Abstract Mineral reaction textures are fundamental archives of geological change. Amphibole reaction rims are among the most widely used to reconstruct pre-eruptive magmatic conditions, traditionally interpreted through changes in pressure, temperature and melt composition. However, these interpretations have largely overlooked the role of deformation, ubiquitous during magma ascent. Here we show that amphibole breakdown is not only thermodynamically sensitive, but also mechanically sensitive. Using electron backscatter diffraction (EBSD) analyses of experimental and natural samples, combined with numerical simulations of crystal rotation under magma flow, we demonstrate that pyroxene nucleates topotactically on amphibole, forming rims, but can later reorient in response to strain. In static experiments, gravitational settling alone produces measurable misorientations that can be tracked over time, while natural samples reveal signatures of externally imposed shear. The resulting rim textures encode evolving strain histories, with crystal misorientation distributions tracking both total strain and variations in rim crystallisation and/or deformation rates. With EBSD-derived crystal orientations now shown to capture both thermodynamic and mechanical histories, amphibole reaction rims emerge as four-dimensional petrological recorders, sensitive to pressure, temperature, composition and strain (P–T–X–ε), providing a powerful unified framework for reconstructing magma evolution and the mechanics of magma transport.

The VEI 4 eruption of La Soufrière Volcano underwent a dramatic transition in style from initial lava dome effusion in December 2020 to hazardous explosive activity in April 2021. Understanding the magmatic processes underpinning such transitions is critical. We provide the first comprehensive dataset of the compositions and volatile contents of melt inclusions from the 2021 explosive activity, including the first measurements of pre-eruptive total CO2 at La Soufrière and estimates of magmatic temperature. We use the petrological method to quantify volatile fluxes to the atmosphere of 5.40 ± 0.60 Mt H2O, 0.37 ± 0.04 Mt CO2, 0.13 ± 0.01 Mt SO2, and 0.33 ± 0.04 Mt HCl. Modelling of volatile saturation pressures indicates final magma storage depths of 2.4–8.9 km (mean 6.4 km) throughout four phases of explosive activity after migration of magma in the lower storage region within the transcrustal mush system. Inferred depths are consistent with recorded seismicity and deformation before and during the explosions.

Improved understanding of the magmatic system of Soufrière Hills Volcano, Montserrat (SHV) is needed to inform future hazard management strategy, and remaining uncertainties include the depth of magma storage and the source of ongoing gas emissions. Eruptive activity between 1995 and 2010 has been proposed to be sourced from either a dual chamber or transcrustal mush-based magmatic system, with volatile solubility models using H2O and CO2 from melt inclusion (MI) glass estimating depths of 5–6 km. To date, published SHV MI volatile data have neglected the vapour bubbles now known to sequester the bulk of MI magmatic carbon. Total CO2 concentrations in SHV magma are therefore underestimated, together with volatile-derived entrapment pressures and inferred magma storage depths. Here, we present a new dataset of volatile (H2O and total CO2) and major element concentrations in plagioclase- and orthopyroxene-hosted SHV MI, that span almost all of the eruptive activity (Phases 1, 2, 4, and 5), and include the first measurement of bubble-hosted CO2 for SHV and indeed the Lesser Antilles Arc. Analyses were conducted using Raman spectroscopy, ion microprobe, and electron probe analysis. Dacitic–rhyolitic MI occur within andesitic whole rock compositions. Volatiles in MI glass are similar to published studies (H2O 2.47–7.26 wt%; CO2 13–1243 ppm). However, bubble-hosted CO2 contributes 9–3,145 ppm, to total inclusion CO2 with 5%–99% (median 90%) of CO2 sequestered within bubbles, and total CO2 concentrations (131–3,230 ppm) are significantly higher than previously published values. Inferred entrapment depths from our dataset range from 5.7 to 17 km – far greater than previous estimates – and support a vertically elongated magmatic system where crystallisation spanned both upper- and mid-crustal depths. Our CO2 measurements enable new estimation of CO2 sources and fluxes. As a total of 4.5 Mt of CO2 was held in SHV magma during the aforementioned phases, the maximum amount of CO2 that can be emitted from a batch of SHV magma is ∼1500–1750 tonnes/day. Measured CO2 fluxes are significantly higher, indicating additional input of CO2 into the system from greater depths. Our study shows that including bubble-hosted CO2 redefines understanding of the SHV plumbing system.

After more than three months of lava dome extrusion, La Soufriere (St Vincent) transitioned to a series of explosive eruptions in April 2021. Here we present a time-series petrologic analysis of the phenocryst and microlite populations during the first c. 48 h of explosivity to constrain ascent conditions and processes that drove changes in behaviour. Primary eruptive products were crystal-rich (45-50 vol%) basaltic andesites with similar phenocryst phase assemblages and compositions. The change in eruptive style is consistent with overpressurization as a consequence of second boiling from anhydrous microlite crystallization. The microlites display variation between the explosive phases, with two populations: (1) 'inherited' - normally zoned high-An plagioclase (> An(70)) + olivine (Fo(62-79)) + clinopyroxene + titanomagnetite, inferred to have crystallized at depths < 15 km and high water pressures; (2) 'juvenile' - unzoned plagioclase (An(45-65)) + clinopyroxene + orthopyroxene + intermediate pyroxene (Wo(12-38)) + titanomagnetite, inferred to have crystallized upon ascent due to decompression and degassing. Scoria from the first explosions featured extensive groundmass crystallization and a significant 'inherited' microlite population. Later explosions had a more abundant 'juvenile' microlite population and lower crystallinity, consistent with more rapid ascent from depth, initiated by decompression following initial blasts and destruction of the lava dome.

A critical challenge during volcanic emergencies is responding to rapid changes in eruptive behaviour. Actionable advice, essential in times of rising uncertainty, demands the rapid synthesis and communication of multiple datasets with prognoses. The 2020–2021 eruption of La Soufrière volcano exemplifies these challenges: a series of explosions from 9–22 April 2021 was preceded by three months of effusive activity, which commenced with a remarkably low level of detected unrest. Here we show how the development of an evolving conceptual model, and the expression of uncertainties via both elicitation and scenarios associated with this model, were key to anticipating this transition. This not only required input from multiple monitoring datasets but contextualisation via state-of-the-art hazard assessments, and evidence-based knowledge of critical decision-making timescales and community needs. In addition, we share strategies employed as a consequence of constraints on recognising and responding to eruptive transitions in a resource-constrained setting, which may guide similarly challenged volcano observatories worldwide.

Soufriere Hills Volcano, Montserrat, erupted from 1995 to 2010, with activity including dome growth, destructive pyroclastic density currents and Vulcanian explosions. Monitoring data, such as gas emissions, show the system is still in a state of unrest. The recent eruptions provide an opportunity to study, in real time, a complex subduction-related subvolcanic transaustal melt-mush reservoir, its magma fluxes, and the timing of crystal and melt storage prior to eruptive paroxysms. How and when mush destabilisation occurs prior to volcanic eruptions continues to be a question of intense debate. Evidence of mafic magma intrusion, a potential eruptive trigger, is preserved in enclaves with quenched and diffuse margins that are mingled with crystal-rich andesite. Here, in this first study of Soufriere Hills Volcano zircon, we report zircon ages and compositions for mafic-intermediate enclaves and host andesites from the most recent dome collapse in 2010 to place temporal constraints on magma reservoir processes. Zircon U-238-Th-230 ages disequilibrium crystallisation ages ranging between c. 2-250 ka constrain the longevity of the magmatic plumbing system. Uniform Hf isotopes, epsilon Hf 11.3 +/- 12 to 14.6 +/- 1.5, indicate invariant compositions that are typical for island arc magma sources. Zircon trace element concentrations and Ti-in-zircon crystallisation temperatures indicate crystallisation in isolated, small-volume, lenses with variable fractions of melt of heterogeneous compositions. We suggest amalgamation of assorted crystal cargoes from these lenses occurred prior to eruption during mush destabilisation triggered by mafic magma recharge. Zircon textures, on the other hand, shed light on recent centimetre-scale magma mingling immediately prior to eruption. Euhedral-subhedral zircon is preferentially preserved in or near quenched contacts of the least-evolved enclave and host andesite. By contrast, reheating of the andesite by the mafic magma recharge in the presence of zircon-undersaturated melts promoted zircon resorption. This led to the formation of subhedral-anhedral corroded zircon that is typical in the host andesite mush. Zircon thus reveals processes ranging from 100,000s of years of andesite storage to short-term partial destruction in response to transient heating and magma mixing events. (C) 2021 The Author(s). Published by Elsevier B.V.

Extrusive and intrusive igneous rocks represent different parts of a magmatic system and ultimately provide complementary information about the processes operating beneath volcanoes. To shed light on such processes, we have examined and quantified the textures and mineral compositions of plutonic and cumulate xenoliths and lavas from Bequia, Lesser Antilles arc. Both suites contain assemblages of iddingsitized olivine, plagioclase, clinopyroxene and spinel with rare orthopyroxene and ilmenite. Mineral zoning is widespread, but more protracted in lavas than xenoliths. Plagioclase cores and olivine have high anorthite (An ≤ 98) and low forsterite (Fo ≤ 84) compositions respectively, implying crystallisation from a hydrous mafic melt that was already fractionated. Xenolith textures range from adcumulate to orthocumulate with variable mineral crystallisation sequences. Textural criteria are used to organize the xenoliths into six groups. Amphibole, notably absent from lavas, is a common feature of xenoliths, together with minor biotite and apatite. Bulk compositions of xenoliths deviate from the liquid line of descent of lavas supporting a cumulate origin with varying degrees of reactive infiltration by evolved hydrous melts, preserved as melt inclusions in xenolith crystals. Volatile saturation pressures in melt inclusions indicate cumulate crystallization over a 162–571 MPa pressure range under conditions of high dissolved water contents (up to 7.8 wt% H 2 O), consistent with a variety of other thermobarometric estimates. Phase assemblages of xenoliths are consistent with published experimental data on volatile-saturated low-magnesium and high-alumina basalts and basaltic andesite from the Lesser Antilles at pressures of 200–1000 MPa, temperatures of 950–1050 °C and dissolved H 2 O contents of 4–7 wt%. Once extracted from mid-crustal mushes, residual melts ascend to higher levels and undergo H 2 O-saturated crystallization in shallow, pre-eruptive reservoirs to form phenocrysts and glomerocrysts. The absence of amphibole from lavas reflects instability at low pressures, whereas its abundance in xenoliths testifies to its importance in mid-crustal differentiation processes. A complex, vertically extensive (6 to at least 21 km depth) magmatic system is inferred beneath Bequia. Xenoliths represent fragments of the mush incorporated into ascending magmas. The widespread occurrence of evolved melts in the mush, but the absence of erupted evolved magmas, in contrast to islands in the northern Lesser Antilles, may reflect the relative immaturity of the Bequia magmatic system.

Mafic enclaves hosted by andesite erupted at the Soufrière Hills Volcano between 1995 and 2010 yield insights into syn-eruptive mafic underplating of an andesite magma reservoir, magma mixing and its role in sustaining eruptions that may be widely applicable in volcanic arc settings. The mafic enclaves range in composition from basalt to andesite and are generated from a hybrid thermal boundary layer at the interface between the two magmas, where the basalt quenches against the cooler andesite, and the two magmas mix. We show, using an analytical model, that the enclaves are generated when the hybrid layer, just a few tens of centimetres thick, becomes buoyant and forms plumes which rise up into the andesite. Mafic enclave geochemistry suggests that vapour-saturated basalt was underplated quasi-continuously throughout the first three eruptive phases of the eruption (the end member basalt became more Mg and V-rich over time). The andesite erupted during the final phases of the eruption contained more abundant and larger enclaves, and the enclaves were more extensively hybridised with the andesite, suggesting that at some time during the final few years of the eruption, the intrusion of mafic magma at depth ceased, allowing the hybrid layer to reach a greater thickness, generating larger mafic enclaves. The temporal trends in mafic enclave composition and abundance suggests that basalt recharge and underplating sustained the eruption by the transfer of heat and volatiles across the interface and when the recharge ceased, the eruption waned. Our study has important implications for the petrological monitoring of long-lived arc eruptions.

Since 2004, the satellite-borne Ozone Mapping Instrument (OMI) has observed sulphur dioxide (SO2) plumes during both quiescence and effusive eruptive activity at Soufrière Hills Volcano, Montserrat. On average, OMI detected a SO2 plume 4–6 times more frequently during effusive periods than during quiescence in the 2008–2010 period. The increased ability of OMI to detect SO2 during eruptive periods is mainly due to an increase in plume altitude rather than a higher SO2 emission rate. Three styles of eruptive activity cause thermal lofting of gases (Vulcanian explosions; pyroclastic flows; a hot lava dome) and the resultant plume altitudes are estimated from observations and models. Most lofting plumes from Soufrière Hills are derived from hot domes and pyroclastic flows. Although Vulcanian explosions produced the largest plumes, some produced only negligible SO2 signals detected by OMI. OMI is most valuable for monitoring purposes at this volcano during periods of lava dome growth and during explosive activity.

Abstract Soufrière Hills Volcano produced prodigious quantities of sulphur dioxide (SO2) gas throughout 1995–2013. An unprecedented, detailed record of SO2 flux shows that high SO2 fluxes were sustained through eruptive pauses and for two years after the end of lava extrusion and are decoupled from lava extrusion rates. Lava extrusion rates have exhibited strong 1- to 2-year cyclicity. Wavelet analysis demonstrates periodicities of c. 5 months and c. 2 years within the SO2 time series, as well as the shorter cycles identified previously. The latter period is similar to the wavelength of cycles in lava extrusion, albeit non-systematically offset. The periodicities are consistent with pressure changes accompanying deformation in a coupled magma reservoir system whereby double periodic behaviour may arise from limited connectivity between two reservoirs. During periods of lava extrusion SO2 is released together with the lava (yielding the c. 2 year period), albeit with some offset. In contrast, when magma cannot flow because of its yield strength, SO2 is released independently from lava (yielding the c. 5 month period). Our results have implications for eruption forecasting. It seems likely that, when deep supply of magma ceases, gas fluxes will cease to be periodic.

The recent study by Devine & Rutherford (2014), ‘Magma storage region processes of the Soufriere Hills Volcano, Montserrat’, published as Chapter 19 of the Geological Society of London Memoir, Volume 39, focused primarily on updating the record of magmatic temperatures recorded by Fe–Ti oxides in the andesite erupted from Soufriere Hills Volcano. However, a key result of the paper was that the compositions of some plagioclase-hosted melt inclusions indicate mixing of mafic magma components into the host andesite and trapping of the mixed melts within phenocrysts. This interpretation is consistent with our earlier work on the chemistry of melt inclusions (Humphreys et al . 2010) and of microlite crystal populations (Humphreys et al . 2009 a , 2013). Humphreys et al . (2010) proposed this interpretation by showing that a subset of Soufriere Hills melt inclusions and matrix glasses has anomalous concentrations of K2O and/or TiO2, as have mafic inclusion matrix glasses (see Humphreys et al . 2010, fig. 2). However, in their Appendix, Devine & Rutherford (2014) suggest that enrichment in K2O can result only from decompression crystallization and not from magma mingling. They also assert that our melt inclusion dataset is fundamentally flawed and subject to faulty corrections for post-entrapment correction; and that our melt inclusions were actually matrix glasses. Finally, Devine & Rutherford (2014) doubt ‘whether or not one can look at melt-inclusion analyses and distinguish the effects of decompression crystallization … from the effects of mingling … with components derived from the injected mafic magma’. Here we address the criticisms of the Devine & Rutherford (2014) Appendix, drawing on additional published data to support our arguments and explanations. We were unable to reproduce some of the key figures of Devine & Rutherford (2014) …

Activity since 1995 at Soufrière Hills Volcano (SHV), Montserrat has alternated between andesite lava extrusion and quiescence, which are well correlated with seismicity and ground deformation cycles. Large variations in SO 2 flux do not correlate with these alternations, but high and low HCl/SO 2 characterize lava dome extrusion and quiescent periods respectively. Since lava extrusion ceased (February 2010) steady SO 2 emissions have continued at an average rate of 374 tonnes/day (± 140 t/d), and incandescent fumaroles (temperatures up to 610 o C) on the dome have not changed position or cooled. Occasional short bursts (over several hours) of higher (∼ 10x) SO 2 flux have been accompanied by swarms of volcano‐tectonic earthquakes. Strain data from these bursts indicate activation of the magma system to depths up to 10 km. SO 2 emissions since 1995 greatly exceed the amounts that could be derived from 1.1 km 3 of erupted andesite, and indicating extensive partitioning of sulfur into a vapour phase, as well as efficient decoupling and outgassing of sulfur‐rich gases from the magma. These observations are consistent with a vertically extensive, crustal magmatic mush beneath SHV. Three states of the magmatic system are postulated to control degassing. During dormant periods (10 3 to 10 4 years) magmatic vapour and melts separate as layers from the mush and decouple from each other. In periods of unrest (years) without eruption, melt and fluid layers become unstable, ascend and can amalgamate. Major destabilization of the mush system leads to eruption, characterized by magma mixing and release of volatiles with different ages, compositions and sources.

22 The andesite lava erupted at the Soufrière Hills Volcano is crystal rich with 33-63% phenocrysts, of 23 plagioclase (65%); amphibole (28%), orthopyroxene (7%) and minor Fe-Ti oxide and clinopyroxene 24 microphenocrysts. The andesite hosts mafic enclaves which have similar mineral phases to the andesite. 25 The enclaves are generally crystal poor but can have up to 27% of inherited phenocrysts from the 26 andesite, the majority of which are plagioclase. The eruption is defined by discrete periods of extrusion 27 called phases, separated by pauses. The enclaves exhibit bulk geochemical trends that are consistent with 28 fractionation. We infer that the intruded mafic liquids of Phases I and II interacted and assimilated 29 plutonic residue remaining from the multiple prior mafic intrusions, while the basaltic liquids from Phases 30 III and V assimilated relatively little material. We also infer a change in the basaltic composition coming 31 from depth. The bulk Fe contents of both magma types are coupled and they both show a systematic inter32 phase variation in Fe content. We interpret the coupled Fe variation to be due to contamination of the 33 andesite from the intruding basalt via diffusion and advection processes, resulting in the erupted andesite 34 products bearing the geochemical imprint of the syn-eruptive enclaves. 35

Lavas from the current eruption of the Soufrière Hills Volcano (SHV), Montserrat exhibit evidence for magma mingling, related to the intrusion of mafic magma at depth. We present detailed field, petrological, textural and geochemical descriptions of mafic enclaves in andesite erupted during 2009–2010, and subdivide the enclaves into three distinct types: type A are mafic, glassy with chilled margins and few inherited phenocrysts; type B are more evolved with high inherited phenocryst content and little glass, and are interpreted as significantly hybridized; type C are composite, with a mafic interior (type A) and a hybrid exterior (type B). All enclaves define tight linear compositional trends, interpreted as mixing between a mafic end member (type A) and host andesite. Enclave glasses are rhyolitic, owing to extensive crystallization during quenching. Type A quench crystallization is driven by rapid thermal equilibration …

Abstract Extrusion during Phase 5 (8 October 2009–11 February 2010) produced significant volumetric and geomorphic changes to the lava dome and surrounding valleys at the Soufrière Hills Volcano, Montserrat. Approximately 74×106 m3 of lava was extruded at an average rate of 7 m3 s−1 during the short period of activity. Addition of lava to the pre-existing dome resulted in a net volumetric increase of up to 38×106 m3. Pyroclastic density current (PDC) and ashfall deposits accounted for the remaining 36×106 m3. A series of thick, blocky lobes were extruded from a central vent. In addition, several short-lived spines and two large shear lobes were also extruded. Significant PDC activity resulted in substantial valley filling of up to 108 m. The large pre-existing dome significantly influenced the growth of lobes, such that many block-and-ash flows were generated from viscous lobes draped over the summit and upper slopes. Geomorphic changes caused by rapid filling of the surrounding valleys aided in both flow avulsion and the emplacement of deposits up to 6 km from the dome. These geomorphic changes have important consequences for hazards from PDCs.

Abstract Sulphur dioxide (SO2) diffusion tube monitoring has been undertaken on Montserrat since 1995, providing a unique and insightful long-term dataset of ground-level SO2 concentrations during the eruption of the Soufrière Hills Volcano (SHV). The monitoring of ground-level SO2 is important to assess the potential of human exposure to high levels of SO2 that may impact on health. Air-quality objectives for SO2 are present in some countries to prevent potential health impacts. Here we summarise diffusion tube monitoring in Montserrat and analyse concentrations with respect to the potential for exposure to levels above recommended levels. We explore relationships that may exist with SO2 flux measurements and volcanic events. Concentrations have been higher during pauses in lava extrusion. Diffusion tube concentrations are highest within 5 km of the volcano and at locations downwind of the plume. Areas where concentrations have exceeded relevant limits have been uninhabited since 1996. The potential for human exposure above recommended limits is, therefore, currently considered low, as the population would not have been exposed to high concentrations for extended periods of time. Full-time occupation and/or long-term exposure in the areas where concentrations exceed the relevant limits would not be advised.