Mixing and Crystal Scavenging in the Main Ethiopian Rift Revealed by Trace Element Systematics in Feldspars and Glasses

Abstract

For many magmatic systems, crystal compositions preserve a complex and protracted history, which may be largely decoupled from their carrier melts. The crystal cargo may hold clues to the physical distribution of melt and crystals in a magma reservoir and how magmas are assembled prior to eruptions. Here we present a geochemical study of a suite of samples from three peralkaline volcanoes in the Main Ethiopian Rift. While whole‐rock data show strong fractional crystallization signatures, the trace element systematics of feldspars, and their relationship to their host glasses, reveals complexity. Alkali feldspars, particularly those erupted during caldera‐forming episodes, have variable Ba concentrations, extending to high values that are not in equilibrium with the carrier liquids. Some of the feldspars are antecrysts, which we suggest are scavenged from a crystal‐rich mush. The antecrysts crystallized from a Ba‐enriched (more primitive) melt, before later entrainment into a Ba‐depleted residual liquid. Crystal‐melt segregation can occur on fast timescales in these magma reservoirs, owing to the low‐viscosity nature of peralkaline liquids. The separation of enough residual melt to feed a crystal‐poor postcaldera rhyolitic eruption may take as little as months to tens of years (much shorter than typical repose periods of 300–400 years). Our observations are consistent with these magmatic systems spending significant portions of their life cycle dominated by crystalline mushes containing ephemeral, small (< 1 km3) segregations of melt. This interpretation helps to reconcile observations of high crustal electrical resistivity beneath Aluto, despite seismicity and ground deformation consistent with a magma body.

Ductility and Compressibility Accommodate High Magma Flux Beneath a Silicic Continental Rift Caldera: Insights From Corbetti Caldera (Ethiopia)

“…For the rhyolite, we use the composition of the Wendo Koshe Pumice at Corbetti (Fontijn et al, ). No major and trace elements compositions for Corbetti's basalts have yet been published, so we compare estimates from the 1.6 Ma Bofa basalts near Aluto volcano (Hutchison et al, ) and a set of post‐caldera basaltic scoria cones at Kone (Iddon et al, ). We use standard values of partial molar, volumes, thermal expansion coefficients, and compressibilities of the oxide components (Spera, ) and find densities of dry melt of 2,335 ± 10 kg m −3 for the Wendo Koshe Pumice, 2,759 ±30 kg m −3 for the Kone Scoria Cones, and 2,810 kg m −3 for the Bofa Basalts.…”

“…Estimates of melt and magma densities in the Main Ethiopian Rift using compositions from Fontijn et al (), Hutchison et al (), and Iddon et al () and standard parameters from Spera (). Volatile estimates are from preliminary melt inclusion studies (Edmonds, pers.…”

“…Therefore, we use finite‐element analysis (FEA) to solve jointly and simultaneously for stresses, strains, and gravity changes in a thermomechanically heterogeneous crust. Employing the optimal location parameters for the elastic point‐source model and the source paramater‐space exploration for the period 2009–2016 presented in Lloyd, Biggs, Birhanu, et al () as starting parameters, the FEA incorporates a physically plausible crustal rheology based on petrological and thermal considerations consistent with a silicic system within a mature magmatic rift (Hutchison et al, ; Iddon et al, ; Keranen et al, ) as well as geophysical constraints on the crustal structure (Lavayssiere et al, ).…”

Low-frequency earthquakes beneath Tullu Moye volcano, Ethiopia, reveal fluid pulses from shallow magma chamber

“…LF earthquakes are detected around numerous volcanoes around the world, although are most common during eruptive episodes (e.g., Iverson et al, 2006;Bell et al, 2017). Earthquakes during these swarms are highly similar to each other and contain long resonating codas which can be the result of tube waves resonating within a magma conduit or resonating cracks close to the surface (Chouet, 1988;Kumagai et al, 2005), for example.…”

Pinpointing Deep Geothermal Upflow in Zones of Complex Tectono‐Volcanic Degassing: New Insights from Aluto Volcano, Main Ethiopian Rift

“…Aluto offers an ideal location to address some of these issues and understand the architecture of a rift‐related alkaline, fault‐controlled geothermal system. The volcanic complex has been a target for geothermal exploration and exploitation for several decades (Gebregzabher, ; Gianelli & Teklemariam, ; Gizaw, ; Hochstein et al, ; Teklemariam et al, ; Valori et al, ) as well as recent geological, geochemical, and geophysical investigations (Biggs et al, ; Braddock et al, ; Hutchison et al, ; Hutchison, Biggs, et al, , Hutchison, Fusillo, et al, , Hutchison, Pyle, et al, ; Iddon et al, ; Nowacki et al, ; Saibi et al, ; Samrock et al, ; Wilks et al, ). Although Aluto is wellstudied compared to many of Ethiopia's geothermal prospects, major uncertainties remain about how geothermal fluids are distributed in the subsurface and how they ascend along the mapped fault zones.…”