Fractionated nickel isotopic compositions in the subducting slab – implications for mantle heterogeneity

• Significant δ 60/58 Ni heterogeneities found in metamorphosed mafic/ultramafic rocks. • Fluid mobile element enrichment correlated with lighter ẟ 60/58 Ni in metagabbros. • Extremely heavy isotopic compositions observed in abyssal serpentinites. • Seafloor alteration cannot explain the ẟ 60/58 Ni range observed. It has been hypothesized that Ni isotope heterogeneities in ultramafic xenoliths, mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) might originate from recycled components. Eclogitic and blueschist facies ultramafic, basaltic, and gabbroic lithologies from the SW Alps have Ni isotopic compositions (δ 60/58 Ni) extending from -0.5 to +0.3 ‰, overlapping well with the values found in oceanic basalts and mantle xenoliths. The Queyras blueschist facies metagabbros display the lightest Ni isotope compositions (-0.5 to +0.2 ‰), correlating negatively with enrichment in fluid mobile elements such as Li and Sb, consistent with isotopic fractionation during metasomatism. The Zermatt eclogites and Allalin gabbros define a more restricted range of δ 60/58 Ni, with metabasalts (-0.2 to +0.1 ‰) lighter than metagabbros (+0.1 to +0.3 ‰); the latter overlapping with published results for typical un-metasomatized lherzolites and harzburgites. Eclogitic serpentinites from Monvisoare similar (δ 60/58 Ni = 0.0 to +0.3 ‰). Only metasediments are isotopically heavy (δ 60/58 Ni = +0.1 to +0.7 ‰), except for one greenschist facies metagabbro from Chenaillet (+0.7 ‰). Published data for seafloor alteration of oceanic crust do not explain the variations. Indeed, abyssal serpentinites from the Atlantic Ocean, included for comparison as a putative Ni-rich recycled protolith, display highly variable δ 60/58 Ni (+0.3 to +1.2 ‰), heavier than any reported MORB, OIB or ultramafic xenoliths, and with an apparent hyperbolic relation with 187 Os/ 188 Os, possibly reflecting ancient sulfide removal or some form of alteration process in antiquity. Subduction of high pressure metamorphic mafic assemblages like those of the SW Alps provide an enriched source component with light δ 60/58 Ni that could contribute to the Ni isotope heterogeneity found in MORB, OIB, and peridotite and pyroxenite xenoliths.