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

Muir, Duncan; Blundy, Jon; Rust, Alison; Hickey, James

doi:10.1093/petrology/egu005

Cited by 45 publications

(53 citation statements)

References 56 publications

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“…The bottom of the present-day seismogenic zone, inferred to be the brittle-ductile transition zone (BDTZ), around Uturuncu volcano is located at depths of 4.5-10 km beneath the ground surface and is shallowest directly beneath Uturuncu (Sparks et al, 2008;Jay et al, 2012;Keyson and West, 2013). Pleistocene eruptions of Uturuncu produced dacites having depths of last equilibration before eruption (see following) that coincide with the present-day BDTZ (Muir et al, 2014b). An estimate of the current temperature distribution between the APBM and the BTDZ, constrained by petrological, experimental, and geophysical data, is summarized in Figure 4.…”

Section: Thermalmentioning

confidence: 94%

“…Experiments determined the preeruptive dacite storage pressures at Uturuncu to be 100 ± 50 MPa (Muir et al, 2014b) at ~870 °C (1143 K). Isotopic data from the Uturuncu dacites indicate a mixture of crustal-derived and differentiated mantle-derived melts (Michelfelder et al, 2014).…”

Section: Petrologymentioning

confidence: 99%

“…The top of the column at z = -6 km marks the upper bound of the BDTZ, for which we set a temperature of T = 387 °C (660 K) (Priestley et al, 2008). This depth corresponds to the upper bound of experimentally determined confining pressures of 100 ± 50 MPa of pre-eruptive (Muir et al, 2014b). However, the temperature is considerably lower than the pre-eruptive storage temperature of the last erupted dacites of Uturuncu at 870 °C (1143 K; Muir et al, 2014a) due to secular cooling of the shallow magmatic reservoir over the past 250 k.y.…”

Section: Initial and Boundary Conditionsmentioning

confidence: 99%

“…The key difference between the two kinds of melt is in their dissolved H 2 O content, which in turn determines the depth at which they would exsolve a water-rich magmatic volatile phase. Uturuncu dacites are water undersaturated, containing ≤4 wt% H 2 O (Muir et al, 2014b) and would not exsolve H 2 O until ~100 MPa confining pressure. Conversely, the andesite Figure 14.…”

Section: Magmatic Columnmentioning

confidence: 99%

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Thermomechanical modeling of the Altiplano-Puna deformation anomaly: Multiparameter insights into magma mush reorganization

et al. 2017

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A 150-km-wide ground deformation anomaly in the Altiplano-Puna volcanic complex (APVC) of the Central Andes, with uplift centered on Uturuncu volcano and peripheral subsidence, alludes to complex subsurface stress changes. In particular, the role of a large, geophysically anomalous and partially molten reservoir (the Altiplano-Puna magma body, APMB), located ~20 km beneath the deforming surface, is still poorly understood. To explain the observed spatiotemporal ground deformation pattern, we integrate geophysical and petrological data and develop a numerical model that accounts for a mechanically heterogeneous and viscoelastic crust. Best-fit models imply subsurface stress changes due to the episodic reorganization of an interconnected vertically extended mid-crustal plumbing system composed of the APMB and a domed bulge and column structure. Measured gravity-height gradient data point toward low-density fluid migration as the dominant process behind these stress changes. We calculate a mean annual flux of ~2 × 10 7 m 3 of water-rich andesitic melt and/or magmatic water from the APMB into the bulge and column structure accompanied by modest pressure changes of <0.006 MPa/yr. Two configurations of the column fit the observations equally well: (1) a magmatic (igneous mush) column that extends to a depth of 6 km below sea level and contains trapped volatiles, or (2) a volatile-bearing hybrid column composed of an igneous mush below a solidified and permeable body that extends to sea level. Volatile loss from the bulge and column structure reverses the deformation, and explains the absence of broad (tens of kilometers) and long-term (>100 yr) residual deformation at Uturuncu. Episodic mush reorganization may be a ubiquitous characteristic of the magmatic evolution of the APVC.

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Section: Thermalmentioning

confidence: 94%

Section: Petrologymentioning

confidence: 99%

Section: Initial and Boundary Conditionsmentioning

confidence: 99%

Section: Magmatic Columnmentioning

confidence: 99%

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Thermomechanical modeling of the Altiplano-Puna deformation anomaly: Multiparameter insights into magma mush reorganization

et al. 2017

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“…Note that all depths in this paper will be with respect to average surface elevation, which is ~4.6 km within 100 km of Uturuncu. In the Central Andes, petrological determination of pre-eruptive conditions implies that magma accumulates at different depths for varying periods of time (e.g., Kay et al, 2010), and the uplift at Uturuncu is believed to be related to partial melt in the APMB where andesitic magmas evolve to dacitic composition prior to ascent to shallower pre-eruptive reservoirs (Muir et al, 2014a(Muir et al, , 2014b.…”

Section: Introductionmentioning

confidence: 99%

Time-dependent deformation of Uturuncu volcano, Bolivia, constrained by GPS and InSAR measurements and implications for source models

Henderson

Pritchard

2017

Geosphere

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New continuous GPS observations near the summit of Uturuncu volcano, Bolivia, indicate an average uplift rate of 2.4 ± 1.9 mm/yr between April 2010 and November 2015, while previous interferometric synthetic aperture radar (InSAR) observations between May 1992 and January 2011 predict an average vertical uplift rate of 7 ± 2 mm/yr. However, the GPS time series is better fit with a timedependent function, such that the uplift rate in 2010 is less than 2 mm/yr and in 2015 may be as high as 9 mm/yr. Motivated by indications of a non-constant rate in the continuous GPS time series, we examine to what extent past InSAR measurements may have been temporally aliased. We present evidence that decreased uplift since 2004 is permitted by available InSAR and is consistent with a lower average GPS rate since 2010. We discuss how these variable rates may affect previously proposed magmatic models at Uturuncu including diapir ascent and reservoir pressurization. In particular, we explore a "dipole" reservoir model consisting of a magma source in the lower crust and sink in the middle crust for which variation in magma supply could explain sub-decadal fluctuations in deformation rate. Inversion of multiple InSAR data sets using homogeneous models constrains the deeper reservoir to 55-80 km depth (lower crust and upper mantle) and the shallower reservoir to 20-35 km. Consistent with previous work, the inferred ratio of volume change between the source:sink (i.e., deep:shallow) reservoirs is up to 10:1. We also incorporate new seismic tomography results in a 3D finite-element model to explore the combined effects of multiple sources and material heterogeneity on surface deformation. An important conclusion from our modeling efforts is that the commonly used diagnostic ratio of maximum radial to vertical surface displacements for shape of the deformation source is affected by both the presence of multiple reservoirs and subsurface heterogeneity. Ultimately, the dipole and diapir models have unresolved shortcomings given the entire suite of available geophysical data, but both provide valuable insight into the conditions for magmatic ascent in the Central Andes.

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Electrical conductivity of hydrous andesitic melts pertinent to subduction zones

Guo

et al. 2017

JGR Solid Earth

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Andesitic magmatism and rocks are widespread at convergent plate boundaries. Electrically conductive bodies beneath subduction zone arc volcanoes, such as the Uturuncu Volcano, Bolivia, may correspond to active reservoirs of H2O‐bearing andesitic magma. Laboratory measurements of electrical conductivity of hydrous andesitic melts are required to constrain the physicochemical conditions of these magma reservoirs in combination with magnetotelluric data. This experimental study investigates electrical conductivity of andesitic melts with 0.01–5.9 wt % of H2O at 1164–1573 K and 0.5–1.0 GPa in a piston cylinder apparatus using sweeping‐frequency impedance spectroscopy. Electrical conductivity of andesitic melt increases with increasing temperature and H2O concentration but decreases with pressure. Across the investigated range of H2O concentration, electrical conductivity varies by 1.2–2.4 log units, indicating stronger influence of H2O for andesitic melt than for rhyolitic and dacitic melts. Using the Nernst‐Einstein equation, the principal charge carrier is inferred to be Na in anhydrous melt but divalent cations in hydrous andesitic melts. The experimental data are regressed into a general electrical conductivity model for andesitic melt accounting for the pressure‐temperature‐H2O dependences altogether. Modeling results show that the conductive layer at >20 km depths beneath the surface of the Uturuncu Volcano could be interpreted by the presence of less than 20 vol % of H2O‐rich andesitic melt (with 6–9 wt % H2O).

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

Cited by 45 publications

References 56 publications

Thermomechanical modeling of the Altiplano-Puna deformation anomaly: Multiparameter insights into magma mush reorganization

Thermomechanical modeling of the Altiplano-Puna deformation anomaly: Multiparameter insights into magma mush reorganization

Time-dependent deformation of Uturuncu volcano, Bolivia, constrained by GPS and InSAR measurements and implications for source models

Electrical conductivity of hydrous andesitic melts pertinent to subduction zones

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