Stepwise variation of Ni isotopes in MORBs from the East Pacific Rise: A response to magma replenishment and differentiation

This study investigates Ni isotopic variations in mid–ocean ridge basalts (MORBs) and associated evolved lavas from the 9–10°N segment of the East Pacific Rise (EPR), to explore the behaviors of Ni isotopes during magma generation and differentiation. The measured δ^60/58Ni range from –0.08‰ to 0.93‰ and vary as a function of MgO content. Samples with MgO content between 9.5 and 6.5 wt% primarily underwent olivine, clinopyroxene, and plagioclase fractionation, exhibiting δ^60/58Ni ranging from 0.10‰ to 0.42‰. Most of these samples have values consistent with the estimated bulk silicate Earth (0.11 ± 0.06‰) and the globally averaged N–MORB (0.13 ± 0.14‰), with the maximum value being notably higher than the reference values. Evolved samples with MgO < 3.5 wt% show a strong enrichment in heavy Ni isotopes (δ^60/58Ni up to 0.93‰), which coincides with the appearance of Fe–Ti oxides on the liquidus.

Rayleigh fractionation models capture the Ni depletion and enrichment in δ^60/58Ni, but fail to replicate the observed isotopic invariance as magma differentiated from the early to intermediate stages. A coupled fractional crystallization–replenishment model incorporating repeated injection of primitive MORB melts better matches the observed δ^60/58Ni variations, suggesting that open–system processes moderate isotopic fractionation. Preferential incorporation of light Ni isotopes into sulfides leads to elevated δ^60/58Ni in the residual melt, while replenishment of primitive melt within the plumbing system can moderate this effect and maintain the δ^60/58Ni of the observed MORBs near the mantle baseline. Late–stage differentiation enhances heavy Ni isotope enrichment in evolved dacitic samples. Fe–Ti oxide crystallization should decrease δ^60/58Ni in the melt, but the separation of immiscible sulfide drives the enrichment of heavy Ni isotopes in the melt. Comparing with the less abundant variably enriched E–MORBs, MORBs from the EPR are commonly depleted in incompatible elements with higher δ^60/58Ni. Such Ni isotope signature reflects derivation from a geochemically homogeneous, depleted mantle source without input from recycled materials or extensive sulfide extraction in the source region. The contrasting Ni isotopic evolution trends between EPR MORBs and ocean island basalts highlight the timing and critical role of sulfide crystallization in controlling Ni isotope fractionation pathways.