Oxygen and sulfur isotopic composition of volcanic sulfate aerosol at the point of emission

Volcanic sulfate aerosol is emitted from the vents of many active volcanoes, but its origin has not yet been explained. We report the first measurements of the isotopic compositions of near- vent volcanic aerosol sulfate and use the combined sulfur and oxygen isotope systematics of the aerosol and magma at Masaya volcano, Nicaragua, to draw preliminary conclusions about the production mechanism of near- source volcanic sulfate. The sulfate aerosol does not display a significant mass- independent oxygen or sulfur isotopic signature, which suggests that primary volcanic aerosol does not contribute to the larger mass- independent anomalies found in some ambient atmospheric aerosols and as preserved in ice cores. The oxygen isotope composition of the sulfate appeared to become heavier on the particle filters with increasing amount of sample collected and suggests that the delta(18) Ovalue of the sulfate aerosol is < 8 parts per thousand. The sulfur isotopic composition of the sulfate did not vary with sample loading on the filters and had a mean delta S-34 value of 7.7 +/- 0.8 parts per thousand, similar to that of the magma ( 6.6 +/- 0.2%). The low Delta O-17 value, the delta O-18 value of the magma ( 6.6 +/- 0.3 parts per thousand) and the high delta O-18 value of atmospheric oxygen ( 23.5 parts per thousand) suggest that known atmospheric oxidation pathways at ambient temperatures are not the major routes of production for this sulfate. Instead, the isotopic systematics of aerosol and magma are consistent with sulfate production either by high- temperature equilibration of the magmatic gas mixture with small amounts of atmospheric oxygen or by direct emission of SO42- from the magma.