Evolution of the Ca isotopic composition of the mantle

Abstract

Calcium stable isotopes are mainly fractionated during low-temperature geochemical processes and have, therefore, the potential to trace weathering and incorporation of recycled carbonated material in the mantle. However, behavior of Ca isotopes during mantle processes over Earth's history is not well understood. In this study, we present Ca isotopic compositions of 27 spinifex-textured and olivine cumulate komatiites ranging in age from 2.41 to 3.55 Ga. The Ca isotopic composition of all spinifex textured komatiites analyzed is similar, with an average δ44/40Ca = 0.92 ± 0.16‰ (n = 7; 2SD) (permil deviation of the 44Ca/40Ca ratio from SRM915a). This value is identical, within error, to the previous estimates of the composition of the mantle. In contrast, olivine cumulates from the Weltevreden Formation in the Barberton Greenstone Belt are isotopically heavier compared to the rest of the komatiites studied. While igneous processes and sea-floor alteration unlikely account for this Ca isotopic signature, crystallization of an early crust from a global magma ocean is a possibility, in agreement with previously obtained O, Os, Nd, Hf isotopic data and trace elements systematics.