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Potassium (K) isotopic compositions for a suite of crustal rocks (plutonic unit, sheeted dike complex, and volcanic sequence) in the Troodos ophiolite, Cyprus, were determined to investigate fluid-transport processes during subduction inception. The gabbros and plagiogranites in the plutonic unit have restricted K isotopic compositions (0.01–0.20 ‰ and −0.17 to 0.02 ‰, respectively), whereas diabases in the sheeted dike complex have variable δ41K values from −0.24 to 1.13 ‰. The δ41K values of the volcanic sequence range from −0.26 to −0.07 ‰ in the lower pillow lavas, and from −0.71 to 0.34 ‰ in the upper pillow lavas. These δ41K values are clearly distinct from those of subducting sediments and altered oceanic crust, ruling out direct crustal input as the cause of the unusually heavy K isotopic compositions. Rather, the high K/Th, K/U, and Sr/Th ratios of the investigated samples, together with binary mixing modelling, reveal that the anomalously heavy K isotopic compositions in Troodos rocks are attributed to the influx of fluids during subduction inception. Notably, the δ41K values in the crustal rocks of the Troodos ophiolite are more variable and extend to higher values than those of arc-front and rear-arc lavas of the Izu arc and arc lavas of Martinique in the Lesser Antilles, indicating continuous K isotope fractionation during slab dehydration. Thus, the heavy K isotopic compositions of the Troodos ophiolite reflect K isotope fractionation during dehydration of a shallow slab within a newly formed subduction system, where heavy K isotopes preferentially enter into the aqueous fluids. Later, progressive breakdown of K-bearing phases at deeper levels results in isotopically lighter hydrous melts, which can be delivered to the arc. [ABSTRACT FROM AUTHOR]

Olivine-rich lavas in the Margi area, Cyprus, are part of the boninitic Upper Pillow Lava series of the Troodos ophiolite. Re-Os isotope compositions of both whole rock and olivine phenocrysts of these lavas have been measured to determine the initial 187Os/188Os isotope compositions of the primary boninitic magma. The Margi lavas have high and variable whole rock Re-Os isotope compositions (187Re/188Os: 1.060–8.613; 187Os/188Os: 0.1307–0.1599), with (87Sr/88Sr) 90Ma (0.705936 ± 0.001021), ε Nd (90 Ma) (5.6 ± 2.5), and δ18O (8.59 ± 0.38 ‰). In contrast, olivine phenocrysts, which host spinel and melt inclusions, have low and constant 187Os/188Os from 0.1301 to 0.1336 and low 187Re/188Os from 0.1350 to 1.557. Initial 187Os/188Os of these olivine phenocrysts (0.1300 ± 0.0041) is lower than that of whole rock (0.1373 ± 0.0135). These facts indicate that the olivine phenocrysts faithfully record initial 187Os/188Os of the primary boninitic magmas, which are comparable to the 187Os/188Os of primitive mantle, whereas whole rock Re-Os isotopes have been affected by secondary processes after olivine crystallization. Further geochemical modeling indicates that, although Os can be mobilized during slab dehydration, the contribution of crustal Os is insufficient to significantly raise the 187Os/188Os of the mantle wedge in a nascent subduction zone. Overall, our findings provide new insights about Os recycling in the early stages of subduction. [ABSTRACT FROM AUTHOR]

The origin of methane in hydrothermal fluids has long been a subject of debate – whether it is abiotic or biotic. In this study, we aim to unravel and quantify the sources of CH 4 in active hydrothermal systems by adopting a holistic approach analyzing well characterized high-temperature hydrothermal fluids (∼230–310 °C) in Iceland. We employ a broad variety of geochemical and isotope indicators, encompassing chemical and isotope compositions of the targeted fluids. These signatures are then compared with results from chemical and isotope kinetic models and data from sedimentary-hosted hydrothermal systems. Carbon species in these fluids include CO 2 (2.60–184 mmol/kg), CH 4 (2.39·10−4–0.325 mmol/kg), dissolved organic carbon (4.78·10−3–0.112 mmol/kg), and CO (1.89·10−6–4.16·10−4 mmol/kg). Carbon and helium isotopes suggest a relatively uniform mantle-derived source of CO 2 (δ13C-CO 2 : −4.80 to −1.50 ‰, CO 2 /3He: 1.49·109–4.14·1010 14C-CO 2 : 0.11–2.42 pMC). Methane, in contrast, has multiple sources. Overall, chemical equilibria among carbon species (CO 2 , CH 4 , CO) is not attained, suggesting kinetic controls. Tritium content (<0.8–1.42 TU) and hydrologic constraints indicate relatively short hydrothermal fluid residence times (∼5–200 years), with occasional inputs from older water components. Within this short timeframe, CH 4 concentrations vary from lower, to significantly higher than those calculated using CO 2 reduction kinetics. The isotope composition (δD-CH 4 : −172 to −138 ‰, δ13C-CH 4 : −32.0 to −24.6 ‰; 14C-CH 4 : 0.36–11.54 pMC) and geochemical and isotope modeling suggest that the majority (>80–90 %) of CH 4 originates from a radiocarbon inactive source, i.e. mantle CH 4 , reduction of mantle CO 2 and/or old organic matter, with relatively small contributions from both marine (<20 %) and terrestrial (<10 %) dissolved organic carbon. Measured isotopic compositions of CH 4 do not match those expected for mantle-derived CH 4 as well as values generated from reduction of mantle-derived CO 2. Instead, differences in δD-CH 4 and δ13C-CH 4 values exist between systems fed by meteoric water and those fed by seawater, challenging the assumption of a uniform CO 2 source and invariable reaction mechanisms. Differences between systems are best explained by variable extent of thermal decomposition and primary variations in the isotope composition of marine and terrestrial organic matter. Also, δD-CH 4 and δ13C-CH 4 values in meteoric water-fed systems closely resemble those in the Öxarfjördur sedimentary-hosted systems. In summary, our data supports a predominant thermogenic origin of CH 4 in both seawater and terrestrial hydrothermal fluids in Iceland. The source of organic matter appears to be a combination of modern dissolved organic carbon and older sedimentary deposits. In addition, some of the hydrothermal systems studied (Krafla, Reykjanes, Theistareykir) which are characterized by low CH 4 concentrations, may contain a significant portion of CH 4 that may originate from CO 2 reduction. [ABSTRACT FROM AUTHOR]

Titanium isotopes are a promising tracer for planetary differentiation processes. The application of this tracer is, however, currently hampered by the lack of a robust estimate for the chondritic reservoir. Here, we conducted an inter-comparison Ti isotope study of three laboratories with the aim of providing an accurate and precise estimate for the chondritic reservoir. While previous estimates may suffer from heterogeneities on the sampling scale, we chose ordinary chondrites to minimise uncertainties associated with the necessary corrections for nucleosynthetic isotope variations in chondrites, and to allow the analysis of sufficiently large sample sizes representative for bulk meteorites. Titanium isotope data reported by the different laboratories are in good agreement with each other. Ordinary chondrites of different subgroups (H, L, LL) and petrologic types (3–6) display identical Ti isotope compositions within uncertainties (average δ49Ti = +0.023 ± 0.009 ‰, 2SE, n = 20; permille deviation of 49Ti/47Ti from the OL-Ti standard). The average Ti isotope composition of ordinary chondrites is within 2SE identical to that of OIBs (+0.029 ± 0.005 ‰, 2SE, n = 52) and all pre 2.7 Ga mafic and komatiitic rocks (+0.019 ± 0.006 ‰, 2SE, n = 58), indicating that the δ49Ti values of the bulk silicate Earth and ordinary chondrites are indistinguishable. Furthermore, our average Ti isotope composition of ordinary chondrites overlaps with those of the Moon, Mars and Vesta, suggesting a homogeneous inner Solar System in terms of mass-dependent Ti isotopes. [ABSTRACT FROM AUTHOR]

Accurately predicting apparent isotope effects of biogeochemical processes can aid the interpretation of laboratory batch experiments and field observations. It has been observed that the apparent instantaneous isotope fractionation factor changes with residual substrate concentration during a reaction process. The change can be attributed to the reversibility of a biogeochemical reaction, which is often overlooked for convenience. Here, we explore the influence of reverse reactions on high-dimensional isotope effects in the framework of Reverse Michaelis-Menten (RMM) kinetics. The apparent instantaneous isotope fractionation factors, as well as the isotope fractionation relationships between two elements of the same nonlabile compound in the RMM model, are functions of specific affinities of the product and substrate to an enzyme and the product-substrate ratios for both isotopes. In our cases, the influence of reaction reversibility is negligible when the substrate consumption rate is less than 63%. The analysis of the simultaneous stable isotope fractionation of two elements in a compound and of the relationship between the associated isotope fractionation factors can serve as a more effective indicator for characterizing the reaction mechanism. [ABSTRACT FROM AUTHOR]

The CM carbonaceous chondrites are key archives for understanding the earliest history of the solar system. Their C-complex asteroid parent body(ies) underwent aqueous alteration, among the products of which are carbonate minerals that can faithfully record the conditions of their formation. In this study we report carbon, triple oxygen and clumped isotope compositions of carbonates in six CM chondrites which span a range in degrees of aqueous alteration (Allan Hills 83100, Cold Bokkeveld, LaPaz Icefield 031166, Lonewolf Nunataks 94101, Murchison, Scott Glacier 06043). Δ17O values range from −2.6 to −1.0 ‰ (±0.1), and where calcite and dolomite co-exist their Δ17O differ by 0.6 permil, suggesting precipitation from distinct fluids. Calculated crystallization temperatures range from 5 to 51 °C for calcite (typically ± 10 °C) and 75 to 101(±15) °C for dolomite. The δ18O VSMOW of the aqueous fluids from which they formed ranges from −6.6 to 2.3 ‰, with no relationship to the δ13C of carbonates. As the population of carbonates in any one CM chondrite can include multiple generations of grains that formed at different conditions, these values represent the mode of the temperature of carbonate formation for each meteorite. We observe that in the more altered meteorites carbonate Δ17O values are lower and formation temperatures are higher. These correlations are consistent with aqueous alteration of the CM chondrites being a prograde reaction whereby the hotter fluids had undergone greater isotope exchange with the anhydrous matrix. Our data are broadly consistent with the closed system model for water/rock interaction, but carbonate mineral formation in the latter stages of aqueous alteration may be linked to fluid movement via fractures. [ABSTRACT FROM AUTHOR]

The magnesium isotopic composition (δ26Mg) of river water is a promising indicator of continental chemical weathering. While many studies have investigated the factors that influence riverine δ26Mg, the impact of hydrology remains unclear. In this study, we collected eighty-four samples of stream water with nearly diurnal resolution in 2018 from a well-monitored, carbonate-rich catchment on the Chinese Loess Plateau. Our results demonstrate that δ26Mg in stream water increases (0.14 ± 0.05 ‰) from dry to wet seasons, but decreases (0.21 ± 0.05 ‰) during rainfall events. These variations closely link to the dissolution and deposition of carbonates (i.e., source-related processes), and the adsorption and desorption of the exchangeable pool. Carbonate dissolution during rainfalls lowers the δ26Mg in stream water, while during rainfall-free periods in wet seasons carbonate deposition elevates the δ26Mg. Conversely, the exchangeable pool, reflecting carbonate weathering in the geological past, cannot be a source of Mg in stream waters, but act as a transfer Mg-pool. At an instantaneous picture, it releases the majority of Mg (>80 %) to stream water, and thereby has buffering effect on riverine Mg isotope. This highlights the significance of considering the buffering effects when studying riverine δ26Mg variations. Overall, our findings suggest that the response of δ26Mg to hydrology is typically associated with extreme hydrologic events and has important implications for tracing continental weathering. [ABSTRACT FROM AUTHOR]

Hydrothermal alteration is crucial in the formation of many ore deposits, with potassium (K) mobilization and cycling being prevalent. Potassic metasomatism of wall rocks generally forms K-bearing minerals, such as hydrothermal feldspar and mica. However, determining the source and redistribution of K (and other elements transported by the same fluid) in hydrothermal systems is challenging. K isotopes offer a potential solution to this problem. This study presents new K isotope data from two K-rich alteration assemblages — K-feldspar and sericite-quartz-pyrite — in the Jiaodong gold province of China. The data covers a compositional range from unaltered granites to syn -magmatic potassic alteration (formation of K-feldspar) and post-magmatic syn -mineralization phyllic alteration (formation of sericite). Potassic alteration in granite correlates with significant K addition, whereas phyllic alteration of earlier phases of magmatic and hydrothermal K-feldspar resulted in K loss. K-feldspar altered granites display similar δ41K values (–0.55 to –0.42 ‰ for whole-rocks and –0.56 to –0.48 ‰ for K-feldspar separates) as unaltered granite (–0.52 to –0.47 ‰). The narrow δ41K range suggests that magmatic fluid exsolution and magmatic-hydrothermal alteration have a minor effect on δ41K of the altered rock. Phyllic alteration of K-feldspar altered precursor rock leads to K loss and elevated δ41K values ranging from –0.36 to –0.19 ‰ for whole-rocks and –0.34 to –0.17 ‰ for sericite mineral separates. As sericite preferentially incorporates 41K, sericite will have higher δ41K values than the precursor K-feldspar, whereas the fluids will have lower δ41K values. Our study demonstrates that hydrothermal alteration may affect the K isotope composition of altered rocks in several ways, contingent on the nature of the involved phases, making K isotopes a promising tool for studying hydrothermal alteration and associated mineralization. [ABSTRACT FROM AUTHOR]

The role of metasomatized mantle in the genesis of magmatic Ni–Cu sulfide deposits in orogenic belts remains vigorously debated. The greatest challenge in clarifying uncertainties is how to track the behavior of metals during metasomatic processes in the mantle. Copper isotopes can shed new light on this enigmatic topic as the transfer of Cu during metasomatic processes can significantly fractionate Cu isotopes. In this regard, we have characterized the Cu isotope and trace-element compositions of fertile and barren mafic–ultramafic intrusions, and coeval basalts in the Eastern Tianshan, Central Asian Orogenic Belt (CAOB). Fertile mafic–ultramafic intrusions are characterized by elevated and highly variable Ba/Nb (40–756, average of 172) and Ba/La (16–219, average of 48) ratios, and exhibit δ65Cu values (−0.97 ‰ to +0.57 ‰) that vary over 1.54 ‰. In contrast, barren intrusions and coeval Permian basalts are characterized by relatively low and invariable Ba/Nb (2–95, average of 29) and Ba/La (0.9–26, average of 8) ratios, and have δ65Cu values that are comparatively less variable (−0.06 ‰ to +0.49 ‰ for δ65Cu) and more similar to bulk silicate Earth (BSE, δ65Cu = 0.06 ± 0.20 ‰). The difference in Cu isotope compositions of fertile and barren intrusions, alongside the variability in Ba enrichment, suggests that a high degree of mantle metasomatism promotes the fertilization of primary magmas in metals and, thus, represents one of the first stages in the development of magmatic Ni–Cu sulfide deposits in convergent margins. [ABSTRACT FROM AUTHOR]

Cave carbonate minerals are an important terrestrial paleoclimate archive. A few studies have explored the potential for applying carbonate clumped isotope thermometry to speleothems as a tool for constraining past temperatures. To date, most papers utilizing this method have focused on mass-47 clumped isotope values (Δ 47) at a single location and reported that cave carbonate minerals rarely achieve isotopic equilibrium, with kinetic isotope effects (KIEs) attributed to CO 2 degassing. More recently, studies have shown that mass-47 and mass-48 CO 2 from acid digested carbonate minerals (Δ 47 and Δ 48) can be used together to assess equilibrium and probe KIEs. Here, we examined 44 natural and synthetic modern cave carbonate mineral samples from 13 localities with varying environmental conditions (ventilation, water level, pCO 2 , temperature) for (dis)equilibrium using Δ 47 -Δ 48 values, in concert with traditional stable carbon (δ13C) and oxygen (δ18O) isotope ratios. Data showed that 19 of 44 samples exhibited Δ 47 -Δ 48 values indistinguishable from isotopic equilibrium, and 18 (95 %) of these samples yield Δ 47 -predicted temperatures within error of measured modern temperatures. Conversely, 25 samples exhibited isotopic disequilibria, 13 of which yield erroneous temperature estimates. Within some speleothem samples, we find Δ 47 -Δ 48 values consistent with CO 2 degassing effects, however, the majority of samples with KIEs are consistent with other processes being dominant. We hypothesize that these values reflect isotopic buffering effects on clumped isotopes that can be considerable and cannot be overlooked. Using a Raleigh Distillation Model, we examined carbon and oxygen isotope exchange trajectories and their relationships with dual clumped isotope disequilibria. Carbon isotope exchange is associated with depletion of both Δ 47 and Δ 48 relative to equilibrium, while oxygen isotope exchange is associated with enrichment of both Δ 47 and Δ 48 relative to equilibrium. Cave rafts collected from proximate locations in Mexico exhibit the largest average departures from equilibrium ( Δ Δ 47 ¯ = −0.032 ± 0.007, Δ Δ 48 ¯ = −0.104 ± 0.035, where ΔΔ i is the measured value – the equilibrium value). This study shows how the Δ 47 -Δ 48 dual carbonate clumped isotope framework can be applied to a variety of tcave carbonate mineral samples, enabling identification of isotopic equilibria and therefore quantitative application of clumped isotope thermometry for paleoclimate reconstruction, or alternatively, constraining the mechanisms of kinetic effects. [ABSTRACT FROM AUTHOR]

The fate of carbon in subduction is explored by considering fluid-rock interactions in a body of metamorphosed ultramafic rocks from the Appalachian belt as potential proxies for metasomatism and carbon recycling (e.g., infiltration, solution, precipitation) in the mantle wedge at convergent margins. Ultramafic rocks may record intense redox variations affecting the exchanges between solid and fluid carbon-bearing phases. The Belvidere Mountain Complex (BMC) in Vermont, USA is an ultramafic body that underwent Ordovician Taconic subduction metamorphism up to 510–520 °C and 0.9–1.3 GPa. Previous investigations indicate that the BMC experienced partial serpentinization and carbon recycling in the subduction zone. Here bulk δ11B, 87Sr/86Sr and δ13C, data, in-situ microRaman spectroscopy and bulk δ13C CH4 and δ2H CH4 on fluid inclusions, and numerical modeling results aim to constrain the origin and formation mechanisms of carbon-bearing solid and fluid phases in the BMC. Bulk 87Sr/86Sr ratios and δ11B of the BMC rocks suggest infiltration of metasediment-derived fluids inside the mafic/ultramafic body. Carbonates present in the studied ultramafic and mafic rocks have δ13C values ranging from −7.26 to −1.27 ‰. This range was found to record progressive reduction of an initial isotopically light carbonate to methane under reducing conditions. MicroRaman spectroscopy on fluid inclusions confirms CH 4 -rich gaseous compositions, along with N 2 , NH 3, and S-H compounds, which support a metasedimentary origin for the fluid infiltrating the BMC. Methane-rich fluid inclusions were observed in partially serpentinized peridotites, in carbonate veins, and in amphibolite bodies associated with the ultramafic body, and as well in the surrounding metasedimentary rock units. The δ13C and δ2H of methane in fluid inclusions show a wide range of values in ultramafic rocks from −45.2 ‰ to −12.6 ‰ for carbon and −226 ‰ to −140 ‰ for hydrogen. The methane stable isotope data suggest the presence of two types of methane preserved in studied samples, formed by high-temperature thermogenic processes of metamorphosed graphitic carbon and by abiotic conversion of more oxidized carbon bearing species in the presence of H 2 -rich fluids produced during metamorphic serpentinization. The BMC also hosts a remarkable example of graphite deposit, with up to 3.90 wt% graphite, along a ∼ 6 m-thick zone within the serpentinized ultramafic body and along lithologic contacts. The δ13C composition of the graphite clusters at −15 ‰ (VPBD), which is consistent with the composition of graphite precipitating from the mixing of the two types of methane-bearing fluids identified in fluid inclusions. The mixing between metasediment-derived and serpentinization-related methane-rich fluids is proposed as the mechanism leading to graphite precipitation. Further complexity is suggested by carbonate reduction evidenced in the samples, which may have locally contributed to the formation of graphite on the precursor carbonate domains. The BMC rocks highlight the variability of carbon recycling processes, including carbon mobilization from multiple solid sources, and mixing of multiple carbon-bearing fluids in a dynamic fluid-rock system, with carbon mobilization and sink adapting to evolving redox conditions. [ABSTRACT FROM AUTHOR]

Large Arctic rivers are an important source of iron (Fe) to the Arctic Ocean, though seasonal variations in the terrestrial source and supply of Fe to the ocean are unknown. To constrain the seasonal variability, we present Fe concentrations and isotopic compositions (δ56Fe) for particulate (>0.22 µm) and colloidal (<0.22 µm–1 kDa) Fe from the Lena River, NE Russia. Samples were collected every month during winter baseflow (September 2012–March 2013) and every 2–3 days before, during and after river ice break-up (May 2015). Iron in particles have isotope ratios lower than crustal values during winter (e.g., δ56Fe Part = −0.37 ± 0.16‰), and crustal-like values during river ice break-up and spring flood (e.g., δ56Fe Part = 0.07 ± 0.08‰), indicating a change in the source of particulate Fe between winter and spring flood. Low isotope values are indicative of mineral dissolution, transport of reduced Fe in sub-oxic, ice-covered sub-permafrost groundwaters and near-quantitative precipitation of Fe as particles. Crustal-like isotopic compositions result from the increased supply of detrital particles from riverbank and soil erosion during river ice break-up and flooding. Iron colloids (<0.22 μm) have δ56Fe values that are comparable to or lower than crustal values during winter (e.g., δ56Fe Col = −0.08 ± 0.05‰) but similar to or higher than crustal values during spring flood (e.g., δ56Fe Col = +0.24 ± 0.11‰). Low δ56Fe ratios for colloidal Fe during winter are consistent with precipitation from isotopically light Fe(II) aq transported in sub-permafrost groundwaters. Higher colloidal δ56Fe ratios during the spring flood indicate that these colloids are supplied from surface soils, where Fe is fractionated via oxidation or organic carbon complexation, similar to during summer. Approximately half of the annual colloidal Fe flux occurs during spring flood while most of the remaining colloidal Fe is supplied during summer months. The total amount of colloidal Fe transported during winter was relatively low. The seasonal variation in colloidal Fe isotope values may be a useful tool to trace the source of colloidal Fe to the Arctic Ocean and monitor future changes in the sources and supply of Fe from the permafrost landscape to the Lena River basin. [ABSTRACT FROM AUTHOR]

The Cambrian explosion is a critical evolutionary milestone in life history, but the mechanistic relationship between the Cambrian explosion, rising atmospheric and oceanic oxygen levels, and the primary productivity remains controversial. Here, we present new mercury (Hg) isotope data from Cambrian marine sediments of the ∼521–514 Ma Yu'anshan and Shuijingtuo Formations (Nanhua Basin, South China), which preserve the famous ∼518 Ma Chengjiang and Qingjiang Cambrian Biotas, respectively. We find evidence that, prior to Cambrian animal diversification, local terrestrial and/or atmospheric inputs are important drivers of Hg enrichment in the shallow shelf Yu'anshan and Shuijintuo Formations. Elevated Hg and total organic carbon (TOC) concentrations of coeval deeper shelf and slope sections of the Niutitang Formation are mainly attributed to upwelling of Hg associated with dissolved organic carbon (Hg-DOC) from pelagic seawater, unrelated to locally high primary productivity. During the deposition of fossiliferous strata, negative shifts of Δ199Hg and δ202Hg values in the shallow water shelf shales from the Shuijingtuo Formation, combined with similar Hg isotope characteristics in the deeper shelf and slope shales, demonstrate a significant shallow and deep-water oxygenation event took place in the Nanhua Basin. A comparable negative shift of Δ199Hg values occurs in coeval Indian Craton sediments, suggesting this oxygenation event could be regional, or possibly even global, in scale. Our newly-collected Hg isotope data provide strong evidence that rising molecular oxygen levels in both surface and deep seawater are associated with enhanced marine primary productivity and could be a critical driver of the Cambrian explosion. Finally, we also argue that Hg isotopes are an emerging and promising redox proxy for studies of Precambrian seawater, but the chemical response of Hg to atmospheric and ocean oxygenation requires further calibration. [ABSTRACT FROM AUTHOR]

Soil is one of the largest reservoirs and re-emission sources of mercury (Hg) on Earth's surface. The accumulation and remobilization of Hg during pedogenesis affect how Hg is transported from terrestrial to aquatic environment and biota, but these processes are poorly understood. Here we present Hg concentration and isotope ratios of soil and bedrock samples from a latosol profile formed through intense weathering of Cenozoic basalts in Zhanjiang, Guangdong Province, China, to trace the sources, transport and transformation processes of Hg during pedogenesis, and to gain insights on the potential impact of tropical soils on global Hg cycling. The entire soil profile shows negative δ202Hg (−2.97‰ ± 0.08‰ to −2.54‰ ± 0.08‰) and negative Δ199Hg (−0.68‰ ± 0.04‰ to −0.43‰ ± 0.04‰), with a gradual downward positive shift of Δ199Hg below 35 cm. An isotopic mixing model based on Δ199Hg suggests a dominant Hg input from atmospheric Hg (Hg Atm) and limited contribution (<36%) of geogenic Hg (Hg Geo) from the bedrock. The strongly negative Δ199Hg throughout the latosol profile suggests that Hg Atm was likely subject to substantial photoreduction on soil surface and migrated downward after incorporation into soils. Moreover, there is a distinct positive shift of δ202Hg alongside a rapid decrease of Hg Geo from the bedrock to the weakly weathered basalt, indicating a substantial loss of Hg Geo with the preferential release of lighter isotopes during the initial weathering, likely due to the dissolution of primary minerals. In addition, the δ202Hg shows a negative correlation with soil pH at some horizons, attributable to the isotopic fractionation during Hg(II) speciation change in soil solution and the selective complexation/adsorption of different Hg(II) species onto mineral surfaces, which are affected by surface charge properties and thus soil pH. Overall, our results provide direct evidence for the accumulation and downward migration of Hg Atm in tropical soils during pedogenesis, as well as the loss of Hg Geo during the weathering of bedrock, which may serve as an underappreciated source of Hg in tropical regions and have a potential impact on the Hg isotope signatures in aquatic environments. [ABSTRACT FROM AUTHOR]

Formation and post-genetic alteration of hydrocarbons provide insights into the dynamic and complex geologic, hydrologic, and microbial history of shallow crustal environments. Clumped isotopologues of methane (e.g., Δ13CH 3 D) have emerged as a proxy for constraining methane formation temperatures in sedimentary basins. However, unrealistically high apparent temperatures and microbial cycling of methane necessitate further investigation into how the generation and biodegradation of hydrocarbons may modify methane clumped isotopologue signatures. This study analyzed and modeled the clumped isotopologues of methane, in addition to traditional gas isotopes, to provide new insights into the origin, thermal maturity, migration, and biodegradation histories of hydrocarbons in the Paradox Basin in the Colorado Plateau. The basin was deeply buried in the geologic past and has been recently incised, leading to rapid denudation, enhanced meteoric circulation, and microbial activity. δ13C CH4 and CH 4 /ΣC 2+ ratios suggest that most natural gases in various reservoirs throughout the basin are thermogenic in origin with variable thermal maturities. However, signatures suggestive of anaerobic oxidation of ethane and propane, and secondary microbial methane generation, exist. In the northeastern part of the basin, Δ13CH 3 D values in reservoirs above and below the Paradox Formation source rocks are consistent with thermodynamic equilibrium, indicating that the thermally mature hydrocarbons equilibrated at ≥160 °C during maximum burial over 30–80 Ma. Disequilibrium Δ13CH 3 D values of natural gas in Paradox Formation reservoirs along the southwestern margin of the basin suggest the presence of low-maturity hydrocarbons consistent with the region's shallower burial history. Models of Δ13CH 3 D values based on the exchange rate of hydrogen isotopes between methane and water and the basin thermal history support that meteoric recharge and microbial activity, following incision/denudation over the past few million years, promoted anaerobic oxidation of hydrocarbons (particularly ethane and propane), biodegradation of crude oil, and generation of secondary microbial methane in shallow reservoirs. [ABSTRACT FROM AUTHOR]

We investigate the chlorine isotope disequilibrium between chlorides in pore fluids and chlorides in seawater and infer its consequences for chlorine isotope exchange between ocean sediment pore fluids and seawater. We illustrate our methodology with pore fluids from two IODP drilled to depths of ≈1000 m (U1456 and U1457) in the Indus River fan dominated by clay-detritus sediments of the western Himalayas. At these two sites, the concentrations of chloride and sodium ions do not show significant changes with depth and remain very close to those of seawater. As a function of depth, however, chlorides show a progressive decrease in 37Cl (downto −2.5 and −1.4 ‰ respectively), while Ca2+ increases and Mg2+ decreases, as commonly observed in clay-rich oceanic sediments. The rate of the δ37Cl decrease is correlated to the lithology and sedimentation rate. Examining the chlorine budget on these two sites, we conclude that: (1) Most of the chlorine (≥96% of the total Cl) is contained in the pore fluids as chlorides, while the chlorine stored in the other sinks (minerals or organochlorine) is generally very minor (representing <4% of the total Cl). (2) These other sinks are too small to sequester the lost 37Cl-enriched chlorine, which could compensate for the decreased δ37Cl of the chlorides observed in the pore fluids (assuming that the pore fluids originally had chlorides with the chlorine isotopic composition of seawater, δ37Cl = 0 ‰). (3) This lost 37Cl-enriched chlorine has been released from the sediment into the overlying seawater, with fluxes of about 20 mol of chloride per square metre per kiloyear. In order to evaluate the consequence of this unbalanced on a global scale, we extend this chlorine budget analysis to 22 other oceanic sites drilled by IODP in a variety of tectonic environments. It is estimated that worldwide oceanic sediment pore fluids contain 1 × 1020 mol of chloride with mean δ37Cl at −2.3 ± 0.1 ‰ and mean chlorinity at 0.290 ± 0.055 mol Cl- per kg of sediment. Most of the calculated values of the lost 37Cl-enriched chlorine flux are between 3.5 and 45.6 mol of chloride per square metre per kiloyear (mean value of 18.3 mol Cl.m−2.kyr−1), corresponding to a typical chloride exchange time between sediments and seawater of between 5.5 and 71 Myr with a mean value of 14 Myr. Importantly, in oceanic sediments, a relationship between the δ37Cl of the chlorides in their pore fluids and the nature of the detrital minerals (smectite, illite, kaolinite, carbonate, quartz, ...) may exists. This relationship should be modulated by the detrital input from the continents to the oceanic sediments and should therefore be a consequence of the climatic conditions on the continents (mainly rain, temperature). The δ37Cl of the seawater should vary according to these detrital inputs. The expected range of variation would be between −0.3 and +0.3 ‰. This is consistent with the range of δ37Cl in seawater predicted by the study of evaporites deposited since 2 Ga (Eggenkamp et al., 2019). [ABSTRACT FROM AUTHOR]

Large-scale helium accumulations are rarely reported in tectonically active continental subduction zones. This study shows geochemical variations in helium and other volatiles (N 2 , CO 2 , Ne and Ar) in hot springs from the India–Asia continental convergent margin (Indus–Yarlung suture, IYS) to the intracontinental extensional region in southern Tibet and identifies a 1200-km-long helium enrichment zone (He > 5000 ppm) within 50 km north of the IYS. In total, 144 gas samples were categorized into four groups: Group A, with dominant N 2 species and He concentrations (0.07–2.31 vol%) 1.4–23 times higher than industrial-grade He resources (0.05–0.1 vol%); Group B, with dominant CO 2 species and high He concentrations (0.05–1.57 vol%); and Groups C and D, depleted in He (<0.05 vol%) and dominated by CO 2 and N 2 , respectively. The gas mixtures exhibit systematic variations from the extensional zone to the continental convergent margin, changing from deep-origin, He-depleted, CO 2 -dominated fluids (Group C) to relatively shallow crust-derived, He-rich, N 2 -dominated gases (Group A). The former exhibits small but detectable amounts of mantle-derived He (2.3 ± 2.6%) and CO 2 (<3.2%), primarily atmosphere-derived N 2 (δ15N = 0.0 ± 0.6‰) and Ar (40Ar/36Ar = 307 ± 25), and crustal CO 2 (82 ± 9% from carbonate and 17 ± 9% from sediments/metasediments) from the subducted Indian slab-carbonate and Tibetan crust. The low He flux of Group C matches the steady release of He from the 70-km-thick crust into the geothermal waters during 0.02–43.5 ka. Shallow calcite precipitation and dissolution reactions cause CO 2 loss and slight He enrichment in the extensional zone. Group A samples at the convergent margin show pure crustal He, more radiogenic 40Ar (40Ar/36Ar = 330 ± 67), and higher sedimentary contributions of 28 ± 23% for N 2 and 37 ± 11% for CO 2. A high He flux near the IYS requires an external source with high 4He/N 2 (∼0.05) and 4He/20Ne (103–105) to provide >62% of the total He flux. This finding is best explained by He production in the south-dipping Gangdese granitic belt and deep crust, accumulation below the accretionary wedge, and episodic liberation along Great Counter thrust structures since 0.7 Ma by Tibetan Plateau uplifts and intense hydrothermal activity. The He-rich fluids mix with the sedimentary and atmosphere-derived N 2 from the accretionary wedge, forming He-rich N 2 -type gases. This study systematically reports large-scale He enrichment along the continental convergent margin for the first time and demonstrates that complex crustal-scale structures in the convergent margin significantly controlling gas geochemistry. These results have profound implications for potential helium resource exploration and He-CO 2 -N 2 recycling in continental subduction zones. [ABSTRACT FROM AUTHOR]

It has been estimated that the acid mine drainage (AMD) impacted Odiel river basin in southern Spain supplies 0.37% and 15% of the global riverine fluxes of Cd and Zn to the oceans, respectively (Sarmiento et al., 2009). However, the behaviour of Cd and Zn in the Ria of Huelva estuary, which connects the Odiel and Tinto watersheds with the Gulf of Cadiz, has yet to be fully investigated. Furthermore, very few studies have investigated Cd and Zn isotope behaviour in estuaries worldwide. This study presents Cd and Zn concentrations and isotopic compositions for the Ria of Huelva estuary and surrounding watersheds, sampled in 2017 and 2019. Sulfide-rich rock samples extracted from three mines yield Cd and Zn isotope compositions that range from –0.14‰ to +0.07‰ (n = 4) for δ114Cd and –0.01‰ to +0.29‰ (n = 4) for δ66Zn. However, a uniform riverine signal of about +0.02‰ for Cd and +0.17‰ for Zn indicates that tracing of individual mining regions using Cd and Zn isotopes is challenging. Limited variability was observed in dissolved δ114Cd values throughout the watershed, including AMD, the estuary, and the Gulf of Cadiz, with a mean value of ±0.00 ± 0.13‰ (n = 25, 2 SD; excludes one AMD outlier, at +0.48‰), including both 2017 and 2019 data. By contrast, δ66Zn values ranged from –0.12‰ to +0.35‰ (n = 28) for the same geographical and temporal scope. In May 2017, a large spill from an abandoned mine, La Zarza, resulted in a drastic increase in the concentrations of trace metals reaching the estuary compared to 2019, but no impact of this mine spill on Cd or Zn isotope compositions is observed. In 2019, an increase in δ66Zn values in the estuary coincided with high pH values (up to pH = 8.8) and chloride concentrations (2.73%), which may reflect an alkaline anthropogenic input from the active neighbouring industrial complex. Overall, Cd concentrations and isotope compositions in the estuary are largely consistent with conservative mixing behaviour. By contrast, Zn behaviour is non-conservative, with removal of 49–97% of dissolved riverine Zn in the estuary during the period 2017–2019, associated with a relatively small isotopic shift to lighter Zn isotope compositions. Removal of Zn to the particulate phase in the Ria of Huelva estuary therefore largely attenuates high riverine Zn fluxes from AMD, indicating that previously estimated Odiel river basin Zn fluxes were overestimated. Nevertheless, the variable but generally light AMD Cd and Zn isotope compositions, coupled with high dissolved metal concentrations, suggest that Cd and Zn isotopes may be useful tracers of regionally averaged AMD inputs to the Gulf of Cadiz and beyond. [ABSTRACT FROM AUTHOR]

Previous work on Cr isotope composition of the lithospheric mantle focused on shallow spinel-facies peridotites and invoked the effects of partial melting and metasomatism. However, little is known on the Cr isotope composition and behavior in the deeper and more voluminous garnet-facies mantle, notably during generation and passage of magmas and fluids. Here, we present Cr isotope data for 26 garnet and ten spinel whole-rock peridotite xenoliths (mainly harzburgites) from the Udachnaya kimberlite on the Siberian craton and minerals from five of them, as well as for minerals of 14 eclogites and four garnet clinopyroxenites from the Dabie orogen in eastern China. Garnet (Gt) and clinopyroxene (Cpx) are major Cr hosts in the mantle. Our ionic modeling suggests the following order of 53Cr/52Cr values in coexisting minerals: garnet ≈ spinel > Cpx > olivine. Cr isotopes are equally partitioned between garnet and Cpx in four peridotites, while garnet is enriched in heavy Cr in one peridotite and all the eclogites (Δ53Cr Gt-Cpx 0.01‰ to 0.27‰, av. 0.10‰), but depleted in heavy Cr in the pyroxenites (Δ53Cr Gt-Cpx −0.03‰ to −0.16‰, av. −0.10‰). The Δ53Cr Gt-Cpx variations are attributed to effects of chemical composition, equilibration temperature and oxygen fugacity on equilibrium Cr isotope fractionation. Further, the anomalous Δ53Cr Gt-Cpx values in the Dabie pyroxenites and one peridotite may indicate disequilibrium inter-mineral isotope fractionation via kinetic diffusion during phase transitions as a result of pressure-temperature (P-T) changes and metasomatism. The whole-rock δ53Cr range in the garnet peridotites (−0.27 to 0.13‰; average −0.08 ± 0.18‰, 2 SD, n = 25, excluding an outlier with −0.86‰) overlaps bulk silicate Earth estimates whereas the spinel peridotites tend to have higher δ53Cr (−0.12 to 0.16‰; av. 0.02 ± 0.20‰, 2 SD, n = 10). While these rocks are residues of high degrees (30–40%) of melt extraction, their range contrasts with limited equilibrium Cr isotope fractionation we calculated for melting residues of both spinel and garnet peridotites (Δ53Cr residue-source < 0.03‰). The δ53Cr values in the spinel peridotites show correlations with modal abundances of Cpx, garnet and spinel, as well as major and trace element composition. We posit that the Cr isotope variations in these peridotites are somehow linked to solid–melt and inter-mineral diffusive exchanges during P-T changes, deformation and ingress of metasomatic media previously documented in the Udachnaya peridotites. The average δ53Cr value (−0.09 ± 0.16‰, 2 SD, n = 31) for garnet peridotites (excluding U9) in this and previous studies is similar to those previously obtained for (mainly off-craton) spinel peridotites and komatiites, and appears to be representative of the Cr isotope composition for bulk mantle. Numerical modeling for mantle magmas derived at different depths and by different melting degrees of garnet and spinel peridotites and eclogites yields a narrow δ53Cr range, possibly affected by oxygen fugacity. [ABSTRACT FROM AUTHOR]

Amorphous precursors to minerals have been observed in laboratory materials and in nature, including across diverse phyla. These metastable phases allow for the incorporation of cations at higher concentrations than classical crystallization pathways, thus, their chemistry and behavior have implications in an array of disciplines. Currently, little is known about the isotopic composition of the anion in amorphous carbonates and how isotopic values evolve during transformation into a mineral. Here, we examined the evolution of isotopic values in amorphous carbonates and mineral transformation products to identify the potential origins of disequilibrium isotopic compositions in carbonate minerals that form from an amorphous precursor. We measured dual carbonate clumped isotopes (13C18O16O - Δ 47 ; 12C18O18O - Δ 48), bulk stable isotope ratios (δ 13C, δ 18O), and chemical and structural data throughout the transformation of amorphous calcium magnesium carbonate (ACMC) into high Mg-calcite (HMC) over 1 year, with crystallization occurring in solutions from 10 to 60 °C. The Δ 47 , Δ 48 , and δ 18O values evolved significantly during transformation, indicating dissolution of ACMC and reprecipitation of HMC. After crystallization, the Δ 47 and Δ 48 values achieved a disequilibrium steady state, while δ 18O values continued to evolve. For the fully crystallized HMC samples, the low temperature samples formed at 10 °C had the greatest extent of oxygen isotope disequilibrium (measured value – equilibrium value = −39‰); conversely; the greatest clumped isotope disequilibrium was observed in high temperature samples formed at 40 and 60 °C (0.068‰ for Δ 47 , 0.072‰ for Δ 48). These results are consistent with a new potential mechanism of disequilibrium clumped isotope values in carbonate minerals. Specifically, the dissolution of ACMC during transformation causes disequilibrium oxygen and clumped isotope values in the dissolved inorganic carbon (DIC) pool. The extent of isotopic disequilibrium in DIC during transformation is temperature dependent, and is recorded by the forming mineral. Isotopic results may also reflect mixing effects, as scanning electron microscopy (SEM) showed ACMC and HMC existing simultaneously during transformation, indicating that ACMC likely progressively dissolves and reprecipitates as the DIC pool isotopically evolves. This may result in heterogeneous isotopic values in HMC. In total, these data suggest a highly dynamic localized environment could exist in biomineralizing organisms and abiotic systems that utilize amorphous precursors to form carbonate minerals, potentially resulting in isotopic values that are not representative of formation temperature. [ABSTRACT FROM AUTHOR]

The stable oxygen and clumped isotope composition of brachiopod calcite are important proxies for the reconstruction of Phanerozoic seawater temperatures and δ18O values. The utility of brachiopods as a temperature archive is nonetheless challenged by indications that their shells precipitate out of isotopic equilibrium with ambient seawater, though the origin of disequilibrium effects has remained elusive. Here, we apply the recently developed dual clumped isotope thermometer (i.e., the simultaneous measurement of Δ 47 and Δ 48 values in CO 2 derived from the acid digestion of carbonates) to modern and fossil brachiopods to investigate disequilibrium signatures recorded in brachiopod calcite. We present evidence that disequilibrium signatures are derived from the combination of two rate-limiting processes: the hydration/hydroxylation of CO 2 , and, potentially, the diffusion of HCO 3 −/CO 3 2− in water. An empirical correction for clumped isotope disequilibrium is presented, based on correlation between measured disequilibrium offsets in the Δ 47 and Δ 48 values of modern brachiopods (Δ 47 disequilibrium = –0.88 × Δ 48 disequilibrium + 0.02). Dual clumped isotope thermometry of Eocene age brachiopods from Seymour Island (∼65 °S) yields temperatures in agreement with previously reported Δ 47 derived temperatures from coeval bivalves. In contrast, uncorrected Δ 47 derived temperatures from the same brachiopods are 6–11 °C colder. Measurement of dual clumped isotope composition alongside δ18O values of brachiopod carbonate should also enable more accurate reconstruction of seawater-δ18O. In comparison with group-specific Δ 47 -temperature calibrations, which correct for an average kinetic bias in Δ 47 , dual clumped isotope thermometry accounts for the magnitude of disequilibrium inherent to an individual sample, facilitating more reliable paleotemperature and seawater-δ18O reconstructions. [ABSTRACT FROM AUTHOR]

The uranium (U) isotope ratio (238U/235U, reported as δ238U) in marine carbonates is an important proxy for reconstructing changes in past oceanic redox conditions, based on the essential assumption that U in carbonates originates from dissolved U(VI) in seawater. However, diagenetic processes may induce the addition of isotopically heavy U(IV) into the carbonates, complicating the application of the U isotope proxy. So far, the valence states of trace amounts of U and its relation with U isotope composition in marine carbonate sediments have not been directly studied. In this study, we have calibrated an ion-exchange chromatographic method to separate U(IV) from U(VI) in geological carbonate samples (e.g., modern and fossil corals, stalagmites, cold seep carbonates) and quantified their contents by mass spectrometry. Our study confirms that modern coral carbonates are faithful archives of U(VI) sourced from seawater. Surprisingly, drill core samples from a modern coral carbonate platform, which bear no sign of alteration inferred from conventional diagenetic proxies (e.g., Mn/Sr), show significant positive correlation between U(IV) fraction and bulk carbonate δ238U, suggesting U(IV) is present in the marine carbonates and drives δ238U toward higher values than that of seawater. We therefore suggest that coupled valence and isotope analyses of U in marine carbonates could provide critical constraints toward reliable reconstruction of marine redox evolution in the Earth's history. [ABSTRACT FROM AUTHOR]

Apollo 15 quartz monzodiorites (QMDs) are reported to contain some of the most deuterium-depleted apatite found in lunar samples. In this study, apatite from six Apollo 15 QMDs, including three samples from 15405 not previously investigated, were analyzed for their H and Cl isotopes. Apatite in 15405 are extremely 2H (or D)-poor, with δD values ranging from −658 ± 53 to −378 ± 113‰, comparable to apatite data from related samples 15403 and 15404. In addition to new H isotope data, the first Cl-isotope data for lunar QMDs are presented. Apatite in 15405 and related samples are enriched in 37Cl with respect to Earth, with measured δ37Cl values ranging from +13 to +37‰. These values are within the reported δ37Cl range for KREEP-rich samples. The fact that the Cl isotopic composition of apatite in QMDs are similar to those in other lunar lithologies, but the H isotopic data are distinct and unique, provides possible further evidence for the existence of a D-poor reservoir in the lunar interior. Raman spectroscopy of the silica polymorph in sample 15405 reveals it to be a mixture of quartz and cristobalite. Based on available experimental data on the stability of various silica phases over a range of pressure and temperature regime, a deep-seated origin in the crust for QMDs may be possible which would support an endogenous origin of the H-Cl isotope systematics of the QMDs. The role of impact-induced transformation of silica phases and its contributing towards low D/H ratio in apatite, however, cannot be ruled out. [ABSTRACT FROM AUTHOR]

The Cedars is an area in Northern California with a chain of highly alkaline springs resulting from CO 2 -charged meteorological water interacting with a peridotite body. Serpentinization resulting from this interaction at depth leads to the sequestration of various carbonate minerals into veins accompanied by a release of Ca2+ and OH– enriched water to the surface, creating an environment which promotes rapid precipitation of CaCO 3 at surface springs. This environment enables us to apply the recently developed Δ 47 -Δ 48 dual clumped isotope analysis to probe kinetic isotope effects (KIEs) and timescales of CO 2 transformation in a region with the potential for geological CO 2 sequestration. We analyzed CaCO 3 recovered from various localities and identified significant kinetic fractionations associated with CO 2 absorption in a majority of samples, characterized by enrichment in Δ 47 values and depletion in Δ 48 values relative to equilibrium. Surface floes exhibited the largest KIEs (ΔΔ 47 : 0.163‰, ΔΔ 48 : −0.761‰). Surface floe samples begin to precipitate out of solution within the first hour of CO 2 absorption, and the dissolved inorganic carbon (DIC) pool requires a residence time of >100 h to achieve isotopic equilibria. The Δ 48 /Δ 47 slope of samples from the Cedars (−3.223 ± 0.519) is within the range of published theoretical values designed to constrain CO 2 hydrolysis-related kinetic fractionation (−1.724 to −8.330). The Δ 47 /δ18O slope (−0.009 ± 0.001) and Δ 47 /δ13C slope (−0.009 ± 0.001) are roughly consistent with literature values reported from a peridotite in Oman of −0.006 ± 0.002 and −0.005 ± 0.002, respectively. The consistency of slopes in the multi-isotope space suggests the Δ 47 -Δ 48 dual carbonate clumped isotope framework can be applied to study CO 2 -absorption processes in applied systems, including sites of interest for geological sequestration. [ABSTRACT FROM AUTHOR]

The equilibrium Mg isotope fractionation factors (ln α) derived from both theoretical calculation and experimental studies are not consistent with the values from natural sample observation. Different cation sites, which have identical coordinate numbers but different bond strengths, contribute equally to ln α in theoretical calculations. These non-equivalent sites might complicate the inter-mineral equilibrium isotope fractionations. Here, we investigate reduced partition function ratios (103ln β) of non-equivalent Mg cation sites (e.g., M1, M2, and M3) in forsterite, orthoenstatite, clinoenstatite, tremolite, omphacite, pyrope and spinel to explore the site-specific effect on the intra- and inter-mineral equilibrium isotope fractionation. Based on all-electron first-principles calculations, 103ln β in different cation sites decreases in the following sequence of spinel M1 > tremolite M3 > orthoenstatite M1 > forsterite M1 > omphacite M1 > tremolite M1 > tremolite M2 > clinoenstatite M1 > forsterite M2 > orthoenstatite M2 > clinoenstatite M2 > pyrope M1. These findings suggest that cation sites with larger polyhedron volumes prefer light Mg isotopes to those with smaller polyhedron volumes inside minerals, and the different contribution of intra-mineral cation sites could result in significant variations of equilibrium Mg isotope fractionation between minerals. The range of ln α between minerals induced by specific sites is defined as the equilibrium isotope fractionation domain. The site-corrected ln α can provide better evaluation of equilibrium fractionation factor of Mg isotopes between spinel and olivine and clarify the equilibrium state between mantle minerals. In addition, we establish spinel-olivine and omphacite-pyrope Mg isotope geothermometers based on the site-specific effect. This study suggests that the site-specific effect could be prevalent in solid materials and thus important for other isotope systems. [ABSTRACT FROM AUTHOR]

Nucleosynthetic isotope variations are powerful tools to investigate genetic relationships between meteorite groups and planets. They are instrumental to assess the early evolution of the solar system, including mixing and reservoir formation in the protoplanetary disk, as well as planet formation. To address these questions, we report high-precision nucleosynthetic Ti isotope compositions of a wide range of bulk meteorites, partially complemented with new Cr isotope data. New Ti isotope data confirm the first order dichotomy observed between carbonaceous chondrites (CC), representing outer solar system compositions, and non-carbonaceous (NC) meteorites from the inner solar system. The data in combination with nucleosynthetic isotope data of other elements (e.g., Cr, Ca) indicate that isotopically heterogeneous reservoirs were also present as sub-reservoirs in the inner disk (NC reservoir), generating two or more clusters i.e., (i) the Vesta-like howardites-eucrites-diogenites (HEDs), mesosiderites, angrites, acapulcoites, lodranites, and brachinites and (ii) the Earth-Mars-like ordinary chondrites (OC), aubrites, enstatite chondrites (EC), winonaites, IAB silicates, rumuruti chondrites (R), Martian and terrestrial samples. These reservoirs likely represent disk substructures such as secondary gaps and ring-structures, created by spiral arms, which were emitted from the growing Jupiter and/or Saturn. The distinct isotopic compositions of these reservoirs may reflect thermal processing of material within the disk in combination with temporal isotopic variations either due to isotopically variable infalling material from a heterogeneous molecular cloud and/or thermal processing during the infall that induced such heterogeneities. Such effects were likely reinforced by thermal processing of the material within the disk itself and by physical size- and density sorting of dust caused by the giant planets, creating gaps and pressure bumps in the disk. Genetic relationships of meteorite groups and their implications on parent body formation are evaluated. New high precision Ti isotope data are consistent with that (i) CH and CB meteorites derive from a common parent body, which most likely accreted material from the same isotopic reservoir as the parent body of CR chondrites, (ii) silicates of IAB irons and winonaites originate from the same parent body, and (iii) mesosiderites and HED meteorites have a common origin on Vesta. The indistinguishable Ti and Cr isotope compositions of HEDs/mesosiderites to acapulcoites are not attributed to a common parent body, because of petrologic and chemical differences in addition to their distinct O isotope compositions. Their parent bodies likely accreted in the same disk region, which showed a higher level of O isotope heterogeneity compared to that of Ti, Cr and other refractory nucleosynthetic tracers. The similarity in Ti isotope compositions of Martian meteorites and OCs indicates that OC-like material belongs to the main building blocks of Mars. [ABSTRACT FROM AUTHOR]

Chromium is an important redox-active element, with significant Cr isotope fractionations driven by redox transformations in the natural environment and quantified through laboratory experiments. Recent work has demonstrated that non-redox-driven transformations can also produce small variations in Cr isotopes, but the role of these processes in controlling environmental Cr isotope variations remains unclear. The highly acidic but biologically diverse acid rock drainage system in Río Tinto, Spain (and affiliated rivers) provides a unique opportunity to examine how non-redox-related processes can substantially affect the isotopic composition of Cr in a natural system. Our results suggest that under conditions where Cr redox transformations are obviated, Cr cycling is largely controlled by formation of Fe-oxyhydroxides and Fe-hydroxysulfates, Cr adsorption, and Cr speciation, with little apparent Cr isotope fractionation between source materials, waters, and sediments. Moderate Cr isotope shifts, however, may be the result of non-redox processes, including adsorption to organic ligands and potentially biological uptake. These results add to a growing body of evidence for the importance of non-redox transformations in contributing to environmental Cr isotope variations, with implications for the use of chromium isotopes in studies of sedimentary Cr isotope records. In particular, this study is a validation of the potential for sulfide weathering in the Proterozoic to deliver relatively unfractionated Cr isotopes to the ocean, despite rising and sustained high oxygen levels. [ABSTRACT FROM AUTHOR]

Serpentinites in subduction zones are important fluid reservoirs for crust-mantle interaction and arc magma formation. The serpentinite-derived fluids can carry fluid-mobile elements such as Ba and potentially affect the geochemical composition of the metamorphic rocks in the subduction zones. However, the Ba isotope behavior during interaction between serpentinite-derived fluids and metamorphic rocks is still poorly understood. The whiteschists of the Dora-Maira Massif in the Western Alps have experienced fluid infiltration by serpentinite-derived fluids during deep subduction. Here we report the Ba isotope data of the whiteschist and its protolith metagranite to investigate the Ba isotope behavior during fluid-rock interaction. The metagranites, which have experienced ultrahigh-pressure metamorphism, have a small variation of δ138Ba (−0.25 to 0.26‰), while the whiteschists display a large Ba isotope variation from −0.99‰ to 0.48‰. Such a large variation in whiteschists cannot be explained by chemical weathering, protolith heterogeneity, or kinetic effects. Instead, considering the negative correlations of δ138Ba with Ba and other fluid-mobile elements (e.g., Sr, Cs, Zn, and Cu) and the geochemical composition features of serpentinite-derived fluids, the large Ba isotope variation in whiteschists is inferred to be induced by the interaction between serpentinite-derived fluid and metagranite during intense fluid infiltration. This is consistent with the observation that the whiteschists with low Ba contents tend to exhibit heavier Mg-Fe isotope compositions but lighter Li isotope compositions, which indicates that the related fluids were derived from the dehydration of mantle wedge serpentinite. The fluid infiltrating process is well explained by a numerical model with high fluid-rock ratios of 4 to 12. Our modeling further suggests that serpentinite-derived fluid has a distinctly higher δ138Ba value than the depleted upper mantle. During fluid-rock interaction, heavy Ba isotopes prefer to be infiltrated into fluids with an apparent fractionation factor (Δ138Ba rock-fluid) of mostly −0.5‰ to −0.1‰ but can be down to −0.8‰ to −0.7‰ required for a few samples. A two-stage dehydration and metasomatism model at different depths can well explain the formation of Dora-Maira whiteschists. Whereas the Ba isotope composition of mantle wedge serpentinite is mainly dictated by crust-derived fluids from the subducting slab, an interaction between the serpentinite-derived fluids and the metagranite protolith resulted in a large Ba isotope variation of whiteschists. In case of a general involvement of serpentinites in subduction zones, serpentinite-derived fluids would significantly affect the Ba isotope compositions of subarc mantle peridotites and also arc lavas. [ABSTRACT FROM AUTHOR]

We undertook Zr isotope measurements on zircon, titanite, biotite, amphibole, and whole rocks from the La Posta pluton (Peninsular Ranges, southern California) together with trace element analyses and U-Pb age measurements to understand the controls on Zr isotope fractionation in igneous rocks, including temperature, crystallization sequence, and kinetic effects. We find large (>0.6‰) Zr isotope fractionations (expressed as δ94/90Zr) between titanite and zircon forming at approximately the same temperature. Using equilibrium fractionation factors calculated from ionic and ab initio models, we infer the controls on Zr isotope evolution to include the relative order in which phases appear on the liquidus, with titanite fractionation resulting in isotopically lighter melt and zircon fractionation resulting in isotopically heavier melt. While these models of Zr fractionation can explain δ94/90Zr variations in zircon of up to ∼1.5‰, crystallization order, temperature and presence of co-crystallizing phases do not explain all aspects of the intracrystalline Zr isotopic distribution in zircons in the La Posta pluton or the large range of Zr isotopic values among zircons (>2‰). Without additional constraints, such as knowledge of co-crystallizing phases and a better understand of the true causes of Zr isotope fractionation, Zr isotopes in zircon remains an ambiguous proxy of magmatic evolution. [ABSTRACT FROM AUTHOR]

The application of carbonate clumped isotope (Δ 47) thermometry in deep-time is often limited by modification of the original temperature signal by thermal resetting. New modeling approaches to estimate the initial isotopic composition of partially reset calcites and maximal burial temperatures, however, open promising avenues in temperature reconstruction. Such approaches strongly depend on laboratory-derived kinetic parameters of calcite materials, which may differ in their microstructure, water content and distribution, and minor and trace element composition, and thus may have different resetting kinetics. The rostra of belemnites, an extinct group of mollusks with a wide temporal and spatial occurrence in the Mesozoic, have been extensively used for deep-time paleoclimate reconstructions using oxygen isotope geochemistry. Belemnites are also important targets for clumped isotope-based temperature reconstructions, but often are found to have reset Δ 47 compositions. Here, we present results from heating experiments on belemnite rostral calcite and optical calcite and provide belemnite-specific kinetic parameters for clumped isotope resetting. We show that belemnite calcite is altered faster and at lower temperatures than optical calcite and all other calcites reported in previous studies. We suggest that fast initial resetting results from oxygen isotope exchange of belemnite calcite with internal skeletal water present as fluid inclusions or organic-derived water, a process completed within 2–4 min at the experimental temperatures used here. Extrapolation to geological timescales using different solid-state bond reordering models shows that belemnite calcite resetting starts at lower burial temperatures than brachiopod, spar, and optical calcites. This susceptibility to thermal resetting results in a measurable (+3 °C) increase of the apparent Δ 47 temperature even under shallow to moderate burial conditions (i.e., 40–50 °C for 106–107 years timescales). Following the overprint to higher apparent Δ 47 temperatures during burial, the belemnite Δ 47 may further re-equilibrate during exhumation resulting in a decrease of apparent Δ 47 temperatures. Such "retrograde resetting" is similar to what is observed for carbonatites and marbles during cooling, and may cause underestimation of the thermal resetting a sample experienced during its geological history. Overall, our results demonstrate the importance of material-specific kinetic parameters and we urge caution when interpreting Δ 47 -derived temperatures of biogenic carbonates from deep-time archives. [ABSTRACT FROM AUTHOR]

Thallium (Tl) isotopes are an emerging tool for tracking the history of molecular oxygen in seawater. Use of Tl isotopes in this manner requires a thorough understanding of the modern Tl isotope cycle, especially within the anoxic settings that typified Earth's past oceans. To this end, we generated Tl concentration and isotope data for waters, particles, and sediments collected during three spring-summer-fall field seasons in Siders Pond, a salt-stratified and sulfide-rich ('euxinic') pond on Cape Cod (Massachusetts, USA). Over short timeframes (i.e., days to weeks), we observed a large degree of variability in the abundance (Tl diss = 9 pM–42 pM) and isotopic composition (ε205Tl diss = –3.9 ± 0.4; 2SD to –0.1 ± 0.7; 2SD) of dissolved Tl in pond surface waters. Thallium drawdown was common and favored removal of the lighter-mass Tl isotope. We surmise that biological Tl uptake plays a role in this phenomenon, but more work is warranted to confirm this hypothesis. Manganese oxides persisted in pond surface waters on many sampling days but had no obvious effect on Tl diss , only a slight effect on particulate ε205Tl values on the day of most prolific Mn oxide accumulation (driving ε205Tl part as high as +0.9 ± 0.8; 2SD). In euxinic waters below the oxycline, rapid drawdown of dissolved Tl was observed on all sampling days together with a complimentary increase in particulate Tl (up to Tl part = 21 pM). Strong associations between Tl and sulfide, and between Tl and other chalcophile metals (Mo and Cd), suggest that Tl is probably removed from euxinic waters in association with particulate sulfides, and seemingly with no resolvable isotopic fractionation effect despite non-quantitative Tl removal. The sediment data track the longer-term Tl cycle in the pond (i.e., across years) and reveal limited isotopic variability. Thallium isotope compositions leached from surface sediments match those predicted for contemporaneous waters, or nearly so, at all depths in the pond (above, within, and below the oxycline). Apparently, only a small fraction of the short-term Tl isotope variability found in the oxic surface waters of Siders Pond gets transferred to anoxic waters, and essentially none is transferred to sediments. Our results reaffirm the notion that Tl isotopes are uniquely capable of tracking long-term sedimentary Mn oxide burial – not mere Mn oxide formation. Our results also verify the ability of sediments formed under reducing conditions to capture the Tl isotope composition of contemporaneous waters. More broadly, the results of our investigation bolster our knowledge of the modern Tl isotope cycle and thereby permit more confident inferences of this cycle in Earth's past. [ABSTRACT FROM AUTHOR]

Melts and fluids derived from subducting/subducted oceanic lithosphere contribute significantly to mantle compositional heterogeneity. Historically metasomatized lithospheric mantle modified by varying materials, especially volatile-rich slab fluids, has been understood to be crucial for the formation of giant ore deposits, thus providing potential prospective areas for mineral exploration. However, it is not straightforward to identify superimposed effects of slab melts and fluids on the sub-continental lithosphere (SCLM) that has experienced long-lived and/or multiple metasomatism. In this study, we present a novel study using Mg and K isotopes, together with radioactive isotopes on mafic post-collisional alkaline potassic rocks (PARs) with the aim of deciphering varying agents contributing to the heterogeneous metasomatism of the SCLM beneath the southeastern Tibetan Plateau. The PARs, formed at ∼36 Ma, are characterized by high abundances of large ion lithophile elements (LILE) but depleted in high field strength elements (HFSE) with high Rb/Sr ratios, indicating a phlogopite-bearing mantle source. We find that features of light Mg (δ26Mg = −0.52 to −0.25‰) and heavy K (δ41K = −0.56 to +0.08‰) isotopes observed in the PARs are largely inherited from their mantle sources with negligible effects of kinetic fractionation, fractional crystallization and crustal level processes. Our modelling reveals that the notable divergence of Sr–Nd–Pb–Mg–K isotope compositions between PAR suites in different tectonic blocks are best understood as resulting from varying contributions of slab-derived components into the overlying mantle lithosphere, including sediment-derived melts, ocean crust-derived supercritical fluids, and marine carbonates. Particularly, their high Th/U ratios and negative correlations between δ26Mg and δ41K values suggest that slab-derived supercritical fluids can dissolve and transport Mg-rich (dolomitic) carbonate into the mantle wedge. [ABSTRACT FROM AUTHOR]

As the earliest-dated solids in our Solar System, calcium-aluminum-rich inclusions (CAIs) provide a record of their formation environment near the young Sun and hold clues to the formation of planetary-scale isotopic reservoirs in the solar protoplanetary disk. Although CAIs from several CV, CK, CM, CO, and ordinary chondrites have been analyzed previously for their Ti isotopic compositions, CAIs from just three CV chondrites have been analyzed for their Cr isotopic compositions, and only a handful of CAIs have been measured for both their Ti and Cr isotopic compositions. We report mass-independent Ti and Cr isotopic anomalies in several CAIs from CV and CK chondrites; this is the first report of the Cr isotopic composition of a CAI from a CK chondrite. With these additional data, we aim to better constrain the compositional range of CAIs in ε50Ti versus ε54Cr space, thereby facilitating the isotopic characterization of the material inherited by the solar protoplanetary disk and the role of CAIs in the formation of distinct planetary-scale isotopic reservoirs in our early Solar System. The narrow range in isotopic anomalies in CAIs when compared to other early-formed refractory inclusions such as platy hibonite crystals (PLACs) and spinel-hibonite inclusions (SHIBs) suggests that CAIs record the mixing of these precursor materials and the averaging of their larger isotopic anomalies. The isotopic composition of CAIs is therefore likely the result of a combination of factors, including mixing of material inherited from their formation region, heterogeneous carrier phase distribution, and thermal processing in the disk. The ε50Ti and ε54Cr isotopic compositions of CAIs are not correlated, further demonstrating that these isotopic anomalies have different carrier phases. The Ti and Cr isotopic compositions of CAIs additionally show that CAIs alone cannot be responsible for the compositional difference between the non-carbonaceous chondritic (NC) and carbonaceous chondritic (CC) isotopic reservoirs but nevertheless do play a role in the formation of these large-scale isotopic reservoirs in the early Solar System. [ABSTRACT FROM AUTHOR]

The 147Sm–143Nd isotope system has long been used as a powerful tracer and geochronometer for a wide range of geological process, yet the robustness of Sm–Nd system has been questioned in the presence of multistage hydrothermal alteration. Here we provide robust evidence for the survival of whole-rock Sm–Nd isotopes from REE redistribution and mineral-scale isotopic resetting during multistage fluid infiltrations in the Lala Fe-Cu-REE deposit in Kangdian region, southwestern China. The Lala deposit is hosted in the ∼1.72–1.68 Ga rift-related Hekou Group and contains variable proportions of REE-rich apatite and fluorite in the ores. Both of the apatite and fluorite grains show textures of extensive metasomatic alteration, during which large amounts of REE, especially LREE, were leached out and mostly re-incorporated into newly-formed monazite, bastnäsite, and xenotime. Quantifying REE distribution budget at thin-section scale confirms that the amounts of REE (1583–5403 ppm) leached from primary apatite and fluorite are compatible with those (1636–5526 ppm) incorporated in the newly crystallized REE minerals, indicating that REE are internally redistributed within investigated samples. The whole ores yield an 18-point 147Sm–143Nd errorchon date of ∼1.71 Ga, which is in accordance with the other analogous Fe-Cu deposits in Kangdian region, and supports a ∼1.71 Ga primary mineralization at Lala. By contrast, SHRIMP 232Th–208Pb dating of texturally constrained monazite yield a crystallization age of ∼0.92 Ga, which corresponds well with the combined monazite-bastnäsite-apatite Sm–Nd isochron ages of representative samples, demonstrating a major REE redistribution and Sm–Nd isotopic resetting event at this time. The results suggest that Sm–Nd mobility was limited to within the whole-rock scale or even within the thin-section scale in the vast majority of cases, such that the whole-ore Sm–Nd isotopic system largely remained closed, despite complete resetting of the major REE minerals during younger fluid infiltration events. Our study exemplifies the apparent "decoupling" of Sm–Nd isotopic systems between macro- and microscales, which can advance our understanding of the complex history of hydrothermal systems. Our findings of a major transition (80–96%) in REE repositories also highlight the economic significance of hydrothermal alteration for modifying ore mineral assemblages, which resembles a natural REE refinement process and helps create beneficial REE resources to meet the society increasing demands. [ABSTRACT FROM AUTHOR]

Microbial anaerobic oxidation of methane (AOM) substantially mitigates atmospheric methane emissions on Earth and is a process to consider for astrobiological targets where methane has been detected. The measurement of doubly substituted, or "clumped", methane isotopes has proven useful in tracing processes of methane formation and oxidation. Both near-equilibrium and extreme disequilibrium methane clumped isotope signatures can be attributed to AOM, but, to date, understanding the mechanistic and environmental controls on those signatures has been lacking. We report measurements of methane clumped isotope compositions of residual methane in AOM-active microbial incubations using sediment slurries from Svalbard and Santa Barbara Channel methane seeps. Incubation experiments of Svalbard sediment slurries resulted in residual methane with very high Δ13CH 3 D and Δ12CH 2 D 2 values up to 19.5‰ and 65.1‰, respectively. We found similarly high Δ13CH 3 D and Δ12CH 2 D 2 values in fluid samples from the Chamorro Seamount, a serpentinite mud volcano in the Mariana forearc, suggesting that minimal reversibility of AOM intracellular reactions leads to kinetic fractionation of clumped isotopologues. When conditions were consistent with a low thermodynamic drive for AOM, however, methane isotopologues approached intra-species quasi-equilibrium. This was clearly observed in isotope exchange experiments with methyl-coenzyme M reductase (Mcr) and in microbial incubations of the Santa Barbara Channel sediment slurries. Using an isotopologue fractionation model, we highlight the critical role of reversibility in controlling the trajectory of gases in Δ13CH 3 D vs. Δ12CH 2 D 2 space during AOM. The near-equilibrium methane isotopologue signatures are generalized as a result of the Mcr-catalyzed intracellular isotope exchange operating under near-threshold free energy conditions, as shown in the deep-biosphere incubations. Our results show that the reversibility of the Mcr-catalyzed reaction is central to understanding the meaning of methane isotopologue ratios affected by microbial production and oxidation. [ABSTRACT FROM AUTHOR]

To assess how chemical weathering processes in large high-relief river systems respond to climatic variability, we studied seasonal changes in radiogenic strontium (87Sr/86Sr) and stable calcium (δ44/40Ca) and magnesium (δ26Mg) isotopes in the Jinsha and Yalong rivers, which drain the southeastern Tibetan Plateau. During the low-runoff season, with discharge (Q) < 2000 m3/s, the river waters reflect the Sr, Ca, and Mg isotope signatures of recharge meltwaters, with additional isotope fractionation signals for Ca and Mg related to secondary mineral precipitation, which might imply that meltwater flushes soil solutions from the soil. During medium-runoff intervals (2000 m3/s < Q < 4000 m3/s), the Sr, Ca, and Mg isotope signatures in the Jinsha river waters are similar to those of the headwaters, which are influenced by evaporite dissolution, while the Yalong is affected by greater carbonate weathering relative to silicate weathering. In both rivers, bedrock dissolution governs the chemical composition of the river waters. During the high-runoff season (Q > 4000 m3/s), storms generate rapid overland flow, which transfers large volumes of soil into the rivers, such that soil weathering plays an important role in regulating riverine chemical compositions. At these times, the riverine Ca and Sr isotope evolution is influenced by secondary mineral dissolution and sediment–water cation exchange. Overall, this study highlights the potential of combining multiple isotope systems (Sr, Ca, Mg) to trace the dynamics of water–rock interaction under variable hydrological conditions. [ABSTRACT FROM AUTHOR]

Various chemical elements including nickel (Ni) exhibit mass-independent isotope heterogeneity on a bulk meteorite level, which is generally accepted to reflect the heterogeneous distribution of presolar carrier(s) from different nucleosynthetic sources. Thus, understanding the nature of the carriers can help decipher the origin of the observed nucleosynthetic variability, which remains elusive. In this study, we present the first high precision measurements of mass-independent and mass-dependent Ni isotope compositions for step-leaches of the CI chondrite Ivuna and Efremovka CAIs supplemented by bulk chondrite measurements. Step-leaches record highly anomalous Ni isotope signatures that can be attributed to at least four diverse nucleosynthetic sources. The most anomalous leachates show either large deficits (up to 0.1 %) in the neutron-poor 58Ni and 60Ni nuclides (L11, thought to contain mainly s-process derived Ni) or minor enrichments and deficits (∼100 ppm) in 60Ni or 64Ni (L6, L8, L9 and L10, all thought to derive mainly from supernovae). Pristine CAIs record Ni isotope compositions typified by enrichments in 58Ni of up to 400 ppm. Our new data for bulk chondrites agree with earlier work and emphasize the appropriateness of using the 62Ni/61Ni ratio for internal normalization. Based on the compositional relations between the step-leaches data, CAIs, and bulk meteorites, we show that 60Ni variability is consistent with being of nucleosynthetic origin as opposed to reflecting variable Fe/Ni ratios in the presence of live 60Fe. Finally, we infer that the observed Ni nucleosynthetic disk variability is predominantly driven by a combination of processes separating different nucleosynthetic carriers in the disk from each other, including thermal processing and size-based sorting. [ABSTRACT FROM AUTHOR]

We present results from chemostatic siderite precipitation experiments conducted using a modified constant composition method at temperatures between 15 and 75 °C and compare these results with previous temperature relationships to obtain a composite low temperature relationship for siderite clumped isotope ratios (Δ 47) from 8.5 to 75 °C: Δ 47 = 0.0364 ± 0.004 ∗ 10 6 T 2 + 0.1672 ± 0.046 ± 95 % C L , R 2 = 0.708 The slope of this line is statistically indistinguishable from those reported for aragonite, calcite, and dolomite clumped isotopes, but differs in its intercept. To test whether this difference results from mineralogical differences in phosphoric acid fractionation factors (AFF) between siderite and Ca-Mg carbonates, we heated siderite to 625 °C at 1 GPa, which yielded a statistically indistinguishable Δ 47 value from that predicted by published clumped isotope calibrations populated by Ca-and Mg-carbonate data at higher temperatures. Thus, the apparent difference between siderite clumped isotope behavior and that of other carbonate minerals may not be well explained by differences in AFF. Despite differences in precipitation methods, our results are entirely consistent with published inorganic and microbially-mediated experimental siderite clumped isotope data, suggesting that siderite Δ 47 -temperature dependence differs from Ca-Mg carbonates for reasons unrelated to precipitation methods. Our isotopic results are also consistent with published isotopic measurements on natural siderites precipitated at known environmental temperatures. Our analysis leads us to conclude that supersaturation-related kinetic effects likely play an important role in clumped isotope fractionation in natural and experimentally precipitated siderite. [ABSTRACT FROM AUTHOR]

Arc basalts typically have higher Fe3+/ΣFe ratios but lighter Fe isotopic compositions than mid-ocean ridge basalts (MORB); however, the reasons for these differences are unclear. Here, we report new Fe isotope data for fresh lavas from the Mariana subduction zone, along with available Fe-B-Sr-Nd isotope and B/Nb ratio data for global arc basalts, to constrain the role of serpentinite dehydration in sub-arc mantle oxidation. The basaltic lavas from the Mariana Arc, like other arc basalts from cold subduction zones in the west Pacific, have lighter Fe isotope compositions (δ56Fe = 0.02 ± 0.02‰; 1SD; n = 9) than the MORB-like δ56Fe values of the middle Mariana Trough basalts (δ56Fe = 0.09 ± 0.02‰; 1SD; n = 8). Notably, the significant negative correlations between the δ56Fe values and the proxy for the addition of serpentinite-derived fluids (i.e., B isotopes and B/Nb ratios) indicate that the serpentinite-derived fluid contribution is a first-order control on the across-arc Fe isotope variations. Dehydration of subducted slab serpentinites or dragged-down forearc serpentinites at sub-arc depths can release isotopically light Fe fluids in the form of Fe2+–SO x complexes. Sulfate-rich fluids containing adequate Fe to metasomatize the sub-arc mantle can cause light Fe isotope compositions and high Fe3+/ΣFe ratios in arc basalts. Alternatively, the low δ56Fe values and high Fe3+/ΣFe ratios of arc magmas may be generated by direct melting of the mélange, which includes dragged-down sulfate-rich forearc serpentinites. Comparatively, serpentinite-derived components are absent at back-arc depths because of the earlier breakdown of serpentine minerals. Accordingly, the back-arc basin basalts exhibit MORB-like δ56Fe-δ11B values and B/Nb–Fe3+/ΣFe ratios. Therefore, the across-arc Fe isotope variations and the decoupling of Fe3+/ΣFe and δ56Fe in global arc magmas can be explained by the different contributions of subducted serpentinites. [ABSTRACT FROM AUTHOR]

The super-chondritic Mo isotopic composition of the continental crust has been attributed to contributions from subducted oceanic slabs and/or sediments, which introduce isotopically light Mo to the mantle. It has been hypothesized that dehydrated oceanic slabs subduct into the deep mantle and contribute to the genesis of ocean island basalts (OIBs). However, OIBs have similar average δ98/95Mo values (−0.22‰± 0.04‰; 2SE; n = 87) to the depleted mantle. Subducted oceanic sediments may create a light-δ98/95Mo reservoir in the subcontinental lithospheric mantle, but this has not been investigated in detail. Understanding the Mo isotopic systematics of the continental crust and mantle requires an understanding of the Mo isotopic composition of lithospheric mantle-derived volcanic rocks. The lithosphere in the Tibetan region has experienced multiple stages of oceanic subduction and is thus ideal for studying magmas derived from sediment-modified lithospheric mantle. Mafic ultrapotassic volcanic rocks (UPVRs) from the Sailipu area on the southern Tibetan Plateau have high Mg# values (67.1–72.5) and enriched Sr–Nd isotopic compositions and are primitive magmas derived from the lithospheric mantle. These rocks have higher Ce/Mo ratios (225–422) and lighter δ98/95Mo values (−0.45‰ to −0.13‰) than the depleted mantle, mid-ocean ridge basalts (MORBs), and OIBs, as well as high ε Hf (t) values at given ε Nd (t) values compared with the terrestrial array (Δε Hf = 4.59–5.73). The involvement of Ca-poor pelagic sediments rather than the lower continental crust and hydrated oceanic slab best accounts for the high Ce/Mo ratios and light δ98/95Mo values of the studied rocks. The Sailipu UPVRs also have higher Nb/Ta (18.9–20.8) and Th/La (2.6–3.4) ratios than arc-related igneous rocks and MORBs, suggesting that their source was phlogopite-bearing lithospheric mantle with a metasomatic vein component (rutile + monazite ± lawsonite). We propose a two-stage geodynamic process to explain the formation of the Sailipu UPVRs: (1) a metasomatic vein component with high Th/La and Ce/Mo ratios formed in the lithospheric mantle during multiple stages of oceanic subduction; (2) high mantle temperatures related to convective thinning of the lithosphere resulted in melting of the vein component and the ambient mantle, which produced hybrid melts of various mantle domains during the post-orogenic stage. The Sailipu UPVRs have enriched Sr–Nd isotopic compositions and high Th/La and Ce/Mo ratios, similar to lamproites that are widespread in the Tethyan orogenic realm from the Mediterranean to the southern Tibetan Plateau. Given that UPVRs with light δ98/95Mo values have also been identified in the Western Alps, we propose that the isotopically light Mo component is widespread in Tethyan lithospheric mantle. This isotopically light Mo in the lithospheric mantle may be a complementary end-member to the isotopically heavier continental crust. The recycling of subducted oceanic sediments has a key role in the deep recycling of Mo. [ABSTRACT FROM AUTHOR]

Diffusion is a significant driver of isotope fractionation at high temperatures, but its precise role is difficult to evaluate in many situations due to the sparsity of data on the isotopic mass dependence of the diffusion coefficient. Such data are lacking particularly for diffusion in minerals. The ratio of diffusion coefficients of two isotopes of the same element in the same material can be characterized as the inverse mass ratio raised to an empirical exponent β , but experiments to determine β are challenging and few measurements have been reported. Here, a method is developed to empirically determine β based on numerical modelling of the diffusion-controlled elemental and isotopic redistribution between minerals during slow subsolidus cooling, and comparison of the results to data from natural mineral pairs. The method is applied to the Ca redistribution between orthopyroxene (opx) and clinopyroxene (cpx), where the elemental partitioning equilibrium as a function of temperature has been well characterized experimentally and Ca isotope fractionation data are available in natural samples that experienced slow cooling. We show that the Ca isotope fractionation during cooling is insensitive to the initial temperature, cooling rate or grain size over a broad range of conditions, provided that diffusion is sufficient to re-homogenize opx grains at near-peak temperatures during early cooling. Furthermore, we find that the Ca isotope fractionation established during cooling is much more strongly dependent on the value of β in opx than the value of β in cpx, as long as cpx dominates the Ca budget in the system. Transient heating events produce fractionation in the opposite sense to that developed during cooling, which can diminish or reverse the isotope fractionation for samples that experienced such events after cooling. Based on the largest Ca isotope fractionation that has been documented between opx and cpx in natural samples, it is inferred that for Ca, β opx = 0.04(1). Despite the relatively small inferred value of β opx , diffusive fractionation of Ca isotopes between opx and cpx is significant over a broad range of cooling rates. Similar behavior during sub-solidus cooling is anticipated for other elements with temperature-dependent inter-mineral partition coefficients, and the method developed here has the potential to be used to place constraints on β for many elements, even in minerals where experimental measurements are not feasible. [ABSTRACT FROM AUTHOR]

Stable barium (Ba) isotopes are emerging as tracers for the recycling of crustal material into the mantle. Small but significant Ba isotope variations have been found in global MORB (δ 138/134Ba values of −0.04‰ to 0.15‰), but the processes causing these Ba isotope variations remain poorly understood. In addition, uncertainties still exist in the estimate of the Ba isotope composition of the depleted upper mantle. Here, we present a systematic study of Ba isotopes for well-characterized MORB glass samples from the South Mid-Atlantic Ridge (SMAR) between 5 and 11°S, which span a wide range of radiogenic isotope ratios and trace element contents. Our results show that the northernmost segment A0 basalts with highly depleted radiogenic isotope compositions have δ 138/134Ba values ranging from −0.02 to 0.05‰. In comparison, the δ 138/134Ba values of the isotopically enriched basalts from the A1-A4 segments vary from 0.02 to 0.11‰. Thus, while small, the enriched components involved can be observed to modify the Ba isotope composition of the mantle source beneath A1-A4 segments. Combining our new measurements with literature data, this study places new constraints on the Ba isotope composition of the depleted upper mantle, as well as the origin of Ba isotope heterogeneity in MORB. The Ba isotope composition of the depleted upper mantle in the absence of recycled components is estimated to have a δ 138/134Ba value of 0.03 to 0.05‰, which can be adopted as the baseline for using Ba isotope ratios as a tracer of mass transfer processes between the crust and mantle. In comparison, global MORB have δ 138/134Ba values ranging from −0.04‰ to 0.15‰. No global correlation is observed between Ba isotope ratios and geochemical parameters that record mantle enrichment, indicating that the Ba isotope variations in MORB cannot be simply ascribed to the addition of some single component, such as sediment. Models for melting and mixing between recycled materials and depleted mantle suggest that the variable Ba isotope compositions of the enriched MORB require a contribution from recycled altered oceanic crust and crustal sedimentary materials. Therefore, these recycled components both play important roles in the chemical budget of the convecting upper mantle, especially for incompatible elements such as Ba. [ABSTRACT FROM AUTHOR]

A growing body of work has demonstrated that the δ2H values of alkenones reflect the δ2H values (δ2H H2O) and / or salinity of the fluid in which they are produced. If so, δ2H alkenone values would act as a surface seawater isotope / salinity proxy, similar to foraminiferal δ18O values, but advantaged in locations with poor carbonate preservation and / or high organic content. Nevertheless, laboratory culture, sediment trap, and water column studies have failed to consistently characterize the effects of temperature, alkenone-producing species, and salinity itself on the δ2H alkenone -salinity and -seawater isotope relationships, and a robust sedimentary alkenone-based calibration remains elusive. Most δ2H alkenone datasets report δ2H C37 , i.e., combined δ2H C37:3 and δ2H C37:2 values, and differ in how they address inter-alkenone fractionation (i.e., α C37:3-C37:2). To constrain controls on alkenone hydrogen isotope systematics in the natural environment, we measured δ2H values of C 37 and C 38 alkenones from 20 open ocean core tops by gas chromatography-stable isotope ratio mass spectrometry after separation of di- and tri-unsaturated forms. Core-top δ2H alkenone data points are currently concentrated in extreme-salinity regions; in combination with our new values from a more moderate range of open ocean δ2H H2O / salinity values, for sedimentary alkenones, we show that 1) mean inter-alkenone hydrogen isotope fractionation is negligible (α C37:3-C37:2 = 1.002 ± 0.006), and therefore that δ2H C37:3 and δ2H C37:2 values can be measured in bulk; 2) temperature and salinity have little impact on alkenone-water fractionation (i.e., α C37-H2O) (mean 0.803 ± 0.010) relative to their expected variability in the ocean; and 3) δ2H C37 and δ2H H2O values are correlated such that statistically identical δ2H C37 -, δ2H C37:3 -, and δ2H C37:2 -δ2H H2O regressions yield a core-top-based calibration of δ2H C37 = 1.44 (± 0.13) * δ2H H2O – 191.62 (± 1.13) ‰. This is indistinguishable from water column calibrations, suggesting a consistent response of environmental δ2H C37 values to changes in δ2H H2O values. This calibration still contains a high amount of scatter (∼ 7 ‰), perhaps attributable to irradiance, growth rate, intra- or interspecies variability, or other factors difficult to constrain in sedimentary material. Nevertheless, when applied to the well-constrained Last Glacial Maximum-to-present mean ocean δ2H H2O change of ∼ 8.8 ‰, it (1.44 ‰ δ2H C37 per 1 ‰ δ2H H2O change) reproduces the mean δ2H C37 Modern – δ2H C37 LGM shift observed from the handful of extant down-core records, legitimizing the observed lack of temperature or salinity effects on α C37-H2O. This suggests that combined δ2H C37:3 + δ2H C37:2 values are a valid proxy for δ2H H2O values in open ocean settings where E. huxleyi and G. oceanica dominate, although additional efforts will be required to refine the core-top calibration for universal use. [ABSTRACT FROM AUTHOR]

The stable (δ18O and δ13C) and clumped (Δ 47) isotope compositions of coral carbonate are valuable archives for paleoclimate reconstructions. However, the Δ 47 -temperature relationships of warm and cold-water corals deviate from that of inorganic carbonate precipitated at equilibrium. Dual clumped isotope thermometry of carbonates (i.e., simultaneous Δ 47 and Δ 48 measurements on a single carbonate) has the potential to achieve more accurate paleotemperature reconstruction, identifying and correcting for kinetically driven isotopic disequilibrium. Here we present the first extensive dual clumped isotope dataset of coral carbonate, spanning a broad range of cold and warm-water coral species. We confirm that corals are enriched in Δ 47 and depleted in Δ 48 relative to equilibrium, a pattern corresponding to the mixing of an equilibrium DIC pool with kinetically derived HCO 3 – produced by hydration and hydroxylation of CO 2. Dual clumped isotope measurements of cold-water corals fall on the initial linear portion of model (IsoDIC) predicted departure from equilibrium. The dual clumped isotope composition of cold-water corals, corrected by the model-predicted Δ 47 /Δ 48 offset slope (−0.78), yield accurate reconstruction of coral growth temperature with a precision of <3 °C at the 68% confidence level. In contrast, disequilibrium offsets in the Δ 47 and Δ 48 of warm-water corals correspond to precipitation from a more equilibrated DIC pool, which we attribute to the action of carbonic anhydrase in the calcifying fluid. It may be possible to correct warm-water coral growth temperatures, using an empirically derived correction (Δ 47 / Δ 48 offset slope of −0.4). Dual clumped isotope thermometry of coral carbonate opens new possibilities to reconstruct both sea surface temperatures and ocean dynamics of intermediate to deep water masses. [ABSTRACT FROM AUTHOR]

Rubidium is a highly fluid-mobile and incompatible element. Rb isotopes have great potential to trace the migration and enrichment of Rb in magmatic and fluid-related processes. In order to decipher the behavior of Rb isotopes during magmatic-fluid interaction, this study presents an estimate of average δ87Rb of the upper continental crust (UCC) and Rb isotope data for highly evolved leucogranites from Kampa, Himalayan orogen. The samples used to estimate the average UCC include 21 granites, 42 loess, and 10 sediments, which show small variations from −0.27‰ to 0.02‰, −0.17‰ to −0.07‰, and −0.31‰ to 0.07‰, respectively. Based on the weighted average δ87Rb of these samples, the estimate of average Rb isotope composition of UCC is −0.14 ± 0.01‰ (2SD/√n, n = 73), which is consistent with the δ87Rb of the bulk BSE (−0.13 ± 0.01‰). A comparison of the Kampa leucogranites with the UCC indicates that the δ87Rb of the Kampa leucogranites (−0.29‰ to 1.20‰) is significantly higher than the average value of the UCC. Such heavy Rb isotope composition cannot be due to chemical weathering, source heterogeneity, fractional crystallization, or partial melting. Instead, because the Kampa leucogranites have been modified by magmatic fluids during magma evolution, the high δ87Rb most likely reflects the signature of magmatic fluids derived from deeper magma. Our study for the first time indicates that the heavy Rb isotope characteristics of highly fractionated leucogranites provide a novel tracer for processes involving fluid activity. This could be useful for understanding the enrichment and mineralization of rare metals related to magmatic fluids. [ABSTRACT FROM AUTHOR]

The compositions of ocean island basalts (OIB) can record signatures of ancient subducted crust that has been recycled through the mantle. Based on their radiogenic isotope, major and trace element compositions, several geochemically-extreme mantle compositions are suggested to host components of recycled crust. Barium (Ba) stable isotopes can serve as a tracer of crustal recycling into the mantle sources of OIB because 1) Ba isotopes are fractionated in surface crustal reservoirs and 2) Ba is highly enriched in crust relative to the mantle. Therefore, the Ba isotopic compositions of the subducted crust could be detected in OIB. Here, we characterized Ba isotopic compositions of fresh OIB from hotspots sampling a range of mantle compositions. We first explore δ 138/134Ba (permil deviation of the 138Ba/134Ba ratio from the standard SRM3104a) in a suite of Hekla samples and demonstrate that magmatic differentiation does not impact the δ 138/134Ba values of OIB over MgO concentrations from 2.3 to 8.0 wt%, and higher MgO lavas are unlikely to experience δ 138/134Ba fractionation. After excluding OIB samples that experienced low-temperature alteration—which may shift the δ 138/134Ba values of samples away from their mantle sources—the δ 138/134Ba values of Samoan (including EM2 and high 3He/4He) lavas range from -0.07 to 0.07‰, while for Cook-Austral (HIMU) lavas the δ 138/134Ba values range from 0.00 to 0.11‰, and for St. Helena (HIMU) samples the δ 138/134Ba values range from 0.02 to 0.05‰. High 3He/4He (> 20 Ra, ratio to atmosphere) lavas from Vestfirðir of Iceland (-0.09 to 0.14‰) and Samoa (-0.02 to 0.01‰) also exhibit Ba isotopic variability. An extreme EM2 lava examined here (143Nd/144Nd = 0.512582) has lighter Ba isotopic composition (-0.07 ± 0.02‰, 2SE) than the depleted mantle sampled by MORB (0.05 ± 0.05‰, 2SD). The Samoan lavas show a possible positive trend between δ 138/134Ba values and 143Nd/144Nd ratios, and can be modeled as a binary mixing between the high 3He/4He (FOZO) mantle and recycled heterogeneous continental crust. In contrast, the extreme HIMU composition (206Pb/204Pb > 21.7) shows heavier Ba isotope signatures (0.11 ± 0.02‰, 2SE); δ 138/134Ba values positively correlate strongly with 206Pb/204Pb ratios in HIMU lavas, consistent with a contribution from recycled oceanic crust. [ABSTRACT FROM AUTHOR]

Triple-oxygen isotope (δ 18O and Δ ′ 17 O) analysis of sulfate is becoming a common tool to assess several biotic and abiotic sulfur-cycle processes, both today and in the geologic past. Multi-step sulfur redox reactions often involve intermediate sulfoxyanions such as sulfite, sulfoxylate, and thiosulfate, which may rapidly exchange oxygen atoms with surrounding water. Process-based reconstructions therefore require knowledge of equilibrium oxygen-isotope fractionation factors (18 α and 17 α) between water and each individual sulfoxyanion. Despite this importance, there currently exist only limited experimental 18 α data and no 17 α estimates due to the difficulty of isolating and analyzing short-lived intermediate species. To address this, we theoretically estimate 18 α and 17 α for a suite of sulfoxyanions—including several sulfate, sulfite, sulfoxylate, and thiosulfate species—using quantum computational chemistry. We determine fractionation factors for sulfoxyanion "water droplets" using the B3LYP/6-31G+(d,p) method; we additionally calculate higher-order method (CCSD/aug-cc-pVTZ and MP2/aug-cc-pVTZ) scaling factors, and we qualitatively estimate the importance of anharmonic zero-point energy (ZPE) corrections using a suite of gaseous sulfoxy compounds. Methodological scaling factors greatly impact 18 α predictions, whereas ZPE corrections are likely small (i.e., ⩽ 1 ‰) at Earth-surface temperatures; existing experimental data best agree with 18 α predictions when including redox state-specific CCSD/aug-cc-pVTZ scaling factors. Theoretical pH- and temperature-specific bulk-solution (i.e., abundance-weighted average of all species) 18 α values yield root-mean-square errors for sulfate/water, sulfite/water, and thiosulfate/water equilibrium of 4.5‰ (n = 18 experimental conditions), 3.7‰ (n = 27), and 2.2‰ (n = 3), respectively. However, sulfate- and sulfite-system agreement improves considerably when comparing experimental results only to SO 3 (OH)−/H 2 O (RMSE = 1.6‰) and SO 2 (OH)−/H 2 O (RMSE = 2.2‰) predictions, rather than bulk solutions. This is particularly true for the sulfite system at high and low pH, when SO 2 (OH)− is not the dominant species. We discuss potential experimental and theoretical biases that may lead to this apparent improvement. By combining 18 α and 17 α predictions, we additionally estimate that sulfate, sulfite, sulfoxylate, and thiosulfate species can exhibit Δ ′ 17 O values as much as 0.199‰, 0.205‰, 0.101‰, and 0.186‰ more negative than equilibrated water at Earth-surface temperatures (reference line slope = 0.5305). This theoretical framework provides a foundation to interpret experimental and observational triple-oxygen isotope results of several sulfur-cycle processes including pyrite oxidation, microbial metabolisms (e.g., sulfate reduction, thiosulfate disproportionation), and hydrothermal anhydrite precipitation. We highlight this with several examples. [ABSTRACT FROM AUTHOR]

Titanium isotopes are a promising novel tracer for magmatic processes. The Ti isotope composition of the Earth's mantle is key for using this tracer on a planetary scale, however, recent studies point to potential Ti isotope variations in mantle rocks. This study presents stable Ti isotope data for well-characterised peridotites from the Horoman massif (Japan) as well as for primitive, orogenic (Variscan and Alpine belts) and South African ultrapotassic rocks. The peridotites originate from a continuous section across the layered Horoman peridotite body and comprise highly depleted harzburgites to fertile lherzolites as well as metasomatically overprinted peridotites. Unlike most primitive mantle-derived magmas, which display very limited Ti isotope variations, δ49Ti (deviation of 49Ti/47Ti from the OL-Ti standard) values of the Horoman peridotites vary from –1.523 ± 0.029 to 0.547 ± 0.015‰ (2SD), spanning a total δ49Ti range of 2.07‰. Heavy, continental crust-like Ti isotope compositions are correlated with Nb/Th and are likely related to metasomatism of the mantle wedge above the Hidaka subduction zone. In such a scenario, isotopically heavy Ti was inherited from the subducted terrigenous sediments and might have been further enhanced by Ti isotope fractionation during mobilisation from the slab. Significant δ49Ti differences between coexisting clino- and orthopyroxene (Δ49Ti opx-cpx = 0.16–0.29‰) and notably light Ti isotope compositions (δ49Ti = −1.523 ± 0.029 to −0.677 ± 0.036‰, 2SD) recorded in highly refractory peridotites might be related to kinetic diffusion-driven isotope fractionation during fluid/melt percolation. The analysed ultrapotassic rocks display distinctly high δ49Ti values (0.114 ± 0.040 to 0.290 ± 0.030‰, 2SD) compared to the majority of primitive mantle-derived magmas. The observed enrichment in heavy Ti isotopes likely originates from recycled continental material characterised by high δ49Ti, in line with their derivation from strongly metasomatised mantle sources. Our results demonstrate the presence of significant small-scale Ti isotope heterogeneity within the upper mantle. [ABSTRACT FROM AUTHOR]

Highly variable molybdenum (Mo) isotope compositions are common in mafic arc magmas above oceanic subduction zones, but Mo isotopes of mafic magmas related to continental deep subduction have not yet been documented. Here, we report for the first time the Mo isotope composition of mafic igneous rocks above a typical continental subduction zone in east-central China, where the North China Block was subducted by the South China Block in the Triassic. Continental basalts of Early Cretaceous age show negative δ98Mo values of −0.98 to −0.16 ‰, significantly lower than the normal mantle value of −0.20 ± 0.01 ‰. These basalts also exhibit arc-like trace element compositions, high (87Sr/86Sr) i ratios of 0.7050 to 0.7058, negative ε Nd (t) values of −15.2 to −10.4, and negative ε Hf (t) values of −18.7 to −7.9. The light Mo isotope signatures are associated with enrichments in not only radiogenic Sr-Nd-Hf isotopes but also melt-mobile incompatible elements in the basalts. This indicates that the mantle source of these basalts would be generated by metasomatic reaction of the mantle wedge peridotite with felsic melts derived from partial melting of the deeply subducted continental crust. Given that continental crust usually exhibits heavy Mo isotope compositions, it would be dehydrated during its subduction to subarc depths. While the dehydration would have released isotopically heavy Mo fluids from the subducting crust, it leaves isotopically light Mo in the residual crust. The dehydrated continental crust underwent partial melting at subarc depths to produce felsic melts with low δ98Mo values, transferring the light Mo isotope signature into the mantle source of the basalts. Therefore, Mo isotopes in mafic igneous rocks are a powerful means to decipher the recycling of crustal components at convergent plate margins. Our study provides the first insight into Mo isotope recycling in continental subduction zones. [ABSTRACT FROM AUTHOR]

The thallium isotopic composition (ε205Tl) of seawater has been identified as a promising proxy for global oceanic redox conditions due to its close association with Mn oxide burial. Currently the preferred archives for past seawater Tl isotope reconstructions are from anoxic (no oxygen detected) and/or sulfidic (no oxygen detected and free sulfide present) environments that may suffer from basin restriction. Here we conduct a comprehensive modern calibration of authigenic sedimentary ε205Tl with core tops covering a wide range of bottom water oxygenation and sediment compositions. We show that quantitative Tl removal in the reducing porewaters (e.g., Mn reduction) at/near the sediment–water interface could allow the sediments to track the water column ε205Tl. Even if the bottom water is not anoxic, a shallow oxygen penetration depth under high export productivity and/or high sedimentation rates may still allow rapid oxygen consumption and buildup of reducing porewaters conducive for complete Tl removal. Thus, applications of the Tl isotope redox proxy can be greatly expanded beyond the anoxic conditions into places with open-ocean connections and high mass accumulation rates for high-resolution temporal reconstructions. We present a decision tree using the sediment enrichment factors of Mn, Ba, and U to determine whether sediment archives are likely to record the seawater ε205Tl. We additionally provide an estimate of authigenic Tl burial fluxes in environments with quantitative Tl removal, which is roughly double the size of the 'euxinic/anoxic' sink in previously published global marine Tl mass balance estimates. [ABSTRACT FROM AUTHOR]