Magma storage prior to the 1912 eruption at Novarupta, Alaska

摘要: New analytical and experimental data constrain the storage equilibration conditions of magmas erupted in 1912 from Novarupta 20th century's largest volcanic event. Phase relations at H2O+CO2 fluid saturation were determined for an andesite (58.7 wt% SiO2) a dacite (67.7 wt%) compositional extremes intermediate erupted. The phase assemblages, matrix melt composition modes natural reproduced experimentally under H2O-saturated (i.e., PH2O=PTOT) negatively sloping region T–P space 930 °C/100 MPa to 960 °C/75 MPa with fO2~NNO+1. are constrained 850 °C/50 MPa 880 °C/25 MPa. If H2O-saturated, these equilibrated (and above) level where co-erupted rhyolite (~100 MPa), suggesting that andesite-dacite magma reservoir was displaced laterally rather than vertically body. Natural mineral compositions also mixed (H2O + CO2) same range PH2O. Thus, model which stratified mafic silicic underlay is consistent findings only if intermediates have XH2O fl=0.7 0.9 extreme compositions, respectively. Disequilibrium features pumice scoria include pristine minerals existing outside their stability fields, zoning titanomagnetite contact ilmenite. Variable rates chemical would eliminate apparent thermal excursion re-distribution time scale days.