A reevaluation of crystal-size distributions in chromite cumulates

Abstract

Although studies have shown that igneous cumulates can form by in situ crystallization without requiring crystal settling, it has not been demonstrated that crystal-size distributions (CSDs) are consistent with such a process. Plots of crystal-size fractions per unit volume vs. crystal 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 trends is similar to previously reported trends for other silicate and oxide phases in crystallizing magmas. However, the lack of smaller grain sizes indicates that the original population was altered during a postnucleation crystalaging period that resulted in the loss of the smaller size fractions, an interpretation consistent with textural and volume-balance evidence. The CSD trends imply that Cr was not concentrated by settling of chromite but was brought to the site of the growing chromite grains. Similarities between the Stillwater data and CSD trends for garnet in metapelites suggest that such trends are a characteristic feature of any geologic system undergoing crystal aging after an initial period of nucleation and crystal growth.