Petrological Insights into Shifts in Eruptive Styles at Volcán Llaima (Chile)

Abstract

Tephra and lava pairs from two summit eruptions (AD 2008 and 1957) and a flank fissure eruption (� AD 1850) are compared in terms of textures, phenocryst contents, and mineral zoning patterns to shed light on processes responsible for the shifts in eruption style during typical eruptive episodes at Volcan Llaima (Andean SouthernVolcanic Zone, Chile).The mineralogy and whole-rock com- positions of tephra and lavas are similar within eruptive episodes, suggesting a common magma reservoir for Strombolian paroxysms and lava effusion. The zoning profiles and textures of plagioclase record successive and discrete intrusions of volatile-rich mafic magma accompanied by mixing of these recharge magmas with the resident basaltic-andesitic crystal mushes that are commonly present at shallow levels in the Llaima system. Each recharge event destabil- izes the plagioclase in equilibrium with the resident crystal mush melt and stabilizes relatively An-rich plagioclase, as is recorded by the numerous resorption zones. Lavas typically have � 15^20 vol. % more phenocrysts than the tephra. Differences in plagioclase and olivine textures and zoning, combined with different phenocryst contents, indicate that a greater volume fraction of recharge magma is present in the explosively erupted magma than in subsequent effu- sively erupted magma. We propose that Strombolian paroxysms at Volcan Llaima are triggered by interactions with large volume frac- tions of recharge magma, which decrease the bulk viscosity and in- crease the volatile contents of the erupted magmas, leading to the conditions required for the fragmentation of basaltic-andesite. Lava effusion ensues from reduced interactions with the recharge magma, after it has partially degassed and crystallized, thereby impeding rapid ascent. This process could be operating at other steady-state basaltic volcanoes, wherein shallow reservoirs are periodically re- filled by fresh, volatile-rich magmas.