Zn Isotope Fractionation Research Articles

Zinc isotope fractionation between Cr-spinel and olivine and its implications for chromite crystallization during magma differentiation

We report the first zinc isotope data (expressed as δ66Zn relative to the JMC-Lyon standard) for chromian spinels (Cr-spinels) and coexisting olivines in oceanic peridotites. All spinel-olivine pairs fall on the 1:1 fractionation line in the diagram of δ66Znspinel versus δ66Znolivine, suggesting equilibrium isotope fractionation. Cr-spinels are always isotopically lighter than coexisting olivines (Δ66Znspinel-olivine = δ66Znspinel − δ66Znolivine = −0.50‰ to −0.33‰; n = 13), which is opposite to the positive Zn isotope fractionation between Al-spinel and olivine observed in cratonic peridotites. The “inverse” Zn isotope fractionation between Cr-spinel and olivine is unlikely to have been caused by low-temperature alteration of the oceanic peridotites, given the lack of correlation between Δ66Znspinel-olivine values and chemical indices of serpentinization and weathering (e.g., LOIs, MgO/SiO2). Ionic model calculation indicates that even considering that Zn occurs at both tetrahedral and octahedral sites in the crystal lattice of spinel, the magnitude of equilibrium fractionation is still inconsistent with the observed Zn isotope fractionation between Cr-spinel and olivine. We suggest a “chemical effect” in which the ZnO bond length (tetrahedral site) in spinel increases when Cr substitutes Al in octahedral site, which is corroborated by the striking negative correlation of δ66Zn with Cr# [molar Cr3+/(Cr3+ + Al)] in natural spinels. Therefore, Zn isotopic compositions of natural spinels can be highly variable depending on their chemical compositions.During magma differentiation, zinc is moderately incompatible in silicate minerals (olivine and pyroxene) but highly compatible in Cr-spinels/chromites that have Zn contents tens of times higher than those of basaltic melts. Given its light Zn isotopic composition, chromite crystallization–if any–can evidently elevate δ66Zn and lower Zn contents of the residual melts. Lunar mare basalts are typically characterized by higher δ66Zn and lower Zn contents relative to terrestrial basalts, but modelling suggests that chromite crystallization during magmatic differentiation is unlikely to account for Zn isotopic and elemental data of lunar mare basalts. Global ocean island basalts (OIBs) and some intraplate alkali basalts have systematically higher δ66Zn and higher Zn contents than those of the normal mantle-derived melts (e.g., mid-ocean ridge basalts; MORBs), which contradicts with a chromite crystallization model. Instead, such signatures can reflect recycling of surface carbonates into the Earth’s deep mantle, reinforcing the application of zinc isotopes as a tracer of deep carbonate recycling.

Improved protocols for Zn purification and MC-ICP-MS analyses enable determination of small-scale Zn isotope variations

In this study we show that following a newly proposed protocol for stable Zn isotope ratio measurements on a ThermoFisher Scientific Neptune Plus MC-ICP-MS significantly improves the long-term external reproducibility down to < ± 0.025‰ 2SD on δ66/64ZnJMC-Lyon for a single static analysis with 90 measurement cycles of 4.2 s integration time each. This protocol includes: (i) stable isotope measurements in medium-mass resolution mode (R > 6500) to prevent the influence of any polyatomic interferences; (ii) the use of a double-spike, which in our case was a previously designed 64Zn-67Zn mixture; (iii) Zn purification with a two-column anion exchange method; (iv) thorough pre-run sample Zn purity checks and signal matching between sample and reference standard solutions; and, (v) pre-run adjustment of sample carrier and desolvating nebulizer system (DNS) gas flows such that the oxide formation rate determined on the 140Ce16O+/140Ce+ ratio remains below 1%. Following these routines, we obtain long-term external reproducibilities of 0.290 ± 0.017‰ 2SD for the IRMM-3702 (n = 145) certified Zn isotope amount reference material (RM) and − 9.974 ± 0.025‰ 2SD for the Alfa Aesar (n = 75) in-house Zn standard solution. The averages of sets of five single measurements of the IRMM-3702 RM and the Alfa Aesar standard solution narrow down the external reproducibility to ± 0.006‰ and ± 0.023‰ (both given as two standard deviations; for pooled data referred to as 2SD*), respectively. Alternatively, five-fold numbers of measurement cycles can reach the same external reproducibility for single measurements. The δ66/64ZnJMC-Lyon compositions of certified natural rock reference materials obtained by means of individual digests and analyses accurately reproduce the literature values, but our data also show that different batches of the BHVO-2 can be isotopically heterogeneous and clearly affect the reproducibility (BIR-1a: 0.247 ± 0.023‰, n = 17; BCR-2: 0.268 ± 0.025‰, n = 12; BHVO-2 #3323: 0.323 ± 0.024‰, n = 6; BHVO-2 #0535: 0.353 ± 0.015, n = 5, all 2SD). In cases where no effect of heterogeneity on reproducibility could be determined, we report the lowest uncertainty for replicate RM digests combined with multiple measurements (JB-2: 0.225 ± 0.012‰ 2SD, n = 3, i = 15). Mixtures between solution digests of the BIR-1a and BHVO-2 in combination with data pooling of five single measurements indeed demonstrate a measurement accuracy of < ± 0.010‰ 2SD*, emphasizing the analytical capability of the proposed method to depict small-scale Zn isotope variations. With the overall analytical improvement, we seek a better discrimination between variable, and potentially overlapping processes causing Zn isotope fractionation in various terrestrial reservoirs.

Zn isotope fractionation during the development of low-humic gleysols from the Mun River Basin, northeast Thailand

Zinc (Zn) is an essential micronutrient released into soils during rock weathering, and Zn isotope composition has been emerging as a powerful tracer involved in the Earth’s surface processes. To explore the soil development in dry tropical regions and evaluate the influence on the global Zn cycles, stable Zn isotope data were presented for the two pristine low-humic gleysol profiles from the Mun River Basin (MRB), northeast Thailand. The samples exhibit the δ66Zn (relative to JMC 3-0749L standard) from −0.69‰ to 0.32‰. Therein, the Fe nodule-containing soils exhibit a light shift of δ66Zn relative to parent red siltstones (Δ66Znnodule-containing soil–p of S1 = −0.60‰) and the other soils (Δ66ZnII-other zones = −0.66‰ – −0.50‰), indicating that the preferential retention of light Zn isotopes in residual soils is attributed to the adsorption of heavy Zn isotopes onto the surface of secondary Fe oxides. Due to the water insufficiency in the MRB, the redox conditions to control the oxide precipitation are driven by the seasonal alteration of groundwater levels. With the recession of groundwater, Fe oxides precipitate and adsorb heavy Zn isotopes. Besides, heavy Zn isotopes are also incorporated in clay fraction. On the contrary, the saturated water is likely to disrupt the interactions of Zn with Fe phases and clay minerals. In addition, low content of organic matter and Mn oxides and the absence of sulfides limitedly contribute to the light Zn pool in soils. Conclusively, the isotopic signature of Zn indicates that Fe oxide-rich soils are likely to control the heavier Zn isotope composition of dissolved load in rivers relative to continental rocks in dry tropical regions.

Zinc isotopic fractionation between aqueous fluids and silicate magmas: An experimental study

Fluids in magmas play a key role in magma differentiation and transportation of economic metals for ore deposits. As a chalcophile and incompatible element, zinc and its isotopes have been increasingly applied to study the magmatic-hydrothermal processes. However, zinc isotopic fractionation between aqueous fluids and magmas has not been well constrained. Here we experimentally determined equilibrium fractionation factors of Zn isotopes between aqueous fluids and silicate magmas (Δ66Znfluid-magma = δ66Znfluid – δ66Znmagma). The results reveal that aqueous fluids are isotopically heavier than the coexisting silicate magmas. No correlation between Δ66Znfluid-magma and temperature or chlorine contents in fluids is observed under our experimental conditions. Instead, Δ66Znfluid-magma is negatively corresponded with NBO/T of the melt (the ratio of non-bridge oxygen and tetrahedron ions), and positively correlated with the molar ratio of Al/(0.5 K + Ca + Fe) in the bulk magmas, suggesting the controlling of silicate composition on Zn isotope fractionation. Our data therefore indicate that the isotopically heavier Zn in the fluids exsolved from magmas may account for the higher δ66Zn of pegmatites and high-silica granitic rocks. Moreover, involvement of magmatic fluids can explain the highly variable and remarkably heavier Zn isotopic signatures of fumaroles, thermal spring waters, and seafloor hydrothermal fluids compared to the igneous rocks. This study provides information that can be used to guide research using Zn isotopes to trace fluid activity and magmatism.

Isotope fractionation of zinc in the paddy rice soil-water environment and the role of 2’deoxymugineic acid (DMA) as zincophore under Zn limiting conditions

Non-traditional stable isotope systems are increasingly used to study micronutrient cycling and acquisition in terrestrial ecosystems. We previously proposed for zinc (Zn) a conceptual model linking observed isotope signatures and fractionations to biogeochemical processes occurring in the rice soil environment and we suggested that 2’deoxymugineic acid (DMA) could play an important role for rice during the acquisition of Zn when grown under Zn limiting conditions. This proposition was sustained by the extent and direction of isotope fractionation observed during the complexation of Zn with DMA synthesised in our laboratory. Here we report a new set of experimental data from field and laboratory studies designed to further elucidate the mechanisms controlling Zn isotope fractionation in the rice rhizosphere and the role of DMA. First, we present acidity (pKa) and complexation (logK) constants for DMA with H+ and Zn2+, respectively, using synthetic 2’deoxymugineic acid and show that they are significantly different from previously published data using isolates from plants. Our new set of thermodynamic data allows for a more accurate calculation of the formation of ZnDMA complexes over pH ranges typically found in the rhizosphere of flooded lowland rice soils and in rice plant compartments (xylem, phloem). We show that at pH > 6.5, Zn is fully complexed by DMA and at pH <4.5 fully dissociated. This has important implications, i.e. that in alkaline paddy soils, DMA can strip Zn from soil solids (organic and inorganic) and that ZnDMA complexes are stable at the root interface if the pH is alkaline and in the phloem and xylem of the rice shoot. Second, we present a new set of Zn isotope data in rice grown in alkaline soils with low Zn availability with and without Zn addition. We used two genotypes not tested to date, i.e. A69–1, tolerant to low Zn supply, and IR26, sensitive to low Zn supply. We confirm previous findings that, in contrast to observations with rice grown in hydroponic studies, no isotopically light Zn is taken up into the shoot irrespective of Zn fertilization and that isotopically heavier Zn is taken up by rice grown in soils with low Zn supply. Third, we determined the isotope fractionation isotherm for Zn during absorption on goethite, representing the iron phase (plaque) typically forming on rice roots, in acidic solution. The negative Zn isotope fractionation observed (i.e., Δ66Zngoethite-solution < 0, where Δ66Zngoethite-solution = (δ66Zn of goethite) – (δ66Zn of solution)) is in contrast to the positive isotope fractionation (Δ66Zngoethite-solution > 0) detected during adsorption of Zn on goethite in alkaline solutions or between root and soil solution in rice grown in alkaline soils. We show that removal of different Zn species from solution and changes in the Zn coordination control extent and direction of isotope fraction during adsorption. Using the new set of results and combining it with recent findings from the literature, we present a refined conceptual model linking biogeochemical processes to Zn isotope fractionation in the rice soil system. Our results confirm the importance of root induced chemical changes in the rhizosphere of rice growing in soils with low Zn availability, demonstrate the unique ability of isotope signatures to deconvolve geochemical processes and conditions in the plant-soil environment and support the hypothesis of an important role of DMA in Zn acquisition under Zn limiting conditions.

Cover Feature: Study on the Isotope fractionation of Zinc in Complexation with Macrocyclic Polyethers (Z. Anorg. Allg. Chem. 6/2021)

The isotope fractionation of Zn was studied by liquid-liquid extraction with aqueous phase of ZnCl2/ HCl and organic phase of crown ethers(CEs), benzo15-crown-5(B15C5), benzo-18-crown-6(B18C6) and Dicyclohexano-24-crown-8(DC24C8)/1,2-dichloroethane. The isotope enrichment factor of the isotope pair 66Zn-64Zn was found to be ϵ66=−0.24 to −0.52‰ in the order of B15C5≥B18C6>DC24C8. The magnitude of ϵ66 was well explained by using the isotope fractionation factors obtained by density functional theory method for Zn chlorides and ZnCl2/CE model molecules. More details can be found in the Article by Toshiyuki Fujii, Chizu Kato, Brandon Mahan, and Frédéric Moynier.

Spatial patterns of Zn, Cd, and Pb isotopic compositions of ground and surface water in mine areas of South Korea reflecting isotopic fractionation during metal attenuation.

As application of multiple metal isotopes can effectively constrain geochemical behavior of contaminants and assess contamination sources and pathways, field-scale studies on the geochemically interlinked fractionation of Zn and Cd isotopes in groundwater are needed. In this study, we collected groundwater samples from multi-level samplers downstream of tailings dumps as well as surface water, ore mineral, precipitate, and tailings samples at the Sambo and Buddeun metallic ore mines in South Korea, and analyzed their Zn, Cd, Pb, and sulfur isotopic compositions. Furthermore, isotopic ratios of ore mineral samples from additional four mines in South Korea (Dangdu, Dongbo, Gomyeong, Samgwang) were compared. A dual isotopic approach using Zn and Cd isotopes was used to assess fractionation processes, and Pb isotopic signatures reflecting their sources were assessed. Increasing trends of δ66Zn and δ114Cd with decreasing Zn and Cd concentrations were observed in groundwater, which was saturated with respect to ZnS (amorphous and sphalerite) and CdS (greenockite). Moreover, for some groundwater samples, δ66Zn showed a positive relationship with δ34SSO4. These results suggest that Zn and Cd are precipitated as sulfide following sulfate reduction. In the plot of δ66Zn against δ114Cd, relatively high and/or increasing δ66Zn in groundwater suggested the effect of fractionation due to sulfide precipitation, while variable and high δ114Cd values suggested the fractionation by adsorption and/or sulfide precipitation, which were based on positive fractionation factors for δ66Zn and δ114Cd during sulfide precipitation and mostly negative and positive fractionation factors for δ66Zn and δ114Cd, respectively, during adsorption. This study shows that the combined use of Zn and Cd isotopes in groundwater can effectively differentiate between adsorption and sulfide precipitation following sulfate reduction in groundwater. Additionally, the 208Pb/206Pb ratios of most water samples reflected those of ore and tailings samples, which verified usefulness of Pb isotopes in water in investigating Pb contamination sources.

Zinc isotopic behavior of mafic rocks during continental deep subduction

Zinc isotopes may act as a new tool of tracking recycling of crustal materials that causes compositional heterogeneity of the mantle. This application relies on an investigation of Zn isotopic behaviors during slab subduction. In this study, we report Zn isotopic compositions for a suite of metabasalts (greenschists, amphibolites, and coesite-bearing eclogites) from the Dabie Orogen (China), which were formed via the subduction of mafic rocks into different depths and up to > 200 km. Three out of eight greenschists are characterized by lighter δ66ZnJMC-Lyon (0.10‰–0.16‰) than those of global basalts (0.28‰ ± 0.05‰), which may be caused by crustal assimilation of the protoliths by sedimentary rocks due to their extremely high 87Sr/86Sr (up to 0.7130) and low εNd values (down to −12.3). The remaining greenschists have relatively low 87Sr/86Sr and their δ66Zn values (0.21‰–0.38‰) overlap the ranges of amphibolites (0.18‰–0.32‰) and coesite-bearing eclogites (0.18‰–0.36‰). There is no correlation between δ66Zn and sensitive indicators of dehydration (Rb/TiO2, Ba/Yb, and H2O+), suggesting that no detectable Zn isotope fractionation has occurred during the deep subduction of mafic rocks even into > 200 km, which is attributed to the limited loss of Zn during prograde metamorphism and dehydration. Thus, Zn isotopic compositions of the deeply subducted mafic rocks are inherited from their protoliths. Considering that these metamorphosed rocks have higher δ66Zn than that of the mantle value by up to 0.2‰, the recycled/subducted mafic crust can incorporate isotopically heavy Zn into the mantle. The subducted slabs may partially melt and generate a metasomatized mantle, resulting in changes of Zn isotopic composition of the hybridized mantle as have been observed in some mantle xenoliths and basaltic lavas.

Zinc source differentiation in hydrothermal vent mollusks: Insight from Zn isotope ratios

Hydrothermal vent represents an extreme environment where metal-enriched fluids are in contact with chemosymbiotic animals. In the present study, Zn isotopic compositions were determined in multiple tissues of three dominant hydrothermal vent mollusks (the mussel Bathymodiolus marisindicus and two gastropods Chrysomallon squamiferum and Gigantopelta aegis) collected from a hydrothermal vent field (Southwest Indian Ridge in the Indian Ocean). We found approximately 1.78‰ differences in the δ66Zn values among the three vent mollusks despite of their similar range of Zn concentrations. The significant variation in the δ66Zn values was considered to be indicative of different Zn uptake sources among the three species as a result of their morphological adaptations. Zinc uptake associated with symbiotic activities may be more relevant in the vent gastropods, whereas Zn uptake from hydrothermal fluids during filter-feeding may also play a role in the vent mussels. However, no significant difference in δ66Zn values was observed among tissues of any of the mollusks, showing the absence of Zn isotope fractionation during internal Zn transport. Our results demonstrated that variable Zn uptake pathways existed among different hydrothermal vent mollusks and could be differentiated by determining the Zn isotopic compositions in their tissues. We also highlight that Zn isotope ratios can be used to track Zn sources to the vent mollusks.

Evaluation of shallow-water carbonates as a seawater zinc isotope archive

Carbonate zinc (Zn) isotopes have increasingly been used to track changes in seawater Zn isotopic composition. A large body of recent work, in particular studies conducted under the GEOTRACES program, have added significantly to our knowledge of how Zn behaves in the water column. However, our understanding of how water-column Zn isotopic signals become incorporated into carbonate sediments is at a much more nascent stage. To help solve this issue, we have measured the Zn isotopic composition of modern surficial and core-top carbonate sediments and seawater from the Bahamas, Panama, and the Persian Gulf. Whereas modern Bahamian seawater has a δ66Zn of 0.14 ± 0.12‰ (2σ), Bahamian carbonates show a large range of δ66Zn—from −0.56 to 1.11‰. The δ66Zn of carbonate mud is ∼0.3‰ higher than that of the Bahamian seawater, potentially due to Zn isotopic fractionation during precipitation. Carbonate ooids and peloids and certain skeletal and microbial aragonites, including gastropods, green algae, stromatolites and precipitates associated with cyanobacterial (Dichothrix) filaments, have δ66Zn indistinguishable, within analytical error, from that of modern seawater. This suggests that these carbonate phases may be promising archives for seawater δ66Zn.

Using Zn isotopes to trace Zn sources and migration pathways in paddy soils around mining area

Paddy soils around mining areas suffer from the great threat of heavy metal pollution. The traditional source-tracing methods based on metal concentrations limit our ability to quantify the sources of heavy metals and trace their transport processes to paddy soils. In this study, Zn isotope compositions of paddy soils in Dabaoshan mine area, a typical sulfide deposit in southern China, have been systematically studied. According to a plot between 1/Zn (i.e. inverse concentration) and δ66Zn value, all the polluted paddy soils fall on the mixing line between acid mine drainage precipitate (AMD-precipitate) and fertilizer while the unpolluted paddy soil falls on the mixing line between fertilizer and bedrock. This indicates the mixing of Zn sources at least three end-members: the mining end-member (i.e. AMD-precipitate), the agricultural end-member (i.e. fertilizer), and bedrock whose geochemical signature is often overprinted by the former two sources around the mining area. The quantitative calculations to apportion the end-member’s contributions show that the mining activity contributes most Zn in the paddy soils with an average of ∼66.2%. The contribution of mining activities has significant spatial variations. Specifically, the mining activities have relatively low impacts on the lower reach and the deep soil. Additionally, the apparent Zn isotope fractionation between AMD and AMD-precipitate (Δ66ZnAMD-precipitate − AMD of −0.35 to −0.08‰) in the tailings dam suggests that Zn cations in AMD coprecipitated with the secondary Fe-bearing minerals (e.g. jarosite and goethite). After being discharged from the tailings dam, Zn is mainly carried by the Fe-oxide minerals and migrated during surface runoff. Our study highlights the contribution of human activities to the Zn pollution in the paddy soils and the key role of Fe-bearing minerals in the migration of Zn. These findings provide a scientific base for the development of policy for pollution control in mining-affected region.

Uptake of trace elements and isotope fractionation of Cu and Zn by birch (Betula pendula) growing on mineralized coal waste pile

Aims of the present study were to determine the extent of the uptake of selected trace elements by birch (Betula pendula Roth) growing on the partly burnt coal waste pile and to evaluate isotopic fractionation of copper (Cu) zinc (Zn). Substrates, birch sap and leaves were collected in the coal waste pile and in a reference area outside the waste pile. The concentration of trace elements in birch sap and leaves both in birch growing on the coal waste pile and at reference sites increase in the range of U, As, Se > Cd, Mo, Cu > Pb > Ba ≫ Zn. However, the uptake of elements by birch is restricted when concentration of metals in substrates are too high.Compared to the isotopic composition of Cu in waste substrates and soils (δ65Cu = – 0.93 to + 1.49‰), this element in sap and leaves (δ65Cu = – 0.58 to −1.64 and – 0.59 to – 2.35‰, respectively) is depleted in heavy isotope regardless of plant-available concentration of Cu in substrates. Compared to the isotopic composition of Zn in substrates (δ66Zn = – 0.20 to 1.43‰), its isotopic composition in sap and leaves (δ66Zn = – 0.85 to + 1.00 and – 0.21 to + 0.57‰, respectively) is depleted in the heavy isotope only when the plant-available concentration of Zn in the substrate and soil is high (>20 mg kg−1). At low plant-available concentration of Zn no isotopic fractionation or enrichment of Zn in the heavy isotope was recorded.

An experimentally-determined general formalism for evaporation and isotope fractionation of Cu and Zn from silicate melts between 1300 and 1500 °C and 1 bar

Vaporisation fractionates elements and their isotopes between condensed phase(s) and the gas phase according to two end-member regimes; kinetic (Langmuir) or equilibrium (Knudsen). The fractionation factor of isotopes i and j of element E between gas and liquid, i/jE(αvap-liq), in the former case depends only on the molar masses of the evaporating species, (Mj/Mi)0.5, whereas, at equilibrium, the fractionation factor is ∼1 at high temperatures. The prevailing regime depends on the production rate of vapour species relative to their rate of transport away from the evaporating surface, for which an accurate theory is lacking. Here, we present results of Cu and Zn isotope fractionation during evaporation of ferrobasaltic melts in the FeO(T)-CaO-MgO-Al2O3-SiO2 (FCMAS) system in a 1 atm gas-mixing furnace at oxygen fugacities (fO2) from 10−8 bar to 10−0.68 bar (air) at temperatures between 1300 and 1500 °C. In air, the fractionation factors are 65/63Cu(αvap-liq) = 0.9972 ± 0.0001 and 66/64Zn(αvap-liq) = 0.9959 ± 0.0002, but increase under reducing conditions, at logfO2 = −8 for 65/63Cu(αvap-liq) (0.9979) and at −3, −5.5 and −8 for Zn, at which 66/64Zn(αvap-liq) = 0.9967, 0.9974 and 0.9978, respectively. This behaviour is caused by slow elemental diffusion through the silicate melt relative to the rate of evaporative loss, thereby depressing i/jE(αvap-liq). When evaporation rates far exceed those for diffusion in the melt, no isotopic fractionation develops in the condensed phase. To account for the 65/63Cu(αvap-liq) and 66/64Zn(αvap-liq) values observed in air, we develop a formalism for mass transfer applicable to any evaporating species over a range of pressures, gas compositions and flow regimes. For natural convection at 1 bar, diffusive transport through the gas is the rate-limiting step, where D(i,j)k is the diffusion rate of isotope i or j through a gas with mean molar mass Mk. Under these conditions i/jE(αvap-liq) is proportional to (Dik/Djk)2/3, and not (Dik/Djk) as previously proposed. Our model predicts i/jE(αvap-liq) in other experiments, and in nature, to a precision of ±0.0005 assuming ideality. The fractionation factors inferred for moderately volatile elements in lunar mare basalts (>0.9994) are inconsistent with a mass transport process, and instead reflect near-equilibrium conditions of vapour loss from the Moon.

Magnesium and zinc stable isotopes as a new tool to understand Mg and Zn sources in stream food webs

AbstractNon‐traditional stable isotopes of metals were recently shown as new dietary tracers in terrestrial and marine mammals. Whether these metal stable isotopes can be used to understand feeding habits in stream food webs is not known yet. In this study, we explored the potential of stable isotopes of essential Mg (δ26Mg) and Zn (δ66Zn) as a new tool in stream ecology. For this purpose, we determined δ26Mg and δ66Zn values of stream organisms and their potential metal sources in upper and lower reaches of two streams in the Lake Biwa catchment, Central Japan. Our goals were (1) to explore variations in δ26Mg and δ66Zn across organisms of different feeding habits and (2) to understand Mg and Zn sources to stream organisms. Overall, δ26Mg and δ66Zn values of organisms were neither related to each other, nor to δ13C and δ15N values, indicating different elemental sources and factors controlling isotopic fractionation depending on element and taxa. Low δ26Mg values in filter‐feeding caddisfly larvae and small gobies indicated aqueous Mg uptake. Higher δ26Mg values in leaf‐shredding crane fly and grazing mayfly larvae suggested Mg isotopic fractionation during Mg uptake from the diet. While the δ26Mg values of stonefly nymphs reflected those of caddisfly larvae as a potential prey, the highest δ26Mg values found in dobsonfly nymphs can be explained by 26Mg enrichment during maturing. δ66Zn values of caddisfly and mayfly larvae indicated Zn was a mixture of aqueous and dietary available Zn, while higher δ66Zn values in crane fly larvae pointed to Zn isotopic fractionation during Zn uptake from plant litter. δ66Zn values in stonefly and dobsonfly nymphs were often in the range of those of caddisfly larvae as their prey, while dragonfly nymphs and small goby were depleted in 66Zn relative to their dietary Zn sources. We conclude that δ26Mg is a promising indicator to assess Mg sources in stream ecology depending on taxa, while the use of δ66Zn is limited due to the complexity in Zn sources.

Archean lithospheric differentiation: Insights from Fe and Zn isotopes

Abstract The Archean continental lithosphere consists of a dominantly felsic continental crust, made of tonalite-trondhjemite-granodiorite (TTG) and subordinate granitoids, and a cratonic lithospheric mantle, made of highly refractory peridotites. Whether they stemmed from the same process of differentiation from the primitive mantle, or were two distinct components that were physically juxtaposed, remains debated. Metal stable isotope ratios are sensitive to magmatic and metamorphic processes and do not evolve with time. Therefore, stable isotope ratios are complementary to radiogenic isotope ratios, and they allow direct comparisons to be made between different terrestrial components without age corrections. Isotopes of iron and zinc, metals ubiquitous in Earth’s lithosphere, can be tracers of lithospheric formation and evolution because they are affected by partial melting (Fe, Zn), redox state (Fe), and the presence of sulfides (Fe, Zn). Here, using stable Fe and Zn isotopic data from Archean samples of the lithospheric mantle and the continental crust, we show that Fe and Zn isotopes define a linear array, best explained by their coupled fractionation behavior during magmatic processes. Our data show that high degrees of partial melting (&amp;gt;30%) during the formation of the cratonic mantle and mafic protocrust, and reworking of the early crust significantly fractionate Fe and Zn isotopes. Conversely, Fe and Zn isotope ratios in the TTG are similar to those in Archean mafic rocks, suggesting an origin by fractional crystallization of basalt, and implying limited Fe and Zn isotopic fractionation, instead of partial melting of mafic crust. Moreover, the absence of Fe and Zn isotope decoupling due to redox effects, melt (fluid)–rock or sediment-rock interaction, and decarbonation indicates that subduction, at least as we understand it now, is not required to explain the Fe and Zn isotope composition of the Archean lithosphere.

Evaporative loss of moderately volatile metals from the superheated 1849 Ma Sudbury impact melt sheet inferred from stable Zn isotopes

The retention of moderately volatile elements on the growing Earth remains a major uncertainty in models of terrestrial accretion. Large impactors were the main carriers of accreted material but their mutual energetic collisions and impacts onto the Earth also caused chemical fractionation for which limited experimental data exist. The objective of this work was to study several moderately volatile elements in the third-largest impact basin preserved on Earth at Sudbury, Ontario. We conducted a new chemostratigraphic transect (n=41) of Zn isotope ratios and concentrations by analysing melt sheet and basin fill samples. The data were compared to common Pb, Cs, Cd and Sb concentration systematics. Within the crystallised melt sheet there are strong trends in the extent of moderately volatile element deficits, Zn isotope composition (δ66/64ZnJMC-L from 0.18 to 0.47‰) and initial Pb isotope composition. The combined evidence suggests that these trends reflect footwall contamination of a melt sheet that had experienced evaporative Zn-loss of up to 75–80%. Accounting for plausible isotopic signatures of target rocks, the maximum mass-dependent Zn isotope fractionation ε was 0.29 ± 0.04‰ (1 s.d.), which translates to modest fractionation factors α=0.99986 to 0.99975. This is comparable to melt fallout-glass and fused sands from nuclear detonation sites. We attribute the observed Zn loss and isotope fractionation to the formation of the impact melt. The rapid formation of a solid lid of breccias upon seawater ingress may have prevented stronger evaporative loss and isotope fractionation. Within the crater fill, there is an up-stratigraphy increase in Zn isotope variability (δ66/64ZnJMC-L from 0.29 to 1.05‰). Combined with evidence for biogenic reduced C, this suggests sedimentation of authigenic particulates within an enclosed crater sea.In the melt sheet, the Zn-Pb and Rb-Cs pairs experienced different extents of maximum evaporative loss (Pb up to 98.4% vs. Zn 78%; and Cs ∼90% vs. Rb ∼30%). The relative loss pattern could reflect evaporation from superheated silicate melt at ∼1,450°C and 1 atm. Loss from super-liquidus melts formed by bolide impacts could have been a significant process shaping the Earth's volatile and moderately volatile inventory.

Large Zn isotope variations in the Ni[sbnd]Mo polymetallic sulfide layer in the lower Cambrian, South China

In South China, black organic-rich shales in the lower Cambrian Niutitang Formation host a NiMo polymetallic sulfide layer that discontinuously extends over ~1600 km. Seawater and hydrothermal origins are among the many suggested hypotheses and are still under debate. In order to discriminate Zn sources, we report Zn isotopes in NiMo polymetallic sulfide layers and their host shales from the Nayong and Zunyi locations in Guizhou province and the Zhangjiajie section in Hunan province. In each section, host organic-rich shales show homogeneous Zn isotope compositions which likely resulted from quantitative scavenging of dissolved Zn from seawater under euxinic conditions. The difference in the average δ66Zn values of organic-rich shales between the two sections in Guizhou (0.76 ± 0.09‰) and one section in Hunan (0.59 ± 0.10‰) might reflect variations of Zn isotope gradient with the depth of seawater. Therefore, the organic-rich sediments need not always represent an isotopically light Zn sink, which is dependent on Zn isotope fractionation in the local basin. However, the δ66Zn values in the NiMo polymetallic sulfide layers are different from those of their host shales, indicating that these sulfide layers did not inherit the Zn isotope signal of seawater. Based on the regular increasing trend in δ66Zn values from Nayong (0.54 ± 0.06‰) to Zhangjiajie (1.34 ± 0.09‰) and the presence of PbZn mineralization in the Dengying/Doushantuo Formations, we argue that hydrothermal fluids associated with PbZn mineralization could be a major source of Zn in NiMo sulfide layers, especially in the Nayong location. A possible model is that the hydrothermal fluids related to MVT-type mineralization got overprinted on a multiple-sourced synsedimentary sulfide-rich layer. We provide additional evidence that Zn isotopes have great potential as a tracer of metal source and can be applied to similar types of mineralization as e.g., the late Devonian Ni-Zn-PGE Nick deposit (Selwyn Basin, Canada) or elsewhere.

A Triple‐Stack Column Procedure for Rapid Separation of Cu and Zn from Geological Samples

We present a new procedure for the separation and purification of Cu and Zn from geological samples. Our procedure employed a single pass, triple‐stack column set‐up. The first column, filled with TRU resin (TrisKem International), quantitatively removed Fe and Ti from sample matrices. A second column, filled with pre‐filter resin (TrisKem International), removed organic compounds. Finally, a third column, filled with anion exchange resin (AG1‐X8, 200–400 mesh, Bio‐Rad), was used to separate Cu and Zn from the remaining matrix. Our procedure required about 50% less acid volume than previously reported methods for Cu and Zn separation, thereby minimising analytical blanks and column running times. Copper and Zn stable isotope ratios were determined by a Thermo Neptune Plus MC‐ICP‐MS using Zn and Cu external normalisation, respectively, in addition to sample‐standard bracketing to correct for instrumental mass bias. We explore the inter‐calibration of Cu and Zn isotope fractionation coefficients during analysis by measuring mixed Cu–Zn solutions with enhanced mass bias variation generated by varying sample gas flow rates. Our results demonstrate that this procedure is useful when variation in instrumental mass bias throughout analytical sequences is insufficient to inter‐calibrate Cu and Zn fractionation coefficients.

Zinc isotopic systematics of the South China Sea basalts and implications for its behavior during plate subduction

Zinc isotopes is a new tracer for deep materials recycling. However, its behavior during plate subduction remains obscure. To address this issue, here we present high precision Zn isotopic compositions (δ66Zn) of basalts recovered from International Ocean Discovery Program (IODP) Expedition 349 in the South China Sea (SCS), which have been influenced by various amounts of subducted materials. Based on (La/Sm)N values, these basalts are classified into N-MORB ((La/Sm)N < 1) and E-MORB ((La/Sm)N ≥ 1) types. The results show that both of the N-MORB and E-MORB type basalts have similar and homogeneous Zn isotopic compositions with δ66Zn ranging from 0.22 to 0.28‰ and 0.24 to 0.31‰, respectively. These values are similar to that of typical MORB reported in the literature (0.28 ± 0.03‰), but are systematically higher than the estimated average value of depleted MORB-type mantle (DMM, 0.18‰).The lack of correlations between δ66Zn and geochemical indicators of magma differentiation (e.g., MgO, SiO2, CaO/Al2O3) suggests that crystal fractionation has a limited effect on Zn isotope fractionation. Even though the mantle sources of the SCS basalts are clearly affected by various amounts of subducted materials, there are no correlations between δ66Zn and indicators of subducted components (e.g., Ce/Pb, Nb/U, Ba/La, 87Sr/86Sr). This implies that insignificant amount of Zn was transported from the subducting slab to the mantle sources of the SCS basalts, so that subducted materials have a limited effect on Zn isotopic compositions of the SCS basalts studied here. Therefore, the observed offset of δ66Zn (~0.10‰) between the mantle and the SCS basalts studied here should be primarily controlled by Zn isotope fractionation during mantle partial melting with no need for contributions from subduction materials.

Contrasting zinc isotopic fractionation in two mafic-rock weathering profiles induced by adsorption onto Fe (hydr)oxides

Zinc isotopes have been increasingly utilized as a proxy for (bio)geochemical cycles of nutrient Zn in the ecosystem like soils, but the behavior and mechanism of Zn isotope fractionation during weathering and soil formation remain to be constrained. Here we reported zinc contents and isotope data for saprolites and laterites from two well-characterized mafic-rock weathering profiles, formed under a subtropical climate (South Carolina, USA) and a tropical climate (Hainan Island, China), respectively. Leaching experiments with basalt and andesite as the starting materials were also performed for comparison. The leaching results revealed no resolvable Zn isotope fractionation during dissolution of silicate minerals in the absence of organic ligands in solutions. The Hainan laterites show strong Zn depletion and have lower δ66ZnLyon values (0.10‰ to 0.27‰) relative to the parent rock (0.31 ± 0.04‰), which are coupled with light copper isotope compositions in the same samples. These observations suggest that Zn mobilization was achieved via metal-organic complexation and Fe-colloids transportation. By contrast, saprolites from the South Carolina are extremely Zn-rich (ten-fold higher concentration than the parent rock) and most samples have higher δ66Zn values (0.10‰ to 0.65‰) compared with the parent rock (0.24 ± 0.04‰). Zinc isotope ratios of the South Carolina profile are positively correlated with iron isotope ratios (δ56Fe) and Fe3+/FeTotal, the latter of which is related to secondary Fe (hydr)oxide formation. These relationships link the high Zn contents and δ66Zn values of saprolites from the South Carolina profile to the substantial adsorption of Zn onto or isomorphic substitution into Fe (hydr)oxides that prefers heavier Zn isotopes. Our study highlights that Fe (hydr)oxides exert a strong influence on the re-distribution of nutrient Zn in natural soils and the (bio)geochemical cycling in natural soil systems.