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 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

Mixing and crystal scavenging in the Main Ethiopian Rift revealed by trace element systematics in feldspars and glasses

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. Whilst whole-rock data shows strong fractional crystallisation 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 crystallised 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 post caldera 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.This project is funded by the Natural Environment Research Council grant NE/L013932/1 (RiftVolc)

Mixing and crystal scavenging in the Main Ethiopian Rift revealed by trace element systematics in feldspars and glasses

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. Whilst whole‐rock data shows strong fractional crystallisation 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 crystallised 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 post‐caldera 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

Examining the past to prepare for the future: Quaternary magma storage in the Main Ethiopian Rift and implications for volcanic hazard and resource potential

Understanding how magma is stored within the crust, and what processes it might undergo, is of paramount concern to Earth scientists. Whilst advances in analytical techniques have revealed increasingly complex pictures of magma storage at arc and mid-ocean ridge settings, we still know relatively little about continental rift volcanism. The Main Ethiopian Rift (MER) is part of the larger East African Rift system, the archetypal example of continental rifting on our planet. Comprehensive geochemical datasets, including new whole-rock, glass (matrix glass and melt inclusions), and mineral analyses, have been generated for several geologically-young volcanic sites across and along the MER. Quaternary eruptive products are typically bimodal. Whole-rock and glass compositions can be modelled effectively by protracted fractional crystallisation of the alkali basalt to produce pantelleritic rhyolites. Despite this seemingly simple geochemical signature, alkali feldspars show trace element (Ba) heterogeneity suggestive of more complex processing. Many are antecrysts, crystallised from a more primitive melt, before being efficiently segregated due to the inherently low viscosity of peralkaline liquids. They are then later entrained from a ‘crystal-mush’ region into more evolved residual melts during large, explosive eruptions. Melt volatile contents are also inconsistent with a simple fractional crystallisation model. Basalts stored both on and off axis are saturated in H2O and CO2 at depths up to ~30 km in the crust. Much of this volcanic CO2 is lost through diffuse surface degassing, measured CO2 fluxes require the intrusion up to 0.14 km3 of basalt beneath the MER each year. On axis a change in S, Cl, and F solubility is recorded over the compositional gap, likely due to a drop-in pressure, as magmas ascend to a shallower storage region (~5-8 km), and the rapid increase in SiO2. The build-up of a pre-eruptive exsolved volatile phase has implications for ore formation and volcanic hazard assessment. The loss of halogens from MER melts likely precludes them from being a viable source of economic REE mineralisation, meanwhile low fluid/melt partition coefficients means that S yields of peralkaline eruptions are predicted to be much higher than their metaluminous counterparts. The state of a magma, in terms of composition or phase, can dramatically influence the geophysical signals observed at the surface. For example, the enhanced compressibility of an exsolved volatile phase will dampen geodetic monitoring signals. Magnetotelluric (MT) surveys are commonly used in Ethiopia, however, have sometimes produced results that contradict other geophysical methods. Forward models of electrical resistivity beneath the MER, informed by geochemical constraints, can help improve MT interpretation. The absence of a conductive body, synonymous with the presence of melt, beneath Aluto volcano can be reconciled with other geophysical evidence if the magma is highly crystalline. The presence of conductive anomalies along and across axis likely reflects the presence of longer-lived, more melt-rich systems, highlighting spatial variation in magmatism common to rift settings. The poor spatial resolution of the MT technique limits its use for volcanic monitoring in the MER. Only the influx of large volumes of melt, or rejuvenation of a crystalline storage system, perhaps prior to larger eruptions, may be detectable on repeat measurements.Funded by NERC RiftVolc projec

Volatile-rich magmas distributed through the upper crust in the Main Ethiopian Rift, Dataset

Dataset contains major element compositions of mineral hosts, and major, minor and volatile element compositions of melt inclusions for basalts and peralkaline rhyolite samples from the Main Ethiopian Rift, as described in the accompanying paper

Disclosing the temperature of columnar jointing in lavas

Columnar joints form by cracking during cooling-induced contraction of lava, allowing hydrothermal fluid circulation. A lack of direct observations of their formation has led to ambiguity about the temperature window of jointing and its impact on fluid flow. Here we develop a novel thermo-mechanical experiment to disclose the temperature of columnar jointing in lavas. Using basalts from Eyjafjallajökull volcano (Iceland) we show that contraction during cooling induces stress build-up below the solidus temperature (980 °C), resulting in localised macroscopic failure between 890 and 840 °C. This temperature window for incipient columnar jointing is supported by modelling informed by mechanical testing and thermal expansivity measurements. We demonstrate that columnar jointing takes place well within the solid state of volcanic rocks, and is followed by a nonlinear increase in system permeability of <9 orders of magnitude during cooling. Columnar jointing may promote advective cooling in magmatic-hydrothermal environments and fluid loss during geothermal drilling and thermal stimulation