The Magmatic Evolution and the Regional Context of the 1835 AD Osorno Volcano Products (41°06’S, Southern Chile)

Morgado, Eduardo; Morgan, Daniel J; Harvey, Jason; Castruccio, Angelo; Brahm, Raimundo; McGee, Lucy E; Parada, Miguel-Ángel; Georgiev, Bogomil and Hammond, Samantha J. (2022). The Magmatic Evolution and the Regional Context of the 1835 AD Osorno Volcano Products (41°06’S, Southern Chile). Journal of Petrology, 63(11), article no. egac105.

DOI: https://doi.org/10.1093/petrology/egac105

Abstract

Osorno volcano (41°06’S, 72°20’W) is a composite stratovolcano of the Central Southern Volcanic Zone of the Chilean Andes. It is the southernmost member of a NE–SW trending alignment of volcanic edifices including La Picada and Puntiagudo volcanoes and the Cordón Cenizos chain. According to contemporary descriptions recorded by Charles Darwin in 1835, two eruptive events occurred: the first during January–February, and the second during November–December 1835 and January 1836. The volcano erupted basaltic andesite lavas and tephra fall deposits (52.4 to 52.9 SiO2 wt. %) which contain phenocrysts of olivine, plagioclase, clinopyroxene, and spinel. The compositions of these phenocryst phases, together with those of olivine-hosted melt inclusions, allowed us to constrain intensive parameters for the pre-eruptive magmas. These varied from 1060 to 1140°C, with an oxygen fugacity buffer of ~ΔQFM +1.1, dissolved water concentrations of up to 5.6 wt. % (average of ~4.2 wt. %) and maximum pressures equivalent to ~7 km depth. Textural relations, such as crystal accumulations and clots, zoning in crystals and other indications of disequilibrium, lead us to infer the involvement of a crystal mush, rich in individual crystals and clots of crystals, which underwent a degree of disaggregation and entrainment into the transiting magma prior to eruption. Comparison of trace element abundances, including rare earth elements, fluid-mobile elements, and relatively fluid-immobile elements, combined with 87Sr/86Sr and 143Nd/144Nd isotope ratios, allow us to consider variations in slab-derived fluid input and the minor role of crustal contamination on the Osorno eruptive products and those from neighbouring volcanic systems. Our results suggest both a greater contribution from slab-derived fluid and a higher degree of partial melting in the systems supplying stratovolcanoes (Osorno, Calbuco, and La Picada) relative to those supplying small eruptive centres built over the major regional Liquiñe-Ofqui Fault Zone.

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