Light calcium isotope anomaly in the Pitcairn mantle plume: a signal of pyroxenite melting

Abstract

The light calcium (Ca) isotope anomaly observed in ocean island basalts (OIBs) compared to mid-ocean ridge lavas has typically been attributed to recycling of carbonate-bearing sediments, partial melting of garnet-rich lithologies or a combination of both. This study presents Ca isotopic data for lavas from Pitcairn Island and nearby seamounts, yielding a δ44/42Ca variation (0.30 ± 0.02 ‰ to 0.40 ± 0.02 ‰) similar to that found in global OIBs. This δ44/42Ca variation cannot be attributed to post-eruption alteration, magmatic differentiation or recycling of carbonate-bearing sediments. Instead, correlations of δ44/42Ca with Sr/Nd and Eu/Eu* suggest that the light Ca isotope anomaly in Pitcairn lavas most likely reflects derivation from a pyroxenite source akin to the lower part of the recycled oceanic crust. Modelling suggests that the low δ44/42Ca end member can be explained by multiple-stage pyroxenite melting without requiring recycled carbonate-bearing sediments, offering new insights into using Ca isotopes to trace crust-mantle interactions.