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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
Gravity anomalies uncovered by a survey in the Central Andes provide evidence for magma transport by diapir, not dyke
The Altiplano-Puna Magma Body (APMB), located in the Central Andes, is the largest known continental, midcrustal zone of partial melt on Earth. Fed by mantle-derived hydrous magmas, this volume of molten rock lies directly beneath the Altiplano-Puna Volcanic Complex (APVC), the biggest ignimbrite province generated in the last 20 million years. At the height its of activity, volcanoes in this area spewed out >12,000 km3 of APMB-sourced lava in episodic supereruptions.
Previous geophysical surveys have detected the presence of 15-30 % interconnected melt in the APMB by virtue of low seismic velocities and very high electrical conductivity. Del Potro and co-workers investigated how this molten rock travels from its storage zones in the mid-crust (>20 km depth) to the surface by conducting a high resolution gravity survey over the entire area above the APMB. By assuming a homogeneous crustal composition, anomalies in the overlying gravitational field can be attributed directly to difference in density when compared with the surrounding rock. Calculations demonstrate that the measured density contrast is consistent with the crust containing, on average, ~ 25 % liquid by volume.
3D imaging of inversions of the gravity anomalies reveal bulbous, vertically elongated structures of 15 km width rooted at the top of the APMB. The distinctive morphology of the gravity anomalies in conflict with magma transport by dyking, instead supporting a theory of magma transport by diapiric ascent. Evolved melts, buoyant due to high SiO2 and H2O contents, accumulate at the top of the APMB and ascend en masse, undergoing chemical and physical differentiation and stratification.
It is notable that the APVC is showing current signs of unrest, as evidenced by large, decade-long, deep-sourced ground deformation at Uturuncu volcano in Bolivi, making Del Potro et al.’s study extremely timely.
del Potro, R, Diez, M, Blundy, JD, Gottsmann, JH & Camacho, AG (2013) ‘Diapiric ascent of silicic magma beneath the Bolivian Altiplano’ Geophysical Research Letters, vol 40, no. 10, pp. 2044-2048. http://dx.doi.org/10.1002/grl.50493