Magma dynamics and time scales during theVEI 5, 2000 BP eruption of El Misti volcano, southern Peru (EM2000BP) are investigated to address cyclic explosive activity at this hazardous volcano The 1·4 km3 of pumice falls and flows have abundant mingled pumice of high-K, calc-alkaline rhyolite and andesite composition Phenocryst zoning and compositions reveal mutual exchange of plagioclase between the two magmas; amphibole in the rhyolite was derived from the andesite. Amphiboles in the andesite are predominantly unrimmed crystals whereas those in the rhyolite mostly exhibit reaction rims. Phase equilibria indicate that the and esite formed at ~900-9508C and 2-3 kbar pressure and was water-saturated with 5·1-6·0wt % H2O, broadly similar to El Misti magmas overall Amphibole, plagioclase, Ti-magnetite, and two pyroxenes were the crystallizing phases. A separate rhyolite magma existed higher in the crust at a temperature of 816±308C and ~5% H2O in which only plagioclase and Fe-Ti oxides were stable. The lack of cognate amphibole in the rhyolite despite H2O saturation requires that it staged above the stability limit of amphibole (<100MPa) Exchange reactions in amphibole (dominantly pargasitic) and trace element partitioning in plagioclase indicate that both andesite and rhyolite magmas were broadly constant in temperature and H2O content. These constraints suggest that the initially separate rhyolite and deeper and esite magmas interacted by an initial and esite recharge event that resulted in mingling and crystal exchange. A period of 50-60 days is required for amphibole introduced into the rhyolite to develop reaction rims owing to decompression. These rims are dominated by plagioclase, a consequence of the Al-rich nature of the amphibole. The lack of reaction rims on amphibole in the andesite implicates a second, more-forceful and voluminous eruption-triggering recharge event during which and esite rose rapidly from source to surface in ≤5 days at ascent rates of at least 0·023 m s-1. Further decompression-driven crystallization is recorded in plagioclase rims and microlite growth that may have contributed to a rapid increase in viscosity leading to explosive eruption. This VEI 5 plinian eruption shares characteristics with other explosive events at El Misti on a time scale of 2000-4000 years, suggesting periodic rechargedriven explosive activity.
Nota bibliográficaFunding Information:
This work has been variously supported by the National Science Foundation (EAR 0087181 to S.d.S.) and the Volcano Disaster Assistance Program (VDAP) of the US Geological Survey. This work was initiated when G.S. was a visiting scientist supported by Oregon State University, Department of Geosciences.