Calcium isotopic fractionation between clinopyroxene and orthopyroxene from mantle peridotites

We report the first observation of Ca isotopic fractionation between co-existing clinopyroxene and orthopyroxene from Kilbourne Hole and San Carlos mantle peridotites. The 44Ca/40Ca in orthopyroxenes is 0.36 to 0.75‰ heavier than that in the co-existing clinopyroxenes. Using these isotopic constraints and the relative proportions of major Ca-bearing minerals in the upper mantle, the estimated 44Ca/40Ca of the upper mantle is 1.05 ± 0.04‰ heavier relative to NIST SRM 915a. This is slightly higher than our average for basalts (0.97 ± 0.04‰ heavier relative to NIST SRM 915a). Combined with published 44Ca/40Ca data on low temperature Ca-bearing minerals (calcite, aragonite and barite), we infer that the inter-mineral fractionation of Ca isotopes at both low- and high temperatures is primarily controlled by the strength of Ca–O bond. Accordingly, the mineral with a shorter Ca–O bond and a smaller Ca coordination number (i.e., stronger Ca–O bond) yields a heavier Ca isotopic ratio (i.e., higher 44Ca/40Ca). Since stable isotopes of major elements, such as Ca and Mg, exhibit small fractionations during igneous processes, the estimate of stable isotopic compositions of the bulk differentiated planetary bodies, including the Earth and the Moon, needs to take into account the relative proportions of major rock-forming minerals and their respective isotopic signatures.