Volatile‐rich magmas distributed through the upper crust in the Main Ethiopian Rift

Iddon, Fiona and Edmonds, Marie (2020) Volatile‐rich magmas distributed through the upper crust in the Main Ethiopian Rift. Geochemistry, Geophysics, Geosystems. ISSN 1525-2027 DOI https://doi.org/10.1029/2019gc008904

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Understanding magma storage and differentiation in the East African Rift underpins our understanding of volcanism in continental rift settings. Here we present the geochemistry of melt inclusions erupted in Main Ethiopian Rift transitional basalts, trachytes and peralkaline rhyolites, produced by fractional crystallisation. Basalts stored on‐ and off‐axis are saturated in an exsolved volatile phase at up to 18 km, in the upper crust. Much of the CO2 outgassed from the magmas is likely lost through diffuse degassing. Observed CO2 fluxes require the intrusion of up to 0.14 km3 of basalt beneath the rift each year. On‐axis peralkaline rhyolites are stored shallowly, at ~4‐8 km depth. In the Daly Gap, magmas saturate in sulfide and an exsolved volatile phase, which promotes magma rise to shallower levels in the crust. Here, magmas undergo further protracted fractional crystallisation and degassing, leading to the formation of a substantial exsolved volatile phase, which may accumulate in a gas‐rich cap. The exsolved volatile phase is rich in sulfur and halogens: their projected loadings into the atmosphere during explosive peralkaline eruptions in the MER are predicted to be substantially higher than their metaluminous counterparts in other settings. The high fraction of exsolved volatiles in the stored magmas enhances their compressibility and must be considered when interpreting ground displacements thought to be caused by magma intrusion at depth, otherwise intruding volumes will be underestimated. Pockets of exsolved volatiles may be present at the roof zones of magma reservoirs, which may be resolvable using geophysical techniques.

Item Type: Article
Uncontrolled Keywords: 2020AREP; IA76
Subjects: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions: 05 - Petrology - Igneous, Metamorphic and Volcanic Studies
08 - Green Open Access
12 - PhD
Journal or Publication Title: Geochemistry, Geophysics, Geosystems
Identification Number: https://doi.org/10.1029/2019gc008904
Depositing User: Sarah Humbert
Date Deposited: 07 May 2020 21:35
Last Modified: 28 Oct 2020 01:01
URI: http://eprints.esc.cam.ac.uk/id/eprint/4731

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