SUBMILLIMETER SCALE ZONATION OF DELTA-O-18 IN QUARTZ AND FELDSPAR, ISLE-OF-SKYE, SCOTLAND

Laser probe analysis of oxygen-isotope ratios in submillimeter domains of quartz and feldspar in hydrothermally altered granite reveals complex intragranular zonation that can be contoured within individual grains. Two samples collected within 10 m of each other in the Maol na Gainmhich Epigranite (MGE), Isle of Skye, Scotland, are similar petrographically, yet display very different deltaO-18 zonation patterns. SK1 has isotopically zoned quartz (deltaO-18 = 1.8-8.2 parts per thousand), with intragranular gradients of deltaO-18 as great as 5 parts per thousand/mm, and individually homogeneous grains of K-feldspar (+/-0.2 parts per thousand) which also display a large range of values (deltaO-18 = -1.7 parts per thousand to -7.3 parts per thousands). Oxygen-isotope gradients of more than 11 parts per thousand/mm were measured in this sample. In contrast, SK4 has homogeneous quartz (deltaO-18 = 5.7 +/- 0.3, n = 14) and a zoned K-feldspar phenocryst (deltaO-18 = -3.3 parts per thousand to -6.9 parts per thousand). Isotopic heterogeneity in quartz correlates with the degree of chlorite alteration in adjacent hornblende grains. The isotopic heterogeneity revealed by the laser probe in single grains of quartz and feldspar is greater than the entire range of values displayed by bulk mineral analysis of samples collected throughout the MGE pluton, although the average values of deltaO-18 obtained by laser probe are identical to the average bulk values. Values in K-feldspar suggest exchange at 400-degrees-C with infiltrating meteoric water (deltaO-18 almost-equal-to -10). The marked differences in zonation profiles between samples on the outcrop scale suggest that permeability and isotope exchange during hydrothermal alteration was highly heterogeneous. These microanalytical data for quartz and feldspar provide the first oxygen-isotope evidence from pervasively altered granites that paleopermeability is caused at least in part by intracrystalline microfractures.