ION MICROPROBE ANALYSIS OF OXYGEN-ISOTOPE RATIOS IN QUARTZ FROM SKYE GRANITE - HEALED MICRO-CRACKS, FLUID-FLOW, AND HYDROTHERMAL EXCHANGE

Quartz grains in hydrothermally altered granites from the Isle of Skye are highly heterogeneous and not equilibrated in oxygen isotope ratio at the 20 mu m scale. Ion microprobe analysis of one grain shows a gr adient of 13 parts per thousand over 400 mu m and a greater range in d elta(18)O than all quartz previously analyzed on the Isle of Skye. Oth er crystals from the same outcrop are homogeneous. Digitized cathodolu minescence images reveal patterns of magmatic zoning and brittle fract uring not otherwise detectable. The ion probe analysis correlates low delta(18)O values on a micro-scale to one set of healed cracks. Thus, quartz exchanges oxygen isotopes primarily by solution and reprecipita tion along fractures, in contrast to more reactive feldspar that appea rs to exchange from the grain boundary inward. Macroscopic models of i sotope exchange are not realistic for these rocks; the flow of hydroth ermal fluids was heterogeneous, anisotropic and crack controlled.