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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.

BACKGROUND In recent years, copper isotopes have been widely applied in supergene environments and biogeochemical processes, acting as novel tracers for soil pollution and biogeochemical cycles during pedogenesis. To date, Cu isotope studies on natural soils have commonly analyzed basalt geostandards for monitoring data quality. However, the contents of copper, matrix ions, and organic matter in the soil and silicate rock are very different. For example, the copper content in silicate rock is >80μg/g, and the copper content in some soils is very low, that is, < 20μg/g. The use of silicate standard materials as reference samples to monitor the data quality of soil samples lacks representativeness. OBJECTIVES To provide new soil standards for high-precision Cu isotope analysis, this study reports high-precision copper isotope data for four soil reference materials (GBW07443, GBW07425, GBW07427, and GBW07389), expressed as δ65Cu relative to NIST SRM 976, which were measured using a multi-collector inductively coupled plasma-mass spectrometer (MC-ICP-MS). METHODS Soil samples were completely dissolved in high-pressure bombs in a muffle furnace. Complete separation of Cu from the matrices was obtained using a strong anion exchange resin (AG MP-1M). The instrument mass bias was corrected using the standard sample-standard method. RESULTS The long-term external reproducibility was higher than ±0.05‰ (n=306, 2SD). Cu isotopic compositions of the four soil reference materials, GBW07443, GBW07425, GBW07427, and GBW07389, from the China National Bureau of Standards were -0.04‰±0.04‰ (n=9, 2SD); -0.07‰±0.05‰ (n=12, 2SD); -0.06‰±0.04‰ (n=12, 2SD); and -0.02‰±0.06‰ (n=12, 2SD), respectively. CONCLUSIONS The δ65Cu values of these reference materials were close to zero and corresponded to the intermediate values of natural soils. Moreover, the sample was easy to obtain, and the experimental results showed uniformity in its chemical and copper isotopic compositions, making it suitable as a standard material for monitoring the reliability of the soil copper isotope chemistry and the mass spectrometry data.

Elucidating metal transport agents is the key to understanding the genesis of deposits and tracking the locations of concealed orebodies. Here, we integrate H-O-S-Cu isotopic data from the shear-zone-hosted Lingyun Cu deposit, China, as a means to fingerprint metal transport agents. Sulfide mineralization can be divided into early and late stages, which consist of chalcopyrite + bornite + quartz veins and chalcopyrite + bornite + ankerite veinlets, respectively. Both δ18Ofluid and δD values of fluid inclusions hosted by quartz (δ18Ofluid: 0.5‰ to 9.9‰, δD: –103.9‰ to –60.1‰) and δ65Cu values of sulfides (–1.85‰ to +0.39‰) from the early stage progressively decrease from the southeastern to northwestern portions of the Lingyun deposit, whereas sulfide δ34S simultaneously shifts toward heavier values (–14.4‰ to 5.0‰). The δ34S and δ65Cu values of sulfides from the late stage have restricted ranges from –11.2‰ to –9.3‰ and –0.30‰ to 0.05‰, respectively. The possibilities of meteoric water addition, water-rock interaction, inter-mineral Cu partitioning, diffusion, and oxidation could be ruled out as reasons for having caused systematic H-O-S-Cu isotope variations. Vapor-liquid separation resulted in preferential incorporation of light Cu, H, and O isotopes into the vapor phase. The decrease in oxygen fugacity in the fluids resulted in a shift toward heavier δ34S values as fluid flowed outward. Vapor-phases are the dominant transport agents for Cu in the Lingyun deposit, which may be widely applicable to shear-zone-hosted deposits. The direction of progressively increasing δ65Cu, δD, and δ18O values and decreasing δ34S values allows identification of potential locations of concealed orebodies. [ABSTRACT FROM AUTHOR]

The stable chromium isotope system has been widely used as a redox proxy to reconstruct the oxygenation history of ocean atmosphere systems. However, the Cr isotope mass balance in modern oceans (i.e., inputs and outputs) remains poorly constrained. To investigate the influence of seawater‐peridotite reaction on the global marine Cr isotope mass balance, we report high‐precision Cr isotope data (δ53Cr) on a series of fresh and altered abyssal peridotites from the Gakkel Ridge and the Southwest Indian Ridge (SWIR). The least altered peridotites give a δ53Cr value of −0.08 ± 0.06‰ (2SD, n = 4) for the oceanic mantle which is consistent with the established δ53Cr of the Bulk Silicate Earth. Compared to fresh peridotites, a subset of altered peridotites exhibit a loss of isotopically light Cr with relatively positive δ53Cr values (up to 0.04‰). These altered peridotites are characterized by significant Cr loss and likely have been subject to serpentinization. By contrast, seafloor weathering has limited influence on the Cr concentrations and isotopic compositions of the altered peridotites. Monte Carlo (MC) simulations of marine alteration suggest a net Cr flux into seawater from altered abyssal peridotites of ∼3.5 × 108 mol/yr, which is on the same order of magnitude as the riverine input flux of 108–109 mol/yr. Furthermore, the MC results suggest that the peridotite‐sourced net Cr flux has a negative δ53Cr signature (−0.33 ± 0.21‰, 2SD). Thus, seawater‐peridotite interactions must be considered when evaluating the modern oceanic Cr isotope mass balance. Plain Language Summary: The stable chromium isotope system is an emerging paleoredox proxy for tracing Earth's atmospheric oxygenation over geological timescales. However, accurately inferring past Earth's atmospheric oxygenation levels using chromium isotopes requires a more precise understanding of the elemental and isotopic cycling of chromium, particularly within the oceans. By studying abyssal peridotites which are rocks found beneath the oceanic crust, we find that specific types of alterations in these rocks, particularly the process of serpentinization, have an impact on the chromium isotopes. These findings provide insights into the mechanisms involved in transporting chromium from the deep Earth mantle to the oceans. We discover that these altered rocks release a flux of isotopically light chromium into the ocean, which is comparable to the amount coming from rivers. Our study sheds new light on the complex interactions between seawater and abyssal peridotites, which potentially exert a profound impact on the global marine Cr isotope mass balance. Key Points: The first data set of stable Cr isotopic compositions for abyssal peridotitesSeafloor weathering has insignificant impact on the Cr isotopic compositions of abyssal peridotitesSeafloor alteration exerts a key control on the Cr isotopic composition of seawater [ABSTRACT FROM AUTHOR]