Volatile‐Rich Magmas Distributed Through the Upper Crust in the Main Ethiopian Rift

Fiona Iddon,Marie Edmonds

doi:10.1029/2019gc008904

Abstract

AbstractUnderstanding 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 crystallization. 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 crystallization 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.

Highlights

An important goal of many studies of volcano-magmatic systems is to extract a record of pre50 eruptive melt chemistry and use it to assess magma differentiation and the conditions of magma storage and processing in the crust
Melt inclusion studies of peralkaline volcanic rocks are scarce in the literature [Barclay et al, 1996; Gioncada and Landi, 2010; Lanzo et al, 2013; Macdonald et al, 2012; Neave et al, 2012] and we have only a very limited picture of differentiation processes, conditions of pre-eruptive storage and the volatile budget of peralkaline volcanic systems
The majority of host olivine cores are in equilibrium with their host melts [Roeder and Emslie, 1970], but there is some variability in olivine forsterite contents for a given melt Mg#

Summary

Introduction

An important goal of many studies of volcano-magmatic systems is to extract a record of pre eruptive melt chemistry and use it to assess magma differentiation and the conditions of magma storage and processing in the crust. Melt inclusion studies of peralkaline volcanic rocks are scarce in the literature [Barclay et al, 1996; Gioncada and Landi, 2010; Lanzo et al, 2013; Macdonald et al, 2012; Neave et al, 2012] and we have only a very limited picture of differentiation processes, conditions of pre-eruptive storage and the volatile budget of peralkaline volcanic systems. The high alkali contents of the melts enhance solubility of volatiles including CO2, H2O, F and Cl [Di Matteo et al, 2004; Scaillet and Macdonald, 2006; Shishkina et al, 2014; Webster et al, 2015]. The systematics of halogen behaviour in peralkaline melts is complex [Webster et al, 2015] and may involve saturation of a low density vapour as well as a brine phase in the shallower parts of the crustal storage system, whereby the melt Cl content becomes buffered at a fixed value, which may be used to estimate magma storage depths [Balcone-Boissard et al, 2016]

Methods

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Discussion

Conclusion