The calcium contents of forsterite crystals equilibrated with silicate melts in the system Na2O-CaO-MgO-Al2O3-SiO2 at 1 atm and 1250°−1450°C were determined by electron microprobe. Over the range of temperatures studied and for melts containing 1.3−18 weight percent CaO, the Ca content of olivine was found to depend linearly upon Ca concentration in the liquid, according to the relation

When evaluated as a Ca-Mg exchange equilibrium, the Ca distribution data show equally simple systematics: the molar ratio of oxides (CaO/MgO)ol/(CaO/MgO)liq is essentially constant at any given temperature and varies in log-linear fashion with 1/T, given by

The simple relationship determined in this study between Ca abundance in olivine and Ca content of coexisting silicate liquid should prove useful in the identification of natural forsteritic olivine crystals that have not equilibrated with their host magmatic liquids.

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