Abstract

Osorno volcano (41 degrees 06'S, 72 degrees 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 Cordon 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 degrees C to 1140 degrees C, with an oxygen fugacity buffer of similar to Delta QFM +1.1, dissolved water concentrations of up to 5.6 wt. % (average of similar to 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 Sr-87/Sr-86 and Nd-143/Nd-144 isotope ratios, allows us to consider variations in slab-derived fluid input and the minor role of crustal contamination on the Osorno eruptive products and those from neighboring 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 centers built over the major regional Liquilie-Ofqui Fault Zone.