A REEVALUATION OF CRYSTAL-SIZE DISTRIBUTIONS IN CHROMITE CUMULATES

Although studies have shown that igneous cumulates can form by in situ crystallization without requiring crystal settling, it has not been d emonstrated that crystal-size distributions (CSDs) are consistent with such a process. Plots of crystal-size fractions per unit volume vs. c rystal size for chromite grains from the Stillwater complex show a log -linear distribution with negative slope at larger sizes and a concave -down distribution at smaller sizes. The log-linear portion of these t rends is similar to previously reported trends for other silicate and oxide phases in crystallizing magmas. However, the lack of smaller gra in sizes indicates that the original population was altered during a p ostnucleation crystal-aging period that resulted in the loss of the sm aller size fractions, an interpretation consistent with textural and v olume-balance evidence. The CSD trends imply that Cr was not concentra ted by settling of chromite but was brought to the site of the growing chromite grains. Similarities between the Stillwater data and CSD tre nds for garnet in metapelites suggest that such trends are a character istic feature of any geologic system undergoing crystal aging after an initial period of nucleation and crystal growth.