The oxygen isotope compositions of calcite, diopside, dolomite, forsterite, garnet, K-feldspar, kyanite, plagioclase, quartz, and wollastonite were analyzed in suites of contact and regional metamorphic rocks using an ion microprobe. Spatial resolution was similar to 10 mu m. Precision, measured as the standard deviation of working standards averaged over the entire project, was 0.13-0.18 parts per thousand for three carbonate standards and 0.11-0.12 parts per thousand for two silicate standards. A total of 1176 analyses (excluding standards) were made of 73 minerals in 23 samples. Both intercrystalline and intracrystalline variability in delta O-18 is greater in contact than in regional metamorphic rocks. Of 27 minerals analyzed in contact metamorphosed rocks, 70% exhibit statistically significant grain-to-grain variability in delta O-18 over areas <= 1.41 cm(2) with the largest range in silicates and carbonates in a single sample of 7.4 parts per thousand (forsterite) and 10.6 parts per thousand (dolomite). Of 88 grains analyzed in two or more places in contact metamorphosed rocks, 32% exhibit statistically significant intracrystalline variability in delta O-18 with the largest range in a single silicate and carbonate grain of 3.1 parts per thousand (forsterite) and 10.1 parts per thousand (dolomite). In contrast, 44% of 45 minerals in regional metamorphic rocks exhibit significant grain-to-grain variability in delta O-18 over areas <= 1.17 cm(2) with the largest range in silicates and carbonates in a single sample of only 1.1 parts per thousand (plagioclase) and 0.9 parts per thousand (calcite). Only 6% of 144 grains analyzed in two or more places in regional metamorphic rocks exhibit significant intracrystalline variability in delta O-18 with the largest range in a single silicate and carbonate grain of only 1.5 parts per thousand (diopside) and 0.7 parts per thousand (calcite). The difference in intercrystalline and intracrystalline variability in delta O-18 between contact and region metamorphic rocks is explained by the longer duration and slower reaction rates of regional metamorphism rather than to differences in temperature. There is no significant difference in intercrystalline and intracrystalline variability in delta O-18 in regional metamorphic rocks among samples from the biotite, garnet, and kyanite zones. Calcite inclusions in forsterite, and calcite and quartz inclusions in garnet either have delta O-18 that is statistically indistinguishable from delta O-18 of the same mineral occurring as nearby matrix grains or have statistically significant lower delta O-18. No reversed isotope fractionations were measured between coexisting mineral pairs. Minerals in individual samples, however, exhibit a wide range in the degree to which they attained and preserve oxygen isotope fractionations consistent with metamorphic temperatures recorded by mineral equilibria. Processes that account for grain-scale departures from isotope exchange equilibrium include: (a) overstepping of prograde mineral reactions, (b) growth zoning in low-diffusivity minerals, (c) interaction of rocks with fluids at the peak of metamorphism and/or during cooling, (d) retrograde mineral reactions, and (e) closed-system isotope exchange between coexisting minerals during cooling. This study provides new information about (1) the degree to which a variety of textural changes experienced by rocks during metamorphism are associated with changes in delta O-18, (2) oxygen isotope homogenization at the outcrop scale among contrasting lithologies, (3) changes in delta O-18 with increasing grade of regional metamorphism, and (4) time scales of metamorphic process. (C) 2014 Elsevier Ltd. All rights reserved.
