Silicic Eruption Transitions in the Laguna del Maule Volcanic Complex, Chile

Abstract:

Laguna del Maule (LdM) is one of the most fertile regions worldwide of rhyolite eruptions with more than 80 events in the last 25 ky (postglacial period). The rhyolite magma comes from discrete, ephemeral batches within a plagioclase-biotite rhyodacitic crystal-rich mush reservoir. We analyze the role of the conditions of magma batches, the conduit processes, and the magma degassing during ascent on the eruption styles and transitions of three postglacial rhyolites of Laguna del Maule. The ignimbrite of Laguna del Maule (rdm) is the first, most silicic, and most voluminous postglacial deposit of LdM. It is the only LdM rhyolite with mafic enclaves, An_40-60 plagioclase antecrysts, and hornblende as the main mafic mineral. The rhyolite of Los Espejos (rle) is the first rhyolite eruption after rdm, it began with a phreatomagmatic phase followed by a subplinian magmatic eruption. The subplinian phase formed a fall deposit which has dense pumice and an increasing fraction of obsidian clasts overlain by a lava flow. Both pyroclast juveniles and lava flow show unzoned plagioclase of An_22-23, a few An_30-40 plagioclase antecrysts and anhedral biotite. The latest LdM rhyolite, Las Nieblas, have a similar eruption sequence and mineral assemblage to rle, but it shows complex-zoned plagioclase phenocrysts of An_20-34, euhedral biotite and no antecrysts. The highly explosive rdm eruption is explained by a high storage temperature (830-875 °C) and subsequent heating by the inputs of mafic-to-intermediate magmas. By contrast, Los Espejos was triggered by cryptic hot fluid/melt recharges after a long (millennial) stable period of cotectic crystallization within the rhyodacitic mush; the lack of antecrysts and the wide An range in the rln plagioclase suggest that melt mobilization within a geochemically and thermally heterogeneous magma batch triggered the eruption. The high anisotropy, high pore connectivity and high permeability of the rle pumice clasts suggest that the explosive-effusive transition was dominated by an increasing total strain and an efficient magma outgassing during ascent. A comparison with recent subplinian eruptions suggests that the increasing formation of pyroclastic obsidians is related to a decrease of the eruption intensity and preludes the formation of lava flow on the order of days.