Archean Pb isotope variability tracks crust-mantle fractionation, granite production, and ore deposit formation
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Various geological processes that affect Earth's crust may be encoded into isotopic tracers preserved in rocks and minerals. The enhanced sensitivity of U, Th, and Pb to crustal fractionation processes allows Pb isotopes to complement information from the Nd and Hf isotope systems. However, melt fractionation, crustal contamination and recycling, hydrothermal fluid flow and fluid-rock interaction, and other processes in the continental crust can lead to mixing of Pb isotopic signatures. Here, we report new Pb isotopic data from granite-K-feldspar and integrate these data with published Pb isotope ratios from granite K-feldspar and Pb-rich ores across the Yilgarn Craton in Western Australia. The aim of this study is to explore how the variability of Pb isotope ratios and derivative parameters can be used to gain information on specific geological processes occurring throughout the crustal column. We develop a model that subdivides different sampling media into chemical process groups and links their initial Pb signatures to Pb source regions and fractionation processes at various locations within the crust and upper mantle. Equilibration of Pb signatures with a primary mantle source reservoir (in part represented by volcanic-hosted massive sulfides) is contrasted with granite formation in the mid to lower crust (granite K-feldspar), and mineralization of ore deposits in the mid to upper crust (Pb-rich ores). Spatial trends similar to those in Nd and Hf isotopic data are recorded by Pb isotopic derivative parameters (μ = source 238U/204Pb, ω = 232Th/204Pb, and Δt - the difference between true sample age and Pb model age) calculated for komatiite-hosted Ni sulfide ores, granite K-feldspar, and volcanic-hosted massive sulfide (VHMS) ores. The significance of subtle differences in absolute values of derivative parameters is supported by the diversity of Pb isotope ratios, Pb model ages, and ∆t as tracked by a statistical metric, quantifying the variability of Pb sources involved in the formation of different chemical process groups. Generally greater variety in an older terrane (Youanmi) documents more ancient and recycled continental crust as compared with more homogeneous Pb isotopic signatures in a younger terrane (Eastern Goldfields Superterrane). The Pb signatures are interpreted, in part, to relate to the timing of source fractionation in the upper mantle with a legacy of this source signal preserved through various depths in the lithospheric section. The least radiogenic VHMS ore samples appear to provide a good approximation of mantle Pb signatures, indicated for example by a deficit in 206Pb and 208Pb relative to the other process groups. A significant heterogeneity recorded in Pb isotopic data from Pb-rich gold ores is explained by the interplay of hydrothermal fluids with diverse sources leading to the mineralization of gold deposits (e.g., leaching of Pb from surrounding rocks or fluid mixing). Such gold ore Pb signatures are distinct from other process groups, which together track sources less heterogeneous in age and/or U and Th.Keywords:
Isotopic signature
Silicic
Stable Fe isotopes provide a potential new tool for tracing the biogeochemical cycle of Fe in soils. Iron isotope ratios in two redoximorphic soils were measured by multicollector inductively coupled plasma mass spectrometry to study the relationships between pedogenic Fe transformation and redistribution processes, and mass‐dependent fractionation of Fe isotopes. Redoximorphic Fe depletion and enrichment zones were sampled in addition to the bulk soil samples. A three‐step sequential extraction procedure was used to separate different Fe pools, which were examined in addition to total soil digests. Significant enrichments of heavy Fe isotopes of about 0.3‰ in δ 57 Fe were found in total soil digests of Fe‐depleted zones compared with bulk soil samples and were explained by the preferential removal of light isotopes, presumably during microbially mediated Fe oxide dissolution under anoxic conditions. Accordingly, pedogenic Fe enrichment zones were found to be slightly enriched in light Fe isotopes. Distinct Fe isotope variations of >1‰ in δ 57 Fe were found between different Fe pools within soil samples, specifically enrichments of light isotopes in pedogenic oxides contrasting with heavy isotope signatures of residual silicate‐bound Fe. Our data demonstrate that pedogenic Fe transformations in redoximorphic soils are linked to isotope fractionation, revealing greater mobility of lighter Fe isotopes compared with heavier isotopes during pedogenesis. No simple quantitative relationship between Fe depletion and isotope fractionation could be inferred, however. Our findings provide new insights into the behavior of Fe isotopes in soils and contribute to the development of Fe isotopes as a tracer for the biogeochemical Fe cycle.
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Biogeochemical Cycle
TRACER
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Abstract To investigate the mechanisms of the offset of seawater radiogenic Nd‐Hf isotope compositions from those of the upper continental crust rocks, combined Nd‐Hf isotope compositions of desert and loess samples from northern China (which integrate a wide range of lithologies and ages of continental rocks) are presented in this study. The results show significant and systematic fractionation of Hf isotopes between fine‐grained detritals/leachates (<5 µm) and coarser fractions (<75 µm) of the same samples. A small but systematic difference of Nd isotope compositions between leachates and detrital silicates is also revealed. Overall, the leaching data either plot along or slightly above the Nd‐Hf seawater array, providing strong direct support that the seawater Nd‐Hf isotope relationship is predominantly generated by weathering of upper continental crust. Our study supports the application of dissolved Hf isotopes as a proxy for different modes of weathering regimes rather than for continental source provenances.
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Nitrogen isotope ratio of nitrate provides a powerful tool to investigate nitrate sources and cycling mechanisms. Although the use of an isotope ratio method for 15N/14N allows identifying the nitrate sources in rivers by estimating a seasonal variation of N-NO3 concentration, however, there are some restrictions. Nitrification, the conversion of NH4+ to NO3-, can proceed with significant nitrogen isotope fractionation, preferentially accumulating 14N in the produced NO3-, and can make it difficult to identify the nitrate source with a high proportion of the isotope δ15N. However, the uptake and assimilation of NH4+ and NO3- have the capability of affecting isotopic compositions of riverine nitrogen compounds, and this may hinder the determination of whether the impact of the nitrate source with a high proportion of the isotope δ15N reduces. In addition, this study demonstrates that nitrate nitrogen concentration may correlate with δ15NNO3 values both positively and negatively. Such correlations are the result of isotope effects during nitrogen transformation processes (e.g. nitrification and assimilation) and isotopic variability in the various nitrate sources. A comparison of NO3- concentration and δ15NNO3 can be used to further distinguish mixing from biological processing. However, in order to get a more precise answer regarding the nitrate sources, it would be useful to take both the data of nitrogen isotopes and data of oxygen isotopes present in nitrates.
Isotopes of nitrogen
Isotopic signature
Nitrogen Assimilation
Isotope Analysis
δ15N
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The last ten years have seen big progress and wide applications of a novel field,non-traditional stable isotope(NTSI)geochemistry,to high temperature geo-science studies.Invention of multi-collector-inductively coupled plasma-mass spectrometry(MC-ICP-MS)led to the big breakthrough of analytical methods for heavy stable isotopes.This contribution summarizes Li,Fe,and Mg isotope studies on igneous rocks and minerals,as representative of NTSI geochemistry.Li isotopes have been widely applied to the studies of mantle geochemistry,recycling of subducted materials,and metamorphism to constrain the source of magma and kinetic diffusion process.Fe isotope fractionation is related to partitioning of multi-valent Fe between Fe-bearing phases,which can occur in the course of mantle metasomatism,partial melting,and fractional crystallization.Mg isotopic compositions of igneous rocks most likely reflect the source signatures.Variation of Mg isotopic ratios of mantle peridotites is trivial and this provides a homogenous background for Mg isotope fractionation in low temperature processes.Furthermore,Cl,Si,Cu,Ca,and U isotopes are also promising in the future geochemical studies.Experimental studies and theoretical simulation for the mechanisms of isotope fractionation provide important guidances for understanding the NTIS data.Experimental studies show that light and heavy isotopes have different migration velocity at high temperature processes such as chemical diffusion,evaporation,and desublimation,which could produce significant kinetic isotope fractionation.Equilibrium isotopic fractionation could occur among mineral,melt,and fluid when chemical environment of the isotopes are different between the phases.Recent thermal diffusion and migration experiments on silicate material reveal a new mechanism of magma differentiation and isotope fractionation.Along a temperature gradient in silicate magma,large elemental variation and isotopic fractionation can occur,by which a wet andesite can even be differentiated to granite.This suggests that thermal migration could be important for continental crustal formation and evolution.If temperature gradient exists long enough during magma differentiation,thermal diffusion can produce significant stable isotope fractionation,which is contrast to the mechanism of traditional kinetic and equilibrium isotope fractionations.Such process can be fingerprinted by positive correlations among multi-stable isotopic systems.Due to thermal diffusion,concentration of material loaded or dissolved in the fluid is a function of Soret coefficient(ST).However,because ST is highly variable and sensitive to lots of factors,the basic physics of thermal diffusion is still poorly understood.As shown by Mg,Ca,and Fe isotope measurement of thermal diffusion experiments,isotope fractionation driven by temperature gradient is independent to the bulk composition and temperature of the system,suggesting that the difference of ST between two isotopes of the same element can be considered as a constant.This can simplify and help the studies on thermal diffusion and ST.
Equilibrium fractionation
Mass-independent fractionation
Isotope Geochemistry
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Metasomatism
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The continental crust is strongly depleted in copper compared with its building blocks-primary arc magmas-and this depletion is intrinsically associated with continental crust formation. However, the process by which Cu removal occurs remains enigmatic. Here we show, using Cu isotopes, that subduction-zone processes and mantle melting produce limited fractionation of Cu isotopes in arc magmas, and, instead, the heterogeneous Cu isotopic compositions of lower crustal rocks, which negatively correlate with Cu contents, suggest segregation or accumulation of isotopically light sulfides during intracrustal differentiation of arc magmas. This is supported by the extremely light Cu isotopic compositions of lower crustal mafic cumulates and heavy Cu isotopic compositions of differentiated magmas in thick continental arcs. Intracrustal differentiation of mantle-derived magmas and subsequent foundering of sulfide-rich mafic cumulates preferentially removes isotopically light Cu, leaving a Cu-depleted and isotopically heavy continental crust.
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Isotopes of chlorine
Isotopic signature
Mass-independent fractionation
Ashing
Isotope Analysis
Equilibrium fractionation
Thermal ionization
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Silicic
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Isotopic signature
Isotopes of nitrogen
Isotope Analysis
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