Constraining magma storage conditions at a restless volcano in the Main Ethiopian Rift using phase equilibria models

Abstract

The Main Ethiopian Rift hosts a number of peralkaline volcanic centres, with many showing signs of recent unrest. Due, in part, to the low number of historical eruptions recorded in the region, volcanism in the Main Ethiopian Rift remains understudied relative to other volcanic settings and conditions of magma storage remain almost entirely unknown. Aluto is one of these restless caldera systems and identifying magma storage conditions is vital for evaluating the risks posed by recent periods of unrest. In this study, we ran ~150 fractional crystallisation models, using the Rhyolite-MELTS thermodynamic software, within the range P = 50-300 MPa, starting H 2 O = 0.5-3 wt% and fO 2 = QFM-2-QFM+1. This represents a realistic range of potential magma storage conditions at Aluto. We assessed the fractionation trends produced using two different starting compositions, which represent different estimates of the parental melt feeding the system. The predicted liquid lines of descent produced by these models are compared with Aluto whole-rock data from the literature, and are presented along with new observations of the natural phase assemblage and erupted mineral compositions to provide information on the magma storage conditions.

The 2011 eruption of Nabro volcano, Eritrea: perspectives on magmatic processes from melt inclusions

“…Other work in the wider East African Rift, examining the Main Ethiopian Rift and Kenyan Rift zones, has reached similar conclusions about crustal mush zones (e.g., Rooney et al 2012a, b;Gleeson et al 2017). These papers indicate that volcanic systems in the Main Ethiopian Rift are characterised by vertically extensive magma plumbing that are likely compositionally zoned.…”

“…In some models, olivine crystallises considerably earlier than plagioclase, which might explain the high CaO and Al 2 O 3 in the olivine inclusions relative to the plagioclase ones. The challenges of modelling alkaline rocks are discussed by Gleeson et al (2017) and Rooney et al (2012a). Rooney et al (2012a) particularly note the problem that MELTS is not well calibrated for fluorapatite, something also evident in our results.…”

Magnetotelluric Image of Transcrustal Magmatic System Beneath the Tulu Moye Geothermal Prospect in the Ethiopian Rift

“…In order to constrain the amount of melt (Figure ) in the recovered electrical conductors (C1)–(C3), we employed laboratory‐based electrical conductivity measurements for basaltic (Pommier & Le‐Trong, ) and peralkaline rhyolitic (Guo et al, ) melts, together with a two‐phase Archie's law to interpret the bulk conductivity of rock with respect to the interconnectivity of the melt fraction (Glover et al, ; Jahrling & Tait, ). Petrological constraints came from geochemical analyses, in addition to results of thermodynamic modeling studies performed to explore magma evolution and storage conditions of igneous rock in the MER (Gleeson et al, ; Trua et al, ; see supporting information). For the lower crustal conductor (C1) we assumed a well‐connected melt phase with an interconnectivity between the Hashin‐Shtrikman upper bound and the parallel model (Glover et al, ).…”

The Electrical Structure of the Central Main Ethiopian Rift as Imaged by Magnetotellurics: Implications for Magma Storage and Pathways

“…Pulse‐like surface deformation events on a number of volcanoes located on the central rift axis have been reported through the analysis of satellite imagery (interferometric synthetic aperture radar, InSAR; Biggs et al, ) and give rise to concern about potential geohazards associated with volcanic activity. The silicic peralkaline volcano Aluto has been the target of a number of geological, geochemical, and geophysical studies undertaken in recent years (Fontijn et al, ; Gleeson et al, ; Hutchison et al, ; Hutchison, Biggs, et al, ; Hutchison, Mather, et al, ; Saibi et al, ; Teklemariam et al, ; Wilks, Kendall, et al, ). It is the site for a hydrothermal power plant with three test drillings awaiting integration into the local power grid.…”

“…There is evidence for pyroclastic density currents with faults dissecting the volcanic complex as seen from exploration wells. Magma composition at depth is thought to be of pantellerite and comendite composition (Fontijn et al, ; Gleeson et al, ; Hutchison, Mather, et al, ). Periods of deformation of Aluto have been reported from InSAR and GPS signal analysis over the past 13 years with rapid uplift events with displacement of about 10–15 cm in 2004 and subsequent slower subsidence followed by another 10 cm uplift at the end of 2008 (Biggs et al, ).…”