Sulfide Dissolution on the Nickel Isotopic Composition of Basaltic Rocks

Global basaltic rocks show large variations in nickel (Ni) isotopic compositions relative to peridotites. The origin of this difference remains unknown. We analyzed Ni isotopic compositions of mineral separates from seven Beiyan peridotite xenoliths and 16 nephelinites from eastern China. Spinel is isotopically heavier, and clinopyroxene is systemically lighter than coexisting olivine and orthopyroxene. An ionic model predicts that olivine, orthopyroxene, and spinel are in Ni isotopic equilibrium, whereas clinopyroxene of metasomatic origin is out of equilibrium. The nephelinites have higher Fe3+/sigma Fe but lower delta Ni-60/58 values than the peridotites. Mantle silicate melting likely leads to enrichment of heavy Ni isotopes in melts, and, importantly, it cannot explain the negative correlation between delta Ni-60/58 and Fe3+/sigma Fe of the nephelinites. Therefore, the light Ni isotopic signature requires the involvement of a low-delta Ni-60/58 component. Sulfide is a minor Ni-rich component in the mantle and is isotopically lighter than the silicates. The relative proportion of Ni from sulfides vs. that from silicates varies in mantle-derived magmas, depending on the sulfur content at sulfide saturation and melting degree. Thus, low-degree melts with high abundance of dissolved sulfides can be variably enriched in light Ni isotopes. We propose that enhanced sulfide dissolution at high oxygen fugacity is a key reason for light Ni isotopic compositions in the nephelinites. This sulfide dissolution model may also be responsible for producing the Ni isotopic variation in global mafic lithologies. When the highly oxidized, low-degree melts react with the peridotitic region, they can impart light Ni isotopic signatures to mantle rocks.