No measurable calcium isotopic variations of back-arc lavas across the Okinawa Trough

Abstract

Calcium (Ca) is an essential element closely connecting to the recycling of marine carbonate at subduction zones. Here, we report Ca isotopic compositions (denote as δ44/40Ca with respect to NIST SRM915a) of back-arc lavas collected across the Okinawa Trough. The δ44/40Ca values showed a narrow range with an average of 0.81 ± 0.08‰ (2SD, n = 13), almost identical to reported values of arc magmas, mid-ocean ridge basalts (MORBs), and other mafic rocks. The MORB-like δ44/40Ca values were dominantly attributed to the mantle partial melting. The accumulation of clinopyroxene and plagioclase has played a negligible role on the Ca isotopes, despite the collected lavas exhibited distinctive rock types ranging from basalt to dacite. The metasomatism by subduction components (slab-derived fluids, sediment melts) and/or crustal assimilation have imprinted trace elemental and HfNd isotopic signals on the Okinawa Trough lavas, while the Ca isotopes have not been significantly modified. Our work suggests that thermal structures and tectonic stages are not pivotal factors constraining the Ca isotopic characteristics of arc and back-arc magmas at subduction zones. The opportunity and possibility to discriminate carbonate Ca isotopic signals in subduction zones are obscured by the highly variable Ca isotopes and carbonate contents of subducted sediments, the homogenization effect of the mantle wedge, variable degrees of sediment melting, and the survival of sediment melts during petrogenesis.