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The magnitude of isotopic fractionation during sublimation of ice remains poorly constrained. Field and laboratory studies that span decades have persistently shown conflicting results. A better understanding of fractionation during sublimation is needed to improve interpretation of alpine hydrology, glaciology, paleoclimate and planetary histories that rely on stable isotopic records stored in icy reservoirs. At the core of the problem is the question of whether sublimation occurs as a layer-by-layer process with no fractionation or whether diffusion within the ice and vapor-ice exchange generate fractionation. We present results from an experiment where we suspended ice spheres in temperatures ranging from -25 to -10 °C in an unsaturated atmosphere and used aRayleigh distillation model to estimate fractionation of the spheres. A small, yet statistically significant and repeatable, isotope fractionation (103lnα18O of ∼ −0.6 ‰ and 103lnα2H between −3.0 and − 4.0 ‰ or α = 0.999, α = 0.9940 to 0.9970, respectively) was found during the sublimation of ice. The results indicate the presence of fractionation yet the values are an order of magnitude smaller than would be predicted for equilibrium fractionation at this temperature. By assuming a porosity for the ice of 0.0005 % - which is typical for ice frozen under these conditions, we estimate an increase in diffusivity relative to solid ice consistent with the observed fractionation values. The results help to reconcile how fractionation estimates during sublimation could vary between experimental and observational studies where porosity is not controlled for and can vary substantially across a continuum from porous firn layers to low porosity ice deep in glaciers. • Closed porosity within ice can increase the diffusivity generating a small but observable isotope fractionation. • Secondary processes such as ice compaction can affect porosity, thus the magnitude of isotope fractionation may vary depending on ice type. • Isotope fractionation during sublimation is much lower than equilibrium fractionation predictions, yet statistically significant. [ABSTRACT FROM AUTHOR]

Here we present in situ Rb Sr and Pb isotopic analysis of K-feldspar from the Shap Granite, (Cumbria, UK) and detrital K-feldspar from the associated Shap Wells Conglomerate Formation, to investigate the potential of the detrital K-feldspar record as an archive of crustal composition. We use a unique, collision cell – multi collector inductively coupled plasma tandem mass spectrometer to make the in situ isotope measurements, using 'chemical resolution' of 87Rb+ and 204Hg+ isobaric interferences on 87Sr+ and 204Pb+, achieved with SF 6 and NH 3 reaction gases, respectively. Using this technique, we successfully demonstrate that the majority of detrital K-feldspar Rb Sr ages, as well as initial Sr and Pb isotope ratios, from the Shap Wells Conglomerate are within uncertainty of those for parental Shap Granite K-feldspar (398 ± 1 Ma, n = 6). Some detrital K-feldspar grains record perturbed Rb Sr ages, reflecting post depositional alteration events. The majority of samples with disturbed Rb Sr ages carry distinct petrographic features evident in SEM imaging, prior to in situ dating, which indicates a means to help screen for reliable ages. We further illustrate the potential value of combined Rb Sr and Pb isotopic information preserved in K-feldspar to estimate the timing and chemical composition of juvenile continental crust formed in the past. Where U Pb has been fractionated during juvenile crust formation, the initial Pb isotope ratio provided by the K-feldspar yields model crustal U/Pb (diagnostic of tectonic setting) and extraction age. The contrast in the U/Pb composition of juvenile crust formed in intraplate or subduction settings allows the model U/Pb ratios of the detrital K-feldspars to be used to constrain the style of juvenile crust formation. Additionally, the initial 87Sr/86Sr isotope ratio measured in detrital K-feldspar can be combined with the calculated crustal extraction age and Rb Sr K-feldspar age to model the Rb/Sr of the juvenile crustal protolith. • Magmatic Rb Sr age and initial Pb isotopic compositions can be preserved in detrital K-feldspars. • Model age and U/Pb composition of crustal protolith determined from coupled Rb Sr and Pb analysis of K-feldspars. • U/Pb composition of crustal protolith can be used to infer tectonic setting of its formation. • Protolith age and K-feldspar initial 87Sr/86Sr ratios used to model protolith 87Rb/86Sr and degree of differentiation. [ABSTRACT FROM AUTHOR]

Benthic foraminifera δ18O increased by 5.4 ‰ through the Cenozoic Era, the past 65 million years of Earth's history. This increase is attributed to growth of global ice sheets and global ocean cooling. On the other hand, δ18O measurements of planktonic foraminifera do not exhibit significant change. These measurements initially were interpreted to suggest cool tropical and subtropical temperatures and introduced the concept of the "cool tropics paradox", in which global bottomwater temperatures were significantly warmer but tropical surface temperatures were not during the early Cenozoic. This lack of change in δ18O of planktonic foraminifera was explained by secondary postburial alteration of the primary planktonic isotope signal. We provide an alternative record of sea surface water characteristics of Cenozoic Era by measuring δ18O of siliceous microfossils. This scarce phase of marine silica, unaltered biogenic opal-A, composed of diatoms, radiolaria and sponge spicules, was separated and purified from deep-sea Cenozoic sediments. We measured δ18O of siliceous microfossils from a set of drilled cores: DSDP 278 from the South Pacific, DSDP 513 from the South Atlantic, and ODP 1050–1053 and DSDP 391 from the subtropical North Atlantic. The purity of the biogenic opal and its mineralogical phase was assessed with SEM, SEM/EDS, XPS, and XRD. The δ18O of biogenic opal-A fluctuates around an average value of 43.0 ‰ through the Cenozoic Era, comparable to the Southern Ocean average of 43.2 ‰ of diatoms and radiolaria from the last 30,000 years which represent the last glacial and interglacial period and suggesting cold surface temperatures of 2–7 °C, both in the subtropical and polar latitudes. Taking into consideration changes in δ18O associated with bottomwater interaction with the lower crust and changes in sea level associated with waxing and waning of ice sheets, we conclude lack of or minimal change in the surface water temperature through the Cenozoic Era. This revives the original idea of the "cool tropics" paradox with a carbonate independent δ18O record and without issues associated with diagenesis of benthic and planktonic foraminifera. [ABSTRACT FROM AUTHOR]

Along the Central Indian Ridge (CIR), the geochemical and isotopic signature of mid-ocean ridge basalts (MORB) from three segments between 8° and 12°S show a FOZO-like enrichment with elevated 3He/4He (R/R A) (> 10 R A), suggesting small-scale upwelling with a deep primordial mantle component. Based on complementary major and trace elements analysis as well as Sr, Nd and Pb radiogenic isotope compositions of MORB samples from the same location, we confirm the presence of a FOZO/C-like enriched signature characterized by high Pb isotope ratios (206Pb/204Pb = 18.1134–19.1481; 207Pb/204Pb = 15.4710–15.6146 and 208Pb/204Pb = 37.8625–39.0332), relatively low 87Sr/86Sr (0.702767–0.702974) and high 143Nd/144Nd (0.512989–0.513118). In agreement with recent seismological studies highlighting an asthenospheric anomaly centered under the Mascarene Basin and flowing beneath the CIR, we propose that this plume-like anomaly named Mascarene Basin Asthenosphere Reservoir (MBAR) is the source of the enriched FOZO/C-like signature observed along this portion of the CIR. Furthermore, our analyses reveal that the source of the MBAR anomaly may have a unique geochemical signature in the Indian Ocean, distinct from other Indo-African plumes such as the Réunion plume. Moreover, the primordial characteristics, and the location of the MBAR (next to the margin of the African LLSVP) suggest that this plume-like anomaly may be a disconnected blob/proto-plume of a broad mantle structure formed by different Indo-African plumes/upwelling (e.g., Afar and Bouvet plumes) and anchored in the lower mantle. • Sr-Nd-Pb isotope and trace element confirm a FOZO/C-like signature for CIR MORB. • Interaction between Mascarene Basin Asthenosphere Reservoir (MBAR) and CIR. • MBAR may be an aborted plume branch from the deep mantle, from the African LLSVP. • MBAR unlikely originated from the same plume branch conduit as the Réunion plume. • MBAR component may show a unique geochemical signature in the Indian Ocean. [ABSTRACT FROM AUTHOR]

Understanding changes in redox conditions in the Earth's geological record is fundamental to unravelling some of the major events of our planet's history, such as the Great Oxidation Event (G.O.E.), biological changes through time related to mass extinctions, or the formation of ore deposits for primordial metal resources. Rocks from uranium deposits are ideal candidates to examine the coupled isotopic fractionation of Tl, Mo and U, as they are fundamentally caused by redox processes, occur in a wide variety of geological settings throughout much of geological history (from about 3 Ga), and result from the interplay between fluids and rocks of variable redox states. Herein we measure δ97Mo (−10.2 to +6.1 ‰), ε205Tl (−9.5 to +5.0 ε-units), and δ238U (−0.72 to 0.30 ‰) for 34 samples from granite-related, intrusive, metamorphic, and Proterozoic unconformity related U deposits. Collectively, these results suggest: 1) in metamorphic and unconformity-related contexts, samples with decreasing ε205Tl values also have decreasing δ238U values, indicative of change in redox conditions, 2) ε205Tl values in the sulfide-rich samples likely reflect the affinity of 205Tl for these minerals, 3) Tl undergoes significant isotopic fractionation during the reduction of U-bearing fluids to precipitate uraninite, and 4) Tl isotopes do not fractionate during secondary remobilization of elements within these systems, and thus retain a record of the conditions of formation, whereas Mo isotopes are heavily fractionated by secondary remobilization. We suggest that the study of U, Mo, and Tl isotopes in U-ore deposits conveys new information related to building blocks of life on Earth and shows great potential for the study of past oxygenation events, thus potentially allowing us to reconstruct the geological and chemical history of our planet. • Uranium and thallium isotopes show potential as proxies for redox reactions in U-ore deposits. • Thallium isotopes do not seem to be affected by alteration and secondary remobilization of uranium. • Molybdenum isotopes in U-ore deposit samples are strongly affected by secondary processes. [ABSTRACT FROM AUTHOR]

High-latitude cold-water coral reefs are particularly vulnerable to climate change due to enhanced CO 2 uptake in these regions. To evaluate their physiological functioning and potential application as pH archives, we retrieved both recent and fossil samples of Lophelia pertusa along the Norwegian margin from Oslofjord (59°N), over to Trondheimsfjord, Sula and Lopphavet (70.6°N). Boron isotope analyses (δ11B) were undertaken using solution-based and laser ablation multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS; LA-ICP-MS), and secondary ion mass spectrometry (SIMS). Epi-fluorescence microscopy was employed to provide a rapid pre-screening routine for structure-specific subsampling in the coral skeleton. This integrated approach enabled us to assess heterogeneities within single specimens, as well as to investigate the role of local environmental influences including recent and past variations. All three mass spectrometry methods show substantial differences in the δ11B of the theca wall (TW) and the centres of calcification (COC's). Micro-bulk subsamples milled from the theca wall of modern specimens originating from different habitats but with comparable seawater pH (8–8.16) gave consistent δ11B values averaging 26.7 (±0.2‰, 2σ, n = 4), while COC subsamples systematically deviated towards lower B/Ca (by ~40%) and depleted δ11B values (minimum 22.7 ± 0.3‰, 2σ), implying a difference of at least 4‰ between TW and COC. SIMS and LA-ICP-MS measurements identified much larger internal heterogeneities with maximum variation of ~10‰ between the distinct skeletal structures; minimal SIMS δ11B values of ~17.3 ± 1.2‰ (2σ) were associated with the pure COC material. Our findings may be interpreted in terms of the occurrence of two main, but likely different, biomineralisation mechanisms in L. pertusa , with the COC's generally exhibiting minimal pH up-regulation, potentially supporting the use of bicarbonate in the early stages of biomineralisation. Furthermore, we highlight the potential utility of L. pertusa for palaeo-proxy studies if targeting the compositionally homogenous TW zones devoid of COC admixtures, which appear to provide highly reproducible measurements. [ABSTRACT FROM AUTHOR]

The Proterozoic Eon spans Earth's middle age during which many important transitions occurred. These transitions include the oxygenation of the atmosphere, emergence of eukaryotic organisms and growth of continents. Since the sulfur and oxygen cycles are intricately linked to most surface biogeochemical processes, these transitions should be recorded in changes to the isotopic composition of marine and terrestrial sulfate minerals. Here we present oxygen (∆17O, δ18O) and sulfur (∆33S, δ34S) isotope records of Proterozoic sulfate from currently available data together with new measurements of 313 samples from 33 different formations bearing Earth's earliest unambiguous evaporites at 2.4 Ga through to Ediacaran aged deposits. This record depicts distinct intervals with respect to the expression of sulfate isotopes that are not completely captured by established intervals in the geologic timescale. The most salient pattern is the muted ∆17O signatures across the GOE , late Proterozoic and Ediacaran with values that are only slightly more negative than modern marine sulfate, contrasting with highly negative values across the mid-Proterozoic and Cryogenian. We combine these results with estimates of atmospheric composition to produce a gross primary production (GPP) curve for the Proterozoic. Through these results we argue that changes in GPP across Earth history likely help account for many of the changes in the Proterozoic Earth surface environment such as rising atmospheric oxygen, large fluctuations in the size of the marine sulfate reservoir and variations in the isotopic composition of sedimentary sulfate. [ABSTRACT FROM AUTHOR]

Several mechanisms could produce the biorelevant organic compounds contained in carbonaceous meteorites. The ratio of heavy-to-light stable isotopes can constrain a compound's formation history because substrates, mechanisms, and physiochemical conditions impact isotope ratios of a reaction's products. Prior stable isotope studies of meteoritic organic compounds have interpreted data via qualitative or semi-quantitative models. Here we create quantitative models (i.e. , explicitly fit to measurements) for hydrogen and carbon isotope compositions of organic compounds in primitive carbonaceous meteorite and use these models to reach broader conclusions regarding the environments, substrates, and chemical processes that contributed to their synthesis. The hydrogen isotope model gathers previous molecular-average δD measurements, sorts them into compounds classes (e.g. , amines, carboxylic acids), and fits the measurements in each class as linear combinations of the non-exchangeable hydrogen moieties, which are determined by their chemical environment (e.g. , R-C H 3 , R-C H (NH 2)(COOH)). In the chondrites studied, methyl hydrogens are the most deuterium-enriched moiety (up to 5000 ‰) and hydrogens attached to α‑carbons are the least. Deuterium enrichment is inversely related to a moiety's acidity and a meteorite sample's degree of aqueous alteration and terrestrial weathering, which suggests that ISM-sourced compounds reacted to form deuterium-enriched organic molecules before parent body accretion or during parent body evolution and that those D enrichments were variably attenuated through exchange with water during aqueous alteration on the parent body and subsequent terrestrial processing. The carbon isotope model tests five potential chemical reaction networks by taking δ13C of reactants for each network and applying isotope effects to determine a range of product δ13C from minimal to complete reactant conversion. The model that fits compounds in the Murchison meteorite most accurately (70% of previous measurements fall within the values predicted by model ±5‰) with the lowest average residuals (6‰) uses an integrated aldehyde network (oxidation, reductive amination, and Strecker synthesis acting on an initial pool of aldehydes and ketones) to produce straight-chain compounds, which then react with aldehydes to create branched-chain compounds. The addition of aldehydes provides a parsimonious fit the carbon isotope contents of compounds that span over 100‰ in δ13C, making it an attractive working hypothesis. [ABSTRACT FROM AUTHOR]

The basaltic suites collected at the Apollo 12 landing site have been interpreted as representing a stratigraphic sequence of volcanic flows emplaced in the Oceanus Procellarum region between approximately 3100–3300 Ma. This study presents Secondary Ion Mass Spectrometry (SIMS) Pb isotopic analyses of samples from each of the basaltic suites, which have been used to constrain precise crystallisation ages and initial Pb isotopic compositions. The new crystallisation ages are consistent with the three main basaltic suites (olivine, pigeonite and ilmenite) being emplaced over a period of approximately 60 million years, and the improved precision of these ages has made it possible to reinterpret the stratigraphic sequence of basalt flows underlying the Apollo 12 landing site. Contrary to previous studies, the three ilmenite basalts are determined as having the oldest ages (with a weighted average of 3187 ± 6 Ma; 2σ) and are, therefore, interpreted as representing the lowest unit in the sequence, underlying the olivine and pigeonite basalts (with an age range constrained by the oldest and youngest pigeonite basalts; 3176 ± 6 Ma and 3129 ± 10 Ma; 2σ). The initial Pb isotopic compositions have been compared with recalculated initial Sr and Nd isotopic compositions, and are consistent with the three main basaltic suites originating from magmatic sources that incorporated different proportions of a common primitive mafic cumulate and the residual trapped liquid fraction remaining after a majority of the lunar magma ocean had crystallised. Our data also demonstrate that the feldspathic basalt (12038) is unique, both in terms of its crystallisation age (3242 ± 13 Ma) and its derivation from a distinct mantle reservoir. [ABSTRACT FROM AUTHOR]

Geochemical signatures preserved within the geologic record can be used to reconstruct past mean temperature and seasonality, but in order to accurately apply any geochemical proxy method in the past, a rigorous study of how the recorded proxy is related to temperature in the modern setting must be conducted. Here, we assess the ability of multiple isotope techniques to correctly record mean annual temperature and seasonality in the bivalve Lucina pensylvanica. We compare subannual-resolution δ18O-based, seasonally-targeted and continuous high-resolution (H.R.) clumped isotope (Δ 47)-based thermometry methods, as well as multiple data treatment methods for each, to determine which approach best matches known modern temperatures (maximum and minimum absolute temperature and annual temperature range), with the goal of defining the ideal sampling scheme for use on fossil shells. In L. pensylvanica shells collected from 7 sites, we observe neither mean temperature nor seasonal biases. Mean annual temperature is best matched by averaging all seasonally-targeted Δ 47 -temperatures. Seasonality is best matched by averaging δ18O carb -based temperatures from all summers and all winters before taking the difference. Of two data treatment approaches applied to the continuous high-resolution Δ 47 -based temperatures, "data optimization" is apparently better at resolving smaller seasonal temperature differences. In contrast, "data smoothing" produces a temperature record unbiased by prior assignment of seasonal extremes and has the simultaneous ability to detect subannual variability in δ18O w. However, accurate application of H.R. Δ 47 methods must balance sampling resolution and growth rate. If sampling resolution is high enough relative to the growth rate (∼1 pt./month or better), we recommend continuous high-resolution Δ 47 -thermometry with data smoothing. If this resolution cannot be achieved due to slow growth rates or insufficient shell size, we recommend pairing subannual δ18O carb -based and seasonally-targeted Δ 47 -based temperature reconstruction to acquire seasonal range in temperature and absolute temperature extremes. • We compare methods of isotopic paleothermometry on the bivalve Lucina pensylvanica. • We find no systematic bias in reconstructed mean or seasonal temperatures using Δ 47. • δ18O paleothermometry is improved by using Δ 47 -based estimates of δ18O w. • Sampling plan must balance growth rate, sampling resolution, analytical precision. [ABSTRACT FROM AUTHOR]

We present beryllium (Be) isotopic ratios ( 10 Be/ 9 Be) for 55 volcanic rocks from 21 Quaternary volcanoes in the northeast Japan arc, including the Hokkaido and central Japan areas (the NEJH). Although Be isotopic ratios in these rocks are much lower than in other arcs reported in the literature, they are much higher than in control samples (ocean island basalt (OIB) and Tertiary basalts) that have no recent addition of 10 Be. Hence, our results indicate the incorporation of a component derived from subducted sediments in the Japan arc magmas. Two kinds of Be isotopic variations were observed in the NEJH: 1) variations among lava flows of different ages in the same volcano; and 2) variations among volcanoes in an along-arc or cross-arc direction. In the first case, there are no apparent differences in Be isotopic ratios for lavas of the same age, but relatively large variations among lavas of different ages, and the degree of variation depends on the regional characteristics, indicating that regional differences in near-surface conditions may affect these Be isotopic variations. In the second case, the progressive decrease in Be isotopic ratios in a cross-arc direction suggests the continuous incorporation of subducted sediment, even in the back-arc region. The regional along-arc Be isotopic variations reflect not only the incorporation of sediments into the arc magma, but also regional tectonic and/or geological and geochemical processes. This is the first systematic study of Be isotopic systems in the NE Japan arc, and we provide direct evidence for the incorporation of sedimentary components from the subducting slab into the arc magma, including in the back-arc region at 300 km from the volcanic front, where the Wadati–Benioff Zone is at a depth of 290 km. [ABSTRACT FROM AUTHOR]

In 2002 a tidally driven seawater exchange remediation strategy was successfully implemented on a severely acidified tropical coastal landscape dominated by acid sulfate soils (ASS) in northern Australia. This study examined changes in the stable sulfur isotope signatures in a range of sulfide and sulfate (SO 4 ) fractions at three sites with different levels of exposure to the tidally driven seawater exchange remediation. δ 34 S in the acid soluble SO 4 fraction (e.g. jarosite) was less depleted in 34 S than the corresponding sulfide, indicating a degree of fractionation during sulfide oxidation and jarosite precipitation. The δ 34 S of jarositic-SO 4 was similar at all three sites indicating the appreciable stability of jarositic-SO 4 even after extended exposure to seawater. δ 34 S of the water soluble, exchangeable and schwertmannitic-SO 4 reflect conditions post remediation and indicate the relative contributions from two potential SO 4 sources – a lighter SO 4 derived from the oxidation of pyrite, and a heavier SO 4 derived from the seawater. The δ 34 S of the contemporary surficial sulfide accumulations also reflect a SO 4 contribution from seawater used for remediation and were isotopically different from the relict sulfides found at depth at all sites. δ 34 S of water soluble sulfate allowed the progress of the remediation to be traced down the soil profile. This study demonstrates the utility of stable sulfur isotope signatures in various sulfide and SO 4 fractions to trace the sulfur geochemical pathways occurring in soils, in this case as a result of the introduction of tidally driven sea water. [ABSTRACT FROM AUTHOR]

We report here data of multiple sulfur isotopes ( 32 S, 33 S, and 34 S) measured by NanoSIMS 50L ion microprobe in Fe–Ni–Cu standards, and pyrite grains recovered from the Neoarchean Abitibi Supracrustal Terrain, Canada. Careful attention to instrument tuning, sample preparation and sample stage positioning results in highly precise and accurate data for 32 S– 33 S– 34 S similar to that of larger radius SIMS instruments. Our results using a multi-Faraday collection system demonstrate that the NanoSIMS 50L instrument is capable of high-precision measurements of multiple isotopes of sulfur capable of resolving mass-independent variations in 33 S (∆ 33 S) on the order of 0.4‰ (2σ) with improvements possible with higher S count rates. Pyrite grains from the Abitibi Supracrustal terrane have a large range of δ 34 S values between − 9.6 and + 6.3‰, and corresponding ∆ 33 S values between − 0.8 and + 1.5‰, consistent with other datasets from Neoarchean sedimentary rocks. The new data indicate the incorporation of atmospheric sulfur affected by mass-independent fractionation (MIF) of 33 S into the Neoarchean marine sulfur cycle and point to subsequent sulfate reducing microorganisms at that time. [ABSTRACT FROM AUTHOR]

High-precision stable Ba isotope ratios are reported in a variety of terrestrial samples, undifferentiated primitive meteorites, and calcium–aluminum-rich inclusions (CAIs) from the Allende chondrite. All whole-rock terrestrial and meteorite samples are isotopically indistinguishable at a 50 parts per million (ppm) level per atomic mass unit (amu). Three CAIs are isotopically light, with δ 138/137 Ba (permil deviation of the 138 Ba/ 137 Ba ratio from a terrestrial standard) values down to − 0.6‰ compared to whole-rock meteorites, whereas the matrix is enriched in heavy isotopes (δ 138/137 Ba: + 0.2‰). Similar light isotope enrichments in CAIs have been previously observed for Eu, Sr, and Ca, while for most other elements CAIs are enriched in the heavier isotopes (e.g. Mg, Fe). Kinetic isotopic fractionation is a possible explanation for the enrichment in the lightest isotopes, either by condensation from a vapor phase enriched in light isotopes by kinetic effects or by kinetic fractionation during non-equilibrium condensation of an undercooled gas as suggested for Ca isotopes. However, the common property of Ba, Eu, and Sr is that they all have a low first ionization potential. We suggest that electromagnetic sorting of ionized species in the early Solar System is a possible alternative mechanism to explain the depletion in heavy isotopes observed in refractory inclusions for those elements. [ABSTRACT FROM AUTHOR]

The significance of magnesium isotope (δ 26 Mg) fractionation in the continental hydrothermal domain is poorly explored. Here, a detailed Mg isotope dataset from various aquifer host rocks and corresponding hydrothermal carbonate precipitates from the Schwarzwald mining area in SW Germany is documented and discussed. This study is motivated by the very considerable existing data set on hydrothermal mineralizations in the study area and the excellent regional control of sampling points. Based on structural, mineralogical and microthermometrical arguments, we here subdivide three clusters of veins: (i) Permian, (ii) Jurassic–Cretaceous and (iii) post-Cretaceous (Cenozoic). The focus is on clusters ii and iii and their corresponding, texturally older hydrothermal fluid inclusion-rich, coarse-grained, low-Mg calcite and dolomite–ankerite solid solutions and younger, oscillatory zoned low-Mg calcite and dolomite–ankerite solid solution crystals precipitated from fluids with temperatures between 50 and 350 °C. In terms of their δ 26 Mg ratios, three characteristic groups of hydrothermal carbonates can be distinguished: (i) Jurassic–Cretaceous veins (δ 26 Mg = − 3.38 to − 0.82‰) in the central Schwarzwald precipitated from fluids containing a significant proportion of sedimentary cover-derived waters. They yield 26 Mg-depleted Mg isotope signatures typical of sedimentary carbonate lithologies. (ii) Jurassic–Cretaceous vein carbonates in the southern Schwarzwald precipitated from mainly basement-derived fluids with 26 Mg-enriched signatures (δ 26 Mg = − 1.22 to + 0.05‰). Supporting evidence for the origin of these fluids comes from Sr–C–Pb isotope systematics and trace element compositional variations of fahlore and sphalerite. The Mg isotope variations are controlled by differences in fluid source characteristics dominate over variations exerted by mineralogical differences (where low-Mg calcites are generally, but not in all cases, more depleted in 26 Mg compared to dolomite–ankerite solid solutions). (iii) Post-Cretaceous (Cenozoic), Rhinegraben-related veins represent a complex tectonic juxtaposition of different aquifer lithologies. As expected, this phase spans the full range from silicate to carbonate Mg isotope signatures (+ 0.45 to − 3.4‰). Magnesium isotope data are supported by textural analyses (optical and cathodoluminescence microscopy), electron microprobe, microthermometry and published radiogenic 87 Sr/ 86 Sr isotope systematics. Our data are not in agreement with a significant temperature-controlled Δ 26 Mg fluid-calcite and no obvious relation between fluid salinity and hydrothermal carbonate δ 26 Mg is found. The results of this study suggest that hydrothermal carbonate magnesium isotope ratios have a significant potential as tracer of hydrothermal fluid sources and corresponding aquifer lithologies. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the initial results of a scientific drilling project to recover core and pressurized fluid samples from a natural CO2 reservoir, near the town of Green River, Utah. The drilling targeted a stacked sequence of CO2-charged Jurassic sandstone reservoirs and caprocks, situated adjacent to a CO2-degassing normal fault. This site has actively leaked CO2 from deep supercritical CO2 reservoirs at depth >2km within the basin for over 400,000years. The project objectives were to gather samples to examine reactive fluid flow in the reservoirs, caprocks and faults, during migration of CO2 through the geological overburden from the deep supercritical CO2 reservoirs. Downhole fluid sampling and fluid element and isotope geochemistry show that the shallow reservoirs are being actively fed by inflow of CO2-saturated brines through the faults. Comparisons of shallow and deep fluid geochemistry suggest that: (i) CO2 and CO2-charged brines co-migrated from the deep reservoirs, (ii) the CO2-saturated brines migrating from depth interact with significant volumes of meteoric groundwater in aquifers in the shallower Permian and Jurassic sandstones, diluting the brine composition, and (iii) that a significant fraction of the CO2 migrating from depth is dissolved in these brine–meteoric water mixtures, with >99% of the CO2 in fluids sampled from the shallow reservoirs being derived during fluid migration, after the fluids left their source reservoir. The 87Sr/86Sr ratio of the brine flowing through the faults is significantly elevated due to the addition of Sr from silicate mineral dissolution during fluid migration. The association of bleached sandstones in the core with CO2-rich fluids supports interpretations from elsewhere that CO2-charged brines with CH4 or H2S reductants can dissolve hematite present within the sediment. Analysis of fluid geochemistry and sandstone petrology suggests that the CO2-rich fluids dissolve carbonate, hematite and gypsum in the reservoirs, as they flow away from the faults. Element and isotope geochemistry of fluid samples from the drillhole and Crystal Geyser constrain mixing models which show that, within the Navajo Sandstone, the reservoir fluids are undergoing complex mixing of: (i) CO2-saturated brine inflowing from the fault, (ii) CO2-undersaturated meteoric groundwater flowing through the reservoir and (iii) reacted CO2-charged brines flow through fracture zones in the overlying Carmel Formation caprock, into the formations above. Such multi-scale mixing processes may significantly improve the efficiency with which groundwaters dissolve the migrating CO2. [Copyright &y& Elsevier]

Abstract: The knowledge of past permafrost conditions is of importance to assess the potential magnitude of changes that periglacial environments may experience as a result of climate warming or disturbance. To assess if past thaw unconformities may be preserved from isotopic and geochemical discontinuities within permafrost, this study investigates the distribution of ground ice, stable water isotopes and major cations in two permafrost cores collected in a hummocky terrain site near Inuvik, Northwest Territories, Canada; a site where the evolution of the active layer during a recent period of permafrost degradation and subsequent aggradation was documented. Based on the high-resolution isotope geochemistry profiles, closed-system Rayleigh-type ionic segregation and isotope fractionation occurred during thermally-induced water migration into shallow permafrost and its freezing along a negative soil temperature gradient. Due to thermally-induced water migration into permafrost, δ18O may not always be able to identify thaw unconformities; however the calculation of the 18O enrichment factors between ice and water (ε18Oi−w) may be used to determine position of thaw unconformities in permafrost, if thaw events are followed by permafrost aggradation. The approach of using ε18Oi−w provides additional information regarding past permafrost conditions that can complement change in cryostructures observed along natural exposures. [Copyright &y& Elsevier]

Abstract: Traditional column chromatography techniques are restricted by several factors, including limitations on column length, resin grain size, elution time, and a general inability to slowly ramp up or down reagent concentrations along a gradient. Likewise, most existing high-performance liquid chromatography (HPLC) systems are not amenable to certain column chromatography techniques that require highly concentrated acids. Here, we outline the development of a Teflon HPLC (T-HPLC) system for application to a wide variety of chromatography problems. The primary factors that set the T-HPLC system apart from any currently available chromatography procedure are the following: 1) a fluid flow path enclosed entirely by Teflon, 2) fully automated elution schemes controlled by computer software, which allows for fresh mixing of reagents for each elution step, and fine scale gradient/ramp elutions, 3) temperature control of the entire system (up to 80°C) for enhanced chemical separations and, 4) a modular design making the system easily adaptable to a variety of separation schemes. The effectiveness of the T-HPLC system is tested on two column techniques that are of particular interest to the geochemistry/cosmochemistry communities. The first application involves the separation of Ni from Mg, which is required for high precision Ni isotopic studies and for investigating the abundance of the extinct 60Fe radionuclide. The T-HPLC system greatly simplifies and improves upon the classical technique. In a single pass on an 80cm long column, we achieve excellent separation of Ni from Mg, with a much improved time frame (10h versus 70h). The second application is the separation of the individual rare earth elements from each other. The isotopic compositions of the multi-isotopic REEs (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb and Lu) may hold important information about nucleosynthetic processes, cosmic-ray exposure effects in meteorites and airless bodies, and mass fractionation effects. For this application, we also developed a computer simulation that uses experimentally determined partition coefficients to simulate an elution curve in order to optimize the actual column elution scheme. Overall, we were able to achieve excellent separation of the multi-isotopic REEs from each other. Although the two applications that we explore are in the fields of geochemistry/cosmochemistry, the modular design and adaptability make the T-HPLC system useful to a wide array of scientific fields and industries. [Copyright &y& Elsevier]

Abstract: Ubiquitous heterogeneity in the Earth''s mantle has been documented by numerous chemical and isotopic analyses of oceanic basalts. Despite the ever-increasing amount of data, the way in which compositional heterogeneity is manifest in the Earth''s mantle, as well as the processes leading to mantle heterogeneity remain fundamental questions. The large amount of available isotope data in oceanic basalts shows that, statistically, only two principal compositional vectors capture the essential features of the data. Care must be taken, however, when estimating the isotopic composition of mantle from basalt samples. This is because partial melting, and melt mixing during melt extraction leads to a biased representation and subdued compositional variability in the basalts relative to their mantle sources. In both ridge and ocean island settings, for example, erupted lavas are expected to be isotopically less depleted than the most depleted source components. Abyssal peridotites indeed range to much more depleted isotope compositions than mid ocean ridge basalts (MORB). The extent of heterogeneity of the MORB mantle source, the depleted mantle, therefore depends on the proportion, as well as differences in composition, age, and sampling of its various depleted and enriched source components. While MORB data thus do not reflect the full extent of mantle heterogeneity, the large amount of trace element and isotope data in ocean island basalts (OIB) suggests that enriched isotope signatures in OIB closely correspond to those of their average enriched mantle components. OIB can therefore be used to trace the geologic reservoirs that exchange mass with the mantle and to identify the geological processes that introduce enriched material into the Earth''s mantle. The generation and subduction of oceanic plates into the deeper mantle, together with small amounts of lower and upper continental crust, appears to be the main process for mantle enrichment. Thereby, erosion and subduction of the lower continental crust accounts for a large part of the enriched isotope signatures in oceanic basalts. Recycling of the upper continental crust, on the other hand, is inferred to be only a minor process, but required to explain the entire spectrum of enriched OIB signatures. Hence a first order geologic process – the generation and subduction of oceanic plates – accounts for the first-order heterogeneity of the Earth''s mantle. Moreover, one of the main processes for establishing the composition of the continental crust – erosion and recycling of the lower continental crust – is also one of the main processes for the generation of mantle heterogeneity. Overall, large-scale chemical cycling between Earth''s two major lithophile element reservoirs, the mantle and the oceanic and continental crust, is responsible for mantle enrichment. Once introduced into the mantle, the heterogeneous materials become stretched, reduced in size and distributed by mantle convection. The isotopic heterogeneity observed in melt inclusions and abyssal peridotites suggests that eventually, the heterogeneity of the mantle sources of oceanic basalts will exist at relatively small scales, certainly on the kilometer scale of the melting region but perhaps even smaller. The way in which mantle heterogeneity is manifest in the source of oceanic basalts is therefore directly related to the fluid dynamics of mantle convection, whereas the timing, nature, and extent of crust–mantle interaction govern the differentiation and compositional evolution of the silicate Earth. [Copyright &y& Elsevier]

Abstract: We introduce a method to analyze element ratios and isotope ratios in mineral overgrowths. This general technique can quantify environmental controls on proxy behavior for a range of cultured biominerals and can also measure compositional effects during seeded mineral growth. Using a media enriched in multiple stable isotopes, the method requires neither the mass nor the composition of the initial seed or skeleton to be known and involves only bulk isotope measurements. By harnessing the stability and sensitivity of bulk analysis the new approach promises high precision measurements for a range of elements and isotopes. This list includes trace species and select non-traditional stable isotopes, systems where sensitivity and external reproducibility currently limit alternative approaches like secondary ion mass spectrometry (SIMS) and laser ablation mass spectrometry. Since the method separates isotopically labeled growth from unlabeled material, well-choreographed spikes can resolve the compositional effects of different events through time. Among other applications, this feature could be used to separate the impact of day and night on biomineral composition in organisms with photosymbionts. [Copyright &y& Elsevier]

Abstract: The Sawayaerdun gold deposit is hosted by Carbonaceous metasediments and is considered to be the largest Muruntau-type gold deposit in the Chinese Tianshan metallogenic belt. Gold mineralization at Sawayaerdun occurs in quartz veins associated with three major hydrothermal events: an early, barren quartz vein stage, middle stage mineralized quartz veins with pyrite and late carbonate (-quartz) veins. The isotopic compositions of quartz and sulfides from the Sawayaerdun gold deposit show some variation but are generally comparable to those of other orogenic-type gold deposits. Fluids trapped in early-stage quartz have a δ18O range of 13.6‰ to 15.4‰, δD of −48‰ to −75‰, δ13C of 0.5‰ to 4.2‰ and δ30Si of −0.2‰ to 0‰. In contrast, isotopic compositions of fluids trapped in middle-stage quartz have δ18O values of 6.7‰ to 14.7‰, δD of −56‰ to −110‰, δ13C of 0.4‰ to 10.1‰ and δ30Si of −0.3‰ to 0‰. Diagenetic and hydrothermal pyrite have similar sulfur (−1.8‰ to 0.9‰) and Pb isotopic values that are associated with host rock compositions. The early-stage, 18O and 13C-rich fluids are probably derived from metamorphic decarbonation of the sedimentary host rock at depth, leading to the precipitation of early barren quartz veins. In the middle stage, a decrease in the regional pressure and temperature regime could have resulted in the incorporation of external fluids into the ore-forming system. These external fluids with isotopic signatures similar to that of the host rock and generally rich in 34S and radiogenic Pb mixed with original ore-forming fluids to generate extensive metal precipitation. Late-stage fluids trapped by calcite veins show isotopic compositions similar to meteoric water, indicating the cessation of hydrothermal fluid circulation at Sawayaerdun occurred at this time. The metallogenetic model illustrated by stable and Pb isotopes is also consistent with fluid inclusion studies in Sawayaerdun. The development of mineralization at Sawayaerdun is strongly linked to fluid mixing, as witnessed by the isotopic signatures of fluids from identified ore-bearing zones. The isotopic compositions of other anomalous zones at Sawayaerdun are similar to those of the mineralized zones, suggesting a high potential for further exploration. [Copyright &y& Elsevier]

Abstract: There is a substantial debate on the mantle sources and ultimate origins of continental flood basalts (CFBs), e.g., whether they are related to deep-seated thermal upwellings (i.e. mantle plumes) in the upper mantle or not. Here we present high-precision isotopic (Sr, Nd, Pb, and Os) whole-rock data on some primitive dike rocks (ferropicrites, meimechites, picrobasalts, and basalts) associated with the Antarctic extension of the Jurassic (~180Ma) Karoo CFB province. The isotopic data together with previously published trace element data show that the parental melts of the studied rocks sampled two distinctive geochemical reservoirs in the deep sub-Gondwanan mantle. The isotopic signatures of the relatively depleted types show evidence of extensive melt extraction in the past and are indistinguishable from those of mid-ocean ridge basalts (MORBs) of the SW Indian Ridge. On the other hand, the relatively enriched type isotopically resembles modern oceanic island basalts (OIBs) and may sample pyroxenitic sources either formed by melt infiltration in the upper mantle or by reaction of peridotite with recycled oceanic crustal components (with up to 15% of sediment material). The depleted types have previously been associated with anomalously hot mantle sources (>1600 °C) and mantle plumes, but their MORB-like isotope compositions are unusual for plume-derived rocks. Instead, our findings are more concordant with a recent model [Coltice, N., Bertrand, H., Rey, P., Jourdan, F., Phillips, B.R., Ricard, Y., 2009. Global warming of the mantle beneath continents back to the Archean. Gondwana Research 15, 254–266] that suggests the generation of the Karoo CFBs in an extensive melting episode caused by internal heating of the upper mantle beneath the Gondwana supercontinent. [ABSTRACT FROM AUTHOR]

Abstract: The oxygen isotope ratio of alpha-cellulose produced by Bristlecone Pine trees from the White Mountains of California was measured for three different 10-year tree-ring sections at near-weekly resolution to investigate how the growing season climate cycle influences the annual to inter-annual isotopic variability of whole-ring samples. We selected sections of wood that grew under periods of generally reduced (155–172 BC) and increased (AD 1106–1122; AD 1827–1834) growth to assess whether or not the environmental parameters that are responsible for ring-width variability, including water availability, also influence the intra-annual cellulosic isotope variability. Each annual ring is characterized by a parabolic function of intraseasonal δ18O change that reflects humidity and temperature-dependent fractionations of H2 18O utilized by the tree during the growth period. Subtle shifts in the shape and amplitude of the seasonal δ18O cycle are attributed to changing growing season length but the differences are not large enough to bias a whole-ring isotopic value. We model the isotope fractionations based on growing season climate variables and tree physiology and find that the measured δ18O is in close agreement with the model-predicted values. The model retains no information on previous winter conditions and thus confirms that these trees draw upon an isotopically homogenized water source. The intraseasonal δ18O cycle arises entirely from fractionations due to summer season temperature and humidity. A persistent issue with dendroclimatic reconstructions is addressing seasonal biases in the record, we find that in this environment where growth and precipitation are separated by many months we are able to capture distinct information on both seasons by coupling average annual values with information on sub-seasonal variability. [Copyright &y& Elsevier]

Isotopic disequilibrium is not as well constrained as equilibrium, hindering interpretation of isotopic variations. Kinetic isotope effects, a subset of disequilibrium, are common in nature and have been assumed to be distinct from equilibrium and mass independent isotope effects based on underdeveloped criteria. Using basic physical principles, we provide needed mechanistic constraints on mass-dependent kinetic isotope effects for the triple oxygen isotope system. We find some kinetic isotope effects yield large isotopic variations, exceeding equilibrium, which could be mistaken for mass independent relationships. Meanwhile, other kinetic isotope effects are found to have triple oxygen isotope relationships that could be mistaken for equilibrium isotope effects. Comparison against prior case studies of thermal decomposition of calcite (CaCO 3) and brucite (Mg(OH) 2) further tests our results. Although oxygen is the focus here, our approach applies to any system with more than two isotopes. [ABSTRACT FROM AUTHOR]

Abstract: The useful yields (ions detected/atom sputtered) of major and trace elements in NIST 610 glass were measured by secondary ion mass spectrometry (SIMS) using Cameca IMS 3f and 6f instruments. Useful yields of positive ions at maximum transmission range from 10−4 to 0.2 and are negatively correlated with ionization potential. We quantified the decrease in useful yields when applying energy filtering or high mass resolution techniques to remove molecular interferences. The useful yields of selected negative ions (O, S, Au) in magnetite and pyrite were also determined. These data allow the analyst to determine if a particular analysis (trace element contents or isotopic ratio) can be achieved, given the amount of sample available and the conditions of the analysis. [Copyright &y& Elsevier]

Abstract: Many marine radiogenic isotope records show both spatial and temporal variations, reflecting both the degree of mixing of distinct sources in the oceans and changes in the distribution of chemical weathering on the continents. However, changes in weathering and transport processes may themselves affect the composition of radiogenic isotopes released into seawater. The provenance of physically weathered material in the Labrador Sea, constrained through the use of Ar–Ar ages of individual detrital minerals, has been used to estimate the relative contributions of chemically weathered terranes releasing radiogenic isotopes into the Labrador Sea. A simple box-model approach for balancing observed Nd-isotope variations has been used to constrain the relative importance of localised input in the Labrador Sea, and the subsequent mixing of Labrador Sea Water into North Atlantic Deep-Water. The long-term pattern of erosion and deep-water formation around the North Atlantic seems to have been a relatively stable feature since 1.5 Ma, although there has been a dramatic shift in the nature of physical and chemical weathering affecting the release of Hf and Pb isotopes. The modelled Nd isotopes imply a relative decrease in water mass advection into the Labrador Sea between 2.4 and 1.5 Ma, accompanied by a decrease in the rate of overturning, possibly caused by an increased freshwater input into the Labrador Sea. [Copyright &y& Elsevier]

New B and Li isotopic data are presented for well-characterized basalts from a transect across the Cascades volcanic arc, S. Washington, for a sediment core on the incoming Juan de Fuca plate, and for dacites from Mt. St. Helens. The basalts define two compositional groups, one (I) having affinities to those from intraplate settings and the other (II) resembling typical volcanic arc calcalkalic lavas. These groups are inferred to have distinct sources. Group I lavas have low Li/Y (0.25–0.6) and B/Nb (∼0.1–0.45), near-constant Ba/Nb (7–18), and show little cross-arc variation in fluid-mobile/immobile element ratios. In contrast, Group II lavas have slightly elevated Li/Y (∼0.5) and higher Ba/Nb (21–100) regardless of location; B/Nb is enriched (to ∼2.2) in the frontal arc, but toward the backarc drops to lower values (<0.4) overlapping with intraplate basalts. δ11B values are among the lowest reported for volcanic arcs. In detail, δ11B exhibits spatial variation, being highest (∼−3‰ to −0.4‰) in frontal arc lavas then decreasing sharply to OIB-like values (−8‰ to −9‰) in the backarc region; no systematic variation with magma types is apparent. δ7Li values (2.5‰ to 5‰) are MORB-like and show no systematic spatial variation. Combined with Sr–Nd–Pb isotopic and other trace element data, these results are consistent with weak slab contributions beneath the frontal arc ranging to negligible inputs below the backarc. This pattern is attributed to progressive dehydration of the subducting slab in an unusually warm subduction zone (SZ); initial inventories of fluid-mobile elements (FMEs) apparently were reduced extensively before the slab reached subarc depths, and essentially depleted as it moved beneath the backarc. Slab contributions appear to be superimposed on a compositionally layered mantle structure consisting of two domains sampled by the distinct basalt groups. Group I lavas appear to form by decompression melting of convectively upwelling deeper mantle that is little modified from an OIB–MORB source composition, except for the relatively small effect on B composition beneath the frontal arc. Group II lavas may form by melting of enriched domains within shallower lithospheric mantle. Because both groups have similar δ11B, production of Group II magmas need not involve modern inputs of slab fluid. This interpretation differs from the standard paradigm for arc magmatism in that flux melting is not invoked as the dominant cause for partial melting; efficient dehydration of the slab may remove the impetus to drive this mechanism. [Copyright &y& Elsevier]

Iwojima volcano, located on the southernmost part of the Izu–Ogasawara arc, is characterized by a magma chemistry, which differs from that of the other volcanoes in the arc, suggesting a larger contribution of subducted component. In order to elucidate the behavior of volatile elements during the subduction process in generating this unusual magma, noble gas and carbon isotope ratios of volcanic volatiles were measured. The noble gas and carbon isotopic characteristics of Iwojima magma are quite distinct from other volcanoes in the Izu–Ogasawara–Mariana arc system: (1) a low 3He/4He ratio of 5.6RA; (2) enrichment in heavy noble gases; and (3) a heavy carbon isotope ratio (+1.5‰) almost equivalent to the highest value for marine carbonates. These unusual features strongly suggest the involvement of slab-derived volatiles in the magma, although the possibility of a shallow origin for the low 3He/4He ratio still remains. Based on a mixing calculation using helium–carbon isotope systematics, an extremely large contribution of slab-derived components (31% for 4He and more than 95% for 12C) is required to explain the unusual features of helium and carbon isotopes of Iwojima. This is in agreement with other petrological and geochemical features which imply a greater role of subducted materials, although the precise relative contributions of each source materials are somewhat different for several of the tracers. This difference is probably caused by different provenance in subducting materials carrying them to melt generation depth in the mantle wedge. The elevated carbon/helium ratio of the slab-derived component also suggests decoupling during the subduction process probably resulting from the preferential loss of helium from the subducting slab beneath the forearc region. Enhanced outputs of slab-derived carbon and helium (3.5×109 and 4.0×103 mol/year) indicate a large input of slab component to the mantle wedge and/or effective devolatilization of slab materials. As for its origin, we believe that the unusual tectonic setting around Iwojima may be responsible: (1) subduction of the Ogasawara Plateau, which brings a significant volume of thick crustal blocks into the mantle wedge beneath Iwojima; (2) flattened subduction as a consequence of the buoyancy of the subducting Ogasawara Plateau; and (3) propagation of the back-arc basin rift axes in the Mariana Trough. In contrast to large-scale subduction of overthickened oceanic crust associated with slab melting, the subduction of a relatively small oceanic plateau may be responsible for the characteristics of the Iwojima magma, in which slab material itself does not play an important role in magma genesis but slab-derived volatiles make a significant contribution. [Copyright &y& Elsevier]

Equilibrium Cr-isotope (53Cr/52Cr) fractionations are calculated using published vibrational spectra and both empirical and ab initio force-field models. Reduced partition function ratios for chromium isotope exchange, in terms of 1000×ln(β53–52), are calculated for a number of simple complexes, crystals, and the Cr(CO)6 molecule. Large (>1‰) fractionations are predicted between coexisting species with different oxidation states or bond partners. The highly oxidized [Cr6+O4]2− anion will tend to have higher 53Cr/52Cr than coexisting compounds containing Cr3+ or Cr0 at equilibrium. Substances containing chromium bonded to strongly bonding ligands like CO will have higher 53Cr/52Cr than compounds with weaker bonds, like [CrCl6]3−. Substances with short Cr-ligand bonds (Cr–C in Cr(CO)6, Cr–O in [Cr(H2O)6]3+ or [CrO4]2−) will also tend have higher 53Cr/52Cr than substances with longer Cr-ligand bonds ([Cr(NH3)6]3+, [CrCl6]3−, and Cr-metal). These systematics are similar to those found in an earlier study on Fe-isotope fractionation (Geochim. Cosmochim. Acta 65 (2001) 2487).The calculated equilibrium fractionation between Cr6+ in [CrO4]2− and Cr3+ in either [Cr(H2O)6]3+ or Cr2O3 agrees qualitatively with the fractionation observed during experimental (probably kinetic) reduction of [CrO4]2− in solution (Science 295 (2002) 2060), although the calculated fractionation (∼6–7‰ at 298 K) does appear to be significantly larger than the experimental fractionation (3.3–3.5‰). Our model results suggest that natural inorganic Cr-isotope fractionation at the earth''s surface may be driven largely by reduction and oxidation processes. [Copyright &y& Elsevier]

Multi-collector inductively coupled plasma mass spectrometry now provides for precise and accurate measurements of Cu isotope ratios. Copper minerals prepared by direct dissolution with and without chromatographic purification yield identical Cu isotope ratios within analytic precision of about 0.04‰ (1σ). Cu isotope ratios have been measured for copper minerals from worldwide magmatic and hydrothermal copper deposits, and for several weathered deposits. Natural variations in δ65Cu values, relative to NBS976, range over 9‰. Chalcopyrite samples from mafic intrusions lie within a narrow range of about 1.5‰, and most cluster tightly between −0.10‰ and −0.20‰. This range lies within the broader black smoker chalcopyrite and iron meteorite ranges, and possibly represents a bulk mantle Cu isotope ratio. Most values for hydrothermal native copper from the Michigan native copper district also show a narrow range just larger than 0.1‰ and suggest a common homogeneous source for Cu in this large hydrothermal system. Later copper sulfide and arsenide minerals from this district range to values more than 2‰ lower than native copper. Ratios for chalcopyrite and bornite from moderate to high-temperature porphyry, skarn, and replacement deposits as a group and within individual deposits exhibit a broad range of values. Variations of nearly 1‰ are observed over distances on the order of 1 m. In some cases, these variations may result from multiple mineralization events or copper remobilization during retrograde or later hydrothermal activity. Fractionations between chalcopyrite and bornite, where they occur in the same sample or in related samples, cluster near 0.4‰, suggesting equilibrium Cu isotope fractionation between them at moderate temperatures. In addition, weathering of hydrothermal copper minerals produces a wide range of values in secondary copper phases. In the supergene environment, cuprite typically has higher values than associated native copper. Therefore, redox states appear to exert a significant control over fractionation at low temperatures. Significant questions remain to be answered. Before the source of copper in hydrothermal environments can be fully addressed, source reservoir Cu ratios need to be determined, and chemical and physical factors that control Cu isotope fractionation must be quantitatively defined. [Copyright &y& Elsevier]

ODP Hole 801C penetrates >400 m into 170-Ma oceanic basement formed at a fast-spreading ridge. Most basalts are slightly (10–20%) recrystallized to saponite, calcite, minor celadonite and iron oxyhydroxides, and trace pyrite. Temperatures estimated from oxygen isotope data for secondary minerals are 5–100 °C, increasing downward. At the earliest stage, dark celadonitic alteration halos formed along fractures and celadonite, and quartz and chalcedony formed in veins from low-temperature (<100 °C) hydrothermal fluids. Iron oxyhydroxides subsequently formed in alteration halos along fractures where seawater circulated, and saponite and pyrite developed in the host rock and in zones of restricted seawater flow under more reducing conditions. Chemical changes include variably elevated K, Rb, Cs, and H2O; local increases in FeT, Ba, Th, and U; and local losses of Mg and Ni.Secondary carbonate veins have 87Sr/86Sr=0.706337–0.707046, and a negative correlation with δ18O results from seawater–basalt interaction. Carbonates could have formed at any time since the formation of Site 801 crust. Variable δ13C values (−11.2‰ to 2.9‰) reflect the incorporation of oxidized organic carbon from intercalated sediments and changes in the δ13C of seawater over time.Compared to other oceanic basements, a major difference at Site 801 is the presence of two hydrothermal silica–iron deposits that formed from low-temperature hydrothermal fluids at the spreading axis. Basalts associated with these horizons are intensely altered (60–100%) to phyllosilicates, calcite, K-feldspar, and titanite; and exhibit large increases in K, Rb, Cs, Ba, H2O, and CO2, and losses of FeT, Mn, Mg, Ca, Na, and Sr. These effects may be common in crust formed at fast-spreading rates, but are not ubiquitous. A second important difference is that the abundance of brown oxidation halos along fractures at Site 801 is an order of magnitude less than at some other sites (2% vs. 20–30%). Relatively smooth basement topography (<100 m) and high sedimentation rate (8 m/Ma) probably restricted the access of oxygenated seawater. Basement lithostratigraphy and early low-temperature hydrothermal alteration and mineral precipitation in fractures at the spreading axis controlled permeability and limited later flow of oxygenated seawater to restricted depth intervals. [Copyright &y& Elsevier]

We report on 55 Nd–Sr–Pb and 23 Hf isotopic compositions for late Miocene to Plio–Quaternary basalts from the Arabian Plate. The sampling profile represents a 2500 km N–S transect along the Red Sea margin from Syria to Yemen. 206Pb/204Pb displays a wide range (from 18.60 to 19.55) from south to north, indicating a pervasive high-μ distribution that questions the previous attribution of this component to the Afar plume. We rather suggest that the high-μ signature resides within the Arabian lithospheric mantle.Compared to basalts from northern and central Arabia (Saudi Arabia to Syria), basalts from southern Arabia (Yemen) display similar ranges for 143Nd/144Nd, 176Hf/177Hf, and 206Pb/204Pb, but are shifted towards compositions more radiogenic in Sr and 208Pb. These distinct time-integrated Rb/Sr and Th/U ratios between north and south are believed to reflect heterogeneous development of the Arabian lithosphere through time and/or result from the thermal effect of the Afar plume on the southern part of the lithosphere. [Copyright &y& Elsevier]

The composition of Kuerti mafic rocks in the Altay Mountains in northwest China ranges from highly geochemically depleted, with very low La, Ta and Nb and high &epsiv;Nd(t) values, to slightly enriched, arc lava-like composition. They display flat to light rare earth element (REE)-depleted patterns and have variable depletions in high field-strength elements (HFSE). These mafic rocks were most probably derived from a variably depleted mantle source containing a subduction component beneath an ancient intra-oceanic backarc basin. Together with the slightly older arc volcanic rocks in the Altay region, the Kuerti mafic rocks display generally positive correlations of their key elemental ratios (e.g., Th/Nb, La/Yb and Th/Yb). These indicate that the more mid-ocean ridge basalt (MORB) component was contained in these magmas, the less arc component was present in their mantle source. Therefore, we propose a two-stage melting evolution model to interpret the compositional evolution of the Kuerti mafic rocks and associated arc volcanic rocks. First, arc basaltic melts were extracted from the hydrated arc mantle wedge beneath Kuerti, leaving behind a mantle source that is variably depleted in incompatible trace elements. Then, mafic rocks were erupted during seafloor spreading in the Kuerti backarc basin from the upwelling asthenospheric mantle. The variably depleted mantle source produced mafic rocks with composition ranging from arc lava-like to more geochemically depleted than MORB. The recognition of Kuerti mafic rocks as backarc basin basalts (BABB) is consistent with the proposed tectonic model that an active backarc basin–island arc system along the paleo-Asian ocean margin was formed in the Altay region during Devonian–Early Carboniferous. New data further indicate that the final orogenic event in the Altay Mountains, i.e. the collision of the north and south continental plates in the region, most probably took place in Late Carboniferous and Permian. [Copyright &y& Elsevier]

Reconstructing fluid evolution history in old sedimentary basins is extremely challenging due to the prolonged evolution, lack of exact age constraints and absence of preserved fluids. By integrating in-situ calcite U Pb dating, fluid inclusion analysis, measurements of major and trace elements and carbon‑oxygen‑strontium isotopes, and molecular composition analysis of petroleum, we have reconstructed the fluid evolution history during the Carboniferous in an Ordovician deeply-buried carbonate reservoir in the Tarim Basin, China, with unprecedented temporal resolution. Five generations of calcite cementation spanning over 37 m.y. are dated in a single calcite vein with U Pb ages ranging from ~353 Ma to ~316 Ma. The initial cement was associated with an intrusion of hyper-saline brine, enriched in REEs. This was followed by influx of meteoric water to precipitate the second and third generations of cement, which are characterized by reduced salinity, anomalously high 87Sr/86Sr ratios and extremely low concentration of REEs. The fourth generation of calcite cement was formed in a deep-fluid system accompanied by oil charge as indicated by the presence of bitumen, oil inclusions and low δ13C and 87Sr/86Sr ratios. The fifth generation of cement was precipitated from equilibrated connate water, and has the lowest salinity and a comparable REE signature to the host rock. Oil charge event occurred around 326 Ma. This study demonstrates the effectiveness of combining in-situ calcite U Pb geochronology, fluid inclusion analysis, elemental and isotopic geochemistry to elucidate high-resolution temporal fluid evolution in old sedimentary basins with a complex tectonic history, providing a new paradigm for studying geofluid evolution. [ABSTRACT FROM AUTHOR]

Noble gases record fluid interactions in multiphase subsurface environments through fractionation processes during fluid equilibration. Water in the presence of hydrocarbons at the subsurface acquires a distinct elemental signature due to the difference in solubility between these two fluids. We find the atmospheric noble gas signature in produced water is partially preserved after hydrocarbons production and water disposal to unlined ponds at the surface. This signature is distinct from meteoric water and can be used to trace oil-field water seepage into groundwater aquifers. We analyse groundwater (n = 30) and fluid disposal pond (n = 2) samples from areas overlying or adjacent to the Fruitvale, Lost Hills, and South Belridge Oil Fields in the San Joaquin Basin, California, USA. Methane (2.8 × 10−7 to 3 × 10−2 cm3 STP/cm3) was detected in 27 of 30 groundwater samples. Using atmospheric noble gas signatures, the presence of oil-field water was identified in 3 samples, which had equilibrated with thermogenic hydrocarbons in the reservoir. Two (of the three) samples also had a shallow microbial methane component, acquired when produced water was deposited in a disposal pond at the surface. An additional 6 samples contained benzene and toluene, indicative of interaction with oil-field water; however, the noble gas signatures of these samples are not anomalous. Based on low tritium and 14C contents (≤ 0.3 TU and 0.87–6.9 pcm, respectively), the source of oil-field water is likely deep, which could include both anthropogenic and natural processes. Incorporating noble gas analytical techniques into the groundwater monitoring programme allows us to 1) differentiate between thermogenic and microbial hydrocarbon gas sources in instances when methane isotope data are unavailable, 2) identify the carrier phase of oil-field constituents in the aquifer (gas, oil-field water, or a combination), and 3) differentiate between leakage from a surface source (disposal ponds) and from the hydrocarbon reservoir (either along natural or anthropogenic pathways such as faulty wells). [ABSTRACT FROM AUTHOR]

Understanding the age and movement of groundwater is important for predicting the vulnerability of wells to contamination, constraining flow models that inform sustainable groundwater management, and interpreting geochemical signals that reflect past climate. Due to both the ubiquity of groundwater with order ten-thousand-year residence times and its importance for climate reconstruction of the last glacial period, there is a strong need for improving geochemical dating tools on this timescale. Whereas 14C of dissolved inorganic carbon and dissolved 4He are common age tracers for Late Pleistocene groundwater, each is limited by systematic uncertainties related to aquifer composition and lithology, and the extent of water-rock interaction. In principle, radiogenic 40Ar in groundwater acquired from decay of 40K in aquifer minerals should be insensitive to some processes that impact 14C and 4He and thus represent a useful, complementary age tracer. In practice, however, detection of significant radiogenic 40Ar signals in groundwater has been limited to a small number of studies of extremely old groundwater (>100 ka). Here we present the first high-precision (<1‰) measurements of triple Ar isotopes (40Ar, 38Ar, 36Ar) in groundwater. We introduce a model that distinguishes radiogenic 40Ar from atmospheric 40Ar by using the non-radiogenic Ar isotopes (36Ar, 38Ar) to correct for mass-dependent fractionation. Using this model, we investigate variability in radiogenic 40Ar excess (Δ40Ar) across 58 groundwater samples collected from 36 wells throughout California (USA). We find that Δ40Ar ranges from ~0‰ (the expected minimum value) to +4.2‰ across three study areas near Fresno, San Diego, and the western Mojave Desert. Based on measurements from a network of 23 scientific monitoring wells in San Diego, we find evidence for a strong dependence of Δ40Ar on aquifer lithology. We suggest that Δ40Ar is fundamentally controlled by the weathering of old K-bearing minerals and thus reflects both the degree of groundwater-rock interaction, which is related to groundwater age, and the integrated flow through different geological formations. Future studies of Late Pleistocene groundwater may benefit from high-precision triple Ar isotope measurements as a new tool to better interpret 14C- and 4He-based constraints on groundwater age and flow. [ABSTRACT FROM AUTHOR]