Submillimeter scale zonation of δ18O in quartz and feldspar, Isle of Skye, Scotland

Abstract

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 δ 18 O zonation patterns. SK1 has isotopically zoned quartz ( δ 18O = 1.8–8.2‰ ), with intragranular gradients of δ 18 O as great as 5 ‰/mm and individually homogeneous grains of K-feldspar (±0.2‰) which also display a large range of values ( δ 18O = −1.7‰ to −7.3‰ ). Oxygen-isotope gradients of more than 11‰/mm were measured in this sample. In contrast, SK4 has homogeneous quartz ( δ 18 O = 5.7 ± 0.3, n = 14) and a zoned K-feldspar phenocryst ( δ 18O = − 3.3‰ to −6.9‰ ). 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 δ 18 O obtained by laser probe are identical to the average bulk values. Values in K-feldspar suggest exchange at 400°C with infiltrating meteoric water ( δ 18O $ ̃ −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.