Significant δ44/40Ca variations between carbonate- and clay-rich marine sediments from the Lesser Antilles forearc and implications for mantle heterogeneity
Calcium isotopes can potentially be used to trace many geological processes. Characterizing the Ca isotopic compositions of marine sediments is a precondition for using Ca isotopes to trace the recycling of marine sediments into the mantle and mantle-derived rocks. The best way to achieve this goal is to investigate different types of marine sediments that are adjacent to subduction zones and would undergo subduction in the future. Here, we present high-precision Ca isotopic data for marine sediments from the Deep Sea Drilling Project (DSDP) Sites 543 and 144 along the Lesser Antilles forearc region, to constrain the Ca isotope signatures of marine sediments with various components and assess their possible effects on δ44/40Ca of the local mantle and arc lavas via subduction. Our results show that the δ44/40Ca values of these marine sediments vary from 0.10 to 1.21‰ and depend on lithology. Carbonate-rich sediments from Site 144 exhibit δ44/40Ca values in the range of 0.10-0.58‰ with a weighted average value of 0.36‰, which is ∼0.6‰ lower than that of the mantle. In comparison, δ44/40Ca of clay-rich sediments from Site 543 range from 0.79 to 1.21‰ with a mantle-like mean value of 0.96‰. The data obtained here suggest that carbonate- and clay-rich marine sediments differ significantly both in Ca isotopic compositions and CaO contents. Recycling of these sediments will result in different effects on the isotopic compositions of the local mantle and probably affect the local arc lavas. Specifically, carbonate-rich sediments from Site 144 with high CaO contents and light Ca isotopic compositions can alter the local mantle to lower δ44/40Ca signatures, which is expected in the southern Lesser Antilles arc lavas.