Investigating the petrogenesis and mineral chemistry of the only recorded Plinian dacite eruption at Llaima volcano, southern Chile

Abstract:

Llaima volcano, located in the Andean Southern Volcanic Zone (SVZ) is one of the largest and most active volcanoes in the region. The composition of erupted material throughout Llaima’s post-glacial history has been predominantly basalt to basaltic andesite. However, at ~10 ka a Llaima had a Plinian eruption that produced the most evolved material observed in the volcano’s post-glacial period. This. dacitic pumice is capped by an andesite and resulted in an estimated 4 km³ of tephra. While several studies briefly address the Llaima Pumice and andesite cap within the context of the post-glacial activity, there are few investigations that focus on the petrogenesis of the dacite eruption. We will report on the composition and pre-eruptive conditions of the Llaima Pumice and the role of mafic recharge in the formation of the dacite tephra and andesite cap. In order to examine the petrogenesis of the Llaima Pumice, I have collected major and trace element data by ICP-MS and will soon collect electron microprobe data. To understand the pre-eruptive storage conditions of the Llaima Pumice, I will use mineral phase chemistry in thermometers and barometers and present the pre-eruptive storage temperatures and depths. I will assess the role of mafic recharge using the most evolved composition of Llaima Pumice and several Llaima basalts in a binary mixing model. The ICP-MS data that has been collected shows the main dacite pumice has an MgO content range of ~0.2 – 1 wt %, whereas the andesite cap is ~1.5 – 2.5 wt % with increasing MgO content up stratigraphy in the dacite pumice, but no clear trend in the andesite cap. CaO content in the dacite pumice ranges from ~0.75 – 3 wt %, but jumps to 4 – 5 wt % in the andesite cap. Again, in the dacite there is a slight correlation with increasing CaO content in stratigraphy, but the andesite cap shows no clear trend. The gradation of more evolved to less evolved magma up section suggests that fractional crystallization played a role in the formation of the dacite pumice. The bimodality of the main pumice and andesite cap suggests that mafic recharge may have encouraged the eruption of the Llaima Pumice. These observations along with the other data that will be collected and processed will provide insight into the geochemical and petrologic evolution of the most evolved erupted material at Llaima volcano.