RHYODACITE MAGMA STORAGE CONDITIONS PRIOR TO THE 3430 YBP CALDERA-FORMING ERUPTION OF ANIAKCHAK VOLCANO, ALASKA

Abstract

This study presents a pre-eruptive magma storage model for the rhyodacite and andesite magmas erupted during the 3430 yBP caldera-forming eruption of Aniakchak volcano, Alaska, derived from phase equilibria experiments and petrological data. The compositions of Fe-Ti oxide pairs from the early erupted Plinian rhyodacite pumice yield core temperatures of 871-900°C, with rims up to ~ 942°C, and fO2 from -10.6 to -11.8 log units. Melt inclusions entrapped in plagioclase phenocrysts have H2O contents between 3 and 5 wt%, estimated by FTIR and electron microprobe volatiles by difference methods, with no detectable CO2. Assuming water saturation, this corresponds to entrapment pressures between ~ 65 and 150 MPa. Phase equilibria results reproduce the natural phase assemblages at PHO MPa at 870-880°C, assuming water saturation. A mismatch in experimental versus natural glass SiO2 and Al2O3, and MELTS models for H2O-undersaturated conditions indicate that the rhyodacite may not have been H2O saturated. MELTS models with XH2O and Ptotal of 125-150 MPa at 870-880°C reproduce the natural groundmass glass Al2 O3 composition best, indicating the magma may have been slightly H2O undersaturated. Those pressures correspond to storage at 4.5-5.4 km depth in the crust. MELTS models and VBD estimates from melt inclusions in titanomagnetite grains from the andesite indicate pre-eruptive conditions of -1,000deg;C and > 110 MPa, corresponding to a minimum residence depth of ~ 4.1 km assuming water saturation or greater if the magma was H2O undersaturated. Previous geochemical studies indicate separate histories of the two magmas, though they retain some evidence that they are ultimately related through fractional crystallization processes. Analogous to the 1912 Novarupta magmas, the rhyodacite and andesite presumably originated within the same crystal mush zone beneath the edifice, yet were separated laterally and underwent different degrees of crustal assimilation. The andesite must have resided in close proximity, with ascent occurring in response to movement of the rhyodacite, and resulting in extensive syn-eruptive mingling. © Springer-Verlag 2006.