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

Abstract

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 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 CO 2 outgassed from the magmas is likely lost through diffuse degassing. Observed CO 2 fluxes require the intrusion of up to 0.14 km 3 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.

Probabilistic Volcanic Hazard Assessment for Pyroclastic Density Currents From Pumice Cone Eruptions at Aluto Volcano, Ethiopia

“…Water contents were estimated from a review of melt inclusion data in peralkaline rhyolites globally, though because the data set is limited, and the likely distribution of these water contents is uncertain, we assume an uninformative uniform distribution between 4 and 8 wt% H 2 O. These values are appropriate for the typical high water contents of pantellerite melts (e.g., Lowenstern and Mahood, 1991;Webster et al, 1993;Wilding et al, 1993;Horn and Schmincke, 2000;Field et al, 2012;Neave et al, 2012), and are consistent with melt inclusion data from Aluto (Iddon and Edmonds, 2020). The product is a histogram of expected H 0 for Aluto (Figure 6), which can be combined with H v during the energy cone simulations at each vent to give H c , where…”

“…a, b and c are regression coefficients related to the location of the convecting/collapsing eruption column boundary within the parameter space. For eruption columns Lowenstern and Mahood (1991); c Wilding et al (1993); d Webster et al (1993); e Horn and Schmincke (2000); f Neave et al (2012); g Field et al (2012); h Iddon and Edmonds (2020); i Gleeson et al (2017); j Woodhouse et al (2013). produced during pumice cone eruptions at Aluto, maximumlikelihood estimation yields: a = 12.78, b = −0.61 and c = 1.1.…”

“…There is an estimated 415,000 people living within 30 km of Aluto, but there is currently no mandated group responsible for the monitoring of volcanic activity in Ethiopia (Vye- Brown et al, 2016). Although the volcano has been the focus of some recent research activity (Biggs et al, 2011;Hutchison, 2015;Hutchison et al, 2015Hutchison et al, , 2016aSamrock et al, 2015;Braddock et al, 2017;Gleeson et al, 2017;Iddon et al, 2017;Wilks et al, 2017;Fontijn et al, 2018;Hübert et al, 2018;McNamara et al, 2018;Clarke et al, 2019;Clarke, 2020;Iddon and Edmonds, 2020), and has recently shown signs of volcanic unrest (Biggs et al, 2011), there is currently no existing hazard assessment or hazard map for Aluto which could be used to mitigate risk during future episodes of unrest and/or eruptions. The most recent volcanism at Aluto, since at least 60 ka, is dominated by pumice cone eruptions, with the most recent known eruption at 400 ± 50 years ago (Hutchison et al, 2016c).…”

Integrated magnetotelluric and petrological analysis of felsic magma reservoirs: Insights from Ethiopian rift volcanoes

“…Analytical solutions for the cementation exponent exist for a fully connected phase, m = 1, and for liquids embedded in an assemblage of perfect spheres, m = 1.5 (Glover, 2010, and references therein). Experimentally determined cementation exponents exist for crystalline mush, m = 1.73 (Jahrling and Tait, 1996), and for saline liquids in freezing permafrost soil, where m increases from 1.81 to 2.1 as water freezes (crystallizes) (Wu et al, 2017).…”

Catching the Main Ethiopian Rift Evolving Towards Plate Divergence

“…In the MER, differentiation (mainly crystal fractionation plus assimilation) has been mainly used to explain the silica-rich peralkaline composition of central volcanoes and the "Dalygap", that is the spatial coincidence of basaltic and rhyolitic products [e.g., 10,18,20,[37][38][39]. Conversely, here we consider the wider process of magma differentiation (since the onset of magma generation), also including the deeper portion of the plumbing systems (19-25 km in the NMER; [40]), where geophysical investigations have detected up to 4% of melt [21,41].…”