Copper Isotope Variations During Magmatic Migration in the Mantle: Insights From Mantle Pyroxenites in Balmuccia Peridotite Massif

Abstract

AbstractThe crust shows large variations in Cu isotopic composition (δ65Cu relative to NIST 976) resulting from redox reactions and other supergene processes. The significant range of δ65Cu in mantle peridotites thus could be ascribed to recycled crustal materials and/or oxidative mantle metasomatism. However, the influence of normal magmatic fractionation processes on δ65Cu variations remains poorly understood, and it is unclear whether the magmatic processes also lead to large δ65Cu variations in mantle rocks. To address the issue, we present bulk‐rock δ65Cu of fresh mantle pyroxenites from the Balmuccia peridotite massif, Northern Italy. These pyroxenites formed by melt‐peridotite reaction and mineral accumulation from MORB‐ to OIB‐like, sulfide‐saturated basic magmas. Mass balance calculations show that sulfide phases host >98 wt % of the Cu budget of bulk rocks (87−484 μg/g). The pyroxenites show significant variations of δ65Cu from −0.66‰ to 0.66‰, which cover the range known for peridotites, including metasomatized ones. Three subsamples from the same pyroxenite layer display >0.3‰ decrease in δ65Cu with progressive differentiation. Nevertheless, δ65Cu does not display correlations with indicators of magmatic differentiation (e.g., Mg# and Cu/Pd) for all pyroxenites in this study. This might be attributed to variable extents of magmatic sulfide segregation for different samples and/or varying δ65Cu of the pyroxenite parent magmas, which were also affected by reactive migration through peridotites. The combined processes can change δ65Cu of evolving magmas and reacted peridotites and lead to Cu isotopic heterogeneity in the mantle, without involvement of oxidative metasomatism or recycled crustal materials.