Ca-Sr-Nd isotopic signatures of mid-ocean ridge basalts from the Central Indian Ridge and implications for recycled materials in the Indian Ocean mantle domain

Mid-ocean ridge basalts (MORBs) from the Indian Ocean have been demonstrated to have more enriched radiogenic isotopic compositions than MORBs from the Pacific and Atlantic Oceans. However, the origin of such anomaly is still under debate. To better constrain the genesis of the isotopic anomaly observed in Indian MORBs, we present a systematic Ca-Sr-Nd isotopic study of a suite of MORBs from the Central Indian Ridge (CIR) between 19 and 20°S and the adjacent Gasitao Ridge. The δ^44/40Ca values of these MORBs vary from 0.70 to 0.89 ‰, most of which are similar to those of reported Pacific and Atlantic MORBs. However, some samples from the northern part of the studied CIR region showed slightly lower δ^44/40Ca values than the average of the reported Pacific and Atlantic MORBs. Notably, these δ^44/40Ca values are correlated with SrNd isotopic ratios, which extend from the depleted MORB mantle to a component with lower δ^44/40Ca and more enriched SrNd isotopic compositions, suggesting the presence of an enriched component in the source of these Indian MORBs. Recent studies have proposed that lower continental crust has similar δ^44/40Ca values to the estimated Bulk Silicate Earth, indicating that the low δ^44/40Ca signatures of some Indian MORBs are unlikely to be produced by delaminated lower continental crust. Garnet in the residue and recycled carbonate-rich marine sediments with oceanic crust can cause lower δ^44/40Ca signatures in mantle-derived rocks. However, trace element ratios and our model calculations suggest that these two factors can hardly induce the correlated Ca-Sr-Nd trends in these Indian MORBs. Instead, recently reported subcontinental lithospheric mantle xenoliths that experienced silicate and/or carbonatite metasomatism, which also show low δ^44/40Ca and enriched SrNd isotopic compositions, correspond with the correlated Ca-Sr-Nd trends in the studied Indian MORBs. Moreover, ancient metasomatized subcontinental lithospheric mantle materials have been observed in abyssal peridotites from the Southwest Indian Ridge. Therefore, delaminated subcontinental lithospheric mantle maybe the most likely cause of the Ca-Sr-Nd isotopic anomaly in Indian MORBs. Our study suggests that combined Ca-Sr-Nd isotopes can be treated as potential tracers to the genesis of mantle heterogeneity.