INSITU OXYGEN ISOTOPE ANALYSIS OF FELDSPAR AND QUARTZ BY ND-YAG LASER MICROPROBE

In situ delta-O-18 analyses have been reproducibly obtained +/- 0.5 parts per thousand (1-sigma) on 400-1200-mu-m diameter spots of plagioclase using a Nd:YAG laser microprobe system and BrF5. Over 100 analyses on plagioclase megacrysts from a single sample of Marcy Anorthosite were made to determine the optimum analytical conditions. The best results are obtained on 400-800-mu-m-thick wafers; less accurate and less precise results were obtained on centimeter-thick slabs. Analyses on some individual wafers display very high precision (below +/- 0.2 parts per thousand, 1-sigma) that are nearly as good as the precision of conventional analyses. However, such high precision cannot yet be systematically obtained in situ, and it is not certain whether it results from especially homogeneous domains within the sample or from differences in operating conditions. Oxygen-leaching and isotope fractionation along the edges of the laser-heated reaction pits may have a minor effect on precision. The combination of sub-millimeter spatial resolution and +/- 0.5 parts per thousand precision has not previously been obtained on feldspar, and provides the means to investigate fluid-rock interactions in a variety of geologic environments. The plagioclase in this study is the first sample for which oxygen isotope homogeneity has been evaluated in situ. The absence of isotopic zonation restricts theories on the roles of fluids associated with the Marcy Anorthosite Massif, New York, U.S.A. In contrast, replicate delta-O-18 analyses in quartz, K-feldspar and plagioclase grains in granite from the Siljan Ring astrobleme, Sweden, vary by > 1 parts per thousand, suggesting the existence of intragranular isotopic heterogeneity resulting from interaction with late meteoric fluids.