On the mechanisms and kinetics of oxygen isotope exchange in quartz and feldspars at elevated temperatures and pressures

Stereological and scanning electron microscope (SEM) analyses of quartz-water and plagioclase feldspar–water interactions have been combined with ¹⁸O/¹⁷O and ¹⁷O/¹⁶O measurements of the associated isotope exchange, in an attempt to develop an understanding of mineral-water interactions at elevated temperatures and pressures. The quartz-water exchange (T = 250 to 600 °C; Ph₂o = 1 to 22 kbar) is dominated by an “Ostwald ripening” recrystallization mechanism, in which reprecipitated quartz develops as faceted overgrowths on original grains that have been rounded by initial solution processes. Correspondingly, there is a linear decrease in specific surface area (S̅) as a function of fractional extent of exchange (f). The unrecrystallized quartz cores exchange through diffusional mechanisms that also become dominant in later stages when recrystallization becomes retarded and increasingly ineffective as a mechanism of isotopic exchange. This transition from solution-precipitation to diffusional mechanisms accounts for a “plateau” pressure effect on exchange rate, observed in later stages of exchange. Substantial recrystallization does not occur in feldspar-water exchange at 500 and 600 °C, Ph₂o = 2 to 15 kbar; the feldspar grains rapidly degrade into smaller fragments and become incoherently coated by a small amount of fine precipitated crystals. Subsequently, isotopic exchange occurs without further textural change and at constant S̅. These observations, the preservation of Al/Si order in albite throughout exchange, the high calculated values of oxygen-diffusion coefficients and possible dependence of these coefficients on (Ph₂o)^½ indicate inhibition of feldspar recrystallization and are consistent with diffusional mechanisms for penetration of water into the feldspar and exchange with shared oxygen atoms in alumino-silica tetrahedra. Equilibrium ¹⁸O/¹⁶O fractionation factors for both quartz-water and albite-water agree well with those measured previously in this laboratory.