Isotope and chemical microsampling: Constraints on the history of an S-type rhyolite, San Vincenzo, Tuscany, Italy

Abstract

Fine scale heterogeneities in strontium isotope ratios, 40Ar- 39Ar ages, and chemical composition have been determined for individual mineral grains and enclaves in the rhyolites of San Vincenzo, Italy. The rhyolites are peraluminous with phenocrysts of alkali feldspar, plagioclase, biotite, and cordierite and have previously been divided into two groups on the basis of whole rock major element chemistry. Group B lavas are distinguished from Group A in having higher MgO and CaO contents as well as containing two pyroxenes and chilled latite enclaves. The Group B lavas show textural evidence for disequilibrium, such as resorption and sieve zones in feldspars. Laser fusion 40Ar- 39Ar dating of individual alkali feldspar crystals suggests that the eruption of these rhyolites took place 4.38 ± 0.04 Ma, although small amounts of excess argon are present in subhedral and cloudy crystals. The groundmass in the Group A sample has a higher initial 87Sr 86Sr (0.725) than that of the Group B (0.713). The Sr isotope compositions of chemically and texturally characterized individual grains reveal marked disequilibrium between minerals and the host glass as well as within-grain zonation. The feldspars and coarse-grained biotites have initial 87Sr 86Sr that are intermediate between those of the Group A and B glasses, with alkali feldspar generally having higher initial 87Sr 86Sr ratios than plagioclase. Chilled latite enclaves, which are present in only the Group B lavas, have relatively low initial 87Sr 86Sr ratios in the range 0.7082–0.7088. The mineral chemistry and isotope data indicate that restite does not form a significant portion of the crystalline assemblage and, therefore, cannot be the source of the strontium isotope disequilibrium. Xenocrysts mixed into the rhyolite from a mafic magma were identified as clinopyroxene megacrysts, clinopyroxene-orthopyroxene clots, orthopyroxene-plagioclase clots, and plagioclase with extensive sieve textures. These grains all have relatively low initial 87Sr 86Sr ratios. Other grains that may be xenocrysts are cordierites with cores containing inclusions of biotite, spinel, and sillimanite, and alkali feldspars with excess 40Ar. The strontium isotope disequilibrium in phenocrysts of plagioclase, alkali feldspar, and biotite is attributed to a changing melt chemistry that was too rapid for strontium diffusive equilibration. A model for the San Vincenzo magma chamber is proposed in which the Sr isotope evolution of the Group B rhyolites is due to magma mixing of a crustal melt with latite magma containing unradiogenic strontium. The variable initial strontium isotope compositions of Group A phenocrysts may be due to assimilation or crystallization along the interface between mingled Group A and B magmas.