Experimental Constraints on Dacite Pre-eruptive Magma Storage Conditions beneath Uturuncu Volcano

Cerro Uturuncu is a long-dormant, compositionally monotonous, effusive dacitic volcano in the Altiplano-Puna Volcanic Complex (APVC) of SW Bolivia. The volcano recently gained attention following the discovery of an similar to 70 km diameter Interferometric Synthetic Aperture Radar (InSAR) anomaly roughly centred on its edifice. Uturuncu dacites, erupted over the past similar to 1 Myr, invariably have a phase assemblage of plagioclase-orthopyroxene-biotite-ilmenite-magnetite-apatite-zircon and rhyolite glass. To better constrain storage conditions of the dacite magmas and to help understand their relationship with the observed deformation, petrological experiments were performed in cold-seal hydrothermal vessels. Volatile-saturated (P-H2O = P-TOTAL and P-H2O + P-CO2 = P-TOTAL) phase equilibria experiments were run between 50 and 250 MPa and 760 and 900 degrees C at fO(2) similar to Ni-NiO. Two synthetic starting compositions were investigated based on a typical Uturuncu dacite whole-rock and its rhyolitic groundmass glass. Pre-eruptive magma storage conditions have been estimated by comparing results from the experiments with natural phase assemblages, modes, and mineral and glass compositions. H2O-saturated experiments constrain storage pressures to 100 +/- 50 MPa, equivalent to 1 center dot 9-5 center dot 7 km below surface. In the dacite, natural phase assemblages are reproduced at 870 degrees C, 100 MPa with both orthopyroxene and biotite stabilized concurrently. Natural glass chemistries are most closely replicated at 50 MPa at 870 degrees C, reflecting the role of decompression crystallization prior to eruption. In H2O-saturated rhyolite experiments the natural phase assemblage is most closely replicated at 870 degrees C, 50 MPa. Isothermal, mixed volatile dacite experiments at 870 degrees C further constrain storage pressures to 110 +/- 10 MPa. Assuming that there has been no dramatic change in the eruptive behaviour of Uturuncu in the last 270 kyr, pre-eruptive storage of dacite magmas at similar to 100 MPa precludes their role in producing the large diameter deformation anomaly. If deformation is a result of magmatic intrusion, then intrusion of less evolved magmas into deeper, mid-crustal storage regions is a more probable explanation. Intrusion within the Altiplano-Puna Magma Body (APMB), the extent of which is roughly coincident with the APVC, is most likely. It is proposed that dacite magmas form from andesitic parents, via fractionation and/or assimilation, within the APMB. Dacites then rise buoyantly to shallow storage levels where they stall and crystallize prior to eruption. Microlites form during subsequent ascent from the storage region to the surface.