Copper isotopic composition of the silicate Earth

Copper isotopes have been successfully applied to many fields in geochemistry, and in particular, as a strongly chalcophile element, the isotope systematics of Cu can be potentially applied as a proxy for crust-mantle and core-mantle differentiation processes. However, to date, the Cu isotopic composition of distinct silicate reservoirs in the Earth, as well as the behaviour of Cu isotopes during igneous processes and slab dehydration are not well constrained. To address these issues, here we report high-precision (±0.05‰; 2SD) Cu isotope data for 132 terrestrial samples including 28 cratonic peridotites, 19 orogenic peridotites, 70 basalts (MORBs, OIBs, arc basalts and continental basalts) and 15 subduction-related andesites/dacites sourced worldwide. The peridotites are classified into metasomatized and non-metasomatized groups, based upon their rare earth element (REE) patterns and the presence or lack of minerals diagnostic of metasomatism (e.g., phlogopite). The metasomatized peridotites span a wide range of δ⁶⁵Cu values from -0.64 to +1.82‰, in sharp contrast to the non-metasomatized peridotites that exhibit a narrow range of δ⁶⁵Cu from -0.15 to +0.18‰ with an average of +0.03±0.24‰ (2SD). Comparison between these two groups of peridotites demonstrates that metasomatism significantly fractionates Cu isotopes with sulfide breakdown and precipitation potentially shifting Cu isotopes towards light and heavy values, respectively. MORBs and OIBs have homogeneous Cu isotopic compositions (+0.09±0.13‰; 2SD), which are indistinguishable from those of the non-metasomatized peridotites within uncertainty. This suggests that Cu isotope fractionation during mantle partial melting is limited, even if sulfides are a residual phase. Compared with MORBs and OIBs, arc and continental basalts are more heterogeneous in Cu isotopic composition. In particular, basalts that were collected from a traverse across the Kamchatka arc over a distance of 200 to 400 km from the trench show a large range of δ⁶⁵Cu from -0.19 to +0.47‰, and samples with higher Ba/Nb ratios tend to be isotopically more heterogeneous. The large Cu isotopic variations in arc and continental basalts most probably reflect the involvement of recycling crustal materials in their sources.Collectively, the dataset obtained in this study suggests that the bulk silicate Earth (BSE) has an average δ⁶⁵Cu value of +0.06±0.20‰ (2SD).