High temperature experiments reveal previous eruptions were characterized by shallow magma storage, a scenario incompatible with the depth of the current anomaly
Cerru Uturuncu, Bolivia, is a continental arc volcano located in the Central Andes. Recent satellite observations of ground deformation in the area have measured uplift of 1 – 2cm per year. This has been accompanied by persistent seismic activity and indications are strong that Uturuncu may be entering a period of unrest and possible magma build up. Inverse modelling of the deformation has indicated a large diameter anomaly at 11 – 17km beneath the volcano.
A new study by Muir and co-workers at the University of Bristol conducted high temperature experiments on the two types of lava primarily from the volcano: rhyolite and dacite. The aim of their work to determine if previous episodes of magma storage are consistent with the depth of the anomaly causing the current deformation, and whether future eruptions would likely be effusive (in continuation of past activity at Uturuncu) or larger-scale explosive events.
The natural mineral assemblages in both types of lava were replicated by experiments at 870ºC at pressures equivalent to 2 – 6 km depth, a similar crustal level to the location of recent earthquakes recorded at the volcano. This experimental evidence precludes the role of dacites and rhyolites in producing the observed anomaly beneath Uturuncu. Instead, the authors propose a model where dacitic magmas are formed from fractional crystallisation in an underlying, deeper magma body before stalling in the shallow crust prior to their effusive eruption.
Muir DD, Blundy JD, Rust AC, & Hickey J (2014) ‘Experimental constraints on dacite pre-eruptive magma storage conditions beneath Uturuncu volcano’. Journal of Petrology, 55(4), 749-767. http://dx.doi.org/10.1093/petrology/egu005
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 ∼70 km diameter Interferometric Synthetic Aperture Radar (InSAR) anomaly roughly centred on its edifice. Uturuncu dacites, erupted over the past ∼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 (PH2O = PTOTAL and PH2O + PCO2 = PTOTAL) phase equilibria experiments were run between 50 and 250 MPa and 760 and 900°C at fO2 ∼ 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 1·9–5·7 km below surface. In the dacite, natural phase assemblages are reproduced at 870°C, 100 MPa with both orthopyroxene and biotite stabilized concurrently. Natural glass chemistries are most closely replicated at 50 MPa at 870°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°C, 50 MPa. Isothermal, mixed volatile dacite experiments at 870°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 ∼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.