The compositional variation of individual volcanic centres over time is key to understanding magmatic processes in the underlying crust. La Soufrière volcano, St. Vincent, Lesser Antilles, has erupted predominantly basaltic andesite magmas for hundreds of thousands of years. Sampling of the recently re-exposed crater walls at La Soufrière reveals that sequentially emplaced crater lavas, feeder dykes and lavas on the volcano’s western flank are low-magnesium basaltic andesites with little variation in major and trace element concentrations. A single La Soufrière crater lava unit is more mafic (8 wt.

Understanding subsurface fluid distribution in volcanic reservoirs is critical for geothermal energy development, critical mineral exploration and forecasting eruptions. Here, we use traveltime tomography to image the seismic velocity structure beneath Aluto volcano, the first pilot geothermal project in Ethiopia, located in the Main Ethiopian Rift. Using seismic data recorded from January 2012 to January 2014, we invert for the 3-D P-wave (Vp), S-wave (Vs) and Vp/Vs ratio. To reduce the non-uniqueness in interpretation, we also compare our results with previously published work on attenuation tomography and magnetotelluric images. Elevated Vp/Vs ratios [at 0 km below sea level (bsl)] around productive geothermal wells suggest high fluid content and/or elevated temperature. Vp/Vs values above 1.8 are observed along the caldera rims and hydrothermal vents, indicating fault and fracture systems as primary fluid conduits. High Vp/Vs below 6 km bsl likely reflects high-temperature areas or the presence of partial melt. In contrast, low Vp/Vs ($\lt 1.5$), low Vp, and average to high Vs beneath the caldera at around 5 km bsl is interpreted as a crystallized body with overpressurized gas volume formed during phase separation and transported upward through fractures and fault systems, accumulating at shallower levels. These findings highlight fluid pathways through the caldera rims and faults, with volatile-rich partial melt at greater depth beneath the caldera centre. Traveltime tomography thus offers a valuable constraints on subsurface fluid distribution and is valuable tool in geothermal exploration.

Anticipating the onset of the 2020/2021 effusive‐explosive eruptive sequence at La Soufrière volcano, St. Vincent was challenging despite the established monitoring networks in operation. Here, we integrate petrological data to decipher retrospectively signs of imminent eruption from available pre‐eruptive monitoring data. Using diffusion chronometry, we estimated the timescales over which magmas transported to the surface. We examined olivine crystals hosted in basaltic andesite scoria, categorizing them into four groups based on their textures (euhedral to anhedral) and core compositions (Fo 73–89 ). Multiply zoned olivine populations are tracked through a multi‐stage journey from depth to surface corresponding to periods of magma ascent and accumulation years before eventual eruption. This correlates temporally with two phases of unrest from monitoring data: (a) a protracted priming phase (lasting more than a decade) manifesting in low‐level seismicity, small crater transformations (rockfalls and new fumaroles) and an elevated CO 2 degassing signal; and (b) a subsequent transition phase, initiating just over a year before eruption with the onset of geophysical unrest in the form of discrete episodes of elevated seismicity and volcano inflation. Our findings provide new insight into the dynamics of magma mobilization at La Soufrière. We demonstrate that magmatic unrest in the roots of the sub‐volcanic system precedes geophysical precursors by years, drawing connections between individually ambiguous surface signals over long timescales. Monitoring strategies optimized to detect early stages of magmatic unrest, such as identifying and locating rarer deep seismicity and routine sampling at the crater plume, could improve future responses to volcanic crises in St. Vincent.

Aluto volcano, situated in the central Main Ethiopian Rift (MER) within the northern part of the East African Rift System (EARS) is seismically active, with indications of unrest detected by InSAR. It hosts Ethiopia's first pilot project for geothermal energy. Despite extensive studies, uncertainties remain about the mechanisms of unrest and the existence of a shallow magma chamber beneath Aluto which could drive the hydrothermal system, and is crucial for understanding its geothermal potential. This study investigates Aluto's magmatic and hydrothermal systems using observations of seismicity in the region. We analyze seismic data from January 2012 to January 2014, locating 2,393 events, which lie predominantly along the Wonji Fault Belt (WFB). Event depths reach up to 40 km beneath Aluto, suggesting the presence of a highly crystallized body at shallow depth, consistent with previous magnetotelluric and gravity studies. Deep crustal seismicity likely relates to fluid and/or magmatic processes. High b‐values of 1.97 ± 0.10 at Aluto indicates the presence of fluids. Seismicity is negligible beneath Silti Debre Zeyt Fault Zone (SDFZ), previously identified as a highly conductive, indicative of melt. Focal mechanisms show normal faulting in the direction of rift extension and full‐moment tensor inversions suggest shear‐failure with fluids potentially activating existing faults. We suggest that the magmatic and hydrothermal systems are connected through pre‐existing faults. Understanding this interaction will enhance our knowledge of the geothermal system, volcanic risk, mechanisms of unrest, and emplacement of geothermal brines.

Many volcanoes erupt a limited compositional range of magmas over their lifetime. The composition of these erupted magmas is thought to be buffered by the crystal-rich mush from which melts are sourced (Blundy, 2022). Identifying the origin of erupted magmas helps us to constrain possible locations of melt accumulation in the mush and improve our interpretation of geophysical signals at restless volcanoes.La Soufrière volcano, St Vincent (Eastern Caribbean), has throughout its lifetime produced predominantly basaltic andesite magmas, including most recently in 2020-21. We explore the origin and phase relations of these erupted magmas by performing a series of high-pressure, high-temperature experiments. Melt extracted from a mush will be multiply-saturated on its liquidus with the mush mineral assemblage at the P-T-fO2-XH2O conditions at the time of segregation. In a system with relatively low thermodynamic variance, for example, five or six independent chemical components (as determined by principal component analysis), a large number of coexisting mineral phases (e.g. plag+cpx+amph+oxides) and a well-constrained fO2, multiple saturation can be reduced to an invariant point on the liquidus of the melt in P-T-H2O space. The approach of finding liquidus multiple saturation for igneous rocks offers a novel magma source thermobarometer and hygrometer.Equilibrium high-pressure, high-temperature experiments were performed at 3-8 kbar and 980-1200ºC, fO2 ≈ Ni-NiO buffer, with initial H2O contents of 2-10 wt%. Plagioclase, clinopyroxene and magnetite are found to be ubiquitous in the melt source region. Amphibole is a peritectic phase and forms on the rim of clinopyroxene, with decreasing temperature, in experiments with high water contents. The peritectic reaction involving amphibole is also observed in St Vincent xenoliths (Brown, 2023). Orthopyroxene is stable at high pressures (8 kbar) and low water contents (≤ 6wt% H2O), with its stability field decreasing with pressure. Five-phase multiple saturation at the liquidus (melt fraction ≥85%) is found for initial H2O contents of 7-8 wt%, at 6 kbar pressure (~22 km depth) and temperatures of 1030-1050ºC. The saturating assemblage is a hornblende-gabbro (cpx+plag+amph+Fe-Ti oxides), consistent with the mineralogy of plutonic xenoliths from historic eruptions of St Vincent (Tollan et al., 2012; Fedele et al., 2021). Mineral compositions in these multiply-saturated runs (e.g. very calcic plagioclase An75-85) are similar to those in the xenoliths. Temperatures agree with mineral geothermometry estimates of the 2020-21 eruption (Weber et al., 2023), suggesting little cooling of the magma during ascent from its source region. Seismicity prior to the 2020-21 eruption is also consistent with mid-crustal source depths (Joseph et al., 2022). Magmas sourced from similar depths can account for the limited compositional diversity of La Soufrière over its volcanic history (Fedele et al., 2021). ReferencesBrown, JR. (2023) Doctoral dissertation, Durham University. Blundy, J. (2022) Journal of Petrology, 63(7), egac054.Fedele et al. (2021) Lithos, 392, p.106150Joseph et al. (2022) Nature Communications, 13(1), p.4129.Tollan et al. (2012) Contributions to Mineralogy and Petrology, 163, pp.189-208.Weber et al. (2023) Geological Society, London, Special Publications, 539(1), pp.SP539-2022.

We introduce three new synthetic basalt reference materials and a new high‐precision set‐up for stable carbon isotope measurement in basaltic glasses using a large‐geometry secondary ion mass spectrometry (SIMS) instrument. The new reference materials, characterised for carbon mass fraction and isotope composition, show homogeneity for in situ analysis for the reported set‐up. Their bulk hydrogen mass fraction and isotope ratios are reported. Our SIMS protocol uses multi‐collection, cycling between concurrent measurements of 12 C and 13 C on electron multipliers, and either 30 Si or 18 O, as a reference mass, on a 10 11 Ω resistor Faraday cup. This set‐up achieves high measurement repeatability for δ 13 C down to ± 0.35‰ 1RSE at 1706 +89 / ‐88 μg g ‐1 CO 2 , with ± 1.00‰ 1RSE or better between 163 +5.1 / ‐5.2 and 267 +8.9 / ‐8.9 μg g ‐1 CO 2 , using a 10 nA primary beam current and a 40 μm analytical pit over a 100 cycle analysis. Carbon blanks were characterised by measuring carbon‐free olivines, allowing for blank corrections on δ 13 C measurements. After blank and instrument mass fractionation corrections, we measure δ 13 C in glasses down to 26.16 +0.85 / ‐0.86 μg g ‐1 CO 2 with a final measurement standard sample deviation of ± 2.97‰ 1 s . We report in situ measurements on an ocean floor basaltic glass and a set of synthetic basaltic glasses to demonstrate our approach. Reference materials and the SIMS set‐up improve the accuracy and precision of δ 13 C measurements in natural basaltic glasses across a wide range of geologically relevant carbon contents.

Determining pressures and temperatures of magmas is crucial for addressing diverse challenges in petrology, geodynamics, and volcanology. However, inherent inaccuracies, especially in barometry, have limited the effectiveness of existing models in unravelling the architecture of crustal igneous systems. In this presentation, I will introduce a novel machine learning model, calibrated using an extensive experimental database, to create regression models for extracting P-T conditions of magmas. Calculations are conducted by considering melt chemistry and the coexisting mineral assemblage as input variables.Our approach is versatile, applicable across a wide range of compositions from basalt to rhyolite, covering pressures from 0.2 to 15 kbar and temperatures ranging from 675 to 1400°C. Testing and optimization demonstrate that the model can recover pressures with a root-mean-square error of 1.1-1.3 kbar and temperature estimates with errors as low as 21°C. This indicates that melt chemistry-mineral assemblage pairs reliably capture magmatic variables across a broader spectrum of conditions than previously thought. We propose that this reliability arises from the relatively low thermodynamic variance in natural magma compositions, despite the presence of numerous oxide components.Applying our model to two cases with well-constrained geophysics - Mount St. Helens volcano (USA) and the Askja caldera in Iceland - we analyse dacite whole-rocks from Mount St. Helens, erupted between 1980-1986. These rocks, inferred to represent liquids extracted from a complex mineral mush, yield melt extraction source pressures that align remarkably well with geophysical constraints. For Askja caldera, our model allows to assign basaltic and rhyolitic magma chemistries to distinct seismic wave speed anomalies, highlighting the potential of our model to bridge the gap between petrology and geophysics. Our model, named MagMaTaB, is accessible through a user-friendly web application.

The volatile contents of silicic crustal magmas inform models for volcanism, crustal growth, degassing behaviour, and hydrothermal ore formation. Volatile saturation pressures of phenocryst-hosted melt inclusions are generally restricted to late-stage, shallow magmas that have undergone significant, ascent-driven degassing of H2O, CO2 and SO2. As an alternative approach, we analyse the volatile contents of rhyolitic melt inclusions (10-30 mu m diameter) in the accessory mineral zircon. We have developed a technique for the reheating of microcrystalline melt inclusions at elevated temperature (950 degrees C) and pressure (100 MPa) prior to analysis, to form homogeneous glassy inclusions without fracturing the host zircon. Analyses of volatiles were performed using secondary ion mass spectrometry and calculated volatile saturation pressures were used to obtain minimum melt inclusion trapping depths. Our results reveal that zircons often grow over an exceptional crustal depth range; many zircons trap melts deeper than 20 km. Significantly, the deepest melt inclusions from porphyry copper deposit-related magmas have high CO2 contents (up to 4000 ppm), indicating CO2-rich sources (fluid molar fraction of CO2 up to 0.95). Zircon crystallisation and the trapping of melt inclusions testify to silicic melt generation over the entire crustal depth range. Deep crustal zircons can be transported to shallow levels by silicic melts percolating through transcrustal mush systems and/or in rapidly ascending volatile-charged magmas.

AbstractArc magmas have higher water contents (2-6 wt.% H2O) than magmas generated in other tectonic environments, with a growing body of evidence suggesting that some deep arc magmas may be ‘super-wet’ (>6 wt.% H2O). Here, we use thermodynamic modelling to show that the behaviour of zirconium during magmatic differentiation is strongly sensitive to melt water contents. We demonstrate that super-wet magmas crystallise zircon with low, homogeneous titanium concentrations (75th percentile <10 ppm) due to a decrease in zircon saturation temperatures with increasing melt H2O. We find that zircon titanium concentrations record a transition to super-wet magmatism in Central Chile immediately before the formation of the world’s largest porphyry copper deposit cluster at Río Blanco-Los Bronces. Broader analysis shows that low, homogeneous zircon titanium concentrations are present in many magmatic systems. Our study suggests that super-wet magmas are more common than previously envisaged and are fundamental to porphyry copper deposit mineralisation.

Primary magmas in volcanic arcs exhibit wide compositional diversity on both local and global scales. Processes responsible for this diversity are generally ascribed to some combination of mantle melting or crustal differentiation processes. One widespread view is that arc magmagenesis is driven by combination of H2O-fluxed and decompression melting of peridotitic mantle wedge, and that primary, mantle-derived melts are high-MgO basalt. However, a variety of other mantle-derived primitive arc magmas, ranging in composition from highMg andesite to picrite, has been recognised and it remains unclear to what extent this diversity can be generated by mantle melting processes modulated, for example, by changes in the thermal state of the mantle wedge or the supply of fluid from the slab. Here we use high pressure and temperature experiments to constrain magma generation conditions of a primitive magnesian (8.8 wt% MgO) basaltic andesite from Klyuchevskoy volcano, Kamchatka arc, Russia. We use an inverse experimental approach to define a multiple saturation point on the liquidus surface of the basaltic andesite. The experimental multiple saturation point defines the pressure and temperature at which an erupted melt could have last been in equilibrium with a polymineralic source rock, such as mantle peridotite, and hence provides a robust estimate of magma source conditions. Equilibrium piston-cylinder experiments were carried out between 0.5 and 1.0 GPa under hydrous conditions (3 to 6 wt% added H2O) at fO2 = Delta NNO+1. We show that Klyuchevskoy basaltic andesite is multiply saturated with the lherzolite assemblage olivine (Fo90) + clinopyroxene + orthopyroxene + Cr-spinel close to its liquidus (>= 95% melt) in the pressure range of 0.6 to 1 GPa (23 to 36 km depth) and 1220-1240 degrees C. Amphibole is present at temperatures just below the multiple saturation point (<= 1200 degrees C). Our results show that basaltic andesite was produced by 8 to 11 wt% partial melting of amphibole-lherzolite source and therefore represents a primary, undifferentiated magma extracted from its source at near-Moho depths. These findings are in a good agreement with geophysical studies of Klyuchevskoy volcano that show magmas are supplied directly from a reservoir at near-Moho depths (25-30 km) through a sub-vertical, pipe-like feeder system. Coeval high-MgO basalts from the volcano may correspond to higher degree mantle melts extracted at slightly greater depths. Our results provide a tight constraint on the thermal structure of the mantle wedge beneath Klyuchevskoy. Experimental temperatures are higher than those calculated from steady-state thermal models suggesting that upwelling asthenosphere might directly impinge the Moho in a similar fashion to mid-ocean ridges. Intra-arc rifting promotes asthenospheric decompression beneath the Central Kamchatka Depression. The presence of amphibole in our experiments at temperatures up to 1200 degrees C indicates that dehydration melting of amphibole peridotite formed by metasomatism of mantle wedge by slab-derived fluids is the primary magma generating process. Amphibole stability exercises an important control on melting conditions. Integrating our results with published multiple-saturation experiments we show that Klyuchevskoy basaltic andesite is one of a family of primary arc magmas whose compositions depend on mantle wedge thermal structure and H2O activity.

The 2020-21 eruption of La Soufriere, St Vincent began with extrusion of a viscous lava dome, which was destroyed upon transition to a major explosive phase. Here we present petrological data to reconstruct the processes leading up to these events. Bulk-rock SiO2 contents range from 52.8 to 55.4 wt%, classifying the lava and the subsequent scoria as basaltic andesite, the latter being slightly more mafic. Macrocrystal chemistry and modes (plag-cpx-opx-tmt-ol) and crystallinity (45-50 vol%) are largely identical for both phases of the eruption. Pyroxenes are homogenous and precipitated mostly from andesitic melts. Conversely, plagioclase shows strong normal zonation resulting from magma ascent and stalling at multiple crustal levels. Clinopyroxene thermobarometry reveals that crystallization predominantly took place between 8 and 13 km depth at temperatures of 997(-35)(+18) degrees C. A lack of evidence for mafic recharge and changes in volatile content and the omnipresence of xenoliths, suggests pre-eruptive destabilization of an andesitic-dacitic melt pocket that disrupted and entrained antecedent mush. Olivine diffusion profiles show that this interaction preceded the onset of eruption. Low dissolved sulfur contents (<= 270 ppm S) place constraints on the total SO2 gas release. Meltmush disruption appears to be a dominant driver of eruptions at La Soufriere.

AbstractCompositions of plagioclase‐melt pairs are commonly used to constrain temperatures (T), dissolved water contents (H2O) and pressures (P) of pre‐eruptive magma storage and transport. However, previous plagioclase‐based thermometers, hygrometers, and barometers can have significant errors, leading to imprecise reconstructions of conditions during plagioclase growth. Here, we explore whether we can refine existing plagioclase‐based hygrothermobarometers with either plagioclase‐melt or melt‐only chemistry (±T/H2O), calibrated using random forest machine learning on experimental petrology data (n = 1,152). We find that both the plagioclase‐melt and melt‐only models return similar cross‐validation root‐mean‐square errors (RMSEs), as the melt holds most of the P‐T‐H2O information rather than the plagioclase. T/H2O‐dependent melt models have test set RMSEs of 25°C, 0.70 wt.% and 76 MPa for temperature, H2O content and pressure, respectively, while T/H2O‐independent models have RMSEs of 38°C, 0.97 wt.% and 91 MPa. The melt thermometer and hygrometer are applicable to a wide range of plagioclase‐bearing melts at temperatures between 664 and 1355°C, and with H2O concentrations up to 11.2 wt.%, while the melt barometer is suitable for pressures of ≤500 MPa. An updated plagioclase‐melt equilibrium model has also been calibrated, allowing the equilibrium anorthite content to be predicted with an error of 5.8 mol%. The new P‐T‐H2O‐An models were applied to matrix glasses and melt inclusions from the 1980 Mount St Helens (USA) and 2014–2015 Holuhraun (Iceland) eruptions, corroborating previous independent estimates and observations. Models are available at https://github.com/kyra‐cutler/Plag‐saturated‐melt‐P‐T‐H2O‐An, enabling assessment of plagioclase‐melt equilibrium and characterization of last‐equilibrated P‐T‐H2O conditions of plagioclase‐saturated magmas.

SUMMARY Dyke propagation is a mechanism for more rapid ascent of felsic magmas through the crust than is possible via diapirs or percolative flow. As it ascends, the magma undergoes complex physical and chemical transformations induced by decompression and cooling. These processes dramatically change the magma density and viscosity, which in turn affect magma ascent rate and the depth at which the dyke arrests. We present a mathematical model of dyke propagation for silicic magmas taking into account the presence of multiple volatile species (H2O and CO2), bubble growth, heat advection and loss, crystallization and latent heat release. We consider conditions for dykes associated with porphyry ore deposits, which may represent an end-member in rapid ascent of felsic magmas from depth. In particular, we simulate the propagation of dykes launched from a deep (900 MPa), volatile-saturated magma source, testing the effects of the magma H2O/CO2 content, temperature and mass on its ascent rate and final emplacement depth. The model predicts short ascent times (hours to days), with a large increase in viscosity at shallow depth, leading to stagnation and solidification of the dyke. Higher initial water content, higher temperature and larger mass of the magma in the dyke promote faster propagation and shallower arrest. Volatile loss from ascending magma remains limited until the stagnation depth, providing a potential mechanism for transfer of deep volatiles to hypabyssal blind intrusions associated with porphyry ore deposits. Our findings are applicable to the problem of silicic magma ascent through the crust more generally.

Intra-arc diversity in volcanic activity and composition is ubiquitous, but its underlying causes remain largely unresolved in many settings. In this work, we examine such variability in the Grenadines archipelago, southern Lesser Antilles arc. Here, juxtaposed volcanic centres exhibit eruptive longevities and chemistries distinct from northern counterparts in the same arc. Our goal is to explain this deviation by investigating variations in magmatic processes using petrological data from erupted crustal xenoliths and lavas, and interpreting these findings within the context of the archipelago's tectonic history and geophysical structure. Textural analyses of xenoliths reveal crystallization over a wide range of pressure-temperature-melt composition conditions in the crust. Mineral phases display discrete compositional trends pointing towards significant inter-island variability in underlying plumbing systems. The geochemical variety of erupted magmas is reminiscent of the entire arc. We speculate that the Grenadines represents the early onset of subduction forming the modern-day Lesser Antilles arc. Extrusive volcanism initiated as submarine activity. Subsequent uplift eroded the original topography of these volcanic centres following the eventual cessation of volcanism in the Neogene. The positioning of the Grenadines on an elevated platform provides rare modern insight into early arc crust formation not commonly preserved in established active arcs.

Experimentally calibrated models to recover pressures, temperatures and water contents of magmas, are widely used in igneous petrology. However, large errors, especially in barometry, limit the capacity of these models to resolve the architecture of crustal igneous systems. Here we apply machine learning to a large experimental database to calibrate new regression models that recover P-T-H2O of magmas based on either melt composition or melt composition plus associated phase assemblage. The method is applicable to compositions from basalt to rhyolite, pressures from 0.2 to 14.5 kbar, temperatures of 675-1425°C, and H2O contents up to 15 wt.%. Testing and optimisation of the model show that pressures can be recovered with root-mean-square-error (RMSE) of 1.2 and 1.0 kbar for the melt-only and melt-phase assemblage models respectively. Errors on temperature estimates are 22-25°C and 1.0-1.3 wt.% for H2O. Our findings demonstrate that melt chemistry is a reliable recorder of magmatic variables. This is a consequence of the relatively low thermodynamic variance of natural magma compositions despite their relatively large number of constituent oxide components. We apply our model to three contrasting cases with well-constrained geophysical information: Mount St. Helens volcano in the Cascades arc (USA), the Altiplano-Puna Volcanic Complex (APVC) in Chile, and the Askja caldera in Iceland. Dacite whole-rocks from Mount St Helens erupted 1980-1986 yield magma source pressures of 3.3-4.3 kbar in excellent agreement with experimental petrology, seismic tomography, magnetotelluric images and earthquake hypocentres. Melt inclusions and matrix glasses record lower pressures, consistent with magma crystallisation during ascent. Glasses and phase assemblages for three large magnitude APVC eruptions (Atana, Toconao, Purico ignimbrites) yield magma storage pressures and temperatures of 1.7-2.4 kbar and 726-789°C, in excellent agreement with previous thermobarometry. While these pressures are shallower than the underlying Altiplano-Puna Magma Body (APMB), rhyolite whole-rock compositions indicate pressures equivalent to the top of the APMB. We suggest that extraction of rhyolitic liquids from the APMB mush, followed by crystallisation at shallower depth preceded each eruption. Magma reservoir depth estimates for historical eruptions from Askja match the location of seismic wave speed anomalies. We show that Vp/Vs anomalies at 5-10 km depth correspond to hot (~1000°C) rhyolite source regions, while basaltic magmas (~1150°C) were stored at 15 km depth under the caldera. These examples illustrate how our model can link petrology and geophysics to better constrain the architecture of volcanic feeding systems. Our model (MagMaTaB) is accessible through a user-friendly web application (https://igdrasil.shinyapps.io/MagMaTaBv23/).

The Okataina Volcanic Centre (OVC) is the most recently active caldera system in the Taupō Volcanic Zone, Aotearoa New Zealand. Although best known for its high rates of explosive rhyolitic volcanism, there are several examples of basaltic to basaltic-andesite contributions to OVC eruptions. These range from minor involvement of basalt in rhyolitic eruptions to the exclusively basaltic 1886 C.E. plinian eruption of Tarawera. To explore the basaltic component supplying this dominantly rhyolitic area, we analyse the textures and compositions (minerals and melt inclusions) of four basaltic eruptions from within and around the OVC that have similar whole rock chemistry, namely: Terrace Rd, Rotomakariri, Rotokawau, and Tarawera. Data from these basaltic deposits provide constraints on the conditions of magma evolution and ascent in the crust prior to eruption, revealing that eruptions sample multiple distinct reservoirs during ascent to the surface. The most abundant basaltic component is generated by cooling-induced crystallisation of a common, oxidised, volatile-rich basaltic melt at various depths within the crust that mixes upon ascent. Despite similar bulk compositions, these four eruptions are texturally distinct from each other as a result of their wide variation in eruption style.

Understanding the crustal structure and the storage and movement of fluids beneath a volcano is necessary for characterizing volcanic hazard, geothermal prospects and potential mineral resources. This study uses local earthquake traveltime tomography to image the seismic velocity structure beneath Nabro, an off-rift volcano located within the central part of the Danakil microplate near the Ethiopia-Eritrea border. Nabro underwent its first historically documented eruption in June 2011, thereby providing an opportunity to analyze its post-eruptive state by mapping subsurface fluid distributions. We use a catalog of earthquakes detected on a temporary seismic array using machine learning methods to simultaneously relocate the seismicity and invert for the three-dimensional P- and S-wave velocity structures (V-P, V-S) and the ratio between them (V-P/V-S). Overall, our model shows higher than average P- and S-wave velocities, suggesting the presence of high-strength, solidified intrusive magmatic rocks in the crust. We identify an aseismic region of low V-P, low V-S, and high V-P/V-S ratio at depths of 6-10 km b.s.l., interpreted as the primary melt storage region that fed the 2011 eruption. Above this is a zone of high V-S, low V-P, and low V-P/V-S ratio, representing an intrusive complex of fractured rocks partially saturated with over-pressurized gases. Our observations identify the persistence of magma in the subsurface following the eruption, and track the degassing of this melt through the crust to the surface. The presence of volatiles and high temperatures within the shallow crust indicate that Nabro is a viable candidate for geothermal exploration.

Magmatic-hydrothermal fluids transport chalcophile metals to the atmosphere as volcanic gases; and to the crust, where they may play a role in the formation of ore deposits. Global volcanic gas datasets show considerable variability in the flux and composition of metals outgassed between volcanoes, but the controls on this variability are unclear. Magmatic chloride is a key ligand for metal transport but magmatic water dominates the exsolved fluid reservoir into which metals partition during crystallisation and decompression. Here we develop models simulating decompression-driven degassing (‘first boiling’) and isobaric crystallisation-driven degassing (‘second boiling’) of magmas to show that while moderate concentrations of chlorine are essential for metal partitioning into the magmatic fluids, magmatic water contents have the greatest potential to control the mass yield of metals carried by exsolving fluids. Our models explain why water-rich magmatic-volcanic systems like Mount Etna (Italy) deliver the largest mass fluxes (per unit of degassing magma) of metals to the atmosphere, whereas relatively dry magmatic-volcanic systems like Yasur (Vanuatu) deliver the smaller mass fluxes. Our results establish the important role of magmatic water in generating large reservoirs of metal-rich aqueous fluids in the shallow crust.

The key processes responsible for the genesis of world-class porphyry copper deposits remain ambiguous. A high water content of parental magmas evolved in the deep crust is posited to be one key control on their formation, yet remains controversial. Here, we report trace element compositions of whole-rocks and zircons that pre-, syn- and post-date mineralisation from several major porphyry copper districts. We find clear temporal trends of decreasing zircon Ti and whole-rock Zr. Across all studied deposits and literature data, we find low zircon Ti and whole-rock Zr concentrations are inherent to intrusive rocks associated with porphyry Cu deposits. According to the Ti-in-zircon thermometer and zircon solubility models, these concentrations are consistent with porphyry Cu-related magmas being cold relative to other intermediate-felsic magmas (<800 degrees C). We perform thermodynamic modelling to demonstrate that such low temperature magma differentiation can be readily explained by elevated water concentrations (>6 wt.%) of porphyry Cu magmas at depth which displaces the liquidus to lower temperatures. By integrating this with the latest zircon solubility models, we show that wet magma differentiation leads to zircon appearing as a relatively early crystallising phase at low temperature. Our study therefore provides new evidence that magmas related to porphyry Cu deposits contain characteristically high water contents (>6 wt.%).