Oxygen Isotope Changes During Mica Alteration

Abstract

Abstract--Oxygen isotope analyses (bOtg) of micas that were artificially depleted in K* indicate little or no isotope exchange during the transformation. The oxidation of iron in K-depleted, iron-rich micas by H202 treatment resulted in 1.6 to 4.6~ decrease in ~O ~8 due to the fact that the equilibrium fracfionation factor is less than the initial difference between the starting 50 ~s of the fluid and micas. The oxygen isotope ratio ofa saponite formed by the weathering ofphlogopite showed a 9.7~ increase in bO ta due to authigenic recrystallization. These results suggest that oxygen isotope ratios can be used to determine the nature of chemical transformation during the weathering of mica to vermiculite and/or smectite.