The Influence of Magma Storage and Ascent Conditions on Laguna del Maule Rhyolite Eruptions

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Abstract
The scarcity of historical rhyolite eruptions means that volcanological and petrological studies of past eruptions are a key tool for assessing the potential for future hazardous activity and improving interpretations of unrest signals. For the last 18 ky, the Laguna del Maule volcanic complex (LdM) in Chile has erupted primarily rhyolites but with differing magma compositions, eruption styles and eruptive volumes. Rapid surface uplift and episodic seismic activity at LdM over the last two decades has emphasized the need to understand both the recent evolution of the magmatic system and the most likely future eruption scenarios. Using mineral composition, geothermobarometry and MELTS modelling, we assess the influence of the magma storage and ascent conditions on the magnitude and styles of three LdM rhyolites. Magmas of the first and largest Plinian-ignimbrite eruption (Laguna del Maule: rdm; >17 km3 DRE) form a distinct mineral assemblage with An28-60 plagioclase, amphibole and quartz derived from a magma plumbing system over a large pressure range (90-350 MPa). We suggest that the rdm eruption was triggered by magma recharge and overpressure within a ~90 MPa magma chamber of high-silica (>76.5 wt.% SiO2) rhyolitic melt. The rdm eruption appears to have reset the LdM storage conditions, such that subsequent rhyolite eruptions have been smaller (3 DRE), involved less evolved melt (2) and produced (sub)Plinian-effusive eruption sequences. Of these, we have studied the earliest (Los Espejos: rle) and the most recent (Las Nieblas: rln). Both have mineral assemblages with An20-34 plagioclase and biotite formed in water-saturated crystal-poor rhyolite batches over limited pressure ranges (ΔP ~ 50 MPa) within a rhyodacitic mush reservoir. We suggest that rle and rln were triggered by injection of water-rich magma and volatile exsolution; the transition to effusive behavior occurred because of changes in magma temperature (rle) or magma decompression rate (rln). Similarities in the limited pressure range and high volatile content inferred for the current low-density magma body with the petrologically-determined source conditions of magmas that fed past subplinian-effusive eruptions suggest that the next LdM eruption will be similar.

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