Calcium isotopic signatures of depleted mid-ocean ridge basalts from the northeastern Pacific

Abstract

A number of high-temperature processes (e.g., melt-rock reactions, metasomatism, partial melting) can produce significant Ca isotopic fractionation and heterogeneity in the mantle, but the mechanism for such fractionation remains obscure. To investigate the effect of mantle partial melting on Ca isotopic fractionation, we reported high-precision Ca isotopic compositions of depleted mid-ocean ridge basalts (MORBs) from the East Pacific Rise and Ecuador Rift in the northeastern Pacific. The δ44/40Ca of these MORB samples exhibit a narrow variation from 0.84‰ to 0.88‰ with an average of 0.85‰±0.03‰, which are similar to those of reported MORBs (0.83‰±0.11‰) and back-arc basin basalts (BABBs, 0.80‰±0.08‰) in literature, but are lower than the estimate value for the bulk silicate Earth (BSE, 0.94‰±0.05‰). The low δ44/40Ca signatures of MORB samples in this study cannot be caused by fractional crystallization, since intermediate-mafic differentiation has been demonstrated having only limited effects on Ca isotopic fractionation. Instead, the offset of δ44/40Ca between MORBs and the BSE is most likely produced by mantle partial melting. During this process, the light Ca isotopes are preferentially transferred to the melt, while the heavy ones tend to stay in the residue, which is consistent with the fact that δ44/40Ca of melt-depleted peridotites increases with partial melting in literature. The behavior of Ca isotopes during mantle partial melting is closely related to the inter-mineral (Cpx and Opx) Ca isotopic fractionation and melting mineral modes. Mantle partial melting is one of the common processes that can induce lower δ44/40Ca values in basalts and Ca isotopic heterogeneity in Earth’s mantle.