Calcium isotopic fractionation during plate subduction: Constraints from back-arc basin basalts

Subduction related metasomatism has been proposed to be one of the most important processes that may cause Ca isotopic heterogeneities in the mantle. Back-arc basin basalts (BABBs) affected by different contributions of subducted materials have various geochemical compositions ranging from MORB-like to arc-like, so that they are ideal media to investigate the behavior of Ca isotopes during plate subduction. Here we report Ca isotopic compositions of a series of BABBs from the Lau Basin, Woodlark Basin and Coriolis Troughs in the southwestern Pacific. Our results show that delta Ca-44/40 of all the BABBs exhibit a narrow range of 0.73 +/- 0.07 to 0.89 +/- 0.06 parts per thousand with an average of 0.80 +/- 0.08 parts per thousand (2SD, N = 21), which are lower than the estimated value of the Bulk Silicate Earth (BSE, 0.94 +/- 0.05 parts per thousand), but are similar to that of MORBs reported in the literature (0.83 +/- 0.11 parts per thousand, 2SD, N = 21). Even though the mantle sources of BABBs studied here have been influenced by various amounts of slab-derived fluids and/or sediments, there is no systematic variation of delta Ca-44/40 with indicators of subduction contributions (e.g., Ce/Pb, Nb/U, Ba/La, Th/Yb, Sr-87/Sr-86 and Nd-143/Nd-144). This suggests that either there is no Ca isotopic discrepancy between subducted components and mantle, and/or the amount of Ca that is transferred from the subducting Pacific slab to the studied BABB mantle sources is insignificant. Water depths at which subducting sediments were deposited control the components of marine sediments that may be subducted, which in turn determine how subduction affects Ca isotopic composition of the mantle. Because the southwestern Pacific has deeper water depths than the carbonate compensation depth, pelagic clays dominate the contribution from the subducted slab rather than carbonates, which leads to the negligible Ca isotopic fractionation during subduction along these trenches. The lack of correlations of delta Ca-44/40 with geochemical indicators of magma differentiation (e.g., MgO, CaO/Al2O3, Sr/Nd, Eu/Eu*) suggests that intermediate-mafic magma differentiation has insignificant effects on Ca isotopic compositions of BABBs. Therefore, we conclude that the observed offset of delta Ca-44/40 (similar to 0.14 parts per thousand) between the mantle and BABBs studied here should be primarily controlled by Ca isotopic fractionation during partial melting without significant contributions from subducted materials. (C) 2019 Elsevier Ltd. All rights reserved.