Numerical modeling of major element distribution between chromian spinel and basaltic melt, with application to chromian spinel in MORBs

A set of empirical equations is developed which allows calculation of chrom ian spinet composition in equilibrium with a basaltic melt under a known se t of conditions. These equations are calibrated with published experimental data for the temperature range from 1,093 to 1,490 degreesC and a pressure of up to 20 kbar. It is demonstrated that the composition of chromian spin et from the experiments, which crystallizes from basaltic, boninitic, and k omatiitic melt and varies from high-Al to high-Cr, and high-Fe3+ can be suc cessfully reproduced using the suggested model. The composition of chromian spinet has been calculated using the glass composition for a set of primit ive basaltic lavas using the suggested set of empirical equations. Good agr eement between the calculated composition and composition of rims of chromi an spinet included in glass is achieved for compositionally diverse spinet from a Hawaiian sample and 15 MORB samples. The Fe2O3 content in the chromi an spinels is, however, often variable and higher than the calculated Fe2O3 content. Additional calculations using 76 published MORB glass analyses re produced most of the MORB chromian spinet range, except for the most Cr- an d Mg-rich. The crystallization of these Cr-rich chromian spinels likely occ urred 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 inc lusions in these Cr- and Mg-rich chromian spinet microphenocrysts.