The liquid composition and crystallization history of the 1979 soufriere magma, St. Vincent, W.I.

Abstract

Electron-microprobe analyses of glassy melt inclusions in phenocrysts of tephra, lava and scoria samples of the basaltic andesite 1979 eruption of the Soufriere volcano, St. Vincent, reveal that the magma is the product of fractional crystallization of alkalic, high-alumina olivine basalt magma. Using differentiation trends defined by melt inclusion/matrix glass analyses, we have constructed a quantitative model of the crystallization history of the 1979 magma. Sulfur analyses of melt inclusions in selected phenocrysts and estimates of volatile contents “by difference” suggest that the Soufriere volcanic system was not closed with respect to volatiles before the 1979 eruption. The volatile content of rare “primitive” melt inclusions in olivine phenocrysts ranges from two to eight weight percent; the sulfur content of these inclusions ranges from 320 to 560 ppm and generally decreases with increasing silica content. The volatile content of common melt inclusions in plagioclase, orthopyroxene and clinopyroxene airfall phenocrysts, on the other hand, is probably less than two weight percent and sulfur contents are below the detection limit of 50 ppm. These melt inclusions contain appreciably lower volatile content than the most primitive olivine melt inclusions, suggesting that they may be samples of largely degassed magma. The low volatile content of most Soufriere 1979 eruption products inferred from microprobe data is consistent with the vulcanian style and small magnitude of the 1979 eruption, which probably involved mainly phreatomagmatic disruption of magma rising and cooling beneath the 1971 lava island. Rhyodacitic glass of rare banded pumices is observed to be the product of fusion of crustal xenoliths.