OXYGEN-ISOTOPE HETEROGENEITY OF PHENOCRYSTS IN RHYOLITE FROM SAN-VINCENZO, ITALY, BY LASER MICROPROBE ANALYSIS

Oxygen isotope ratios of phenocrysts separated from a rhyolite of San Vincenzo, Italy, were measured using the laser-probe technique. The ph enocrysts (<3 mm diameter of alkali-feldspar, plagioclase, and quartz) were picked from a 0.5 to 1 mm thick section mounted on an adhesive t ape. One to 2 mg samples were used for oxygen isotope analysis, while the major element chemistry of a small portion separated from the same phenocryst was determined by electron microprobe. The accuracy and re producibility of the oxygen isotopic ratio (measurement error +/-0.2 p arts per thousand) and chemical composition obtained for each separate d phenocryst are comparable to those of conventional stable isotopic a nd chemical analyses. The delta(18)O values of the feldspars range fro m +11.7 to 14.6 parts per thousand (SMOW), while those of the quartz r anges from +13.8 to 14.4 parts per thousand. The delta(18)O values of the feldspars decrease with increasing Or component. The large variati on of oxygen isotope ratios may be attributed to oxygen isotopic heter ogeneity of the source magma during crystallization, caused by magma m ixing and assimilation.