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Significant volumes of magma can be intruded into the crust during continental break‐up, influencing rift evolution by altering the thermo‐mechanical structure of the crust and its response to extensional stresses. Rift magmas additionally feed surface volcanic activity and can be globally significant sources of tectonic CO2 emissions. Understanding how magmatism may affect rift development requires knowledge on magma intrusion depths in the crust. Here, using data from olivine‐hosted melt inclusions, we investigate magma dynamics for basaltic intrusions in the Main Ethiopian Rift (MER). We find evidence for a spatially focused zone of magma intrusion at the MER upper‐lower crustal boundary (10–15 km depth), consistent with geophysical datasets. We propose that ascending melts in the MER are intruded over this depth range as discrete sills, likely creating a mechanically weak mid‐crustal layer. Our results have important implications for how magma addition can influence crustal rheology in a maturing continental rift. Plain Language Summary: Continental rifting, the break‐up of continents to form new ocean basins, is a key component in the tectonic cycle that affects Earth's surface environment. The rifting process is aided by magmatic activity in its final stages, which weakens the crust by heating it. This is believed to facilitate present‐day rifting in Ethiopia, where we find rift‐related volcanoes. The depth of magma storage in the rifting crust will determine how heat is distributed, and therefore how the physical properties of the crust are altered. Here we study melt inclusions, small pockets of magmas trapped within growing crystals beneath rift volcanoes. Using the concentrations of CO2 and H2O in melt inclusions we infer the pressures (and therefore depths) that they formed. Our results demonstrate that magmas rising through the Ethiopian crust consistently stall at a depth range of 10–15 km beneath the surface. Furthermore, the diverse chemical composition of our melt inclusions show that magmas are stored in multiple small bodies versus a larger mixed magma reservoir. This study therefore provides new insights into how magmas are stored in the Ethiopian crust before volcanic eruptions and suggests that rising magmas may produce a weak layer in the middle of the rifting crust. Key Points: We determine magma storage conditions in the Main Ethiopian Rift through geochemical analysis of olivine‐hosted melt inclusionsVolatile saturation barometry reveals that basaltic melts are focused at 10–15 km depth in the Ethiopian crustGeochemical heterogeneity in melt inclusions suggests that magma storage is likely to occur in semi‐discrete sills [ABSTRACT FROM AUTHOR]

Abstract Significant volumes of magma can be intruded into the crust during continental break‐up, influencing rift evolution by altering the thermo‐mechanical structure of the crust and its response to extensional stresses. Rift magmas additionally feed surface volcanic activity and can be globally significant sources of tectonic CO2 emissions. Understanding how magmatism may affect rift development requires knowledge on magma intrusion depths in the crust. Here, using data from olivine‐hosted melt inclusions, we investigate magma dynamics for basaltic intrusions in the Main Ethiopian Rift (MER). We find evidence for a spatially focused zone of magma intrusion at the MER upper‐lower crustal boundary (10–15 km depth), consistent with geophysical datasets. We propose that ascending melts in the MER are intruded over this depth range as discrete sills, likely creating a mechanically weak mid‐crustal layer. Our results have important implications for how magma addition can influence crustal rheology in a maturing continental rift.

We consider here the validity, accuracy, and precision of rhyolite-MELTS modelling in inferring the pre-eruptive magma storage conditions for the caldera-forming 25.5 ka Oruanui and 232 CE (1.72 ka) Taupō eruptions at Taupō volcano, New Zealand as proposed by Pamukçu et al. (2020: Contrib Mineral Petrol 175: 48). There are four major issues with the modelling as used. First, the model is inappropriately applied for the Taupō event as this magma does not contain phenocrystic quartz. Second, the products of both eruptions, as is typical for virtually all Taupō Volcanic Zone rhyolites, contain plagioclase but lack sanidine, leading to increased model errors beyond those stated. Third, temperatures utilised for modelling crystallisation histories for both magmas are in conflict with independent measures of magmatic temperatures for these rocks. Fourth, glass compositions for each of these eruptions individually fall within analytical uncertainty yet yield model pressures that vary over hundreds of MPa, in part due to contrasting analytical protocols. We conclude that rhyolite-MELTS is too imprecise (errors of ± ~ 100 MPa or more for the methods applied) for quartz + 1 feldspar systems like at Taupō volcano specifically and the Taupō Volcanic Zone in general to be applied with confidence to deriving relative or absolute depths of magma storage. [ABSTRACT FROM AUTHOR]

Abstract The 2018 lower East Rift Zone (LERZ) eruption and the accompanying collapse of the summit caldera marked the most destructive episode of activity at Kı̄lauea Volcano in the last 200 years. The eruption was extremely well‐monitored, with extensive real‐time lava sampling as well as continuous geodetic data capturing the caldera collapse. This multiparameter data set provides an exceptional opportunity to determine the reservoir geometry and magma transport paths supplying Kı̄lauea’s LERZ. The forsterite contents of olivine crystals, together with the degree of major element disequilibrium with carrier melts, indicates that two distinct crystal populations were erupted from Fissure 8 (termed high‐ and low‐Fo). Melt inclusion entrapment pressures reveal that low‐Fo olivines (close to equilibrium with their carrier melts) crystallized within the Halema’uma’u reservoir (∼2‐km depth), while many high‐Fo olivines (>Fo81.5; far from equilibrium with their carrier melts) crystallized within the South Caldera reservoir (∼3–5‐km depth). Melt inclusions in high‐Fo olivines experienced extensive post‐entrapment crystallization following their incorporation into cooler, more evolved melts. This favored the growth of a CO2‐rich vapor bubble, containing up to 99% of the total melt inclusion CO2 budget (median = 93%). If this CO2‐rich bubble is not accounted for, entrapment depths are significantly underestimated. Conversely, reconstructions using equation of state methods rather than direct measurements of vapor bubbles overestimate entrapment depths. Overall, we show that direct measurements of melts and vapor bubbles by secondary‐ion mass spectrometry and Raman spectroscopy, combined with a suitable H2O‐CO2 solubility model, is a powerful tool to identify the magma storage reservoirs supplying volcanic eruptions.

The 2018 lower East Rift Zone (LERZ) eruption and the accompanying collapse of the summit caldera marked the most destructive episode of activity at Kı̄lauea Volcano in the last 200 years. The eruption was extremely well‐monitored, with extensive real‐time lava sampling as well as continuous geodetic data capturing the caldera collapse. This multiparameter data set provides an exceptional opportunity to determine the reservoir geometry and magma transport paths supplying Kı̄lauea's LERZ. The forsterite contents of olivine crystals, together with the degree of major element disequilibrium with carrier melts, indicates that two distinct crystal populations were erupted from Fissure 8 (termed high‐ and low‐Fo). Melt inclusion entrapment pressures reveal that low‐Fo olivines (close to equilibrium with their carrier melts) crystallized within the Halema'uma'u reservoir (∼2‐km depth), while many high‐Fo olivines (>Fo81.5; far from equilibrium with their carrier melts) crystallized within the South Caldera reservoir (∼3–5‐km depth). Melt inclusions in high‐Fo olivines experienced extensive post‐entrapment crystallization following their incorporation into cooler, more evolved melts. This favored the growth of a CO2‐rich vapor bubble, containing up to 99% of the total melt inclusion CO2 budget (median = 93%). If this CO2‐rich bubble is not accounted for, entrapment depths are significantly underestimated. Conversely, reconstructions using equation of state methods rather than direct measurements of vapor bubbles overestimate entrapment depths. Overall, we show that direct measurements of melts and vapor bubbles by secondary‐ion mass spectrometry and Raman spectroscopy, combined with a suitable H2O‐CO2 solubility model, is a powerful tool to identify the magma storage reservoirs supplying volcanic eruptions. Plain Language Summary: Pockets of frozen magma trapped within olivine crystals, termed "melt inclusions," can provide information about the depths at which magma is stored beneath the surface prior to a volcanic eruption. This is because the amount of CO2 and H2O that can be dissolved in a melt is dependent on the pressure, and therefore the depth. We examine melt inclusions from lava flows produced during the 2018 eruption of Kı̄lauea Volcano. Previous work, based on geophysics, has shown that magma is stored in two main reservoirs at Kı̄lauea, located at ∼1–2‐ and ∼3–5‐km depth. However, because many melt inclusions host almost all of their CO2 within a vapor bubble, which is rarely measured, previous petrological estimates of magma storage depths at Kı̄lauea do not align with the depths of the two reservoirs identified by geophysics. In this study, we measure the amount of CO2 in the glass and the bubble using secondary‐ion mass spectrometry and Raman spectroscopy, respectively. By adding these two measurements together, we can reconstruct the amount of CO2 that was present when melt inclusions were trapped. Calculated depths align remarkably well with geophysical estimates, and demonstrate that the 2018 eruption was supplied by both magma storage reservoirs. Key Points: Petrological, gaseous and geophysical observations can be reconciled by a model where Fissure 8 was supplied from two summit storage reservoirs (∼1–2‐ and 3–5‐km depth)Extensive post‐entrapment crystallization of melt inclusions within high‐Fo olivines (Fo > 81.5) caused ∼90% of the CO2 to enter the vapor bubbleRaman analyses of vapor bubbles combined with choice of a suitable H2O‐CO2 solubility model is required to accurately determine magma storage depths [ABSTRACT FROM AUTHOR]