Calcium fractionation in zoned plagioclase from the Tobacco Root batholith, southwestern Montana

Abstract

Electron microprobe analyses for Si, Al, Na, Ca, K, and Fe in twenty samples of zoned plagioclase from the Tobacco Root batholith, Montana, indicate that partitioning of CaO between the melt and the growing plagioclase grain followed a logarithmic distribution. The analytical data also indicate a change in the logarithmic fractionation factor as the grains grew and the melt became more granitic. This is further substantiated by an increase in the initial fractionation factor in the more granitic samples.