SYSTEMATICS OF CALCIUM PARTITIONING BETWEEN OLIVINE AND SILICATE MELT: IMPLICATIONS FOR MELT STRUCTURE AND CALCIUM CONTENT OF MAGMATIC OLIVINES

Abstract

A systematic characterization of the chemical factors that control calcium partitioning between olivine and melt in a magmatic environment was undertaken using experiments performed on compositionally simple systems (CaO-MgO-SiO2, CaO-MgO-Al2O3-SiO2, CaO-MgO-Al2O3-SiO2-Cr2O3, CaO-MgO-Al2O3-SiO2-TiO2, CaO-MgO-Al2O3-SiO2-Na2O, CaO-MgO-Al2O3-SiO2-FeO, CaO-MgO-Al2O3-SiO2-FeO-Na2O) over a wide range of temperature (1050-1530 °C) at one bar pressure. The calcium concentration of olivines is shown to be dependent not only on the forsterite content of the olivine but to a large extent on melt composition. For a fixed CaO content of the melt, these results show that the CaO concentration of olivine is strongly sensitive to the amount of alumina, alkali and ferrous iron present in the coexisting melt. Oxygen fugacity and temperature are not found directly to affect Ca partitioning. It is thus proposed that the systematic variations of the calcium content of olivine may be used as an "in-situ chemical potentiometer" of the lime activity of the melt. Based upon these data in synthetic systems, an empirical model describing Ca partitioning between olivine and melt is developed. When applied to natural olivines this model reproduces their Ca content, where melt composition is known, to within ±10% relative. The model may therefore be used to predict changes in melt composition during olivine crystallization and/or to assess whether an olivine is in equilibrium with its host magma. Finally, the wide range of Ca partitioning observed at fixed crystal composition confirms that minor element partitioning between crystal and melt cannot be predicted from the physical characteristics of the crystal alone, and that the non-ideality of the melt has to be taken into account.