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The upper mantle under the Afar Depression in the East African Rift displays some of the slowest seismic wave speeds observed globally. Despite the extreme nature of the geophysical anomaly, lavas that erupted along the East African Rift record modest thermal anomalies. We present measurements of major elements, H2O, S, and CO2, and Fe3+/ΣFe and S6+/ΣS in submarine glasses from the Gulf of Aden seafloor spreading center and olivine‐, plagioclase‐, and pyroxene‐hosted melt inclusions from Erta Ale volcano in the Afar Depression. We combine these measurements with literature data to place constraints on the temperature, H2O, and fO2 of the mantle sources of these lavas as well as the initial and final pressures of melting. The Afar mantle plume is C/FOZO/PHEM in isotopic composition, and we suggest that this mantle component is damp, with 852 ± 167 ppm H2O, not elevated in fO2 compared to the depleted MORB mantle, and has temperatures of ∼1401–1458°C. This is similar in fO2 and H2O to the estimates of C/FOZO/PHEM in other locations. Using the moderate H2O contents of the mantle together with the moderate thermal anomaly, we find that melting begins at around 93 km depth and ceases at around 63 km depth under the Afar Depression and at around 37 km depth under the Gulf of Aden, and that ∼1%–29% partial melts of the mantle can be generated under these conditions. We speculate that the presence of melt, and not elevated temperatures or high H2O contents, are the cause for the prominent geophysical anomaly observed in this region. Plain Language Summary: The mantle under the Afar Depression and Gulf of Aden in Northeastern Africa is geophysically distinct from the mantle elsewhere on Earth. Typically, these geophysical distinctions are thought to arise from elevated temperatures, but the composition of lavas that erupted in this region demonstrate that the mantle is only moderately warm and cannot fully explain the geophysical nature of the mantle in this region. We produce new measurements of submarine pillow glasses erupted from Erta Ale volcano and find that in addition to being somewhat warm, the mantle in this region is also somewhat hydrated compared to the mantle that feeds mid‐ocean ridge volcanoes but is not substantially different in bulk oxidation state. These conditions together produce a region of partial melt that exists between 93 km and 63–36 km depth under the surface. We speculate that this lens of melt can explain the geophysical observations of the mantle in this region. Key Points: The Afar mantle plume has water contents of 852 ± 167 ppm H2O and is not substantially more oxidized than the depleted MOR mantleMelting in this region begins around 93 km depth and extends to 63 km under the Afar Depression and to 37 km under the Gulf of AdenThis melt lens is sustained by the Afar plume and may explain the extreme slowness of seismic wave speeds in the mantle in this region [ABSTRACT FROM AUTHOR]

East Africa hosts significant reserves of untapped geothermal energy. Exploration has focused on geologically young (<1 Ma) silicic calderas, yet there are many sites of geothermal potential where there is no clear link to an active volcano. The origin and architecture of these systems are poorly understood. Here, we combine remote sensing and field observations to investigate a fault‐controlled geothermal play located north of Lake Abaya in the Main Ethiopian Rift. Soil gas CO2 and temperature surveys were used to examine permeable pathways and showed elevated values along a ∼110 m high fault, which marks the western edge of the Abaya graben. Ground temperatures are particularly elevated where multiple intersecting faults form a wedged horst structure. This illustrates that both deep penetrating graben bounding faults and near‐surface fault intersections control the ascent of hydrothermal fluids and gases. Total CO2 emissions along the graben fault are ∼300 t d−1; a value comparable to the total CO2 emission from silicic caldera volcanoes. Fumarole gases show δ13C of −6.4‰ to −3.8‰ and air‐corrected 3He/4He values of 3.84–4.11 RA, indicating a magmatic source originating from an admixture of upper mantle and crustal helium. Although our model of the North Abaya geothermal system requires a deep intrusive heat source, we find no ground deformation evidence for volcanic unrest or recent volcanism along the graben fault. This represents a key advantage over the active silicic calderas that typically host these resources and suggests that fault‐controlled geothermal systems offer viable prospects for geothermal exploration. Key Points: First conceptual model of a fault‐controlled magmatic geothermal resource in the East African RiftFocus on North Abaya in the Main Ethiopian Rift and show that deep hydrothermal upflow is concentrated along a major graben bounding faultMagmatic CO2 emissions along this fault are ∼300 t d−1 and comparable to average values from the world's subaerial volcanoes [ABSTRACT FROM AUTHOR]

Quantifying the global volcanic CO2 output from subaerial volcanism is key for a better understanding of rates and mechanisms of carbon cycling in and out of our planet and their consequences for the long‐term evolution of Earth's climate over geological timescales. Although having been the focus of intense research since the early 1990s, and in spite of recent progress, the global volcanic CO2 output remains inaccurately known. Here we review past developments and recent progress and examine limits and caveats of our current understanding and challenges for future research. We show that CO2 flux measurements are today only available for ~100 volcanoes (cumulative measured flux, 44 Tg CO2/year), implying that extrapolation is required to account for the emissions of the several hundred degassing volcanoes worldwide. Recent extrapolation attempts converge to indicate that persistent degassing through active crater fumaroles and plumes releases ~53–88 Tg CO2/year, about half of which is released from the 125 most actively degassing subaerial volcanoes (36.4 ± 2.4 Tg CO2/year from strong volcanic gas emitters, Svge). The global CO2 output sustained by diffuse degassing via soils, volcanic lakes, and volcanic aquifers is even less well characterized but could be as high as 83 to 93 Tg CO2/year, rivaling that from the far more manifest crater emissions. Extrapolating these current fluxes to the past geological history of the planet is challenging and will require a new generation of models linking subduction parameters to magma and volatile (CO2) fluxes. Key Points: Progress in determining subaerial volcanic CO2 flux has been significantChallenges remain with regard to extrapolations through time and global coverage of measurements and with regard to diffuse tectonic degassing and dynamic nature of volcanic degassingVolcanic and tectonic contributions are <2% of current anthropogenic contributions [ABSTRACT FROM AUTHOR]

Compact mass spectrometers (MSs) can provide simultaneous multispecies analysis with high sensitivity and precision. For volcanic gas monitoring in situ, instrumental mass spectrometry requires reliability and ruggedness combined with low power usage and portability. The Volcanic Gas Analytical Monitor (VGAM) is capable of quantitative molecular analysis of a variety of atmospheric and volcanic gases in a single sensor by ion trap mass spectrometry. These gases include H2, He, H2O, N2, O2, Ar, NO, N2O, CO, CO2, H2S, SO, SO2, and CH4. Unlike previous field instruments using magnetic sector and quadrupole MSs with vacuums backed by compact turbomolecular pumps, the VGAM uses a low‐power autoresonant ion trap MS and nonevaporable getter‐ion pumped vacuum that operates at only 25‐W total power, which is often the power requirement for just the MS. Ratiometric mass spectral response is combined with total pressure measurements to report absolute partial pressures. Data are generated in real time and are recorded to internal flash memory. Relatively low power (<1 W) data telemetry to remote sites is possible. Analysis of volcanic plumes, fumaroles, and solfatara fields is accomplished by direct inlet of gases, after trapping as much excess water vapor in both the external and internal foreline as possible. We report herein on the VGAM autorun and postprocessing procedures, initial calibration results for CO2, and the results of a brief field deployment at Sulfur Banks solfatara field, Kilauea Volcano, Hawaii. Key Points: We developed a low‐power mass spectrometer‐based detector for long‐term volcanic monitoring of a variety of gases simultaneouslyData are generated in real time and are recorded to internal flash memory, with possible telemetry to remote sitesWe report herein on the VGAM autorun and postprocessing procedures, initial calibration results for CO2, and the results of a brief field deployment at Sulfur Banks solfatara field, Kilauea Volcano, Hawaii [ABSTRACT FROM AUTHOR]

Magma transfer from the mantle to the crust in arcs is an important step in the global cycling of elements and volatiles from Earth's interior to the atmosphere. Arc intrusive rocks dominate the total magma mass budget over extrusive rocks. However, their total volume and rate of addition is still poorly constrained, especially in continental arcs. We present lateral (forearc to backarc) and depth‐dependent (volcanics to deep crust) magma volume additions and arc‐wide magma addition rates (MARs) calculated from three continental arc crustal sections preserving magma flare‐up periods. We observe an increase in volume addition with depth and less magma added in the forearc (~15%) and backarc (~10% to 30%) compared to the main arc. Crustal‐wide MARs for each section are remarkably similar and around 0.7–0.9 km3/km2/Ma. MARs can be used to estimate CO2 fluxes from continental arcs. With initial magma CO2 contents of 1.5 wt.%, global continental arc lengths, and MARs, we calculate changes in C (Mt/year) released from continental arcs since 750 Ma. Calculated present‐day global C fluxes are similar to values constrained by other methods. Throughout the Phanerozoic, assuming equal durations of flare‐up and lull magmatism, calculated continental CO2 flux rates vary between 4 and 18 Mt C/year with highest values in the Mesozoic. These fluxes are considered minima since the intake of mantle and/or crustal carbon is not considered. Magmatic episodicity in continental arcs and changes in arc thickness and width are critical to consider when calculating MARs through time. Plain Language Summary: The transfer of magma from Earth's interior to the crust and surface via volcanic eruptions transports elements and volatiles, which can ultimately form economically important ore deposits and release gases to the atmosphere. However, the amount and timescales of magma addition to the crust, especially in continental settings, is poorly understood. In this study, we use exposed igneous rocks solidified at different depths in the crust and deposited during volcanic eruptions to determine the amount and timescales of magma addition. We use these newly constrained rates to calculate the total amount of CO2 gas released to the atmosphere by emplacement and crystallization of magma in the crust. Our calculations agree well with present‐day published estimates of CO2 degassing from continental arcs and allow us to extrapolate CO2 fluxes throughout Earth History to track relative changes in CO2 degassing to the atmosphere. Key Points: Depth‐dependent (volcanics to deep crust) and lateral (forearc to backarc) magma volume addition rates are calculated from exposed continental crustal sectionsVolume addition increases with depth and is lower in the forearc and backarc compared to the main arc, while crustal‐wide, long‐term rates are similar between arcsAddition rates are used to calculate global CO2 fluxes from continental arcs; calculated values are similar to global CO2 fluxes estimated by other methods [ABSTRACT FROM AUTHOR]