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

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 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. Key Points: Alkali basalts are stored both on‐ and off‐axis at depths >15 km in the crust; on‐axis peralkaline rhyolites are stored at between ~4–8 km depthSaturation of mafic magmas in a sulfide and exsolved volatile phase in the Daly Gap promotes buoyancy and rise of magmas to shallower reservoirs, where protracted fractional crystallization and degassing occursCaldera‐forming, explosive eruptions in the MER may give rise to large emissions of SO2 and halogens, which may have significant environmental impacts [ABSTRACT FROM AUTHOR]