Calcium isotope compositions of subduction-related leucite-bearing rocks: Implications for the calcium isotope heterogeneity of the mantle and carbonate recycling in convergent margins

Abstract

Large Ca isotope variations recorded in mantle xenoliths and mantle-derived igneous rocks have been widely used to decode the origin of mantle heterogeneity, especially to trace recycled sedimentary carbonates. However, it is still unclear whether and how the subduction of sedimentary carbonate causes significant Ca isotope fractionation in the mantle in correspondence of convergent margins. In this study, we present Ca isotope data for sixteen silica-undersaturated ultrapotassic leucite-bearing rocks and their clinopyroxene (Cpx) phenocrysts from the “Colli Albani” volcano (Italy), and for six carbonate-bearing sediments from nearby Apennine Chain. These subduction-related volcanic rocks have been widely accepted as deriving from a mantle source that involves recycled carbonated sediments indicated by their enriched incompatible trace elements and radiogenic isotopes, and olivine geochemistry. The δ44/40Ca values (normalized to NIST SRM 915a) of leucite-bearing rocks are uniform (from 0.64 to 0.79 ‰, with one exception of 0.86 ‰) with an average of 0.72 ± 0.03 ‰, 2SE, N = 16, which is 0.12 ± 0.03 ‰ (2SE) and 0.22 ± 0.03 ‰ (2SE) lower than MORBs (0.84 ± 0.02 ‰, 2SE, N = 25) and BSE (0.94 ± 0.01 ‰, 2SE, N = 14) respectively. In contrast, the carbonate-bearing sediments show variable and notably low δ44/40Ca values, ranging from 0.44 to 0.77 ‰. The δ44/40Ca of Cpx (0.70 to 0.76 ‰) is indistinguishable from their host rocks (0.73 to 0.76 ‰). In addition, the lack of correlations between δ44/40Ca and SiO2, CaO, Ni and P2O5, and the Ca isotopic simulation result indicate that Ca isotope fractionation caused by fractional crystallization is insignificant. Given that the Ca isotope fractionation is limited during the melting of spinel wehrlite-like mantle source (within ∼0.04 to 0.11 ‰), the slightly low δ44/40Ca values of the Italian leucite-bearing rocks require 10 % to 20 % addition of carbonated sediments in their mantle source. This indicates that recycled carbonated sediments can cause slight δ44/40Ca heterogeneity (∼0.1 to 0.2) of the mantle wedge in local/regional scales.