NUMERICAL MODELING OF MAJOR ELEMENT DISTRIBUTION BETWEEN CHROMIAN SPINEL AND BASALTIC MELT, WITH APPLICATION TO CHROMIAN SPINEL IN MORBS

Abstract

A set of empirical equations is developed which allows calculation of chromian spinel composition in equilibrium with a basaltic melt under a known set of conditions. These equations are calibrated with published experimental data for the temperature range from 1,093 to 1,490 C and a pressure of up to 20 kbar. It is demonstrated that the composition of chromian spinel from the experiments, which crystallizes from basaltic, boninitic, and komatiitic melt and varies from high-Al to high-Cr, and high-Fe3+ can be successfully reproduced using the suggested model. The composition of chromian spinel has been calculated using the glass composition for a set of primitive basaltic lavas using the suggested set of empirical equations. Good agreement between the calculated composition and composition of rims of chromian spinel included in glass is achieved for compositionally diverse spinel from a Hawaiian sample and 15 MORB samples. The Fe2O3 content in the chromian spinels is, however, often variable and higher than the calculated Fe2O3 content. Additional calculations using 76 published MORB glass analyses reproduced most of the MORB chromian spinel range, except for the most Cr- and Mg-rich. The crystallization of these Cr-rich chromian spinels likely occurred from a more primitive Al-poor melt than that of the 76 MORB glasses. An example of a more primitive glass can occasionally be found as glass inclusions in these Cr- and Mg-rich chromian spinel microphenocrysts.