Calcium Stable Isotopes of Tonga and Mariana Arc Lavas: Implications for Slab Fluid-Mediated Carbonate Transfer in Cold Subduction Zones

Abstract

Carbonation of oceanic lithosphere occurs widely during seawater alteration, and subducted carbonated oceanic lithosphere may play a key role in carbon recycling in subduction zones. Here we investigate the Ca isotopic composition of arc lavas from the Tonga rear arc and Mariana arc, western Pacific, to explore the effect of fluid-mediated carbonate transfer in subduction zones. Fresh basalts (0.84 +/- 0.01 parts per thousand, 2sd, n = 3) and dacites (0.84 +/- 0.10 parts per thousand, 2sd, n = 9) from the Tonga rear arc show indistinguishable delta Ca-44/40, reflecting negligible Ca isotopic fractionation during differentiation of hydrous arc magmas. More importantly, arc lavas from both the Tonga rear arc (0.84 +/- 0.09 parts per thousand, 2sd, n = 12) and Mariana arc (0.79 +/- 0.12 parts per thousand, 2sd, n = 9) display MORB-like delta Ca-44/40 values. The MORB-like delta Ca-44/40 of arc lavas indicates that the carbonates released from altered oceanic lithosphere (AOL) do not significantly modify the Ca isotopic composition of the mantle wedge, although extensive volcanic CO2 degassing at both arcs suggests that slab fluids might introduce abundant carbonate into the depleted mantle wedge. These results could be attributed to a limited Ca budget in the slab fluids added to the mantle wedge and/or homogenization effect of variable delta Ca-44/40 for the slab fluids. At cold subduction zones, a fraction of carbonates from the AOL may survive during slab dehydration and recycle into the deep mantle.