The behavior of nickel isotopes during mantle melting

Abstract

Mantle-derived mafic rocks show relatively large variations in nickel (Ni) isotopes and mostly isotopically light compared to the bulk silicate Earth (BSE). Whether this signature is due to the source heterogeneity or controlled by melting processes has been a debatable issue. Here, we analyzed Ni isotopic compositions of 18 intraplate basalts from the Sabzevar region, northern Iran and 16 serpentinized peridotites from Cyprus and South China. The Sabzevar basalts were likely sourced from a sulfur-free/barren mantle domain with recycled pyroxenites involved, manifested by their high Cu contents (127–265 ppm), Fe_T/Mn (58.2–77.4) and Zn/Fe_T ratios (11.9–16.4). The Ni isotopic compositions of the Sabzevar basalts (average δ^60/58Ni = +0.15 ± 0.08 ‰; 2SD) are slightly heavier than the BSE value (average δ^60/58Ni = +0.11 ± 0.06 ‰; 2SD), consistent with equilibrium Ni isotope fractionation during mantle silicate melting as predicted by ionic model calculations. Our new data of serpentinized peridotites (average δ^60/58Ni = +0.16 ± 0.06 ‰; 2SD), together with previously reported data for oceanic sediments and metabasalts, suggest that recycled lithologies have mantle-like or relatively heavy Ni isotopic compositions. Thus, the isotopically light Ni in mafic rocks is unlikely to be caused by crustal recycling-induced mantle heterogeneity. Rather, the light Ni isotopic signature is caused by the dissolution of recycled sulfides into the mantle melts. We suggest two sulfide dissolution models (“dissolve and go” and “dissolve and equilibrium”) to describe the δ^60/58Ni and Cu systematics in terrestrial basalts. In both models, the initial sulfide content (S_initial) and oxygen fugacity (fO₂) exert a major control on Ni isotopic compositions of resulting melts. These two parameters vary among different geological settings. Basalts derived from mantle sources with high S_initial and fO₂ are characterized by light Ni isotopic compositions, whereas basalts sourced from sulfur-free/barren and reduced mantle domains are likely characterized by heavy Ni isotopic compositions, aligning with the characteristic observed in natural samples.