The mechanism of cation and oxygen isotope exchange in alkali feldspars under hydrothermal conditions

Abstract

The mechanism of re-equilibration of albite in a hydrothermal fluid has been investigated experimentally using natural albite crystals in an aqueous KCl solution enriched in 18O at 600°C and 2 kbars pressure. The reaction is pseudomorphic and produces a rim of K-feldspar with a sharp interface on a nanoscale which moves into the parent albite with increasing reaction time. Transmission electron microscopy (TEM) diffraction contrast and X-ray powder diffraction (XRD) show that the K-feldspar has a very high defect concentration and a disordered Al, Si distribution, compared to the parent albite. Raman spectroscopy shows a frequency shift of the Si-O-Si bending vibration from ~476 cm−1 in K-feldspar formed in normal 16O aqueous solution to ~457 cm−1 in the K-feldspar formed in 18O-enriched solution, reflecting a mass-related frequency shift due to a high enrichment of 18O in the K-feldspar silicate framework. Raman mapping of the spatial distribution of the frequency shift, and hence 18O content, compared with major element distribution maps, show a 1:1 correspondence between the reaction rim formed by the replacement of albite by K-feldspar, and the oxygen isotope re-equilibration. The textural and chemical characteristics as well as the kinetics of the replacement of albite by K-feldspar are consistent with an interface-coupled dissolution-reprecipitation mechanism.