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+/ΣFe but lower δ60/58Ni 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 δ60/58Ni and Fe3+/ΣFe of the nephelinites. Therefore, the light Ni isotopic signature requires the involvement of a low‐δ60/58Ni 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. Plain Language Summary: There is scant information on why global basaltic rocks show large Ni isotopic variations relative to peridotites. We present Ni isotopic data for mineral separates from peridotite xenoliths as well as nephelinites from eastern China. The olivine, orthopyroxene, and spinel are in Ni isotopic equilibrium, whereas clinopyroxene of metasomatic origin is out of equilibrium. Compared to the normal mantle value, the nephelinites have lower Ni isotope values, which are negatively correlated with Fe3+/ΣFe. This signature cannot be produced by mantle silicate melting. Instead, the data suggest enhanced dissolution of isotopically light sulfide into the mantle melts at high oxygen fugacity. Recycling of carbonate sediments can induce incipient mantle melting and increase the oxygen fugacity. The oxidized, low‐degree melts, as represented by the nephelinites, have higher sulfur content at sulfide saturation, leading to a higher proportion of sulfide‐derived Ni in the melts, and thus lighter Ni isotopic compositions. When such melts react with the peridotitic region, they can impart light Ni isotopic signatures to mantle rocks, and potentially lead to the newly formed minerals being out of Ni isotopic equilibrium with primary ones. This sulfide dissolution model may also be responsible for producing the Ni isotopic variation in global mafic lithologies. Key Points: The clinopyroxene is enriched in lighter Ni isotopes, and is out of equilibrium with other silicate minerals due to metasomatic originThe nephelinites have lower δ60/58Ni values, because enhanced sulfide dissolution at high fo2 introduces light Ni isotopes to the meltsSulfide dissolution is an important step in producing the Ni isotopic variations observed in global mafic rocks [ABSTRACT FROM AUTHOR]