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Sulphur and chlorine geochemistry of mafic to intermediate tephras from the Chilean Southern Volcanic Zone (33-43°S) compared with those from the Central American Volcanic Arc

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  • Chemistry
  • Earth Science

Abstract

Here we present the first systematic investigation of volatile geochemistry along the Southern Volcanic Zone (SVZ) of Chile. Holocene olivine-hosted melt inclusions in the most mafic tephras sampled from 16 volcanoes along the volcanic front of the SVZ were analysed for pre-eruptive sulphur, chlorine and major element contents. These results are combined with trace element compositions of the host whole rocks. The highest fractionation-corrected gas contents occur in the least-degassed melt inclusions from small peripheral cones from both the transitional and the southern-central SVZ, reaching ~3000 μg/g S and 1400 μg/g Cl, while the lowest abundances of ~1000 μg/g S and ~600 μg/g Cl were found in the central SVZ at Volcán Lonquimay, Volcán Llaima, and Volcán Villarrica. Chlorine co-varies with trace element indicators for the degree of melting and/or source enrichment. The highest Cl enrichment is present in the melts with least degree melting, underscoring its incompatible behaviour. The lowest Cl contents are found in high-degree melts from the most depleted mantle sources. The size of the volcanic edifices correlates inversely with Cl abundances in the melt. In comparison with other subduction zones, the SVZ melt inclusions exhibit Cl abundances in the same range as most of those from the Central American and those from the Marianas arcs. In contrast, S concentrations are lower at the SVZ, with the exception of small peripheral cones of Los Hornitos next to Cerro Azul, Cabeza de Vaca near Osorno, and Apagado. The lower S contents are likely to be related to the generally more evolved nature of the SVZ lavas, compared to those from Central America and the Marianas.

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