RB AND SR DIFFUSION IN ALKALI FELDSPARS, WITH IMPLICATIONS FOR COOLING HISTORIES OF ROCKS

Volume diffusion of Sr and Rb have been measured in orthoclase feldspar as well as Sr diffusion in albite, using isotope exchange, hydrothermal solutions, and analysis by ion microprobe. For water pressures of 1000 bars, the kinetics of diffusion parallel to the c-axis are given by the Arrhenius relations [GRAPHICS] Strontium transport in the Or is isotropic to within a factor of four. The measurements use a new procedure in which only the elements in question are present in the starting fluid. Cooling rates of igneous rocks can now be determined from closure temperatures, material balance, and the Rb-Sr dating method, based on the departures of the mineral data from the whole-rock isochron, for cooling rates less than approximately 100-degrees-C/my.