First-principles investigations of equilibrium calcium isotope fractionation between clinopyroxene and Ca-doped orthopyroxene

Equilibrium fractionation factors of Ca isotopes among mantle minerals are essential for using Ca isotopes as an important tracer in mantle geochemistry. Using the vibrational frequencies obtained by the first-principles calculations based on density functional theory (DFT), we calculated equilibrium fractionation factors of Ca isotopes between two major Ca-bearing minerals, clinopyroxene (cpx) and orthopyroxene (opx). Our results show that opx has much higher 44Ca/40Ca than its co-existing cpx even at high temperatures (e.g., 1273 K), agreeing with the observations on mantle peridotites. In addition to the well-known temperature effect, our calculations for the first time reveal that the inter-mineral Ca isotope fractionation factor between opx and cpx also has a strong dependence on the chemical composition of opx (i.e., Ca content). Specifically, it increases substantially with decreasing Ca/Mg (atomic ratio) of opx when Ca/Mgopx is lower than 1/15. Such compositional effect on inter-mineral Ca isotope fractionation provides a convincing interpretation to the different isotope fractionations between coexisting opx and cpx observed in Kilbourne Hole and San Carlos peridotites.