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

The crust shows large variations in Cu isotopic composition (delta Cu-65 relative to NIST 976) resulting from redox reactions and other supergene processes. The significant range of delta Cu-65 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 delta Cu-65 variations remains poorly understood, and it is unclear whether the magmatic processes also lead to large delta Cu-65 variations in mantle rocks. To address the issue, we present bulk-rock delta Cu-65 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 mu g/g). The pyroxenites show significant variations of delta Cu-65 from -0.66 parts per thousand to 0.66 parts per thousand, which cover the range known for peridotites, including metasomatized ones. Three subsamples from the same pyroxenite layer display >0.3 parts per thousand decrease in delta Cu-65 with progressive differentiation. Nevertheless, delta Cu-65 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 delta Cu-65 of the pyroxenite parent magmas, which were also affected by reactive migration through peridotites. The combined processes can change delta Cu-65 of evolving magmas and reacted peridotites and lead to Cu isotopic heterogeneity in the mantle, without involvement of oxidative metasomatism or recycled crustal materials.