The mafic Curacautín ignimbrite of Llaima volcano, Chile

Abstract

Mafic volcanism accounts for 80% of magmas erupted on Earth. Although the majority of these eruptions are effusive to Strombolian and fountain-fed, large explosive mafic eruptions do occur. This work uses the deposits and pyroclast textures from the 12.6 ka Curacautín ignimbrite eruption of Llaima volcano to constrain the conditions that drove this mafic explosive eruption and extrapolate the findings to provide insights into the conditions that promote large-volume, mafic explosive volcanism elsewhere. The Curacautín ignimbrite (Ci) consists of four massive coarse ash to lapilli tuff flow units; Unit 1 is at least 30 m thick in proximal exposures, and Units 2–4 range from 1 to 4 m thick. New 14C dates and field observations suggest the Ci is the result of a single eruptive episode at ~12.6 ka. A lack of fall deposits and presence of abundant clast agglutination suggests the Ci eruption was a boil over event. We estimate the proximal Ci tephra volume to be between 6 and 9 km3 (equivalent to 3.5–4.5 km3 DRE), which is less than previous estimates. Even with our lower estimate, the Ci is still larger than the Masaya Triple Layer, Pucón ignimbrite, Tarawera 1886, and Etna 122 BCE mafic eruptions.Average vesicularities of pyroclasts range from 43 to 71%, and all but one exposure have vesicularities ≤56%. Average phenocryst content is ≤1–3%, but plagioclase microlite crystallinities are between 29 and 44%, with volumetric number densities between 8.21 × 106 and 1.84 × 107 mm−3. Such high microlite content suggests high disequilibrium resulting from rapid magma ascent and decompression. We interpret that the combination of rapid ascent and increased magma viscosity due to the crystallization of microlites caused gases to remain coupled with the Ci magma. This, in combination with ash textures, suggests the Ci eruption explosivity was driven by brittle fragmentation. Assuming that mass eruption rates exceeded 2.0 × 10 8 kg s − 1 to produce complete column collapse, we estimate an eruption duration of ~15–17 h. This study further supports the interpretation that extensive microlite nucleation from rapid ascent can lead to large mafic explosive eruptions.