Calcium isotopic compositions of mid-ocean ridge basalts

Abstract:

Previous studies have demonstrated that Earth’s mantle has heterogeneous calcium isotopic compositions. But the reason why mantle has its heterogeneity remains uncertain. In general, δ^44/40Ca values of mantle xenolith samples have a variation of >0.45‰. While ultramafic rocks, especially dunites, have higher δ^44/40Ca values than volcanic rocks, and there is a positive correlation between δ^44/40Ca and Ca/Mg. These phenomena imply that the heterogeneity of Ca isotopic compositions of mantle xenolith samples might result from different degrees of melt extraction, as indicated by large Ca isotopic fractionation between co-existing clinopyroxene and orthopyroxene.

However, because ancient marine carbonate has its own unique calcium isotopic characteristics, recycling of even a small amount of ancient marine carbonates into the mantle could also cause the heterogeneity of Ca isotopes in Earth’s mantle. This could be the reason why oceanic island basalts (OIB) have lighter Ca isotopic compositions than the mantle xenolith. Thus, the lighter Ca isotopic compositions in the mantle source cannot only be ascribed to magmatic processes. Therefore, it is more important to know calcium isotopic characteristics during partial melting and oceanic crust contamination.

Mid-ocean ridge basalts (MORB) are formed from the partial melts of the upper mantle and are rarely affected by crustal contamination. Different types of MORB, including D-MORB, N-MORB and E-MORB, have experienced different degrees of partial melting and contamination of enriched end-members. Here we report calcium isotopic characteristic of different types of MORB, we believe it will be very helpful to understand the behaviors of Ca isotopes during partial melting and it is possible to provide further information to discover the reason why calcium isotopic compositions is heterogeneous in Earth’s mantle.

This work was supported by Natural Science Foundation of China (No. 41373007, No. 41490632 and No. 91328204) and State Key Laboratory of Isotope Geochemistry grants (SKLIG-JY-12-01, SKLIG-KF-12-01 and SKLIG-KF-12-02).