Oxygen Isotope Changes during Mica Alteration

-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. Key Words--Mica, Oxygen isotopes, Potassium, Recrystallization, Saponite, Weathering. I N T R O D U C T I O N Oxygen and hydrogen isotope composi t ions of ocean sediments (Savin and Epstein, 1970) and Quaternary soils (Lawrence and Taylor, 1971) that consist largely of authigenic minerals are in isotopic equil ibrium with the coexisting waters. According to Savin and Epstein (1970) detrital clays, however, show no major oxygen isotope exchange with the coexisting waters, but these authors were unable to make a definitive statement about hydrogen isotope exchange of detri tal clays in sea water. Unweathered minerals in soil profiles developed on igneous and sedimentary rocks also undergo no appreciable oxygen or hydrogen isotope exchange with meteoric water in the weathering environment (Lawrence and Taylor, 1972). Thus, O18/O ~6 ratios may be used to distinguish authigenic minerals from detrital components because detrital minerals originating from igneous and metamorphic rocks have lower O~s/O ~6 ratios than most minerals formed at low temperatures (Hoers, 1980). The above statement, however, may not be applicable to assemblages found at high latitudes and high altitudes where meteoric waters are depleted in O ~8 (S. M. Savin, Case-Western Reserve Universi ty, Cleveland, Ohio, personal communicat ion, 1984). Quaternary soils commonly contain appreciable quantities of detrital micas and their t ransformation products, as well as varying amounts ofauthigenic minerals. Detrital micas transform to vermiculite and then to minerals of the smectite group (Jackson et aL, 1952). Copyright 9 1985, The Clay Minerals Society These mineralogical changes result in the replacement o f K + from micas by hydrated cations, with concurrent decrease in layer charge. The decrease in layer charge results from composi t ional changes such as the oxidat ion of iron, the ejection of structural cations such as Mg 2+, Fe 3+, and A13§ (Veith and Jackson, 1974), and the incorporat ion of Si in the layers (Sridhar and Jackson, 1974). The clays formed from the alteration of micas swell in water (Sridhar et al., 1972) and commonly have aluminous and/or ferruginous coatings (Roth et al., 1969). Although the changes in chemical composi t ion during the above t ransformations have been studied by many workers, the extent of isotope exchange between the aluminosil icate f ramework and the associated waters has received little or no study. The objective o f the present study is to determine the extent of oxygen isotope exchange in various structural layers during the weathering of micas to expanding layer silicates, both in nature and in the laboratory. E X PE RIM E N T A L