Calcium isotope compositions of arc magmas: Implications for Ca and carbonate recycling in subduction zones

Abstract

Calcium (Ca) is a fluid-mobile element and plays a pivotal role in regulating carbon cycling through the precipitation of calcium carbonate. Calcium isotopes have been proposed as a promising tracer for deep carbon recycling because of the higher Ca content and distinctive Ca isotope composition in carbonate relative to the mantle. To test the effect of carbonate recycling on Ca isotope composition in arc magma and determine the fate of carbonate in subduction zones, we analyzed a suite of magmatic rocks from Central America, Southern Lesser Antilles, and Central-eastern Aleutian arcs. These three island arcs have variable thermal regimes from intermediate to cool and sediment types from carbonate-rich to carbonate-poor. Thirty-six arc magmas display δ44/40Ca values from 0.70‰ to 1.00‰ which are similar to the range of fresh mid-ocean ridge basalts (MORBs). Their average δ44/40Ca (0.82 ± 0.12‰, 2SD, n = 36) is lower than the estimates of Bulk Silicate Earth (~0.94‰, BSE) and can be ascribed to the effect of mantle partial melting. Despite strong evidence of C and Sr isotopes that various degrees of carbonate metasomatism occurred in the source of these rocks, they do not show Ca isotope variation. Based on the flux model, the buffering effect of the mantle wedge can homogenize the different carbonate Ca isotope signals at subduction zones where carbonate recycling efficiency is lower than 30% or the carbonates are mainly brought by carbonate-bearing altered oceanic crust. For subduction zones with carbonate recycling efficiency higher than 30%, the MORB-like δ44/40Ca in arc magmas may also reflect the storage of Ca in the mantle wedge and the retention of Ca in the descending slab beyond sub-arc depth. Combined with the fact that carbonate Ca isotope signals have been observed in OIBs and intra-plate igneous rocks, our new data indicates that most subducted Ca may finally enter the deep mantle via corner flow or within the descending slab.