fairly uniform but still reveal a small degree of isotopic heterogeneity in the Paleoproterozoic crust enclosing the complex. The small isotopic differences observed in the two types of rapakivi granites (biotite-hornblende granite and biotite granite) may result either from an isotopically heterogeneous lower crustal source or, more likely, from contamination of the granitic magma derived from a lower crustal source during prolonged residence at upper crustal levels.

  17. Archean crustal evolution in the Southern São Francisco craton, Brazil: Constraints from U-Pb, Lu-Hf and O isotope analyses

    NASA Astrophysics Data System (ADS)

    Albert, Capucine; Farina, Federico; Lana, Cristiano; Stevens, Gary; Storey, Craig; Gerdes, Axel; Dopico, Carmen Martínez

    2016-12-01

    In this study we present U-Pb and Hf isotope data combined with O isotopes in zircon from Neoarchean granitoids and gneisses of the southern São Francisco craton in Brazil. The basement rocks record three distinct magmatic events: Rio das Velhas I (2920-2850 Ma), Rio das Velhas II (2800-2760 Ma) and Mamona (2750-2680 Ma). The three sampled metamorphic complexes (Bação, Bonfim and Belo Horizonte) have distinct εHf vs. time arrays, indicating that they grew as separate terranes. Paleoarchean crust is identified as a source which has been incorporated into younger magmatic rocks via melting and mixing with younger juvenile material, assimilation and/or source contamination processes. The continental crust in the southern São Francisco craton underwent a change in magmatic composition from medium- to high-K granitoids in the latest stages, indicating a progressive HFSE enrichment of the sources that underwent anatexis in the different stages and possibly shallowing of the melting depth. Oxygen isotope data shows a secular trend towards high δ18O (up to 7.79‰) indicating the involvement of metasediments in the petrogenesis of the high potassium granitoids during the Mamona event. In addition, low δ18O values (down to 2.50‰) throughout the Meso- and Neoarchean emphasize the importance of meteoritic fluids in intra-crustal magmatism. We used hafnium isotope modelling from a compilation of detrital zircon compositions to constrain crustal growth rates and geodynamics from 3.50 to 2.65 Ga. The modelling points to a change in geodynamic process in the southern São Francisco craton at 2.9 Ga, from a regime dominated by net crustal growth in the Paleoarchean to a Neoarchean regime marked by crustal reworking. The reworking processes account for the wide variety of granitoid magmatism and are attributed to the onset of continental collision.

  18. Petrogenesis and tectonic implications of Late Devonian arc volcanic rocks in southern Beishan orogen, NW China: Geochemical and Nd-Sr-Hf isotopic constraints

    NASA Astrophysics Data System (ADS)

    Guo, Qian-Qian; Chung, Sun-Lin; Xiao, Wen-Jiao; Hou, Quan-Lin; Li, Shan

    2017-05-01

    Late Devonian (ca. 370 Ma) volcanic rocks provide important information about the nature of magmatism during the tectonic transition between the Early and Late Paleozoic in the Beishan orogen, southern Central Asian Orogenic Belt. They are predominantly an andesitic-dacitic-rhyolitic assemblage, characterized by alkali contents ranging from slightly calcic to slightly alkaline. The rhyolitic rocks are generally ferroan, whereas the andesitic rocks are magnesian. These volcanic rocks exhibit similar trace element characteristics to those of continental arcs. Strongly negative εNd(t) values (- 2.8 to - 3.6) and high Sr isotopic compositions (initial 87Sr/86Sr = 0.7036-0.7108) suggest that they are mainly derived from an ancient crust. However, the positive zircon εHf(t) values (+ 1.4 to + 16.4) support the role of juvenile components in their genesis, indicating the significant input of new mantle-derived magmas. These characteristics imply a hybrid derivation, from an ancient crustal source with the addition of juvenile materials during magma genesis, or perhaps heterogeneous contamination or hybridization during magma emplacement. Combined with the regional geology, our results indicate that the Late Devonian magmatism resulted from a southward retreat of the subduction zone, which records significant continental crustal growth in a transitional arc or an accretionary arc setting. The distinct geochemical compositions, especially the Nd-Hf isotope decoupling of the Dundunshan volcanic rocks, imply a significant change in the geodynamic setting in the Late Paleozoic.

  19. 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

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  1. Geochronological, geochemical and Sr-Nd-Hf isotopic constraints on the petrogenesis of Late Cretaceous A-type granites from the Sibumasu Block, Southern Myanmar, SE Asia

    NASA Astrophysics Data System (ADS)

    Jiang, Hai; Li, Wen-Qian; Jiang, Shao-Yong; Wang, He; Wei, Xiao-Peng

    2017-01-01

    The Late Cretaceous to Paleogene granitoids occur widespread in the Sibumasu block within Myanmar (SE Asia), which show a close association with tin-tungsten mineralization. However, the precise timing, petrogenesis and tectonic significance of these granitoids are poorly constrained so far. In this study, we present a detailed study on geochronology, elemental and Sr-Nd-Hf isotopic geochemistry for the Hermyingyi and Taungphila granites in southern Myanmar, with the aim of determining their petrogenesis and tectonic implications. LA-ICP-MS U-Pb dating of zircon grains from the two granites yield ages of 69-70 Ma, indicating a Late Cretaceous magmatic event. These granitic rocks are weakly peraluminous and belong to the high-K calc-alkaline series. They are both characterized by high SiO2, K2O + Na2O, FeOT/(FeOT + MgO) and Ga/Al ratios and low Al2O3, CaO, MgO, P2O5 and TiO2 contents, enriched in Rb, Th, U and Y, but depleted in Ba, Sr, P, and Eu, suggesting an A-type granite affinity. Moreover, they display prominent tetrad REE patterns and non-CHARAC trace element behavior, which are common in late magmatic differentiates with strong hydrothermal interaction or deuteric alteration. The granites belong to A2-type and probably formed at a high temperature and anhydrous condition. They have zircon εHf(t) values from - 12.4 to - 10.0 and whole-rock εNd(t) values from - 11.3 to - 10.6, with Paleoproterozoic TDM2 ages (1741-1922 Ma) for both Hf and Nd isotopes. Geochemical and isotopic data suggest that these A-type granites were derived from partial melting of the Paleoproterozoic continental crust dominated by metaigneous rocks with tonalitic to granodioritic compositions, without significant input of mantle-derived magma and followed by subsequent fractional crystallization. By integrating all available data for the regional tectonic evolution in SE Asia and adjacent regions, we attribute the formation of the Late Cretaceous A-type granites to a back-arc extension

  2. Timing and tectonic implications of the Pan-African Bangangte syenomonzonite, West Cameroon: Constraints from in-situ zircon U-Pb age and Hf-O isotopes

    NASA Astrophysics Data System (ADS)

    Tchouankoue, Jean Pierre; Li, Xian-Hua; Belnoun, Rose Noel Ngo; Mouafo, Lucas; Ferreira, Valderez Pinto

    2016-12-01

    The Bangangte pluton is a SW-NE elongated (5 × 20 km) massif located in the southeastern part of the Pan-African North Equatorial Fold Belt in Cameroon, consisting of two units with dominant monzonites in the south and syenites in the north. SIMS U-Pb zircon dating yields consistent emplacement ages of 585 ± 4 Ma and 583 ± 4 Ma for the southern unit and the northern unit, respectively. The Bangangte rocks display typical shoshonitic compositions characterized by Na2O + K2O > 5 wt%, K2O/Na2O ∼2, enrichment in LILE and LREE, but depletion in HFSE. Rocks from both units have similar O-Hf isotopes, with the monzonite zircons from the southern unit showing slightly higher δ18O (7.0 ± 0.4‰) but lower εHf(t) (-15.3 ± 1.4) value than the syenite zircons from southern unit (δ18O = 6.0 ± 0.4‰; εHf(t) = -14.0 ± 2.0). They were generated by partial melting of an enriched mantle source metasomatized by previous subduction processes, accompanied by crystal fractionation of pyroxene, Ti-Fe oxides and apatite, as well as crustal contamination to varying degrees. These rocks display a transitional geochemical feature of the subduction-related and within-plate shoshonites, suggesting that they were most likely emplaced in a post-collisional setting at the waning stage of the Pan-African orogeny.

  3. 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

  4. 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

  5. Geochronological, elemental and Sr-Nd-Hf-O isotopic constraints on the petrogenesis of the Triassic post-collisional granitic rocks in NW Thailand and its Paleotethyan implications

    NASA Astrophysics Data System (ADS)

    Wang, Yuejun; He, Huiying; Cawood, Peter A.; Srithai, Boontarika; Feng, Qinglai; Fan, Weiming; Zhang, Yuzhi; Qian, Xin

    2016-12-01

    New U-Pb geochronological, petrologic, elemental and Sr-Nd-Hf-O isotopic data for the granites from the Inthanon and Sukhothai zones in NW Thailand in conjunction with correlations with SW China are presented to constrain the age and position of the Paleotethys Ocean in this region and the associated assembly of Southeast Asia. The geochronological data show that the granitic rocks in the Inthanon and Sukhothai zones, herein named Group 1 and Group 2 granites, respectively, yield similar crystallization ages of 230-200 Ma. Group 1 samples are characterized by monzogranite and granite with I- and S-type geochemical affinity and Group 2 samples by I-type monzogranite and granodiorite. They have generally similar chondrite-normalized REE and PM-normalized multi-element patterns but distinct Sr-Nd-Hf-O isotopic compositions. Group 1 samples have slightly higher initial 87Sr/86Sr ratios (0.7111-0.7293) but lower εNd(t) values (- 11.1 to - 14.1) than those of Group 2 samples (87Sr/86Sr(i) = 0.7073-0.7278 and εNd(t) = - 8.3 to - 11.0). Group 1 samples show the lower εHf(t) values (- 5.4 to - 18.2), older TDM (1.62-2.40 Ga) and higher δ18O values (+ 7.95 to + 9.94) than those of Group 2 samples (εHf(t) of - 11.1 to + 4.80, TDM of 0.96-1.95 Ga and δ18O of + 4.95 to + 7.98) for the Triassic crystallization zircons. These geochemical signatures are similar to the Kwangsian and Indosinian granites in the South China and Indochina blocks but distinct from those of the Gangdese I-type granite and Sibumasu Paleozoic granite. Our data suggest that Group 1 samples mainly originated from the early Paleozoic supracrustal rocks containing metapelite and metavolcanic components, which had previously experienced the surface weathering. Group 2 samples were derived from a hybridized source of an old metamorphic and a newly underplated mafic component. Synthesis of our data with available regional observations indicates that the Inthanon zone represents the main suture zone of the

  6. An autochthonous Avalonian basement source for the latest Ordovician Brenton Pluton in the Meguma terrane of Nova Scotia: U-Pb-Hf isotopic constraints and paleogeographic implications

    NASA Astrophysics Data System (ADS)

    Duncan Keppie, J.; Gregory Shellnutt, J.; Dostal, Jaroslav; Fraser Keppie, D.

    2017-06-01

    The Ediacaran-Ordovician Meguma Supergroup was thrust over Avalonia basement prior to the intrusion of post-Acadian, ca. 370 Ma, S-type granitic batholiths. This has led to two main hypotheses regarding the original location of the Meguma terrane, a continental rise prism bordering either NW Africa or Avalonia. On the other hand, the pre-Acadian, ca. 440 Ma Brenton pluton has yielded the following U/Pb LA-ICP-MS zircon data: (1) 448 ± 3 Ma population peak inferred to be the intrusive age and (2) ca. 550 and 700 Ma inherited ages common to both Avalonia and NW Africa. In contrast, Hf isotopic analyses of zircon yielded model ages ranging from 814 to 1127 Ma with most between 940 and 1040 Ma: such ages are typical of Avalonia and not NW Africa. The ages of the inherited zircons found within the Brenton pluton suggest that it was probably derived by partial melting of sub-Meguma, mid-crustal Avalonian rocks, upon which the Meguma Supergroup was deposited. Although Avalonia is commonly included in the peri-Gondwanan terranes off NW Africa or Amazonia, paleomagnetic data, faunal provinciality, and Hf data suggest that, during the Ediacaran-Early Cambrian, it was an island chain lying near the tropics (ca. 20-30 °S) and was possibly a continuation of the Bolshezemel volcanic arc accreted to northern Baltica during the Ediacaran Timanide orogenesis. This is consistent with the similar derital zircon population in the Ediacaran-Cambrian Meguma Supergroup and the Dividal Group in northeastern Baltica.

  7. 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.

  8. Alxa Block Provenance of Ediacaran (Sinian) Sediments in the Helanshan Area: Constraints from Hf Isotopes and U-Pb Geochronology of Detrital Zircons

    NASA Astrophysics Data System (ADS)

    Xiaopeng, D.

    2016-12-01

    The tectonic relationship between the Alxa Block and the North China Craton has long been controversial. The Helanshan area lies at the western margin of the Ordos Block and east of the Alxa Block (Fig.a), and it contains rocks of the lower Zhengmuguan and upper Tuerkeng formations that belong to the Ediacaran system. The Zhengmuguan Formation is made up of abyssal facies rocks including dolomite and glacial conglomerate with dropstones, and the Tuerkeng Formation consists of silty slate of the neritic facies. A discontinuity marks the boundary between the Tuerkeng Formation and the Early Cambrian Suyukou Formation, which is composed mainly of pebbly sandstone towards the base and sandstone towards the top, representing a change in sedimentary facies from terrestrial to littoral.The Neoproterozoic U-Pb ages of zircons from the Ediacaran and Early Cambrian sediments peak at 818 ± 4 Ma (n = 88) and 905 ± 8 Ma (n = 20), consistent with the Neoproterozoic age peaks found in the Precambrian basement of the Alxa Block(Fig.b). There are few Neoproterozoic zircons in the Neoproterozoic strata of the Langshan area, and there are no reports of Neoproterozoic zircons in the Zhuozishan area, northwest of Helanshan, or in the western margin of the neighboring Ordos Basin. A number of Neoproterozoic zircons are found in the Middle Cambrian to Middle Ordovician strata of the Niushoushan area. And while Niushoushan is part of the Hexi Corridor, it did not amalgamate with the NCC before the Early-Middle Cambrian. Therefore, the Neoproterozoic and Early Cambrian sediments in Helanshan record information about Neoproterozoic magmatic events in the Alxa Block, and indicate an Alxa Block provenance(Fig.c).The Hf isotopic characteristics of the Neoproterozoic zircons from the Ediacaran Zhengmuguan Formation in the Helanshan area (eHf(t) = -7.812 to 3.274, TDMC = 2211-1578 Ma, n = 10) are similar to those Neoproterozoic igneous zircons from the Langshan area (eHf(t) = -1.105 to 5

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

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-08-01

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

  11. Detrital zircon U-Pb ages and Hf isotopic constraints on the terrigenous flexural basins of the Western Alps and their paleogeographic implications

    NASA Astrophysics Data System (ADS)

    Chu, Y.; Lin, W.; Faure, M.; Wang, Q.

    2015-12-01

    Detrital zircons from Cretaceous mica schist, Eocene sandstone and Oligocene siltstone of the Western Alps fall into three main separable age clusters at 610-540 Ma, 490-430 Ma, and 340-280 Ma that correspond to the Cadomian, Caledonian (Ordovician) and Variscan (Carboniferous) events widespread in Western and Central Europe. Hf isotopic results indicate that these three magmatic and tectonic episodes did not give rise to significant production of juvenile crust. A small but distinguishable group of Triassic zircon, around 250-200 Ma which is considered to derive from the Southern Alps, has been detected in the Oligocene "Schistes à Blocs" formation and the Brianconnais flysch. In contrast, this age group is absent in Eocene sandstones. In agreement with sedimentological studies, our results show that the main source areas of the Eocene flysch were probably located in the European continent. The arrival of detritus from the Internal Zone occurred in Early Oligocene, coeval with the tectonic rotation from northwestward to westward in the propoagation of allochthonous units. Based on previous studies and our new data, the Penninic Frontal Thrust zone was likely a paleorelief during the Eocene that accounts for the lack of detritus from the Internal Zone. Contemporary sediments were accumulated in the foredeep of the Adriatic Plate. From Oligocene time onwards, the blockage was cut through after a regional uplifting, and thus the Internal Zone started to provide detritus into the western flexural basins.

  12. Detrital zircon U-Pb ages and Hf isotopic constraints on the terrigenous sediments of the Western Alps and their paleogeographic implications

    NASA Astrophysics Data System (ADS)

    Chu, Yang; Lin, Wei; Faure, Michel; Wang, Qingchen

    2016-11-01

    Detrital zircons from Cretaceous micaschist, late Eocene-earliest Oligocene sandstone and early Oligocene siltstone of the Western Alps fall into three main separable age clusters at 610-540 Ma, 490-430 Ma, and 340-280 Ma that correspond to the Cadomian (Neoproterozoic), Ordovician, and Variscan (Carboniferous) events widespread in western and central Europe. Hf isotopic results indicate that these three magmatic and tectonic episodes did not give rise to significant production of juvenile crust. A distinguishable group of Triassic zircons, around 250-200 Ma which is considered to derive from the Southern Alps, has been detected in the early Oligocene "Schistes à Blocs" formation and the Briançonnais "Flysch Noir". In contrast, this age group is absent in late Eocene to earliest Oligocene sandstones. In agreement with sedimentological studies, our results show that the main source areas of the Eocene sandstone were probably located in the European continent. The arrival of detritus from the Internal Zone occurred in early Oligocene, coeval with the tectonic rotation from northwestward to westward in the propagation of allochthonous units. Based on previous studies and our new data, we argue that the Briançonnais Zone was likely a paleorelief since the middle Eocene that accounts for the lack of detritus from the Adriatic units. Contemporary sediments were accumulated in the foredeep of the Adriatic plate. From Oligocene time onward, the blockage was cut through after a regional uplifting, and thus, the Internal Zone started to provide detritus into the western flexural basins.

  13. 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

  14. Role of crustal contribution in the early stage of the Damara Orogen, Namibia: new constraints from combined U-Pb and Lu-Hf isotopes from the Goas Magmatic Complex

    NASA Astrophysics Data System (ADS)

    Milani, Lorenzo; Kinnaird, Judith; Lehmann, Jeremie; Naydenov, Kalin; Saalmann, Kerstin; Frei, Dirk; Gerdes, Axel

    2014-05-01

    The tholeiitic to calcalkaline Goas intrusive Complex of Namibia reflects the Pan-African plate convergence between the Congo and Kalahari Cratons and marks the first Pan-African magmatic event in the inland branch of the Damara Orogeny. We present new laser-ablation ICP-MS zircon U-Pb geochronology coupled with single-zircon Hf isotopic data obtained on Goas samples, in order to constrain the age of emplacement and investigate the crustal contribution on the magma sources. New ages on magmatic and detrital zircons on a pegmatitic and two metapsammitic samples are also presented, and help in constraining the major geotectonic events which affected the Goas magma sources through time. The new ages bracket the magmatic event between 580 Ma and 545 Ma, providing a better constraint on the timing record of the magmatic suite. Data, furthermore, show that the complex has been emplaced in a relatively short time, with a continuous magmatic activity from early tholeiitic metagabbro/hornblendite to main diorite or granite bodies with calcalkaline affinity. Hf isotopes analysis on zircons show invariably negative ɛHft values (from -34.4 to -3.8), indicating a significant crustal residence time with long-term reworking of multiple and mixed Archean to Mesoproterozoic components. Although the role of multiple crustal components is apparent from the wide range of ɛHft distribution, values of single intrusions cluster within relatively small ranges. The subchondritic data attest that no significant Pan-African juvenile magma was involved in the magmatogenesis. A subduction environment, although plausible, it is not inevitably disclosed by the new data. As we suggest magma underplating as a likely heating source for the Goas magmatism, the prolonged crustal residence time and the apparent lack of juvenile components suggest that the magma below the Pan-African active margin was unable to pass through the lithosphere, but managed to heat up sections of the crust sufficiently to

  15. In-situ zircon U-Pb age and Hf-O isotopic constraints on the origin of the Hasan-Robat A-type granite from Sanandaj-Sirjan zone, Iran: implications for reworking of Cadomian arc igneous rocks

    NASA Astrophysics Data System (ADS)

    Honarmand, Maryam; Li, Xian-Hua; Nabatian, Ghasem; Neubauer, Franz

    2017-01-01

    The Lower Permian Hasan-Robat syenogranite occurs as a single pluton and intruded the Upper Carboniferous-Lower Permian sandstones and dolomitic limestones in the central part of the Sanandaj-Sirjan zone. This syenogranitic intrusion shows A-type granitic affinity and is a good representative of Early Permian igneous activity in Iran. SIMS U-Pb zircon analyses indicate a crystallization age of 294.2 ± 2.5 Ma for the Hasan-Robat A-type granite. In-situ Lu-Hf and oxygen isotope analyses of magmatic zircons were carried out to infer the magma sources and evolution of the Hasan-Robat A-type syenogranite. The Hf-O zircon isotopic compositions are relatively homogeneous, with nearly chondritic ɛHf(t) values of -0.8 to +2.4 corresponding to two-stage zircon Hf model ages of 1.15-1.36 Ga. The δ18O values of zircon range from +7.6 to +8.6‰. The Hf model ages of the Hasan-Robat zircons is within the range of those reported from the Cadomian granitoids in Iran. The isotopic features of the Hasan-Robat syenogranite are in good agreement with Hf isotopic values and Hf and Nd model ages reported from the Cadomian arc magmatic suites in Iran. Thus, partial melting of these Cadomian igneous rocks would be the favorite source for the Hasan-Robat syenogranitic magma during the opening of the Neotethys Ocean and separation of Iranian terranes from the northern margin of Gondwana.

  16. Subduction-related magmatism and crustal role in the early stage of the Damara Orogen, Namibia: new constraints from combined U-Pb and Lu-Hf isotopes from the Goas Magmatic Complex

    NASA Astrophysics Data System (ADS)

    Milani, L.; Kinnaird, J.; Lehmann, J.; Naydenov, K.; Saalmann, K.; Frei, D.; Gerdes, A.

    2013-12-01

    Based on new U-Pb ages on zircons, the tholeiitic to calcalkaline Goas intrusive Complex marks the first magmatic Pan-African event in the inland branch of the Damara Orogen at ca. 575 Ma. The Goas orogenic event reflects the Pan-African collision between the Congo and Kalahari Cratons. The new ages provide a better constraint on the evolution of the orogenic suite, indicating that the Goas Complex has been emplaced in a relatively short time, not exceeding 25-30 Ma, with continuous magmatic activity from early tholeiitic metagabbro/hornblendite to main bodies with calcalkaline affinity. The absence of a time gap between the tholeiitic metagabbro/hornblendites and the calcalkaline products is documented. Our new geochemical and geochronological data concur with previous tectono-stratigraphic models by supporting the existence of an orogenic event with subduction of the Kalahari Craton below the Congo Craton, closure of the interposed ocean between the two cratons and subsequent continent-continent collision. Hf isotopes resulted in invariably negative ɛHft values (-34.4 to -3.8). This is indicative of relevant crustal contribution also within the early calcalkaline magmatism. A prolonged crustal residence time and the apparent lack of a substantial juvenile contribution imply that the mantle-derived magmas below the former active margin were unable to pass through the lithosphere, but managed to heat-up sections of the lower crust sufficiently to invoke large-scale melting and reworking processes in the upper crust. A notable mixing between different crustal components is apparent from the wide range of ɛHft distribution, although the values of single intrusions usually cluster within relatively small ranges. This suggests that the different intrusions reflect heterogeneous feeding crustal portions, but that each single event formed by melting of a rather uniform crustal material, which would indicate storage in a proper magmatic chamber, contamination with

  17. Palaeoproterozoic continental arc magmatism, and Neoproterozoic metamorphism in the Aravalli-Delhi orogenic belt, NW India: New constraints from in situ zircon U-Pb-Hf isotope systematics, monazite dating and whole-rock geochemistry

    NASA Astrophysics Data System (ADS)

    Kaur, Parampreet; Zeh, Armin; Chaudhri, Naveen

    2017-04-01

    Presently, the extent, origin and petrogenesis of late Palaeoproterozoic (ca. 1.85 Ga) magmatism in the north-central Aravalli-Delhi orogenic belt, NW India and subsequent metamorphic overprints are poorly constrained. Results of new in situ zircon U-Pb-Hf isotope analyses in combination with whole-rock elemental and isotopic data provide the first hard evidence that granitoid magmatism occurred in a continental magmatic arc setting between 1.86 and 1.81 Ga. The Hf-Nd model ages of 3.0-2.6 Ga and inherited zircon grains of 3.3-2.5 Ga indicate abundant reworking of Archaean crust. Flat HREE patterns with negative Eu anomalies furthermore reveal that the granitoids were generated from garnet-free and plagioclase-rich sources at shallow depths. Significant isotope variation among granitoid samples (εHft = -3.7 to -9.0; εNdt = -4.8 to -7.9) indicate that the reworked Archaean crust was not completely homogenised during the Palaeoproterozoic. This is best reflected by zircon Hf-isotope variation of ca. 9.5 epsilon units within the oldest granitoid sample. Zircon grains from this sample define three discrete Hf-isotope groups at εHf1.86Ga = -8.9, -4.8 and -1.6. These are interpreted to result from mixing of zircon-saturated magmas derived from three distinct sources within the crust prior to solidification. A monazite U-Pb isochron age of 868 ± 4 Ma from one of the granitoid samples furthermore indicates that the Aravalli fold belt was affected by an important post-magmatic overprint, perhaps related to the widespread metasomatic, granulite metamorphic and/or magmatic events during the same time span.

  18. Zircon U-Pb ages and Hf-O isotopes, and whole-rock Sr-Nd isotopes of the Bozhushan granite, Yunnan province, SW China: Constraints on petrogenesis and tectonic setting

    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

    2015-03-01

    The Bainiuchang silver-polymetallic ore deposit is a super-large deposit in the western part of the South China tungsten-tin province (or the Nanling tungsten-tin province). The deposit is spatially and temporally associated with the Bozhushan granite pluton. Our new data indicate that the Bozhushan granitoids formed at 86-87 Ma. The granitoids are geochemically consistent with A-type granite. The Bozhushan pluton consists predominantly of biotite granite that is characterized by weakly peraluminous to metaluminous compositions and high alkali contents (Na2O + K2O = 7.51-9.06 wt.%). The granitic rocks are enriched in large-ion lithophile elements (LILE) Rb, Th, U, and K, but relatively depleted in Ba and Sr. In addition, they have high Zr + Nb + Ce + Y contents (310-478 ppm) and high 10,000× Ga/Al ratios (2.7-3.1). The temperatures of the parental magmas for the Bozhushan granites are estimated to be 790-842 °C based on the zircon saturation thermometer. Isotopically, the Bozhushan granites are characterized by elevated initial 87Sr/86Sr ratios (0.7126-0.7257) and low εNd values (-11.2 to -12.4), and high δ18O values (7.91-9.58‰) and low εHf values (-9.5 to -6.1) for zircon crystals, which indicate a dominant continental crustal source. The two-stage Hf model ages vary from 1.53 to 1.86 Ga. The isotopic compositions support the interpretation that the granitic rocks formed by melting of the Meso- and Neoproterozoic metasedimentary basements of the Cathaysia block. These results, together with geological records in the other parts of the western Cathaysia block, suggest that the formation of the Bozhushan A-type granites is related to lithospheric extension and asthenospheric upwelling that are associated with the change of plate motion in Late-Cretaceous.

  19. 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

  20. Mid-Neoproterozoic intraplate magmatism in the northern margin of the Southern Granulite Terrane, India: Constraints from geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Deeju, T. R.; Santosh, M.; Yang, Qiong-Yan; Pradeepkumar, A. P.; Shaji, E.

    2016-11-01

    The northern margin of the Southern Granulite Terrane in India hosts a number of mafic, felsic and alkaline magmatic suites proximal to major shear/paleo-suture zones and mostly represents magmatism in rift-settings. Here we investigate a suite of gabbros and granite together with intermediate (dioritic) units generated through mixing and mingling of a bimodal magmatic suite. The massive gabbro exposures represent the cumulate fraction of a basic magma whereas the granitoids represent the product of crystallization in felsic magma chambers generated through crustal melting. Diorites and dioritic gabbros mostly occur as enclaves and lenses within host granitoids resembling mafic magmatic enclaves. Geochemistry of the felsic units shows volcanic arc granite and syn-collisional granite affinity. The gabbro samples show mixed E-MORB signature and the magma might have been generated in a rift setting. The trace and REE features of the rocks show variable features of subduction zone enrichment, crustal contamination and within plate enrichment, typical of intraplate magmatism involving the melting of source components derived from both depleted mantle sources and crustal components derived from older subduction events. The zircons in all the rock types show magmatic crystallization features and high Th/U values. Their U-Pb data are concordant with no major Pb loss. The gabbroic suite yields 206Pb/238U weighted mean ages in the range of 715 ± 4-832.5 ± 5 Ma marking a major phase of mid Neoproterozoic magmatism. The diorites crystallized during 206Pb/238U weighted mean age of 724 ± 6-830 ± 2 Ma. Zircons in the granite yield 206Pb/238U weighted mean age of 823 ± 4 Ma. The age data show broadly similar age ranges for the mafic, intermediate and felsic rocks and indicate a major phase of bi-modal magmatism during mid Neoproterozoic. The zircons studied show both positive and negative εHf(t) values for the gabbros (-6.4 to 12.4), and negative values for the diorites (-7

  1. 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

  2. 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

  3. Tracing Proterozoic arc mantle Hf isotope depletion of southern Fennoscandia through coupled zircon U-Pb and Lu-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Petersson, Andreas; Bjärnborg, Karolina; Scherstén, Anders; Gerdes, Axel; Næraa, Tomas

    2017-07-01

    Constraints on the composition of the depleted mantle Sm-Nd and Lu-Hf crust formation ages have a long history of scientific debate. When calculating mantle extraction ages, and constructing crustal growth models, a linear evolution of incompatible trace elements in a depleted mantle since > 4 Ga is routinely used. Mantle depletion however varies regionally and over time and subduction of sediments and oceanic crust renders a mantle-wedge variously enriched relative to a modelled depleted mantle. Here we show that primitive mantle-derived subduction related gabbroic intrusions from southern Fennoscandia have Hf isotope compositions that are enriched relative to a MORB-like linear depleted mantle evolution curve. Extrapolation of primitive Paleoproterozoic gabbro suites enables the construction of a regional mantle evolution curve, providing improved constraints on model ages, crustal residence times and the fraction of juvenile versus reworked continental crust. Convergent margins are assumed to be one of the main sites of continental crust growth, and using an overly depleted mantle source yield model ages that are too old, and hence cumulative crustal growth models show too much crust generation early in the Earth's history. The approach of using the Hf isotope composition of zircon from primitive subduction related gabbroic intrusions as a proxy for mantle Hf isotope composition, piloted in this study, can be applied to other convergent margins.

  4. Isotopic constraints on planetary evolution

    NASA Technical Reports Server (NTRS)

    Depaolo, D. J.

    1986-01-01

    Direct observations are considered that have been made on terrestrial and lunar materials in the search for answers to questions such as: when did the major structural elements (core, mantle, and crust) come into being and evolved. Measurements of isotope ratios of Pb, Sr, Nd, Hf, Ar, and other elements provide information about the internal evolution of the earth and moon since their formation 4.5 Gyr ago. The existing evidence indicates the core formed more than 4.4 Gyr ago, possibly while the earth was still accreting. Though the moon crust is similarly very old, the moon ceased to differentiate after about 1 Gyr whereas earth differentiation has continued to the present. Cataclysmic episodes have occurred, the largest 2.8 Gyr ago, when about half of the present continental mass formed.

  5. The initial Hf isotopic composition of the Earth

    NASA Astrophysics Data System (ADS)

    Bouvier, A.; Boyet, M. M.; Vervoort, J. D.; Patchett, P. J.

    2011-12-01

    One area of considerable activity in trying to understand the formation and evolution of Earth's crust is the isotopic analysis of Hf in parallel with Sm-Nd and U-Pb zircon studies, either to constrain early crustal growth and evolution [1], or as a complement to detrital zircon studies [2]. The 176Lu decay constant deduced from early planetary and Earth materials have different values. It has been suggested that a period of irradiation in the early Solar System affected the 176Hf production rate in meteoritic and planetary materials [3,4]. In this scenario, the initial Hf isotopic composition of the Solar System and the Earth would be ~4 ∈Hf units lower, affecting tremendously the interpretation of the differentiation history of the early Earth. We investigated Lu-Hf compositions of calcium-aluminum-rich inclusions, the oldest known objects of the Solar System dated at 4568 Ma [5], to assess the possibility of neutrino irradiation in the solar nebula. Here we report high-precision 176Lu-176Hf systematics of leached and unleached, and spiked and unspiked, bulk fractions and mineral separates of 6 individual CAIs from 2 CV3 chondrites. Isotopic analyses were carried out by Neptune MC-ICPMS at ASU. Analytical details are in [6,7]. The unspiked Hf fractions reveal stable isotope anomalies of μ178Hf= 20 ± 6 and μ180Hf= 31 ± 9 (2SD) for the CAI B4 fractions (n=3) and μ178Hf= -4 ± 10 and μ180Hf= 2 ± 10 (n=2) for BCR-2 relative to the JMC 475 Hf standard. Further high-precision analysis of unspiked Sm and Nd fractions of the samples will be made to correct from nucleosynthetic or neutron capture anomalies [8]. Such Hf stable isotopic anomalies predict no more than 50ppm correction on 176Hf/177Hf. At this stage, we have thus regressed together the spiked and unspiked Hf compositions of CAI fractions (n=13) for isochron calculations. The slope of the Lu-Hf isochron is 0.0882 ± 0.0026 (2SD) which corresponds to a 176Lu decay constant value of 1.852 (± 0.052) ×10

  6. Archean Lithosphere Beneath Arctic Canada: Lu-Hf Isotope Systematics for Kimberlite-Hosted Garnet-Peridotites From Somerset Island

    NASA Astrophysics Data System (ADS)

    Schmidberger, S. S.; Simonetti, A.; Francis, D.; Gariepy, C.

    2001-05-01

    Knowledge of the age of lithospheric mantle underlying the continents provides valuable constraints for the timing of formation and stabilization of Archean cratons. This study reports Lu-Hf isotopic data for garnet-peridotites, and their constituent garnets, from the Nikos kimberlite (100 Ma) on Somerset Island in the Canadian Arctic obtained using a Micromass IsoProbe multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS) at GEOTOP-UQAM. The low temperature peridotites (<1100 C; 80-150 km) exhibit a significant range in 176Lu/177Hf ratios (0.03-0.05) and are characterized by radiogenic 176Hf/177Hf(0.1Ga) isotopic compositions (0.28294-0.28419) corresponding to \\epsilonHf(0.1Ga) values of +8 to +52. In contrast, 176Lu/177Hf ratios (0.004-0.04) are lower for the high temperature peridotites (>1100 C; 160-190 km) and their 176Hf/177Hf(0.1Ga) isotopic compositions (0.28265-0.28333; \\epsilonHf(0.1Ga)=-2 to +22) are less radiogenic than those of the shallow xenoliths. A Lu-Hf isochron for six peridotites yields a mid Archean age of 3.4\\pm0.3 Ga and an initial 176Hf/177Hf ratio of 0.28101\\pm24. The remaining peridotites (n=9), in contrast, are characterized by extremely high (+35) initial \\epsilonHf(3.4Ga) values, which correlate negatively with their 176Lu/177Hf ratios, suggesting addition of Hf as a result of metasomatic interaction with the host kimberlite. The garnets from the low temperature (3.4 Ga old) peridotites are characterized by high 176Lu/177Hf ratios and define an errorchron age of 1.4\\pm0.2 Ga, which may reflect re-equilibration of Hf during kimberlite magmatism.

  7. 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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  9. The Mongol-Okhotsk Ocean subduction-related Permian peraluminous granites in northeastern Mongolia: Constraints from zircon U-Pb ages, whole-rock elemental and Sr-Nd-Hf isotopic compositions

    NASA Astrophysics Data System (ADS)

    Zhao, Pan; Xu, Bei; Jahn, Bor-ming

    2017-08-01

    Northeastern Mongolia represents a unique area that experienced evolution of both the Mongol-Okhotsk Ocean (MOO) and the Central Asian Orogenic Belt (CAOB). In order to better understand the evolution of MOO and its effect on the CAOB, we performed zircon U-Pb dating and Hf isotope, and whole-rock elemental and Sr-Nd isotopic analyses on three Permian peraluminous granitic plutons in northeastern Mongolia. Zircon dating result revealed that granitoids of East Monhhaan, South Monhhaan and Tuvshinshiree plutons were emplaced at 254 ± 3, 265 ± 5, and 273 ± 3 Ma, respectively. The result provides Permian ages for the Permian-Triassic volcanic-plutonic belt in northeastern Mongolia. Geochemical analysis suggests that all the granitoids are of peraluminous S-type granites. Elemental and Sr-Nd-Hf isotopic data indicate that these S-type granites were generated from mainly clay-poor greywacke with some contribution of juvenile mantle component. The Permian S-type granites emplaced likely in an active margin related to the subduction of the Mongol-Okhotsk Ocean. The protracted intermittent extension during the long-lived compression was considered as a likely geodynamic mechanism for the upwelling of mantle-derived basaltic magma and melting of crustal materials.

  10. The Hf-Nd Isotopic Record of Southwest - no Evidence for Early Continental Crust

    NASA Astrophysics Data System (ADS)

    Fisher, C. M.; Vervoort, J. D.

    2013-12-01

    The nature and timing of the growth of continental crust has generated debate for decades, with numerous models for the rate of crustal growth being suggested. Ultimately, two end-member models have been proposed. One favors rapid and early differentiation of continental crust and a complimentary depleted mantle, while the other proposes an early basaltic proto-crust with true continental crust and depleted mantle forming many hundreds of millions of years later. Radiogenic isotope systems (e.g., Nd and Hf) are crucial in this debate, as they provide constraints on ancient fractionation of Sm from Nd and Lu from Hf, signaling mantle depletetion, relative to the Chondritic Uniform Reservior (CHUR). Southwest Greenland contains some of the best-studied Eoarchean rocks on Earth. As such, these rocks provide one of the best vantage points on the long-standing question of the growth of early continental crust. In order to address the question of the timing of early crustal growth, our approach is to focus on the zircon record from this area, as it is clearly the most robust repository of age and Hf isotopic information. We have conducted Laser Ablation Split Stream analysis on the U-Pb and Hf isotopic composition of twelve samples of the least-altered meta-igneous rocks from the Amitsoq gneiss from the Isukasia and Nuuk regions of southwest Greenland. This analytical approach allows us to unambiguously determine the age and Hf isotopic composition on the same zircon volume. Further, we have conducted new isotope-dilution whole rock Sm-Nd and Lu-Hf solution MC-ICPMS analyses of these same samples. Our results show that zircon from rocks ranging in age from ~3.8 Ga to ~3.6 Ga have initial ɛHf values within analytical uncertainty (~1.5 ɛHf units) of CHUR, consistent with previous age and Hf isotope studies conducted in the region. Thus, we conclude that these samples from southwest Greenland (which contains one of the largest tracts of Eoarchean crust) contain no

  11. 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

  12. Middle Neoproterozoic (ca. 705-716 Ma) arc to rift transitional magmatism in the northern margin of the Yangtze Block: Constraints from geochemistry, zircon U-Pb geochronology and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Wang, Ruirui; Xu, Zhiqin; Santosh, M.; Xu, Xianbing; Deng, Qi; Fu, Xuehai

    2017-09-01

    The South Qinling Belt in Central China is an important window to investigate the Neoproterozoic tectono-magmatic processes along the northern margin of the Yangtze Block. Here we present whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes of a suite of Middle Neoproterozoic intrusion from the Wudang Uplift in South Qinling. Zircon LA-ICP-MS U-Pb ages reveal that these rocks were formed at ca. 705-716 Ma. Geochemical features indicate that the felsic magmatic rocks are I-type granitoids, belong to calcic- to calc-alkaline series, and display marked negative Nb, Ta and Ti anomalies. Moreover, the enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs), combined with depletion of heavy rare earth elements (HREEs) support that these rocks have affinity to typical arc magmatic rocks formed in Andean-type active continental margins. The REE patterns are highly to moderately fractionated, with (La/Yb)N = 5.13-8.10 in meta-granites, and 2.32-2.35 in granodiorite. The granitoids have a wide range of zircon εHf(t) values (-29.91 to 14.76) and zircon Hf two-stage model ages (696-3482 Ma). We suggest that the ca. 705-716 Ma granitoids were sourced from different degrees of magma mixing between partial melting of the overlying mantle wedge triggered by hydrous fluids released from subducted materials and crustal melting. The hybrid magmas were emplaced in the shallow crust accompanied by assimilation and fractional crystallization (AFC). Both isotopic and geochemical data suggest that the ca. 705-716 Ma felsic magmatic rocks were formed along a continental arc. These rocks as well as the contemporary A-type granite may mark a transitional tectonic regime from continental arc to rifting, probably related to slab rollback during the oceanic subduction beneath the northern margin of Yangtze Block.

  13. 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

  14. Hf Isotope Compositions in MORB from the Arctic MAR

    NASA Astrophysics Data System (ADS)

    Blichert-Toft, J.; Andres, M.; Tomza, U.; Schilling, J.

    2001-12-01

    We report on 176Hf/177Hf for 52 MAR basalt glasses from 66-78° N. Spreading rate, ridge axis elevation, and degree of melting progressively decrease northwards to very low values. On southern Kolbeinsey Ridge (66-69° N), ɛ Hf increases from +14.5 north of Iceland to +19.5 at the Spar FZ, where typical N-MORB erupt (ɛ Nd=+11, (La/Sm)n=0.5, (Lu/Hf)n=2.8). This gradient reflects mixing of the Iceland plume with depleted upper mantle. On northern Kolbeinsey Ridge (69-71° N), ɛ Hf decreases steeply from +19.5 at the Spar FZ to +15 at the Jan Mayen FZ, reflecting mixing between depleted upper mantle and the Jan Mayen plume, located 160 km to the east. On Mohns Ridge (71-73.5° N), ɛ Hf increases abruptly from +10 at Jan Mayen platform (ɛ Nd=4.5) to +24.3 at the intersection with Knipovich Ridge, where N-MORB erupt with ɛ Nd=+10, (La/Sm)n=0.6, and (Lu/Hf)n=1.3. The ɛ Hf gradient over Mohns Ridge reflects the northward dispersion of the Jan Mayen plume and its mixing with depleted upper mantle. Along Knipovich Ridge (73.5-78° N), which runs NW parallel to and on the fringe of the Barents Sea-Svalbard continental shelf break, ɛ Hf scatters between +20 and +23, a relatively small and highly radiogenic range compared to that of +7.2 to +10 for ɛ Nd. Overall, the Hf isotope latitudinal profile is similar to that of Nd and opposite those of Pb and Sr. A major distinction between the Hf and Nd isotope profiles is the magnitude of the maxima observed where N-MORB are present. The 176Hf/177Hf maximum at 73.5° N (ɛ Hf=24) is significantly higher than that at 69° N (ɛ Hf=19), whereas the opposite is observed for 143Nd/144Nd. In Nd-Hf isotope space, three distinct mixing trends are observed, stacked significantly above the global mantle array. The farthest displaced Jan Mayen-Mohns trend converges at a sharp angle with the mantle array at ɛ Nd=5. The southern and northern Kolbeinsey trends are superimposed, run subparallel to the mantle array, and intersect the Jan

  15. 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

  16. 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

  17. The Mesozoic metamorphic-magmatic events in the Medog area, the Eastern Himalayan Syntaxis: constraints from zircon U-Pb geochronology, trace elements and Hf isotope compositions in granitoids

    NASA Astrophysics Data System (ADS)

    Dong, Hanwen; Xu, Zhiqin; Li, Yuan; Liu, Zhao; Li, Huaqi

    2015-01-01

    Based on the regional geological mapping, several granitoid intrusions had been discovered in the Eastern Himalayan Syntaxis (EHS). In order to constrain their petrogenesis and discuss their relations with the regional tectonics, we carried out U-Pb dating, trace elements and Hf isotope geochemistry studies on zircons separated from the granitoid rocks, in the area of the EHS. In this contribution, the granitoid rocks are mainly composed of diorites (X20-1-6) and granitic gneissic rocks (X2-15-1). The U-Pb zircon dating of diorites yields a crystallization age of 193.8 ± 2.0 Ma. These zircon have ɛ Hf( t) values ranging from -6.48 to -0.05, indicating an involvement of ancient crustal materials in the generation of these igneous rocks. The zircons from the Medog granitic gneissic rock commonly show zoning structures. The REE patterns and abundances of the inherited cores are different from those of the oscillatory rims. The LA-ICP-MS U-Pb zircon in situ analyses indicate that: (1) the zircon cores give multi-stage magmatic event ages ranging from 516 to 1,826 Ma, of which six ages are converged on 1,330-911 Ma, it is considered that the migmatitic gneiss has been formed in this time, and (2) while the zircon rims yield 206Pb/238U weighted mean ages of 217.4 ± 3.0 Ma (MSWD = 3.2), which was interpreted to represent the ages of the Triassic anatexis. Their ɛ Hf( t) values range from -18.98 to -8.36 and -14.22 to 8.72, respectively. The timing of the anatexis in the Medog area is coeval with the widespread metamorphism in Lhasa terrane.

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

    NASA Astrophysics Data System (ADS)

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

    2005-04-01

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

  19. Petrogenesis and tectonic implications of the Early Paleozoic intermediate and mafic intrusions in the South Qinling Belt, Central China: Constraints from geochemistry, zircon U-Pb geochronology and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Wang, Ruirui; Xu, Zhiqin; Santosh, M.; Liang, Fenghua; Fu, Xuehai

    2017-08-01

    The characteristics and tectonic implications of the Early Paleozoic alkaline magmatic belt in the South Qinling Belt, which was originally part of the northern Yangtze Block prior the Devonian, have remained elusive. Whether this magmatic belt is related to rifting of the passive continental margin, to back-arc extension in the active continental margin, or to mantle plume activity is debated. Understanding the origin and geodynamic significance of this magmatic belt can provide new constraints on the Early Paleozoic tectonic evolution of the northern Yangtze Block. Here we present zircon U-Pb data from a suite of nepheline syenite, quartz syenite, diabase, and gabbro from the northern margin of the Yangtze Block which show an age range of ca. 435-440 Ma. The εHf(t) values of the intermediate rocks up to 16.59 suggest magma generation from depleted mantle sources and new crustal growth. Geochemically, the syenites showing high total alkali contents and are enriched in LREE, LILE (Rb, Ba, and K), and HFSE (Th, U, Nb, Ta, Zr, and Hf), with depletion in Sr, P, and Ti. The intermediate and mafic magmatic rocks were generated through magmas sourced from the subcontinental lithospheric mantle metasomatized by asthenospheric mantle and underwent fractional crystallization without significant crustal contamination. The magmatic suite represents a significant phase of crustal extension in the northern margin of the Yangtze Block.

  20. 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

  1. Petrology, geochemistry, zircon U-Pb dating and Lu-Hf isotope of granitic leucosomes within felsic gneiss from the North Qaidam UHP terrane: Constraints on the timing and nature of partial melting

    NASA Astrophysics Data System (ADS)

    Shengyao, Yu; Jianxin, Zhang; Deyou, Sun; del Real, Pablo García; Yunshuai, Li; Xilin, Zhao; Kejun, Hou

    2015-03-01

    Granitic leucosomes are widely distributed within felsic gneiss in the North Qaidam ultrahigh-pressure (UHP) metamorphic terrane in western China, which is crucial to understanding the relationships between partial melting, metamorphic evolution and orogenic processes. We have applied petrology, whole-rock geochemistry and Sr-Nd isotope, zircon U-Pb geochronology, trace element composition and Lu-Hf isotope of these granitic leucosomes to determine the nature and timing of partial melting of these rocks. Anatexis of the felsic gneiss is evidenced by (1) highly cuspate, elongated feldspar grains along quartz-quartz and quartz-feldspar boundaries, (2) cuspate wedge-shaped pockets of K-feldspar + quartz + plagioclase ± muscovite along the boundaries of quartz and/or plagioclase, and (3) felsic veinlets of K-feldspar + quartz ± plagioclase ± muscovite along grain boundaries. Major elements (FeOT, MnO, MgO and TiO2) as well as LREEs, HREEs and HFSEs are mainly retained in the melanosomes, whereas the large-ion lithophile elements (LILEs, e.g., Rb, Ba, K, Sr, Pb) are preferentially partitioned into the granitic leucosomes. Three discrete U-Pb ages are recorded in the zoned zircons from the melanosomes and granitic leucosomes. The inherited magmatic (pre-metamorphic) zircon cores from melanosomes and granitic leucosomes contain quartz + feldspar inclusions and record a Neoproterozoic protolith age of approximately ~ 950 Ma. The unzoned zircon mantles in the melanosomes and granitic leucosomes show characteristics similar to metamorphic zircons, in terms of such as remarkably flat heavy rare earth element (HREE) patterns, an absence of obviously negative Eu anomalies, and low Th/U ratios. These zircon mantles record an eclogite-facies metamorphic age of 444-449 Ma. The last discrete age at 433-435 Ma is preserved in anatectic zircon rims, which display pronounced oscillatory zoning, and contain felsic mineral inclusions of K-feldspar + plagioclase + quartz. The

  2. 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

  3. 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

    2017-03-01

    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

  4. 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

  5. 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

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

    NASA Astrophysics Data System (ADS)

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

    2015-11-01

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

  7. Combined U Pb and Hf isotope LA-(MC-)ICP-MS analyses of detrital zircons: Comparison with SHRIMP and new constraints for the provenance and age of an Armorican metasediment in Central Germany

    NASA Astrophysics Data System (ADS)

    Gerdes, Axel; Zeh, Armin

    2006-09-01

    Uranium-lead ages obtained by LA-ICP-MS analyses of zircon cores from a high-grade Armorican metasediment from the Mid-German Crystalline Rise, Central Germany, yield results which are identical to, but more precise than those previously obtained by SHRIMP dating. This is mainly due to the fact that SHRIMP analyses are more sensitive than LA-ICP-MS analyses to common Pb contamination on the surface of the grain mount. The new U-Pb ages, in combination with in-situ Hf isotope analyses of zircon, provide the first evidence that detrital zircons within Armorican sediments crystallized in both juvenile and evolved magmatic rocks during the Archaean at 2.7-2.9 Ga, the Palaeoproterozoic at 1.8-2.1 Ga, and the Neoproterozoic/Early Palaeozoic at 500-720 Ma. In addition, zircons were formed at ca. 1.0 Ga by remelting of Palaeoproterozoic crust during the Grenville orogeny. The U-Pb dataset shows an age gap between 1.8 and 1.0 Ga, which is characteristic of Armorican sediments, and indicates that the metasediment protolith is younger than Late Cambrian. In addition, the data support previous conclusions that sediments constituting the Armorican terrane assemblage were derived from three crustal sources. Dominant sources were the Avalonian-Cadomian belt (ca. 45%), situated at the northern margin of Gondwana during the Neoproterozoic, and the West-African and/or eastern Amazonian cratons (ca. 50%). The Grenville belt was a minor source (< 5%). Variation of ɛHf( t) values of the Neoproterozoic/Early Paleozoic zircons indicates two periods of increased juvenile magma formation, one at 595-575 Ma and a second at 515-500 Ma. The older event is coeval with the formation of the Avalonian-Cadomian magmatic arc, whereas the younger event can be related to the break-up of the northern Gondwana margin in Cambrian/Ordovician times. In between, at around 545 Ma, only recycling of older crustal material took place.

  8. An Assessment of the Total Uncertainly in Hf Isotope Analyses by LA-MC-ICPMS

    NASA Astrophysics Data System (ADS)

    Vervoort, J. D.; Dufrane, S. A.; Hart, G. L.

    2007-12-01

    The analysis of Hf isotopes in zircon by LA-MC-ICPMS is a promising new analytical method that is increasingly being used by many laboratories. The total uncertainty of these laser Hf isotopic analyses, however, has several potential components that should be fully accounted for in assigning total analytical uncertainty. Currently, this full respresentation of uncertainty is not widely done by the laser Hf community. The sources of potential uncertainty in a laser Hf isotope analysis include: 1) uncertainty during the Hf analysis (within-run uncertainty); 2) uncertainty in the correction of 176Yb and 176Lu on 176Hf (included in this must be the allowance for the differences in mass bias behavior between Yb/Lu and Hf, which are not equivalent, sensu stricto); 3) matrix effects between zircon grains (this effect has been demonstrated for U-Pb zircon analysis and may be significant for some zircon grains); 4) any instrumental biases in the determination of Hf isotopic composition (most MC- ICPMS analyses of the JMC 475 Hf standard, for example, do not give exactly 176Hf/177Hf=0.282160 and require some normalization to a common value, which is difficult to do with laser Hf analyses). Furthermore, the determination of initial Hf isotope values requires both precise and accurate age determinations and accurate 176Lu/177Hf ratios in addition to the above factors that contribute uncertainty in present-day Hf isotope ratios. The effect of 176Lu/177Hf uncertainty on the initial values is small because of the very low ratio in zircon but the age correction is highly significant because the CHUR reference varies by ~2.3 ɛHf units/100 my. For zircons with a highly complex growth history (e.g., early Archean and Hadean zircons) linking precise and accurate age determinations to the Hf isotopic analysis is especially challenging and represents a potentially large source of uncertainty for these zircons. Because only in-run uncertainty can be determined during a single analysis

  9. 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

  10. 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.

  11. 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

  12. OIB signatures in basin-related lithosphere-derived alkaline basalts from the Batain basin (Oman) - Constraints from 40Ar/39Ar ages and Nd-Sr-Pb-Hf isotopes

    NASA Astrophysics Data System (ADS)

    Witte, M.; Jung, S.; Pfänder, J. A.; Romer, R. L.; Mayer, B.; Garbe-Schönberg, D.

    2017-08-01

    Tertiary rift-related intraplate basanites from the Batain basin of northeastern Oman have low SiO2 (< 45.6 wt.%), high MgO (> 9.73 wt.%) and moderate to high Cr and Ni contents (Cr > 261 ppm, Ni > 181 ppm), representing near primary magmas that have undergone fractionation of mainly olivine and magnetite. Rare earth element systematics and p-T estimates suggest that the alkaline rocks are generated by different degrees of partial melting (4-13%) of a spinel-peridotite lithospheric mantle containing residual amphibole. The alkaline rocks show restricted variations of 87Sr/86Sr and 143Nd/144Nd ranging from 0.70340 to 0.70405 and 0.51275 to 0.51284, respectively. Variations in Pb isotopes (206Pb/204Pb: 18.59-18.82, 207Pb/204Pb: 15.54-15.56, 208Pb/204Pb: 38.65-38.98) of the alkaline rocks fall in the range of most OIB. Trace element constraints together with Sr-Nd-Pb isotope composition indicate that assimilation through crustal material did not affect the lavas. Instead, trace element variations can be explained by melting of a lithospheric mantle source that was metasomatized by an OIB-type magma that was accumulated at the base of the lithosphere sometimes in the past. Although only an area of less than 1000 km2 was sampled, magmatic activity lasted for about 5.5 Ma with a virtually continuous activity from 40.7 ± 0.7 to 35.3 ± 0.6 Ma. During this period magma composition was nearly constant, i.e. the degree of melting and the nature of the tapped source did not change significantly over time.

  13. Accurate Hf isotope determinations of complex zircons using the "laser ablation split stream" method

    NASA Astrophysics Data System (ADS)

    Fisher, Christopher M.; Vervoort, Jeffery D.; DuFrane, S. Andrew

    2014-01-01

    The "laser ablation split stream" (LASS) technique is a powerful tool for mineral-scale isotope analyses and in particular, for concurrent determination of age and Hf isotope composition of zircon. Because LASS utilizes two independent mass spectrometers, a large range of masses can be measured during a single ablation, and thus, the same sample volume can be analyzed for multiple geochemical systems. This paper describes a simple analytical setup using a laser ablation system coupled to a single-collector (for U-Pb age determination) and a multicollector (for Hf isotope analyses) inductively coupled plasma mass spectrometer (MC-ICPMS). The ability of the LASS for concurrent Hf + age technique to extract meaningful Hf isotope compositions in isotopically zoned zircon is demonstrated using zircons from two Proterozoic gneisses from northern Idaho, USA. These samples illustrate the potential problems associated with inadvertently sampling multiple age and Hf components in zircons, as well as the potential of LASS to recover meaningful Hf isotope compositions. We suggest that such inadvertent sampling of differing age and Hf components can be a significant cause of excess scatter in Hf isotope analyses and demonstrate that the LASS approach offers a robust solution to these issues. The veracity of the approach is demonstrated by accurate analyses of 10 reference zircons with well-characterized age and Hf isotopic composition, using laser spot diameters of 30 and 40 µm. In order to expand the database of high-precision Lu-Hf isotope analyses of reference zircons, we present 27 new isotope dilution-MC-ICPMS Lu-Hf isotope measurements of five U-Pb zircon standards: FC1, Temora, R33, QGNG, and 91500.

  14. Zircon Hf isotope evidence for an enriched mantle source for the Bushveld Igneous Complex

    NASA Astrophysics Data System (ADS)

    Alex Zirakparvar, N.; Mathez, Edmond. A.; Scoates, James S.; Wall, Corey J.

    2014-09-01

    We use the Hf isotope composition of zircon from the Bushveld Complex to better understand the source of its parent magmas. The data set, which consists of 141 individual LA-ICP-MS analyses from 11 samples encompassing the entire cumulate stratigraphy, shows that the parent magmas had a Hf isotope composition unlike that of the depleted mantle at 2.06 Ga. Specifically, sample average ɛHf(present) values range from -55.3 to -52.5 (ɛHf(2.06 Ga) = -9.0 to -6.8) and are surprisingly homogeneous. This homogeneity is difficult to reconcile with direct assimilation of crustal material by Bushveld parent magmas because it would require that each batch of magma had assimilated just the right amount of material to all acquire the same Hf isotopic composition. Also, calculations suggest that simple mixing of regional crust into a primitive, mantle-derived liquid cannot account for both the presumed Hf and major elemental concentrations and the 176Hf/177Hf ratio of the Bushveld magmas. Rather, the Hf data are consistent with generation of these magmas by partial melting in a sub-continental mantle lithospheric source with an unradiogenic Hf isotopic composition equal to that of the Bushveld parent magmas. Several possibilities for the development of such a source are explored using the new Hf isotope data.

  15. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2007-06-01

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

  17. Initial subduction of the Paleo-Pacific Oceanic plate in NE China: Constraints from whole-rock geochemistry and zircon U-Pb and Lu-Hf isotopes of the Khanka Lake granitoids

    NASA Astrophysics Data System (ADS)

    Liu, Kai; Zhang, Jinjiang; Wilde, Simon A.; Zhou, Jianbo; Wang, Meng; Ge, Maohui; Wang, Jiamin; Ling, Yiyun

    2017-03-01

    Northeast China is located in the eastern part of the Central Asian Orogenic Belt (CAOB) and was influenced by Paleo-Pacific subduction during the Mesozoic. Abundant granitoids from the late Paleozoic to early Mesozoic in NE China record this process, including the Khanka Lake granitoids, which resulted in extensive growth of continental crust in the area. However, the question of how and when the Paleo-Pacific tectonic system began to affect NE China is still highly controversial. The Khanka Lake granitoids can be subdivided into two main components based on their geochemical characteristics, namely granodiorite and syenogranite. The granodiorite has a U-Pb age of 249 Ma and is adakite-like (enriched in LREE and LILEs with high Mg#, Sr, La/Yb, Sr/Y and Na2O/K2O), with zircon εHf(t) values of - 0.65 to 1.61, produced by the magma mixing between melting of the lower continental crust and juvenile basaltic magma. The syenogranite has zircon U-Pb ages of 209 to 199 Ma and geochemical features of highly fractionated I-type granites, with high SiO2, total alkalis and low Mg (and Mg#), Fe, Cr and Ni, and positive zircon εHf(t) of 1.72 to 5.12, indicating an origin from remelting of juvenile crust. The granitoids were intruded by felsic veins between 195 and 184 Ma with positive zircon εHf(t) from 0.57 to 5.32. The εHf(t) values of the granitoids become more positive as the zircon U-Pb ages become younger, suggesting continuous melting of juvenile crust during subduction. It is concluded that the Khanka Lake granitoids record the early stage of subduction of the Paleo-Pacific Oceanic plate, which commenced at least ca. 250 Ma ago.

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

    NASA Astrophysics Data System (ADS)

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

    2013-11-01

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

  19. Iron Isotope Constraints on Planetesimal Core Formation

    NASA Astrophysics Data System (ADS)

    Jordan, M.; Young, E. D.

    2016-12-01

    The prevalence of iron in both planetary cores and silicate mantles renders the element a valuable tool for understanding core formation. Magmatic iron meteorites exhibit an enrichment in 57Fe/54Fe relative to chondrites and HED meteorites. This is suggestive of heavy Fe partitioning into the cores of differentiated bodies. If iron isotope fractionation accompanies core formation, we can elucidate details about the history of accretion for planetary bodies as well as their compositions and relative core sizes. The equilibrium 57Fe/54Fe between metal and silicate is necessary for understanding observed iron isotope compositions and placing constraints on core formation. We measure this fractionation in two Aubrite meteorites, Norton County and Mount Egerton, which have known temperatures of equilibration and equilibrated silicon isotopes. Iron was purified using ion-exchange chromatography. Data were collected on a ThermoFinnigan NeptuneTM multiple-collector inductively coupled plasma-source mass spectrometer (MC-ICP-MS) run in wet plasma mode. The measured fractionation Δ57Femetal-silicate is 0.08‰ ± 0.039 (2 SE) for Norton County and 0.09‰ ± 0.019 (2 SE) for Mount Egerton, indicating that the heavy isotopes of Fe partition into the metallic phase. These rocks are in isotopic equilibrium at a temperature of 1130 K and 1200 K ± 80 K, respectively. The concentration of the heavy isotopes of iron in the metallic phase is consistent with recent experimental studies. Using our measured metal-silicate Fe isotope fractionation and the resulting temperature calibration, while taking into account impurities in the metallic phase and temperatures of equilibration, determine that core formation could explain the observed difference between magmatic iron meteorites and chondrites if parent bodies have small cores. In order to verify that Rayleigh distillation during fractional crystallization was not a cause of iron isotope fractionation in iron meteorites, we measured

  20. What Hf isotopes in zircon tell us about crust-mantle evolution

    NASA Astrophysics Data System (ADS)

    Iizuka, Tsuyoshi; Yamaguchi, Takao; Itano, Keita; Hibiya, Yuki; Suzuki, Kazue

    2017-03-01

    The 176Lu-176Hf radioactive decay system has been widely used to study planetary crust-mantle differentiation. Of considerable utility in this regard is zircon, a resistant mineral that can be precisely dated by the U-Pb chronometer and record its initial Hf isotope composition due to having low Lu/Hf. Here we review zircon U-Pb age and Hf isotopic data mainly obtained over the last two decades and discuss their contributions to our current understanding of crust-mantle evolution, with emphasis on the Lu-Hf isotope composition of the bulk silicate Earth (BSE), early differentiation of the silicate Earth, and the evolution of the continental crust over geologic history. Meteorite zircon encapsulates the most primitive Hf isotope composition of our solar system, which was used to identify chondritic meteorites best representative of the BSE (176Hf/177Hf = 0.282793 ± 0.000011; 176Lu/177Hf = 0.0338 ± 0.0001). Hadean-Eoarchean detrital zircons yield highly unradiogenic Hf isotope compositions relative to the BSE, providing evidence for the development of a geochemically enriched silicate reservoir as early as 4.5 Ga. By combining the Hf and O isotope systematics, we propose that the early enriched silicate reservoir has resided at depth within the Earth rather than near the surface and may represent a fractionated residuum of a magma ocean underlying the proto-crust, like urKREEP beneath the anorthositic crust on the Moon. Detrital zircons from world major rivers potentially provide the most robust Hf isotope record of the preserved granitoid crust on a continental scale, whereas mafic rocks with various emplacement ages offer an opportunity to trace the Hf isotope evolution of juvenile continental crust (from εHf[4.5 Ga] = 0 to εHf[present] = + 13). The river zircon data as compared to the juvenile crust composition highlight that the supercontinent cycle has controlled the evolution of the continental crust by regulating the rates of crustal generation and intra

  1. Sm-Nd, Lu-Hf and Nb-Zr Constraints on the Early Differentiation of the Moon

    NASA Astrophysics Data System (ADS)

    Mezger, K.; Munker, C.; Scherer, E. E.

    2002-12-01

    The dominant chemical and mineralogical differentiation process on the Moon is most likely associated with the formation and subsequent crystallisation of a magma ocean. The major evidence in favour of the existence of a magma ocean is the ancient anorthositic crust that once covered the lunar surface. The complementary heavier minerals that crystallised as cumulates from the magma ocean must have included olivine, orthopyroxene, clinopyroxene, spinel, ilmenite, and possibly garnet. Partial melts from these differentiated silicate and oxide layers gave rise to younger mare basalts that now cover parts of the Moon's surface. Radioactive parent-daughter systems such as 147Sm-143Nd, 176Lu-176Hf and 92Nb-92Zr can provide important time constraints on early lunar differentiation processes, particularly if parent-daughter pairs are sufficiently fractionated during magmatic processes. Therefore we obtained Sm-Nd, Lu-Hf and Nb-Zr isotope data for a variety of well dated lunar samples including KREEP basalts, low and high-Ti mare basalts and lunar soils. 92Zr/90Zr and 96Zr/90Zr in mare basalts agree with the chondritic value within the analytical errors of +/-0.5 and +/-1.5 ɛ-units (2σ ), respectively. The absence of 92Zr-isotope anomalies and the presence of small 182W anomalies in mare basalts [1] confine the crystallisation age of the magma ocean to ca. 4.52 - 4.51 Ga (assuming initial 182Hf/180Hf of 1x10-4 and 92Nb/93Nb of 1x10-3). The initial ɛHf (ɛNd) range from -1.9 (-2.7) in the KREEP rocks to +16.6 (6.6) in high-Ti mare basalts. The initial Hf and Nd isotope ratios define two trends that deviate significantly from the terrestrial Hf-Nd array. The initial 176Hf-177Hf values for mare basalts (+7.3 to +16.6 ɛ) are consistent with the data from Beard et al. [2] (0 to +8 ɛ) using the revised 176Lu decay constant [3], but expand the known compositional range of mare basalts. The high ɛHf compared to the ɛNd in the low-Ti basalts requires that garnet played an

  2. 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.

  3. 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

  4. Formation of Paleoproterozoic eclogitic mantle, Slave Province (Canada): Insights from in-situ Hf and U-Pb isotopic analyses of mantle zircons [rapid communication

    NASA Astrophysics Data System (ADS)

    Schmidberger, Stefanie S.; Heaman, Larry M.; Simonetti, Antonio; Creaser, Robert A.; Cookenboo, Harrison O.

    2005-12-01

    In-situ Hf isotope analyses and U-Pb dates were obtained by laser ablation-MC-ICP-MS for a zircon-bearing mantle eclogite xenolith from the diamondiferous Jericho kimberlite located within the Archean Slave Province (Nunavut), Canada. The U-Pb zircon results yield a wide range of ages (˜2.0 to 0.8 Ga) indicating a complex geological history. Of importance, one zircon yields a U-Pb upper intercept date of 1989 ± 67 Ma, providing a new minimum age constraint for zircon crystallization and eclogite formation. In contrast, Hf isotope systematics for the same zircons display an intriguing uniformity, and corresponding Hf depleted mantle model ages range between 2.1 ± 0.1 and 2.3 ± 0.1 Ga; the youngest Hf model age is within error to the oldest U-Pb date. The Jericho eclogites have previously been interpreted as representing remnants of metamorphosed oceanic crust, and their formation related to Paleoproterozoic subduction regimes along the western margin of the Archean Slave craton during the Wopmay orogeny. Hf isotope compositions and U-Pb results for the Jericho zircons reported here are in good agreement with a Paleoproterozoic subduction model, suggesting that generation of oceanic crust and eclogite formation occurred between 2.0 and 2.1 Ga. The slightly older Hf depleted mantle model ages (˜2.1 to 2.3 Ga) may be reconciled with this model by invoking mixing between 'crustal'-derived Hf from sediments and more radiogenic Hf associated with the oceanic crust during the ˜2 Ga subduction event. This results in intermediate Hf isotope compositions for the Jericho zircons that yield 'fictitiously' older Hf model ages.

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

    NASA Astrophysics Data System (ADS)

    Zhang, Yu; Meng, Fanxue; Niu, Yaoling

    2016-10-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2013-01-01

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

  7. Impact of glacial activity on the weathering of Hf isotopes - Observations from Southwest Greenland

    NASA Astrophysics Data System (ADS)

    Rickli, Jörg; Hindshaw, Ruth S.; Leuthold, Julien; Wadham, Jemma L.; Burton, Kevin W.; Vance, Derek

    2017-10-01

    Data for the modern oceans and their authigenic precipitates suggest incongruent release of hafnium (Hf) isotopes by chemical weathering of the continents. The fact that weathering during recent glacial periods is associated with more congruent release of Hf isotopes has led to the hypothesis that the incongruency may be controlled by retention of unradiogenic Hf by zircons, and that glacial grinding enhances release of Hf from zircons. Here we study the relationship between glacial weathering processes and Hf isotope compositions released to rivers fed by land-terminating glaciers of the Greenland Ice Sheet, as well as neighbouring non-glacial streams. The weathered source rocks in the studied area mostly consist of gneisses, but also include amphibolites of the same age (1.9 Ga). Hafnium and neodymium isotope compositions in catchment sediments and in the riverine suspended load are consistent with a predominantly gneissic source containing variable trace amounts of zircon and different abundances of hornblende, garnet and titanite. Glacially sourced rivers and non-glacial streams fed by precipitation and lakes show very unradiogenic Nd isotopic compositions, in a narrow range (ɛNd= -42.8 to -37.9). Hafnium isotopes, on the other hand, are much more radiogenic and variable, with ɛHf between -18.3 and -0.9 in glacial rivers, and even more radiogenic values of +15.8 to +46.3 in non-glacial streams. Although relatively unradiogenic Hf is released by glacial weathering, glacial rivers actually fall close to the seawater array in Hf-Nd isotope space and are not distinctly unradiogenic. Based on their abundance in rocks and sediments and their isotope compositions, different minerals contribute to the radiogenic Hf in solution with a decreasing relevance from garnet to titanite, hornblende and apatite. Neodymium isotopes preclude a much stronger representation of titanite, hornblende and apatite in solution, such as might result from differences in dissolution rates

  8. Post-collisional magmatism: Crustal growth not identified by zircon Hf-O isotopes

    NASA Astrophysics Data System (ADS)

    Couzinié, Simon; Laurent, Oscar; Moyen, Jean-François; Zeh, Armin; Bouilhol, Pierre; Villaros, Arnaud

    2016-12-01

    The combination of U-Pb, Lu-Hf and O isotopic analyses in global zircon databases has recently been used to constrain continental crustal growth and evolution. To identify crust-forming events, these studies rely on the assumption that new crust is formed from depleted mantle sources. In contrast, this work suggests that post-collisional mafic magmas and their derivatives represent a non-negligible contribution to crustal growth, despite having zircons with "crust-like" Hf-O isotopic characteristics. We address this paradox and its implications for crustal evolution on the basis of a case study from the Variscan French Massif Central (FMC). The late stages of continental collisions are systematically marked by the emplacement of peculiar mafic magmas, rich in both compatible (Fe, Mg, Ni, Cr) and incompatible elements (K2O, HFSE, LREE) and displaying crust-like trace element patterns. This dual signature is best explained by melting of phlogopite- (and/or amphibole-) bearing peridotite, formed by contamination of the mantle by limited amounts (10-20%) of crustal material during continental subduction shortly preceding collision. Mass balance constraints show that in melts derived from such a hybrid source, 62-85% of the bulk mass is provided by the mantle component, whereas incompatible trace elements are dominantly crustal in origin. Thereby, post-collisional mafic magmas represent significant additions to the crust, whilst their zircons have "crustal" isotope signatures (e.g. - 2 < εHft < - 9 and + 6.4 < δ18O < + 10 ‰ in the FMC). Because post-collisional mafic magmas are (i) ubiquitous since the late Archean; (ii) the parental magmas of voluminous granitoid suites; and (iii) selectively preserved in the geological record, zircons crystallized from such magmas (and any material derived from their differentiation or reworking) bias the crustal growth record of global zircon Hf-O isotopic datasets towards ancient crust formation and, specifically, may lead to an

  9. Garnet effect on Nd-Hf isotope decoupling: Evidence from the Jinfosi batholith, Northern Tibetan Plateau

    NASA Astrophysics Data System (ADS)

    Huang, Hui; Niu, Yaoling; Mo, Xuanxue

    2017-03-01

    The initial Nd and Hf isotope ratios of a 420 Ma post-collisional dioritic-granitic batholith from the Northern Tibetan plateau define a negative trend above and orthogonal to the ԐHf(t)-ԐNd(t) terrestrial array. This uncommon trend offers an insight into the origin of the puzzling Nd-Hf isotope decoupling in the crustal rocks. On this trend, samples depleted in heavy rare earth elements (HREEs, i.e., [Dy/Yb]N ≫ 1) deviate most from the terrestrial array whereas samples with flat HREEs (i.e., [Dy/Yb]N ≥ 1) deviate less or plot within the terrestrial array, pointing to the controlling effect of garnet in the magma source. Ancient garnet-bearing residues after melt extraction will have elevated Lu/Hf ratios and can evolve with time to produce high ԐHf(t) at a low ԐNd(t) value. Mixing of melts derived from such source lithologies (high Lu/Hf) with melts possessing a within-terrestrial array Nd-Hf isotopic composition (low Lu/Hf) best explains the observed trend orthogonal to the terrestrial array. The samples from the Jinfosi batholith with the most decoupled Nd-Hf isotope compositions require a larger degree (> 40%) and ancient (i.e., ≥ 1.8 Gyr) previous melt extraction from their source. It follows that the ancient melts with depleted HREEs complementary to those garnet-bearing residues should have low ԐHf values and plot below the terrestrial array, which is indeed shown by some Archean/Paleoproterozic TTGs.

  10. 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.

  11. 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.

  12. 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

  13. A method for combined Sr-Nd-Hf isotopic analysis of <10 mg dust samples: implication for ice core science

    NASA Astrophysics Data System (ADS)

    Ujvari, Gabor; Wegner, Wencke; Klötzli, Urs

    2017-04-01

    Aeolian mineral dust particles below the size of 10-20 μm often experience longer distance transport in the atmosphere, and thus Aeolian dust is considered an important tracer of large-scale atmospheric circulation. Since ice core dust is purely Aeolian in origin, discrimination of its potential source region(s) can contribute to a better understanding of past dust activity and climatic/environmental causes. Furthermore, ice core dust source information provides critical experimental constraints for model simulations of past atmospheric circulation patterns [1,2]. However, to identify dust sources in past dust archives such as ice cores, the mineralogy and geochemistry of the wind-blown dust material must be characterized. While the amount of dust in marine cores or common terrestrial archives is sufficient for different types of analyses and even for multiple repeat measurements, dust content in ice cores is usually extremely low even for the peak dusty periods such as the Last Glacial Maximum (LGM) (5-8 mg dust/kg ice; [3]). Since the most powerful dust fingerprinting methods, such as REE composition and Sr-Nd-Pb isotopic analyses are destructive there is a clear need to establish sequential separation techniques of Sr, Nd, Pb and other REEs to get the most information out of small (5-10 mg) dust samples recovered from ice cores. Although Hf isotopes have recently been added as a robust tool of aerosol/dust source discrimination (e.g. [4,5,6,7]), precise Hf isotopic measurements of small (<10 mg) dust samples are still challenging due to the small Hf amounts (on the order of 1-10 ng) and often compromised by potential problems arising during ion exchange chemistry. In this pilot study an improved method for chemical separation of Sr, Nd and Hf by Bast et al. [8] was applied, which allows the precise isotope analysis of sub-ng amounts of Hf by MC-ICPMS. This ion exchange chromatography procedure has been combined with established methods of separating and

  14. Origin of Mesoproterozoic A-type granites in Laurentia: Hf isotope evidence

    NASA Astrophysics Data System (ADS)

    Goodge, John W.; Vervoort, Jeffrey D.

    2006-03-01

    Granitic rocks are commonly used as a means to study chemical evolution of continental crust. In particular, their isotopic compositions reflect the relative contributions of mantle and crustal sources in their genesis. In Laurentia, a distinctive belt of Mesoproterozoic A-type or "anorogenic" granites of ˜ 1.4 Ga age was emplaced within composite, heterogeneous Proterozoic crust. Zircons are an ideal mineral to constrain the granite petrogenetic history because they are repositories of both age (U-Pb geochronology) and tracer (Lu-Hf isotopic) information. We measured the Hf isotope composition of zircons from 31 previously dated A-type granites intruding Proterozoic basement provinces from the southwest U.S. to the upper mid-continent. Isotopic compositions for all granites are broadly similar, with average 176Hf/ 177Hf(i) ratios of 0.281871-0.282153. Averages for granites within different crustal provinces yield present-day ɛHf values between - 31.9 and - 21.9. Initial ɛHf values discriminate the granites by age of the 2.0-1.6 Ga crust which they intrude, but are independent of intrusion age, as follows (basement formation ages in parentheses): southern Granite-Rhyolite (1.5-1.3 Ga), + 7.0 ± 0.9; central Yavapai (1.8-1.7 Ga), + 5.4 ± 0.9; western Yavapai (1.8-1.7 Ga), + 3.3 ± 1.1; Granite-Rhyolite (1.5-1.3 Ga), + 1.4 ± 0.6; Mojave (1.8-1.7 Ga), + 0.2 ± 0.8; and Penokean (1.9-1.8 Ga), - 0.1 ± n/d. The narrow ranges of Hf isotopic signatures within these regional groupings of granites reflect the age and isotopic composition of the basement provinces they intrude. Granites in the southern Granite-Rhyolite and central Yavapai provinces have the highest initial ɛHf, reflecting their more juvenile sources, whereas Mojave and Penokean granites show contributions from more evolved crustal sources. Simple calculations indicate that all the granites represent dominantly crustal melts; although a mantle contribution cannot be ruled out, if present it must be

  15. 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

  16. Distribution of p-process 174Hf in early solar system materials and the origin of nucleosynthetic Hf and W isotope anomalies in Ca-Al rich inclusions

    NASA Astrophysics Data System (ADS)

    Peters, Stefan T. M.; Münker, Carsten; Pfeifer, Markus; Elfers, Bo-Magnus; Sprung, Peter

    2017-02-01

    Some nuclides that were produced in supernovae are heterogeneously distributed between different meteoritic materials. In some cases these heterogeneities have been interpreted as the result of interaction between ejecta from a nearby supernova and the nascent solar system. Particularly in the case of the oldest objects that formed in the solar system - Ca-Al rich inclusions (CAIs) - this view is confirm the hypothesis that a nearby supernova event facilitated or even triggered solar system formation. We present Hf isotope data for bulk meteorites, terrestrial materials and CAIs, for the first time including the low-abundance isotope 174Hf (∼0.16%). This rare isotope was likely produced during explosive O/Ne shell burning in massive stars (i.e., the classical "p-process"), and therefore its abundance potentially provides a sensitive tracer for putative heterogeneities within the solar system that were introduced by supernova ejecta. For CAIs and one LL chondrite, also complementary W isotope data are reported for the same sample cuts. Once corrected for small neutron capture effects, different chondrite groups, eucrites, a silicate inclusion of a IAB iron meteorite, and terrestrial materials display homogeneous Hf isotope compositions including 174Hf. Hafnium-174 was thus uniformly distributed in the inner solar system when planetesimals formed at the <50 ppm level. This finding is in good agreement with the evidently homogeneous distributions of p-process isotopes 180W, 184Os and possibly 190Pt between different iron meteorite groups. In contrast to bulk meteorite samples, CAIs show variable depletions in p-process 174Hf with respect to the inner solar system composition, and also variable r-process (or s-process) Hf and W contributions. Based on combined Hf and W isotope compositions, we show that CAIs sampled at least one component in which the proportion of r- and s-process derived Hf and W deviates from that of supernova ejecta. The Hf and W isotope

  17. Lu-Hf and Sm-Nd isotope systematics of Korean spinel peridotites: A case for metasomatically induced Nd-Hf decoupling

    NASA Astrophysics Data System (ADS)

    Choi, Sung Hi; Mukasa, Samuel B.

    2012-12-01

    We determined the Lu-Hf and Sm-Nd isotope compositions of spinel peridotite xenoliths in alkali basalts from Baengnyeong and Jeju islands, South Korea, to constrain the timing of melt-depletion events and stabilization of the lithospheric mantle beneath the region. Equilibration temperatures estimated by two-pyroxene thermometry range from 780 to 950 °C, and from 960 to 1010 °C for Baengnyeong and Jeju peridotites, respectively. The Baengnyeong peridotite clinopyroxenes are characterized by extremely radiogenic Hf in association with isotopically less extreme Nd, resulting in strong Nd-Hf decoupling referenced to the mantle array. This is in stark contrast to the well-correlated isotopic compositions of Hf and Nd in the Jeju peridotite clinopyroxenes, which plot along the Nd-Hf mantle array. The Hf abundances and isotopic compositions of the Baengnyeong clinopyroxenes were less affected by relatively recent secondary enrichments that overprinted the light rare earth element abundances and Nd isotopes, causing the decoupling of the Nd-Hf isotopes. The Nd-Hf isotopic compositions of the Jeju peridotites appear to have been re-equilibrated, probably as a result of efficient diffusion at the relatively higher temperatures of the Jeju peridotites. Lu-Hf tie lines for clinopyroxene and orthopyroxene from four of the Korean peridotites have negative slopes on a Lu-Hf isochron diagram, yielding negative ages. This is interpreted to indicate recent isotopic exchange in orthopyroxene by reaction with metasomatic agents having low 176Hf/177Hf components. Secondary overprinting in orthopyroxene was facilitated by the considerably lower Hf concentrations than in co-located clinopyroxene. Baengnyeong lherzolite clinopyroxenes yield a Lu-Hf errorchron age of 1.9 ± 0.1 Ga, which is independently supported by a model Os age (based on Re depletion or TRD) of 1.8 Ga on a refractory Baengnyeong peridotite. We interpret this age range to mark the time of stabilization of the mantle

  18. Lu-Hf and Sm-Nd isotope systematics of Korean spinel peridotites: A case for metasomatically induced Nd-Hf decoupling

    NASA Astrophysics Data System (ADS)

    Choi, S.; Mukasa, S. B.

    2012-12-01

    We determined the Lu-Hf and Sm-Nd isotope compositions of spinel peridotite xenoliths in alkali basalts from Baengnyeong and Jeju islands, South Korea, to constrain the timing of melt-depletion events and stabilization of the lithospheric mantle beneath the region. Equilibration temperatures estimated by two-pyroxene thermometry range from 780 to 950°C, and from 960 to 1010°C for Baengnyeong and Jeju peridotites, respectively. The Baengnyeong peridotite clinopyroxenes are characterized by extremely radiogenic Hf in association with isotopically less extreme Nd, resulting in strong Nd-Hf decoupling referenced to the mantle array. This is in stark contrast to the well-correlated isotopic compositions of Hf and Nd in the Jeju peridotite clinopyroxenes, which plot along the Nd-Hf mantle array. The Hf abundances and isotopic compositions of the Baengnyeong clinopyroxenes were less affected by relatively recent secondary enrichments that overprinted the light rare earth element abundances and Nd isotopes, causing the decoupling of the Nd-Hf isotopes. The Nd-Hf isotopic compositions of the Jeju peridotites appear to have been re-equilibrated, probably as a result of efficient diffusion at the relatively higher temperatures of the Jeju peridotites. Lu-Hf tie lines for clinopyroxene and orthopyroxene from four of the Korean peridotites have negative slopes on a Lu-Hf isochron diagram, yielding negative ages. This is interpreted to indicate recent isotopic exchange in orthopyroxene by reaction with metasomatic agents having low 176Hf/177Hf components. Secondary overprinting in orthopyroxene was facilitated by the considerably lower Hf concentrations than in co-located clinopyroxene. Baengnyeong lherzolite clinopyroxenes yield a Lu-Hf errorchron age of 1.9 ± 0.1 Ga, which is independently supported by a model Os age (based on Re depletion or TRD) of 1.8 Ga on a refractory Baengnyeong peridotite. We interpret this age range to mark the time of stabilization of the mantle

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

    NASA Astrophysics Data System (ADS)

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

    2016-10-01

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

  20. 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

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  2. Extreme Hf and light Fe isotopes in Archean komatiites - a remnant of very early mantle depletion?

    NASA Astrophysics Data System (ADS)

    Nebel, O.; Sossi, P.; Campbell, I. H.; Van Kranendonk, M. J.

    2014-12-01

    Hafnium isotope signatures in some Archean komatiites (ca. 3.5-3.0 billion years old) require a mantle source with a time-integrated Lu/Hf that exceeds average modern depleted mantle. Investigation of the timing and locus of parent-daughter fractionation in their mantle sources potentially constrains differentiation processes in the early Earth and their subsequent distribution and storage. In addition, they may help to constrain the Hf isotope evolution of the greater depleted mantle. In order to shed light on these processes, we discuss radiogenic Hf isotopes in conjunction with stable Fe isotope systematics in Archean komatiites from the Pilbara craton in Western Australia. Our findings indicate that, after careful evaluation of the effects of alteration, pristine samples are characterised by initial 176Hf/177Hf, which lie above the age-corrected depleted mantle, as a consequence of ancient melt extraction. Iron isotope systematics for these samples further point to a mantle source that is isotopically lighter than average modern depleted mantle, which is also consistent with melt-depletion. Taken together, these observations require a component of an old, super-depleted reservoir in the komatiite mantle source(s) that survived in the mantle for possibly hundreds of millions of years. The Lu/Hf of this refractory mantle appears to be complementary to, and therefore contemporaneous with, the first terrestrial crust, as preserved in Hadean (i.e., > 4 Ga) detrital zircon cores, which may indicate a causal relationship between them. We will discuss implications for very early mantle dynamics and the formation of very early mantle reservoirs.

  3. Major Change in Hf Isotopic Composition of Deep-Ocean Sediments at About 120 Ma

    NASA Astrophysics Data System (ADS)

    Marini, J.; Chauvel, C.

    2001-12-01

    We report Hf isotopic compositions of sediments collected in the West Pacific in front of the Izu-Bonin arc during ODP Leg 185, Hole 1149. The drilled sedimentary pile is 408 m thick, covers a time range from Valanginien (about 133 Ma) to present-day and consists in a succession of biogenic cherts and siliceous and pelagic clays. All samples with ages up to 117Ma have positive and almost constant Epsilon Hf at about +6. At present only one chert sample from the lower part of the hole was analyzed. It has an age of 123Ma and a much lower Epsilon Hf at ?4. This shift in Hf isotopic composition is also seen in data reported by Pearce et al. (1999) for sediments from Site 801B located some 1800 km Southeast on the Pacific plate. Hf isotopic studies of Fe-Mn and Mn crusts show that the present-day Pacific seawater has rather constant Epsilon Hf of +5 (Albarede et al., 1997, David et al., 2001). This is interpreted to represent the composition of ambient deep waters and to be little influenced by material of continental origin. Since all samples from Hole 1149 with ages up to 117Ma have Hf isotopes similar to the value measured in present-day crusts, it appears that the source of Hf in the Pacific deep ocean remained similar over this time interval. However, a major shift occurred at 120 Ma; older sediments have Hf isotopes with a much stronger continental signature. We suggest that a major change in the budget of Hf in seawater occurred at that time. The composition of seawater changed from continental dominated to mantle dominated. Processes that might explain such a drastic shift include: (1) the onset of massive volcanism on the Ontong-Java plateau and other Pacific plateaus at 120 Ma coupled with the enhanced activity of mid-ocean ridges wordwide. These processes are considered to be responsible for the major transgressions reported in the Cretaceous; (2) alternatively, drastic changes in ocean-water circulation could have the same effect, but this interpretation

  4. Lu-Hf isotope systematics of the Hadean-Eoarchean Acasta Gneiss Complex (Northwest Territories, Canada)

    NASA Astrophysics Data System (ADS)

    Guitreau, Martin; Blichert-Toft, Janne; Mojzsis, Stephen J.; Roth, Antoine S. G.; Bourdon, Bernard; Cates, Nicole L.; Bleeker, Wouter

    2014-06-01

    The Acasta Gneiss Complex (AGC) is a remnant Hadean-Eoarchean terrane composed of strongly deformed polyphase mafic to felsic gneisses which preserve a multi-stage history of magmatic emplacement, inheritance, and subsequent tectono-thermal modifications. The complexities encountered in such an old terrane fragment have been documented in previous geochronological studies of the AGC (e.g. zircon U-Pb, 147Sm-143Nd), and are evident also in its Lu-Hf isotope systematics. Here, we report new Lu-Hf isotope whole-rock measurements which show that some AGC gneisses were severely disturbed by migmatization and associated mineral segregation, while others preserve their Lu-Hf isotope systematics relatively intact with mostly near- to sub-chondritic initial 176Hf/177Hf ratios. Results reveal identifiable Eoarchean and later (Paleoarchean) magmatic events at around 3960 Ma and again at 3600 Ma, with a major metamorphism of the complex at 3750 Ma. The oldest and least disturbed gneisses have a Lu-Hf regression age of 3946 ± 87 Ma, in good agreement with U-Pb zircon geochronology. A role of yet older crust (4000-4200 Ma) in the formation of the AGC is also evident, but seems not to have influenced to first order the Lu-Hf isotope systematics of the 3960 Ma group. The ca. 3960 Ma group is proposed to be representative of its mantle source based on the absence of correlation between εHf(t) and Ce/Pb. It is further suggested that these two parameters show that the ca. 3600 Ma gneisses were sourced in part from a mafic lithology belonging to the 3960 Ma group, and that multiple sources (mantle and crust) were involved in AGC formation. The identification of preserved Lu-Hf isotope systematics in AGC gneisses means that complementary geochemical and isotopic studies bearing on the petrogenesis of pre-3900 Ma rocks are possible. Despite its history of strong deformation and alteration, carefully selected domains within the AGC carry surviving information about the evolution of the

  5. 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.

  6. The Lu-Hf isotope system in the Acasta gneiss complex (NWT, Canada)

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

    The Acasta gneiss complex (AGC) is an outcrop of poly-phase Hadean/Eoarchean rocks that records a multi-stage history. We explore the complexities encountered in such an old crustal remnant at the mineral- to outcrop scale and discuss the preservation and disturbance of the Lu-Hf isotope system in these rocks. Twenty-one new Lu-Hf isotope whole rock measurements are used in combination with previous data to show that some samples have been disturbed by metamorphic garnet growth and/or migmatization/mineral segregation while others clearly preserve their Lu-Hf isotope systematics. Two main Archean magmatic events to be identified at around 3600 and 3960 Ma and an almost purely metamorphic event at about 3750 Ma. Least-contaminated gneisses indicate a Lu-Hf isochron age range of 3929 ± 84 to 3945 ± 91 Ma for the oldest rocks that range from mafic to felsic. This age discrepancy relative to 3960 Ma is likely due to crust assimilation. This process is significant in both magmatic groups in the AGC as also observed by previous authors. However, our study demonstrates that crustal contamination is widespread but not ubiquitous and, according to assimilation calculations, the least contaminated samples identified here indicate a near-CHUR Hf isotope composition for the mantle source of 3960 Ma group. Comparison between Hf isotopes in >3.9 Ga detrital zircons from North-American Archean meta-sedimentary basins and those in Acasta gneisses indicate a possible connection that would in turn imply that the AGC was volumetrically significant. Analogy with western Australian Hadean zircon bearing meta-sedimentary belts would point out to local effects in early crustal record in favor of progressive (but not continuous) crustal growth as independent crustal segments in the Hadean-Eoarchean.

  7. 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

  8. Sm-Nd and Lu-Hf isotope composition of chondritic components

    NASA Astrophysics Data System (ADS)

    Bouvier, A.; Vervoort, J. D.; Patchett, P. J.; Gopel, C.

    2009-12-01

    The 146Sm-142Nd, 147Sm-143Nd and 176Lu-176Hf radiogenic isotopic systems are widely used as chronometers and tracers of planetary evolution. These involve refractory lithophile elements and thus it is assumed that the average Sm-Nd and Lu-Hf composition of bulk terrestrial planets should be the same as that of chondrites (CHUR). We previously revised the CHUR compositions with 0.1960 ±0.0004 for 147Sm/144Nd and with 0.0336 ±0.0001 for 176Lu/177Hf using unequilibrated ordinary (OC) and carbonaceous (CC) chondrites [1], and proposed these should apply to the bulk silicate Earth (BSE). Recent studies suggest that BSE may have a super-chondritic Sm/Nd (~5%) and Lu/Hf (~10%) composition and could explain the Nd and Hf isotopic systematics of Earth and planetary materials [2, 3]. Here, we present additional Sm-Nd and Lu-Hf compositions of chondrites and chondritic components to evaluate potential isotopic heterogeneities present in the protoplanetary disk. Isotopic analyses were carried out by Neptune MC-ICPMS at ASU. Analytical details are in [1, 4]. We extend our study to homogenized whole-rock (WR) powders of 4 equilibrated OC to investigate the scale of Lu-Hf isotopic heterogeneities as consequences of thermal metamorphism on the OC parent bodies (PB) [1]. Their 147Sm/144Nd and 176Lu/177Hf vary from 0.1954 to 0.1969, and 0.0298 to 0.0341 respectively indicating that open metasomatism associated with crystallization of phosphate [1] occurred at least at the cm scale on the OCPB. We also present the first Lu-Hf and coupled Sm-Nd isotopic data of 6 single or pooled chondrules, and 2 calcium aluminum-rich inclusions (CAIs) from 5 type 3 OC & CC. The 147Sm/144Nd and 176Lu/177Hf ranges are 0.1956-0.1969, and 0.0331-0.0341 respectively for chondrules, and 0.1947-0.2147, and 0.0392-0.0501 respectively for CAIs. The chondrules are within the range of our earlier Sm/Nd and Lu/Hf CHUR-BSE estimates but the CAIs have significantly higher values, especially for Lu/Hf. Thus

  9. Probing Archean lithosphere using the Lu-Hf isotope systematics of peridotite xenoliths from Somerset Island kimberlites, Canada

    NASA Astrophysics Data System (ADS)

    Schmidberger, Stefanie S.; Simonetti, Antonio; Francis, Don; Gariépy, Clément

    2002-04-01

    A knowledge of the Hf isotopic composition of the subcontinental lithosphere beneath Archean cratons is essential to constrain the Hf isotope budget of the Earth's mantle. Hf isotopic measurements were obtained by MC-ICP-MS for a suite of refractory peridotite xenoliths and constituent garnets from the Nikos kimberlite (100 Ma) on Somerset Island in order to constrain the isotopic composition and age of the lithosphere beneath the northern Canadian craton. The low-temperature Nikos peridotites (<1100°C), which represent the shallow Somerset lithosphere, are characterized by higher 176Lu/ 177Hf ratios (0.03-0.05) and Hf isotopic values ( 176Hf/ 177Hf (0.1Ga)=0.28296-0.28419) than the deep-seated high-temperature peridotites (>1100°C; 0.004-0.03, 0.28265-0.28333, respectively). These differences in Hf isotope signatures suggest that shallow and deep subcontinental lithosphere beneath Somerset Island represent isotopically distinct domains and do not share a common petrogenetic history. The Lu-Hf isotope systematics of the shallow low-temperature peridotites define a positively sloped line that plot along a 2.8 Ga reference isochron. A number of these peridotites are characterized by highly radiogenic Hf isotopic compositions suggestive of long-term radiogenic ingrowth (billions of years). These findings are consistent with an interpretation that the shallow Somerset lithosphere (to depths of ˜150 km) stabilized in the Archean. The majority of the high-temperature peridotites plot closer to the composition of the host kimberlite. Although the observed isotopic variation may be attributed in part to kimberlite-related Hf addition, it is possible that these deep-seated xenoliths represent younger mantle. The superchondritic 176Lu/ 177Hf ratios observed for a number of the shallow low-temperature peridotites indicate strong fractionation of Lu and Hf, suggesting mantle root formation in the garnet stability field (depths >80 km). The Hf isotope compositions for the

  10. 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.

  11. Small Volume Isotopic Analysis of Zircon Using LA-MC-ICP-MS U-Pb and Lu-Hf and Sub-ng Amounts of Hf in Solution

    NASA Astrophysics Data System (ADS)

    Bauer, A.; Horstwood, M. S.

    2016-12-01

    Crust-mantle evolution studies are greatly informed by zircon U-Pb and Lu-Hf isotopic datasets and the ease with which these data can now be acquired has seen their application become commonplace. In order to deconvolute geochemical change and interpret geologic variation in complexly zoned zircons, this information is most ideally obtained on the smallest volume of zircon by successive SIMS U-Pb and LA-MC-ICP-MS Lu-Hf isotopic analyses. However, due to variations in zircon growth zone geometry at depth, the Lu-Hf analysis may not relate to the lower volume U-Pb analysis, potentially causing inaccuracy of the resultant age-corrected Hf isotope signature. Laser ablation split-stream methods are applied to be certain that U-Pb and Lu-Hf data represent the same volume of zircon, however, the sampling volume remains relatively large at 40x30µm1. Coupled ID-TIMS U-Pb and solution MC-ICP-MS Lu-Hf work traditionally utilize whole-zircon dissolution ( 10-50ng Hf), which has the potential to homogenize different zones of geologic significance within an analysis. Conversely, modern ID-TIMS U-Pb methods utilize microsampling of zircon grains, often providing < 5ng Hf, thereby challenging conventional Lu-Hf acquisition protocols to achieve the required precision. In order to obtain usable precision on minimal zircon volumes, we developed laser ablation methods using successive 25um spot U-Pb and Lu-Hf ablation pits with a combined depth of 18um, and low-volume solution introduction methods without Hf-REE separation utilizing Hf amounts as low as 0.4ng, while retaining an uncertainty level of ca. 1 ɛHf for both methods. We investigated methods of Yb interference correction and the potential for matrix effects, with a particular focus on the accurate quantification of 176Lu/177Hf. These improvements reduce the minimum amount of material required for U-Pb and Hf isotopic analysis of zircon by about an order of magnitude. 1Ibanez-Mejia et al (2015). PreRes, 267, 285-310.

  12. 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

  13. Combined Nd and Hf isotope evidence for deep-seated source of Isua lavas

    NASA Astrophysics Data System (ADS)

    Rizo, Hanika; Boyet, Maud; Blichert-Toft, Janne; Rosing, Minik

    2011-12-01

    We present the first combined 146,147Sm- 142,143Nd and 176Lu- 176Hf study of mafic rocks (amphibolites) from the western part of the Isua Supracrustal Belt (ISB, SW Greenland). Sm-Nd and Lu-Hf whole-rock isochrons yield identical ages within error: 3.72 ± 0.08 and 3.67 ± 0.07 Ga, respectively. 142Nd measurements confirm and extend the database of 142Nd excesses in Isua samples (μ 142Nd = 7-16 ppm relative to the terrestrial Nd standard), indicating that early-differentiated reservoirs escaped complete homogenization by mantle convection until the Archean. Combined 146,147Sm- 142,143Nd systematics suggest differentiation of the Isua amphibolite reservoir at maximum ages of ~ 4.53, ~ 4.47, and ~ 4.32 Ga. These ages are relative to, respectively, ordinary-chondritic, enstatite-chondritic, and higher-than-chondritic mantle compositions. The intercept of the Sm-Nd whole-rock isochron is in accordance with 142Nd results and consistent with a superchondritic initial 143Nd/ 144Nd ratio (ɛ 143Nd 3.7Ga = + 1.41 ± 0.98). In contrast, the corresponding initial ɛ 176Hf 3.7Ga = - 1.41 ± 0.57 is subchondritic. Since Lu/Hf and Sm/Nd fractionate similarly during mantle processes, the Sm-Nd and Lu-Hf isotope systems display incongruent relative parent-daughter behavior in the source of Isua amphibolites. Based on high-pressure and -temperature phase partition coefficients, we propose a model that satisfies 147Sm- 143Nd, 176Lu- 176Hf, and 142Nd results, as well as trace element characteristics. A deep-seated source composed largely of magnesium perovskite (98% MgPv) and containing 2% calcium perovskite satisfactorily explains the Nd and Hf isotopic discordance observed for Isua amphibolites. The negative HFSE anomalies characterizing Isua basalts likewise could have been inherited from such an early (4.53-4.32 Ga) deep mantle cumulate. Since the ~ 4 Ga old Acasta Gneisses (Northwest Territories, Canada) have radiogenic 176Hf/ 177Hf (Scherer et al., 2010), dissociation of the

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

    NASA Astrophysics Data System (ADS)

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

    2005-12-01

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

  15. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2004-01-01

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

  17. 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.

  18. 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

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  20. 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.

  1. U-Pb ages and Hf isotope compositions of zircons in plutonic rocks from the central Famatinian arc, Argentina

    NASA Astrophysics Data System (ADS)

    Otamendi, Juan E.; Ducea, Mihai N.; Cristofolini, Eber A.; Tibaldi, Alina M.; Camilletti, Giuliano C.; Bergantz, George W.

    2017-07-01

    The Famatinian arc formed around the South Iapetus rim during the Ordovician, when oceanic lithosphere subducted beneath the West Gondwana margin. We present combined in situ U-Th-Pb and Lu-Hf isotope analyses for zircon to gain insights into the origin and evolution of Famatinian magmatism. Zircon crystals sampled from four intermediate and silicic plutonic rocks confirm previous observations showing that voluminous magmatism took place during a relatively short pulse between the Early and Middle Ordovician (472-465 Ma). The entire zircon population for the four plutonic rocks yields coherent εHf negative values and spreads over several ranges of initial εHf(t) units (-0.3 to -8.0). The range of εHf units in detrital zircons of Famatinian metasedimentary rocks reflects a prolonged history of the cratonic sources during the Proterozoic to the earliest Phanerozoic. Typical tonalites and granodiorites that contain zircons with evolved Hf isotopic compositions formed upon incorporating (meta)sedimentary materials into calc-alkaline metaluminous magmas. The evolved Hf isotope ratios of zircons in the subduction related plutonic rocks strongly reflect the Hf isotopic character of the metasedimentary contaminant, even though the linked differentiation and growth of the Famatinian arc crust was driven by ascending and evolving mantle magmas. Geochronology and Hf isotope systematics in plutonic zircons allow us understanding the petrogenesis of igneous series and the provenance of magma sources. However, these data could be inadequate for computing model ages and supporting models of crustal evolution.

  2. Hf isotope evidence for effective impact melt homogenisation at the Sudbury impact crater, Ontario, Canada

    NASA Astrophysics Data System (ADS)

    Kenny, Gavin G.; Petrus, Joseph A.; Whitehouse, Martin J.; Daly, J. Stephen; Kamber, Balz S.

    2017-10-01

    We report on the first zircon hafnium-oxygen isotope and trace element study of a transect through one of the largest terrestrial impact melt sheets. The differentiated melt sheet at the 1.85 Ga, originally ca. 200 km in diameter Sudbury impact crater, Ontario, Canada, yields a tight range of uniform zircon Hf isotope compositions (εHf(1850) of ca. -9 to -12). This is consistent with its well-established crustal origin and indicates differentiation from a single melt that was initially efficiently homogenised. We propose that the heterogeneity in other isotopic systems, such as Pb, in early-emplaced impact melt at Sudbury is associated with volatility-related depletion during the impact cratering process. This depletion leaves the isotopic systems of more volatile elements more susceptible to contamination during post-impact assimilation of country rock, whereas the systems of more refractory elements preserve initial homogeneities. Zircon oxygen isotope compositions in the melt sheet are also restricted in range relative to those in the impacted target rocks. However, they display a marked offset approximately one-third up the melt sheet stratigraphy that is interpreted to be a result of post-impact assimilation of 18O-enirched rocks into the base of the cooling impact melt. Given that impact cratering was a more dominant process in the early history of the inner Solar System than it is today, and the possibility that impact melt sheets were sources of ex situ Hadean zircon grains, these findings may have significance for the interpretation of the early zircon Hf record. We speculate that apparent εHf-time arrays observed in the oldest terrestrial and lunar zircon datasets may be related to impact melting homogenising previously more diverse crust. We also show that spatially restricted partial melting of rocks buried beneath the superheated impact melt at Sudbury provided a zircon crystallising environment distinct to the impact melt sheet itself.

  3. Early Differentiation of the Crust-Mantle System: a Hf Isotope Perspective

    NASA Astrophysics Data System (ADS)

    Scherer, E.; Munker, C.; Mezger, K.

    2001-12-01

    The Lu decay constant recently determined by Scherer et al. 2001 (i.e., 1.865 x 10-11 yr-1) agrees with the results of the two latest physical counting experiments (1.86 x 10-11 yr-1; Dalmasso et al 1992, Nir-El and Lavi 1998), but is ca. 4 percent lower than the decay constants that have been used throughout the Hf isotope literature (e.g., 1.94 x 10-11, Tatsumoto et al., 1981; 1.93 x 10-11 Sguigna et al, 1982). In addition to making Lu-Hf ages older by ca. 4 percent, the revised decay constant also shifts the calculated initial epsilon Hf values of early Archean and Hadean rocks and zircons that are used to constrain crust-mantle differentiation in the early Earth. The initial epsilon Hf values for low-Lu/Hf samples such as zircons and evolved felsic rocks shift downward by 2-4 epsilon units, primarily due to the shift in the position of the CHUR evolution curve rather than that of the samples themselves. Mafic rocks, such as komatiites have higher Lu/Hf ratios that are closer to that of CHUR and therefore their initial epsilon Hf values do not shift as much (up to 1.3 epsilon units lower or 0.4 epsilon units higher). Using the old decay constant, some early Archean rocks (e.g., Amitsoq gneisses; Vervoort et al., 1996, Vervoort and Blichert-Toft, 1999) seemed to have very high initial epsilon Hf values (up to +6), implying that the upper mantle was moderately depleted in the early Archean and that a substantial volume of crust was produced in the Hadean. However, when recalculated with the new decay constant, the data suggest that the mantle was only slightly depleted, requiring less early crust extraction, and allowing a later date for the onset of significant crust production. In contrast, the extremely low recalculated epsilon Hf values of Earth's oldest zircons (Amelin et al., 1999, Amelin et al., 2000) indicate that Earth's first crust formed at or before 4.3 Ga, and that this crust remained intact long enough (>200 million years) to evolve to such low

  4. 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

  5. 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.

  6. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  8. 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.

  9. The first Lu-Hf zircon isotope data for gabbro-diorite-tonalite associations of the Urals

    NASA Astrophysics Data System (ADS)

    Ronkin, Yu. L.; Smirnov, V. N.; Ivanov, K. S.; Gerdes, A.

    2017-01-01

    The Lu-Hf isotope systematics of zircon from the gabbro-plagiogranite association (gabbro, diorite, tonalite, and plagiogranite), which is one of the most typical associations of igneous rocks in the Urals, was studied for the first time. The isotope study yielded a unified age limit of 433 Ma, which corresponds to the time of formation of this rock association. The younger "rejuvenated" ages characterize superimposed thermal impact events, induced by the volcanic arc activity, as well as collisional and postcollisional processes. Here, the initial 176Hf/177Hf( t) ratio in the studied zircon from gabbro and plagiogranite corresponds in fact to a highly LILE-depleted (DM) mantle.

  10. Hf Isotope Systematics of Archean Anorthosites: Manfred Complex, Yilgarn Craton, Western Australia

    NASA Astrophysics Data System (ADS)

    Souders, K.; Sylvester, P.

    2016-12-01

    Archean anorthosite complexes represent a minor, yet distinct rock type found within many Archean terranes. These mantle-derived melts are commonly found in layers withassociated leucogabbro, gabbro, and ultramafic units of similar origin. Most Archean anorthosites are intensely deformed and metamorphosed yet preserved igneous minerals have been identified within several complexes. It has become obvious that Archean anorthosites contain zircon crystals, which can be used to establish robust crystallization ages for anorthosite complexes. These minerals are also ideal targets for in situ Lu-Hf isotopic analysis to further characterize the source of Archean anorthosites and provide insight into the formation and evolution of the continental crust during the Archean. The ca. 3.7 Ga Manfred Complex is exposed northeast of Mount Narryer within the Narryer Gneiss Terrane, Yilgarn Craton, Western Australia. The layered anorthosite gabbro-ultramafic intrusion outcrops in pods and lenses, engulfed by granitic gneisses [1, 2, 3]. We have sampled anorthosites, leucogabbros and gabbros from the Manfred Complex and determined their age by LA-ICPMS U-Pb zircon geochronology. Zircons separated from these rocks give ages of 3.63 Ga to 3.73 Ga. LA-MC-ICPMS Lu-Hf isotope analyses were performed by focusing the laser spot directly on top of the U-Pb analysis location for each zircon grain. Initial Hf isotope compositions of zircon grains from the Manfred complex range from ca. ɛHf +2 to -3. This range suggests contributions from both depleted mantle and more ancient crustal sources to the parent magma of the Manfred Complex. [1] Kinny et al. (1988) Prec. Res. 38, 325-341. [2] Myers (1988) Prec. Res. 38, 309-323. [3] Williams & Myers (1987) WA Geol. Surv. Rpt. 22, 32 pp.

  11. 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

  12. Hf-Nd isotopic variability in mineral dust from Chinese and Mongolian deserts: implications for sources and dispersal

    PubMed Central

    Zhao, Wancang; Sun, Youbin; Balsam, William; Lu, Huayu; Liu, Lianwen; Chen, Jun; Ji, Junfeng

    2014-01-01

    Mineral dust provenances are closely related to the orogenic processes which may have distinct Hf-Nd isotopic signatures. Here we report the clay-sized (<2 μm) Hf-Nd isotope data from Asian dust sources to better constrain the source and transport dynamics of dust deposition in the North Pacific. Our results show that there is a more positive radiogenic Hf isotopic composition with clay-sized fractions than the corresponding bulk sample and a decoupling of the Hf-Nd couplets in the clay formation during the weathering process. The clay-sized Hf-Nd isotopic compositions of the desert samples from the Sino-Korean-Tarim Craton (SKTC) are different from those of the Gobi and deserts from the Central Asian Orogeny Belt (CAOB) due to varying tectonic and weathering controls. The Hf-Nd isotopic compositions of dust in the North Pacific central province (NPC) match closely with those from the Taklimakan, Badain Jaran and adjacent Tengger deserts, implying that the NPC dust was mainly transported from these potential sources by the westerly jet. Our study indicates that dusts from the CAOB Gobi deserts either didn't arrive in NPC or were quantitatively insignificant, but they were likely transported to the North Pacific margin province (NPM) by East Asian winter monsoon. PMID:25060781

  13. 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.

  14. 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.

  15. Rapid sample digestion by fusion and chemical separation of Hf for isotopic analysis by MC-ICPMS.

    PubMed

    Ulfbeck, David; Baker, Joel; Waight, Tod; Krogstad, Eirik

    2003-02-06

    A new method for rapid sample digestion and efficient chemical separation of Hf and REE from rock samples for precise isotopic analysis is presented. Samples are digested by fusion in the presence of a lithium borate flux at 1100 degrees C and dissolved whilst molten in dilute nitric or hydrochloric acid. Prior to chemical separation using ion exchange techniques, Li and B from the flux material and Si from the sample are separated from the remaining major elements, REE and high field strength elements (HFSE) in the sample by Fe-hydroxide co-precipitation. The chemical separation of Hf is a two-stage procedure designed to first remove the remaining matrix elements (e.g. Fe, Ba) in the sample using standard cation exchange techniques, followed by separation of Hf from the REE and HFSE on TEVA extraction chromatographic resin. Hf yields are >90% and total procedural blanks are ca. 50 pg. Hf isotope ratios of a synthetic standard solution and replicate digestions of international rock standards BHVO-1 and BCR-1 measured on multi-collector inductively coupled plasma mass spectrometer (MC-ICPMS) reproduce similarly to Hf during analysis: (176)Yb/(176)Hf<0.0001; (176)Lu/(176)Hf<0.00001; Ti/Hf<0.05. This technique also provides a means of separating Nd from the REE fraction for isotopic analysis and, potentially, may be adapted for measurement of Lu/Hf ratios by isotope dilution techniques.

  16. The Importance of Complete Sample Dissolution and Spike-Sample Equilibration on Lu-Hf Isotope Studies

    NASA Astrophysics Data System (ADS)

    Mahlen, N. J.; Beard, B. L.; Johnson, C. M.; Lapen, T. J.

    2005-12-01

    Lu-Hf geochronology has gained attention due to its potential for precisely determining the age of garnet growth in a wide variety of rocks. A unique aspect of Lu-Hf analysis, however, is the disparate chemical behavior of Hf and Lu. For example, Hf is soluble in HF and Lu is soluble in HCl, which can create problems for spike-sample equilibration during dissolution as discussed by Unruh et al. 1984 JGR 89:B459 and later by Beard et al. 1998 GCA 62:525. Although partial dissolution may appear as an attractive means to preferentially dissolve garnet relative to refractory inclusions such as rutile and zircon, our investigations have shown that incomplete spike-sample equilibration may occur in such approaches. This leads to erroneous Lu and Hf contents that can adversely affect Lu-Hf isochron ages and calculated initial Hf isotope compositions. Dissolution of whole-rock powders using hot plates (low-pressure) or short-term microwave dissolution may produce inaccurate Lu-Hf isotope and concentration results, whereas high-temperature and -pressure dissolution in traditional Teflon steel-jacketed (Parr) bombs produces precise and accurate results. The greatest disparity in Lu-Hf isotope and concentration systematics of whole-rock powders among dissolution methods occurs for zircon- and garnet-bearing samples. In contrast, Sm-Nd isotope results are not affected by these different dissolution methods. Lu-Hf isochrons involving garnet may be affected by the dissolution method in a manner similar to that observed for whole-rock powders. Incomplete dissolution of garnet generally increases the measured Lu/Hf ratios, potentially increasing the precision of the isochron. In a number of lithologies, however, including garnet-bearing eclogites and amphibolites, significant errors may be introduced in the Lu-Hf age using hot plates (low-pressure) or short-term microwave dissolution, as compared to those obtained using high-temperature and -pressure dissolution bombs. These

  17. Properties of even 168-178Hf isotopes using IBM-1 and SEF

    NASA Astrophysics Data System (ADS)

    Abed Al-Jubbori, Mushtaq; Ajaj Al-Mtiuty, Khalaf; Saeed, Khaliel. I.; Sharrad, Fadhil I.

    2017-08-01

    The properties of the ground and excited-state (γ- and β- bands) of 168-178Hf nuclei have been studied. The ratio and Eγ (I → I - 2)/I have been calculated as a function of the spin (I) to determine the ground-state evolution. The results indicate that these isotopes have a rotational property SU(3). The energy levels for the ground-state, γ- and β- bands of 168-178Hf have been calculated using the Interacting Boson Model and Semi Empirical Formula (SEF). The parameters of the best fit to the measured data are determined. The behavior of energy and B(E2) ratios in the ground state band are examined.

  18. Chronology and Isotopic Constraints on Lunar Evolution

    NASA Astrophysics Data System (ADS)

    Snyder, G. A.; Borg, L. E.; Nyquist, L. E.; Taylor, L. A.

    Isotopic systematics of lunar rocks indicate three major, distinct, reservoirs in the Moon: (1) the urKREEP residuum of a global lunar magma ocean with high 238U/204Pb (µ) >500, high Rb/Sr and thus elevated 87Sr/86Sr, and low Sm/Nd and consequent negative ɛNd values; (2) a “primordial” deep mantle source with µ values more typical of Earth, low Rb/Sr and 87Sr/86Sr, high Sm/Nd, and extremely positive ɛNd values, and positive to variable ɛW values; and (3) a shallower mantle reservoir that has similar µ values to the second, intermediate Nd values, low to intermediate 87Sr/86Sr, and chondritic ɛW values. The vast majority of lunar samples can be modeled by mixing these three reservoirs. A possible fourth source, with µ values from 35 to 100, is represented by a few early crustal rocks, the ferroan anorthosites. Ferroan anorthosites, ostensibly the earliest lunar crustal rocks, exhibit a range of ages from 4.56 to 4.29 Ga and initial ɛNd values (0.9 to 3.1). These ages are inconsistent with derivation of all these rocks from a short-lived magma ocean, as suggested by 182W and 142Nd anomalies in lunar highland rocks and basalts. The positive Nd values of the ferroan anorthosites indicate time-integrated LREE-depletion, which is also inconsistent with direct derivation from a progressively LREE-enriched magma ocean. Instead, the derivation of ferroan anorthosites may involve convective overturn of a magma ocean and consequent mixing of LREE-enriched, plagioclase-rich, lower crust with underlying LREE-depleted, mafic cumulate sources. Later modification of this early anorthositic crust involved serial KREEP basalt magmatism, ponding in the crust, and crystallization of highland alkali suite and magnesian suite plutons from 4.4 to 3.9 Ga. The end of this major period of crustal evolution roughly coincides in time with a fall-off in large basin-forming impacts. Argon-40 argon-39 analyses of a variety of lunar samples at the different landing sites have allowed

  19. 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

  20. Subduction Contributions in the Trans-Mexican Volcanic Belt: Implications from Lava Chemistry and Hf-Nd-Pb Isotopes

    NASA Astrophysics Data System (ADS)

    Cai, Y.; Goldstein, S. L.; Langmuir, C. H.; Gómez-Tuena, A.; Lagatta, A.; Straub, S. M.; Martín Del Pozzo, A.

    2007-05-01

    Despite thick continental crust, near primitive lavas erupt throughout the Trans-Mexican Volcanic Belt (TMVB). In order to distinguish and better constrain subduction contributions and effects of crustal contamination, we analyzed samples representing subducting sediments from DSDP Site 487, and Quaternary lavas from stratovolcanoes and cinder cones, including alkaline "high-Nb" lavas from the Sierra Chichinautzin Volcanic Field (SCVF) showing negligible subduction signature in its trace element chemistry and representing melts of the mantle wedge. Our primary observations and implications are: (1) The high-Nb SCVF `intraplate' lavas define a linear trend along the "Nd-Hf mantle-crust array", defining the composition of the mantle wedge. (2) Popocatepetl and Nevado de Toluca stratovolcanoes show the highest Nd and Hf isotope ratios, higher than the `intraplate' lavas, indicating their sources are more "depleted mantle-like" than the regional mantle wedge. (3) The Popo and Toluca chemical and isotopic trends sharply contrast with Pico de Orizaba, which shows classic indications of crustal contamination (e.g. high 207Pb/204Pb, low Nd-Hf isotope ratios), consistent with contamination by local Precambrian crust. (4) Higher Nd-Hf isotopes in Popo and Toluca lavas also correlate with lower Pb isotope ratios, and lower Lu/Hf and Zr/Hf. Together, these data indicate contributions from subducted Pacific oceanic crust and hydrothermal sediment. (5) Popo and Toluca are also enriched in Th/LREE compared with `intraplate' lavas, reflecting subducted sediment contributions. (6) Nd-Hf isotope ratios of hydrothermal sediment from DSDP Site 487 lie on the "seawater array", with high Hf isotope ratios compared to the "mantle-crust array". Popo and Toluca Nd-Hf isotopes display a shallower slope than the "intraplate lava Nd-Hf array", reflecting contributions from hydrothermal sediment. Popocatepetl and Toluca lavas therefore avoid substantial crustal contamination of mantle wedge

  1. 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.

  2. Sr-Nd-Hf-Pb Isotope Ratios in Recent NE Lau Lavas

    NASA Astrophysics Data System (ADS)

    Todd, E.; Gill, J. B.; Freymuth, H.

    2009-12-01

    Sr-Nd-Hf-Pb isotope ratios in recently erupted NE Lau lavas are being measured and results will be presented. The recent W. Mata boninites lie slightly west of the northern termination of the Tongan volcanic front in an area where previously dredged boninites are enriched in LREE and HFSE. They are interpreted as containing an OIB-type mantle source component fluxed by slab-derived fluids (Falloon et al., 2007; Pearce et al., 2007). However, their Sr-Nd-Pb isotope ratios do not match any known component from the Samoan plume, pelagic sediment, or Louisville Ridge-derived volcaniclastic sediment. Their most distinctive isotopic characteristics are low 87Sr/86Sr and high 206Pb/204Pb relative to Nd-Hf isotope ratios. They are displaced toward HIMU-type basalts from the Cook-Austral islands (e.g., Tubuai) and have been interpreted as derived from diverse HIMU, EMI, and EMII mantle source components that are carried southward from beneath the Pacific Plate into the NE Lau Basin. Basalts from the NE Lau Spreading Center have more of a Indian-MORB source than do the boninites, and are similar to the <3 Ma OIB-type basalts from Fiji (Gill, 1984 and unpub.). If so, then some diverse enriched “plums” melt out of southward-advecting MORB-source “pudding” over as little as 100 km, whereas others persist for ~500 km to Fiji. Data for the recently erupted samples will test these interpretations, and will be compared to characteristics of the earliest (7.4 Ma) boninitic lavas in the region that have less of the OIB component. References: T. J. Falloon, L. Danyushevsky, A. J. Crawford, R. Maas, J. D. Woodhead, S. Eggins, S. Bloomer, D. J. Wright, S. K. Zlobin, and A. R. Stacey. Multiple mantle plume components involved in the petrogenesis of subduction-related lavas from the northern termination of the Tonga Arc and northern Lau Basin: Evidence from the geochemistry of arc and backarc submarine volcanics. Geochemistry, Geophysics, Geosystems, 8(Q09003):45, 2007. J. B. Gill

  3. Linking in-situ Hf isotopes in zircon with in-situ Pb isotopes in plagioclase: a microanalytical approach to characterize Archean anorthosite petrogenesis

    NASA Astrophysics Data System (ADS)

    Souders, K.; Sylvester, P.; Myers, J.

    2011-12-01

    Multiple isotope systems are often used to distinguish petrogenetic processes and determine the age and source of magmatic systems. Advances in laser ablation multi-collector ICPMS instrumentation have allowed Earth scientists to determine accurate and precise isotope ratios of minerals in-situ. Most studies have focused on measuring isotopes that are abundant within a mineral (e.g. Hf in zircon) but the integration of multiple ion counters into the collector configuration of MC-ICPMS instruments has provided the ability to measure isotope ratios of minor elements (e.g. Pb in plagioclase) in-situ. These abilities allow for an alternative approach to igneous petrogenesis. Instead of isotopic analysis of bulk samples, in-situ methods can be utilized to target specific domains preserved in individual minerals. Analysis of co-magmatic minerals in igneous rocks using multiple isotopic systems can be linked to solve a range of petrologic problems. As an example, we present in-situ analyses by LA-MC-ICPMS for Pb isotope compositions of preserved igneous plagioclase megacrysts and Hf isotope compositions of zircon grains from the 2936 Ma Fiskenæsset and 2914 Ma Nunataarsuk anorthosite complexes, southwestern Greenland, two of the best-preserved Archean anorthosites in the world. For both Fiskenæsset and Nunataarsuk, the initial Pb isotope compositions of plagioclase megacrysts and the initial ɛHf compositions of zircon grains extend beyond analytical uncertainty suggesting multiple sources contributed to the parent magma for both anorthosite complexes. Initial ɛHf of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ɛHf units. Plagioclase Pb isotope compositions from both anorthosite complexes share a depleted mantle end member yet diverge from this point: Fiskenæsset toward a high-μ, more radiogenic Pb crustal composition and Nunataarsuk toward a low-μ, less radiogenic Pb

  4. Decoupling of Hf-Nd isotope ratios in early Archean rocks from southern West Greenland - primary or secondary disturbance?

    NASA Astrophysics Data System (ADS)

    Hoffmann, J.; Munker, C.; Polat, A.; Rosing, M.

    2010-12-01

    In contrast to the strongly coupled behaviour of the Lu-Hf and Sm-Nd systems in Phanerozoic and Proterozoic rocks, early Archean rocks, e.g. those from SW-Greenland, exhibit an apparent decoupling of both systems [e.g., 1]. This apparent decoupling may either indicate metamorphic disturbance or, alternatively, mirror early differentiation processes such as magma ocean crystallisation involving cumulate segregation [1]. To evaluate the issue, we performed combined measurements of Hf-Nd isotope compositions together with major and trace element measurements for well preserved >3.72 to >3.80 Ga old tholeiitic metabasalts and gabbros from the inner western part and the southwestern unit of the Isua Supracrustal Belt, SW-Greenland. A careful evaluation of alteration based on major and trace elements reveals pristine magmatic trends, and therefore near chondritic to moderately depleted initial ɛHf values of -0.2-+2.7 and initial ɛNd values of 0.0-+3.0 indeed most likely characterize the early Archean mantle sources. These values confirm a decoupling of the Hf-Nd systematics, but are in marked contrast to more depleted signatures in nearby boninite-like metabasalts of the Garbenschiefer unit [2]. In Sm-Nd and Lu-Hf isochron space, the Isua metabasalts yield early Archean ages, in agreement with emplacement ages inferred from crosscutting relationships with tonalites. Trace element compositions of the metabasalts indicate a source overprint by subduction components. It is likely, that the source overprint may have caused partial decoupling of ɛHf-ɛNd values, due to selective addition of Nd as in modern subduction settings. In this case, the most radiogenic initial ɛNd and ɛHf isotope values characterize the most depleted mantle sources, and less radiogenic values might reflect addition of more enriched subduction components. A comparison of our data with Hf-Nd isotope data from the literature for early Archean rocks from Greenland reveals an overlap in initial Hf

  5. 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

  6. Recoupling the Nd-Hf isotope record of the early Earth? Evidence from the Pilbara craton, Western Australia

    NASA Astrophysics Data System (ADS)

    Kemp, T.; Vervoort, J. D.; Smithies, H.; Hickman, A.; Van Kranendonk, M. J.

    2011-12-01

    Geochemical evidence has long been used to argue for mass exchange between the earliest terrestrial crust and mantle, yet the apparently decoupled Nd-Hf isotope record preserved by the oldest rocks implicates differentiation processes unlike those associated with modern plate tectonics. This debate has, however, been staged in areas of great geological complexity, where the veracity of whole rock radiogenic isotope data is questionable, or involved inferences from ancient detrital zircons whose provenance is enigmatic. Here, we report the Nd and Hf isotope compositions of samples from the Paleoarchaean to Neoarchaean Pilbara craton, a superbly preserved, archetypal granite-greenstone belt in Western Australia. We have analyzed Nd-Hf isotopes in mafic rocks and Hf isotopes in zircons of interleaved felsic units to monitor the effects of whole-rock isotope disturbance and to track the evolution of crust and mantle reservoirs. Mafic samples range from ~3.52 Ga komatiites and basalts of the Warrawoona Group, whose eruption is attributed to mantle plume activity, through to younger (3.12-2.95 Ga) basalts, calc-alkaline basalts and boninites similar to those of intra-oceanic volcanic arcs. Felsic rocks (3.48-2.85 Ga) include dacite and rhyolite units from the greenstone belts and their coeval plutonic counterparts in the granitic complexes. Additionally, the Hf isotope composition of the oldest (3.65-3.73 Ga) detrital zircons from intra-cratonic basins were measured to explore the pre-3.5 Ga history of the craton and to test whether this developed on an older, but unexposed, continental substrate. Results are summarized as follows. Collectively, the Pilbara samples exhibit greater variability in initial 143Nd/144Nd than in 176Hf/177Hf, as has been observed in other ancient Archaean terranes (J. Vervoort et al. Nature 379,1996). Epsilon Hf values of the mafic samples increase from +1.1 at 3.53 Ga to +3.0 at 3.2 Ga, before falling sharply to -3.3 by 2.95 Ga. Epsilon Nd

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

    NASA Astrophysics Data System (ADS)

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

    2009-12-01

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

  8. 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

  9. 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.

  10. 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.

  11. Hf Isotope Evidence for Subducted Basalt and Sediment Contributions to the Eastern Trans-Mexican Volcanic Belt

    NASA Astrophysics Data System (ADS)

    Cai, Y.; Tuena, A. G.; Capra, L.; Straub, S. M.; Goldstein, S. L.; Langmuir, C. H.

    2005-12-01

    Magmas generated at thick crust continental arcs often have enriched continental crust-like trace element patterns and Pb-Sr-Nd isotope ratios that are intermediate to both upper mantle and crustal compositions. Thus it is difficult to distinguish between contributions from (a) the subducted basalt and the upper mantle wedge, and (b) subducted sediment and the continental crust. These issues have been the focus of major controversy. Here we show evidence for subduction contributions to lavas in a classic thick crust environment. In Eastern Trans-Mexican Volcanic Belt, the upper continental crust is 30 km to 45 km thick. However, primitive mafic lavas erupt on many sites across the arc. We have analyzed the subducting sediments as represented by DSDP 487, located seaward of the trench, where the lower third of the sediment column has strongly hydrothermal pelagic features and the upper two-thirds is composed of terrigenous sediments. The pelagic sediments have distinctive features that could be used to identify a subduction component in the volcanics, including high REE/Hf, negative Ce anomalies, and Nd-Hf isotopes that lie on the "seawater array" and offset from the "mantle-crust" array. We have focused on a unique series of lavas from volcano Nevado de Toluca, located southwest of Mexico City. These lavas show negative Ce anomalies coupled with low REE/Hf and Zr/Nd ratios. Hf-Nd isotope ratios show a shallow trend compared to the mantle-crust array, consistent with a pelagic component. In addition, Hf isotopes show a striking positive correlation with Ce anomalies that trend toward the pelagic sediment compositions. These and other observations provide clear evidence for a component from subducted sediment in the lavas. In addition, there is a negative correlation of Lu/Hf and Hf isotopes that requires a mixing endmember with MORB-like Hf isotope ratios but with lower than MORB Lu/Hf. This indicates a melt from eclogitic subducted basalt. Compared to other

  12. Core formation conditons in planetesimals: constraints from isotope fractionation experiments.

    NASA Astrophysics Data System (ADS)

    Guignard, J.; Quitté, G.; Toplis, M. J.; Poitrasson, F.

    2016-12-01

    Planetesimals are small objects (10 to 1000 km) early accreted in the history of the solar system which show a wide variety of thermal history due to the initial amount of radiogenic elements [1] (26Al and 60Fe), from a simple metamorphism to a complete metal-silicate differentiation. Moreover, isotope compositions of siderophile element, e.g. Fe, Ni, and W in meteorites spread on a range that can be attributed to the process of core-mantle segregation. We therefore performed isotope fractionation experiments of nickel and tungsten between metal and silicate in a gas-mixing (CO-CO2) vertical furnace, at different temperatures (from 1270°C to 1600°C), oxygen fugacity (from IW+2 to IW-6) and annealing times (from 20 minutes to 48 hours). The starting silicate is an anorthite-diopside eutectic composition glass, synthesize from the respective oxides. The starting metal is either a nickel or tungsten wire according to the element to study. After each experiment, metal and silicate are mechanically separated and digested in acids. Nickel and Tungsten separation have been made according to the methods developed by [2] and [3] and isotopes measurements have been made using a high resolution MC-ICP-MS (Neptune; Thermofisher©). Results show evidence for a strong kinetic isotope fractionation during the first annealing times with a faster diffusion of lightest isotopes than heaviest. Similar mechanism has been already highlighted for iron isotope fractionation between silicate and metal [4]. Chemical and isotopic equilibrium is also reached in our experiments but the time required dependent on the conditions of temperature and oxygen fugacity. Therefore, at equilibrium, metal-silicate isotope fractionation has also been quantified as well its temperature dependence. These experimental data can be used in order to bring new constraints on the metal silicate segregation in the planetesimals early accreted. [1] Lee T., et al., GRL, 3, 41-44 (1976) [2] Quitté G., and Oberli

  13. Xe isotopic constraints on deep volatile cycling over Earth history

    NASA Astrophysics Data System (ADS)

    Parai, R.; Mukhopadhyay, S.

    2016-12-01

    The modern deep Earth volatile budget consists of primordial volatiles retained in the planet throughout accretion, volatile species generated by radiogenic ingrowth (e.g., 40Ar produced by 40K decay), and volatiles subducted into the mantle from surface reservoirs over time. The noble gases are unique volatile tracers, in that they are themselves chemically inert, but are thought to be trapped within hydrous alteration phases in the subducting slab. The noble gases thus provide a tracer of transport between the deep Earth and surface reservoirs reflecting pressure and temperature conditions in the subducting slab. Constraints on the flux of noble gases from surface reservoirs into the deep Earth can accordingly be used to provide insight into the limits on cycling and the evolving distribution of major volatile species in terrestrial reservoirs over time. Xe isotope systematics in mantle-derived rocks show that 80-90% of the mantle Xe budget is derived from recycling of atmospheric Xe (e.g., Mukhopadhyay, 2012; Tucker et al., 2012; Parai et al., 2012; Peto et al., 2013; Parai and Mukhopadhay, 2015), indicating that atmospheric Xe is retained in subducting slabs beyond depths of magma generation in subduction zones over Earth history. However, the isotopic composition of atmospheric Xe has changed over time: ancient atmospheric gases trapped in Archean quartz indicate that the atmospheric Xe isotopic composition at 3.5 Ga was less fractionated than the modern atmospheric composition (Pujol et al., 2011). Here we take the evolution of the atmospheric Xe isotopic composition into account to test various models of volatile outgassing and regassing into the Earth's mantle. 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. Models in which the deep Earth transitions from a net outgassing to net

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

  16. 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.

  17. 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

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

    PubMed

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

    2010-05-11

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

  19. 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.

  20. 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

  1. Hf-Nd isotope evidence for a transient dynamic regime in the early terrestrial mantle

    PubMed

    Albarede; Blichert-Toft; Vervoort; Gleason; Rosing

    2000-03-30

    Modern basalts have seemingly lost all 'memory' of the primitive Earth's mantle except for an ambiguous isotopic signal observed in some rare gases. Although the Earth is expected to have reached a thermal steady state within several hundred million years of accretion, it is not known how and when the initial chemical fractionations left over from planetary accretion (and perhaps a stage involving a magma ocean) were overshadowed by fractionations imposed by modern-style geodynamics. Because of the lack of samples older than 4 Gyr, this early dynamic regime of the Earth is poorly understood. Here we compare published Hf-Nd isotope data on supracrustals from Isua, Greenland, with similar data on lunar rocks and the SNC (martian) meteorites, and show that, about 3.8 Gyr ago, the geochemical signature of the Archaean mantle was partly inherited from the initial differentiation of the Earth. The observed features seem to indicate that the planet at that time was still losing a substantial amount of primordial heat. The survival of remnants from an early layering in the modern deep mantle may account for some unexplained seismological, thermal and geochemical characteristics of the Earth as observed today.

  2. 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.

  3. Isotopic constraints on methane's global sources and ENSO-dependence

    NASA Astrophysics Data System (ADS)

    Schaefer, Hinrich; Mikaloff Fletcher, Sara; Veidt, Cora; Lassey, Keith; Brailsford, Gordon; Bromley, Tony; Dlugokencky, Ed; Englund Michel, Sylvia; Miller, John; Levin, Ingeborg; Lowe, Dave; Martin, Ross; Vaughn, Bruce; White, James; Nichol, Sylvia

    2017-04-01

    Atmospheric levels of the potent greenhouse gas methane (CH4) have been rising since the industrial revolution, except for a plateau during the early 2000s. Stable carbon isotopes in methane (delta-13CH4) provide constraints on the budget changes associated with the plateau's onset and its end. We present a reconstruction of annual global delta-13CH4 averages based on a global network of stations, whose trends are indicative of global methane source and sink activity. A box model analysis shows that from the mid-1990s methane emissions with the characteristic thermogenic delta-13CH4 signature reduced, implying persistently lower emissions from fossil fuel productions as the cause of the plateau. However, variations in hydroxyl, the main CH4 sink, provide an equably plausible explanation for the plateau onset that may also account for strong variability in emission-vs-removal rates during the plateau period. In contrast, the renewed CH4 rise since 2006 can only be explained by increasing emissions with a biogenic isotope signature, i.e. agriculture or wetlands. We present correlation studies that test whether ENSO activity controls atmospheric delta-13CH4, and by extension methane levels, through tropical wetland emissions.

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

    PubMed

    Pourmand, Ali; Dauphas, Nicolas

    2010-05-15

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

  5. 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.

  6. New Hf Isotope Perspectives on Magma Sources Beneath the Colorado Plateau, Western U.S.A. (Invited)

    NASA Astrophysics Data System (ADS)

    Reid, M. R.; Bouchet Bert Manoz, R.; Blichert-Toft, J.; Ramos, F. C.

    2009-12-01

    Oligocene and younger volcanism in the southwestern U.S. locally penetrates the Colorado Plateau (CP), an elevated region of thick (120-150 km) continental lithosphere. As judged from major element inversions, melts responsible for several young volcanic fields may have been extracted at depths (mostly <100 km) well within present or recently thermally reactivated mantle lithosphere, yet at high temperatures (Lee et al., 2009). Given the tectonic integrity of the CP, it should be possible to distinguish chemical features acquired by the lithosphere when it formed 1.68-1.74 b.y. ago from those imposed by metasomatism related to shallow Farallon plate subduction and/or contributions from the sublithospheric mantle. New Hf (and Nd, as warranted) isotope data for 82, mostly magnesian, CP volcanic rocks quadruple the number of Hf isotope analyses for this region and enable us to delineate distinct Nd-Hf isotopic signatures and trends for individual volcanic fields. Collectively, the volcanic rocks are more radiogenic and therefore different from the isotopic characteristics of mafic lavas associated with other portions of the same-aged crustal terranes to the northeast in the Rocky Mountain region. Most samples define a narrower range of Hf isotope ratios (ɛHf = +1 to +8) than the widely variable Nd isotope ratios (ɛNd = -4 to +6), in contrast to oceanic basalts (ɛHf ~ -2 to +11 over the same range). Shallow-sloping trends defined by most volcanic fields appear to diverge from a common mantle component (ɛNd = +7; ɛHf = +8) that lies somewhat below the mantle array. The enriched end-members for these arrays generally lie above the mantle array, are quite diverse in their isotopic characteristics, and could represent lithospheric domains admixed with recycled sediment and/or variably metasomatized by ancient melt infiltration. Melt-induced Lu/Hf fractionation (measured relative to time-integrated ratios) correlates well with apparent Sm/Nd fractionation, a trend that

  7. The Mg-Sr-Nd-Hf Isotopic Compositions of Beiyan Mantle Xenoliths: Implications on the Melt/Fluid-Rock Interaction

    NASA Astrophysics Data System (ADS)

    Li, P.; Lu, Y.; Li, S.

    2016-12-01

    It has been widely confirmed that mantle metasomatism plays an important role in the refertilization of lithospheric mantle. However, the mechanism of metasomatism including the nature and source of metasomatic flux is still difficult to be thoroughly revealed. We present an integrated study of whole-rock major and trace elements and Mg-Sr-Nd-Hf isotopes for mantle xenoliths and host basalts at Beiyan, Shandong Province in eastern China. The primary objective is to clarify the periods of metasomatism and provide constrains on the interaction between xenolith and melt/fluid. Based on the mineral component, Beiyan xenoliths were subdivided into four types: lherzolite, cpx-rich lherzolite, wehrlite and olivine clinopyroxenite. These samples show variable Mg# values, decreasing from 89.4 90.2 in lherzolites, 85.9 89.6 in cpx-rich lherzolites, 81.2 87.3 in wehrlites, till 77.7 79.4 in olivine clinopyroxenites. Low Mg# values, LREE-enriched patterns and the occurrence of hydrous minerals such as phlogopite, amphibole, feldspar and carbonates such as calcite, ankerite and reacted texture suggest subsequent melt/fluid addition. Relative to the Mg isotopic composition of lherzolites (δ26Mg = -0.21±0.06‰ (2SD, n=3)), which are similar to the average δ26Mg of normal mantle (-0.25±0.07‰ [1]), δ26Mg of cpx-rich lherzolites (-0.18±0.07‰ (2SD, n=4)) are heavier, and δ26Mg of wehrlites (-0.31±0.07‰ (2SD, n=9)) are lighter. Combining with the Sr-Nd-Hf isotopic results, at least two-stage melt/fluid-rock interactions occurred in Beiyan xenoliths, which formed the isotopic heterogeneous. The distinct heavy Mg isotopic compositions and fertile Sr-Nd-Hf isotopic compositions in cpx-rich lherzolites indicate that they are the products of interaction between lherzolite and fluid derived from subducted Pacific slab which may inherit heavy Mg isotopic compositions of abyssal peridotite. The basaltic melt metasomatism with light Mg isotopic (δ26Mg = -0.54‰ to -0.40

  8. Early Earth differentiation processes investigated through the short-lived 146Sm-142Nd and 182Hf-182W isotope systems

    NASA Astrophysics Data System (ADS)

    Rizo Garza, H. L.; Walker, R. J.; Carlson, R.; Touboul, M.; Horan, M. F.; Puchtel, I. S.; Boyet, M.; Rosing, M. T.

    2016-12-01

    The earliest history of Earth is difficult to capture due to the scarcity and high degrees of alteration of ancient terrains. Nevertheless, short-lived isotope systems can provide important constraints on early geological processes. The 182Hf-182W (t1/2 = 8.9 Ma) and 146Sm-142Nd (t1/2=103 Ma) systems are variably sensitive to differentiation processes that occurred during the first 50 Ma and 500 Ma of Earth's history, respectively. Eoarchean mantle-derived rocks from the Isua supracrustal belt (southwest Greenland) show well-resolved anomalies in both 182W (+5 to +21 ppm) and 142Nd (-10 to +15 ppm) compared to terrestrial standards. While there is evidence that W was mobilized in the crust accessed by the Isua suite, W and Nd isotopic anomalies are interpreted to primarily reflect Hadean processes that affected the mantle precursors of these rocks. Variations in 142Nd do not correlate with those of 182W, however, suggesting different mechanisms for the origin and retention of the isotopic anomalies present in these two systems. The 142Nd data, combined with 143Nd data for the long-lived 147Sm-143Nd system, are interpreted to reflect silicate crystal-liquid fractionation 4.3 Ga ago. Variations of 182W in the Isua lavas mantle source could also have been produced as a result of Hf/W fractionation caused by silicate differentiation during the lifetime of 182Hf, but these processes would have to somehow be decoupled from the processes that affected Sm/Nd. Alternatively, 182W variability could be the result of combined early Hf/W fractionations caused by metal-silicate segregation within discrete mantle domains and late accretion processes. Variations in 182W and 142Nd observed in the Isua supracrustal rocks are similar to those observed in the ancient rocks from Nuvvuagittuq Greenstone Belt, the Acasta Gneiss Complex, and the Saglek Block, suggesting that similar processes affected diverse early Earth rocks.

  9. ~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

  10. 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

  11. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2013-04-01

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

  13. 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.

  14. Zircon Lu-Hf isotopes in high-alumina orthopyroxene megacrysts from the Neoproterozoic Rogaland Anorthosite Province, SW Norway: A window into the Sveconorwegian lower crust

    NASA Astrophysics Data System (ADS)

    Sauer, Simone; Slagstad, Trond; Andersen, Tom; Kirkland, Christopher L.

    2013-04-01

    The Rogaland Anorthosite Province consists of three massif-type anorthosite bodies and an associated layered Bjerkreim-Sokndal intrusion. The rocks were emplaced between ca. 950 and 920 Ma, following the Sveconorwegian orogeny at ca. 1000 Ma. The anorthosites are commonly thought to have formed by melting of the lower continental crust. They lack zircon or other dateable minerals, so geochronological constraints come from zircon crystals found within opx megacrysts in the rocks. The opx megacrysts contain ca. 7.7 wt.% Al2O3, and are therefore high-alumina opx megacrysts (HAOM). Pressure estimates range from ca. 11 to 13 kbar which implies that the HAOM crystallised in the lower crust and that their zircon cargo has probably been shielded from interaction with the middle and upper crust. If the assumption of a lower-crustal source is correct, the zircons may be viewed as probing the Hf isotopic composition of the lower crust in SW Norway. Zircons were separated from HAOM in the Egersund-Ogna anorthosite and yield a U-Pb age of 948±3 Ma, supporting Andersen and Griffin's (2004) suggestion of onset of magmatism prior to the commonly accepted ca. 930 Ma age for these rocks. Lu-Hf isotopic analyses of the zircons yield a ɛHf948 Ma value of +3.8 ± 3 (2σ), which is interpreted to represent the composition of the lower crust in SW Norway. Three crustal components (or, more correctly, their lower-crustal counterparts) were considered as potential source rocks for the anorthosite magma that formed the Egersund-Ogna body: i) ca.1500 Ma Telemarkian magmatic rocks that cover large tracts of S Norway; ii) ca.1260 Ma magmatic rocks that are geographically widespread in south and central Scandinavia, but volumetrically minor; iii) 1050 to 1020 Ma magmatic rocks in the 30-40 km-wide and nearly 200 km-long Sirdal Magmatic Belt in SW Norway. The isotopic data show that 1500 Ma crust cannot be a major component of the lower-crustal source, whereas both 1260 Ma and 1050-1020 Ma

  15. Plume versus plate origin for the Shatsky Rise oceanic plateau (NW Pacific): Insights from Nd, Pb and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Heydolph, Ken; Murphy, David T.; Geldmacher, Jörg; Romanova, Irina V.; Greene, Andrew; Hoernle, Kaj; Weis, Dominique; Mahoney, John

    2014-07-01

    Shatsky Rise, an early Cretaceous igneous oceanic plateau in the NW Pacific, comprises characteristics that could be attributed to either formation by shallow, plate tectonic-controlled processes or to an origin by a mantle plume (head). The plateau was drilled during Integrated Ocean Drilling Program (IODP) Expedition 324. Complementary to a recent trace element study (Sano et al., 2012) this work presents Nd, Pb and Hf isotope data of recovered lava samples cored from the three major volcanic edifices of the Shatsky Rise. Whereas lavas from the oldest edifice yield fairly uniform compositions, a wider isotopic spread is found for lavas erupted on the younger parts of the plateau, suggesting that the Shatsky magma source became more heterogeneous with time. At least three isotopically distinct components can be identified in the magma source: 1) a volumetrically and spatially most common, moderately depleted component of similar composition to modern East Pacific Ridge basalt but with low 3He/4He, 2) an isotopically very depleted component which could represent local, early Cretaceous (entrained) depleted upper mantle, and 3) an isotopically enriched component, indicating the presence of (recycled) continental material in the magma source. The majority of analyzed Shatsky lavas, however, possess Nd-Hf-Pb isotope compositions consistent with a derivation from an early depleted, non-chondritic reservoir. By comparing these results with petrological and trace element data of mafic volcanic rock samples from all three massifs (Tamu, Ori, Shirshov), we discuss the origin of Shatsky Rise magmatism and evaluate the possible involvement of a mantle plume (head).

  16. Hf and Nd Isotopic and REE Investigations of Magnetite in a Proterozoic IOCG system: Fingerprinting Sources and Timing of Mineralisation

    NASA Astrophysics Data System (ADS)

    Schaefer, B. F.

    2016-12-01

    The Stuart Shelf on the margin of the Gawler Craton, South Australia, contains numerous economic and sub-econmic IOCG mineralised systems, including the giant Olympic Dam Cu-Au-U deposit. Hematite and magnetite have played a critical in the genesis of all of these deposits, and increasingly it appears that magnetite has been in equilibrium with either the final mineralised assemblage or was critical in transporting metals during the ore forming event. 14 magnetites and one hematite from three separate styles of iron oxide mineralisation associated with the Prominent Hill Cu-Au deposit were selected for detailed analysis. The REE and isotopic separations were all conducted by low blank wet chemistry and isotopes determined by TIMS (Nd) and MC-ICPMS (Hf). Magnetites associated with skarn style mineralsiation proximal to the ore body are unformly depleted in REE, whereas hematite within the ore and magmatic magmatites and whole rock gabbros from the nearby 1590Ma White Hill Gabbro intrusion are all relatively LREE enriched and display a comparable range in REE. Significantly however, magnetite separates almost invariably display more evolved Hf isotopic signatures than the host lithologies adjacent the economic mineralisation (dacites and metasediments at Prominent Hill mine) implying that the magnetites were sourcing their REE inventory dominantly from the local crust rather than a mantle derived source. In contrast, the magmatic magnetites from the White Hill Complex display Nd and Hf isotopes which are slightly more primitive, recording a greater relative mantle component, however still requiring a significant crustal input. Significantly, the hematite which contains the Au mineralisation preserves ɛNd (1590) = -4.04 and ɛHf (1590) = -6.05 essentially identical to the magmatic magnetites and their host gabbros in the White Hill complex and the basalts and dacites of the host Gawler Range Volcanics (ɛNd (1590) = -7.10 - -3.72 and ɛHf (1590) = -7.69 - -1

  17. Hf isotope evidence for variable slab input and crustal addition in basalts and andesites of the Taupo Volcanic Zone, New Zealand

    NASA Astrophysics Data System (ADS)

    Waight, Tod E.; Troll, Valentin R.; Gamble, John A.; Price, Richard C.; Chadwick, Jane P.

    2017-07-01

    Crustal contamination complicates the identification of primary mantle-derived magma compositions in continental arcs. However, when crustal processes and components are well characterised, it is possible to extrapolate through continental arc magma compositional arrays towards the Hf and Nd isotope compositions of uncontaminated primary magmas. This is because of the similar behaviour of Hf and Nd during fractional crystallisation and mantle melting, and the subsequent limited variation in Hf/Nd in mantle-derived magmas and in many crustal lithologies, resulting in linear contamination trends for Hf-Nd isotopes. Here we present new Hf isotope data for a selection of volcanic rocks and crustal lithologies from the Taupo Volcanic Zone (TVZ), New Zealand and propose that the scatter in Hf-Nd isotopes indicates heterogeneity in the parental magmas prior to interactions with crustal lithologies. The observed variations likely represent variability in primary magma compositions as a result of different degrees of sediment addition at the slab-wedge interface. Coupled variations in isotopic composition, LILE/HFSE ratios (e.g. Rb/Zr and Ba/La) and SiO2 also clearly indicate that shallower level crustal interactions have occurred. Andesites from Ruapehu Volcano have more consistent parental magma compositions, and require greater amounts of a source sediment contribution. Notably, the compositions of older Ruapehu lavas can be modelled by interactions between mantle-derived magmas and lower crustal granulites, whereas younger lavas have probably interacted more with mid- to shallow crustal meta-sedimentary greywacke-argillite lithologies of the Permian to Cretaceous composite Torlesse Terrane. Hf-Nd isotopic compositions of meta-igneous granulite xenoliths from Mt. Ruapehu are consistent with previous interpretations that they are derived from oceanic crust that underlies the Torlesse meta-sediments. The results indicate that interactions with sediments at both the slab

  18. Bringing organic carbon isotopes and phytoliths to the table as additional constraints on paleoelevation

    NASA Astrophysics Data System (ADS)

    Sheldon, N. D.; Cotton, J. M.; Hren, M. T.; Hyland, E. G.; Smith, S. Y.; Strömberg, C. A. E.

    2015-12-01

    A commonly used tool in paleotectonic and paleoaltimetry studies is the oxygen isotopic composition of authigenic carbonates formed that formed in lakes or soils, with both spatial (e.g., shoreline to mountain top) or temporally resolved records potentially providing constraints. However, in many cases there is a substantial spread in the oxygen isotope data for a given time period, often to the point of allowing for essentially any interpretation of the data depending upon how they have been used by the investigator. One potential way of distinguishing between different potential paleotectonic or paleoaltimetric interpretations is to use carbon isotope and plant microfossil (phytolith) analyses from the same paleosols to screen the oxygen isotope data by looking for evidence of evaporative enrichment. For example, if both inorganic (carbonate) and organic carbon isotopes are measured from the same paleosol, then in it possible to determine if the two isotope record equilibrium conditions or if they record disequilibrium driven by kinetic effects. In the former case, the oxygen isotope results can be considered reliable whereas in the latter case, the oxygen isotope results can be considered unreliable and could be culled from the interpretation. Similarly, because the distribution of C4 plants varies as a function of temperature and elevation, the presence/absence or abundance of C4 plant phytoliths, or of carbon isotope compositions that require a component of C4 vegetation can also be used to constrain paleoelevation by providing a maximum elevation constraint. Worked examples will include the late Miocene-Pliocene of Catamarca, Argentina, where phytoliths and organic carbon isotopes provide a maximum elevation constraint and can be used to demonstrate that oxygen isotopes do not provide a locally useful constraint on paleoelevation, and Eocene-Miocene of southwestern Montana where organic matter and phytoliths can be used to select between different potential

  19. 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

  20. Erosion of Archean continents: The Sm-Nd and Lu-Hf isotopic record of Barberton sedimentary rocks

    NASA Astrophysics Data System (ADS)

    Garçon, M.; Carlson, R. W.; Shirey, S. B.; Arndt, N. T.; Horan, M. F.; Mock, T. D.

    2017-06-01

    Knowing the composition, nature and amount of crust at the surface of the early Earth is crucial to understanding the early geodynamics of our planet. Yet our knowledge of the Hadean-Archean crust is far from complete, limited by the poor preservation of Archean terranes, and the fact that less attention has been paid to the sedimentary record that tracks erosion of these ancient remnants. To address this problem and get a more comprehensive view of what an Archean continent may have looked like, we investigated the trace element and Sm-Nd, Lu-Hf isotopic records of Archean metasedimentary rocks from South Africa. We focused our study on sandstone and mudstone from drill core in the Fig Tree Group (3.23-3.26 Ga) of the Barberton granite-greenstone belt, but also analyzed the 3.4 Ga Buck Reef cherts and still older (3.5-3.6 Ga) meta-igneous rocks from the Ancient Gneiss Complex, Swaziland. Based on principal component analysis of major and trace element data, the Fig Tree metasedimentary rocks can be classified into three groups: crustal detritus-rich sediments, Si-rich sediments and Ca-, Fe-rich sediments. The detritus-rich sediments have preserved the Sm-Nd and Lu-Hf isotopic signatures of their continental sources, and hence can be used to constrain the composition of crust eroded in the Barberton area in the Paleoarchean period. Based on Sm/Nd ratios, we estimate that this crust was more mafic than today, with an average SiO2 content of 60.5 ± 2 wt.%. This composition is further supported by isotopic mixing calculations suggesting that the sedimentary source area contained equal proportions of mafic-ultramafic and felsic rocks. This implies that the Archean crust exposed to weathering was more mafic than today but does not exclude a more felsic composition at depth. Neodymium and Hf crustal residence ages show that the eroded crust was, on average, ∼300-400 Ma older than the deposition age of the sediments, which highlights the importance of intracrustal

  1. 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

  2. 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

  3. The Proterozoic of NW Mexico revisited: U-Pb geochronology and Hf isotopes of Sonoran rocks and their tectonic implications

    NASA Astrophysics Data System (ADS)

    Solari, L. A.; González-León, C. M.; Ortega-Obregón, C.; Valencia-Moreno, M.; Rascón-Heimpel, M. A.

    2017-07-01

    Several Proterozoic basement units crop out in the Sonora State of NW Mexico, and the same can be correlated with crustal provinces of southern Laurentia in the neighboring southwestern USA. Zircon U-Pb and Hf isotopic determinations in more than 300 grains separated from igneous and metaigneous rocks from these units indicate that the crystalline basement in Sonora is made up of different components, which are from west to east: (1) The Caborca-Mojave province to the west, characterized by the so-called Bámori Complex, have U-Pb ages between 1696 and 1772 Ma, with moderately juvenile to slightly evolved ɛHf values, yielding T DM ages of ca. 2.1-2.4 Ga; (2) in the intermediate area, east of Hermosillo, the Palofierral and La Ramada orthogneiss units yield an age of 1640 and 1703 Ma, respectively, both having juvenile ɛHf with the Palofierral overlapping the depleted mantle curve at ca. 1.65 Ga; and (3) in the northeastern Sonora, samples from the southern extension of the Mazatzal province, represented by the Pinal Schist, yielded ages between 1674 and 1694 Ma, with moderately juvenile to juvenile ɛHf values and a T DM age of ca. 1.9 Ga. In addition, a suite of post-tectonic granites was also studied in Caborca (San Luis granite) as well as in northeastern Sonora (Cananea granite), both yielding ages of ca. 1.44 Ga with moderately juvenile ɛHf values ranging from -1 to +8 and T DM dates of ca. 1.8-1.9 Ga and 1.6-1.7 Ga, respectively. These two isotopically contrasting provinces may imply the existence of a Proterozoic paleo-suture. However, if the Palofierral gneiss, of which the Hf signature straddles the depleted mantle array, is taken as the source for the 1.44 Ga Cananea granite, then the location of such a suture zone should lay farther south than the proposed trace of the Mojave-Sonora megashear.

  4. Production, chemical and isotopic separation of the178m2Hf high-spin isomer

    NASA Astrophysics Data System (ADS)

    Oganessian, Yu. Ts.; Hussonnois, M.; Brianôcon, Ch.; Karamian, S. A.; Szeglowski, Z.; Ledu, D.; Meunier, R.; Constantinescu, M.; Kim, J. B.; Constantinescu, O.

    1997-05-01

    The 178m2Hf with its long-lived (T1/2=31 y), high-spin Iπ = 16+, isomeric state, is a challenge for new and exotic nuclear physics studies. The 178m2Hf isomer has been produced in microweight quantities using the 176Yb(α,2n) nuclear reaction, by irradiation with a high-intensity beam using the U-200 cyclotron in Dubna. Radiochemistry and mass separation methods have been developed, with the aim to separate and purify the produced Hf material. Thin targets of isomeric hafnium-178 on carbon backings have been prepared and used in experiments with neutron, proton and deuteron beams.

  5. 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.

  6. 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

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

    PubMed

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

    2014-12-05

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

  8. Reconciliation of the excess 176Hf conundrum in meteorites: Recent disturbances of the Lu-Hf and Sm-Nd isotope systematics

    NASA Astrophysics Data System (ADS)

    Bast, Rebecca; Scherer, Erik E.; Sprung, Peter; Mezger, Klaus; Fischer-Gödde, Mario; Taetz, Stephan; Böhnke, Mischa; Schmid-Beurmann, Hinrich; Münker, Carsten; Kleine, Thorsten; Srinivasan, Gopalan

    2017-09-01

    The long-lived 176Lu-176Hf and 147Sm-143Nd radioisotope systems are commonly used chronometers, but when applied to meteorites, they can reveal disturbances. Specifically, Lu-Hf isochrons commonly yield dates up to ∼300 Myr older than the solar system and varying initial 176Hf/177Hf values. We investigated this problem by attempting to construct mineral and whole rock isochrons for eucrites and angrites. Meteorites from different parent bodies exhibit similar disturbance features suggesting that a common process is responsible. Minerals scatter away from isochron regressions for both meteorite classes, with low-Hf phases such as plagioclase and olivine typically being most displaced above (or left of) reference isochrons. Relatively Hf-rich pyroxene is less disturbed but still to the point of steepening Lu-Hf errorchrons. Using our Lu-Hf and Sm-Nd data, we tested various Hf and Lu redistribution scenarios and found that decoupling of Lu/Hf from 176Hf/177Hf must postdate the accumulation of significant radiogenic 176Hf. Therefore early irradiation or diffusion cannot explain the excess 176Hf. Instead, disturbed meteorite isochrons are more likely caused by terrestrial weathering, contamination, or common laboratory procedures. The partial dissolution of phosphate minerals may predominantly remove rare earth elements including Lu, leaving relatively immobile and radiogenic Hf behind. Robust Lu-Hf (and improved Sm-Nd) meteorite geochronology will require the development of chemical or physical methods for removing unsupported radiogenic Hf and silicate-hosted terrestrial contaminants without disturbing parent-daughter ratios.

  9. 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

  10. Rate of growth of the preserved North American continental crust: Evidence from Hf and O isotopes in Mississippi detrital zircons

    NASA Astrophysics Data System (ADS)

    Wang, Christina Yan; Campbell, Ian H.; Allen, Charlotte M.; Williams, Ian S.; Eggins, Stephen M.

    2009-02-01

    Detrital zircons from the Mississippi River have been analyzed for U-Th-Pb, Lu-Hf and O isotopes to constrain the rate of growth of the preserved North American continental crust. One hundred and forty two concordant zircon U/Pb dates on grains mounted in epoxy, obtained by Excimer laser ablation ICP-MS method, resolved six major periods of zircon crystallization: 0-0.25, 0.3-0.6, 0.95-1.25, 1.3-1.5, 1.65-1.95 and 2.5-3.0 Ga. These age ranges match the ages of the recognized lithotectonic units of the North American continent in the hinterland of the Mississippi River. Ninety-six zircons mounted on tape, which show no age zonation and were within 7.5% of concordance, were selected to represent the six U/Pb age time intervals and analyzed for Lu-Hf and O isotope by laser ablation MC-ICP-MS and SHRIMP II, respectively. The δ 18O values of the zircons show a small step increase in the maximum δ 18O values at the Archean-Proterozoic boundary from 7.5‰ in the Archean to 9.5‰, and rarely 13‰, in the Proterozoic and Phanerozoic. However, the average value of δ 18O in zircons changes little with time, showing that the increase in the maximum δ 18O values between 2.5 and 2.0 Ga, which can be attributed to an increase in the sediment content of the source regions of younger granitoids, is largely balanced by an increase in zircons with anomalously low δ 18O, which can be attributed to hydrothermally altered crust in the granitoid source region. ɛHf i values for the zircons range from 13.1 to -26.9. Zircons derived from juvenile crust, which we define as having mantle δ 18O (4.5-6.5‰) and lying within error of the Hf depleted mantle growth curve, are rare or absent in the Mississippi basin. The overwhelming majority of zircons crystallized from melted pre-existing continental crust, or mantle-derived magmas that were contaminated by continental crust. The average time difference between primitive crust formation and remelting for each of the recognized

  11. 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.

  12. Open system Hf isotope homogenization by a DISPOREP process under amphibolite-facies conditions, an example from the Limpopo Belt (South Africa)

    NASA Astrophysics Data System (ADS)

    Zeh, Armin; Gerdes, Axel

    2013-04-01

    Isotope homogenization in metamorphic rock is a prerequisite for precise isochrone dating. However, whether or not homogenisation occurs during a metamorphic overprint dependent on several parameters and processes, which compete with each other and comprise at least (i) volume diffusion, (ii) dissolution-re-precipitation, (iii) intergranular diffusive or fluid enhanced transport, and (iv) metamorphic mineral reaction(s). Isotope homogenisation is commonly reached in high-grade (granulite-facies) metamorphic rocks, where diffusion is fast, and mineral reactions and dissolution-re-precipitation accompanied or maintained by a melt phase, but it is incomplete in low-grade to amphibolite-facies rocks, in the presence of an aqueous fluid phase. This holds true, in particular, for the Lu-Hf isotope system, which is mainly controlled by accessory zircon, which is very resistant against dissolution in aqueous fluids and has slow diffusivity for Hf, U, Pb. Thus zircon often maintains it primary U-Pb-Hf isotope composition obtained during previous magmatic crystallisation (i.e, magmatic grains in orthogneisses or detrital magmatic grains in paragneisses), even under very high-grade metamorphic conditions >1000° C. However, results of recent isotope studies show, that the U-Pb and Lu-Hf isotope systems of zircon-bearing ortho- and paragneisses can homogenize completely (on hand specimen scale) even under amphibolite facies T - P conditions of

  13. 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.

  14. Stable Isotope Constraints on the Formation of Moon

    NASA Astrophysics Data System (ADS)

    Magna, T.; Dauphas, N.; Righter, K.; Camp, R.

    2017-05-01

    The development of high-precision techniques to measure stable isotope compositions of a number of elements which, in the past, were considered homogeneous, implicated a new fresh look at the origin of Moon.

  15. Budget of Methyl Bromide in the Atmosphere: Isotopic Constraints

    NASA Astrophysics Data System (ADS)

    Bill, M.; Miller, L. G.; Rhew, R. C.; Goldstein, A. H.

    2001-12-01

    Bromine radicals contribute significantly to stratospheric ozone loss through coupled reactions with ClO, HO2, and NO2 radicals. Bromine is approximately 40-100 times more effective, atom for atom, at destroying ozone than chlorine. For instance, Br coupled reactions are responsible for 30 to 50% of the total ozone loss in the polar vortex. The largest source of bromine to the stratosphere is methyl bromide (CH3Br). CH3Br has a concentration in the troposphere of about 10 pptv, a total estimated lifetime of 0.6 to 0.9 years, and has the highest concentration of any long lived organobromine. Unlike chlorofluorocarbons, which are produced entirely by humans, methyl bromide is produced by both anthropogenic and natural processes. We are developing the use of stable isotopes to constrain the budget of CH3Br through quantification of the source signatures and the isotopic fractionations associated with sinks. The largest natural sources appear to be biological production in oceans ( ~35%), biomass burning ( ~13%), and salt marshes ( ~10%). Thus far, the only natural emissions to be isotopically characterized is from salt marsh plants. Carbon isotopic ratios of CH3Br emitted from the salt marsh have a strong diurnal variation from -65\\permil during daytime when emission rates are highest to -12\\permil at night when emissions are ~13% of the daytime rates. The \\delta13C weighted mean of salt marsh emission is -43\\permil CH3Br. Anthropogenically produced CH3Br is used for fumigation of soils, harvested crops and structures, and represents approximately 30% of the total source flux. The fumigation of harvested crops and structures constitutes approximately one third of the anthropogenic source to the atmosphere and, because the release rate of applied CH3Br approaches 100%, it should have a mean isotopic composition equal to industrially manufactured CH3Br, -54.4\\permil. However during soil fumigation, some of the CH3Br is consumed insitu causing isotopic fractionation

  16. Mantle transition zone input to kimberlite magmatism near a subduction zone: Origin of anomalous Nd-Hf isotope systematics at Lac de Gras, Canada

    NASA Astrophysics Data System (ADS)

    Tappe, Sebastian; Graham Pearson, D.; Kjarsgaard, Bruce A.; Nowell, Geoff; Dowall, David

    2013-06-01

    Late Cretaceous-Eocene kimberlites from the Lac de Gras area, central Slave craton, show the most extreme Nd-Hf isotope decoupling observed for kimberlites worldwide. They are characterized by a narrow range of moderately enriched Nd isotope compositions (ɛNd(i)=-0.4 to -3.5) that contrasts strongly with their moderately depleted to highly enriched ɛHf(i) values (+3.9 to -9.9). Although digestion of cratonic mantle material in proto-kimberlite melt can theoretically produce steep arrays in Nd-Hf isotope space, the amount of contaminant required to explain the Lac de Gras data is unrealistic. Instead, it is more plausible that mixing of compositionally discrete melt components within an isotopically variable source region is responsible for the steep Nd-Hf isotope array. As development of strongly negative ΔɛHf requires isotopic aging of a precursor material with Sm/Nd≫Lu/Hf for billion-year timescales, a number of models have been proposed where ancient MORB crust trapped in the mantle transition zone is the ultimate source of the extreme Hf isotope signature. However, we provide a conceptual modification and demonstrate that OIB-type domains within ancient subducted oceanic lithosphere can produce much stronger negative ΔɛHf during long-term isolation. Provided that these OIB-type domains have lower melting points compared with associated MORB crust, they are among the first material to melt within the transition zone during thermal perturbations. The resulting hydrous alkali silicate melts react strongly with depleted peridotite at the top of the transition zone and transfer negative ΔɛHf signatures to less dense materials, which can be more easily entrained within upward flowing mantle. Once these entrained refertilized domains rise above 300 km depth, they may become involved in CO2- and H2O-fluxed redox melting of upper mantle peridotite beneath a thick cratonic lid. We argue that incorporation of ancient transition zone material, which includes

  17. 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

  18. Identifying the complex melting reaction from 20 Ma to 14 Ma in Tsona leucogranite in Southern Tibet: geochemistry, zircon U-Pb chronology and Hf isotopes evidence

    NASA Astrophysics Data System (ADS)

    Shi, Qingshang; Zhao, Zhidan; Liu, Dong; Zhu, Di-Cheng

    2017-04-01

    The Miocene leucogranites, the record of the evolution of the Himalayan-Tibetan Orogen, extensively intruded the Greater Himalayan Sequence (GHS), and distributed along the South Tibetan Detachment System (STDS) (Guo and Wilson, 2012). Here we present a study of geochemistry, zircon U-Pb chronology and Hf isotopes on the Yamarong leucogranites from Tsona area, Eastern Himalaya, to explore the petrogenesis of the rocks, including melting condition and mechanism, and source of fluid within the magmatism through time. Our new results include: (1) The age of the Yamarong leucogranites range from 14 Ma to 20 Ma (YM1510-1 = 19.7 ± 0.1 Ma, n = 13; YM1502-1 = 17.5 ± 0.1 Ma, n = 12; YM1412 =14.2 ± 0.1 Ma, n = 18), which suggest that the anataxis processes have lasted for more than 6 Ma. (2) The geochemical features are different between the rocks with changing ages, especially between 20 Ma and 17 Ma. The Rb/Sr value of 20 Ma leucogranites (4.1-6.84) is lower than that of 17 Ma samples (5.12-19.02). The 20 Ma leucogranites have higher Ba contents (188-337 ppm) than that of 17 Ma rocks (50-158ppm), which exhibit different trends in the Rb/Sr versus Ba plot, and reveal different melting reaction from 20 Ma to 17 Ma. (Inger and Harris, 1993) (3) The ɛHf(t) isotopes of 20 Ma leucogranites are lower (average ɛHf(t) = -12.5) than that of 17 Ma ones (average ɛHf(t) = -10), which implies differential dissolution of inherited zircon during two partial melting events possibly due to different fluid contribution (Gao et al., 2017); (4) The positive linear relationship of LREEs versus Th in the rocks, with relatively higher contents of Th and LREEs in the 20 Ma, and lower in the 17 Ma leucogranites, which suggests the relationship were mostly controlled by monazite. And this further indicates more monazite was dissolved from the source region in the early stage (˜20Ma) than the later (17Ma) (Gao et al., 2017). In summary, our study provides new evidence for the complex melting

  19. U-Pb isotopic ages and Hf isotope composition of zircons in Variscan gabbros from central Spain: evidence of variable crustal contamination

    NASA Astrophysics Data System (ADS)

    Villaseca, Carlos; Orejana, David; Belousova, Elena; Armstrong, Richard A.; Pérez-Soba, Cecilia; Jeffries, Teresa E.

    2011-03-01

    Ion microprobe U-Pb analyses of zircons from three gabbroic intrusions from the Spanish Central System (SCS) (Talavera, La Solanilla and Navahermosa) yield Variscan ages (300 to 305 Ma) in agreement with recent studies. Only two zircon crystals from La Solanilla massif gave slightly discordant Paleoproterozoic ages (1,848 and 2,010 Ma). Hf isotope data show a relatively large variation with the juvenile end-members showing ɛHfi values as high as +3.6 to +6.9 and +1.5 to +2.9 in the Navahermosa and Talavera gabbros, respectively. These positive ɛHfi values up to +6.9 might represent the composition of the subcontinental mantle which generates these SCS gabbros. This ɛHfi range is clearly below depleted mantle values suggesting the involvement of enriched mantle components on the origin of these Variscan gabbros, and is consistent with previous whole-rock studies. The presence of zircons with negative ɛHfi values suggest variable, but significant, crustal contamination of the gabbros, mainly by mixing with coeval granite magmas. Inherited Paleoproterozoic zircons of La Solanilla gabbros have similar trace element composition (e.g. Th/U ratios), but more evolved Hf-isotope signatures than associated Variscan zircons. Similar inherited ages have been recorded in zircons from coeval Variscan granitoids from the Central Iberian Zone. Granitic rocks have Nd model ages (TDM) predominantly in the range of 1.4 to 1.6 Ga, suggesting a juvenile addition during the Proterozoic. However, Hf crustal model ages of xenocrystic Proterozoic zircons in La Solanilla gabbro indicate the presence of reworked Archean protoliths (TDM2 model ages of 3.0 to 3.2 Ga) incorporated into the hybridized mafic magma.

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

    NASA Astrophysics Data System (ADS)

    Turchyn, A. V.; Rennie, V.

    2012-12-01

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

  1. Sr isotopic constraints on the process of glauconitization

    NASA Astrophysics Data System (ADS)

    Clauer, Norbert; Keppens, Eddy; Stille, Peter

    1992-02-01

    Sr isotopic data of Holocene glauconitic pellets suggest a twofold glauconitization process that is strongly linked to the detrital clay minerals from surrounding mud which act as precursors until the K2O content of the glauconitic material reaches 4%. Above that, a detectable sea-water influence becomes progressively predominant. The process seems to start with a dissolution-crystallization reaction in equilibrium with the precursor, which is followed by a maturation process in isotopic equilibrium with sea-water Sr, while the K2O content increases further. This chemical evolutionary process may also occur for other types of clay minerals.

  2. Oxygen isotope constraints on the alteration temperatures of CM chondrites

    NASA Astrophysics Data System (ADS)

    Verdier-Paoletti, Maximilien J.; Marrocchi, Yves; Avice, Guillaume; Roskosz, Mathieu; Gurenko, Andrey; Gounelle, Matthieu

    2017-01-01

    We report a systematic oxygen isotopic survey of Ca-carbonates in nine different CM chondrites characterized by different degrees of alteration, from the least altered known to date (Paris, 2.7-2.8) to the most altered (ALH 88045, CM1). Our data define a continuous trend that crosses the Terrestrial Fractionation Line (TFL), with a general relationship that is indistinguishable within errors from the trend defined by both matrix phyllosilicates and bulk O-isotopic compositions of CM chondrites. This bulk-matrix-carbonate (BMC) trend does not correspond to a mass-dependent fractionation (i.e., slope 0.52) as it would be expected during fluid circulation along a temperature gradient. It is instead a direct proxy of the degree of O-isotopic equilibration between 17,18O-rich fluids and 16O-rich anhydrous minerals. Our O-isotopic survey revealed that, for a given CM, no carbonate is in O-isotopic equilibrium with its respective surrounding matrix. This precludes direct calculation of the temperature of carbonate precipitation. However, the O-isotopic compositions of alteration water in different CMs (inferred from isotopic mass-balance calculation and direct measurements) define another trend (CMW for CM Water), parallel to BMC but with a different intercept. The distance between the BMC and CMW trends is directly related to the temperature of CM alteration and corresponds to average carbonates and serpentine formation temperatures of 110 °C and 75 °C, respectively. However, carbonate O-isotopic variations around the BMC trend indicate that they formed at various temperatures ranging between 50 and 300 °C, with 50% of the carbonates studied here showing precipitation temperature higher than 100 °C. The average Δ17O and the average carbonate precipitation temperature per chondrite are correlated, revealing that all CMs underwent similar maximum temperature peaks, but that altered CMs experienced protracted carbonate precipitation event(s) at lower temperatures than

  3. Sr-Nd-Hf-Pb isotopic evidence for modification of the Devonian lithospheric mantle beneath the Chinese Altai

    NASA Astrophysics Data System (ADS)

    Yu, Yang; Sun, Min; Huang, Xiao-Long; Zhao, Guochun; Li, Pengfei; Long, Xiaoping; Cai, Keda; Xia, Xiaoping

    2017-07-01

    Intensive Devonian felsic magmatism is recorded within the southwestern Mongolian collage system of the Central Asian Orogenic Belt (CAOB). The voluminous magmas have isotopic compositions of juvenile materials from the mantle, thus manifesting significant mantle-crust interaction and continental growth at this time. Here, we present systematic Sr-Nd-Hf-Pb isotopic data for the Devonian mafic intrusions in the Chinese Altai, a key region within the southwestern Mongolian collage system to decipher the evolution of the mantle during this important tectonothermal event. The Keketuohai gabbro (409 ± 5 Ma) and type I mafic dykes (376 ± 5 Ma) within the Habahe complex have high (87Sr/86Sr)i, (206Pb/204Pb)t, (207Pb/204Pb)t and (208Pb/204Pb)t ratios, and decoupled Nd-Hf isotopic compositions; e.g., low εNd(t) values (- 2.5 to + 5.4) combined with high εHf(t) (+ 2.6 to + 15.1) values. These rocks have low Ba/La and high La/Yb and Th/Yb ratios, and are enriched in Pb, the light rare earth elements (LREE) and Th. They formed from magmas generated from the depleted lithospheric mantle metasomatised by hydrous melts from subducted sediments. In comparison, the gabbroic samples from the Habahe complex (369 ± 3 Ma) are enriched in the LREE, Th and Ba and have high La/Yb, Th/Yb and Ba/La ratios. They do not show significant Pb anomalies, and have depleted isotopic compositions that include low initial 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios and high εNd(t) (+ 7.4 to + 7.8) and εHf(t) (+ 13.4 to + 15.3) values. These rocks are thought to have formed from magmas derived from the lithospheric mantle metasomatised by hydrous melts from subducted oceanic crust. The type II mafic dykes within the Habahe complex are depleted in the LREE and Th, have high Ba/La ratios, and are enriched in Pb, Ba, Sr, and U. They have positive εNd(t) (+ 7.6 to + 8.1) and εHf(t) (+ 14.1 to + 15.4) values, high initial 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios

  4. Archean to Middle Proterozoic evolution of Baltica subcontinental lithosphere: evidence from combined Sm-Nd and Lu-Hf isotope analyses of the Sandvik ultramafic body, Norway

    NASA Astrophysics Data System (ADS)

    Lapen, Thomas J.; Medaris, L. Gordon; Johnson, Clark M.; Beard, Brian L.

    2005-09-01

    Combined Sm-Nd and Lu-Hf age and isotope data indicate that Mg- and Cr-rich ultramafic rocks at Sandvik, Western Gneiss Region (WGR), Norway, originated from depleted Archean lithospheric mantle that was chemically and physically modified in Middle Proterozoic time. The Sandvik outcrop consists of garnet peridotite and garnet-olivine pyroxenite and thin garnet pyroxenite layers. These contain two principal mineral assemblages: an earlier porphyroclastic assemblage of grt + opx + cpx ± ol (1,200-1,000°C, 40-50 kbar) and a later kelyphitic assemblage of grt + spl + am ± opx ± ol (700-750°C; 12-18 kbar). A CHUR Hf model age indicates a period of melt extraction at ca. 3.3 Ga for garnet peridotite, reflecting extremely high Lu/Hf ratios and very radiogenic present-day 176Hf/177HfHf=+2,165). Lu-Hf garnet-cpx-whole rock ages of two olivine-bearing samples (garnet peridotite and garnet-olivine pyroxenite) from the outcrop are ca. 1,255 Ma, whereas two olivine-free garnet pyroxenites yield Lu-Hf ages of ca. 1,185 Ma. All Sm-Nd garnet-cpx-whole rock ages of these samples are significantly younger (ca. 1,150 Ma for garnet peridotite and ca. 1,120 Ma for garnet pyroxenite). The isotope systematics indicate that the Lu-Hf ages represent cooling from an earlier period of formation/recrystallization for garnet peridotite whereas they probably reflect formation/recrystallization ages of the garnet pyroxenite. The Sm-Nd ages and isotope systematics of the garnet peridotite samples are consistent with an episode of LREE metasomatism, perhaps facilitated by a fluid of carbonatitic composition that strongly decoupled Sm-Nd and Lu-Hf. The Sm-Nd ages of the garnet pyroxenite may represent either LREE metasomatism or cooling, and, like the peridotites, Lu-Hf ages are older than Sm-Nd ages. The age data, as well as the inferred Nd isotope composition of the fluid that affected the olivine-bearing samples, suggest that these rocks were not in contact during the LREE metasomatic

  5. Hf Isotopic Geochemistry of Zircon by Multiple Collector-Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS): A New Chemical Procedure

    NASA Astrophysics Data System (ADS)

    Goolaerts, A.; Mattielli, N.; Weis, D.; Scoates, J. S.

    2001-12-01

    Zircon is the major reservoir of Hf in most crustal rocks and is characterized by low Lu/Hf. In situ analysis of zircon by laser ablation MC-ICP-MS is currently gaining favor in the geochemical community due to the precise spatial control of analysis required for zircons with complex internal morphology. However, the disadvantages of the laser ablation technique include lower precision than MC-ICP-MS, increased isobaric interferences, important memory effects, and poor control of Lu-Hf fractionation. We present a new chemical and analytical method for determining the Hf isotopic composition of zircon specifically designed for a new-generation MC-ICP-MS instrument (Nu Plasma). After dissolution, our chemical procedure involves chromatographic separation in two stages with an overall yield of ~80%: Hf-Zr are isolated from the REE using AG50W X8 resin and Lu-Yb are isolated from the lighter REE using HDEHP resin. Routine isotopic analysis of 176Hf/177Hf on the standard JMC-475 gives an internal precision of 19 ppm and an external precision of 40 ppm with a sensitivity of 20 volts/ppm for an uptake of 80 μ l/min. The average 176Hf/177Hf value for JMC-475 is 0.282162 +/- 3 (2se), which falls within error of the accepted value of 0.282163 +/- 9. The isotopic composition of Lu was carried out using a mixed Lu-Yb standard solution (AAS, Alfa Aesar) of 50 ppb with an average value for 175Lu/176Lu of 37.72 +/- 2 (2sd). Preliminary results from low-U (<100 ppm) zircon in monzonitic intrusions of the 1.43 Ga Laramie anorthosite complex, Wyoming, USA, demonstrate the extreme sensitivity of Hf isotopes in magmatic zircon to crustal contamination. Initial ɛ Hf values vary from -1 to +4 and are positively correlated with the whole rock Zr concentrations (299-2143 ppm) and negatively correlated with initial 87Sr/86Sr (0.70749-0.71295) for a constant value of initial ɛ Nd (-2.5) in the whole rocks. These correlations clearly reflect the effects of contamination by Archean crustal

  6. 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

  7. 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

  8. 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

  9. 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

  10. 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.

  11. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  13. Isotopic constraints of mantle derived carbonatitic melts from Calatrava, Spain

    NASA Astrophysics Data System (ADS)

    Humphreys, E. R.; Bailey, K.; Hawkesworth, C. J.; Wall, F.; Avanzinelli, R.

    2010-12-01

    Carbonatite volcanism is typically associated both spatially and temporally with alkaline, ultramafic volcanism (Woolley & Church, 2005). Recent discoveries in Calatrava, Spain illustrate the activity of carbonatite in the source melts of leucitite volcanism. Melilitite pyroclastic lapilli tuffs also show a clear association with volcanic carbonate. Carbonatitic activity has been shown to initiate at depths greater than 100km (Humphreys et al., 2010) despite a maximum estimate of lithospheric thickness of 80km. The presence of aragonite and abundant mantle xenoliths in many deposits are clear indication of the rapid emplacement rates of such magmas. Carbonatitic activity in the source of the leucitite melts is indicated by carbonate inclusions within olivine xenocrysts and the presence of occasional carbonatite lenses. The composition of lead and strontium isotope ratios in the bulk rock, and spatially resolved analysis of carbonate from the groundmass and from inclusions demonstrates a genetic affinity between the inclusions and the related bulk rock composition. Lead and strontium isotopic analysis suggest that such melts do not represent the composition of convecting asthenospheric mantle. 87Sr/86Sr (0.7055-0.7068) values are higher than those of MORB and most OIB. Lead isotope ratios show a trend displaced to higher 207Pb/204Pb relative to MORB and OIB. Carbonate inclusions have less radiogenic lead values than the more radiogenic bulk rocks. Our data indicate that carbonatitic activity in the mantle is intrinsic in the generation of the leucitite lava. However, petrographic and isotopic evidence suggest a complex melt history. Olivine xenocrysts are not in equilibrium with the host leucitite, despite inclusions within olivine showing an isotopic affinity to the bulk rock. We suggest that in this example, alkaline magmatism was induced by the presence of CO2 in the mantle source. Isotopic evidence shows that the mantle producing such melts was not

  14. 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

  15. 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.

  16. 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.

  17. 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.

  18. 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

  19. Mantle and crustal sources of Archean anorthosite: a combined in situ isotopic study of Pb-Pb in plagioclase and Lu-Hf in zircon

    NASA Astrophysics Data System (ADS)

    Souders, A. Kate; Sylvester, Paul J.; Myers, John S.

    2013-01-01

    Isotopic analyses of ancient mantle-derived magmatic rocks are used to trace the geochemical evolution of the Earth's mantle, but it is often difficult to determine their primary, initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration. We present in situ analyses by LA-MC-ICPMS for the Pb isotopic compositions of igneous plagioclase (An75-89) megacrysts and the Hf isotopic compositions of BSE-imaged domains of zircon grains from two mantle-derived anorthosite complexes from south West Greenland, Fiskenæsset and Nunataarsuk, which represent two of the best-preserved Archean anorthosites in the world. In situ LA-ICPMS U-Pb geochronology of the zircon grains suggests that the minimum crystallization age of the Fiskenæsset complex is 2,936 ± 13 Ma (2σ, MSWD = 1.5) and the Nunataarsuk complex is 2,914 ± 6.9 Ma (2σ, MSWD = 2.0). Initial Hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 ɛHf units. In terms of Pb isotopic compositions of plagioclase, both anorthosite complexes share a depleted mantle end member yet their Pb isotopic compositions diverge in opposite directions from this point: Fiskenæsset toward a high-μ, more radiogenic Pb, crustal composition and Nunataarsuk toward low-μ, less radiogenic Pb, crustal composition. By using Hf isotopes in zircon in conjunction with Pb isotopes in plagioclase, we are able to constrain both the timing of mantle extraction of the crustal end member and its composition. At Fiskenæsset, the depleted mantle melt interacted with an Eoarchean (~3,700 Ma) mafic crust with a maximum 176Lu/177Hf ~0.028. At Nunataarsuk, the depleted mantle melt interacted with a Hadean (~4,200 Ma) mafic crust with a maximum 176Lu/177Hf ~0.0315. Evidence from both anorthosite complexes provides support for the long-term survival of ancient mafic crusts that, although unidentified at the

  20. 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.

  1. Arc magmatism associated with steep subduction: Insights from trace element and Sr-Nd-Hf-B isotope systematics

    NASA Astrophysics Data System (ADS)

    Zhang, Yunying; Yuan, Chao; Sun, Min; Long, Xiaoping; Wang, Yunpeng; Jiang, Yingde; Lin, Zhengfan

    2017-03-01

    Subduction zones are the major sites for elemental cycling via slab dehydration and subsequent mantle metasomatism and melting. However, the nature of slab fluids associated with steep subduction remains largely unknown. To clarify this issue, we present an integrated study for Late Paleozoic (318-312 Ma) intermediate dykes from the Beishan orogenic collage, NW China. The dykes consist mainly of dioritic and granodioritic rocks. The dioritic dykes exhibit typical subduction-like geochemical signatures, together with relatively high Mg#, high ɛNd(t) and ɛHf(t), and low initial Sr isotopes, suggesting that they originated probably from a subduction-modified mantle. The granodioritic dykes exhibit high Mg#, high Sr/Y, La/Yb, and Na2O/K2O ratios, low Y and Yb contents, and mid-ocean ridge basalt-like Sr-Nd isotopes and high zircon ɛHf(t), similar to slab-derived adakite, indicating that they were likely formed by partial melting of subducted oceanic crust. The coeval adakitic and normal dioritic dykes reflect a thermal anomaly that was probably caused by rollback of subducted oceanic slab. The dioritic dykes have δ11B values from -7.7 to -6.4‰, whereas the adakitic dykes have relatively high δ11B values from -6.9 to -4.4‰. The δ11B values of adakitic dykes are lower than those of typical altered oceanic crust, in agreement with the expected loss of 11B from subducted oceanic slab during early subduction. Results of a mixing model suggest that the mantle source of the dioritic dykes has been hybridized by 11B-depleted fluids expelled from a highly dehydrated slab at deep depth, owing to the high-angle dip of the subducting oceanic slab.

  2. 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.

  3. 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.

  4. 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

  5. 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.

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

    USGS Publications Warehouse

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

    1998-01-01

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

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

    NASA Astrophysics Data System (ADS)

    Cai, Keda; Sun, Min; Xiao, Wenjiao

    2014-05-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    USGS Publications Warehouse

    Morgan, J.W.

    1985-01-01

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

  10. The Magmatic Structure of Mt. Vesuvius: Isotopic and Thermal Constraints

    NASA Astrophysics Data System (ADS)

    Civetta, L.; D'Antonio, M.; de Lorenzo, S.; Gasparini, P.

    2002-12-01

    Mt. Vesuvius is an active volcano famous for the AD 79 eruption that destroyed Pompeii, Herculaneum and Stabiae. Because of the intense urbanization around and on the volcano, the risk today is very high. Therefore, the knowledge of the structure and behavior of the magmatic system is fundamental both for the interpretation of any change in the dynamics of the volcano and for prediction of eruptions. A review of available and new isotopic data on rocks from Mt. Vesuvius, together with mineralogical and geochemical data and recent geophysical results, allow us to constrain a thermal modeling that describes history and present state of Mt. Vesuvius magmatic system. This system is formed by a "deep", complex magmatic reservoir where mantle-derived magmas arrive, stagnate and differentiate. The reservoir extends discontinuously between 10 and 20 km of depth, is hosted in densely fractured crustal rocks, where magmas and crust can interact, and has been fed more than once since 400 ka. The hypothesis of crustal contamination is favored by the high temperatures reached by crustal rocks as a consequence of repetitive intrusions of magma. From the "deep" reservoir magmas of K-basaltic to K-tephritic to K-phonotephritic composition rise to shallow depths where they stagnate at 3-5 km of depth before plinian eruptions, and through crystallization and mixing processes with the residual portion of the feeding systems, generate isotopically and geochemically layered reservoirs. Alternatively, during "open conduit" conditions deep, volatile-rich magma batches rise from the "deep" reservoir to less than 1 km of depth and mix with the crystal-rich, volatile-poor resident magma, triggering eruptions.

  11. Neodymium isotope constraints on the tectonic evolution of East Gondwana

    NASA Astrophysics Data System (ADS)

    Harris, N. B. W.; Bartlett, J. M.; Santosh, M.

    East Gondwana incorporates a collage of polymetamorphic terrains with long-lived tectonic histories from the Early Archaean to the Neoproterozoic. The oldest cratonic areas have been identified in South India (north of the Palghat-Cauvery shear zone) and East Antarctica (the Napier Complex). These terrains are remnants of an East Gondwana craton that underwent initial crustal growth during the Early Archaean and granulite-facies metamorphism at ˜2.5 Ga. Both were virtually unaffected by the Pan-African orogeny (1.1-0.5 Ga). In contrast, Proterozoic terrains were subject to high-grade metamorphism during the Pan-African event. On the basis of published Nd model ages, a direct correlation can be made between southern Madagascar (south of the Ranotsara shear zone), southern India (the Madurai Block and Kerala Khondalite Belt) and the Highland/Southwestern Complex of Sri Lanka, which comprise a Later Archaean-Palaeoproterozoic (3.2-2.0 Ga) mobile belt that may extend eastwards into East Antarctica. The youngest period of crustal growth in East Gondwana has been recognised at 1.5-0.8 Ga from isotopic studies of the Mozambique Belt of East Africa, the Vijayan Complex of Sri Lanka and the Yamato-Belgica Complex/Sør Rondane Mountains of East Antarctica. Small slivers of terrain of intermediate age (1.9-1.2 Ga) have been recognised in South India (Achankovil metasediments) and Sri Lanka (Wanni Complex) that may represent mixed-age contributions to clastic sedimentary basins.

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

    NASA Astrophysics Data System (ADS)

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

    2015-01-01

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

  13. 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.

  14. Within-Site Variation in Feather Stable Hydrogen Isotope (δ2Hf) Values of Boreal Songbirds: Implications for Assignment to Molt Origin

    PubMed Central

    Bayne, Erin M.; Sólymos, Péter; Foster, Kenneth R.; Godwin, Christine M.; Krikun, Richard; Pyle, Peter; Hobson, Keith A.

    2016-01-01

    Understanding bird migration and dispersal is important to inform full life-cycle conservation planning. Stable hydrogen isotope ratios from feathers (δ2Hf) can be linked to amount-weighted long-term, growing season precipitation δ2H (δ2Hp) surfaces to create δ2Hf isoscapes for assignment to molt origin. However, transfer functions linking δ2Hp with δ2Hf are influenced by physiological and environmental processes. A better understanding of the causes and consequences of variation in δ2Hf values among individuals and species will improve the predictive ability of geographic assignment tests. We tested for effects of species, land cover, forage substrate, nest substrate, diet composition, body mass, sex, and phylogenetic relatedness on δ2Hf from individuals at least two years old of 21 songbird species captured during the same breeding season at a site in northeastern Alberta, Canada. For four species, we also tested for a year × species interaction effect on δ2Hf. A model including species as single predictor received the most support (AIC weight = 0.74) in explaining variation in δ2Hf. A species-specific variance parameter was part of all best-ranked models, suggesting variation in δ2Hf was not consistent among species. The second best-ranked model included a forage substrate × diet interaction term (AIC weight = 0.16). There was a significant year × species interaction effect on δ2Hf suggesting that interspecific differences in δ2Hf can differ among years. Our results suggest that within- and among-year interspecific variation in δ2Hf is the most important source of variance typically not being explicitly quantified in geographic assignment tests using non-specific transfer functions to convert δ2Hp into δ2Hf. However, this source of variation is consistent with the range of variation from the transfer functions most commonly being propagated in assignment tests of geographic origins for passerines breeding in North America. PMID:27806037

  15. Within-Site Variation in Feather Stable Hydrogen Isotope (δ2Hf) Values of Boreal Songbirds: Implications for Assignment to Molt Origin.

    PubMed

    Nordell, Cameron J; Haché, Samuel; Bayne, Erin M; Sólymos, Péter; Foster, Kenneth R; Godwin, Christine M; Krikun, Richard; Pyle, Peter; Hobson, Keith A

    2016-01-01

    Understanding bird migration and dispersal is important to inform full life-cycle conservation planning. Stable hydrogen isotope ratios from feathers (δ2Hf) can be linked to amount-weighted long-term, growing season precipitation δ2H (δ2Hp) surfaces to create δ2Hf isoscapes for assignment to molt origin. However, transfer functions linking δ2Hp with δ2Hf are influenced by physiological and environmental processes. A better understanding of the causes and consequences of variation in δ2Hf values among individuals and species will improve the predictive ability of geographic assignment tests. We tested for effects of species, land cover, forage substrate, nest substrate, diet composition, body mass, sex, and phylogenetic relatedness on δ2Hf from individuals at least two years old of 21 songbird species captured during the same breeding season at a site in northeastern Alberta, Canada. For four species, we also tested for a year × species interaction effect on δ2Hf. A model including species as single predictor received the most support (AIC weight = 0.74) in explaining variation in δ2Hf. A species-specific variance parameter was part of all best-ranked models, suggesting variation in δ2Hf was not consistent among species. The second best-ranked model included a forage substrate × diet interaction term (AIC weight = 0.16). There was a significant year × species interaction effect on δ2Hf suggesting that interspecific differences in δ2Hf can differ among years. Our results suggest that within- and among-year interspecific variation in δ2Hf is the most important source of variance typically not being explicitly quantified in geographic assignment tests using non-specific transfer functions to convert δ2Hp into δ2Hf. However, this source of variation is consistent with the range of variation from the transfer functions most commonly being propagated in assignment tests of geographic origins for passerines breeding in North America.

  16. Water mass circulation and weathering inputs in the Labrador Sea based on coupled Hf-Nd isotope compositions and rare earth element distributions

    NASA Astrophysics Data System (ADS)

    Filippova, Alexandra; Frank, Martin; Kienast, Markus; Rickli, Jörg; Hathorne, Ed; Yashayaev, Igor M.; Pahnke, Katharina

    2017-02-01

    The Labrador Sea is one of the key areas for deep water formation driving the Atlantic thermohaline circulation and thus plays an important role in Northern Hemisphere climatic fluctuations. In order to better constrain the overturning processes and the origins of the distinct water masses, combined dissolved Hf-Nd isotopic compositions and rare earth element (REE) distribution patterns were obtained from four water depth profiles along a section across the Labrador Sea. These were complemented by one surface sample off the southern tip of Greenland, three shallow water samples off the coast of Newfoundland, and two deep water samples off Nova Scotia. Although light REEs are markedly enriched in the surface waters off the coast of Newfoundland compared to north Atlantic waters, the REE concentration profiles are essentially invariant throughout the water column across the Labrador Sea. The hafnium concentrations of surface waters exhibit a narrow range between 0.6 and 1 pmol/kg but are not significantly higher than at depth. Neodymium isotope signatures (ɛNd) vary from unradiogenic values between -16.8 and -14.9 at the surface to more radiogenic values near -11.0 at the bottom of the Labrador Sea mainly reflecting the advection of the Denmark Strait Overflow Water and North East Atlantic Deep Water, the signatures of which are influenced by weathering contributions from Icelandic basalts. Unlike Nd, water column radiogenic Hf isotope signatures (ɛHf) are more variable representing diverse weathering inputs from the surrounding landmasses. The least radiogenic seawater ɛHf signatures (up to -11.7) are found in surface waters close to Greenland and near the Canadian margin. This reflects the influence of recirculating Irminger Current Waters, which are affected by highly unradiogenic inputs from Greenland. A three to four ɛHf unit difference is observed between Denmark Strait Overflow Water (ɛHf ∼ -4) and North East Atlantic Deep Water (ɛHf ∼ -0

  17. 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

  18. Constraints on the development of the early continental crust from isotopic data

    NASA Technical Reports Server (NTRS)

    Jacobson, S. B.

    1985-01-01

    Aspects of the origin and development of the early (AE) continential crust are addressed by radiogenic isotope and trace element studies. The most important ones are: (1) at what time did the earliest continental crust form; (2) what was its composition; (3) by what processes did it grow and by what processes was it destroyed; (4) what were the rates of production and destruction as a function of time during this time period? Nd is isotopic data on the oldest terrestrial rocks indicate that the mantle at this time had already suffered substantial depletion in incompatible elements due to earlier continent forming events. Isotopic data on young volcanic rocks derived from the depleted mantle show no evidence of this early history. The observed isotopic patterns of Nd, Sr, Hf and Pb through time together with the presently observed age spectrum of crustal rocks are considered. These patterns can be modelled by a transport model in which the continental growth and destruction rates are allowed to vary as a function of time. It is suggest that the mass of the continents at 3.8 AE ago was about 25% of the current continental mass. However, due to the very high recycling rates obtained in the early Archean only a few percent of this crust has been preserved up to the present.

  19. Constraints on the development of the early continental crust from isotopic data

    NASA Technical Reports Server (NTRS)

    Jacobson, S. B.

    1985-01-01

    Aspects of the origin and development of the early (AE) continential crust are addressed by radiogenic isotope and trace element studies. The most important ones are: (1) at what time did the earliest continental crust form; (2) what was its composition; (3) by what processes did it grow and by what processes was it destroyed; (4) what were the rates of production and destruction as a function of time during this time period? Nd is isotopic data on the oldest terrestrial rocks indicate that the mantle at this time had already suffered substantial depletion in incompatible elements due to earlier continent forming events. Isotopic data on young volcanic rocks derived from the depleted mantle show no evidence of this early history. The observed isotopic patterns of Nd, Sr, Hf and Pb through time together with the presently observed age spectrum of crustal rocks are considered. These patterns can be modelled by a transport model in which the continental growth and destruction rates are allowed to vary as a function of time. It is suggest that the mass of the continents at 3.8 AE ago was about 25% of the current continental mass. However, due to the very high recycling rates obtained in the early Archean only a few percent of this crust has been preserved up to the present.

  20. 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.

  1. 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

  2. Evidence for Slab Melt Contributions to the Mexican Volcanic Belt and Other Young Hot Slab Arcs from Lu-Hf Isotopes

    NASA Astrophysics Data System (ADS)

    Goldstein, S. L.; Cai, Y. M.; Langmuir, C. H.; Lagatta, A.; Straub, S. M.; Gomez-Tuena, A.; Martin Del Pozzo, A.

    2007-12-01

    Despite major advances in delineating the processes that govern magma generation at convergent margins, the problem persists of distinguishing slab, mantle wedge, and crustal contributions. A corrollary question is whether there is significant melting of subducted ocean crust. Especially in thick crust regions, the importance of crustal versus mantle contributions to lavas represents a long-standing fundamental issue in arc magma geochemistry. We show that frontal arc magmas from the Central Mexican Volcanic Belt (CMVB), including the large andesitic stratovolcanoes Popocatepetl and Nevado de Toluca, display negligible crustal contamination, and contain substantial contributions from melting of subducted Pacific ocean crust. Despite ca. 50 km thick continental crust, the CMVB erupts near primitive lavas including "high-Nb" alkaline basalts that show negligible "subduction signatures" in their trace element patterns. These "high-Nb" basalts define the regional mantle wedge composition in isotope-trace element space. The "normal" calcalkaline lavas form a negative correlation between Hf isotopes and Lu/Hf. One endmember is like the high Nb basalts representing the regional mantle wedge. The other endmember has higher Hf isotopes (approaching values of Pacific MORB) and very low Lu/Hf of less than 0.04 (e.g. compared to typical values of ca. 0.2 in Pacific MORB). The low Lu/Hf values require low degree partial melting of a source rich in garnet. The high Hf isotopes require a depleted mantle source with isotopes like Pacific MORB. Together the Lu-Hf data indicate a substantial component derived from melting of eclogitic Pacific ocean crust. A key feature of the data is that the stratovolcano lavas showing the largest slab melt signature also show the highest Hf isotope ratios and thus are more "depleted mantle-like" than the regional mantle wedge. Thus, the integrated data allow us to clearly distinguish between mantle and crustal sources in the CMVB and point to

  3. Petrology and geochemistry Toro Ankole kamafugite magmas: isotopic constraints.

    NASA Astrophysics Data System (ADS)

    Muravyeva, N. S.; Belyatsky, B. V.

    2009-04-01

    samples resembles the kimberlite trend. The temperature and oxygen fugacity of olivine-spinel equilibrium were calculated (Ballhaus et al., 1991) use data on mineral compositions. The pressure of phenocrysts crystallization was estimated with the Cpx-geobarometer. The obtained results show that the kamafugites crystallization passed over the wide temperature interval - 1300-716оC and fO2 exceeding buffer QFM (0.8<ΔQFM<3.6), that is considerably higher the same of most basalts and unchanged upper mantle material. The presence of the sulfate microinclusions in the mafurite olivine confirms the correct results, obtained by olivine-spinel method. High oxygen fugacity values for the primitive melts may be mainly explained by the special features of the upper mantle composition where metasomatism processes were intensively developed and usually accompanied by oxidation of the source. Sr and Nd isotope signatures for kamafugites form a slightly enriched relative to BSE cluster (87Sr/86Sr = 0.704629 - 0.705356; 143Nd/144Nd = 0.512488 - 0.512550). Some inverse correlation with major element contents is observed: 87Sr/86Sr - CaO, 143Nd/144Nd - Mg#, 87Sr/86Sr - Mg#. Sr-Nd isotope data of Bunyaruguru kamafugites suggest that its mantle source composition is nearly EM1. But the range of Pb composition for investigated is 206Pb/204Pb: 18.998 - 19.566; 207Pb/204Pb: 15.686 - 15.737; 208Pb/204Pb: 39.303 - 40.264. On these data the mantle source composition for studied kamafugites is close to EM2 or Dupal characteristics. Reference: Ballhaus C., Berry R.F., Green D.H. High pressure experimental calibration of the olivine-orthopyroxene-spinel oxygen geobarometer: implications for the oxidation state of the upper mantle 1991. Contrib. Mineral. Petrol. vol.107, p.27 - 40.

  4. 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.

  5. Crustal contamination and mantle source characteristics in continental intra-plate volcanic rocks: Pb, Hf and Os isotopes from central European volcanic province basalts

    NASA Astrophysics Data System (ADS)

    Jung, S.; Pfänder, J. A.; Brauns, M.; Maas, R.

    2011-05-01

    We report new Os-Pb-Hf isotope data for a suite of alkaline to basaltic (nephelinites, basanites, olivine tholeiites to quartz-tholeiites) lavas from the Miocene Vogelsberg (Germany), the largest of the rift-related continental volcanic complexes of the Central European Volcanic Province (CEVP). 187Os/ 188Os in primitive (high-MgO) alkaline lavas show a much wider range than has been observed in alkaline basalts and peridotite xenoliths from elsewhere in the CEVP, from ratios similar to those in modern MORB and OIB (0.1260-0.1451; 58.9-168 ppt Os) to more radiogenic ratios (0.1908 and 0.2197; 27.6-15.1 ppt Os). Radiogenic Os is associated with high ɛ Hf and ɛ Nd, low 87Sr/ 86Sr and does not correlate with Mg ∗ or incompatible trace elements (e.g. Ce/Pb), suggesting the presence of a radiogenic endmember in the mantle rather than crustal contamination as the source of radiogenic Os. This contrasts with another high-Mg alkaline lava characterized by highly radiogenic 187Os/ 188Os (0.4344, 10.3 ppt Os), lower ɛ Hf and ɛ Nd, higher 87Sr/ 86Sr, and Pb isotope signatures than the other alkaline lavas with similar trace element composition suggestive of contamination with crustal material. Hafnium (ɛ Hf: +8.9 to +5.0) and Pb isotope compositions ( 206Pb/ 204Pb: 19.10-19.61; 207Pb/ 204Pb: 15.56-15.60) of the alkaline rocks fall within the range of enriched MORB and some OIB. The Vogelsberg tholeiites show even more diverse 187Os/ 188Os, ranging from 0.1487 in Os-rich olivine tholeiite (31.7 ppt) to ratios as high as 0.7526 in other olivine-tholeiites and in quartz-tholeiites with lower Os concentrations (10.3-2.0 ppt). Low- 187Os/ 188Os tholeiites show Pb-Hf isotope ratios ( 206Pb/ 204Pb:18.81; 207Pb/ 204Pb: 15.61; ɛ Hf: +2.7) that are distinct from those in alkaline lavas with similar 187Os/ 188Os and originate from a different mantle source. By contrast, the combination of radiogenic Os and low 206Pb/ 204Pb and ɛ Hf in the other tholeiites probably reflects

  6. Production cross sections of niobium and tantalum isotopes in proton-induced reactions on (nat)Zr and (nat)Hf up to 14 MeV.

    PubMed

    Murakami, M; Haba, H; Goto, S; Kanaya, J; Kudo, H

    2014-08-01

    Production cross sections of Nb and Ta isotopes in the proton-induced reactions on (nat)Zr and (nat)Hf, respectively, were measured up to 14 MeV using a stacked-foil technique. The observed nuclides in the (nat)Zr(p,x) reactions were (90g,91m,92m,95m,95g,96)Nb, (95)Zr, and (87g,88)Y. In the (nat)Hf(p,x) reactions, (175,176,177,178,179)Ta and (175)Hf were observed. The obtained cross sections for each nuclide were compared with the previously reported data and with the theoretical cross sections calculated by the TALYS-1.4 code. Thick-target yields of the observed nuclides were deduced from the measured production cross sections. Copyright © 2014 Elsevier Ltd. All rights reserved.

  7. Evidence of early Archean crust in northwest Gondwana, from U-Pb and Hf isotope analysis of detrital zircon, in Ediacaran surpacrustal rocks of northern Spain

    NASA Astrophysics Data System (ADS)

    Naidoo, Thanusha; Zimmermann, Udo; Vervoort, Jeff; Tait, Jenny

    2017-06-01

    The Mora Formation (Narcea Group) is one of the oldest Precambrian supracrustal successions in northern Spain. Here, we use U-Pb and in situ Hf isotope analysis on detrital zircon to determine its age and provenance. The youngest U-Pb dates constrain the maximum depositional age of the Mora Formation at 565 ± 11 Ma. Results indicate: (1) a dominant Ediacaran zircon population (33%; 565-633 Ma, Cadomian) within a spectrum of Neoproterozoic ages (40%; 636-996 Ma); and (2) smaller Mesoproterozoic (5%; 1004-1240 Ma), Palaeoproterozoic (11%; 1890-2476 Ma) and Archean (11%; 2519-3550 Ma) populations. Results here do not point to one specific cratonic source area; instead, detritus may have been derived from the West African craton and Amazonia, or even the concealed Iberian basement. The lack of 1.3-1.8 Ga grains suggests exclusion of the Sahara Craton as a major source, but this is not certain. This mixed composition favours a complex source history with reworking of detritus across terrane/craton boundaries. Hafnium isotope compositions indicate a range of crustal and juvenile sources, with initial ɛHf values between -15.8 and 11.1, and Hf model ages from 0.8 to 3.7 Ga. For Neoproterozoic zircons (80%), juvenile components (ɛHf(i) +10) may be related to Rodinia fragmentation and the onset of an active margin setting leading to the Cadomian orogeny. Palaeoproterozoic to Paleoarchean grains (20%) all have negative ɛHf values and Meso- to Eoarchean Hf model ages. This indicates an early (Archean) sialic crustal component for northwestern Gondwana.

  8. Effect of reagent vibrational excitation and isotope substitution on the stereo-dynamics of the Ba + HF → BaF + H reaction

    NASA Astrophysics Data System (ADS)

    Zhao, Juan; Luo, Yi

    2011-04-01

    Based on an extended London—Eyring—Polanyi—Sato (LEPS) potential energy surface (PES), the Ba + HF reaction has been studied by the quasi-classical trajectory (QCT) method. The reaction integral cross section as a function of collision energy for the Ba + HF → BaF + H reaction is presented and the influence of isotope substitution on the differential cross sections (DCSs) and alignments of the product's rotational angular momentum have also been studied. The results suggest that the integral cross sections increase with increasing collision energy, and the vibrational excitation of the reagent has great influence on the DCS. In addition, the product's rotational polarization is very strong as a result of heavy-heavy-light (HHL) mass combination, and the distinct effect of isotope substitution on the stereodynamics is also revealed.

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

    NASA Astrophysics Data System (ADS)

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

    2011-12-01

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

  10. Coupled zircon Lu-Hf and U-Pb isotopic analyses of the oldest terrestrial crust, the >4.03 Ga Acasta Gneiss Complex

    NASA Astrophysics Data System (ADS)

    Bauer, Ann M.; Fisher, Christopher M.; Vervoort, Jeffrey D.; Bowring, Samuel A.

    2017-01-01

    The Acasta Gneiss Complex of the Northwest Territories, Canada, contains some of the earliest terrestrial continental crust and thus provides a critical sample set for characterization of crust-forming processes on the early Earth. Here we report the results of a combined Lu-Hf and U-Pb isotopic study of zircons from predominantly felsic orthogneisses from the Acasta Gneiss Complex that crystallized between ∼4.0 and 2.9 Ga, many of which contain complex zoning and therefore require an analytical treatment suited to distinguish amongst compositionally distinct age and Hf isotopic domains. To ensure the reliability of the analyses and of subsequent geologic interpretations, we employed the laser ablation split-stream (LASS) technique to concurrently measure the Lu-Hf and U-Pb isotopic systems in zircon. Our results confirm prior findings of precursor Hadean crust (>4.0 Ga) in the source of these rocks and the continued involvement of this reservoir until ∼3.6 Ga. We present evidence for the input of relatively more juvenile material at ∼3.6 Ga, which we suggest corresponds to a fundamental change in the source of the magmas. This study extends the lower bound of the published Acasta Hf isotopic record from 3.6 Ga to 2.9 Ga and demonstrates that the ∼3.6 Ga-2.9 Ga interval is largely represented by reworking of relatively juvenile ∼3.6 Ga crust and the diminution of the >4.0 Ga crustal signal. Significantly, there is no evidence that rocks within the Acasta Gneiss Complex were derived from a strongly depleted mantle.

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

    NASA Astrophysics Data System (ADS)

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

    2010-11-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2010-12-01

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

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

    NASA Astrophysics Data System (ADS)

    Nawrat, Joscha; Bahlburg, Heinrich; Axel, Gerdes

    2013-04-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2012-12-01

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

  16. Assessing the isotopic evolution of S-type granites of the Carlos Chagas Batholith, SE Brazil: Clues from U-Pb, Hf isotopes, Ti geothermometry and trace element composition of zircon

    NASA Astrophysics Data System (ADS)

    Melo, Marilane G.; Lana, Cristiano; Stevens, Gary; Pedrosa-Soares, Antônio C.; Gerdes, Axel; Alkmin, Leonardo A.; Nalini, Hermínio A.; Alkmim, Fernando F.

    2017-07-01

    The Carlos Chagas batholith (CCB) is a very large ( 14,000 km2) S-type granitic body formed during the syn-collisional stage of the Araçuaí orogen (southeastern Brazil). Zircons extracted from the CCB record a wide range of U-Pb ages (from 825 to 490 Ma), indicating a complex history of inheritance, magmatic crystallization and partial melting during the evolution of the orogeny. Magmatic zircons (ca. 578-588 Ma) are marked by similar Hf isotope compositions and REE patterns to those of inherited cores (ca. 825-600 Ma), indicating that these aspects of the chemical signature of the magmatic zircons have likely been inherited from the source. The U-Pb ages and initial 176Hf/177Hf ratios from anatectic and metamorphic zircon domains are consistent with a two-stage metamorphic evolution marked by contrasting mechanisms of zircon growth and recrystallization during the orogeny. Ti-in-zircon thermometry is consistent with the findings of previous metamorphic work and indicates that the two metamorphic events in the batholith reached granulite facies conditions (> 800 °C) producing two generations of garnet via fluid-absent partial melting reactions. The oldest metamorphic episode (ca. 570-550 Ma) is recorded by development of thin anatectic overgrowths on older cores and by growth of new anatectic zircon crystals. Both domains have higher initial 176Hf/177Hf values compared to relict cores and display REE patterns typical of zircon that grew contemporaneously with peritectic garnet through biotite-absent fluid partial melting reactions. Hf isotopic and chemical evidences indicate that a second anatectic episode (ca. 535-500 Ma) is only recorded in parts from the CCB. In these rocks, the growth of new anatectic zircon and/or overgrowths is marked by high initial 176Hf/177Hf values and also by formation of second generation of garnet, as indicated by petrographic observations and REE patterns. In addition, some rocks contain zircon crystals formed by solid

  17. 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

  18. 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.

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

    NASA Astrophysics Data System (ADS)

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

    2008-12-01

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

  20. 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

  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. Lead isotopic signatures in Antarctic marine sediment cores: a comparison between 1M HCl partial extraction and HF total digestion pre-treatments for discerning anthropogenic inputs.

    PubMed

    Townsend, A T; Snape, I; Palmer, A S; Seen, A J

    2009-12-20

    Sensitive analytical techniques are typically required when dealing with samples from Antarctica as even low concentrations of contaminants can have detrimental environmental effects. Magnetic Sector ICP-MS is an ideal technique for environmental assessment as it offers high sensitivity, multi-element capability and the opportunity to determine isotope ratios. Here we consider the Pb isotope record of five marine sediment cores collected from three sites in the Windmill Islands area of East Antarctica: Brown Bay adjacent to the current Australian station Casey, Wilkes near the abandoned US/Australian Station and McGrady Cove lying midway between the two. Two sediment pre-treatment approaches were considered, namely partial extraction with 1M HCl and total dissolution involving HF. Lead isotope ratio measurements made following sediment partial extraction provided a more sensitive indication of Pb contamination than either Pb concentrations alone (irrespective of sample pre-treatment method) or isotope ratios made after HF digestion, offering greater opportunity for discrimination between impacted and natural/geogenic samples and sites. Over 90% of the easily extractable Pb from sediments near Casey was anthropogenic in origin, consisting of Pb from major Australian deposits. At Wilkes impact from discarded batteries with a unique isotopic signature was found to be a key source of Pb contamination to the marine environment with ~70-80% of Pb being anthropogenic in origin. The country and source of origin of these batteries remain unknown. Little evidence was found suggesting contamination at Wilkes by Pb originating from the major US source, Missouri. No definitive assessment could be made regarding Pb impact at McGrady Cove as the collected sediment core was of insufficient depth. Although Pb isotope ratio signatures may indicate anthropogenic input, spatial concentration gradients at nearby Brown Bay suggest contamination at McGrady Cove is unlikely. We recommend

  3. 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

  4. 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.

  5. 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

    2013-04-01

    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). 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 Ma; re-calculated in Kinoshita et al., 2012) should 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 "low-TiO2" amphibolites; this is attributable to an ancient multi-stage history of their mantle source as indicated by rare-earth element patterns. Modeling shows that the 142Nd/144Nd deficits could have developed in response to a re-fertilization episode within a mantle domain depleted by primordial crust extraction at 4510 Ma.

  6. 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

  7. 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.

  8. 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.

  9. The origin of an oceanic plateau: Isotope geochemistry (Sr, Nd, Pb and Hf) of volcanic rocks from IODP Site U1347 and ODP Site 1213 (Hf data) on the Shatsky Rise (Northwest Pacific)

    NASA Astrophysics Data System (ADS)

    Heydolph, K.; Geldmacher, J.; Hoernle, K.

    2011-12-01

    K.HEYDOLPH1*, J.GELDMACHER2, 1 ,K.HOERNLE1 1IFM-GEOMAR, Wischhofstr. 1-3. D-24148 Kiel, Germany, (*correspondence: kheydolph@ifm-geomar.de) 2 Integrated Ocean Drilling Program, Texas A&M University, 1000 Discovery Drive, College Station, Texas 77845-9547 (geldmacher@iodp.tamu.edu) The submarine Shatsky Rise plateau, a unique large igneous province (LIP) in the northwest Pacific Ocean ca. 1500 km east of Japan, is the only large intraoceanic plateau, which formed during the Late Jurassic to Early Cretaceous at a time period with numerous reversals of the Earth's magnetic field. These magnetic reversals combined with bathymetric data allow a detailed reconstruction of the tectonic history. Accordingly the three main volcanic edifices Tamu, Ori and Shirshov massifs formed by massive volcanism during a short time span along a southwest - northeast trending, rapidly spreading triple junction. Therefore, the magnetic and bathymetric data suggest that the Shatsky Rise formed through the interaction of a mantle plume head with a ridge [1, 2]. We present new Sr, Nd and Pb (double spike) and for the first time Hf isotope data from volcanic rocks of relatively fresh basaltic lava flows from recent IODP Exp. 324 Site U1347 and ODP Leg 198 Site 1213 (Hf data) both located on Tamu massif the southernmost (oldest) volcanic edifice of Shtasky Rise. Initial 176Hf/177Hf and 143Nd/144Nd isotopic compositions are fairly uniform throughout the entire holes ranging between 0.283076 to 0.283100 and 0.512903 to 0.512981 respectively, showing neither distinct MORB nor intraplate (plume) affinity. Relatively unradiogenic 87Sr/86Sr data ranging from 0.70276 to 0.70296 mostly overlaps with Pacific MORB like values. In a Nd vs Hf isotope plot they form a tight cluster at the edge of the Pacific MORB field below the present-day Hf-Nd mantle array. Although initial Pb double spike 206Pb/204Pb and 208Pb/204Pb isotopic compositions for Site U1347 range from 18.13 to 18.46 and 37.71 to 37

  10. 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

  11. 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

    2014-01-07

    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.

  12. Oxygen Isotopic Constraints on the Genesis of Carbonates from Martian Meteorite ALH84001

    NASA Technical Reports Server (NTRS)

    Leshin, Laurie A.; McKeegan, Kevin D.; Carpenter, Paul K.; Harvey, Ralph P.

    1998-01-01

    Ion microprobe oxygen isotopic measurements of a chemically diverse suite of carbonates from Martian meteorite ALH84001 are reported. The delta(sup 18)O values are highly variable, ranging from +5.4 to + 25.3%, and are correlated with major element compositions of the carbonate. The earliest forming (Ca-rich) carbonates have the lowest delta(sup 18)O values and the late-forming (Mg-rich) carbonates have the highest delta(sup 18)O values. Two models are presented which can explain the isotopic variations. The carbonates could have formed in a water rich environment at relatively low, but highly variable temperatures. In this open-system case the lower limit to the temperature variation is approx. 125 C, with fluctuations of over 250 C possible within the constraints of the model. Alternatively, the data can be explained by a closed-system model in which the carbonates precipitated from a limited amount of CO2-rich fluid. This scenario can reproduce the isotopic variations observed at a range of temperatures, including relatively high temperatures (less than 500 C). Thus the oxygen isotopic compositions do not provide unequivocal evidence for formation of the carbonates at low temperature. Although more information is needed in order to distinguish between the models, neither of the implied environments is consistent with biological activity. Thus, we suggest that features associated with the carbonates which have been interpreted to be the result of biological activity were most probably formed by inorganic processes.

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

    NASA Astrophysics Data System (ADS)

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

    2016-04-01

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

  14. 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.

  15. 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

  16. 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

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

    NASA Astrophysics Data System (ADS)

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

    1988-04-01

    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 ( Δ qz - mt=10.0‰) within 2 3 meters of the orthogneiss contact to 600° C ( Δ 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 δ 18Owr value of 8.0±0.6‰. The greater Δ 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 α-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 ( D 0), consistent with the experimentally determined values, that will result in reasonable cooling rates for the Wind River iron formation. The discrepancy between the

  18. Tracking the influence of a continental margin on growth of a magmatic arc, Fiordland, New Zealand, using thermobarometry, thermochronology, and zircon U-Pb and Hf isotopes

    NASA Astrophysics Data System (ADS)

    Scott, J. M.; Cooper, A. F.; Palin, J. M.; Tulloch, A. J.; Kula, J.; Jongens, R.; Spell, T. L.; Pearson, N. J.

    2009-12-01

    Geothermobarometric, radiogenic isotopic and thermochronologic data are used to track the influence of an ancient continental margin (Western Province) on development of an adjacent Carboniferous-Cretaceous magmatic arc (Outboard Median Batholith) in Fiordland, New Zealand. The data show a record of complicated Mesozoic Gondwana margin growth. Paragneiss within the Outboard Median Batholith is of Carboniferous to Jurassic age and records burial to middle crustal depths in Late Jurassic-Early Cretaceous during subduction-related plutonism and arc thickening. In contrast, Western Province metasedimentary rocks in the area of study immediately west of the Outboard Median Batholith are Late Cambrian-Early Ordovician in age, recrystallized at the amphibolite facies in the Late Devonian-Early Carboniferous and exhibit no evidence for Mesozoic textural or isotopic reequilibration. A phase of deformation, between 128 and 116 Ma deformed, exhumed, and cooled the Outboard Median Batholith to greenschist facies temperatures, while large parts of the Western Province underwent ≥9 kbar metamorphic conditions. Zircon grains from Mesozoic inboard plutons are isotopically more evolved (ɛHf(t) = +2.3 to +4.0) than those in the Outboard Median Batholith (ɛHf(t) = +9.4 to +11.1). The contrasting zircon Hf isotope ratios, absence of S-type plutons or Proterozoic-Early Paleozoic inherited zircon, and the apparent absence of Early Paleozoic metasedimentary rocks indicates that the Outboard Median Batholith is unlikely to be underlain by the Western Province continental lithosphere. The new data are consistent with the Outboard Median Batholith representing an allochthonous (although not necessarily exotic) arc that was juxtaposed onto the Gondwana continental margin along the intervening Grebe Mylonite Zone.

  19. Do S-type granites commonly sample infracrustal sources? New results from an integrated O, U-Pb and Hf isotope study of zircon

    NASA Astrophysics Data System (ADS)

    Appleby, Sarah K.; Gillespie, Martin R.; Graham, Colin M.; Hinton, Richard W.; Oliver, Grahame J. H.; Kelly, Nigel M.

    2010-07-01

    In contrast to I-type granites, which commonly comprise infracrustal and supracrustal sources, S-type granites typically incorporate predominantly supracrustal sources. The initial aim of this study was to identify the sources of three Scottish Caledonian (~460 Ma) S-type granites (Kemnay, Cove and Nigg Bay) by conducting oxygen, U-Pb and Hf isotope analyses in zircon in order to characterise one potential end-member magma involved in the genesis of the voluminous late Caledonian (~430-400 Ma) I-type granites. Field, whole-rock geochemical and isotopic data are consistent with the generation of the S-type granites by melting their Dalradian Supergroup country rocks. While Hf isotope compositions of magmatic zircon, U-Pb data of inherited zircons, and high mean zircon δ18O values of 9.0 ± 2.7‰ (2SD) and 9.8 ± 2.0‰ for the Kemnay and Cove granites support this model, the Nigg Bay Granite contains zircons with much lower δ18O values (6.8 ± 2.1‰), similar to those found in Scottish I-type granites. This suggests that the Nigg Bay Granite contains low-δ18O material representing either altered supracrustal material, or more likely, an infracrustal source component with mantle-like δ18O. Mixing trends in plots of δ18O vs. ɛHf for S-type granite zircons indicate involvement of at least two sources in all three granites. This pilot study of Scottish Caledonian S-type granites demonstrates that, while field and whole-rock geochemical data are consistent with local melting of only supracrustal sources, the oxygen isotopic record stored in zircon reveals a much more complex petrogenetic evolution involving two or more magma sources.

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

    NASA Astrophysics Data System (ADS)

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

    2008-12-01

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

  1. Characterising the continental crust factory: new insights into the roots of an island arc from Hf isotopes in rutile (Kohistan complex, Pakistan)

    NASA Astrophysics Data System (ADS)

    Ewing, Tanya; Müntener, Othmar; Schaltegger, Urs

    2017-04-01

    Island arcs are one of the primary sites of generation of new continental crust. As such, a question of fundamental importance to models of continental growth is to what extent island arc magmas are strictly juvenile melts derived directly from the mantle, versus potentially incorporating a significant recycled continental component, for example from subducted sediment. The Kohistan complex (northeastern Pakistan) preserves a remarkably complete ˜50 km thick cross-section through an exhumed Jurassic-Cretaceous island arc. It affords a rare opportunity to study the evolution of island arc magmatism from subduction initiation, through intra-oceanic subduction, to arc-continent collision. In this study, we investigate the ultramafic-mafic Jijal Complex, which preserves part of the plutonic roots of the Kohistan complex formed over ˜20 Ma of intra-oceanic subduction. The Jijal Complex is volumetrically dominated by ultramafic rocks and garnet-bearing gabbros whose petrogenesis is controversial. Garnet formation has variously been attributed a prograde metamorphic origin1, a magmatic origin recording crystallisation at high pressures2,3, or a restitic origin following partial melting4. We have characterised the source of the Jijal Complex using in situ LA-MC-ICPMS determination of the Hf isotope composition of rutile from garnet gabbros, which are zircon-free. This work exploits the superior sensitivity of the Neptune Plus, coupled with an improved analytical protocol, to improve precision of this novel technique and permit in situ analysis of rutile with only ˜10-30 ppm Hf. Rutile occurs included in early-formed minerals such as clinopyroxene and garnet, indicating crystallisation at high pressures and temperatures. Rutile from all samples, collected across ˜3 km of former crustal depth, has indistinguishable Hf isotope compositions close to depleted mantle values. Integrating the new Hf isotope data for rutile with previously published whole rock Nd-Sr isotope

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

    PubMed

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

    2007-07-15

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

  3. Petrogenesis of the Cretaceous Zhangzhou batholith in southeastern China: Zircon U-Pb age and Sr-Nd-Hf-O isotopic evidence

    NASA Astrophysics Data System (ADS)

    Chen, Jing-Yuan; Yang, Jin-Hui; Zhang, Ji-Heng; Sun, Jin-Feng; Wilde, Simon A.

    2013-03-01

    Whole-rock geochemical and Sr-Nd-Hf isotopic data and in situ zircon U-Pb and Hf-O isotopes have been determined for mafic (gabbro and diorite) and felsic (I- and A-type granites) rocks from the Zhangzhou batholith in southeastern (SE) China, in order to constrain their source and petrogenesis. The batholith consists of gabbro, diorite, granodiorite, monzogranite and alkali feldspar granite, with mafic microgranular enclaves in the monzogranite. Zircon SIMS and LA-ICP-MS U-Pb dating gives consistent emplacement ages of 107-97 Ma for these rocks, establishing that the mafic and felsic magmas were coeval. The gabbros and diorites have relatively high MgO contents (up to 5.2 wt.%) at low silica concentrations (up to 49.9 wt.%), with relatively homogeneous whole-rock initial 87Sr/86Sr ratios (~ 0.706), negative ɛNd(t) values of - 3.4 to - 2.7, chondrite-like ɛHf(t) values of - 0.3 to + 0.5, zircon ɛHf(t) values of - 0.8 to + 3.2 and δ18O values of + 5.0 to + 6.1‰, indicating that they were derived by partial melting of an enriched subcontinental lithospheric mantle, coupled with olivine and pyroxene fractionation. The calc-alkaline granodiorites and monzogranites are metaluminous and have relatively high SiO2 and low MgO contents. They have whole-rock initial 87Sr/86Sr ratios of 0.706, ɛNd(t) ratio of - 3.0 to - 4.0, ɛHf(t) values of - 2.0 to + 0.3 and zircon ɛHf(t) values of - 4.4 to + 0.3, with Nd and Hf model ages of about 1.3 Ga, indicating that they were mainly derived by partial melting of old continental crustal materials. The alkali feldspar granites have geochemical features similar to A-type granite, with relatively high K2O + Na2O and Nb contents and FeO/MgO and Ga/Al ratios. They have distinct isotopic compositions from the associated mafic rocks, with ɛNd(t) values of - 4.9, ɛHf(t) values of - 3.2 to - 3.5, zircon ɛHf(t) values of - 5.1 to + 1.0 and δ18O values of + 5.1‰ to + 6.3‰. They are depleted in Sr, Ba and Eu, indicating that they

  4. 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.

  5. 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.

  6. Tracing the metasomatic and magmatic evolution of continental mantle roots with Sr, Nd, Hf and and Pb isotopes: A case study of Middle Atlas (Morocco) peridotite xenoliths

    NASA Astrophysics Data System (ADS)

    Wittig, Nadine; Pearson, D. Graham; Duggen, Svend; Baker, Joel A.; Hoernle, Kaj

    2010-02-01

    We studied clinopyroxenes from spinel-facies peridotite xenoliths sampled by the Quaternary intra-plate volcanism of the Middle Atlas (Morocco) and present new trace element and Sr-Nd-Hf isotope data. However, we focus in particular on Pb isotope data and 238U/ 204Pb and 232Th/ 204Pb ratios of these clinopyroxenes. This data allows us to investigate: (a) the timing of metasomatic events, (b) the prevalence and persistence of elevated 238U/ 204Pb, 232Th/ 238U and 232Th/ 204Pb in continental mantle roots and (c) the 238U/ 204Pb and 232Th/ 204Pb composition of putative basaltic melts generated from such metasomatised sub-continental lithospheric mantle (SCLM). Incompatible trace element concentrations in these clinopyroxenes are elevated, marked by high-field strength element depletion and fractionated elemental ratios (e.g., U/Nb, Zr/Hf) most consistent with enrichment due to carbonatitic liquids. Sr, Nd and Hf isotopes have an affinity to HIMU. U, Th and Pb abundances in the clinopyroxenes generally exceed estimates of primitive mantle clinopyroxene. Pb isotope compositions of these clinopyroxenes are radiogenic and vary between 206Pb/ 204Pb = 19.93-20.25, 207Pb/ 204Pb = 15.63-15.66 and 208Pb/ 204Pb = 39.72-40.23. These Pb isotope systematics result in generally negative Δ7/4 but positive Δ8/4; setting these samples distinctly apart from typical HIMU. These Pb isotope compositions are also distinct from the associated host volcanic rocks. 238U/ 204Pb and 232Th/ 204Pb of these clinopyroxenes, which range from 26 to 81 and 136 to 399, respectively, are elevated and more extreme than estimates of MORB- and HIMU-source mantle. The Pb isotope evolution of the clinopyroxenes suggests that the metasomatic enrichment is younger than 200 Ma, which discounts the volcanic activity due to the opening of the Atlantic and the onset of the collision of the African and Eurasian plates as processes generating the lithophile element and isotope composition of this continental

  7. Spot U-Pb and Hf Isotope Analyses of Detrital Zircons from the Khondalites in the Western Block of the North China Craton

    NASA Astrophysics Data System (ADS)

    Xia, X.; Sun, M.; Zhao, G.; Luo, Y.

    2005-12-01

    High-grade metapelitic rocks, named khondalites, occur along the north margin of the Ordos Terrane, the Western Block of the North China Craton. Their provenance is considered to be the basement of this terrane, which is covered by Mesozoic-Cenozoic basin sediments. Detrital zircons from khondalites of Jining and Wulashan Complexes were analyzed by LA-ICPMS in this study. Most of the detrital zircons yield U-Pb ages of 1.9-2.1 Ga, showing that Paleoproterozoic rocks were predominant in their provenance. Their maximum depositional age and metamorphic ages are constrained to be 1.84 and 1.81 Ga, respectively. Hf isotope analysis was conducted on detrital zircons from the Wulashan khondalite. ?Hf values vary between -8 and +9, suggesting crystallization of zircons from magmas derived from underlying old crust and juvenile materials from the mantle. The lowest ?Hf values with different ages define an evolutionary line that intersects the depleted mantle line at about 2.6 Ga in a ?Hf vs. time diagram. This implies that rocks extracted from the depleted mantle at about 2.6 Ga may be predominant in the lower crust. All detrital zircons from one sample have U-Pb ages exclusively at about 2.0 Ga and possess positive ?Hf from +1 to +9, clearly recording a significant juvenile crustal growth event at about 2.0 Ga. The above results suggest that the Ordos Terrane of the Western Block may have a major crustal extraction at about 2.6 Ga and a significant juvenile crustal growth at about 2.0 Ga. This is in striking contrast with the Eastern Block, where Archean rocks are important in the basement. Therefore, our data clearly demonstrate that the Ordos Terrane developed separately from the Eastern Blocks, until they collided to form the coherent North China Craton.

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

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

    NASA Astrophysics Data System (ADS)

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

    2016-11-01

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

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

    SciTech Connect

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

    1989-02-01

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

  11. 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

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

    NASA Astrophysics Data System (ADS)

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

    2015-12-01

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

  13. 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

    2017-04-01

    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).

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

    NASA Astrophysics Data System (ADS)

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

    2016-05-01

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

  15. Composition and isotopic constraints on the petrogenesis of alkaline arc lavas: Lihir Island, Papua New Guinea

    SciTech Connect

    Kennedy, A.K.; Hart, S.R.; Frey, F.A. )

    1990-05-10

    The SiO{sub 2}-undersaturated lavas from Lihir island, Papua New Guinea, like most arc lavas are highly enriched in Sr, Ba, K, Rb, and Cc and depleted in Hf, Ta, Nb, and Ti relative to ocean floor basalts and oceanic island basalts. These alkali-rich lavas have arc trace element signatures and Nd, Sr, and Pb isotopic systematics. However, they are not a product of present-day subduction, as this volcanism has tapped mantle which was enriched by prior subduction episodes. The narrow range of Pb isotopic compositions suggest a cogenetic origin for these lavas. During the fractionation of the primitive Lihir lavas, elements normally considered incompatible (i.e., the light rare earth elements (LREE), Rb, Th, and P) have high bulk solid/melt partition coefficients (0.15-1.5). Relatively higher partition coefficients during formation of the evolved lavas produced crossing rare earth element (REE) patterns, and primitive lavas have higher incompatible elements abundances than evolved lavas. The Lihir lavas have lower alkali, Sr, Ba, K, Rb, Cs, and LREE abundances than other Tabar-Feni lavas. They are derived from a less enriched mantle source rather than by a higher degree of melting of a source similar to that of the other islands. The similarity of Sm/Nd ratios of these undersaturated arc lavas to those of tholeiitic and calc-alkaline arc lavas and the moderate chondrite-normalized La/Yb (la/Yb{sub cn} = 3-7) indicates that there has been limited enrichment of the LREE relative to the heavy REE during generation of the arc-modified source mantle. The alkaline nature of these lavas reflects their generation, in a tensional tectonic environment, from a fossil arc mantle region that has undergone extreme arc enrichment of alkali and alkaline earth elements during two earlier subduction episodes.

  16. Isotopic constraints on the genesis and evolution of basanitic lavas at Haleakala, Island of Maui, Hawaii

    NASA Astrophysics Data System (ADS)

    Phillips, Erin H.; Sims, Kenneth W. W.; Sherrod, David R.; Salters, Vincent J. M.; Blusztajn, Jurek; Dulai, Henrietta

    2016-12-01

    To understand the dynamics of solid mantle upwelling and melting in the Hawaiian plume, we present new major and trace element data, Nd, Sr, Hf, and Pb isotopic compositions, and 238U-230Th-226Ra and 235U-231Pa-227Ac activities for 13 Haleakala Crater nepheline normative basanites with ages ranging from ∼900 to 4100 yr B.P. These basanites of the Hana Volcanics exhibit an enrichment in incompatible trace elements and a more depleted isotopic signature than similarly aged Hawaiian shield lavas from Kilauea and Mauna Loa. Here we posit that as the Pacific lithosphere beneath the active shield volcanoes moves away from the center of the Hawaiian plume, increased incorporation of an intrinsic depleted component with relatively low 206Pb/204Pb produces the source of the basanites of the Hana Volcanics. Haleakala Crater basanites have average (230Th/238U) of 1.23 (n = 13), average age-corrected (226Ra/230Th) of 1.25 (n = 13), and average (231Pa/235U) of 1.67 (n = 4), significantly higher than Kilauea and Mauna Loa tholeiites. U-series modeling shows that solid mantle upwelling velocity for Haleakala Crater basanites ranges from ∼0.7 to 1.0 cm/yr, compared to ∼10 to 20 cm/yr for tholeiites and ∼1 to 2 cm/yr for alkali basalts. These modeling results indicate that solid mantle upwelling rates and porosity of the melting zone are lower for Hana Volcanics basanites than for shield-stage tholeiites from Kilauea and Mauna Loa and alkali basalts from Hualalai. The melting rate, which is directly proportional to both the solid mantle upwelling rate and the degree of melting, is therefore greatest in the center of the Hawaiian plume and lower on its periphery. Our results indicate that solid mantle upwelling velocity is at least 10 times higher at the center of the plume than at its periphery under Haleakala.

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

    PubMed

    McCulloch, M T

    1996-03-01

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

  18. 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

  19. 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

  20. Minor Sulfur Isotope Constraints on the composition of Earth's Archean atmosphere

    NASA Astrophysics Data System (ADS)

    Claire, M.

    2016-12-01

    Minor sulfur isotope anomalies in the sedimentary record are direct recorders of ancient chemical reactions that occurred in the atmosphere, and therefore form the most direct proxy for Archean atmospheric composition. The mere presence of mass-independently fractionated sulfur isotopes (MIF-S) in the rock record has resolved nearly a century's worth of debate by constraining atmospheric oxygen to trace levels prior to 2.4 billion years ago, and indirectly indicates the presence of a dominant reducing gas, likely H2 or CH4. The MIF-S database has grown substantially in the past decade, and reveals complex time- and facies-dependent changes in MIF-S magnitudes. The structure within the sedimentary MIF-S record suggests that constraints beyond this simple "on-off" switch for atmospheric O2 are possible once we understand the mechanisms that generate and preserve the signal in the rock record. Recently, I proposed an initial quantitative framework for predictions of atmospheric MIF-S [1], but concluded that new measurements of MIF-S generation mechanisms were needed to provide robust constraints. Since then, identification of MIF-S arising from SO2 photoexcitation [2], and updated absorption cross-sections for SO2 and SO [3-4] provide critical new ground-truth on all 4 isotopes of sulfur. Furthermore, breakthroughs in coupled photochemical-climate modeling have enabled better predictions of UV transparency within hazy atmospheres [5] such as those that might have dominated in the Archean [6-8]. I will present 1-D photochemical modeling results based on these new fundamental constraints, in comparison with MIF-S data from the Archean, to interpret the steady-state composition of the Archean atmosphere and time-dependent perturbations to it. In particular, Δ36S/Δ33S resulting from perturbations to atmospheric species will be discussed as a key tool for constraining the composition of the reducing atmosphere. [1] Claire et al. (2014) GCA; [2] Whitehill et al., PNAS

  1. Mesoproterozoic arc magmatism in SE India: Petrology, zircon U-Pb geochronology and Hf isotopes of the Bopudi felsic suite from Eastern Ghats Belt

    NASA Astrophysics Data System (ADS)

    Dharma Rao, C. V.; Santosh, M.; Zhang, Zeming; Tsunogae, Toshiaki

    2013-10-01

    The southern segment of the Eastern Ghats Mobile Belt (EGMB) in India was an active convergent margin during Mesoproterozoic, prior to the final collision in Neoproterozoic during the assembly of the Rodinia supercontinent. Here we present mineralogical, whole-rock geochemical, zircon U-Pb and Hf isotopic data from a granitoid suite in the Bopudi region in the EGGB. The granitoid complex comprises quartz monzodiorite with small stocks of rapakivi granites. The monzodiorite, locally porphyritic, contains K-feldspar megacrysts, plagioclase, quartz, biotite and ortho-amphibole. The presence of mantled ovoid megacrysts of alkali feldspar embaying early-formed quartz, and the presence of two generations of the phenocrystic phases in the rapakivi granites indicate features typical of rapakivi granites. The K-feldspar phenocrysts in the rapakivi granite are mantled by medium-grained aggregates of microcline (Ab7 Or93), which is compositionally equivalent to the rim of Kfs phenocryst and Pl (An23-24 Ab75). The geochemistry of both the granitoids shows arc-like features for REE and trace elements. LA-ICP-MS zircon analyses reveal 207Pb/206Pb ages of 1582 (MSWD = 1.4) for the rapakivi granite 1605 ± 3 Ma (MSWD = 3.9) for the monzodiorite. The zircons from all the granitoid samples show high REE contents, prominent HREE enrichment and a conspicuous negative Eu anomaly, suggesting a common melt source. The zircons from the monzodiorite have a limited variation in initial 176Hf/177Hf ratios of 0.28171-0.28188, with ɛHf(t) values of -2.2 to +2.8. Correspondingly, their two-stage Hf isotope model ages (TDM2) ranging from 2.15 to 2.47 Ga probably suggest a mixed source for the magma involving melting of the Paleoproterozoic basement and injection of subduction-related juvenile magmas. The prominent Mesoproterozoic ages of these granitoids suggest subduction-related arc magmatism in a convergent margin setting associated with the amalgamation of the Columbia-derived fragments

  2. Particle-number conserving analysis for the systematics of high-K pair-broken bands in Hf and Lu isotopes (170⩽A⩽178)

    NASA Astrophysics Data System (ADS)

    Zhang, Z. H.; Lei, Y. A.; Zeng, J. Y.

    2009-09-01

    Within the framework of the particle-number conserving (PNC) formalism, one-quasiparticle and low-lying high-K pair-broken (multiquasiparticle) bands systematically observed in Hf and Lu isotopes (170⩽A⩽178) are analyzed consistently. The PNC method deals with the cranked shell model with pairing interaction, in which the Pauli blocking effects are exactly accounted for, and the pairing interaction strength is determined by the experimental odd-even difference in binding energies. With an appropriate Nilsson level scheme that best fits the experimental bandhead energies of the one-quasiparticle bands, the experimental moments of inertia (MOIs) of these one-quasiparticle and multiquasiparticle bands (including configuration and frequency dependences, signature splitting, etc.) can be well reproduced without any additional free parameter. In most cases, the PNC formalism supports the configuration assignments in earlier works. the PNC calculation also reveals that the experimental systematics of low-lying high-K pair-broken bands in Hf and Lu isotopes are intimately related to the subshell effects near the Fermi surfaces of both protons and neutrons.

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

    NASA Technical Reports Server (NTRS)

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

    1992-01-01

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

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

    USGS Publications Warehouse

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

    1992-01-01

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

  5. 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.

  6. The tectono-sedimentary evolution of North Helvetic Flysch basin in Central Alps as revealed by detrital zircon U-Pb age dating and Hf isotope geochemistry

    NASA Astrophysics Data System (ADS)

    Lu, Gang; Winkler, Wilfried; Willett, Sean; Rahn, Meinert

    2017-04-01

    Palaeogene syn-tectonic volcanic products sparsely occur in the North Helvetic Flysch, which mainly deposited in the Northern Alpine Foreland Basin. However, the volcaniclastic provenance of the North Helvetic Flysch and its counterparts in Central Alps is still a matter of debate. We investigate the Early Oligocene (Rupelian) turibidite deposits to evaluate their temporal and genetic relationship with the hypothetical magmatic provinces and basement. Detrital zircons from several representative localities (Haute-Savoie, Alpe de Taveyannaz, Glarus and Trento area) have been dated by LA-ICP-MS analysis methods. The obtained age patterns are compared with trace element analysis and 176Hf/177Hf isotope of detrital zircons, which indicate the magmatic environment of zircons crystallization. The ages of detrital zircons show two major populations: a large dominance (92%) of pre-Alpine zircons (Cadomian, Caledonian, Variscan and post-Variscan, ca. > 252 Ma) as commonly observed in other Alpine Flysch formations, which derive from the basement and sedimentary cover of the South Alpine and Austroalpine units. Few Neo-Alpine ones (8%) in the range from Late Eocene to Early Oligocene ( 39.8±0.7-29.7±0.8 Ma) occur, which match the geochronologic data of the Adamello ( 42-33Ma), Biella ( 34-31Ma) and Bergell ( 32-29 Ma) intrusions. With regard to the REE and Hf isotope signatures, it appears that the volcanic fragments were derived from related dykes and surficial extrusions. In addition, the discovery of the minor Neo-Alpine zircons may be due to low zircon production in the volcanic belt along the Palaeo-Insubric line. However, a long distance transport of the syn-sedimentary volcanic material and mixing with various Alpine basement and cover sources is also suggested.

  7. Sr-Nd-Hf-Pb isotope geochemistry of basaltic rocks from the Cretaceous Gyeongsang Basin, South Korea: Implications for basin formation

    NASA Astrophysics Data System (ADS)

    Choi, S.; Kwon, S.; Lee, D.

    2013-12-01

    To better understand the formative mechanism of the Cretaceous Gyeongsang Basin in South Korea, we determined the geochemical compositions of Early Cretaceous syntectonic basaltic rocks intercalated with basin sedimentary assemblages. Two distinct compositional groups appeared: tholeiitic to calc-alkaline basalts from the Yeongyang sub-basin and high-K to shoshonitic basaltic trachyandesites from the Jinju and Uiseong sub-basins. All collected samples exhibit patterns of light rare earth element enrichment and chondrite-normalized (La/Yb)N ratios ranging from 2.4 to 23.6. In a primitive-mantle-normalized spidergram, the samples show distinctive negative anomalies in Nb, Ta, and Ti and a positive anomaly in Pb. The basalts exhibit no or a weak positive U anomaly in a spidergram, but the basaltic trachyandesites show a negative U anomaly. The basalts have highly radiogenic Sr [(87Sr/86Sr)i = 0.70722-0.71145], slightly negative ɛNd, positive ɛHf [(ɛNd)i = -2.7 to 0.0; (ɛHf)i = +2.9 to +6.4], and radiogenic Pb isotopic compositions [(206Pb/204Pb)i = 18.20-19.19; (207Pb/204Pb)i = 15.60-15.77; (208Pb/204Pb)i = 38.38-39.11]. The basaltic trachyandesites are characterized by radiogenic Sr [(87Sr/86Sr)i = 0.70576-0.71119] and unradiogenic Nd, Hf, and Pb isotopic compositions [(ɛNd)i = -14.0 to -1.4; (ɛHf)i = -17.9 to +3.7; (206Pb/204Pb)i = 17.83-18.25; (207Pb/204Pb)i = 15.57-15.63; (208Pb/204Pb)i = 38.20-38.70]. The 'crust-like' signatures, such as negative Nb-Ta anomalies, elevated Sr isotopic compositions, and negative ɛNd(t) and ɛHf(t) values, of the basaltic trachyandesites resemble the geochemistry of Early Cretaceous mafic volcanic rocks from the southern portion of the eastern North China Craton. Considering the lower-crust-like low U/Pb and high Th/U ratios and the unradiogenic Pb isotopic compositions, the basaltic trachyandesites are considered to be derived from lithospheric mantle modified by interaction with melts that originated from foundered eclogite

  8. Sr-Nd-Hf-Pb isotope geochemistry of basaltic rocks from the Cretaceous Gyeongsang Basin, South Korea: Implications for basin formation

    NASA Astrophysics Data System (ADS)

    Kwon, Sun-Ki; Choi, Sung Hi; Lee, Der-Chuen

    2013-09-01

    To better understand the formative mechanism of the Cretaceous Gyeongsang Basin in South Korea, we determined the geochemical compositions of Early Cretaceous syntectonic basaltic rocks intercalated with basin sedimentary assemblages. Two distinct compositional groups appeared: tholeiitic to calc-alkaline basalts from the Yeongyang sub-basin and high-K to shoshonitic basaltic trachyandesites from the Jinju and Uiseong sub-basins. All collected samples exhibit patterns of light rare earth element enrichment and chondrite-normalized (La/Yb)N ratios ranging from 2.4 to 23.6. In a primitive-mantle-normalized spidergram, the samples show distinctive negative anomalies in Nb, Ta, and Ti and a positive anomaly in Pb. The basalts exhibit no or a weak positive U anomaly in a spidergram, but the basaltic trachyandesites show a negative U anomaly. The basalts have highly radiogenic Sr [(87Sr/86Sr)i = 0.70722-0.71145], slightly negative εNd, positive εHf [(εNd)i = -2.7 to 0.0; (εHf)i = +2.9 to +6.4], and radiogenic Pb isotopic compositions [(206Pb/204Pb)i = 18.20-19.19; (207Pb/204Pb)i = 15.60-15.77; (208Pb/204Pb)i = 38.38-39.11]. The basaltic trachyandesites are characterized by radiogenic Sr [(87Sr/86Sr)i = 0.70576-0.71119] and unradiogenic Nd, Hf, and Pb isotopic compositions [(εNd)i = -14.0 to -1.4; (εHf)i = -17.9 to +3.7; (206Pb/204Pb)i = 17.83-18.25; (207Pb/204Pb)i = 15.57-15.63; (208Pb/204Pb)i = 38.20-38.70]. The “crust-like” signatures, such as negative Nb-Ta anomalies, elevated Sr isotopic compositions, and negative εNd(t) and εHf(t) values, of the basaltic trachyandesites resemble the geochemistry of Early Cretaceous mafic volcanic rocks from the southern portion of the eastern North China Craton. Considering the lower-crust-like low U/Pb and high Th/U ratios and the unradiogenic Pb isotopic compositions, the basaltic trachyandesites are considered to be derived from lithospheric mantle modified by interaction with melts that originated from foundered

  9. 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

  10. 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

  11. 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

  12. Isotopic Constraints on the Genesis of Carbonates in Martian Meteorite ALH 84001

    NASA Technical Reports Server (NTRS)

    Leshin, Laurie A.

    1999-01-01

    Oxygen isotopic analyses in approximately 20 micrometer spots in a chemically diverse suite of carbonates from ALH 84001 show highly variable delta(exp 18)O values from +5.4 to +25.3%. The isotopic data are correlated with the major element composition of the carbonate. The earliest forming (Ca-rich) carbonates have the lowest delta(exp 18)O values and the late-forming Mg-rich carbonates have the highest delta(exp 18)O values. Two models that can explain the isotopic variation were investigated. The carbonates could have formed in a water-rich environment at relatively low, but highly variable temperatures. In this open-system case the lower limit to the temperature variation is approximately 125 C, with fluctuations of over 250 C possible within the constraints of the model, depending on fluid composition. Alternatively the data can be explained by a closed-system model in which carbonates precipitated from a limited amount of a CO2-rich fluid. This scenario can reproduce the range of isotopic values observed, even at relatively high temperatures (greater than 500 C). Thus, the oxygen isotopic compositions do not provide unequivocal evidence for formation of the carbonates at low temperature. Neither of these scenarios is consistent with a biological origin of the carbonates and their associated features. Olivine from ALH 84001 occurs as clusters within orthopyroxene adjacent to fractures containing disrupted carbonate globules and feldspathic shock glass. The inclusions are irregular in shape and range in size from approximately 40 micrometers to submicrometer. The olivine exhibits a limited range of chemical composition from Fo(sub 65) to Fo(sub 66). We measured delta(exp 18)O values of the olivine to be +5.1 +/- 1.4%, indistinguishable within uncertainty from the host orthopyroxene. The data suggest that the olivine formed at high temperature (greater than 800 C), and is probably unrelated to carbonate formation. Instead the olivine probably formed by

  13. 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.

  14. 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. Copyright © 2014 Elsevier Ltd. All rights reserved.

  15. 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.

  16. 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

  17. 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.

  18. Isotopic constraints on the genesis and evolution of basanitic lavas at Haleakala, Island of Maui, Hawaii

    USGS Publications Warehouse

    Phillips, Erin H.; Sims, K.W.W.; Sherrod, David R.; Salters, Vincent; Blusztajn, Jurek; Dulaiova, Henrieta

    2016-01-01

    To understand the dynamics of solid mantle upwelling and melting in the Hawaiian plume, we present new major and trace element data, Nd, Sr, Hf, and Pb isotopic compositions, and 238U–230Th–226Ra and 235U–231Pa–227Ac activities for 13 Haleakala Crater nepheline normative basanites with ages ranging from ∼900 to 4100 yr B.P. These basanites of the Hana Volcanics exhibit an enrichment in incompatible trace elements and a more depleted isotopic signature than similarly aged Hawaiian shield lavas from Kilauea and Mauna Loa. Here we posit that as the Pacific lithosphere beneath the active shield volcanoes moves away from the center of the Hawaiian plume, increased incorporation of an intrinsic depleted component with relatively low 206Pb/204Pb produces the source of the basanites of the Hana Volcanics. Haleakala Crater basanites have average (230Th/238U) of 1.23 (n = 13), average age-corrected (226Ra/230Th) of 1.25 (n = 13), and average (231Pa/235U) of 1.67 (n = 4), significantly higher than Kilauea and Mauna Loa tholeiites. U-series modeling shows that solid mantle upwelling velocity for Haleakala Crater basanites ranges from ∼0.7 to 1.0 cm/yr, compared to ∼10 to 20 cm/yr for tholeiites and ∼1 to 2 cm/yr for alkali basalts. These modeling results indicate that solid mantle upwelling rates and porosity of the melting zone are lower for Hana Volcanics basanites than for shield-stage tholeiites from Kilauea and Mauna Loa and alkali basalts from Hualalai. The melting rate, which is directly proportional to both the solid mantle upwelling rate and the degree of melting, is therefore greatest in the center of the Hawaiian plume and lower on its periphery. Our results indicate that solid mantle upwelling velocity is at least 10 times higher at the center of the plume than at its periphery under Haleakala.

  19. 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

  20. 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

  1. Zircon U-Pb geochronology and Hf isotopes from the Sanbagawa Metamorphic Belt, Western Shikoku, Japan: evidence for the prevalence for the Late Cretaceous protoliths

    NASA Astrophysics Data System (ADS)

    Walia, Monika; Knittel, Ulrich; Suzuki, Shigeyuki; Nishizaka, Naoki; Kimura, Kazunari; Lee, Yuan-Hsi; Lee, Hao-Yang

    2017-04-01

    Sanbagawa Metamorphic Belt lies to the south of Median Tectonic Line and is exposed on Kyushu, Shikoku and Honshu Islands in Japan. This belt has been the focus of many studies in recent years since the discovery of young detrital zircon grains (80 - 95 Ma). Samples for this study come from a 2000 m bore hole from north-western Shikoku drilled in an area considered to be part of the Jurassic to Early Cretaceous part of the Sanbagawa Belt. Dating of single zircon grains using the LA-ICP-MS U-Pb dating method shows that all but one sample contain zircons younger than 100 Ma and thus the protoliths are younger than the previously accepted age of metamorphism of the Sanbagawa Belt at ca. 110 Ma. The single sample that contains only zircons dated at 136 ± 3 Ma, apparently is of volcanic origin and could be a clast representing the source of 130-140 Ma zircons of the sample taken about 120m above this sample. In addition, three surface samples were analyzed. Two of these also contain zircons younger than 100 Ma, whereas the third sample contains only zircons older than 159 Ma. Hf-isotope values for the younger age group 82-116 Ma, ɛHf(T) range from -2.4 to +9.6. Zircon grains of 127-146 Ma ages have more positive ɛHf(T) values of +11.5 to +19.0 indicating depleted mantle source. We envisage these zircons to have been derived from the ocean side of a magmatic arc. All grains in the range 215-250 Ma are characterized by negative ɛHf(T) ranging from -2.3 to -15.2, suggesting re-melting of already existent crust. Within the detrital zircon populations contained in the Sanbagawa meta-sediments age groups are recognized that are also known from SE China. However, compared to those from mainland China, zircons from the Sanbagawa meta-sediments are usually characterized by higher ɛHf(T) values suggesting higher input of material derived from the depleted mantle.

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

    NASA Astrophysics Data System (ADS)

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

    2016-11-01

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

  3. 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

  4. Constraints on post-depositional isotope modifications in East Antarctic firn from analysing temporal changes of isotope profiles

    NASA Astrophysics Data System (ADS)

    Münch, Thomas; Kipfstuhl, Sepp; Freitag, Johannes; Meyer, Hanno; Laepple, Thomas

    2017-09-01

    The isotopic composition of water in ice sheets is extensively used to infer past climate changes. In low-accumulation regions their interpretation is, however, challenged by poorly constrained effects that may influence the initial isotope signal during and after deposition of the snow. This is reflected in snow-pit isotope data from Kohnen Station, Antarctica, which exhibit a seasonal cycle but also strong interannual variations that contradict local temperature observations. These inconsistencies persist even after averaging many profiles and are thus not explained by local stratigraphic noise. Previous studies have suggested that post-depositional processes may significantly influence the isotopic composition of East Antarctic firn. Here, we investigate the importance of post-depositional processes within the open-porous firn (≳ 10 cm depth) at Kohnen Station by separating spatial from temporal variability. To this end, we analyse 22 isotope profiles obtained from two snow trenches and examine the temporal isotope modifications by comparing the new data with published trench data extracted 2 years earlier. The initial isotope profiles undergo changes over time due to downward advection, firn diffusion and densification in magnitudes consistent with independent estimates. Beyond that, we find further modifications of the original isotope record to be unlikely or small in magnitude (≪ 1 ‰ RMSD). These results show that the discrepancy between local temperatures and isotopes most likely originates from spatially coherent processes prior to or during deposition, such as precipitation intermittency or systematic isotope modifications acting on drifting or loose surface snow.

  5. Geochronologic, Isotopic, and Trace Element Constraints on Zircon Recycling in Sierra Crest Intrusive Suites, Sierra Nevada Batholith, USA

    NASA Astrophysics Data System (ADS)

    Miller, J. S.; Lackey, J.; Memeti, V.; Hirt, W. H.; Wooden, J. L.

    2011-12-01

    Owing to its ubiquity and chemical properties, zircon is the primary tool for obtaining age information from felsic igneous rocks. Numerous geochronologic studies in ancient and recent plutonic and volcanic rocks over the last decade have shown: (1) that assemblages of zircons from single hand samples rarely crystallized at the same time; (2) that zircons from single hand samples may have variable geochemistry and isotopic compositions requiring distinct growth environments. Nonetheless, the conditions under which diverse assemblages of magmatic zircons crystallize, are dispersed and then gathered within such magma systems are less well understood. We have initiated a systematic geochemical (trace element) and isotopic (δ18O, 176Hf/177Hf) investigation of zircons from dated plutons within several of the Cretaceous Sierran Crest zoned intrusive suites of the Sierra Nevada Batholith to better understand melt sources and zircon recycling within these large magma systems. Preliminary O and Hf isotopic results indicate that zircon assemblages between different intrusive suites have variable isotopic compositions with multiple crustal and mantle sources involved. Preliminary trace element analysis of some of the zircon suites also show appreciable variability but important trends have emerged from the data. In particular, there are pronounced differences between trace element patterns and ratios of the youngest, interior, K-feldspar megacryst-bearing granodiorites (e.g. Cathedral Peak, Mono Creek, Whitney) and the older outer units of the intrusive suites, which are generally more mafic granodiorites and tonalites. In contrast to the more mafic outer units, zircons from the inner megacryst-bearing intrusions are overwhelmingly characterized by low Ti-in-zircon (Tzrn,Ti) model temperatures (regardless of uncertainties in aTiO2), low concentrations of MREEs, high Yb/Gd, low Th/U, high Hf, and high Eu/Eu*. These characteristics, and generally low Zr contents of the

  6. Geochronological, geochemical, and Sr-Nd-Hf isotopic characteristics of Cretaceous monzonitic plutons in western Zhejiang Province, Southeast China: New insights into the petrogenesis of intermediate rocks

    NASA Astrophysics Data System (ADS)

    Liu, Liang; Qiu, Jian-Sheng; Zhao, Jiao-Long; Yang, Ze-Li

    2014-05-01

    We present comprehensive petrological, geochemical, and Sr-Nd-Hf isotopic data for the Matou and Dalai plutons in western Zhejiang Province, Southeast China, with the aim of constraining the petrogenesis of monzonites and to offer new insights into the deep processes of interaction between crustal- and mantle-derived magmas beneath SE China. The Matou pluton comprises quartz monzonite, whereas the Dalai pluton consists of quartz monzodiorite. Zircon U-Pb ages obtained by laser ablation-inductively coupled plasma-mass spectrometry show that both plutons were emplaced at 99-101 Ma. Rocks of both plutons are intermediate to silicic, metaluminous to weakly peraluminous, subalkaline, and K-rich in composition. Samples of the plutons are enriched in large ion lithophile (e.g., Rb, K, and Pb) and light rare earth elements, depleted in high-field strength elements (e.g., Nb, Ta, and Ti), and have small negative or no Eu anomalies. In addition, the rocks have high Mg# values (up to 53.9), high zircon ɛHf(t) values (up to - 1.4), and low Nb/U and Ta/U ratios. Geochemical evidence suggests that both depleted asthenospheric and metasomatically enriched mantle components were involved in the formation of these monzonitic rocks. The presence of inherited zircons with Palaeoproterozoic ages and zircons with unusually low ɛHf(t) values (- 12.9) in the Matou quartz monzonites indicates that ancient crustal materials were also involved in their petrogenesis. In combination with the presence of abundant mafic microgranular enclaves (MMEs) with spheroidal to ellipsoidal-ovoidal shapes and xenocrysts within the more diffused enclaves, and the results of trace element modelling, we suggest that the Matou quartz monzonites were generated by mixing between mantle-derived mafic magmas and crustally derived silicic magmas. The Dalai pluton is relatively homogeneous and contains fewer MMEs than the Matou pluton. Zircons from the Dalai pluton show no inherited components, indicating that

  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. Constraints on Late Tertiary Elevation of the Colorado Plateau From Carbonate Clumped- Isotope Thermometry

    NASA Astrophysics Data System (ADS)

    Huntington, K. W.; Wernicke, B. P.; Eiler, J. M.

    2009-05-01

    Topography is a first-order expression of the buoyancy of the lithosphere, and the timing and pattern of elevation change can place fundamental constraints on mantle flow and continental dynamics. We investigate the timing of Colorado Plateau uplift using clumped-isotope thermometry to independently constrain both the temperature and isotopic composition of ancient surface waters based on the 13C-18O bond enrichment in carbonates. Analyses of ancient lake sediments from the plateau interior and adjacent lowlands are compared to signals recorded by modern sediments collected over 3 km of elevation in the region. Comparison of modern and ancient samples deposited near sea level provides an opportunity to quantify the influence of climate on changes in temperature, and therefore more accurately assess the contribution from changes in elevation. Both modern and ancient (Miocene-Pliocene) carbonates record near-surface spring/summer lake water temperatures that vary strongly with elevation. Modern and ancient lake carbonate temperature lapse rates of -4.2±0.7°C/km and -4.1±0.6°C/km, respectively, suggest that little if any post-16 Ma change in elevation of the southern plateau is required to explain the data. Agreement of δ18O data for modern and ancient surface waters supports this interpretation. The zero-elevation intercept of the ancient trend is 7.7±2.0°C warmer than the modern trend, indicating significant cooling due to climate change since Late Miocene time. The temperature data are permissive of up to 450 m of uplift or 250 m of subsidence of the plateau interior since 6 Ma, but do not support km-scale changes. Combined with previous constraints, the data suggest that most uplift of the south-central plateau occurred during Late Cretaceous/earliest Tertiary time, favoring uplift mechanisms such as crustal thickening by channel flow, hydration of the mantle lithosphere due to volatile flux from the Laramide flat slab, or dynamic topography associated with

  9. Neoproterozoic granitic magmatism along the Ailao Shan-Red River belt: U-Pb zircon geochronology, Lu-Hf isotopes and tectonic implications

    NASA Astrophysics Data System (ADS)

    Chen, Xiaoyu; Liu, Junlai; Qi, Yinchuan; Fan, Wenkui; Burg, Jean-Pierre

    2017-04-01

    The Neoproterozoic tectonic characteristics of the high grade metamorphic massifs along the Ailao Shan-Red River belt are debated. Controversies are on 1) whether the massifs were parts of the Yangtze block to the northeast or 2) parts of the Indochina block to the southwest and 3) the magmatic rocks represent arc magmatism or rifting linked to break-up of the Rodinia supercontinent. This study presents new and precise LA-ICP-MS U-Pb age dating and geochemical and Hf isotopic analyses of granitic intrusions along the Ailao Shan-Red River belt in an attempt to elucidate the Neoproterozoic magmatic evolution of this belt. In general, zircon U-Pb ages of the studied granitic rocks are between 804 and 724Ma, with a weighted mean of ca. 770 Ma, thus confirming Neoproterozoic magmatism. All samples plot into the peraluminous domain, indicating a major crustal resource. In consistency with these conclusions, most of the Neoproterozoic granitoids show negative ɛHf (t) values near the chondrite line. A few samples possess low positiveɛ Hf (t) values, being signatures of mantle sources. It is therefore concluded that the Neoproterozoic magmatism along the ASRR belt originated from mantle sources with important contributions through anatexis of ancient lower crust. Discrimination diagrams of tectonic settings suggest continental arc magmatism. Neoproterozoic magmatism is widely reported along the margins of the Yangtze block, especially in the northern margin. However, there are fewer reports about Neoproterozoic magmatic activity along the southern and southwestern margins. The geochronology spectrum and geochemisty of the studied Neoproterozoic granitic rocks are similar to those along the western margin of the Yangtze block. The present study, combined with previous results, suggests that oceanic subduction contributed to the generation of the arc magmatisms along the western and southwestern margin of the Yangtze plate and along the ASRR belt (as part of the

  10. 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.

  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. 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.

  13. Formation and tectonic evolution of Southeastern China and Taiwan: Isotopic and geochemical constraints

    NASA Astrophysics Data System (ADS)

    John, B. M.; Zhou, X. H.; Li, J. L.

    1990-11-01

    The southern part of China consists of the Proterozoic Yangtze Craton and the Phanerozoic South China foldbelts (including the Himalayan foldbelt of Taiwan). Models for continental growth have been many and controversial. Isotopic and geochemical data from Mesozoic and younger granitoids and sediments are used here to place constraints on the tectonic evolution of Southeastern China and to evaluate whether the young Phanerozoic foldbelts are representative of old rejuvenated landmass, whether they characterize crustal accretion through successive subduction processes, hence suggesting a net growth of continental mass, or whether they represent some intermediate situation. Available Sm-Nd isotopic data for Phanerozoic granitoids and metasediments from the South China foldbelts and Taiwan invariably show Proterozoic model ages ( TDM) ranging from 1 to 2.5 Ga, with a mean of 1.54 ± 0.30 (1σ) Ga. All rocks have negative ɛNd(T) values (-2 to -15), suggesting variable but important contributions of old continental materials to the sources of the Phanerozoic rocks. Obviously they do not represent wholesale growth of continental mass. The granitoids of the South China foldbelts have multiple origins as viewed from their chemical and isotopic characteristics. Secular geochemical variation has been established for inland granitoids of Proterozoic to Mesozoic age. Their high ISr values (0.710-0.737) suggest that most granitoids are essentially anatectic products of ancient continental crust. This is compatible with remelting via microcontinental collision or terrane accretion. Repeated intracrustal reprocessing by partial fusion and differentiation has undoutedly contributed to important mineralization. Mesozoic granitoids (excluding A-type alkaline granites) in the coastal region and Taiwan show relatively low ISr(0.705-0.710) and high ɛNd values, implying that greater amounts of mantle components have been added to ancient continental material in the generation of these

  14. Lithium isotope variations in lavas and olivine phenocrysts from the Cook-Austral Islands: Constraints on sample alteration and the true Li-isotope signature of HIMU mantle

    NASA Astrophysics Data System (ADS)

    Lassiter, J. C.; Hauri, E. H.; Hart, S. R.; Blusztajn, J.; Chan (Deceased), L.

    2008-12-01

    olivine separate suggests that olivine phenocrysts may be affected by kinetic affects such as diffusive influx of 6Li during magma cooling. However, the majority of olivine/whole rock pairs are within error, confirming a lack of olivine/melt isotopic fractionation and the ability of olivines to robustly record magmatic δ7Li values. Olivine δ7Li values correlate broadly with whole rock radiogenic isotopes (Pb, Nd, Hf). HIMU lavas are characterized by moderately elevated δ7Li (up to 6.2‰), though not as high as previously proposed for HIMU mantle. The moderately elevated δ7Li values are consistent with derivation of HIMU lavas from a source that contains dehydrated recycled oceanic crust that has not suffered severe loss of Li during dehydration. This study suggests that recycled oceanic crust does not completely lose its heavy Li-isotope fingerprint upon subduction into the mantle, consistent with the model of [2], but in contradiction to the model of [1] for Li behavior during slab dehydration. [1] Zack, et al., EPSL, 208, 279-290, 2003. [2] Marshall, et al., Geochim. Cosmochim. Acta, 70, 4750-4769, 2006.

  15. Thallium Isotope Constraints on Hydrothermal Water Fluxes at Mid-Ocean Ridge Axes and Flanks

    NASA Astrophysics Data System (ADS)

    Rehkamper, M.; Nielsen, S. G.; Alt, J. C.; Butterfield, D. A.

    2004-12-01

    The hydrothermal circulation that occurs at mid-ocean ridge axes and flanks has profound effects on the chemical budgets of the oceans but our understanding of the relevant fluxes is incomplete. Here, we use new thallium (Tl) isotope and concentration data for hydrothermal fluids and rocks from ODP Hole 504B to obtain independent estimates of the high- and low-temperature (T) hydrothermal water fluxes at spreading axes and ridge flanks. Seawater is characterized by relatively uniform Tl isotope compositions and concentrations of ɛ 205Tl = -6 and 65 pmol/kg, respectively (ɛ 205Tl represents the deviation of the 205}Tl/{203Tl ratio of a sample from the standard in parts per 10,000). In contrast, high-T hydrothermal fluids from ridges axes display ɛ 205Tl = -2±1, indistinguishable from unaltered mantle rocks. The correlation of Tl and Cl abundances indicates an average Tl content of 10-25 nmol/kg for high-T endmember fluids. The low-T alteration of the upper volcanic zone of ODP Hole 504B is associated with Tl-uptake from seawater. The isotope fractionation that occurs during the uptake generates Tl-rich rocks that have ɛ 205Tl as low as -16. The sheeted dike complex displays low Tl contents due to leaching of the rocks by high-T hydrothermal fluids. Taken together, these observations indicate that high-T vent fluids do not acquire significant Tl from the altered Tl-rich rocks of the volcanic section. With this constraint, the high-T axial water flux can be calculated, assuming that Tl is leached with an efficiency of 60-95% from 1.0-1.4 km of sheeted dikes and upper gabbros, which have a Tl concentration of 3±1 ppb. These parameters yield a high-T water flux of 0.2-2.5 x 1013 kg/yr, equivalent to a heat flux of 0.1 to 1.2 TW. This result is in excellent agreement with other geochemical estimates of high-T water fluxes, e.g., those based on Li isotopes (Chan et al., 2002) or the Sr isotope profile of ODP Hole 504B (Teagle et al., 2003). If the total axial

  16. The closure of Palaeo-Tethys in the Sanjiang area, southwestern China: New insights from zircon U-Pb and Hf isotope data of granitoids

    NASA Astrophysics Data System (ADS)

    Dong, G.; Mo, X.; Wang, W.; Sun, Z.

    2016-12-01

    There are several granitoids associated with two sub-parallel suture zones in the Sanjiang area, southwestern China, which were interpreted to represent the magmatic expression of the tectonic evolution of Palaeo-Tethys during Late Palaeozoic to Early Mesozoic times. The granitoids are mainly peraluminous, high-K calc-alkaline with sub-parallel REE patterns and strong negative Eu anomalies, showing S-type characteristics. Recent geochronological and geochemical data of granotoids in the belts are adding some evidences on the Palaeo-Tethys evolution. Here we present new LA-ICP-MS ziron U-Pb ages and Hf isotopic data in the S type granitic intrusions from Lincang to Yacha to constrain on the times of the tectonic evolution. Measured ages of 200Ma - 230 Ma in the Lincang granite are equitant of the Wandashan granite (219Ma) and Yacha granite (220Ma) to the northern extension and granitic intrusions in eastern Myanmar (published ages of 210-220Ma) to the southern extension. There are also some I type granitoids (282-284 Ma) to the east in Jinghong and granodiorite (239 Ma) to the north in Baomaxueshan, which suggest the existence of island arc system and subduction setting. The occurrence of bimodal volcanism (Xiaodingxi Fm.) in Jinghong and middle Triassic andesite (Manghuai Fm.) with ages of 236-248 Ma supports this proposal. Thus, it has been confirmed that the S type granitoids from Lincang to Yacha formed under collsional setting during late Triassic period and the formation process lasted for 30 Ma approximately. Further, the new zircon Hf isotope data indicated that the granitoids should be formed by underlain Palaeoproterozoic crust with ɛHf model ages of 1900-2400 Ma. Therefore, the S type granitoids from Lincang to Yacha were formed during continental collision between the Baoshan-Gengma Terrane and the Lanping-Simao Terrane after subduction of the Paleo-Tethyan Oceanic Plate, which is suggesting the closure of the Paleo-Tethyan Ocean.

  17. 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.

  18. 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

  19. The Malvinas (Falkland) Islands revisited: The tectonic evolution of southern Gondwana based on U-Pb and Lu-Hf detrital zircon isotopes in the Paleozoic cover

    NASA Astrophysics Data System (ADS)

    Ramos, Victor A.; Cingolani, Carlos; Junior, Farid Chemale; Naipauer, Maximiliano; Rapalini, Augusto

    2017-07-01

    The first U-Pb and Hf-Lu isotopic data of detrital zircons from Devonian strata of the Malvinas (Falkland) Islands allow re-evaluation of different hypotheses regarding their location before the breakup of Gondwana. Of the various published hypotheses there are only two that have gained support. Adie's hypothesis involves a rotation of 180 degrees of the islands and a large displacement of Patagonia, independently of South America, whereas Borrello's hypothesis assumes a relative fixed position of the islands with respect to South America over time. The first hypothesis has traditionally been evaluated by highlighting the similarities of the geology of the Malvinas Islands with similar rocks cropping out in South Africa. In this paper we test those hypotheses that led to correlate the islands with the Cape System, based on geological, paleomagnetic and geochronological data. However, new isotopic data compared with contemporaneous Patagonian rocks, together with the present knowledge of offshore features of the Malvinas Plateau, suggest that its correlation with South Africa is not as compelling. Although there is no conclusive evidence, the simplest hypothesis based on the present available datasets, favors a closer relationship with Patagonia. No doubt that more research is needed in order to elucidate the paleogeography of the Malvinas Islands before the opening of the South Atlantic Ocean. xml:lang="es"

  20. 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.

  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. 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

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

    NASA Astrophysics Data System (ADS)

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

    2014-05-01

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

  4. 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.

  5. New constraints on kinetic isotope effects during CO2(aq) hydration and hydroxylation: Revisiting theoretical and experimental data

    NASA Astrophysics Data System (ADS)

    Sade, Ziv; Halevy, Itay

    2017-10-01

    CO2 (de)hydration (i.e., CO2 hydration/HCO3- dehydration) and (de)hydroxylation (i.e., CO2 hydroxylation/HCO3- dehydroxylation) are key reactions in the dissolved inorganic carbon (DIC) system. Kinetic isotope effects (KIEs) during these reactions are likely to be expressed in the DIC and recorded in carbonate minerals formed during CO2 degassing or dissolution of gaseous CO2. Thus, a better understanding of KIEs during CO2 (de)hydration and (de)hydroxylation would improve interpretations of disequilibrium compositions in carbonate minerals. To date, the literature lacks direct experimental constraints on most of the oxygen KIEs associated with these reactions. In addition, theoretical estimates describe oxygen KIEs during separate individual reactions. The KIEs of the related reverse reactions were neither derived directly nor calculated from a link to the equilibrium fractionation. Consequently, KIE estimates of experimental and theoretical studies have been difficult to compare. Here we revisit experimental and theoretical data to provide new constraints on oxygen KIEs during CO2 (de)hydration and (de)hydroxylation. For this purpose, we provide a clearer definition of the KIEs and relate them both to isotopic rate constants and equilibrium fractionations. Such relations are well founded in studies of single isotope source/sink reactions, but they have not been established for reactions that involve dual isotopic sources/sinks, such as CO2 (de)hydration and (de)hydroxylation. We apply the new quantitative constraints on the KIEs to investigate fractionations during simultaneous CaCO3 precipitation and HCO3- dehydration far from equilibrium.

  6. 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

    2017-04-01

    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

  7. 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

  8. 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

  9. 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

  10. 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

  11. Juvenile crust formation in the Zimbabwe Craton deduced from the O-Hf isotopic record of 3.8-3.1 Ga detrital zircons

    NASA Astrophysics Data System (ADS)

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

    2017-10-01

    Hafnium and oxygen isotopic compositions measured in-situ on U-Pb dated zircon from Archaean sedimentary successions belonging to the 2.9-2.8 Ga Belingwean/Bulawayan groups and previously undated Sebakwian Group are used to characterize the crustal evolution of the Zimbabwe Craton prior to 3.0 Ga. Microstructural and compositional criteria were used to minimize effects arising from Pb loss due to metamorphic overprinting and interaction with low-temperature fluids. 207Pb/206Pb age spectra (concordance >90%) reveal prominent peaks at