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Magnetite is common in various magmatic and hydrothermal ore deposit types, and its trace element geochemistry has become increasingly used in ore genesis studies and mineral exploration. While fractional crystallization has been shown to influence the chemistry of igneous magnetite, the extent to which this process regulates the trace element composition of hydrothermal magnetite remains poorly understood. In this study, we analyzed trace elements in hydrothermal magnetite from the Baijian Fe-(Co) skarn deposit in eastern China and used Rayleigh and equilibrium fractionation modeling to demonstrate the importance of magnetite precipitation in controlling fluid and magnetite chemistry during Fe skarn mineralization. The Baijian Fe-(Co) skarn deposit has three stages of magnetite. From early Mag-1 to later Mag-2 and Mag-3, the concentrations of compatible elements (Ni and V) decrease, whereas those of incompatible elements (Zn, Mn, and Co) increase. There are obvious trends of increasing incompatible/compatible element ratios (e.g., Co/Ni, Zn/V, and Zn/Ni) and decreasing compatible/incompatible element ratios (e.g., V/Mn, Ni/Mn, and V/Co) from Mag-1 to Mag-3, with strong correlations between each of these ratios. Such systematic trace element variations in successive stages of magnetite can be best explained by increasing degrees of fractional crystallization with time. The wide range of incompatible/compatible element ratios (spanning 2-4 orders of magnitude) in Mag-2 and Mag-3 suggests that magnetite crystallization follows a process akin to Rayleigh fractionation. Results from this study highlight the significant role that magnetite crystallization during skarn formation has on the trace element chemistry of this mineral. Moreover, as the crystallization of magnetite progresses, the Co/Fe ratio of residual hydrothermal fluids is elevated, which favors the precipitation of Co in late-stage sulfides. This process helps to explain why some Fe skarn deposits, as well as magnetite-rich iron oxide-apatite and iron oxide-copper-gold deposits, are potentially important economic sources for Co, currently necessary as one component in Li-ion batteries. [ABSTRACT FROM AUTHOR]

Late-Pan-African granitic pegmatites in Malawi host gem mineralization (tourmaline, beryl/aquamarine/heliodor). We use major and trace element chemistry of mica and tourmaline as proxies to describe the geochemical characteristics and to analyze the evolution of the pegmatite-forming melts. Trace element contents and ratios of pegmatitic micas and tourmalines show characteristic fractionation trends. Mica from highly fractionated pegmatite typically shows high Rb, Cs, Zn, Nb, Ta, F, and Li concentrations but low Ni, Co, V, Ti and Sc concentrations. In their less fractionated counterparts, these compositional patterns are largely reversed. Exceptions in these element patterns are related to the presence or absence of other phases that may fractionate specific elements more strongly than mica. Tourmaline shows similar fractionation trends in major and trace elements. The observed patterns indicate fractional crystallization as the dominant process of melt evolution. A near exponential decrease of alkali element ratios, such as K/Rb and K/Cs, and an increase in Rb, Cs and Li in white mica from the less to the more strongly differentiated zones suggest Rayleigh fractional crystallization. The modelling of these element ratios shows that in different pegmatite bodies the least differentiated zone formed at a fractionation coefficient of F = 0.35–0.5. Zones of intermediate fractionation show F = 0.85–0.9. Gem mineralization is associated with the most highly fractionated pegmatites or pegmatite zones (F = ~ 0.99). These highly fractionated pegmatites show strong enrichment of Li, Rb and Cs in mica and tourmaline forming from melts rich in incompatible elements. The crystallization of gem phases depended on this highly enriched environment. [ABSTRACT FROM AUTHOR]

In this work, we investigated in situ isotopic compositions of antimony (Sb) minerals from two substages of the ore deposits near Pezinok (Slovakia). The δ123Sb values of the primary Sb minerals range from −0.4 and +0.8‰ and increase progressively along the precipitation sequence. In the substage II, the early-formed gudmundite (FeSbS) shows in all sections the lowest δ123Sb values, followed by berthierite (FeSb2S4), stibnite (Sb2S3), and valentinite (Sb2O3) with the heaviest δ123Sb values. A similar trend was observed for the substage III, from the initially-formed stibnite, followed by kermesite (Sb2S2O), valentinite, senarmontite (both Sb2O3), and schafarzikite (FeSb2O4). The evolution can be rationalized by a Rayleigh fractionation model with a starting δ123Sb value in the fluid of +0.3‰, applying the same mineral-fluid fractionation factor to all minerals. Thus, the texturally observed order of mineralization is confirmed by diminishing trace element contents and heavier δ123Sb values in successively crystallized Sb minerals. Antimony in substage III was likely supplied from the oxidative dissolution of stibnite that formed earlier during substage II. The data interpretation, although limited by the lack of reliable mineral-fluid fractionation factors, implies that Sb precipitation within each substage occurred from an episodic metal precipitation, likely associated with a similar Sb isotope fractionation between fluid and all investigated Sb minerals. Large isotopic variations, induced by precipitation from a fluid as a response to temperature decrease, may be an obstacle in deciphering the metal source in hydrothermal ore deposits. However, Sb isotopes appear to be an excellent instrument to enhance our understanding on how hydrothermal systems operate. [ABSTRACT FROM AUTHOR]

Understanding climate change at the spatiotemporal scales necessary to improve climate projections requires proxy records that complement sparse and often contradictory observational temperature data sets. Massive long‐lived corals have tremendous potential in this regard, continuously recording information about ocean conditions as they grow. Nevertheless, extracting accurate ocean temperatures from corals is challenging because factors other than temperature influence skeletal chemistry. Here, we tested the ability of the coral Sr‐U thermometer to accurately capture annual sea surface temperatures (SSTs) in the subtropical Atlantic, where year‐to‐year temperatures vary by ∼1°C. Using laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS), we generated sufficient U/Ca – Sr/Ca pairs from a slow‐growing (1−2 mm/yr) Siderastrea siderea coral to calculate annual Sr‐U values. With the fine‐scale spatial resolution attained using the laser, skeleton accreted during both fast and slow growing times of the year was represented in our sampling. The resulting 30‐year‐long Sr‐U record tracked the amplitude and timing of annual SST to within ±0.2°C of observations (r = −0.71), whereas the Sr/Ca record did not (r = 0.23). Furthermore, Sr‐U corrected for Sr/Ca offsets among adjacent skeletal elements approximately 1 mm apart. These offsets are equivalent to differences of 2–3°C if typical Sr/Ca–SST calibrations are applied. Our observations indicate that Sr‐U can accurately constrain decadal‐to‐multidecadal variability and secular SST trends in regions where this information is urgently needed. Plain Language Summary: The chemistry of coral skeletons can be used to estimate past ocean temperatures and fill gaps in the instrumental record where observations are sparse. However, the accuracy of single element‐to‐calcium ratio thermometers is limited by biological factors that influence coral composition. A new coral thermometer, Sr‐U, corrects for biological effects on Sr/Ca, a widely used coral paleothermometer. Here, we evaluate whether Sr‐U can reconstruct temperatures in a slow‐ and seasonal‐growing coral from the Bahamas. Unlike Sr/Ca, Sr‐U tracks the observed average annual sea surface temperatures recorded at this site over a 30‐year period. Our results reveal consistent offsets in Sr/Ca values among neighboring parts of the coral skeleton that grew at the same time. These offsets translate to 2–3°C differences among parts of the coral that grew at the same temperature. However, we find no such difference in Sr‐U values between these skeletal elements. We conclude that Sr‐U can accurately reconstruct past ocean temperatures with a greater degree of confidence than those derived from Sr/Ca. Key Points: Our results confirm that Rayleigh fractionation dominates elemental ratio variability of a Siderastrea coral from the BahamasSr/Ca differences between neighboring skeletal elements, equivalent to 2–3°C, are resolved by Sr‐USr‐U in this Bahamas coral tracks annual seawater temperature amplitude and variability over a 30‐year period and a <1°C interannual range [ABSTRACT FROM AUTHOR]

This study evaluated the effectiveness of using organic carbon materials (i.e., woodchips) to remove nitrate from groundwater. The results of our flow-through column experiment, which was conducted over 1.6 years, suggested that denitrifying bacteria reduce nitrate by using it as an electron acceptor and woodchips as an electron donor. The nitrate removal rates were sufficiently high (0.39–1.04 mmol L−1 day−1) during the operation of the column. Denitrification process was supported by fractionation of nitrogen and oxygen isotopes (δ15N and δ18O), with the δ15N and δ18O values enriched from 7.4‰ and 22.3‰ to 21.2‰ and 30.4‰, respectively. Enrichment factors (ε ) for 15 N and 18O were calculated using the Rayleigh fractionation model, with values of − 13.2‰ for ε15N and − 7.1‰ for ε18O. The fractionation ratio of 15 N to 18O was 1.9:1, confirming denitrification. The most abundant bacterial genera in the soil used for inoculation were Enterobacter (86.7%), Nitrospira (1.8%), and Arthrobacter (1.5%), while those in the column effluent were Macrococcus (37.1%), Escherichia (14.7%), and Shigella (14.6%), indicating that bacterial communities changed in response to geochemical conditions in the column. This study suggests that nitrate in groundwater can be effectively removed using woodchip-based passive treatment systems and that information on isotopic fractionation and denitrifying bacteria can be key tools to understand denitrification. [ABSTRACT FROM AUTHOR]

Tibetan Plateau (TP) receives moistures from different directions, so variations of isotope elevation gradient on different sides may exist. In this study, we systematically monitored and modeled the elevation effect of stable isotopes in precipitation of the eastern margin of the Tibetan Plateau, where we collected 265 water samples for stable isotope (δ2H and δ18O) analyses, of which 70 are eventful rainfall samples, and 195 are surface water samples, for comparison. Alpine lakes on the mountain peaks were used to reconstruct the average isotope composition of the local precipitation based on the evaporation effect. An elevation gradient of −4.2‰ km−1 (R2=0.93) was established for the eastern marginal region of TP. This elevation gradient was further confirmed by fractionation assessment using the Rayleigh fractionation model. This is the highest ever reported for the TP and it is the highest as compared to those of all the other slopes as well as that of the interior part of the TP. Precipitation isotopes in the interior of TP show a lower elevation gradient, which is likely caused by significant local moisture recycling. This paper also discusses the limitations of using river water samples to construct the elevation gradient. [ABSTRACT FROM AUTHOR]

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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. • Power law relationship is found between density and radius of Uranus moons. • Density varies with ice/rock ratio in the moon. • Ice/rock fractionation follows a Rayleigh law for distillation. • Ice sublimation during accretion of planetesimals can explain the fractionation. [ABSTRACT FROM AUTHOR]

Redox reactions control the mobility and bioavailability of selenium (Se) in biogeochemical systems, both modern and ancient. Se isotope ratio measurements (e.g., 82Se/76Se) have been developed to enhance understanding of biogeochemical transformations and transport of Se. Stable isotope ratios of many elements are known to be powerful indicators of redox reactions, and shifts in 82Se/76Se have been observed for Se reduction reactions. However, Se isotope shifts caused by naturally relevant oxidation reactions have not been published. Here, we report Se isotope fractionation factors for oxidation of Se(IV) by birnessite. Experiments were conducted at pH = 4.0 and 5.5, with two types of birnessite of contrasting composition at two concentrations of suspended birnessite. The results are consistent with a single 82Se/76Se fractionation factor, for all times during all experiments, of 0.99767 (±0.0035 2 s.d.). Expressed as ε, the fractionation is 2.33‰ (±0.08‰).

The evaporation process plays a crucial role in the geochemical transport of volatile elements, such as antimony (Sb). However, there is limited understanding regarding the behavior of Sb isotopes during evaporation and their fractionation patterns, which hinders its application in the environment. To address this knowledge gap, a series of experiments was conducted to investigate the evaporation of Sb (V) chloride solution and its impact on the loss of Sb and Sb stable isotope fractionation. Our results demonstrate that the evaporation of Sb chloride solution results in a substantial loss of Sb and induces significant fractionation of Sb stable isotopes. The extent of Sb loss is shown to be influenced by both temperature and the concentration of hydrochloric acid, with higher temperatures and concentrations promoting greater rates of evaporative Sb loss. Following five cycles of repeated evaporation, the Sb isotope ratio of the residual solution increased by up to 1.47‰, indicating pronounced fractionation wherein the lighter Sb species preferentially evaporated into the vapor phase. The observed fractionation of Sb can be well described by the Rayleigh fractionation model. Interestingly, the heating temperature and the concentration of hydrochloric acid do not affect the fractionation factor of Sb in the studied range. The fractionation factor between vapor and aqueous (α vapor-aqueous) is estimated to be 0.99927 (−0.73‰ in Δ'123Sb vapor-aqueous). Moreover, the experimental data obtained can be elucidated by a kinetic isotope fractionation model involving collisional diffusion of SbO 2 Cl during evaporation at the vapor-aqueous boundary. These laboratory experiments provide valuable insights for utilizing Sb isotopes in the study of Sb pollution tracing, hydrothermal mineralization, and volcanic activities. Furthermore, they may contribute to enhancing our understanding of Sb geochemical cycles, especially high temperature associated processes. [ABSTRACT FROM AUTHOR]

The potential utilization and development of the Ba isotope tool depend on an accurate δ137/134Ba determination of the samples. During the chemical purification, whether the adsorption process on the surface of the ion-exchange resin could lead to the Ba isotopic fractionation and the degree of fractionation directly influence the accurate δ137/134Ba determination. In the present work, first-principles calculations based on the density functional theory were used to quantify the Ba isotopic equilibrium fractionation factor between the aqueous solution and the resin in the acid leaching process. By constructing and optimizing the geometric configurations of Ba-containing species, (Ba(H2O)n2+, Ba(H2O)nCl2, Ba(H2O)n(NO3)2, and the adsorbed Ba2+ on the surface of the resin, extracting the harmonic vibrational frequencies, we finally at 298 K obtained the fractionations, Δ137/134Basoln-ads = 0.07‰, Δ137/134BaBa(H2O)nCl2-ads = 0.05‰, and Δ137/134BaBa(H2O)n(NO3)2-ads = 0.02‰. Overall, there were almost no Ba isotope fractionations during leaching. Although the Ba isotope fractionation can be magnified by the Rayleigh fractionation process in purification, the difference in δ137/134Ba between the initial and final stages did not exceed 0.060‰ (or 0.045‰) when leaching the standard sample with HCl or HNO3, which is equal to or less than the accuracy of Ba isotopic analysis. At a common yield of 89.75%, Ba isotopic fractionation induced by incomplete recovery was 0.015‰ for HCl (or 0.011‰ for HNO3). Finally, if the influence of an incomplete recovery on the δ137/134Ba determination needs to be ignored, the recovery is suggested to be not less than 67% for HCl (or 46% for HNO3). [ABSTRACT FROM AUTHOR]

The K/Cs ratios of K-feldspars from granitic pegmatites are compared to models derived from the Rayleigh equation. The K/Cs and K/Rb ratios of K-feldspars and micas exhibit decreasing values when plotted against their Cs or Rb contents across cogenetic suites of granites and pegmatites and from margin to core of individual bodies. The trends in elemental ratios conform to Rayleigh fractionation for the crystallization of feldspars and micas from a silicate melt. Within two individual pegmatite bodies, the K/Cs ratio of K-feldspar initially falls more rapidly than the Rayleigh model predicts. That might reflect a local increase in the concentration of Cs relative to K due to the pile-up of incompatible elements in a boundary layer of melt adjacent to the crystal growth front. The addition of an aqueous solution to the Rayleigh model (i.e., the simultaneous crystallization of K-feldspars from melt and from aqueous solution) predicts high and increasing K/Cs ratios of K-feldspars that are not observed in natural rock suites, except when K-feldspars crystallize in miarolitic cavities or when primary K-feldspar recrystallizes to microcline perthite in an open hydrothermal system. In those cases, the Cs content of K-feldspars falls to nil because of the high solubility of Cs in aqueous solution and low compatibility of Cs in K-feldspar. Otherwise, the observed patterns of K/Rb or K/Cs in K-feldspar and micas in pegmatites conform to crystal-melt fractionation in which an aqueous solution plays no part. From the viewpoint of the geochemistry of Cs in pegmatites, these observations give support to the model proposed by Cameron et al. (1949) and endorsed by Jahns (1953a, 1953b). [ABSTRACT FROM AUTHOR]

Pingos are indicators of modern and past conditions of permafrost. In total, 1,620 pingos have been identified on the Yamal and Gydan peninsulas in western Siberia. The main purpose of this study is to consider the distribution of stable isotopes in pingo ice cores formed under conditions of open and closed systems. Two pingos from ice cores of different origin in the continuous permafrost zone of northwest Siberia have been considered: the Messoyaha‐1 pingo (10.5 m in height) and the Pestsovoe pingo (17 m in height). Drilling of the ice core was performed with continuous sampling of an undisturbed frozen core. Ice formation was estimated according to the Rayleigh fractionation in a closed‐system versus an open‐system framework. For the Pestsovoe pingo, a pronounced decrease in δ18O values with corresponding increase in dexc with depth indicates a closed system upon freezing of the lake talik from the top down. For the Messoyakha‐1 pingo, the values of δ18O and δ2Н showed a weak tendency to decrease with depth, with values of dexc varying randomly. Ice that was segregated in the overlying and underlying sediments had similar values of δ18O and δ2Н and a low slope. Isotopically nonequilibrium ice formation was established for ice which had been segregated in a closed system and for ice cores formed in an open to semiclosed system. The vacuum mechanism of water suction from the surrounding lake or lake talik may have played a significant role during the formation of the upper ice core of the Messoyakha‐1 pingo and its additional growth. [ABSTRACT FROM AUTHOR]

Tellurium stable isotope compositions and abundances (δ128/126Te relative to SRM 3156) are reported for 43 ordinary, enstatite, and Rumuruti chondrites, which together with results from a companion study on carbonaceous chondrites are used to assess the origin of volatile element fractionations in chondrites. Whereas Te isotope variations among carbonaceous chondrites predominantly reflect mixing between isotopically light chondrules/chondrule precursors and CI-like matrix, Te isotope variations among non-carbonaceous chondrites mainly result from Te redistribution during parent body thermal metamorphism. The enstatite chondrites in particular display increasingly heavy Te isotopic compositions and decreasing Te concentrations with increasing degree of metamorphism, indicating migration of isotopically light Te from the strongly metamorphosed inner parts towards the cooler outer regions of the parent bodies. By contrast, ordinary and Rumuruti chondrites display less systematic Te isotope variations, implying more localized redistribution of Te during parent body thermal metamorphism. We also report Te stable isotope data for 19 terrestrial mantle-derived rocks. Peridotites with Al 2 O 3 contents close to those inferred for the bulk silicate Earth (BSE) exhibit uniform δ128/126Te values, which we interpret to represent the Te isotopic composition of the BSE. This composition overlaps with the Te isotope composition of some volatile-rich carbonaceous chondrites (most notably CM chondrites), but also with that of enstatite chondrites. Comparison of the Te results to Se isotopes and Se/Te ratios shows that due to uncertainties in the composition of the BSE and the isotopic composition of bulk chondrite parent bodies, neither Te isotopes alone nor the combined Se-Te elemental and isotopic systematics can distinguish between a carbonaceous and enstatite chondrite-like late veneer, which is the presumed source of Se and Te in the BSE. Together, the results of this study illustrate that the relative abundances and mass-dependent isotope compositions of volatile elements like Se and Te are modified by physical and chemical processes occurring after planetary accretion, which severely complicates their use as genetic tracers. A corollary of this is that contrary to prior proposals the Se-Te systematics are not contradicting an inner solar system origin of the late veneer, as it has been inferred using nucleosynthetic isotope anomalies of other elements. [ABSTRACT FROM AUTHOR]

Coral skeletons are one of the best archives for past ocean seawater (SW) chemistry and isotopes. However, the distribution coefficients of major and minor elements in coral skeletons are not well determined. In this study, we launched an experiment to determine the distribution coefficients of multiple elements in corals’ skeletons by changing Ca concentrations in SW (CaSW). Two scleractinian corals, Pocillopora damicornis and Acropora cervicornis were cultured in modified Gulf of Eilat water (Red-Sea) with CaSW of approximately 10, 15, 20, and 25 mM. After almost three months, the newly grown skeletons were analyzed for the following elements: Li, Na, Mg, K, Sr, and Ba. Their ratios to Ca in the coral skeleton (El/Cacoral) increased linearly with El/CaSW (with R2 values above 0.98), crossing the origin and thus indicating constant distribution coefficient for each element over the experimental range of El/CaSW. The values of DEl were in good agreement with values reported for corals collected in natural seawater. However, differences were observed between the two species, and both were slightly deviating from inorganic aragonite D values. These deviations are well explained by Rayleigh fractionation process in the calcifying fluid (assuming it is mainly seawater). This was observed both for elements with D > 1 (Ba and Sr) and D < 1 (Li, Mg, Na, and K). P. damicornis showed open system behavior (∼20% of its Ca utilized) while A. cervicornis showed more closed calcifying reservoir (∼50% of its Ca utilized). The finding that the distribution coefficients of the six minor and trace elements are constant for a given species, should help in the reconstruction of past seawater chemistry based on multielement measurements in fossil corals. In particular, Na/Cacoral can be used to reconstruct past ocean Ca concentrations and with El/Cacoral ratios for other elements, their concentrations for the Cenozoic can be reconstructed.

Water stable isotopes (δ2H and δ18O) can record surface water evaporation, which is an important hydrological process for understanding watershed structure and function evolution. However, the isotopic estimation of water evaporation losses in the mountain watersheds remains poorly explored, which hinders understanding spatial variations of hydrological processes and their relationships with the temperature and vegetation. Here we investigated δ2H, δ18O, and d-excess values of stream water along an altitude gradient of 2130 to 3380 m in Guan'egou mountain watershed at the east edge of the Qinghai-Tibet Plateau in China. The mean δ2H (− 69.6 ‰ ± 2.6 ‰), δ18O (− 10.7 ‰ ± 0.3 ‰), and d-excess values (16.0 ‰ ± 1.4 ‰) of stream water indicate the inland moisture as the major source of precipitation in study area. Water stable isotopes increase linearly with decreasing altitudes, based on which we estimated the fractions of water evaporation losses along with the altitude and their variations in different vegetations. This study provides an isotopic evaluation method of water evaporation status in mountain watersheds, the results are useful for further understanding the relationship between hydrological processes and ecosystem function under the changing climate surrounding the Qinghai-Tibet Plateau. [ABSTRACT FROM AUTHOR]

Pegmatites often exhibit regional as well as internal zonation. In this study, we use major and trace element and Li–B isotopic composition of muscovite and tourmaline from internally zoned and un-zoned S-type pegmatites and their host granites to characterize the geochemical and isotopic fractionation associated with their formation. The internally zoned pegmatites comprise three distinct textural zones, namely wall, intermediate and core. Muscovite and tourmaline occur in all the zones of these pegmatites. The trace element concentrations/ratios of muscovite from the three zones form well-defined differentiation trends marked by enrichment of incompatible elements such as Rb, Cs, Sr, B, Zn, Nb, Ta, P and the depletion of Ni, Co, V, Sc, Ti, Ba from the wall zone, through the intermediate zone to the core zone. This is suggestive of a strong role of fractional crystallization in producing the compositional diversity in the internally zoned pegmatites. Alkali element ratios such K/Rb and K/Cs in muscovite exhibit near exponential decline from the wall to the core zone which is suggestive of Rayleigh-type fractional crystallization. Fractional crystallization modelling reveals that the formation of the wall zone requires < 69% crystallization, the intermediate zone 85–95% and the core zone ca. 99% crystallization, leading to extreme enrichment of Rb and Cs and other incompatible elements. Muscovites and tourmaline from the un-zoned pegmatites display similar compositional trends as the internally zoned ones, but with a significant compositional gap between the host granite and the pegmatite. Lithium isotopic composition of muscovites and B-isotopic composition of tourmalines become progressively lighter from wall to core zone of the internally zoned pegmatite and from the granite to the pegmatite for the un-zoned pegmatites. This is suggestive of an important role of vapour exsolution in the formation of the pegmatites. Taken together, the geochemical and isotopic trends in the pegmatites can be explained by Rayleigh fractional crystallization operating in tandem with vapour exsolution. [ABSTRACT FROM AUTHOR]

Although carbonate rocks are important for carbonate-hosted Pb Zn deposits, there is no effective constraint on the role of carbonate rocks during the Pb Zn mineralization process. This paper uses the giant Huize Pb Zn deposit (>7 Mt. metals @ 25–35 % Pb + Zn) as a case study and employs zinc isotopes to gain new insights into such issue. The Huize deposit, located in the Sichuan-Yunnan-Guizhou (SYG) Pb Zn metallogenic province, is the largest carbonate-hosted deposit in the western Yangtze Block, SW China. Zinc isotope compositions of sphalerite, altered carbonate rocks and fresh carbonate rocks from the Huize deposit are of +0.39 ‰ to +0.68 ‰ (mean + 0.52 ‰, n = 20), −0.25 ‰ to +0.34 ‰ (mean + 0.07 ‰, n = 11) and + 0.35 ‰ to +0.71 ‰ (mean + 0.51 ‰, n = 6), respectively. The initial δ66Zn value of hydrothermal fluids calculated by the Rayleigh fractionation simulation is +0.4 ‰. Such zinc isotope signatures are significantly different from those of potential source rocks, such as the basement Proterozoic metamorphic rocks (δ66Zn = +0.10 ‰ to +0.34 ‰, mean + 0.22 ‰), the host Upper Ediacaran to Middle Permian sedimentary rocks (δ66Zn = −0.24 ‰ to +0.35 ‰, mean + 0.1 ‰) and the cover Upper Permian Emeishan basalts (δ66Zn = +0.30 ‰ to +0.44 ‰, mean + 0.35 ‰). Hence, it is likely that the carbonate rocks acted zinc source and provided heavy zinc isotopes for the Pb Zn mineralization. The ore-forming fluids extracted the heavy zinc isotopes of the ore-hosting carbonate rocks, resulting in the zinc isotopes of the altered carbonate rocks being far lighter than those of the marine carbonate rocks (δ66Zn = +0.24 ‰ to +1.32 ‰). The sulfur isotope compositions (+12.9 ‰ to +15.6 ‰, mean + 14.3 ‰, n = 20) of sphalerite from the Huize deposit also suggested that the source of reduced sulfur is the marine sulfate minerals within the ore-hosting carbonate rocks. Therefore, carbonate rocks play key roles in the formation of carbonate-hosted Pb Zn deposits: 1) provide ore-forming elements and ore-hosting space through dissolution; 2) cause isotope variation (e.g., zinc isotopes) by isotope exchange; 3) act as a carbonate buffer to adjust physicochemical conditions (e.g., pH) via cyclical carbonate dissolution, CO 2 degassing and recrystallization. On the other hand, the zinc isotope mapping shows that the western part of the SYG Pb Zn metallogenic province has lighter zinc isotopes than those of the eastern part. This suggests that the migration pathway of hydrothermal fluids is likely west to east, which provides a new clue for regional Pb Zn ore prospecting. The outcomes have important benefits for understanding the genesis of Pb Zn deposits, the role of carbonate rocks and the application of zinc isotopes. [Display omitted] • Zinc isotope fractionation was jointly controlled by fluid mixing and Rayleigh fractionation. • Heavy zinc isotopes were extracted from carbonate rocks through water/rock interaction. • Carbonate rocks played a key role in the formation of carbonate-hosted Pb Zn deposits. • Zinc isotope mapping provides a new clue for regional Pb Zn ore prospecting. [ABSTRACT FROM AUTHOR]

The chance of apparent isotope anomalies either in closed systems at equilibrium or during Rayleigh processes is investigated. The term 'apparent' is chosen to highlight that only mass-dependent fractionation (MDF) processes are considered. A goal is finding analytical expressions that could be used for dealing with the complexity of real systems. For example, the formula for calculating the isotopic fractionation in systems with any desired number of phases involved is identified. Terrestrial samples could exhibit artificially low MIF signatures because Δ is not invariant with the choice of the reference material. An alternative parameter Δ* only depending on β is therefore introduced. The potential transfer of sample points in a three-isotope plot to parallel lines during reversible processes is also investigated. Larger apparent anomalies are expected in systems lacking an infinite reservoir and for large enrichment factors. Pyrite is stable beyond the conditions for a reducing environment and MIF signatures from Archaean sediments could be compatible with reversible processes involving oxidised species. Isotope anomalies in pyrite from the Dresser Formation, Australia, are compatible with post-depositional incorporation of 2–5% sulphur of atmospheric origin. Overall, observed delta values could mainly be related to changes in the local absolute isotope abundances. [ABSTRACT FROM AUTHOR]

Here we present SIMS data for a suite of Zimmi sulphide-bearing diamonds that allow us to evaluate the origin and redox-controlled speciation of diamond-forming fluids for these Neoproterozoic eclogitic diamonds. Low δ13C values below −15‰ in three diamonds result from fluids that originated as carbon in the oceanic crust, and was recycled into the diamond-stable subcratonic lithospheric mantle beneath Zimmi during subduction. δ13C values between −6.7 and −8.3‰ in two diamonds are within the range for mantle-derived carbon and could reflect input from mantle fluids, serpentinised peridotite, or homogenised abiogenic and/or biogenic carbon (low δ13C values) and carbonates (high δ13C values) in the oceanic crust. Diamond formation processes in eclogitic assemblages are not well constrained and could occur through redox exchange reactions with the host rock, cooling/depressurisation of CHO fluids or during H 2 O-loss from CHO fluids. In one Zimmi diamond studied here, a core to rim trend of decreasing δ13C (−23.4 to −24.5‰) and decreasing [N] is indicative of formation from reduced CH 4 -bearing fluids. Unlike mixed CH 4 -CO 2 fluids near the water maximum, isochemical diamond precipitation from such reduced CHO fluids will only occur during depressurisation (ascent) and should not produce coherent fractionation trends in single diamonds that reside at constant depth (pressure). Furthermore, due to a low relative proportion of the total carbon in the fluid being precipitated, measurable carbon isotopic variations in diamond are not predicted in this model and therefore cannot be reconciled with the 1‰ internal core-to- rim variation. Consequently, this Zimmi eclogitic diamond showing a coherent trend in δ13C and [N] likely formed through oxidation of methane by the host eclogite, although the mineralogical evidence for this process is currently lacking. [ABSTRACT FROM AUTHOR]

Stable mercury (Hg) isotope ratios are an emerging tracer for biogeochemical transformations in environmental systems, but their application requires knowledge of isotopic enrichment factors for individual processes. We investigated Hg isotope fractionation during dark, abiotic reduction of Hg(II) by dissolved iron(Fe)(II), magnetite, and Fe(II) sorbed to boehmite or goethite by analyzing both the reactants and products of laboratory experiments. For homogeneous reduction of Hg(II) by dissolved Fe(II) in continuously purged reactors, the results followed a Rayleigh distillation model with enrichment factors of -2.20 ± 0.16‰ (ε 202 Hg) and 0.21 ± 0.02‰ (E 199 Hg). In closed system experiments, allowing reequilibration, the initial kinetic fractionation was overprinted by isotope exchange and followed a linear equilibrium model with -2.44 ± 0.17‰ (ε 202 Hg) and 0.34 ± 0.02‰ (E 199 Hg). Heterogeneous Hg(II) reduction by magnetite caused a smaller isotopic fractionation (-1.38 ± 0.07 and 0.13 ± 0.01‰), whereas the extent of isotopic fractionation of the sorbed Fe(II) experiments was similar to the kinetic homogeneous case. Small mass-independent fractionation of even-mass Hg isotopes with 0.02 ± 0.003‰ (E 200 Hg) and ≈ -0.02 ± 0.01‰ (E 204 Hg) was consistent with theoretical predictions for the nuclear volume effect. This study contributes significantly to the database of Hg isotope enrichment factors for specific processes. Our findings show that Hg(II) reduction by dissolved Fe(II) in open systems results in a kinetic MDF with a larger ε compared to other abiotic reduction pathways, and combining MDF with the observed MIF allows the distinction from photochemical or microbial Hg(II) reduction pathways.

The biogeochemical cycling of cadmium (Cd) and its isotopes in the surface ocean is dominated by biological uptake into phytoplankton, while regeneration of the sinking particulate Cd governs the shapes of Cd profiles in the deeper ocean. Additionally, the water mass circulation plays an important role in the redistribution of Cd and its isotopes. Superimposed on this general cycle, it has recently been argued that cadmium sulphide (CdS) precipitation can occur in oceanic Oxygen Deficient Zones (ODZ). This has been proposed to account for the decrease in Cd/PO 4 ratios and the positive Cd isotope excursion in seawater compared to sinking particles. To assess whether CdS precipitation results in a fractionation of Cd isotopes, we performed cadmium–sulphide precipitation experiments under controlled low-oxygen conditions in low (pure water) and high ionic strength (synthetic seawater) water, with variable reaction times and cadmium/sulphide ratios. Enrichment in light Cd isotopes is systematically observed in the precipitated CdS phase, in agreement with recent ab initio simulations of isotope effects associated with Cd speciation in aqueous solution. Our experimental results follow a closed-system Rayleigh fractionation model with the fractionation factor ( α Cdsol-CdS ) for 112 Cd/ 110 Cd decreasing with increasing salinity – from 1.00026 for pure water to 1.00014 for a salinity twice that of modern seawater, indicating a salinity control on the magnitude of Cd isotope fractionation. We propose that this fractionation, varying with salinity, is controlled by isotope equilibrium between the various Cd aqueous species present in solution, with preferential involvement of free Cd 2+ ion in CdS formation. The magnitude of Cd isotope fractionation in seawater ( α Cdsol-CdS = 1.00016) determined in our study is in accord with the Cd isotope shift observed in modern oceanic ODZ and attributed to removal by CdS. Our experiments show that CdS precipitation could have significant local impact on the cycling of Cd, and further demonstrates the utility of Cd isotopes as tracers of the sequestration of cadmium into sulphides in restricted euxinic basins, such as the Black Sea. [ABSTRACT FROM AUTHOR]

Because of the inability of depleted cratonic peridotites to effectively buffer oxygen fugacities when infiltrated by CHO or carbonatitic fluids, it has been proposed recently ( Luth and Stachel, 2014 ) that diamond formation in peridotites typically does not occur by rock-buffered redox reactions as previously thought but by an oxygen-conserving reaction in which minor coexisting CH 4 and CO 2 components in a water-rich fluid react to form diamond (CO 2 + CH 4 = 2C + 2H 2 O). In such fluid-buffered systems, carbon isotope fractionation during diamond precipitation occurs in the presence of two dominant fluid carbon species. Carbon isotope modelling of diamond precipitation from mixed CH 4 - and CO 2 -bearing fluids reveals unexpected fundamental differences relative to diamond crystallization from a single carbon fluid species: (1) irrespective of which carbon fluid species (CH 4 or CO 2 ) is dominant in the initial fluid, diamond formation is invariably associated with progressive minor (<1‰) enrichment of diamond in 13 C as crystallization proceeds. This is in contrast to diamond precipitation by rock-buffered redox processes from a fluid containing only a single carbon species, which can result in either progressive 13 C enrichment (CO 2 or carbonate fluids) or C 13 depletion (CH 4 fluids) in the diamond. (2) Fluid speciation is the key factor controlling diamond δ 13 C values; as X CO 2 (X CO 2 = CO 2 /[CO 2 + CH 4 ]) in the initial fluid increases from 0.1 to 0.9 (corresponding to an increase in f O 2 of 0.8 log units), the carbon isotope composition of the first-precipitated diamond decreases by 3.7‰. The tight mode in δ 13 C of − 5 ± 1 ‰ for diamonds worldwide places strict constraints on the dominant range of X CO 2 in water-rich fluids responsible for diamond formation. Specifically, precipitation of diamonds with δ 13 C values in the range −4 to −6‰ from mantle-derived fluids with an average δ 13 C value of −5‰ (derived from evidence not related to diamonds) requires that diamond-forming fluids were relatively reduced and had methane as the dominant carbon species (X CO 2 = 0.1–0.5). Application of our model to a recently published set of in-situ carbon isotope analyses for peridotitic diamonds from Marange, Zimbabwe ( Smit et al., 2016 ), which contain CH 4 fluid inclusions, allows us to perfectly match the observed co-variations in δ 13 C , δ 15 N and N content and at the same time explain the previously counter-intuitive observation of progressive C 13 enrichment in diamonds that appear to have grown from a fluid with methane as the dominant carbon species. Similarly, the almost complete absence in the published record of progressive C 13 depletion trends within diamonds likely reflects ubiquitous precipitation from CH 4 - and CO 2 -bearing water-rich fluids, rather than diamond formation exclusively by carbonate-bearing and CH 4 -free oxidized fluids or melts. [ABSTRACT FROM AUTHOR]

• Analytical expressions describing the residual liquid composition in Rayleigh fractionation are derived for ternary mixtures with constant relative volatilities. • The analytical expressions are verified by calculating the liquid compositions for multi-stage flash cascades. • The analytical expressions produce results with better precision compared to applying numerical methods directly on the system of differential equations. • The analytical expressions are expected to cast new light on other fields involving Rayleigh fractionation. The Rayleigh equation is an important fundamental equation in distillation, isotope separation, hydrology and many other fields. However, it is only applicable to binary systems. In this work, analytical expressions are derived describing the composition of the residual liquid in Rayleigh fractionation for ternary systems with constant relative volatilities. The expressions are in the form of parameter equations, and the range of the parameter is analyzed from a mathematical perspective. Two methods for calculating the compositions of the residual liquid and the distillate are presented. The analytical equations are verified by calculating the liquid compositions for multi-stage flash cascades. The compositions of the streams involved in Rayleigh fractionation are investigated with the analytical equations and the results are compared with those in an equilibrium flash process. It indicates that Rayleigh fractionation achieves a better degree of separation than equilibrium flash. The physical significance of the parameter in the equations is interpreted based on the residue curve. Finally, an example is given to show that the analytical expressions produce results with better precision compared to applying numerical methods directly on the system of differential equations. The analytical equations for ternary mixtures are expected to cast some new light on other fields involving Rayleigh fractionation and to promote the development of many disciplines. [ABSTRACT FROM AUTHOR]

Using stable isotope data obtained on multiple aliquots of diamonds from worldwide sources, it has been argued that carbon and nitrogen in diamond are decoupled. Here we re-investigate the carbon–nitrogen relationship based on the most comprehensive microbeam data set to date of stable isotopes and nitrogen concentrations in diamonds (n = 94) from a single locality. Our diamond samples, derived from two kimberlites in the Chidliak Field (NE Canada), show large variability in δ 13 C (− 28.4 ‰ to − 1.1‰, mode at − 5.8‰), δ 15 N (− 5.8 to + 18.8‰, mode at − 3.0‰) and nitrogen contents ([N]; 3800 to less than 1 at.ppm). In combination, cathodoluminescence imaging and microbeam analyses reveal that the diamonds grew from multiple fluid pulses, with at least one major hiatus documented in some samples that was associated with a resorption event and an abrupt change from low δ 13 C and [N] to mantle-like δ 13 C and high [N]. Overall, δ 13 C appears to be uncorrelated to δ 15 N and [N] on both the inter- and intra-diamond levels. Co-variations of δ 15 N–log[N], however, result in at least two parallel, negatively correlated linear arrays, which are also present on the level of the individual diamonds falling on these two trends. These arrays emerge from the two principal data clusters, are characterized by slightly negative and slightly positive δ 15 N (about − 3 and + 2‰, respectively) and variable but overall high [N]. Using published values for the diamond-fluid nitrogen isotope fractionation factor and nitrogen partition coefficient, these trends are perfectly reproduced by a Rayleigh fractionation model. Overall, three key elements are identified in the formation of the diamond suite studied: (1.) a low δ 13 C and low [N] component that possibly is directly associated with an eclogitic diamond substrate or introduced during an early stage fluid event. (2.) Repeated influx of a variably nitrogen-rich mantle fluid (mildly negative δ 13 C and δ 15 N). (3.) In waning stages of influx, availability of the mantle-type fluid at the site of diamond growth became limited, leading to Rayleigh fractionation. These fractionation trends are clearly depicted by δ 15 N–[N] but are not detected when examining co-variation diagrams involving δ 13 C. Also on the level of individual diamonds, large (≥ 5‰) variations in δ 15 N are associated with δ 13 C values that typically are constant within analytical uncertainty. The much smaller isotope fractionation factor for carbon (considering carbonate- or methane-rich fluids as possible carbon sources) compared to nitrogen leads to an approximately one order of magnitude lower sensitivity of δ 13 C values to Rayleigh fractionation processes (i.e. during fractionation, a 1‰ change in δ 13 C is associated with a 10‰ change in δ 15 N). As a consequence, even minor heterogeneity in the primary isotopic composition of diamond forming carbon (e.g., due to addition of minor subducted carbon) will completely blur any possible co-variations with δ 15 N or [N]. We suggest this strong difference in isotope effects for C and N to be the likely cause of observations of an apparently decoupled behaviour of carbon and nitrogen isotopes in diamond. [ABSTRACT FROM AUTHOR]

Soils from two sites viz. Kotumsar and Tirathgarh, located ∼5 km apart in a tropical reserve forest (18°52′N, 81°56′E) in central India, have been explored for soil organic carbon (SOC) content, its mean residence time (MRT) and the evolution of stable carbon isotopic composition (δ 13 C). SOC stocks in the upper 30 cm of soil layers are ∼5.3 kg/m 2 and ∼3.0 kg/m 2 ; in the upper 110 m are ∼10.7 kg/m 2 and ∼7.8 kg/m 2 at Kotumsar and Tirathgarh, respectively. SOC decreases with increasing depth. Bomb carbon signature is observed in the upper ∼10 cm. Organic matters in the top soil layers (0–10 cm) have MRTs of the order of a century which increases gradually with depths, reaching 3500–5000 yrs at ∼100 cm. δ 13 C values of SOC increase with depth, the carbon isotopic fractionation is obtained to be −1.2‰ and −3‰ for soils at Kotumsar and Tirathgarh, respectively, confirmed using Rayleigh isotopic fractionation model. The evolution of δ 13 C in soils was also studied using a modified Rayleigh fractionation model incorporating a continuous input into the reservoir: the depth profiles of δ 13 C for SOC show that the input organic matter from surface into the deeper soil layers is either insignificant or highly labile and decomposes quite fast in the top layers, thus making little contribution to the residual biomasses of the deeper layers. This is an attempt to understand the distillation processes that take place in SOC, assess the extent of decomposition by microbes and effect of percolation of fresh organic matter into dipper soil layers which are important for stable isotope based paleoclimate and paleovegetation reconstruction and understanding the dynamics of organic carbon in soils. [ABSTRACT FROM AUTHOR]