ABSTRACT

Oxygen isotope ratios of phenocrysts separated from a rhyolite of San Vincenzo, Italy, were measured using the laser-probe technique. The phenocrysts (<3 mm diameter of alkali-feldspar, plagioclase, and quartz) were picked from a 0.5 to 1 mm thick section mounted on an adhesive tape. 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 reproducibility of the oxygen isotopic ratio (measurement error ±0.2‰) and chemical composition obtained for each separated phenocryst are comparable to those of conventional stable isotopic and chemical analyses. The δ¹⁸O values of the feldspars range from +11.7 to 14.6‰ (SMOW), while those of the quartz ranges from +13.8 to 14.4‰. The δ¹⁸O values of the feldspars decrease with increasing Or component. The large variation of oxygen isotope ratios may be attributed to oxygen isotopic heterogeneity of the source magma during crystallization, caused by magma mixing and assimilation.