Rayleigh Fractionation Research Articles

Density of Uranus moons: Evidence for ice/rock fractionation during planetary accretion

Current models suggest the five regular moons of Uranus formed in a single stage from a primary planetary disk or a secondary impact disk. Using latest estimates of moon masses (Jacobson, 2014), we find a power-law relationship between size and density of the moons due to varying rock/ice ratios caused by fractionation processes. This relationship is better explained by mild enrichment of rock with respect to ice in the solids that aggregate to form the moons following Rayleigh law for distillation (Rayleigh, 1896) than by differential diffusion in the disk, although the two mechanisms are not exclusive. Rayleigh fractionation requires that moon composition and density reflect their order of formation in a closed-system circumplanetary disk. For Uranus, the largest and densest moons Titania and Oberon (R ∼ 788 and 761 km, respectively) first formed, then the mid-sized Umbriel and Ariel (585 and 579 km), satellites in each pair forming simultaneously with similar composition, and finally the small rock-depleted Miranda (236 km). Fractionation likely occurred through impact vaporization during planetesimal accretion. This mechanism would add to those affecting the composition of accreting planets and moons in disks such as temporal/spatial variation of disk composition due to temperature gradients, advection, and large impacts. In the outer solar nebula, Rayleigh fractionation may account for the separation of a rock-dominated reservoir, and an ice-dominated reservoir, currently represented by CI carbonaceous chondrite/type-C asteroids and comets, respectively. Potential consequences for Uranus moons' composition are discussed.

The Role of Hydrocarbons in the Genesis of Mississippi-Valley-Type (MVT) Zn–Pb Deposits: Insights from In Situ Sulfur Isotopes of Sphalerite from the Southwestern Margin of the Yangtze Block, SW China

The coexistence of numerous Mississippi-Valley-type (MVT) Zn–Pb deposits and (paleo) oil/gas reservoirs in the world suggests a close genetic relationship between mineralization and hydrocarbon accumulation. Xuequ–Shandouya middle MVT Zn–Pb deposits are mainly hosted in the Lower Cambrian Maidiping Member siliceous dolostone on the southwestern margin of the Yangtze Block, accompanied by large amount of bitumen in the orebodies. Therefore, this type of Zn–Pb deposit is a natural laboratory for studying the relationship between the mineralization and the accumulation of paleo-oil/gas reservoirs. The deposit is characterized by spheroidal and concentric banded sphalerite. In situ sulfur isotope studies are carried out to determine the sulfur sources, sulfate reduction mechanisms, and role of hydrocarbons in the zinc–lead mineralization process. According to the mineral paragenesis and relative temporal relationship, two mineralization stages (1 and 2) are identified. An in situ sulfur isotope analysis of spheroidal and concentric banded sphalerite particles from Stage 2 shows that there are the two following types of sulfur isotopes in the sphalerite: one with relatively invariable δ34S values in the core (+8.31 to +9.30‰), and the other with a gradual increase from the core margin (core) to the rim (+0.39 to +16.18‰). These two types reflect that they may have formed in different times, with first type forming in the early period of Stage 2, while the second type was formed in the late period of Stage 2. The sulfur isotopic data suggest the sulfur source of evaporated sulfate minerals and multiple formation mechanisms for reduced sulfur (H2S). In the early period of Stage 2 mineralization, the sulfate reduction mechanism is mainly a mixture of bacterial sulfate reduction (BSR) and/or thermochemical sulfate reduction (TSR), while a very small amount may come from the thermal decomposition of organic compounds (DOCs). In the late period of Stage 2, TSR is dominant, and the gradual increase in the δ34S value may be related to Rayleigh fractionation. The oil/gasreservoir not only acts as a reducing agent to provide the required hydrogen sulfide for zinc–lead mineralization through TSR or BSR, but also provides reduced sulfur for mineralization through the thermal decomposition of organic compounds directly.

Geochemical behavior of riverine magnesium isotopes in the Yarlung Tsangpo River Basin, southern Tibetan Plateau

The riverine magnesium (Mg) isotope composition is generally controlled by sources and fractionation processes. However, it remains unclear in which cases these factors are predominant at the basin scale. In this study, we investigated the major elements, trace elements, and Mg and strontium (Sr) isotope ratios in the Yarlung Tsangpo River (YTR) Basin, southern Tibetan Plateau (TP), to explore the geochemical behavior of Mg isotopes and the dominant controlling factors. Riverine Mg2+ was shown to derive primarily from the weathering of silicates and carbonates. Riverine δ26Mg, which is first influenced by the mixing of different lithological sources, was detected ranging from −1.63 ‰ to −0.52 ‰. According to the saturation indexes of common Mg-bearing secondary minerals, the basin was divided into two zones. Above Shannan City (Zone I), where river waters mostly were oversaturated with minerals, positive correlations were observed between δ26Mg and the Ca/Mg and Sr/Mg ratios. This indicated that, after mixing, secondary minerals (talc and chlorite), which tend to preferentially incorporate 24Mg, may be formed in most of waters, leading to an increase in riverine δ26Mg values. The Rayleigh and batch fractionation models were fitted with factors ranging from 0.9975 to 0.9997. Below Shannan City (Zone II), where river waters mostly were unsaturated with minerals, riverine δ26Mg was positively correlated with 87Sr/86Sr and negatively correlated with the Ca/Mg, Sr/Mg, and HCO3/Na ratios. This suggested that the riverine δ26Mg were influenced by carbonate dissolution with faster kinetic rates. Our analysis of riverine δ26Mg values in the YTR Basin may be a reference for interpreting the relationships between δ26Mg and δ7Li in river systems worldwide. In minerals-oversaturated waters, secondary mineral formation was shown to lead to isotope fractionation, resulting in a positive correlation between the two parameters, whereas in minerals-unsaturated waters, the δ26Mg and δ7Li values were negatively correlated, possibly due to the predominance of mineral dissolution.

A Comparative Study of Cave System Calcium Isotope Ratios: Implications for Quantitative Reconstruction of Paleorainfall From Speleothems

AbstractVariations in speleothem calcium isotope ratios (δ44Ca) are thought to be uniquely controlled by prior carbonate precipitation (PCP) above a drip site and, when calibrated with modern data, show promise as a semi‐quantitative proxy for paleorainfall. However, few monitoring studies have focused on δ44Ca in modern cave systems. We present a multi‐year comparative study of δ44Ca, carbon isotopes (δ13C), and trace elemental ratios from cave drip waters, modern calcite, and host rocks from two cave systems in California—White Moon Cave (WMC) and Lake Shasta Caverns (LSC). Drip water and calcite δ44Ca from both caves indicate PCP‐driven enrichment, and we used a simple Rayleigh fractionation model to quantify PCP variability over the monitoring period. Modern calcite trace element and δ44Ca data positively correlate at WMC, but not at LSC, indicating a shared PCP control on these proxies at WMC but not at LSC. At both WMC and LSC, we observe an inverse relationship between PCP and rainfall amounts, though this relationship is variable across individual drip sites. Our modeled data suggest that WMC experiences ∼20% more PCP than LSC, consistent with the fact that WMC receives less annual rainfall. This work supports speleothem δ44Ca as an independent constraint on PCP that can aid in the interpretation of other hydrologically sensitive proxies and provide quantitative estimates of paleorainfall. Additional, long‐term monitoring studies from a variety of climate settings will be key for understanding δ44Ca variability in cave systems more fully and better constraining the relationship between PCP and rainfall.

Multi-tracer evidence of hydrology and primary production controls on dissolved organic matter composition and stability in the semi-arid aquatic continuum

Autochthonous dissolved organic matter (Auto-DOM) produced by a biological carbon pump using dissolved inorganic carbon (DIC) from carbonate weathering plays an important role in carbon cycling within inland waters. However, little is known regarding how environmental conditions impact the composition and fate of organic matter, especially in surface waters of the semi-arid Loess Plateau, which is enriched in DIC by significant carbonate weathering. To obtain novel insight, we combined hydrochemistry, isotopic composition (δ2H, δ18O, and δ13CDIC), Rayleigh fractionation model, optical spectroscopy (absorbance and fluorescence), and Fourier transform ion cyclotron resonance mass spectrometry measurements to elucidate how primary production and hydrology impact the composition of DOM and the stability of the resulting Auto-DOM throughout the river–reservoir–wetland aquatic continuum of the Bahe River in the carbonate-mineral-rich semi-arid Loess Plateau where carbonate weathering is significant. The Rayleigh fractionation model results indicated that watershed DIC is primarily consumed through aquatic primary production rather than CO2 degassing. Further investigation revealed that primary production and evaporation co-occurred in this watershed. With the enrichment of the stable water isotope δ2H, the relative abundances of the allochthonous compounds decreased and the relative abundances of the autochthonous substances increased, suggesting that the terrestrial signal of riverine DOM decreased while autochthonous production increased along the flow pathway. In addition, associations between optical and molecular characteristics among DOM samples from different water bodies revealed that the stability ratio (Fmax(C2/(C2 + C4))) of Auto-DOM to the ratio of carboxylic-rich alicyclic molecules showed a consistent trend, suggesting that phytoplankton-derived and biomineralized C2 compounds are potentially recalcitrant DOM in inland waters. We conclude that hydrology and primary production affect the source, composition, and, potentially, the stability of DOM in DIC-enriched surface waters of the semi-arid Loess Plateau, which may lead to a more humic-like DOM composition in inland water and export this lower bioavailability DOC to the ocean in the long term.

End-Member Mixing Model for Methane Carbon Isotope Fractionation During Shale Gas Desorption and Its Application.

Unlike conventional natural gas reservoirs, shale gas development involves systematic changes in methane carbon isotopes that cannot be effectively described by existing isotope fractionation models and mechanisms. Therefore, based on fundamental theories such as Rayleigh fractionation, mass transfer flow, and mass conservation, this study established isotopic fractionation equations for methane in adsorbed and free gas. By considering adsorbed and free gases as two end-members and using an isotope mixing model, a fractionation model for methane carbon isotopes during shale gas desorption was constructed. This model quantifies the isotopic fractionation effects during shale gas desorption and elucidates the mechanism of methane carbon isotope fractionation. Using on-site desorbed gas content and isotope data, parameter fitting and model calculations were conducted to characterize methane carbon isotope variations throughout the process of shale core field desorption. The results show a pattern of "initially negative and then turning positive," consistent with those of physical simulation experiments. It was clarified that differences in mixing the two end-members and isotopic fractionation play key roles in the variation of methane carbon isotopic composition in shale gas. By applying the methane carbon isotope fractionation model, the contribution of adsorbed gas during shale gas production was explored. It was found that in the early stage of development, the adsorbed gas in Well JY 1 was negligible. After nearly seven years of development, the contribution of adsorbed gas in the later stage has only reached nearly 15%, indicating that the production contribution of adsorbed gas is still less than 0.3 million cubic feet per day. The open flow of Well JY 6-2 is more conducive to the production of adsorbed gas, but the production capacity is still mainly contributed by free gas, indicating that the shale gas production capacity in the later stage in the Jiaoshiba gas field is still primarily dominated by free gas.

Zirconium isotope tracing of the magmatic-hydrothermal transition

Zirconium (Zr) isotope has significant potential for tracing magmatic processes, particularly late-stage fractionation involving fluids. However, the fractionation of Zr isotopes between magma and fluid is still unclear. In-situ Zr isotope analyses of zircons from three granite-pegmatite suites in the Tibetan plateau show a wide range of δ94/90Zr values (IPGP-Zr) between −0.23 ‰ and +1.38 ‰, with the highest values in hydrothermally altered zircons from pegmatites. A coupled dissolution-reprecipitation process of zircons in a zircon-undersaturated melts is the most plausible mechanism to explain the strong enrichment of heavy Zr isotopes in hydrothermally altered zircons. During the formation of granitic pegmatite, exsolution of F-Na-Si-bearing fluids can enhance zircon solubility and will significantly remove Zr from the melt. Faster diffusion of 90Zr into the fluid channel will result in preferential accumulation of light Zr isotopes in the fluid, and drive the residual melt towards heavier Zr isotopes under disequilibrium conditions. Later, the reprecipitated zircons will inherit the heavy Zr isotope composition of the melt. Rayleigh fractionation models suggest fluid-melt fractionation factors for Zr isotope of about 0.99911–0.99930. This shows the major controls of fluid on Zr isotope fractionation during crystal-magma-fluid differentiation. Thus Zr isotope can be a powerful tracer of the magmatic-hydrothermal transition.

Interaction between intermittent river and groundwater in arid and semiarid basin considering the influence of evaporation on isotopic compositions

Study region: Tabu Basin is located on a typical-deserted steppe in Inner Mongolia, China and plays a significant role in maintaining ecological stability of northern China.Study focus: Among rivers worldwide, 51–60 % cease to flow for at least one day annually, especially in arid and semiarid areas. Understanding interactions between intermittent rivers (IRs) and groundwater (GW) is crucial for protecting water resources and sustaining ecological balance. A comprehensive framework was developed to analyze these interactions.New hydrological insights for the region: Unlike previous studies, this study considered seasonal differences in hydrochemical and isotopic compositions of IR and GW, as well as the intensive influence of evaporation. Multitemporal datasets were employed to analyze the IR–GW interaction across spatial, temporal, and profiling scales. Hydrochemical evidence and isotopic analysis indicated the input of meltwater with depleted isotopic signatures to GW and GW recharge to IR. The isotopic compositions followed the order of rainfall < GW <IR; the higher isotopic compositions of the IR were mainly attributed to evaporation. A formula for minimizing the influence of evaporation was derived based on Rayleigh fractionation. Using the MixSIAR model, the GW and rainfall contributions to the IR were 63.8 % and 36.2 %, respectively. The GW was consisted of 55.3 % meltwater and 44.7 % summer rainfall. The integration of multiple methods offers an effective approach for studying IR–GW interactions in arid and semiarid areas.

Kinetic nitrogen isotope effects of 18 amino acids degradation during burning processes

Understanding the nitrogen isotopic variations of individual amino acids (AAs) is essential for utilizing the nitrogen isotope values of individual amino acids (δ15N-AA) as source indicators to identify proteinaceous matter originating from biomass combustion processes. However, the nitrogen isotope effects (ε) associated with the degradation of individual amino acids during combustion processes have not been previously explored. In this study, we measured the nitrogen isotope values of residual free amino acids -following a series of controlled combustion experiments at temperatures of 160–240 °C and durations of 2 min to 8 h, as described in Part 1. δ15N values of proline, aspartate, alanine, valine, glycine, leucine, and isoleucine are more positive than their initial δ15N values after prolonged combustion. Variations in δ15N values of the most AAs conform to the Rayleigh fractionation during combustion and their nitrogen isotope effects (ε) are greatly impacted by their respective combustion degradation pathways. This is the first time the ε values associated with the degradation pathways of AAs during combustion have been characterized. Only the ε values associated with Pathway 1 (dehydration to form dipeptide) and 2 (simultaneous deamination and decarboxylation) are found to be significant and temperature-dependent, ranging from + 2.9 to 6.4‰ and + 0.9‰ to + 3.8‰, respectively. Conversely, ε values associated with other pathways are minor. This improves the current understanding on the degradation mechanisms of protein nitrogen during biomass burning.

Multi-isotopes (δD, δ18Owater, 87Sr/86Sr, δ34S and δ18Osulfate) as indicators for groundwater salinization genesis and evolution of a large agricultural drainage lake basin in Inner Mongolia, Northwest China

Groundwater salinization, a major eco-environmental problem in arid and semi-arid areas, can accelerate soil salinization, reducing crop productivity and imbalances in ecosystem diversity. This study classified water samples collected from the Ulansuhai Lake basin into five clusters using self-organizing maps (SOM). On this basis, multiple isotopes (δ18Owater, δD, 87Sr/86Sr, δ18Osulfate and δ34S) and isotopic models (Rayleigh fractionation and Bayesian isotope mixing models) were used to identify and quantify the genesis and evolution of groundwater salinization. The results showed that the samples were brackish or saline water, and the hydrochemical types were dominated by Na + K-Cl (SO4). It has been proved that the processes associated with groundwater salinization in the Ulansuhai Lake basin were dominated by water-rock interaction and human inputs. Among them, evaporite dissolution contributed substantially to groundwater salinity. Furthermore, salt inputs from human activities cannot be negligible. Based on the model calculations, evaporite dissolution accounted for the most significant proportion of all sources, with a mean value of 53 %. In addition, human inputs from regular agricultural activities (28 % from sewage and manure and 8 % from fertilizers) constituted another vital source of groundwater salinization associated with extensive agricultural activities in the study area. This study's results can deepen our understanding of the genesis of groundwater salinization and the evolution of the agricultural drainage lake basin. This knowledge will assist the Environmental Protection Department in developing effective policies for groundwater management in the Yellow River Basin.

Source and fate of atmospheric iron supplied to the subarctic North Pacific traced by stable iron isotope ratios

The availability of dissolved iron (Fe) limits primary production over some regions of the surface ocean, especially in regions such as the subarctic North Pacific. In this work, we use Fe stable isotope ratios (δ56Fe) in bulk and size-fractionated marine aerosol particles and dissolved Fe of surface seawater in the subarctic North Pacific on Japanese GEOTRACES cruise GP02 (Summer 2017) as a tracer to clarify the relative contribution of combustion and natural Fe in both marine aerosol particles and surface seawater. The bulk aerosols collected in the coastal regions of both East Asia and western North Pacific have total δ56Fe values that are as low as −0.5 ‰ when compared to crustal (+0.1 ‰), with both the water-soluble phase and the fine particles even more fractionated (as low as −1.9 and −2.8 ‰, respectively). The negative correlation between the aerosol δ56Fe signatures and the enrichment factors of Fe and other elements dominated by anthropogenic sources (e.g., lead and cadmium) in these coastal regions indicates the presence of Fe emitted from high-temperature combustion sources, such as coal combustion and metal smelting. In these regions, combustion Fe accounts for 4–13 and 13–45 % of the total and water-soluble aerosol Fe, respectively. The results demonstrate that soluble aerosol Fe sourced from combustion Fe can be equivalent to that sourced from natural dust Fe in these coastal regions. By contrast, the aerosol particles in pelagic regions were near crustal δ56Fe in all particle size fractions, indicating the dominance of natural Fe and little to no combustion Fe. The relationships among the fractional Fe solubility, major ion concentration, Fe species, and δ56Fe indicate that the presence of combustion Fe is the dominant reason for the solubility increase in the coastal regions and that the atmospheric processing of mineral dust during transport is more important in the pelagic regions. The dissolved Fe of the surface seawater at 10 m depth had a consistently higher δ56Fe by up to +1.5 ‰ than that of the simultaneously collected water-soluble aerosol Fe. The pattern of the elevated δ56Fe in the surface seawater corresponds to decreasing Fe concentrations and can be approximated by Rayleigh fractionation; we attribute these elevated surface δ56Fe values to the effect of biological uptake. New Fe fluxes from both the atmosphere and deeper depths are limited at least in summer compared with the biological uptake in the open ocean of the subarctic North Pacific.

Antimony Isotope Fractionation during Kinetic Sb(III) Oxidation by Antimony-Oxidizing Bacteria Pseudomonas sp. J1.

Antimony (Sb) isotopic fractionation is frequently used as a proxy for biogeochemical processes in nature. However, to date, little is known about Sb isotope fractionation in biologically driven reactions. In this study, Pseudomonas sp. J1 was selected for Sb isotope fractionation experiments with varying initial Sb concentration gradients (50-200 μM) at pH 7.2 and 30 °C. Compared to the initial Sb(III) reservoir (δ123Sb = 0.03 ± 0.01 ∼ 0.06 ± 0.01‰), lighter isotopes were preferentially oxidized to Sb(V). Relatively constant isotope enrichment factors (ε) of -0.62 ± 0.06 and -0.58 ± 0.02‰ were observed for the initial Sb concentrations ranging between 50 and 200 μM during the first 22 days. Therefore, the Sb concentration has a limited influence on Sb isotope fractionation during Sb(III) oxidation that can be described by a kinetically dominated Rayleigh fractionation model. Due to the decrease in the Sb-oxidation rate by Pseudomonas sp. J1, observed for the initial Sb concentration of 200 μM, Sb isotope fractionation shifted toward isotopic equilibrium after 22 days, with slightly heavy Sb(V) after 68 days. These findings provide the prospect of using Sb isotopes as an environmental tracer in the Sb biogeochemical cycle.

Generation of abnormal distributions of inter- and intramolecular δ13C compositions in hydrocarbons from gold tube pyrolysis of an Australian torbanite

Abnormal stable carbon isotopic (δ13C) compositions, deviating from the conventional order of δ13C1 < δ13C2 < δ13C3, are frequently observed in natural gas reservoirs. For thermogenic gas, these anomalies, such as δ13C1 < δ13C3 < δ13C2 or δ13C1 > δ13C2 > δ13C3, have multiple formation mechanisms including gas mixing in conventional systems and desorption processes of gaseous hydrocarbons in unconventional shale gas systems due to their inconsistency with Rayleigh fractionation processes. Considering distinct reaction pathways (e.g., C1 polymerized to C2), these aberrant δ13C signatures are often construed as intrinsic hallmarks of extensively evolved natural gas. However, on the basis of gas generation simulation, not all findings exhibit abnormal δ13C values, hinting at multifaceted and intricate mechanisms governing the isotopic fractionation of alkane gas components. This study conducted gold tube pyrolysis of an Australian torbanite, revealing four distinct types of δ13C anomalies in hydrocarbon classes. Polycyclic aromatic hydrocarbons (PAHs) exhibited δ13C values more negative than co-occurring n-alkanes. δ13C3 displayed a negative trend shift from EasyRo = 3.5 %, resulting in a partially δ13C-reversed gas (δ13C1 < δ13C3 < δ13C2) formed at EasyRo ≈ 4.1 %. Moreover, intramolecular δ13C3 (both terminal and central carbons, termed δ13Ca and δ13Cb, respectively) reversed alongside the overall δ13C3 trend. Additionally, the evolution of site preference in δ13C3 (termed ‰SP = δ13Ca – δ13Cb) transitioned from progressively negative to positive. The results of this study demonstrate that the potential conversion between saturated hydrocarbons, aromatic hydrocarbons, and alkane gases is at least partially responsible for δ13C anomalies, but also that gaseous hydrocarbons formed from other fractions at high maturity cannot be ruled out, such as kerogen and pyrobitumen.

Temporal monitoring of fumarole composition at Santorini volcano (Greece) highlights a quiescent state after the 2011–2012 unrest

Santorini Island (Greece) is an active volcano which has alternated between dormant and active periods over the last 650,000 years with the latest volcanic unrest occurring in 2011–2012. Here we report a geochemical survey of fumarolic gases collected at Nea Kameni islet located in the center of the caldera over the period 2015–2022 in order to study the activity of the volcano and changes in hydrothermal conditions. This period is marked by the absence of significant geochemical anomalies compared to the unrest of 2011–2012, implying that no new magma upwelling has occurred. This is evident from the low CO2/CH4 ratio and H2 concentration of fumaroles. An increase of the atmospheric contribution in gases after the 2011–2012 unrest suggests a decrease of the deep gas flow and the chemical and C-He-isotope compositions are compatible with a model of Rayleigh fractionation in which CO2 dissolves in water at decreasing temperatures over time. These results are consistent with temperature estimates obtained using the H2/N2 geothermometer, seismic and geodetic evidences. This implies a slowing of the degassing of the hydrothermal/volcanic system and a cooling of the magma injected at shallow depth in 2011–2012. All these conclusions support a quiescent state of the Santorini volcano over the period 2015–2022.

Lower crustal control in the iron isotope systematics of plutonic xenoliths from Adak Island, Central Aleutians, with implications for arc magma geochemistry

We present bulk-rock and mineral Fe isotope data of ultramafic to mafic xenoliths and basaltic to andesitic lavas from Adagdak Volcano (Adak Island, Central Aleutians) to study the effects of early differentiation on the Fe isotopic evolution of island arc basalts and their crystallization products. The Fe isotope composition of ultramafic cumulate xenoliths increases from dunite (δ56Fe = –0.09 to –0.02 ‰) to clinopyroxenite (δ56Fe = +0.06 to +0.09 ‰), consistent with higher modal proportions of clinopyroxene (δ56Fe = –0.05 to +0.11 ‰) relative to olivine (δ56Fe = –0.10 to +0.06 ‰) in the latter. Mid-crustal cumulate amphibole gabbro and hornblendite cumulates also record heavier Fe isotope compositions (δ56Fe = +0.04 to +0.08 ‰) due to the abundance of isotopically heavy amphibole (δ56Fe = +0.07 to +0.09 ‰) and magnetite (δ56Fe = +0.11 to +0.13 ‰) in these rocks. High inter-mineral fractionations observed in spinel-olivine and spinel-clinopyroxene pairs (Δ56Fespl-ol = +0.12 to +0.28 and Δ56Fespl-cpx = +0.06 to +0.19) suggest that spinel is not recording equilibrium crystallization conditions for the ultramafic assemblages, likely due to subsolidus Fe-Mg exchange. Our data also include Fe isotope measurements of one mantle dunite (δ56Fe = +0.03 ± 0.05 ‰). Five Adagdak lavas, spanning from basalts to andesites, yield a narrow range of δ56Fe between +0.03 and +0.06 ‰. Our results highlight the potential of amphibole in driving the Fe isotope depletion trends observed in many erupted arc lavas, as amphibole hosts 28–99 % of the FeOT budget in the amphibole gabbro and hornblendite cumulates. This is also supported by single-crystal synchrotron Mössbauer spectroscopy of two amphibole grains, the first from an amphibole gabbro and the second from a hornblendite, which yield Fe3+/ΣFe ratios of 0.55 ± 0.06 and 0.58 ± 0.02, respectively. Water content and hydrogen isotope compositions determined by secondary-ion mass spectrometry from the same amphibole grains indicate partial dehydrogenation. Using Rayleigh fractionation modeling to account for oxidation during post-crystallization dehydrogenation, we calculate magmatic Fe3+/ΣFe ratios of 0.41 ± 0.04 for the amphibole gabbro and 0.30 ± 0.05 for the hornblendite. These data are then used to estimate an appropriate Fe force constant for Adagdak amphibole and quantitatively evaluate the effects of amphibole fractionation. Through a fractional crystallization model, we show how arc melts may experience periods of increasing δ56Fe during olivine-dominated fractionation, followed by decreasing δ56Fe once magnetite and amphibole saturate as cumulate phases. Notably, this dichotomy between fractionation of isotopically light versus heavy cumulate assemblages and its effects on the Fe isotope evolution of arc magmas is not captured by the Adagdak lava record, highlighting the utility of cumulates in chronicling the early isotopic evolution of magmatic systems.

Magmatic–Hydrothermal Evolution at the Barren Wushan Pluton, Southeast China: Insight into Controls on Mineralization Potential

Abstract A-type granites typically exhibit enrichment and mineralization of critical metals such as molybdenum and tin, essential for emerging technologies. However, the key factors influencing their mineralization potential remain elusive. The scarcity of studies on barren systems impedes the understanding of this question. Here, a detailed melt and fluid inclusion study was conducted on the barren Wushan pluton to reconstruct its magmatic evolution and magmatic–hydrothermal transition and explore the factors controlling the metallogenic potential of Mo and Sn in A-type granites. The Wushan pluton displays apparent lithological zoning consisting of two major phases, i.e., medium-grained seriate to porphyritic alkali feldspar granite and fine-grained porphyritic granite. Miarolitic cavities are widely developed in each lithofacies. The silicate melt inclusions from two granitic phases are rhyolitic, with moderate F contents (0.06–0.53 wt %) and depleted H2O contents (2.0–3.5 wt %). Melt inclusions show a wide range of incompatible element contents, such as Cs (9–1977 μg/g) and Rb (268–2601 μg/g), suggesting that Wushan has undergone a high degree of magma evolution. Mo behaves incompatibly in the magmatic evolution, and its content is enriched with the increasing degree of fractional crystallization, but remains constant after the Cs content exceeds 50 μg/g. Rayleigh fractionation model suggests that a large amount of Mo is extracted from fluid exsolution, which restrains Mo from further enrichment. In contrast, Sn behaves as a mildly incompatible element during the entire magmatic evolution history. The contents of Sn increase slowly compared to the trend of Mo, and the maximum contents reach ~30 μg/g in the highly evolved melts. The separation and crystallization of Sn-bearing minerals such as biotite, magnetite, and titanite inhibit the enrichment of Sn. Intermediate-density (ID-type) fluid inclusions hosted in the miarolitic quartz, representing the initial fluid exsolving from magma, display high Mo but low Sn concentrations. Constrained from two assemblages of coexisting ID-type fluid and melt inclusions, the fluid/melt partition coefficients of metals are obtained, with DMo, fluid/melt at 16–19, while DSn, fluid/melt is only about 1. The comparison between Mo-mineralized and barren intrusions worldwide shows that the metal contents in melts and fluids are not fundamentally different. The mineralized intrusions are characterized by the lower melt viscosity and the development of apophyses, both of which facilitate the extraction of metals and fluids from large magma chambers, followed by their concentration into a small rock volume. Consequently, it appears that physical and structural conditions rather than chemical compositions play a crucial role in the Mo mineralization process. Enrichment of Sn in melts is necessary but not decisive for Sn mineralization, whereas Sn enrichment in the initial exsolving fluid determines the Sn mineralization potential of a given granitic system. Compared to Sn enrichment in source melting and fractional crystallization which commonly enhance final Sn fertility in the highly evolved melts, the efficiency of Sn partitioning between melt and fluid plays a fundamental role in converting melt fertility into Sn-enriched fluids and thereby high mineralization potential of the magmatic–hydrothermal system. Our findings suggest a prospect for Mo exploration in the coastal A-type granite belt in South China, while the potential for Sn mineralization is expected to be limited.

Sphalerite Records Cd Isotopic Signatures of the Parent Rocks in Hydrothermal Systems: A Case Study From the Nayongzhi Zn–Pb Deposit, Southwest China

AbstractMetal stable isotopes (e.g., Zn, Cd, and Cu) have been used to track metal sources in different types of hydrothermal systems. However, metal isotopic variations in sulphides could be triggered by various factors such as mineral precipitation and fluid mixing. Thus, tracking the metal sources of hydrothermal systems is still a big challenge for metal isotopes. In this study, we investigated the Cd isotopic systematics of sphalerite from the Nayongzhi Zn–Pb deposit, which is a Mississippi Valley‐type (MVT) deposit in the Sichuan–Yunnan–Guizhou mineralization province (SYGMP). We reinterpreted the published S isotope data for the SYGMP and found that the large S isotopic variations were controlled by Rayleigh fractionation between sulphide and reduced S. As such, a model that involves mixing of a metal‐rich fluid with a reduced S pool formed by thermochemical sulfate reduction (TSR) can explain the ore formation in the Nayongzhi deposit. Based on this model, no Cd isotopic fractionation was observed due to its low solubility in fluids during mixing, and thus the Cd isotopic variations of sphalerite were inherited from the source rocks. The large range of Zn/Cd ratios and uniform Cd isotopic compositions of the sulphides are similar to those of igneous rocks but different from those of sedimentary rocks, indicating that Zn and Cd were derived mainly from basement rocks (e.g., migmatite, gneiss, and granulite). Our results reaffirm that metal stable isotopes, particularly Cd isotope compositions of sphalerite, are powerful geochemical tracers for investigating the formation mechanisms of ore deposits.

Equilibrium Fractionation of Stable Strontium Isotopes During Adsorption to Mn Oxides

AbstractUnderstanding the stable strontium isotope fractionation between seawater and ferromanganese crusts/nodules is crucial for reconstructing seawater Sr isotopic history. This study explores stable Sr isotope fractionation during adsorption to manganese oxides, as preliminary experiments indicated their higher Sr adsorption capacity compared to iron oxides. We observed that equilibrium in concentration and isotopic composition can be achieved within a day. The lighter isotope (86Sr) is preferentially adsorbed, with the fractionation extent showing no significant change with respect to reaction time, pH, or Sr concentration under low ionic strength conditions (−0.14‰). Equilibrium fractionation processes, rather than Rayleigh fractionation, more accurately represent the isotopic composition trends of dissolved and adsorbed Sr. Experiments in synthetic seawater demonstrated greater Sr isotope fractionation (−0.20‰) compared to low ionic‐strength solutions. This fractionation during adsorption by manganese oxides in synthetic seawater is consistent with the isotopic discrepancy between seawater and ferromanganese nodules, suggesting that Mn oxide adsorption is the primary mechanism for Sr accumulation in ferromanganese deposits. These results imply that ferromanganese deposit Sr isotopic compositions could be proxies for reconstructing seawater Sr isotopic history, which is influenced by carbonate burial and continental weathering, reflecting the global carbon cycle changes.

Continental Arc Accumulation Mafic Rocks in the Mid-Upper Crust: Constraints From the Early Eocene Hornblende Gabbro–Diorite in the Tengchong Block, Southeastern Extension of Tibet

Abstract Magmatic activity in the syn-collision stage is key for net crustal growth. To understand the mechanism of accretion–differentiation and compositional change of the continental crust, it is important to focus on the magmatic activity during the syn-collision stage. Early Eocene mafic–ultramafic rock assemblages found in the western part of the Tengchong Block resulted from a continuous series of arc magmatic evolution, thoroughly recording the continental arc magmatic system during the subduction of the Neo-Tethys Ocean and syn-collision of the Indian-Asian continents. Early Eocene hornblende gabbro–diorite in the Tengchong Block formed at 53 Ma, and the primitive magma was derived from an enriched mantle source due to the enriched Nd–Hf isotopes. The amphibole and biotite thermobarometer measurements indicate that the mafic magma reservoirs in the Tengchong Block occurred at a mid-upper crust. Petrography, amphibole Fe/Mg exchange coefficient (KD), Rayleigh fractionation, and equilibrium melt calculation indicate that the Early Eocene hornblende gabbro–diorite in the Tengchong Block was created due to plagioclase-dominated accumulation at the mid-upper crust level. Based on the calculation, the corresponding amphibole equilibrium melt is more silicic (dacitic–rhyolitic in composition) than the bulk rocks, indicating a more evolved composition in the mid-upper crust. Three types of plagioclases reveal the multi-recharging and dissolution–reprecipitation promoting the further evolution of these mafic rocks. Therefore, this study concludes that magma recharge and plagioclase-dominated accumulation processes may be important mechanisms for the formation and evolution of mafic magma and the further crustal differentiation at the mid-upper crust level in a continental margin arc.

δ2H isotopic offsets in xylem water measurements under cryogenic vacuum distillation: Quantifying and correcting wood‐water hydrogen exchange influences

AbstractStable isotopes δ18O and δ2H are used to infer vegetation water sources. In some studies, significant xylem water δ2H offsets from potential source waters have been observed. The offsets appear to be more prevalent with cryogenic vacuum distillation (CVD) of plant water. Hypothesized mechanisms for these offsets include changes during plant water uptake and transport, and methodological problems. We propose that a large portion of the offsets are due to hydrogen isotope exchange between xylem water and non‐crystalline hydroxyl groups of wood cellulose and hemicellulose during CVD. We present a method for estimating the hypothesized isotopic exchange between wood tissues and water, which is the result of Rayleigh and equilibrium fractionation. To estimate the exchange, we use published wood properties for North American tree species and isotope chemical relationships as a function of moisture content, CVD temperature and water extraction efficiency. A simple model of exchange between xylem water and hydroxyl groups captures the range of observations in studies in which CVD and non‐CVD methods were compared. To evaluate the model, we compared observed δ2H offsets (sw‐excess) values from two field datasets (90°C, n = 364, and 170°C, n = 43) to δ2H offsets estimated with our chemical model. We found good agreement between observed and estimated δ2H offsets for samples extracted at 90°C (r2 = 0.69) but not for samples extracted at 170°C (r2 = 0.20). The offset may be eliminated by increasing the extraction temperature to 229°C or by adding a standard sufficient to raise the moisture content to &gt;150%. A correction can also be approximated by applying a theoretical calculation based on the extraction temperature, moisture content and water extraction efficiency.