Zn Isotope Fractionation Research Articles

Sources of the Laoxiongdong carbonate-hosted Pb–Zn deposit in Southwest China: constraints from S–Pb–Zn isotopic compositions

The Sichuan–Yunnan–Guizhou (SYG) Pb–Zn mineral province, which has significant Pb–Zn repositories in China, is situated in the western Yangtze Block. Despite decades of research, the Pb–Zn source of deposits is still disputed between the basement rocks, sedimentary wall rocks, or the Emeishan flood basalts. The newly-discovered Laoxiongdong Pb–Zn deposit is hosted by the Late Ediacaran Dengying Formation in the SYG province. Moreover, the Laoxiongdong orebodies lie between regional deep faults and close to the Proterozoic basement and Emeishan basalts. Hence, this deposit represents a good case study on the ore-forming material source for the regional Pb–Zn mineralization. The Laoxiongdong Pb–Zn ores are massive, vein, or disseminated and have mainly sphalerite, galena, pyrite, quartz, and calcite. In this paper, we present new sulfide S–Pb–Zn isotope data of the deposit. The sulfide δ34SCDT values (+ 11.8 to + 16.5 ‰) suggest that the reduced sulfur was mainly sourced from evaporites in the Late Ediacaran–Cambrian sequences. Pb isotopic ratios (206Pb/204Pb = 18.004–18.107, 207Pb/204Pb = 15.652–15.667, and 208Pb/204Pb = 38.037–38.248) suggest that the lead metal was primarily originated from the basement rocks. The sphalerite δ66Zn values (+ 0.16 to + 0.37 ‰) are also highly similar (within error) to those of basement rocks (+ 0.10 to + 0.34 ‰), suggesting a basement-rocks zinc source with minor contributions from the carbonate host rocks and Emeishan flood basalts. The narrow sphalerite Zn isotopic range (0.21 ‰) also indicates that the Zn isotopic fractionation between the sphalerite and initial fluid was limited during the sphalerite ore precipitation. Therefore, we propose that both the Late Ediacaran Dengying Formation rocks and Proterozoic basement rocks were important ore-forming material source for the Laoxiongdong deposit, whereas the Emeishan flood basalts represent only a minor ore-material source.

Zinc isotopic variation of water and surface sediments from the German Elbe River

Recent studies suggested the use of the isotopic composition of Zn as a possible tracer for anthropogenic Zn emissions. Nevertheless, studies mainly focused on sampling areas of a few km2 with well-characterized anthropogenic Zn emissions. In contrast, this study focused on analyzing a large sample set of water and sediment samples taken throughout the course of the Elbe River, a large, anthropogenically impacted river system located in Central Europe. The primary objective was to evaluate the use of the isotopic composition of Zn to trace anthropogenic Zn emission on a large regional scale. In total 18 water and 26 surface sediment samples were investigated, covering the complete course of over 700 km of the German Elbe between the German/Czech border and the German North Sea, including six tributaries. Stable isotope abundance ratios of Zn were assessed by multi-collector inductively coupled plasma mass spectrometry (MC ICP-MS) in water filtrates (<0.45 µm) and total digests of the sieved surface sediment fraction (<63 µm) after analyte/matrix separation using Bio-Rad AG MP-1 resin via a micro-column approach and application of a 64Zn/67Zn double spike. Measured isotopic compositions of δ66Zn/64ZnIRMM-3702 ranged from −0.10 ‰ to 0.32 ‰ for sediment samples, and from −0.51 ‰ to 0.45 ‰ for water samples. In comparison to historical data some tributaries still feature high mass fractions of anthropogenic Zn (e.g. Mulde, Triebisch) combined with δ66Zn/64ZnIRMM-3702 values higher than the lithogenic background. The dissolved δ66Zn/64ZnIRMM-3702 values showed a potential correlation with pH. Our results indicate that biogeochemical processes like absorption may play a key role in natural Zn isotopic fractionation making it difficult to distinguish between natural and anthropogenic processes.

Fractionation of Zn isotopes in terrestrial ferromanganese crusts and implications for tracing isotopically-heterogeneous metal sources

Zinc liberated from geological sources during surface weathering is the main source of Zn to the surface environment, and is ultimately sequestered by sinks such as Mn oxides and oxyhydroxides. This behaviour has been applied to geochemical mineral exploration studies and terrestrial ferromanganese crusts have been shown to have anomalously-high Zn/Mn ratios around known Zn-Pb ore deposits. This poses the question: if Zn isotopic compositions of these Fe-Mn crusts can also discriminate the underlying anomalous metal sources from the surrounding non-mineralized lithologies. Isotopic fractionation during fractionation onto Mn oxides in well-studied marine Fe-Mn crusts has shown that isotopically-heavy Zn is preferentially-adsorbed in that environment, which has a comparatively isotopically-homogeneous Zn reservoir. However, isotopic fractionation behaviour in terrestrial Fe-Mn crusts that may have more heterogeneous Zn sources is unknown. Here, we present the first Zn and Pb isotope study of terrestrial Fe-Mn crusts to test if Zn isotopic compositions can be used to delineate isotopically-different lithological metal sources.Our data from Fe-Mn crusts that formed overlying isotopically-variable lithologies; hydrothermal polymetallic (Zn-Pb) deposits, and non-mineralized Archean rocks show that, while adsorption of Zn onto Mn oxyhydroxides in terrestrial Fe-Mn crusts generally favours isotopically-heavy Zn, the magnitudes of isotopic fractionation varies in Fe-Mn crusts overlying mineralized and non-mineralized areas. Fractionation factors (Δ66/64Zncrust – source up to 0.54‰) in crusts overlying non-mineralized lithological sources with a δ66Zn value of 0.30‰ are comparable with the predicted equilibrium fractionation onto birnessite. Conversely, magnitudes of Δ66/64Zncrust – source up to 0.37‰ in crusts overlying isotopically-light Zn-Pb mineralization, that has a mean δ66Zn value of -0.15‰, are much lighter than non-mineralized counterparts with similar Zn/Mn ratios. This difference may be explained by changes in hydrogeochemical complexation of Zn, with localized chloro-complexes that carry isotopically-light Zn from the isotopically-light mineralization.These findings demonstrate that different lithological Zn sources that are isotopically-variable give rise to isotopic heterogeneity in the surface Zn reservoir. This can be reflected by differing fractionation patterns following adsorption onto Mn oxyhydroxides in terrestrial Fe-Mn crusts that overly different Zn sources. Furthermore, Pb isotope compositions of the Fe-Mn crusts also closely reflect that of the underlying geology. This highlights that the use of Zn isotopes may be a valuable means of vectoring towards anomalous metal sources such as ore deposits.

Cycling of zinc and its isotopes across multiple zones of the Southern Ocean: Insights from the Antarctic Circumnavigation Expedition

Zinc (Zn) is an essential micronutrient, playing an important role in several key processes in marine phytoplankton. Here, we present the first high-resolution depth profiles for dissolved Zn and δ66Zn from all major zones of the Southern Ocean, collected during the Antarctic Circumnavigation Expedition in Austral Summer 2016/2017. The dataset reveals that Zn cycling changes between different regions of the Southern Ocean. Within the Antarctic Circumpolar Current (ACC), Zn cycling is closely linked to phosphate (PO4), governed by uptake and regeneration, seasonal mixing, and upwelling. Here, uptake by phytoplankton is associated with a very small fractionation (α = 0.99995), resulting in slightly elevated surface δ66Zn (up to +0.67‰), overlying a shallow subsurface δ66Zn minimum (+0.36‰ at ∼200 m). South of the ACC, a partial coupling of Zn and silicate (Si) results in a shift of the Zn isotope systematics and a deep enrichment of Zn and Si. Two possible mechanisms could potentially cause this change: 1) reversible scavenging onto sinking particulates, or, 2) association of isotopically heavy Zn with diatom frustules. We also observe the effects of regional processes on Zn in the surface Southern Ocean: firstly, natural Fe fertilization near the Balleny Islands appears to lead to reduced Zn uptake (relative to phosphate) by phytoplankton, that is associated with a greater apparent Zn isotope fractionation than elsewhere in the Southern Ocean (α = 0.99987); secondly, meltwater inputs from the Mertz Glacier add small amounts of isotopically light Zn to surface waters near the Antarctic shelf. Overall, we propose that the lack of distinct δ66Zn signatures transported in intermediate waters of the lower latitude global oceans is due to near-complete uptake of Zn by phytoplankton in the surface Southern Ocean with only a small isotope fractionation, in contrast to elements like cadmium and silicon.

Extreme Mg and Zn isotope fractionation recorded in the Himalayan leucogranites

High-silica granites (typically > 70 wt.% SiO2) represent the products of extreme crustal differentiation, but whether their distinctive chemical compositions reflect source lithology or are produced via magmatic differentiation is commonly difficult to discriminate. To provide new insights into this issue, here we present high-precision Mg and Zn isotope data for high-silica, peraluminous leucogranites from the Himalayan orogen. The samples are subdivided into two-mica leucogranite, tourmaline leucogranite and garnet-bearing leucogranite based on their mineral assemblages. The two-mica leucogranites, representing the least evolved Himalayan leucogranites, have similar δ66ZnJMC 3-0749L (mean = 0.31 ± 0.06‰) but heavier δ26MgDSM-3 values (mean = −0.01 ± 0.12‰) relative to more mafic igneous rocks. This indicates that they were formed by anatexis of weathered silicates, consistent with the well-acknowledged metasedimentary source of the Himalayan leucogranites. In contrast, the more evolved tourmaline leucogranites and garnet-bearing leucogranites have lower δ26Mg and higher δ66Zn values compared with the two-mica leucogranites. The extremely low δ26Mg (−1.32‰ to −0.54‰) and high δ66Zn values (0.35–0.69‰) of garnet-bearing leucogranites vary systematically with indices of granitic differentiation (e.g., Zr/Hf, K/Rb, Eu/Eu*, 1/TiO2). Although exsolution of chlorine-rich fluid may result in elevated δ66Zn values, it is unlikely to explain the low δ26Mg signatures of the same samples. Analysis of major Mg-bearing minerals suggests that substantial segregation of tourmaline and/or Fe-Ti oxide could have driven the differentiated leucogranites towards very low δ26Mg values. In this regard, the slightly lower δ26Mg values (mean = −0.17 ± 0.06‰) of tourmaline leucogranites relative to the two-mica leucogranites may also reflect that the former were more differentiated than the latter. Thus, although source heterogeneity may be responsible for the Mg isotopic variations observed in some high-silica granites, our study implies that high-silica granites could be remarkably heterogeneous in terms of Mg isotopes primarily as a result of prolonged fractional crystallization at the late stage of melt evolution. The anomalously light Mg and heavy Zn isotopic signatures of garnet-bearing leucogranites highlight that Mg and Zn isotopes may be treated as important makers of highly fractionated granites in future studies.

Zinc isotope analyses of singularly small samples (&lt;5 ng Zn): Investigating chondrule-matrix complementarity in Leoville

The potential complementarity between chondrules and matrix of chondrites, the Solar System's building blocks, is still a highly debated subject. Complementary superchrondritic compositions of chondrite matrices and subchondritic chondrules may point to formation of these components within the same reservoir or, alternatively, to mobilization of elements during secondary alteration on chondrite parent bodies. Zinc isotope fractionation through evaporation during chondrule formation may play an important role in identifying complementary relationships between chondrules and matrix and is additionally a mobile element during hydrothermal processes. In an effort to distinguish between primary Zn isotope fractionation during chondrule formation and secondary alteration, we here report the Zn isotope data of five chondrule cores, five corresponding igneous rims and two matrices of the relatively unaltered Leoville CV3.1 chondrite. The detail required for these analyses necessitated the development of an adjusted Zn isotope analyses protocol outlined in this study.

Zinc, cadmium and sulfur isotope fractionation in a supergiant MVT deposit with bacteria

Zinc (Zn) and cadmium (Cd) are two important biological-sensitive elements, and their isotopes are strongly fractionated during biological processes. To develop the potential application of Zn and Cd isotopes for tracing the contribution of bacteria to metal mineralization, in this study we investigate for the first time the ranges and causes of isotope fractionation of Zn and Cd in a large hydrothermal system with bacteria from the world-class Jinding Zn-Pb deposit, southwest China. Extremely negative δ34SCDT values of sulfides (−48.6‰ to +7.7‰) obtained in this study and literature reveal the dominant biogenic sulfur associated with bacterial sulfate reduction. Both Zn and Cd isotope ratios are highly variable and significantly lighter compared with most of Zn-Pb ore deposits worldwide. The extremely negative δ66ZnJMC 3-0749L (down to −0.69‰) exceeds the range of Rayleigh fractionation during inorganic precipitation of ZnS. Zinc isotope ratios display a well-defined positive correlation with Zn/Cd ratios and the samples with lightest δ66Zn have lowest Zn/Cd ratios. This reflects that sphalerites formed in the early stages of mineralization already have extremely light Zn isotopic compositions. There is a negative relationship between δ34SCDT and δ66Zn, indicating that lighter Zn isotope ratios are related to stronger bacterial sulfate reduction, which results in heavier sulfur isotope ratios. Since organic compounds (e.g., Zn-carboxylate) in fluids are strongly enriched in heavy isotopes relative to free Zn2+ and Zn-sulfide, sulfides formed in the bacteria-rich hydrothermal systems would acquire light δ66Zn. Therefore, the extremely negative δ34SCDT and δ66Zn ratios demonstrate for the first time that bacterial metabolisms not only induced sulfate reduction, but also were directly involved in the formation of metals (e.g., Zn) in a large hydrothermal system. A similar process may have also partly contributed to the substantial fractionation of cadmium isotopes in the hydrothermal system. Overall, our studies show that zinc and cadmium isotopes may act as an effective tool in deciphering the specific role of organisms in formation of sulfides in hydrothermal systems and other settings.

Volatile loss under a diffusion-limited regime in tektites: Evidence from tin stable isotopes

Tektites are glasses derived from near-surface continental crustal rocks that were molten and ejected from the Earth's surface during hypervelocity meteorite impacts. They are among the driest terrestrial samples, although the exact mechanism of water loss and the behaviour of other volatile species during these processes are debated. Based on the difference in magnitude of the Cu and Zn isotopic fractionations in tektites, and the difference of diffusivity between these elements, it was suggested that volatile loss was diffusion-limited. Tin is potentially well suited to testing this model, as it has a lower diffusivity in silicate melts than both Cu and Zn, but a similar volatility to Zn. Here, we analysed the Sn stable isotopic composition in a suite of seven tektites, representing three of the four known tektite strewn fields, and for which Zn and Cu isotopes were previously reported. Tin is enriched in the heavier isotopes (≥2.5‰ on the 122Sn/118Sn ratio) in tektites, correlated with the degree of Sn elemental depletion in their respective samples as well as with Cu and Zn isotope ratios, implying a common control. While the isotope fractionation of Sn, Cu and Zn is a result of volatility, the magnitude of isotope fractionation is strongly moderated by their relative rates of diffusion in the molten tektite droplets. An Australasian Muong Nong-type tektite analysed has the least Sn depletion and Sn isotope fractionation, consistent with these samples being more proximal to the source and experiencing a shorter time at high temperatures.

Cu and Zn isotope fractionation during extreme chemical weathering

Copper and Zn are trace metal micronutrients whose stable isotope systematics are receiving increasing attention as possible paleoenvironmental tracers. However, to realise this potential, their behaviour during chemical weathering must be better constrained. We present coupled Cu and Zn isotope data for a well-characterised Indian laterite weathering profile, which includes a full suite of samples from unaltered greywacke bedrock to indurated lateritic duricrust. This sample set provides an exceptional opportunity to interrogate Cu and Zn isotope compositions during an extreme example of chemical weathering. Despite their occurrence in different host phases within the parent greywacke, Cu and Zn isotopes behave coherently during weathering. We observe preferential loss of heavy isotopes at increasing degrees of alteration, with 0.6‰ total variability in δ66Zn and 0.9‰ in δ65Cu. Given the absence of evidence for CuS or ZnS phases in the parent lithology, we attribute the liberation of heavy isotopes to organic complexation in the aqueous phase and/or incorporation of light isotopes in secondary aluminous Fe-oxides. Strong enrichment of both metals is also associated with a peak in Mn at a previously identified paleo-water table horizon. This dataset confirms that weathering under oxygenated conditions releases isotopically heavy Cu, regardless of the host phase. Meanwhile, Zn isotopes are only fractionated to any significant extent at the most extreme degrees of chemical weathering reached during lateritization. We conclude that the isotopic composition of the weathering-derived input of Zn to rivers should be largely insensitive to climate change on geological timescales.

Experimental determination of Zn isotope fractionation during evaporative loss at extreme temperatures

Abstract Zinc isotopes fractionate during evaporation, and thus can potentially be used to calculate the proportion of volatile elemental loss from objects such as tektites, nuclear fallout melt glasses formed from silicate soils, and rocks from the Moon. The utility of the Zn isotope system in constraining the magnitude of volatile loss depends on accurate knowledge of its fractionation behavior during evaporation, i.e., the Zn isotope fractionation factor. Here, we present new results of Zn isotope analyses of experimentally heated soil samples, together with analyses of environmentally derived fallout melt glasses, and australite tektites, to better constrain how and to what extent Zn isotopes fractionate during thermally-driven evaporation. Major and trace element analyses of melt glass derived from the experimental heating of rhyolitic and arkosic soils demonstrates that elemental fractionation progressively becomes more important with increasing temperature and time. Initial Zn isotope ratios in the rhyolitic and arkosic soils are similar to upper continental crustal (UCC) values but become increasingly isotopically heavy as more Zn is lost from the heated glass, with a maximum δ66Zn value of +1.49 ± 0.05‰. The progressive loss of Zn from the rhyolitic soil is consistent with a fractionation factor (α) of 0.99879 ± 13. After loss of a labile component from the arkosic soil further evaporative loss of Zn from the silicate rich residue is also consistent with an α of ∼0.9988. This fractionation factor is not sensitive to either the heating temperature or duration. The fallout melt glass, derived from near surface nuclear weapons tests, and natural tektite samples are both relatively enriched in isotopically heavy Zn compared to the UCC. Although the fallout melt glass is largely comprised of the same rhyolitic soil used in our experiments the extent of Zn isotope fractionation is lower; consistent with an α of 0.9997. This is similar to estimated values of α associated with loss of Zn from Trinitite and lunar rocks. It is more difficult to constrain a value for α from australite tektites because the location of the impact site is not well constrained. Based on our analyses, we estimate a range of α values between 0.9988 and 0.9997, which is broadly consistent with experimental data and analyses of fallout melt glass. In all cases the measured Zn isotope fractionation factors are much lower than would be expected from evaporative loss of Zn in a high vacuum. We hypothesize that, for Zn, the value of α is dependent on the localized gas pressure and composition, with the degree of isotopic fractionation decreasing from high vacuum (

Cu and Zn isotope fractionation during oceanic alteration: Implications for Oceanic Cu and Zn cycles

The behaviours of Cu, Zn and their isotopes during seafloor fluid-peridotite interaction and its influence on oceanic Cu-Zn cycles were investigated by analyzing a series of unaltered to strongly altered oceanic peridotites. Most of the altered peridotites have δ 66 Zn values indistinguishable from those of the unaltered ones (0.19 ± 0.05‰; 2SD), suggesting that seafloor alteration generally induces a small Zn isotope fractionation. A few altered samples that seem to have gained Zn have elevated δ 66 Zn, probably due to adsorption by ubiquitous secondary phases (e.g. goethite). Almost all altered peridotites show Cu loss (up to 90%) and have Cu isotope ratios that are shifted toward heavy values relative to the unaltered ones. Given that Cu is organically-complexed in fluids over a wide range of temperature and such fluids have strong preferences for the heavy isotopes, alteration and weathering alone are unlikely to account for the substantial loss of Cu coupled with elevated δ 65 Cu. An alternative mechanism is that initial Cu in peridotites was partly lost by alteration and weathering, and then Cu was added via adsorption by secondary mineral phases preferring the heavy Cu isotopes. This is confirmed by leaching experiments that were designed to extract the adsorbed Cu fractions in secondary phases. The results show that the leachates and the residua have, respectively, higher and lower δ 65 Cu values in comparison with the bulk peridotites, demonstrating that adsorption is an important mechanism for generating heavy copper isotope enrichments in the altered peridotites. In any case, mass balance considerations indicate that oceanic alteration must have resulted in a net input of isotopically light Cu from oceanic peridotites into the ocean. Recent estimates found that the Cu isotopic composition of the modern ocean is imbalanced, because all characterized sinks are isotopically light relative to characterized input fluxes, which requires an additional, isotopically light input. Our results indicate that seafloor alteration of abyssal peridotites is an isotopically light input (δ 65 Cu = −0.4‰ to −0.1‰) to the modern ocean, up to ∼1‰ lighter than the riverine input (∼0.7‰). First-order flux estimates suggest that this process exerts an important control on the marine budget of Cu. Most of Zn released during seafloor alteration has δ 66 Zn similar to that of mantle peridotites and this input is thus slightly lighter than the riverine input (∼0.33‰). Current estimates of modern oceanic Zn isotope cycles require consideration of this isotopically light source.

Zn isotope fractionation during uptake into marine phytoplankton: Implications for oceanic zinc isotopes

The extreme scarcity of zinc (Zn) in the euphotic zone, coupled to deep enrichments, is consistent with biological uptake at the surface and regeneration at depth. In the context of a nutrient-type depth profile so clearly shaped by uptake into phytoplankton, the growing dataset for Zn isotopes presents a challenge. These data either show very minor isotope effects associated with extreme depletion, or enrichment of the light isotopes in the upper ocean. In contrast, culturing of eukaryotes in the laboratory suggests that light Zn isotopes are preferentially taken up into diatoms and coccoliths, implying that Zn depletion at the surface should be associated with extremely heavy residual dissolved signals.Here we present the first Zn isotope measurements for cultured marine cyanobacteria and compare these data to those for eukaryotic diatoms grown under identical conditions. Of the four cyanobacteria cultured, belonging to the genera Synechococcus and Prochlorococcus, three preferentially take up light Zn into the cell, with a variability that is not fundamentally different between pro- and eukaryotic phytoplankton. We also observe only very subtle differences between Zn/P and Fe/P uptake ratios for these three cyanobacteria groups relative to diatoms grown under the same conditions. A fourth strain exhibits preferential uptake of heavy Zn isotopes, and very high Zn/P ratios. Overall, we speculate that the observed variability among cyanobacteria may be related to the molecular structure of their photosynthetic light harvesting apparatus, adapted to significantly different light niches.These new and published culture data support the hypothesis that cellular δ66Zn in culture might largely be controlled by the organic ligands that bind Zn in the medium. Given that the Zn-binding ligands in the ocean have thermodynamic stability constants that are orders of magnitude smaller than the EDTA used in culture media, the surprisingly subtle Zn isotope variability in some parts of the surface ocean may be reconciled with culture data by the lesser, near zero, preference of these weaker complexes for heavy Zn isotopes.“This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean - GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.”

Homogeneous Zn isotopic compositions in the Maozu Zn-Pb ore deposit in Yunnan Province, southwestern China

The Sichuan-Yunnan-Guizhou (SYG) metallogenic province, southwestern Yangtze Block, is one of the most important Zn-Pb repositories in China, with more than 200 million tons (Mt) of Zn-Pb ores. The source regions for the regional Zn-Pb mineralization in the SYG are still controversial, mostly in regard to the Emeishan flood basalts, basement rocks and host rocks. The Maozu Zn-Pb deposit is geographically close to the SYG center, which could also indicate that it is closer to the regional Zn-bearing fluid center than those deposits along the SYG margin. However, the origin of the ore-forming Zn in the Maozu deposit has not been constrained until now. In this study, the Zn and S isotopic compositions and Fe and Cd concentrations of the sphalerite from the Maozu deposit are investigated to evaluate the Zn isotopic fractionation during sphalerite precipitation, the source regions of the ore-forming zinc and sulfur, and the center of the regional Zn-bearing fluid system. It is suggested that the Zn isotopic fractionation between the sphalerite and original fluid during sphalerite precipitation could be limited due to the striking decrease in the temperature of the incoming fluid. The Zn isotopic variation of the sphalerite from four mining levels in the Maozu deposit is really small (−0.06 to +0.23‰), which could be attributed to heterogeneous Zn isotopic compositions in original incoming fluid rather than Zn isotopic fractionation during sphalerite deposition. As such, in the Maozu deposit, the Zn isotopic compositions of original hydrothermal fluid could approximately be represented by the Zn isotopic signals documented in sphalerite (−0.06 to +0.23‰). We found that the Zn isotopic signatures of incoming fluid are largely overlapped with those of the basement rocks (+0.10 to +0.34‰), signifying that the ore-forming Zn could be predominantly sourced from the basement rocks. However, in the Maozu deposit, the ore-forming sulfur is most likely derived from the sulfur-bearing evaporates hosted in the country rocks via thermochemical sulfate reduction (TSR). The contrasting sources of ore-forming zinc and sulfur suggest the mixing of Zn-bearing fluid from the basement rocks and sulfur-bearing fluid from the host rocks, which could be responsible for the sharp decrease in the temperature of the incoming fluid in the Maozu deposit. Combined with the results of previous studies, the integrally lower Zn isotopic compositions of sphalerite in the Maozu deposit compared to those deposits along the SYG margin may demonstrate that the Maozu deposit is closer to the regional Zn-bearing fluid center. If this is the case, Zn isotopes could be an effective proxy to trace the Zn source of Zn-enriched ore deposits and to identify the regional Zn-bearing fluid center of Zn-enriched metallogenic provinces.

Mantle Zn Isotopic Heterogeneity Caused by Melt‐Rock Reaction: Evidence From Fe‐Rich Peridotites and Pyroxenites From the Bohemian Massif, Central Europe

AbstractTo investigate the effect of melt‐rock reaction on Zn isotope fractionation and mantle Zn isotopic heterogeneity, we analyzed Zn isotopic compositions of peridotites, pyroxenites, and mineral separates from the Bohemian Massif, Central Europe. The Mg‐lherzolites (Mg# = 90.9 to 89.1, FeOT = 7.9 to 9.0 wt %) are melting residues with only moderate metasomatism and have δ66Zn from 0.11 to 0.20‰. In contrast, the Fe‐rich peridotites (Mg# = 88.2 to 80.3, FeOT = 10.0 to 14.5 wt %) and pyroxenites have larger ranges of δ66Zn from 0.11 to 0.31‰ and −0.33 to 0.42‰, respectively. Large disequilibrium intermineral Zn isotope fractionation occurs in the Fe‐rich peridotites and pyroxenites with Δ66ZnOpx‐Ol = −0.50‰, Δ66ZnGrt‐Ol = −0.55 to −0.39‰, Δ66ZnGrt‐Opx = −0.28 to −0.05‰, and Δ66ZnGrt‐Cpx = −0.50 to 0.12‰. Combined with their low SiO2 contents and radiogenic Sr‐Nd‐Os isotopic compositions, the high δ66Zn of the Fe‐rich peridotites is attributed to reaction between Mg‐lherzolites and percolating SiO2‐undersaturated basaltic melts that incorporated isotopically heavy crustal components. Crystallization of the isotopically heavy percolating melts migrating through the lithospheric mantle yield the high‐δ66Zn pyroxenites. The low δ66Zn of the pyroxenites and large intermineral Zn isotopic disequilibrium may result from kinetic Zn isotope fractionation during melt‐rock reaction. Collectively, these observations indicate that melt‐rock reaction can cause intermineral Zn isotopic disequilibrium and significant Zn isotopic heterogeneity in the mantle. This study thus highlights the potential use of Zn isotopes to trace melt‐rock reaction events in the mantle.

Zn Isotope Fractionation in the Oyster Crassostrea hongkongensis and Implications for Contaminant Source Tracking.

Variations in stable isotope ratios have been used to trace sources of contaminants as well as their biogeochemical pathways in the environment. In this study, we investigated the influences of internal redistribution among tissues and ambient water conditions on Zn isotope fractionation in oysters. There was no significant difference in Zn isotope ratios during in vivo Zn transportation among various oyster tissues. Estuarine oysters were exposed to additional Zn either at different salinities or at different Zn concentrations, following which the Zn isotope ratios in the oysters were measured. Results showed no significant difference in δ66/64Zn values in the oysters exposed at different salinities. Tissue Zn accumulation increased with increasing Zn levels in water over the 30 day exposure. Within this period, there was a nearly 0.3‰ difference in averaged δ66/64Zn values in the exposed oysters compared to the initial δ66/64Zn values in the oysters prior to exposure. However, there was no evidence of significant difference in δ66/64Zn values in oysters exposed at different Zn levels, with postexposure signatures similar to the δ66/64Zn values of the Zn solution added. Our results suggested that the δ66/64Zn values measured in the oysters were approaching the δ66/64Zn values of the "source" faster with increasing Zn concentrations added in the water. This study highlighted the absence of Zn isotope fractionation during Zn internal distribution and in vivo transport in oysters. The calculation of the contributions of different Zn sources demonstrated that oysters can be a sentinel animal for Zn source tracking in marine environments.

The Fate of Zn in Agricultural Soils: A Stable Isotope Approach to Anthropogenic Impact, Soil Formation, and Soil-Plant Cycling.

The supplementation of Zn to farm animal feed and the excretion via manure leads to an unintended Zn input to agricultural systems, which might compromise the long-term soil fertility. The Zn fluxes at three grassland sites in Switzerland were determined by a detailed analysis of relevant inputs (atmospheric deposition, manure, weathering) and outputs (seepage water, biomass harvest) during one hydrological year. The most important Zn input occurred through animal manure (1076-1857 g ha-1 yr-1) and Zn mass balances revealed net Zn accumulations (456-1478 g ha-1 yr-1). We used Zn stable isotopes to assess the importance of anthropogenic impacts and natural long-term processes on the Zn distribution in soils. Soil-plant cycling and parent material weathering were identified as the most important processes, over the entire period of soil formation (13 700 years), whereas the soil pH strongly affected the direction of Zn isotopic fractionation. Recent anthropogenic inputs of Zn only had a smaller influence compared to the natural processes of the past 13 700 years. However, this will probably change in the future, as Zn stocks in the 0-20 cm layer will increase by 22-68% in the next 100 years, if Zn inputs remain on the same level as today.

Two episodes of sulfide mineralization at the Yuhuang-1 hydrothermal field on the Southwest Indian Ridge: Insight from Zn isotopes

The newly discovered Yuhuang-1 hydrothermal field (YHF) on the Southwest Indian Ridge primarily contains two different sulfide distribution areas: the southwest sulfide (SWS) and the northeast sulfide (NES) distribution areas. In this study, Zn isotope compositions and element ratios of Zn, Fe, Cu, and Cd in the surface sulfides were analyzed to characterize Zn isotopic fractionation, ore-forming conditions, and mineralization processes of the YHF. The results indicate that sulfide-rich samples in the YHF exhibit intensive Zn isotopic fractionation, with δ66Zn values ranging from −0.43‰ to 1.24‰. These values are not only larger than those currently reported for seafloor sulfides but are also larger than the values reported for the deposits on land. Remarkably, the obtained δ66Zn value (−0.43‰) is the lowest one observed in seafloor sulfides (−0.26‰–1.17‰), currently. A study of Zn isotope systematics revealed that Zn isotope variations of sulfide-rich samples in the YHF were controlled by the sphalerite and pyrite content; the study also revealed that early minerals are rich in light Zn isotopes, but later minerals are rich in heavy Zn isotopes. A comparative study of previous data on Zn isotopes shows that Zn isotope fractionations in the YHF are likely attributed to Rayleigh fractionation, caused by mineral precipitation and multiple generations of pyrite. Further, Zn/Cd ratios of sulfide-rich samples indicate that the ore-forming temperature of the silicified sulfide-rich samples in the SWS probably formed under lower temperatures as compared with the silicified sulfide-rich chimney in the NES. Furthermore, sulfur and Zn isotope compositions in the SWS are significantly different from those in sulfide-rich samples of the NES, which suggests pervasively different metallogenic conditions, further indicating that they are probably products of separate mineralization episodes.

Zinc isotopic compositions of migmatites and granitoids from the Dabie Orogen, central China: Implications for zinc isotopic fractionation during differentiation of the continental crust

Some high silica granitic rocks (SiO2 > 70 wt%) show larger Zn isotopic variations (δ66ZnJMC 3-0749L) than less silicic granitic rocks, for which the reasons are unclear. Here we report Zn isotopic data for leucosomes (L, melt) and melanosomes phases (M, residue) in migmatites as well as granitoids from the Dabie orogenic belt, central China. Leucosomes always have higher δ66Zn than their associated melanosomes, with Δ66ZnL–M (δ66ZnL – δ66ZnM) varying widely from 0.08‰ ± 0.07‰ to 0.76‰ ± 0.07‰. High (Th/U)L/M (Th/UleucosomeTh/Umelanosome) ratios of four migmatite samples indicate modification of the leucosomes by fluid exsolution. Rayleigh fractionation modeling suggests that fluid exsolution could be responsible for the elevated δ66Zn in the four leucosomes. Three samples with large Δ66ZnL–M values and positive Eu anomalie, also have high Δ66ZnL-M values, probably due to accumulation of isotopically heavy plagioclase. Equilibrium Zn isotopic fractionation caused by partial melting is modeled to be 0.14‰–0.18‰ (Δ66Znmelt–residual) at 704–820 °C, consistent with the measured diatexite data (Δ66ZnL-M = 0.14‰ ± 0.07‰). The remaining two samples that were not significantly affected by fluid exsolution or plagioclase accumulation yield Δ66ZnL-M values larger than those expected in equilibrium melting and may reflect disequilibrium fractionation between melt and residue during the rapid melt segregation from the source. Our study confirms that Zn isotopes could be significantly fractionated during crustal anatexis and the magnitude of fractionation can be further enhanced by late-stage magmatic differentiation. Therefore, zinc isotope may act as a potential indicator of crust differentiation.

A global assessment of Zn isotope fractionation in secondary Zn minerals from sulfide and non-sulfide ore deposits and model for fractionation control

We investigated extent and direction of Zn isotope fractionation in secondary zinc minerals formed during low temperature hydrothermal and/or supergene oxidation of primary sulfide deposits. Zinc isotope data have been obtained from non-sulfide zinc mineral separates (willemite - Zn2SiO4, smithsonite - ZnCO3, hemimorphite - Zn4(Si2O7)(OH)2·H2O, hydrozincite - Zn5(CO3)2(OH)6, and sauconite - Na0.3Zn3(Si,Al)4O10(OH)2·4H2O) collected from several Zn deposits in Ireland, Belgium, Poland, Namibia, Peru, Yemen and Zambia. The data are compared with Zn isotope compositions measured on Zn sulfides collected in the same areas and/or derived from the existing literature, to establish the controls of direction and likely extent of any fractionations. We find that willemite has the greatest compositional variability, with measured δ66ZnJCM-Lyon values ranging from −0.42 to 1.39‰, spanning the entire range of terrestrial variation in Zn isotopes recorded to date. Overall, significant fractionations in positive and negative directions are recorded relative to the precursor phase (primary sphalerite or an earlier secondary phase), with primary sphalerite falling in a relatively narrow range of isotopic values (approximately −0.1 to +0.4‰). Most of the data observed on willemite, hemimorphite and hydrozincite can be explained with a model of isotopic fractionation, in which partial dissolution of primary sphalerite is followed by precipitation of an initial secondary phase that preferentially incorporates heavy Zn isotopes. Smithsonite, instead, preferentially incorporates light Zn isotopes. This reflects the variation in the Zn-x bond strengths of these secondary phases with respect to the original sulfides. We also observed that isotope compositions do not depend only on the difference between the fractionation factors of the involved phases but also on the amount of the secondary mineral precipitated after dissolution of primary sulfide, and that the greatest fractionations occur when only small amounts of secondary mineral are precipitated. Progressive precipitation from migrating fluids that form phases enriched in heavy zinc isotopes would lead to a gradual decrease in the δ66Zn values of such phases, and the fluids involved, in time and space. Strong negative isotopic shifts are almost only observed for late crystallizing phases, such as those in vugs. These are interpreted to reflect precipitation from residual, isotopically-light fluids that are the inevitable highly-fractionated product of the above-described process. Where a more complete replacement of primary sulfide has occurred, such as in the high-grade core of non-sulfide zinc orebodies, there is limited net isotopic fractionation because dissolved primary zinc is nearly quantitatively reprecipitated locally. In addition, in only one case (Yanque, Peru) we observed that the fringes of non-sulfide zinc deposit were characterized by isotopically fractionated compositions, with highly negative values implying extensive precipitation (earlier, or elsewhere) of isotopically heavy secondary phases. The higher-grade ore zones, where complete breakdown of primary sulfides and quantitative reprecipitation of zinc have occurred, show instead less fractionated compositions.

Zinc isotope fractionation during the inorganic precipitation of calcite – Towards a new pH proxy

Zinc was co-precipitated with calcite in mixed-flow reactors at 25 °C and 6.1 ≤ pH ≤ 8.5 to quantify Zn isotope fractionation between calcite and the reactive fluid. The results suggest that the difference between the isotopic composition of the solid and the fluid (Δ66Zncalcite-fluid = δ66Zncalcite − δ66Znfluid) increases by about 0.6‰ as the solution pH decreases from 8.5 to 6.1. In contrast, based on Zn aqueous speciation and the theoretical values of the reduced partition function ratios for zinc species, lnβ, the isotopic fractionation between calcite and aqueous Zn2+, Δ66Zncalcite - Zn2+, remains constant at 0.58 ± 0.05‰ over the entire pH range investigated. The constant value of Δ66Zncalcite - Zn2+ suggests that irrespective of the solution pH, the same Zn aqueous species, likely free Zn2+ ions, interacts with calcite surface sites during the growth of this mineral via ion by ion attachment. The enrichment of calcite in 66Zn is consistent with the formation of mononuclear, inner-sphere tetrahedral Zn surface complexes at the calcite surface and the increase of Zn coordination to 6 following its incorporation in the crystal lattice with no further isotopic fractionation. Overall, the results suggest that Zn isotopic composition of natural calcite has the potential to shed light on the prevailing pH at the time of calcite formation in the geological past.