Chemical separation and isotopic measurement of germanium using hexapole-collision cell-MC-ICPMS were developed in various Fe–Ni, ZnS and silicates matrices in order to investigate the potentiality ...of Ge as an isotopic tracer of planetary differentiation and rock-forming processes. Analytical procedures are described for the critical step of silicate dissolution in HF+HNO3 medium, as well as for Ge chemical purification using a single cationic-exchange resin step for Fe–Ni and ZnS matrices, and two anionic and cationic resin steps for silicate matrices. Germanium isotopic measurements using MC-ICPMS were performed with appropriate Ar+H fluxes in the collision cell to eliminate argide interferences on Ge masses. Three methods of mass bias correction, including sample standard bracketing, external Ga mass bias correction using the exponential law, and the empirical “regression method”, give similar results and demonstrate the use of Ga as an appropriate element for mass bias correction of Ge. Results are presented as delta values with respect to JMC Ge standard, and NIST3120a Ge standard for comparison. We show a long-term 2SD reproducibility of less than 0.24‰ on the δ74Ge.
These analytical methods have been applied to Fe-meteorites, sphalerite (ZnS) deposits, and geostandard silicates ranging from ultramafic to basaltic to granitic compositions, and to an iron formation composition. Fe-meteorites and terrestrial silicate samples display small variations of δ74GeJMC=+1.77±0.22‰ and +0.89±0.16‰ (2SD reproducibility), respectively. This contrasts with the large variations seen in low-temperature rocks, such as the ZnS ores (δ74GeJMC=−0.37 to −1.69‰), and banded iron formations (IF-G Isua, δ74GeJMC=+1.38‰). A slight δ74GeJMC–NBO/T negative tendency in silicate samples indicates that polymerisation of silicate melt would control the small Ge isotope fractionation among mantle silicates.
A comparison of δ74Ge values of iron meteorites and Earth silicate mantle opens new perspectives in deep Earth processes. On the basis of theoretical metal-silicate isotopic equilibrium processes, the low δ74Ge of silicate Earth cannot reconcile one-stage process of core–mantle segregation. It is proposed that the δ74Ge(JMC) value of silicate earth samples of +0.89±0.16‰ (or δ74Ge(NIST3120a)=+0.53‰) represents the composition of the accessible Earth modern mantle. The δ74Ge of the silicate mantle in equilibrium with the core at time of core formation would be distinct to that of the present mantle in result of distinct thermodynamic parameters, e.g. fO2, pressure, inducing changes in coordination and valence state of Ge in the silicate crystallographic structure. In addition, the light isotopic composition of the Earth's mantle could result from reverse diffusive processes induced by an increase in oxidation state at the end of core formation. This would have some implications on core formation modelling and the use of Ge isotopes for tracing the origin of deep mantle plumes.
► Ge chemistry and MC-ICPMS isotopic analyses of metal, silicate, sulphide matrices. ► Ge isotopic composition of Earth's mantle, iron meteorites, terrestrial sulphides. ► Small range of Ge isotopic variations for high-temperature core and mantle processes. ► Large Ge isotopic variations for superficial low-temperature processes. ► δ74Ge in the Earth's mantle is not in equilibrium with the core.
The increasing worldwide demand in germanium (Ge) is driving renewed research for understanding its geological cycle and the factors controlling its concentration in minerals. The advent of accurate, ...high-resolution trace element analysis by LA-ICP-MS, as well as the advances in MC-ICP-MS technique for Ge isotopes in sulphides, has enhanced studies in this field. Ge isobaric interferences, standard calibration and data interpretation remain outstanding issues needing to be addressed for more precise and comprehensive LA-ICP-MS analyses.
An integrated mineralogical and geochemical study was carried out on typical sphalerite (ZnS) samples from the main Ge deposit in western Europe: the vein-type Zn–Ge–Ag–(Pb–Cd) deposit of Noailhac – Saint-Salvy (Tarn, France). In situ coupled measurements of trace elements and S isotopes were performed using LA-ICP-MS and SIMS, respectively, together with bulk Ge isotopes by MC-ICP-MS. Principal component analyses revealed element clusters antithetically distributed within distinct zoning types in sphalerite: sector zonings are enriched in Cu, Ge, Ga, Sb and As, whereas rhythmic bandings (dark brown bands primarily) are enriched in Fe, Cd, In and Sn. This typical distribution points to crystallographic controls on trace element uptake during sphalerite growth, occurring with concomitant microscale variations in fluid compositions at the fluid–crystal interface. Regardless of the zoning type, in all spots, Cu contents approach the sum of tri- and tetravalent cations (Ge, Ga, In, etc.) so that Cu could provide charge-balance for the entire set of coupled substitution mechanisms responsible for the incorporation of the whole range of trace elements in this sphalerite. Strong binary correlations suggest direct substitutions as Zn2+↔(Fe2+, Cd2+) and coupled substitutions as 2Zn2+↔Cu++Sb3+, 3Zn2+↔Ge4++2Ag+, and 3Zn2+↔In3++Sn3++□ (vacancy) despite no clear evidence for the presence of Sn4+.
δ74GeNIST3120a in bulk sphalerite varies from −2.07±0.37‰ to +0.91±0.16‰ (2σ SD) and positively correlates with bulk Ge content. This indicates considerable Ge isotopic fractionation within sphalerite during low-T hydrothermal deposition and zoning processes, associated with possible microscale open system fluid mixing. The trace element features in sphalerite from Saint-Salvy compared with those of other deposits confirm their use as discriminators among genetic types of ores (e.g., high In contents for magmatic-related deposits, and Ge for low-temperature deposits).
Understanding the evolution of metal in the protoplanetary disk is necessary to constrain the first steps of metal-silicate segregation and the early stages of the evolution of the protoplanetary ...disk. We measured the siderophile elemental compositions (PGE, Ni, Co, Fe, Cu, Ga, Ge) of individual metal grains in H ordinary chondrites by laser ablation inductively coupled plasma mass spectrometry to investigate their formation. We analyzed unequilibrated ordinary chondrites (H3) to constrain processes affecting the metal before accretion, and inferred the effects of metamorphism by comparing their elemental compositions to those of equilibrated chondrites (H4–H6). Our results highlight large variations of refractory (Re, Os, W, Ir, Ru, Mo, Pt) and moderately volatile siderophile element (Pd, Au, Ga, Ge) concentrations among metal grains in H3 samples that permit to classify them according to their Ge/Ir ratios and HSE contents. These intergrain variations are progressively homogenized in H4–H6 samples due to their increasing degrees of metamorphism. To constrain the origin of the metal, we modeled its evolution during melting and crystallization. Our melting model of a single metallic precursor containing 1.5 wt% C and up to 12 wt% S reproduces well the observed range of siderophile element compositions in the metal. Metal grains show a range of W, Mo, and Ga compositions that we interpret to reflect various local (grain-scale) oxidation states during the melting event(s) due to the heterogeneous distribution of various oxidizing components within the precursors. The very similar HSE compositions of H and L/LL metal grains suggests that the variations of bulk metal abundance and HSE concentrations observed among the different classes of ordinary chondrites (H, L, LL) result from the heterogeneous physical distribution of a relatively chemically homogeneous metal component among OC parent bodies, and not from a chemical (sensu lato) gradient between H and LL chondrites.
Ordinary chondrites (OCs) are classified into three groups, according to their oxidation state, which increases from the H to L to LL groups. This is demonstrated by the decrease in metal content ...(H = ∼8 vol%, L = ∼4 vol%, and LL = ∼2 vol%), and by a positive correlation between Δ17O and %Fa through the OC sequence. Compared to other chondrites, OCs exhibit the largest variation in oxidation state, but there is an ongoing debate on the processes that control this variation. To constrain the causes of the variations in the oxidation state with respect to the associated nebular versus parent bodies processes, we investigated the elemental and isotopic variations of germanium (moderately siderophile and volatile) in the bulk sample, as well as in the metal, silicate and sulfide phases, over a range of petrographic types for the H, L, and LL ordinary chondrites.
We found that δ74/70Gemetal is a proxy for the δ74/70Gebulk composition and that each OC group is distinguishable by their δ74/70Gemetal, which increases from −0.51 ± 0.09‰ for H chondrites, −0.31 ± 0.06‰ for L chondrites, and, finally, to −0.26 ± 0.09‰ for LL chondrites (2σ SD). Additionally, the OC sequence exhibited a positive correlation, from H to L to LL, between δ74/70Gemetal and %Fa, as well as oxygen isotopes (δ17O, δ18O and Δ17O), that was not a consequence of a “size sorting effect” on chondrules (i.e., chondrule mixing) or metamorphic processes in the parent bodies but, rather, was the result of nebular processes. We propose that the correlation between the δ74/70Ge values and %Fa, Δ17O, δ18O can be explained by an increasing proportion of accreted hydrated phyllosilicates, from the H, L to LL groups, with high δ74/70Ge and Δ17O. We found that 10 to 15% of phyllosilicates, with a composition of Ge = 4–7 ppm and δ74/70Ge = 3–2.5‰, is needed to change the δ74/70Ge from H to LL, which corresponds to a Δ17O ≈ 8–7‰. This value agrees with the Δ17O ≈ 7‰ composition of the accreted nebular component reported by Choi et al. (1998). During thermal metamorphism, phyllosilicates destabilize, liberating germanium that will be incorporated in the metal, then leading to its high δ74/70Ge signature.
High-temperature metamorphism can explain the lack of δ74/70Gemetal variation with the petrologic type in the OC, even for the type 3 chondrites (T ≈ 675 °C), implying a complete reaction even at low petrologic types. In addition, metal-silicate re-equilibration in response to thermal metamorphism results in a decrease in Δ74/70Gemetal-silicate from 0.33‰ to 0.06‰, within the H chondrite group, which is interpreted as the result of δ74/70Gesilicate variation. The mean positive Δ74/70Gemetal-silicate fractionation factor of +0.22 ± 0.36‰ (error propagation on individual error) also displays a remarkable similarity to the direction of isotopic fractionation with other germanium isotopic metal-silicate datasets, such as the magmatic iron meteorites, the Earth silicate reservoirs. We propose that the Δ74/70Gemetal-silicate and the negative δ74/70Ge values of OCs are inherited from metal-silicate melting and partial exchange before planetesimal accretion in a light isotope-enriched gas. Finally, the δ74/70Gemetal-Δ17Osilicate correlation between the IIE iron meteorites and OCs, provides new evidence for the existence of a highly reduced HH group.
Metal stable isotopes (e.g., Zn, Cd, and Cu) have been used to track metal sources in different types of hydrothermal systems. However, metal isotopic variations in sulphides could be triggered by ...various factors such as mineral precipitation and fluid mixing. Thus, tracking the metal sources of hydrothermal systems is still a big challenge for metal isotopes. In this study, we investigated the Cd isotopic systematics of sphalerite from the Nayongzhi Zn–Pb deposit, which is a Mississippi Valley‐type (MVT) deposit in the Sichuan–Yunnan–Guizhou mineralization province (SYGMP). We reinterpreted the published S isotope data for the SYGMP and found that the large S isotopic variations were controlled by Rayleigh fractionation between sulphide and reduced S. As such, a model that involves mixing of a metal‐rich fluid with a reduced S pool formed by thermochemical sulfate reduction (TSR) can explain the ore formation in the Nayongzhi deposit. Based on this model, no Cd isotopic fractionation was observed due to its low solubility in fluids during mixing, and thus the Cd isotopic variations of sphalerite were inherited from the source rocks. The large range of Zn/Cd ratios and uniform Cd isotopic compositions of the sulphides are similar to those of igneous rocks but different from those of sedimentary rocks, indicating that Zn and Cd were derived mainly from basement rocks (e.g., migmatite, gneiss, and granulite). Our results reaffirm that metal stable isotopes, particularly Cd isotope compositions of sphalerite, are powerful geochemical tracers for investigating the formation mechanisms of ore deposits.
Plain Language Summary
Metal stable isotopes, particularly Cd isotopes, have been widely used in investigating the metal sources, fluid evolution, and formation mechanisms of ore deposits. Here, we studied the Cd isotopic compositions of sphalerite from the Nayongzhi Zn–Pb deposit in the Sichuan–Yunnan–Guizhou mineralization province. The range of δ114/110CdNIST‐3108 value is smaller in the Nayongzhi deposit (−0.16–0.21‰), but the published S isotopic composition has significant variation (11.8–33.0‰). We found that the large S isotopic variations were controlled by Rayleigh fractionation between sulphide and reduced S. Thus, a mineralization model of the Nayongzhi deposit has been proposed, which involves the mixing of a metal‐rich fluid with a reduced S pool formed by thermochemical sulfate reduction. Based on this model, no Cd isotopic fractionation was observed due to its low solubility in fluids during mixing. Therefore, the Cd isotopic variations of sphalerite were inherited from the source rocks. Combining the Zn/Cd ratios and Cd isotopic composition characteristics of the sulphides, igneous rocks, and sedimentary rocks, it is indicated that Zn and Cd were derived mainly from basement rocks (e.g., migmatite, gneiss, and granulite).
Key Points
S isotopic variations caused by Rayleigh fractionation between sulphide and reduced S; Cd isotopic variations inherited from source rocks
Zn/Cd ratios and Cd isotopic compositions reveal that Zn and Cd were dominantly derived from basement rocks
The Nayongzhi deposit was formed by mixing between metal‐rich and reduced sulfur ore‐forming fluids
Synchrotron-based microscale X-ray absorption near edge structure spectroscopy (μ-XANES) has been combined with X-ray fluorescence (μ-XRF) mapping to investigate Ge, Cu and Fe oxidation states in ...compositionally zoned Ge-rich sphalerite from the Saint-Salvy deposit (France). The present study aims at improving our understanding of substitution mechanisms and trace element uptake relative to Ge isotope fractionation in sphalerite. K-Edge XANES records of various Ge-, Cu- and Fe-bearing sulphides are presented for comparison with sphalerite, and ab initio calculations at the Ge K-edge complete our experimental data. The Ge K-edge spectra of the Ge-bearing sphalerite are identical to those of germanite, renierite and briartite, indicating the presence of tetrahedrally-coordinated Ge4+. In addition, Cu and Fe K-edge spectra suggest the presence of Cu+ and Fe2+, respectively, in the tetrahedral site. No significant differences in the oxidation states of Ge, Cu and Fe were observed within or between the zoning types or between the samples. The intake of Ge4+ in sphalerite may therefore occur in the tetrahedral divalent metal site via coupled substitutions charge-balanced by monovalent elements such as 3Zn2+↔Ge4++2Cu+, resulting in a strong Ge–Cu elemental correlation, or, when Ge does not correlate with monovalent elements, through the creation of lattice vacancies such as 2Zn2+↔Ge4++□(vacancy). The tetravalent state of Ge is compatible with temperature-related Ge isotopic fractionation and can explain the large range of δ74Ge measured in the Saint-Salvy sphalerite. Moreover, the exceptional enrichment in Ge and the large variations in the ‘bulk’ Ge contents in these sphalerites do not appear to be related to charge effects but would instead result from the effect of temperature-related partitioning.
Silicon in iron meteorite metal PACK, Andreas; VOGEL, Ingo; ROLLION-BARD, Claire ...
Meteoritics & planetary science,
October 2011, Letnik:
46, Številka:
10
Journal Article
Recenzirano
Odprti dostop
– We report Si concentrations in the metal phases of iron meteorites. Analyses were performed by secondary ion mass spectrometry using a CAMECA 1270 ion probe. The Si concentrations are low ...(0.09–0.46 μg g−1), with no apparent difference in concentration between magmatic and nonmagmatic iron meteorites. Coexisting kamacite and Ni‐rich metal phases have similar Si contents. Thermodynamic calculations show that Fe,Ni‐metal in equilibrium with silicate melts at temperatures where metal crystallizes should contain approximately 100 times more Si than found in iron meteorites in this work. The missing Si may either occur as tiny silicate inclusions in metal or it may have diffused as Si‐metal into surrounding silicates at low temperatures. In both cases, extensive low‐temperature diffusion of Si in metal is required. It is therefore concluded that low Si in iron meteorites is a result of subsolidus reactions during slow cooling.
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•Cd isotope compositions of the largest hydrothermal system from the SYG.•Light Cd isotopes preferentially enriched in galena rather than sphalerite.•Cd was derived from the mixing ...between the Emeishan basalts and sedimentary rocks.•Cd isotope as a powerful geochemical tracer for hydrothermal systems.
The Sichuan–Yunnan–Guizhou (SYG) Zn–Pb metallogenic zone in SW China contains >400 carbonate-hosted hydrothermal Zn–Pb deposits. Some of these, such as the Huize, Tianbaoshan, and Daliangzi deposits, are super-large deposits with significant reserves of Cd, Ge, and Ag. However, the sources of these metals remain controversial. This study investigated the Cd isotopic geochemistry of the Huize deposit, the largest Zn–Pb deposit in the SYG area. Sphalerites formed at three stages in the deposit have different colors: black or dark brown (Stage I), red (Stage II), and light-yellow (Stage III). The δ114/110Cd values of the sphalerites are in the order Stage III < Stage I < Stage II. Kinetic isotopic fractionation is likely the key factor causing the lower δ114/110Cd values in the early formed Stage I sphalerites than in later-formed Stage II sphalerites, with cooling of ore-forming fluids being responsible for the still lower values of the Stage III sphalerites. In galena, the δ114/110Cd values are inversely correlated with Cd contents and tend to be higher in high-Zn galena. We speculate that Cd isotopic fractionation was significant during the precipitation of sphalerite and galena, with light Cd isotopes being enriched in galena rather than sphalerite. Comparison of the Cd isotopic signatures and Zn/Cd ratios of different endmembers suggests that the δ114/110Cd values and Zn/Cd ratios of sphalerite from the Huize deposit, as well as other large-scale deposits from the SYG area, are lie in those range of Emeishan basalts and sedimentary rocks and the mean δ114/110Cd values of these deposits show good negative correlation with 1/Cd, suggesting that the ore-forming materials of these deposits were derived from the mixing of Emeishan basalts and sedimentary rocks. This study demonstrates that Cd isotopes can be useful proxies in elucidating ore genesis in large Zn–Pb deposits.
The use of molybdenum as a quantitative paleo-atmosphere redox sensor is predicated on the assumption that Mo is hosted in sulfides in the upper continental crust (UCC). This assumption is tested ...here by determining the mineralogical hosts of Mo in typical Archean, Proterozoic, and Phanerozoic upper crustal igneous rocks, spanning a compositional range from basalt to granite. Common igneous sulfides such as pyrite and chalcopyrite contain very little Mo (commonly below detection limits of around 10 ng/g) and are not a significant crustal Mo host. By contrast, volcanic glass and Ti-bearing phases such as titanite, ilmenite, magnetite, and rutile contain significantly higher Mo concentrations (e.g., up to 40 µg/g in titanite), and can account for the whole-rock Mo budget in most rocks. However, mass balance between whole-rock and mineral data is not achieved in 4 out of 10 granites analyzed with in-situ methods, where Mo may be hosted in undetected trace molybdenite. Significant Mo depletion (i.e., UCC-normalized Mo/Ce < 1) occurs in nearly every granitic rock analyzed here, but not in oceanic basalts or their differentiates (Greaney et al., 2017; Jenner and O’Neill, 2012). On average, granites are missing ∼60% of their expected Mo contents. There are two possible reasons for this: (1) Mo partitions into an aqueous magmatic vapor/fluid phase that is expelled from cooling plutons, and/or (2) Mo is partitioned into titaniferous phases during partial melting and fractional crystallization of an evolving magma. The first scenario is likely given the high solubility of oxidized Mo. However, correlations between Mo/Ce and Nb/La in several plutonic suites suggest fractionating phases such as rutile or Fe-Ti oxides may sequester Mo in lower crustal rocks or in subducting slabs in arc settings.
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