Initial Isotope Ratios Research Articles

Vegetation and location of water inflow affect evaporation in a subtropical wetland as indicated by the deuterium excess method

AbstractEvaporation represents a principal form of water loss from water bodies, but quantifying evaporation with transpiring vegetation is challenging, owing to the difficulties in disentangling evaporation from transpiration. As the two processes have different effects on water isotope signatures (i.e., oxygen and hydrogen stable isotope ratios expressed by δ18O and δ2H), we used both the δ18O and δ2H values of water samples to calculate their deuterium excess (d) values, which are proportionate to fraction of evaporation loss (Ef) of water being sampled. Comparing with a single isotope method, the d method holds the advantage that it does not require knowledge of the initial isotope ratios of source water. Limitations of the d method, for example, effects of prior‐evaporation of water before entering water bodies, can be checked and corrected. The sampling locations throughout our study site differed in vegetation coverage, water depth, and distance to water inflow (or discharge gate of water inflow), which are three important factors in water management. This sampling design offered us a unique opportunity to quantify the influences of the above three factors on Ef. We found the following: (1) There was a spatiotemporal pattern in δ18O distribution. (2) The Ef was negatively related to the vegetation coverage, positively related to distance to the water source inflow but nonsignificantly related to water depth. Therefore, vegetation and distance to the water inflow are two important considerations for the control of evaporative water loss, and evaporation calculated by the d method can lead to more informed water management.

Early Solar System Solar Wind Implantation of 7Be into Calcium-Alumimum Rich Inclusions in Primitive Meteortites

The one time presence of short-lived radionuclides (SLRs) in Calcium-Aluminum Rich inclusions (CAIs) in primitive meteorites has been detected. The solar wind implantation model (SWIM) is one possible model that attempts to explain the catalogue of SLRs found in primitive meteorites. In the SWIM, solar energetic particle (SEP) nuclear interactions with gas in the proto-solar atmosphere of young stellar objects (YSOs) give rise to daughter nuclei, including SLRs. These daughter nuclei then may become entrained in the solar wind via magnetic field lines. Subsequently, the nuclei, including SLRs, may be implanted into CAI precursors that have fallen from the main accretion flow which had been destined for the proto-star. This mode of implanting SLRs in the solar system is viable, and is exemplified by the impregnation of the lunar surface with solar wind particles, including SLRs. X-ray luminosities have been measured to be 100,000 times more energetic in YSOs, including T-Tauri stars, than present-day solar luminosities. The SWIM scales the production rate of SLRs to nascent SEP activity in T-Tauri stars. Here, we model the implantation of 7Be into CAIs in the SWIM, utilizing the enhanced SEP fluxes and the rate of refractory mass inflowing at the X-region, 0.06 AU from the proto-Sun. Taking into account the radioactive decay of 7Be and spectral flare variations, the 7Be/9Be initial isotopic ratio is found to range from 1 × 10−5 to 5 × 10−5.

Monte Carlo sampling for error propagation in linear regression and applications in isochron geochronology

Geochronology is essential for understanding Earth’s history. The availability of precise and accurate isotopic data is increasing; hence it is crucial to develop transparent and accessible data reduction techniques and tools to transform raw mass spectrometry data into robust chronological data. Here we present a Monte Carlo sampling approach to fully propagate uncertainties from linear regressions for isochron dating. Our new approach makes no prior assumption about the causes of variability in the derived chronological results and propagates uncertainties from both experimental measurements (analytical uncertainties) and underlying assumptions (model uncertainties) into the final age determination. Using synthetic examples, we find that although the estimates of the slope and y-intercept (hence age and initial isotopic ratios) are comparable between the Monte Carlo method and the benchmark “Isoplot” algorithm, uncertainties from the later could be underestimated by up to 60%, which are likely due to an incomplete propagation of model uncertainties. An additional advantage of the new method is its ability to integrate with geological information to yield refined chronological constraints. The new method presented here is specifically designed to fully propagate errors in geochronological applications involves linear regressions such as Rb-Sr, Sm-Nd, Re-Os, Pt-Os, Lu-Hf, U-Pb (with discordant points), Pb-Pb and Ar-Ar.

Disturbance of the Sm-Nd isotopic system by metasomatic alteration: A case study of fluorapatite from the Sin Quyen Cu-LREE-Au deposit, Vietnam

The Neoproterozoic (840 Ma) Sin Quyen deposit in northwestern Vietnam contains replacement Cu-LREE-Au orebodies in Proterozoic metasedimentary rocks. In this deposit, LREE-bearing minerals include allanite-(Ce), monazite-(Ce), chevkinite-(Ce), and fluorapatite. Fluorapatite from orebodies has undergone variable degrees of metasomatic alteration. Samarium-neodymium isotopic analyses were conducted on altered fluorapatite, and also on allanite-(Ce) and monazite-(Ce), to investigate whether such metasomatism can affect the Sm-Nd isotope system. Allanite-(Ce) and monazite-(Ce) have Sm-147/Nd-144 ratios ranging from 0.0359 to 0.0549, and Nd-143/Nd-144 ratios from 0.51147 to 0.51172. Their initial Nd-143/Nd-144 values at the time of mineralization range from 0.51126 to 0.51148, but mostly cluster between 0.51135 and 0.51145. Thus, the primary ore-forming fluids were relatively homogeneous in their Sm-Nd isotopic compositions. In the Sm-147/Nd-144 vs. Nd-143/Nd-144 diagram, the compositions of allanite-(Ce) and monazite-(Ce) generally plot along a Sm-Nd isochron of 840 Ma, implying that the Sm-Nd isotopic systems of these minerals were either closed or only slightly modified. In contrast, altered fluorapatite crystals have Sm-147/Nd-144 ratios varying from 0.0667 to 0.1348, and Nd-143/Nd-144 ratios from 0.51160 to 0.51199. The calculated initial Nd-143/Nd-144 ratios range widely from 0.51114 to 0.51141, with most values lower than those of the allanite-(Ce) and monazite-(Ce). In the Sm-147/Nd-144 vs. Nd-143/Nd-144 diagram, their compositions mostly plot below the 840-Ma Sm-Nd isochron. Petrographic observations and trace elemental analyses show that metasomatic modification of fluorapatite grains led to increases of their Sm/Nd ratios. The unaltered domains in the grains have Sm/Nd ratios varying from 0.114 to 0.200, with an average value of 0.161; whereas the altered domains have Sm/Nd ratios varying from 0.111 to 0.254, with an average value of 0.183. The increased Sm/Nd ratios can cause the calculated initial Nd-143/Nd-144 ratios to be lower than actual initial isotopic ratios, and can also result in compositional deviations from the reference Sm-Nd isochron. This study demonstrates that the traditionally assumed inert Sm-Nd isotopic system can be metasomatically disturbed due to changes in the Sm/Nd ratio. Therefore, care must be taken when interpreting the Sm-Nd isotopic data from apatite/apatite-rich rocks that have undergone metasomatic alteration.

A reappraisal of the evolution of the palaeo-Pacific margin of Gondwana from the Pb and Os isotope systematics of igneous rocks from the southern Adelaide fold belt, South Australia

We have undertaken Pb and Os isotopic analyses of igneous rocks from the southern Adelaide fold belt to further understanding of the Neoproterozoic – early Palaeozoic evolution of the palaeo-Pacific margin of Gondwana. The new Pb isotope data and a reappraisal of existing geochronological data suggest that layered mafic intrusions at Black Hill and Cambrai may have intruded much earlier than previously thought. Intrusion may even have occurred prior to deposition of the Normanville and Kanmantoo Group sediments and the onset of what is traditionally regarded as the Delamerian Orogeny. We tentatively term this earlier event the Hallett Cove orogeny. However, equivalent ages are common in the formerly adjacent Ross Orogen. When the Pb isotope signatures of the suites of gabbros, and I-, S-, and A-type granites are compared at 487Ma they all span a very large range (206Pb/204Pbi=15.4–18.7, 207Pb/204Pbi=15.4–15.7, 208Pb/204Pbi=33.0–38.8) compared with other formerly adjacent terranes or the subsequent Lachlan fold belt. Similarly, 187Os/188Osi ratios span 0.122–1.17 but include one gabbro and one rhyolite that have subchondritic 187Os/188Osi ratios. When taken together, the combined data do not support models in which the observed range in initial isotope ratios reflects major and variable mid to upper crustal assimilation of magmas derived from the contemporary asthenospheric mantle. Rather, it appears that magmas parental to the gabbros and A-type rocks were derived from subduction-modified lithospheric mantle that has a median metasomatic age of 1.4Ga. Large-scale Pb isotopic homogenization, as proposed for the Lachlan fold belt did not occur in the Adelaide fold belt.

Oceanic mafic magmatism in the Siletz terrane, NW North America: Fragments of an Eocene oceanic plateau?

The Siletz terrane, a predominantly mafic accreted oceanic terrane, is located in the Cascadia forearc region of Oregon, Washington and Vancouver Island. The terrane represents a late Palaeocene–Eocene large igneous province that consists of pillow lavas, massive flows and intrusive sheets. Previously it has been proposed that the Siletz terrane represents either an accreted oceanic plateau, hotspot island chain, backarc basin, island arc, or a sequence of slab window volcanics. A province-wide geochemical reassessment of the terrane, including new high precision Sr-Pb-Nd-Hf isotope data, has been used to assess the validity of the proposed tectonomagmatic models for the Siletz terrane. The trace element data show little evidence of crustal contamination, or an arc signature, and the samples have rare earth element (REE) patterns that are flat to light REE enriched. These features are similar to other oceanic plateaus such as the Ontong Java and the Caribbean. Initial isotope ratios range from 206Pb/204 Pb: 18.751 to 19.668, 207Pb/204Pb: 15.507 to 15.661, 208Pb/204Pb: 38.294 to 39.2128, 176Hf/177Hf: 0.28300 to 0.28316 (εHf: 9.0 to 14.5), 143Nd/144Nd: 0.51282 to 0.51299 (εNd: 5.0 to 8.1) and 87Sr/86Sr: 0.70302 to 0.70380. These data are consistent with a mantle source of the Siletz terrane that appears to have been heterogeneous and slightly enriched. The enriched signature has characteristics of both EM2 and HIMU components and this, combined with a calculated mantle potential temperature well above ambient mantle, indicates derivation of the Siletz magmatism from a mantle plume, possibly the Yellowstone Hotspot. We therefore conclude that the Siletz terrane represents an accreted oceanic plateau.

Late Mesozoic magmatism and metallogeny in NE China: The Sandaowanzi–Beidagou example

ABSTRACTThe Sandaowanzi (>22t Au) and Beidagou (>5t Au) tellurium–gold deposits are located in the northeastern Central Asian Orogenic Belt (Heilongjiang Province, NE China). The ore-hosting volcanic rocks unconformably overly monzogranite and were intruded by adakitic granodiorite. In this study, we report new-age, geochemical, and Sr–Nd–Pb isotopic data to elucidate the genetic link between the igneous rocks and the Te–Au mineralization. New-age data indicate that local magmatism occurred in the Early Jurassic (ca. 177.2 Ma) and Early Cretaceous (ca. 118.7 − 122.0 Ma). Geochemically, the igneous rocks are enriched in LREEs, Pb, K, and U, and depleted in Nb, P, and Ti, showing calc-alkaline affinity. The Early Jurassic monzogranite rocks are featured by 87Sr/86Sr = 0.7111−0.7118; εNd(t) = −4.6 to −4.7; 206Pb/204Pb = 18.098−18.102, 207Pb/204Pb = 15.558−15.580, and 208Pb/204Pb = 37.781−37.928, whereas the Early Cretaceous adakitic granodiorite contains: 87Sr/86Sr = 0.7071−0.7073; εNd(t) = − 3.4 to −3.2; 206Pb/204Pb = 17.991−18.080, 207Pb/204Pb = 15.483−15.508, and 208Pb/204Pb = 37.938−37.985. Initial isotopic ratios for the Early Cretaceous volcanic rocks: 87Sr/86Sr = 0.7061−0.7087; εNd(t) = − 3.6 to −2.9; 206Pb/204Pb = 18.136−18.199, 207Pb/204Pb = 15.512−15.628, and 208Pb/204Pb = 38.064−38.155. The pyrite, chalcopyrite, and telluride grains yielded δ34S of −6.52 ‰ to 2.13 ‰ (mean = − 0.82 ‰) and δ13CPDB of the calcite samples are in the range of −6.64 ‰ to −5.24 ‰, implying the ore materials were derived from mantle. The geochemical and isotopic results indicate that primary melts of Late Mesozoic magmatic rocks have features by partial melting of the continental crust. The adakitic rocks may have been the products of the thickened lower crustal delamination and the subsequent asthenospheric upwelling during the intra-continental extension in NE China. Regionally, intrusive activity and molybdenum mineralization during the Jurassic was affected by subduction setting, whereas gold mineralization was controlled by the Early Cretaceous tectonothermal events associated with a superposition extension.

Exotic lamproites or normal ultrapotassic rocks? The Late Miocene volcanic rocks from Kef Hahouner, NE Algeria, in the frame of the circum-Mediterranean lamproites

The late Miocene (11–9Ma) volcanic rocks of Kef Hahouner, ~40km NE of Constantine (NE Algeria), are commonly classified as lamproites in literature. However, these rocks are characterized by an anhydrous paragenesis with plagioclase and Mg-rich olivine phenocrysts, set in a groundmass made up of feldspars, pyroxenes and opaque minerals. Thus, we classify the Kef Hahouner rocks as ultrapotassic shoshonites and latites, having K2O>3wt.%, K2O/Na2O>2.5, MgO>3–4wt.%, SiO2<55–57wt.% and SiO2/K2O<15.All the investigated samples show primitive mantle-normalized multi-element patterns typical of orogenic (arc-type) magmas, i.e. enriched in LILE (e.g. Cs, Rb and Ba) and LREE (e.g. La/Yb=37–59) with respect to the HFSE, peaks at Pb and troughs at Nb and Ta. Initial isotopic ratios are in the range of 87Sr/86Sr=0.70874–0.70961, 143Nd/144Nd=0.51222–0.51223, 206Pb/204Pb=18.54–18.60, 207Pb/204Pb=15.62–15.70 and 208Pb/204Pb=38.88–39.16.The Kef Hahouner volcanic rocks show multi-element patterns similar to the other circum-Mediterranean lamproites and extreme Sr, Nd and Pb isotopic compositions. Nevertheless, the abundant plagioclase, the presence of Al-rich augite coupled with high Al2O3 whole rock compositions (9.6–21.4wt.%), and the absence of phlogopite are all at inconsistent with the definition of lamproite. We reviewed the rocks classified as lamproites worldwide, and found that many of these rocks, as for the Kef Hahouner samples, should be actually defined as “normal” potassic to ultrapotassic volcanic rocks. Even the grouping of lamproites into “orogenic” and “anorogenic” types appears questionable.

Statistical clumped isotope signatures.

High precision measurements of molecules containing more than one heavy isotope may provide novel constraints on element cycles in nature. These so-called clumped isotope signatures are reported relative to the random (stochastic) distribution of heavy isotopes over all available isotopocules of a molecule, which is the conventional reference. When multiple indistinguishable atoms of the same element are present in a molecule, this reference is calculated from the bulk (≈average) isotopic composition of the involved atoms. We show here that this referencing convention leads to apparent negative clumped isotope anomalies (anti-clumping) when the indistinguishable atoms originate from isotopically different populations. Such statistical clumped isotope anomalies must occur in any system where two or more indistinguishable atoms of the same element, but with different isotopic composition, combine in a molecule. The size of the anti-clumping signal is closely related to the difference of the initial isotope ratios of the indistinguishable atoms that have combined. Therefore, a measured statistical clumped isotope anomaly, relative to an expected (e.g. thermodynamical) clumped isotope composition, may allow assessment of the heterogeneity of the isotopic pools of atoms that are the substrate for formation of molecules.

Modeling of sputtering of the ice surfaces under impact of H+ ions: Redistribution of the h and o isotopes applied to the satellites of Jupiter

Using computer modeling, the current models for sputtering coefficient calculation of the ice surfaces under the impact of H+ ions are statistically analyzed and the sputtering coefficients and their confidence intervals are calculated over a wide range of ion energies. It was established that the approximation model (Fama et al., 2008) with a calculated confidence interval of ±20% is less sensitive to the variable parameters. The sputtering coefficients of water ice p = 0.94, T = 80 K) are calculated under the impact of H+ ions for energies from several eV to 10 keV and the results are verified with experimental data. The maximum sputtering coefficient is 0.9 H2O/ion at the energy of the incident H+ ions of 200 eV. The modeling of the sputtering coefficients dependence of H2O molecules from the ice surface temperature showed that they weakly vary in the temperature range of 40–100 K and increase with an increase in the surface temperature. At maximum of distribution under T = 40–100 K, the sputtering coefficient Y(E = 200 eV) is 0.9 H2O/ion at T = 200 K — 1.1 H2O/ion. The distribution by kinetic energy of the dispersed H2O molecules and H and O atoms are modeled for the energy of 1–100 keV of the incident H+ ions. These results may be applied for modeling of variable isotopic compositions of the exosphere of the Jupiter’s satellites in the course of sputtering. The calculated ratios of the sputtering coefficients of H, D, 18O, and 16O from the surface for the Jupiter’s satellites (Europe, Ganymede, Callisto) under the impact of H+ ions for energy from several eV to 10 keV are (1.7 ± 0.3) × 10−5 and 0.18 ± 0.03, respectively. These ratios are distinct from the initial isotope ratios on the surface of the Jupiter’s satellites. This significant difference may lead to the redistribution of isotopes on the surface of the Jupiter’s satellites. The variation in the D/H ratio on the surface of the Jupiter’s satellites depends on the flux density of the radiated ions.

Laboratory studies of carbon kinetic isotope effects on the production mechanism of particulate phenolic compounds formed by toluene photooxidation: a tool to constrain reaction pathways.

In this study, we examined compound-specific stable carbon isotope ratios for phenolic compounds in secondary organic aerosol (SOA) formed by photooxidation of isotope-label-free toluene. SOA generated by photooxidation of toluene using a continuous-flow reactor and an 8 m(3) indoor smog chamber was collected on filters, which were extracted with acetonitrile for compound-specific analysis. Eight phenolic compounds were identified in the extracts using a gas chromatograph coupled with a mass spectrometer, and their compound-specific stable carbon isotope ratios were determined using a gas chromatograph coupled with a combustion furnace followed by an isotope ratio mass spectrometer. The majority of products, including methylnitrophenols and methylnitrocatechols, were isotopically depleted by 5-6‰ compared to the initial isotope ratio of toluene, whereas the isotope ratio for 4-nitrophenol remained identical to that of toluene. On the basis of the reaction mechanisms proposed in previous reports, stable carbon isotope ratios of these products were calculated. By comparing the observed isotope ratios with the predicted isotope ratios, we explored possible production pathways for the particulate phenolic compounds.

Timing and extent of Mg and Al isotopic homogenization in the early inner Solar System

The first million years of the Solar System is a key period when the first solids were formed from the nebula gas. The chronology of the different processes at the origin of these solids is still largely unknown and relies strongly on the assumption made of homogeneous distribution for short-lived radioactive nuclides such as 26Al. This assumption is questioned. In this study, in situ 26Al–26Mg isotope systematics was studied with high precision in 12 calcium, aluminum-rich inclusions (CAIs) (1 type A, 2 type B, 5 type C, and 4 fine grained spinel-rich), 2 amoeboid olivine aggregates (AOAs), and 2 Al-rich chondrules from Efremovka and Vigarano. The (26Al/27Al)i in these early Solar System solids (the subscript ‘i’ stands for the initial isotope ratio obtained from the mineral 26Al isochron) range from ∼6.5×10−5 to 0.2×10−5 with δMgi⁎26 from −0.08 to +0.37‰. The (26Al/27Al)i and δMgi⁎26 of CAIs and chondrules can be explained by formation of their precursors from a homogeneous reservoir (Solar System Initial, noted hereafter SSI) with initial magnesium isotopic composition of δMgSSI⁎26=−0.052±0.013‰ and initial (26Al/27Al)SSI abundance of (5.62±0.42)×10−5. The high precision magnesium isotope data obtained in the present study and literature data allows identifying a few epochs of formation/reprocessing of CAIs. The time periods of these epochs correspond well with the median life times of the pre-main sequence evolution of stars of Solar mass if we anchor the (Al26/Al27)SSI=(5.62±0.42)×10−5 to the beginning of class I phase. This provides a natural explanation to the range of (26Al/27Al)i – (∼6 to 0.02) × 10−5 seen in corundum grains, FUN (Fractionation and Unidentified Nuclear Effects) CAIs, ultrarefractory CAIs, normal CAIs, and chondrules, and suggests a possible relationship between the astrophysical conditions and the formation of these early solids. Corundum grains, FUN CAIs, ultrarefractory CAIs would have formed during the class 0 of the pre main-sequence, during the collapse of the parent molecular cloud, before homogenization was complete while normal CAIs and chondrules formed later during class I to the beginning of class III. Alternatively, if one wants to explain the present data without calling for an early homogenization of Al and Mg isotopes in the forming Solar System, at least four distinct isotopic reservoirs are required to produce by mixing the range observed in δMgi⁎26 and (26Al/27Al)i. There is no obvious astrophysical explanation of the existence of such distinct reservoirs, which makes this hypothesis quite ad hoc and thus very unlikely.

Tchabal Gangdaba massif in the Cameroon Volcanic Line: a bimodal association

Tchabal Gangdaba (TG) volcanic massif, which is a part of the continental sector of the Cameroon Volcanic Line (CVL), is dated between 34.4 and 25.1 Ma. It displays mafic lavas (picrobasalt and basanite, 41–43 wt % SiO2) and felsic lavas (rhyolite, 68–73 wt % SiO2). The lack of intermediate rocks evidences a pronounced Daly gap between 43 and 68 wt % SiO2, which corresponds to an important time span of 3.4 Ma. It is interpreted as due to extensive fractional crystallization under peculiar thermodynamical conditions. Felsic lavas yield strong negative anomalies in Ba, Sr and Eu (0.1 < Eu/Eu* < 0.3) reflecting alkali feldspar and plagioclase fractionation and in Ti reflecting titanomagnetite and ilmenite fractionation. All TG rocks are enriched in LREE and mildly fractionated, which suggests enriched mantle sources. Sr initial isotopic ratios ranging from 0.7033 to 0.7059, eNd from +2.89 to +4.64 and Pb isotopic ratios of 18.988 < 206Pb/204Pb < 19.998, 15.592 < 207Pb/204Pb < 15.673 and 38.948 < 208Pb/204Pb < 39.648 are typical of an enriched mantle source close to HIMU and FOZO. A discrete Sr crustal leaching is evidenced.

Modelling the Spatial and Temporal Trends of Carbon Isotope Fractionation of VOC Mixtures in the Unsaturated Zone

The transport of volatile organic contaminant (VOC) mixtures in the unsaturated zone is typically controlled by a combination of transport (i.e., advection, dispersion, and diffusion) phenomena, phase change (i.e., volatilization, dissolution) and reaction processes (i.e., sorption, biodegradation). As revealed from field and laboratory studies, biodegradation is probably the most important attenuation mechanism of VOCs in the unsaturated zone since it transforms organic contaminants to harmless products. However, in field applications, it is usually difficult, time consuming, or expensive to distinguish and quantify biodegradation among the different natural phenomena and processes, such as diffusion, dispersion, volatilization and sorption, without the aid of numerical modeling. Recent experimental studies showed large variations in space and time in the 13C/12C ratio for many organic compounds and suggested the possibility of using carbon isotope fractionation as a tool to assess biodegradation of organic contaminants in the unsaturated zone. The processes that were indicated to likely have the largest effect on the observed isotope ratio variations are (i) diffusion, which results in molecules with lighter isotopes to diffuse slightly faster than molecules with heavier isotopes, and (ii) biodegradation as a consequence of a preferential breakdown of the chemical bonds of the lighter isotopes. This study used numerical modeling of VOC mixture transport in the unsaturated zone to quantify the contribution of diffusion and biodegradation to carbon isotope fractionation of individual compounds, and to evaluate the use of 13C/12C ratio as a possible tool to assess biodegradation of organic contaminants. The numerical model incorporated transport of carbon isotope fractions of selected organic compounds and used data from a controlled fuel source emplacement field experiment and parameters from laboratory-scale studies. Costeffective modelling of the transport and fate of volatile organic contaminant mixtures was realized with the use of the recently developed constituent averaging technique [1]. Results indicated that (i) both diffusion and biodegradation lead to enrichment in 13C compared to the initial isotope ratio in locations within and close to the contaminant source, (ii) biodegradation lead to enrichment in 13C while diffusion resulted in smaller 13C/12C ratio in locations further away from the source, and (iii) the above effect of both processes on the isotope ratio was increased as a function of time. Modelling results showed that the use of carbon isotope fractionation at the field represents a promising tool for assessing and possibly quantifying biodegradation of organic contaminants in the unsaturated zone.

Petrogenesis of the Bao'anzhai granite and associated Mo mineralization, western Dabie orogen, east-central China: Constraints from zircon U–Pb and molybdenite Re–Os dating, whole-rock geochemistry, and Sr–Nd–Pb–Hf isotopes

Recently, many Mo deposits genetically related to emplacement of Early Cretaceous granites have been found in the Dabie–Qinling belt. A typical intrusion that combines magmatism and metallogenesis, the Bao'anzhai granite, yields a zircon 238U–206Pb age of 123.2 ± 1.1 Ma and a molybdenite Re–Os isochron age of 122.5 ± 2.7 Ma. This granite is characterized by high silica and alkali, but low Mg, Fe, and Ca. It is enriched with light rare earth elements (REEs) and large-ion lithophile elements (LILEs, Rb, K, Th, U) but depleted of heavy REEs, high field strength elements (HFSEs, Nb, Ta, Ti, and Y), and Sr. This high-K granite has medium initial 87Sr/86Sr ratios (0.706518–0.707116) and low initial Pb isotopic ratios [(206Pb/204Pb)i, 16.423–16.699; (207Pb/204Pb)i, 15.285–15.345; (208Pb/204Pb)i, 37.335–37.633], and is characterized by low ϵNd(t) and ϵHf(t) values (−14.92 to −14.22 and −21.67 to −19.19, respectively). These data indicate that this pluton is a high-K calc-alkaline fractionated I-type granitite. It was generated by partial melting of the Yangtze lower crust, which is probably similar to Neoproterozoic TTG-like magmatic rocks at the north Yangtze Block under a non-thickened lower crust environment (<35 km). The ores also have low radiogenic Pb isotopes (206Pb/204Pb, 16.592–17.674; 207Pb/204Pb, 15.300–15.476; 208Pb/204Pb, 37.419–37.911) and low Re content in molybdenite (5.693–10.970 ppm), suggesting a crustal magmatic source for the metallogenic minerals in the Mo deposit.

Numerical modeling of geochemical variations caused by crustal relamination

Geochemical consequences of composite diapirs formed in subduction zones have been studied using a thermomechanical numerical model of an ocean‐continent subduction zone. This model includes dehydration of subducted crust, aqueous fluid transport, partial melting, and melt emplacement. Subduction of crustal material to sublithospheric depth results in the formation of a tectonic rock mélange composed of basalt, sediment, and hydrated /serpentinized mantle. At asthenospheric depth, this rock mélange may evolve into partially molten diapirs and rise through the mantle prior to emplacement (relamination) at crustal levels. We have investigated the composition and the geochemical evolution of liquids derived from such composite diapirs by analyzing the differing proportions of the crustal end‐members in the source, i.e., basalt and sediment. Our results show that the proportions of the components (in the diapiric mélange) are limited to short‐range variations within an interval of Xb [=volume fraction of basalt/(basalt + sediment)] = 0.4 − 0.8, yielding melt with a relatively stable granodioritic major element composition. Hence, granodioritic melt is transported by rising composite diapirs to crustal levels, contributing to the growth of the continental crust. In addition to this, we have calculated Sr and Nd isotopic initial ratios of the diapiric mélange as a function of time, based on the fraction of the components in the mélange. Liquids derived from composite diapirs inherit the geochemical characteristics of the composite source and show distinct temporal variations of radiogenic isotopes depending on the changing values of Xb. Partial melting of composite diapirs is therefore expected to produce melt with a constant major element composition but substantial changes in terms of radiogenic isotopes.

Incorporation of36Cl into Calcium-Aluminum-Rich Inclusions in the Solar Wind Implantation Model

We consider the short-lived radionuclide (SLR) ³⁶Cl in calcium-aluminum-rich inclusions (CAIs) found in primitive meteorites with the solar wind implantation model. In this model, SLRs are produced via nuclear reaction with solar energetic particles (SEPs) interacting with gaseous targets in the protosolar atmosphere in T-Tauri stars. These SLRs are captured by the solar wind and then implanted in CAI precursor materials, which have dropped from the funnel flow leading onto the protostar. This method of incorporating SLRs into solar system materials is currently active in our solar system and has been measured with SLRs from the solar wind being implanted on the lunar surface. T-Tauri stars are capable of SEP fluxes <i>&#x7e;</i>10⁵ greater than contemporary SEP fluxes. Here we scale the production rate of ³⁶Cl to the ancient SEP activity. From the enhanced production rates and the refractory mass inflow rate at 0.06&#x2009;AU from the protosun, we model the ancient ³⁶Cl content in CAIs. We find the initial isotopic ratio of ³⁶Cl/³⁵Cl to range from about 1 &#xd7; 10^&#x2212;5 to 5 &#xd7; 10^&#x2212;5 and the concentration of ³⁶Cl to range from about 3 &#xd7; 10¹³ to 1.5 &#xd7; 10¹⁴ atoms g^&#x2212;1.

Geochemistry of 1.78 Ga A-type granites along the southern margin of the North China Craton: implications for Xiong'er magmatism during the break-up of the supercontinent Columbia

The Palaeoproterozoic Luoling granites occur along the southern margin of the North China Craton. They are rich in silica and total alkalis with SiO2 contents ranging from 65.18 to 72.72 wt.%, K2O from 4.68 to 6.62 wt.%, and Na2O from 1.35 to 4.88 wt.%. They have high Fe*[FeOt/(FeOt + MgO)] ranging from 0.84 to 0.95 wt.% and low MnO (0.03–0.09 wt.%), MgO (0.27–1.55 wt.%), CaO (0.36–2.04 wt.%), TiO2 (0.4–1.12 wt.%), and P2O5 (0.04–0.36 wt.%). Geochemically, they show typical characteristics of A-type granites, such as high contents of alkalis (i.e. high K2O + Na2O, with K2O/Na2O > 1), Rb, Y, Nb, and REEs (except for Eu); high FeOt/MgO and Ga/Al ratios; and low CaO, Al2O3, and Sr contents. New secondary ion mass spectroscopy (SIMS) zircon U–Pb ages reveal that the Luoling granites were emplaced at 1786 ± 7 Ma and thus were approximately coeval with Xiong'er volcanic rocks in the area. Their negative bulk-rock initial Nd and zircon initial Hf isotopic ratios suggest that they have affinities to EM-I-type mantle and both are the products of Xiong'er magmatism during the Palaeoproterozoic. We regard them as produced under a continental rift setting during the Palaeoproterozoic, genetically related to the break-up of the Columbia supercontinent.

PLANETARY-SCALE STRONTIUM ISOTOPIC HETEROGENEITY AND THE AGE OF VOLATILE DEPLETION OF EARLY SOLAR SYSTEM MATERIALS

Isotopic anomalies in planetary materials reflect both early solar nebular heterogeneity inherited from presolar stellar sources and processes that generated non-mass-dependent isotopic fractionations. The characterization of isotopic variations in heavy elements among early solar system materials yields important insight into the stellar environment and formation of the solar system, and about initial isotopic ratios relevant to long-term chronological applications. One such heavy element, strontium, is a central element in the geosciences due to wide application of the long-lived 87 Rb– 87 Sr radioactive as a chronometer. We show that the stable isotopes of Sr were heterogeneously distributed at both the mineral scale and the planetary scale in the early solar system, and also that the Sr isotopic heterogeneities correlate with mass-independent oxygen isotope variations, with only CI chondrites plotting outside of this correlation. The correlation implies that most solar system material formed by mixing of at least two isotopically distinct components: a CV-chondrite-like component and an O-chondrite-like component, and possibly a distinct CI-chondrite-like component. The heterogeneous distribution of Sr isotopes may indicate that variations in initial 87 Sr/ 86 Sr of early solar system materials reflect isotopic heterogeneity instead of having chronological significance, as interpreted previously. For example, given the differences in 84 Sr/ 86 Sr between calcium aluminum inclusions and eucrites (e 84 Sr > 2), the difference in age between these materials would be ∼6 Ma shorter than previously interpreted, placing the Sr chronology in agreement with other long- and short-lived isotope systems, such as U–Pb and Mn–Cr.

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

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, Fiskenaesset 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 Fiskenaesset 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 eHf 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: Fiskenaesset 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 Fiskenaesset, 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 surface to date, could still be present within the Fiskenaesset and Nunataarsuk regions.