Phase equilibrium of a high-SiO2 andesite at f 0 2=RRO: implications for Augustine Volcano and other high-f 0 2 arc andesites

Understanding the impact of magmatic plumbing systems on explosive volcanic activity is important for hazard management. This study describes phase equilibria experiments using a high-silica andesite (HSA; -SiO2 = 62.5 wt%) from the 2006 eruption of Augustine Volcano, Alaska. Experiments were conducted under -H2O saturated conditions, f 0 2 = RRO (Re-ReO2 or Ni-NiO + 2), at pressures of 50-200 MPa (PTotal = PH2O), and at temperatures of 800-1060 degrees C. Run durations varied from 23 to 539 h, inversely scaled with temperature. The natural Augustine HSA phase assemblage (plagioclase, two pyroxenes, Fe-Ti oxides, magnesio-hornblende) was reproduced at 860-880 degrees C and 120-200 MPa. Comparing experimental and natural glass and plagioclase compositions further refined those conditions to 870 degrees C and 120-170 MPa. Crystallization of euhedral quartz was accompanied by biotite and small amounts of cummingtonite at T = 850 degrees C. The relatively high temperature appearance of these typically low-T phases indicates that higher f 0 2 could enhance their stability. The storage conditions estimated from our experiments compare well with previous magma plumbing system models for Augustine from geophysical and petrological data. The refined experimental pressure range suggests a storage depth of 4.6-6.6 km, assuming a crustal density of 2650 kg/m3. The strong petrological and geochemical similarities between the products of the 2006, 1986, and 1976 eruptions suggest that the Augustine magmatic system had generally consistent crystallization conditions for the HSA lithology during that > 30-year time interval. The experimental results broad implications for understanding higher f 0 2 magmas at andesitic arc volcanoes.