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

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 delta Ca-44/40 of these MORB samples exhibit a narrow variation from 0.84 parts per thousand to 0.88 parts per thousand with an average of 0.85 parts per thousand +/- 0.03 parts per thousand, which are similar to those of reported MORBs (0.83 parts per thousand +/- 0.11 parts per thousand) and back-arc basin basalts (BABBs, 0.80 parts per thousand +/- 0.08 parts per thousand) in literature, but are lower than the estimate value for the bulk silicate Earth (BSE, 0.94 parts per thousand +/- 0.05 parts per thousand). The low delta Ca-44/40 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 delta Ca-44/40 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 delta Ca-44/40 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 delta Ca-44/40 values in basalts and Ca isotopic heterogeneity in Earth's mantle.