Explosive style, magma degassing and evolution in the Chaimilla eruption, Villarrica volcano, Southern Andes

Abstract

Villarrica (Chile) is a basaltic stratovolcano, cur- rently in an open-conduit condition. It now has relatively fre- quent Strombolian and effusive eruptions, but it had large explosive eruptions in prehistoric times. Among them, the most recent eruption was Chaimilla, which occurred about 3100 years ago and produced deposits that indicate complex, multiphase eruptive dynamics. Significant differences in min- eralogy and glass compositions of the erupted scoria suggest the eruption was fed by two distinct magma batches with similar bulk compositions but distinct crystallization and degassing histories. The lower sequence scoria has a complex crystal assemblage with several crystal populations produced by mixing between a relatively degassed magma containing Fo75-79 olivine, normally or reversely zoned plagioclase (An70-94) and augite (type 1 magma), and a subordinate vol- ume of more-primitive and more volatile-rich magma rising from depth (type 2 magma) and carrying normally zoned pla- gioclaseandhigher-Mg(Fo81-85)olivinecrystals.Type2mag- ma was the main component emitted during the larger and more explosive eruptive phase that deposited the upper sequence. The Chaimilla eruption occurred under closed- vent conditions and was fed by water-rich magmas. When comparedwiththe petrologicalfeaturesofthe magma current- ly erupted at Villarrica, which has slightly more-evolved bulk compositions, lower crystal content and lower water content, these results suggest that the evolution in eruptive style of the volcano from highly explosive to a lava lake/Strombolian ac- tivity corresponds to significant changes in the shallow plumbing system (which is now at much shallower depths); these plumbing-system changes were not associated with sig- nificant changes in the parental magma compositions.