DETERMINING CRYSTAL-MUSH STORAGE CONDITIONS AND RHYOLITE FORMATION FOR THE 2011-12 PUYEHUE-CORDÓN CAULLE ERUPTION

Crystal-mush storage and rhyolite formation are important processes to study as they contribute to massive explosive eruptions and are ultimately linked to each other. Crystal-mush systems create a silicic melt lens cap from the interstitial melt of a crystal-rich sponge. The melt lens cap is then tapped to produce explosive silicic eruptions. Studies on old, exposed systems have highlighted the importance of crystal-mush storage but constraining these processes in an active volcanic system may help to refine models of melt-crystal segregation and storage conditions.

Puyehue-Cordon Caulle (PCC) is an active volcanic complex located in the Southern Andean Volcanic Zone that provides ideal conditions to assess crystal-mush dynamics and rhyolite formation. PCC produced three rhyodacitic eruptions in the last century with the most recent 2011-12 eruption exhibiting significant episodes of uplift and subsidence prior to the eruption. Such deformation has been attributed to a shallow crystal-mush system. Published barometry data shows the rhyodacite was stored at shallow crustal levels (5-7 km). Petrologic evidence of crystal-mush may exist in the presence of mafic enclaves we identified in the effusive rhyodacite.

We interpret the mafic enclaves as part of a crystal-mush due to their high crystallinity (40-55% of >100 µm phenocrysts) and cumulate-like textures in the form of intricately intergrown phenocrysts. While their bulk composition is primitive (<52 wt% SiO₂, 5-9.3 wt% MgO), the interstitial glass in the enclaves is almost identical in major and trace elements to the host rhyodacite glass pointing toward a direct genetic relationship and presenting one of the largest compositional gaps on record. Our preliminary data shows PCC exhibiting a ~20 wt% SiO₂ compositional gap between the enclaves and host rhyodacite in a single eruption, which points towards efficient rhyolite formation. Other stratocones such as Mt Mazama present half the magnitude in gaps at a more evolved end-member. Mineral-scale data will provide constraints on storage conditions as well as thermal histories and thus provides insight on large-scale crystal-mush dynamics. Utilizing both mafic enclave and deformation data will gain us a new understanding on rhyolite formation and storage conditions for explosive volcanic systems.