The lack of visual observations and difficulty recreating ambient conditions in the laboratory mean deep subaqueous volcanic eruptions (>500 m water depth) remain enigmatic. However, the pressure and temperature dependency of water speciation and solubility in volcanic melts and glasses, and water's high diffusion rate, offers a potential window into the processes and conditions during deep subaqueous volcanism. The 2012 submarine eruption of Havre Volcano is the ideal laboratory in which to use water species distributions to understand submarine eruption processes. Post-eruption AUV mapping, and ROV observation and sampling generated an unprecedented amount of contextual information for a deep submarine eruption including on the magma ascent conditions, eruption sequence, and magma fragmentation. Here we present high spatial resolution synchrotron-FTIR measurements of water speciation in glassy ash (125-500 mu m) from several subunits (1, 2 and 3) of a widespread ash with lapilli deposit produced in the 2012 eruption. Measurements record OH depletion profiles around vesicles, along with H2Omol enrichment profiles. In several thick vesicle walls far-field regions unaffected by processes at the bubble margins have also been identified. By focusing on OH concentrations, which is effectively non-diffusing below the glass transition, we can remove the effects of secondary rehydration and calculate the pressure and water depth at which ash grain quenched. Far-field regions record quench depths within the 650-900 m water depth range for pre- and post- eruption vent depth, suggesting ash grains equilibrated at vent depth. OH depletion profiles produce quench pressures on vesicle margins typically corresponding to 200-400 m water depth shallower, suggesting a 2-4 MPa decompression before ash quench. Diffusion modelling suggests that at eruption temperature the observed OH depletion profiles formed through exsolution in <2 s. The difference in quench pressure within the ash is not reflective of a difference in the depth of quenching. Rather it is reflective of the rapidity of the quench process relative to the diffusion rate. The results imply transient low-pressure conditions formed during ash generation in the 2012 Havre eruption. We discuss several physical processes in the context of submarine volcanism that may explain the formation of transient low-pressure regions. The results have implications for how we understand submarine eruption processes and highlight the potential of high resolution FTIR for studying submarine volcanism.

EDITORIAL article Front. Earth Sci., 14 April 2023Sec. Marine Geoscience Volume 11 - 2023 | https://doi.org/10.3389/feart.2023.1184033

Deep subaqueous volcanic eruptions (> 500 m below sea level (mbsl)) remain enigmatic due to a lack of visual observations and difficulty recreating ambient conditions in the lab. Eruptive activity at West Mata seamount in May 2009 remains one of two deep subaqueous eruptions to have ever been filmed. A distinct low-intensity eruptive style, termed bubble escape activity, was observed at Hades vent (1200 mbsl) characterized by the ascent and implosion of 0.2 -1 m diameter volatile-filled vapor bubbles (Resing et al., 2011). Video of this volcanic activity is used to constrain simple numerical models and produce the first subaqueous eruption actualistic model driven by visual observations. Bubble escape activity occurs in three stages defined by changing exsolved volatile and lava behavior. During Stage 1, vapor bubble ascent in a magma filled conduit drives either ductile or brittle deforma-tion of the lava surface at the vent, depending on the timescale of lava cooling prior to bubble escape activity. Fragmentation of the lava during Stage 2 culminates with the vapor bubble coming into direct contact with the ambient water. At this point, Stage 3, bubbles implode through rapid condensation and contraction of the exsolved volatile phase, due to rapid heat loss from the vapor bubble to the ambient water. Numerical modeling of exsolved volatile expansion during conduit ascent to vents across the ocean depth range has identified a transition in exsolved volatile expansion characteristics at 2 -5 MPa. This transition would produce a fundamental change in eruption processes, from which the characteristics and depth range of deep and shallow end members of bubble escape activity are defined. Bubble escape activity highlights implosive behavior driven by underpressure that develops during exsolved volatile contraction as a key, but often overlooked, component of both pyroclastic and effusive subaqueous volcanism across the entire ocean depth range. This stands in contrast to overpressure driving subaerial explosive eruptions. The fact that exsolved volatiles can expand, contract, or maintain an approximately constant volume in subaqueous volcanism also calls for the careful application of terminology (e.g. explosive) to describe subaqueous eruption processes. (c) 2021 Elsevier B.V. All rights reserved.

The Okinawa Trough (OT) is an incipient continental back-arc basin that extends ~1200 km south from Kyushu to Taiwan, and can be split in to three segments, the Northern (NOT), Middle (MOT), and Southern (SOT). We present petrological descriptions, major and trace element, and Sr–Nd isotopic data for a bimodal sample suite of lavas and pumice collected from various seafloor knolls and ridges in the Io and Iheya grabens in the MOT, and from a volcanic ridge in the Yaeyama graben and an isolated knoll in the SOT. Samples were dredged during the R/V Sonne HYDROMIN1 and 2 cruises in 1988 and 1990, respectively. The mafic end member ranges from ~50 to 56 wt% SiO 2 and records a subduction signature, with MORB normalised multi-element plots showing enrichment of incompatible elements and depletion of Nb and Ta. Both tholeiitic and calc-alkaline trends are recorded with basalts from Yaeyama graben and a single mafic enclave from Iheya graben showing tholeiitic affinities, while basalts erupted in Iheya graben and three mafic enclaves from Io graben are calc-alkaline. Major and trace element data also suggest at least two groups of basalt in Iheya Graben, defined by the relative depletion in incompatible elements, and enrichment in MgO, Co, Sr of one group vs the other. The silica end member ranges from ~71 to 77 wt% SiO 2 , with all samples recording the same subduction signature present in the mafic end member. However, there is significant diversity in the incompatible elements of the

The Okinawa Trough (OT) is an incipient continental back-arc basin that extends from Kyushu in the north to Taiwan in the south. The Okinawa Trough can be split in to three segments, the Northern (NOT), Middle (MOT), and Southern (SOT) with active back-arc volcanism restricted to volcanic centres located in en-echelon grabens the MOT and SOT. Previous studies have shown magmatism in the OT is bimodal (basaltic to rhyolitic), with at least two types of silicic melts inferred to form through pure fractional crystallisation from basalt and by fractional crystallisation along with minor crustal assimilation (Shinjo and Kato, 2000). Here we present petrological descriptions, along with major, trace element and Sr–Nd isotopic data for 75 silicic end member samples recovered as both lava and pumice, collected during the R/V Sonne HYDROMIN1 and 2 cruises in 1988 and 1990, respectively. Samples were dredged from various seafloor knolls and ridges located in the Io and Iheya grabens and from Izena Hole in the MOT, and from a single volcanic ridge in the Yaeyama graben and a single isolated knoll in the SOT. Results show a chemically highly diverse silicic end member magmas, with at least four identifiable groups based on differences in the degree of enrichment of incompatible elements (LREE, K, Rb, Ba, etc.). Each group contains at least one dense lava sample suggesting the chemical diversity is a primary feature of magmatism in the Okinawa Trough rather than a result of the floating in of pumiceous material from various locations. Using petrological descriptions and the chemistry of samples along with MELTS modelling we plan to calculate magma formation conditions and identify any evidence of magma mixing or crustal assimilation. In doing so we hope to provide a model to explain the diversity of silicic magma chemistry in the MOT and SOT. Shinjo, R., and Kato, Y. (2000). Geochemical constraints on the origin of bimodal magmatism at the Okinawa Trough, an incipient back-arc basin. Lithos 54, 117–137. doi:10.1016/S0024-4937(00)00034-7.

Understanding clast dispersal from subaqueous volcanism is hampered by uncertainty in the source and extent of seafloor deposits. Extensive samplingin situof seafloor deposits from the 2012 submarine eruption of Havre volcano provides an ideal opportunity to assess subaqueous dispersal. The 2012 Havre eruption produced 14 lavas/domes, a pumice raft, and three seafloor clastic deposits. At Havre the source of clastic deposits can be confidently identified, and deposit thickness, grain size, and distribution are also well-constrained. We examine a seafloor deposit termed subunit 3 (S3) generated in the 2012 Havre eruption to investigate dispersal of fine lapilli and ash, and the eruption conditions that generated this deposit. Subunit 3 is the third from bottom of four subunits that make up the Ash with Lapilli unit. Subunit 3 is composed of ash with highly elongate shapes, unique within the 2012 Havre deposits. It thickens and coarsens toward Lava G, also generated in the 2012 eruption, located on the southwest wall of Havre caldera. Lava G is the only lava produced during the 2012 Havre eruption that has a glassy carapace with elongated vesicles and a fibrous texture. We infer the source of unit S3 is Lava G, due to the spatial pattern of deposit thinning and fining away with distance from this lava, and the morphological and microtextural similarity of ash with the Lava G carapace rock. Grain size and transport distance of ash from S3 are used to test a simple 1D model addressing both clast dispersal by a buoyant thermal plume above an explosive eruption, and by penetrative convection during effusive lava emplacement. Comparison of calculated maximum dispersal distances with grain size and transport distance show that a jet forming eruption generating a turbulent plume is required to generate S3. We suggest that S3 was generated by hybrid explosive-effusive activity during the effusion of Lava G. Using model results we calculate maximum clast dispersal distances across a range of grain sizes for both dispersal mechanisms. The calculated maximum clast dispersal distance has wide implications globally for interpretation of ash deposits from subaqueous eruptions.

The majority of Earth's volcanic eruptions occur beneath the sea, but the limited number of direct observations and samples limits our understanding of these unseen events. Subaerial eruptions lend some insight, but direct extrapolation from the subaerial to the deep sea is precluded by the great differences in pressure, thermal conditions, density and rheology, and the interplay among them. Here we present laboratory fragmentation experiments that mimic deep-sea explosive eruptions and compare our laboratory observations with those from the kilometre-deep submarine eruption of Havre Volcano, Kermadec Arc, New Zealand, in 2012. We find that the Havre eruption involved explosive fragmentation of magma by a pressure-insensitive interaction between cool water and hot magma, termed an induced fuel–coolant interaction. The laboratory experiments show that this water–magma interaction is initiated by the formation of cracks in cooling magma into which the water coolant can infiltrate, driving explosive fragmentation. Explosive submarine eruptions have previously been considered unlikely because stabilization of a vapour film at the magma–water contact was thought to be a key requirement but is suppressed at depths exceeding 100 m. However, here we demonstrate that these induced fuel–coolant interactions between magma and water can occur in a range of wet environments regardless of pressure, from the subaerial to the deep sea, and may operate on different planets, as well as apply to materials other than magma and water. Interactions between magma and water can drive explosive fragmentation eruptions of the type seen in the Havre volcanic eruption, New Zealand, in 2012, even under submarine conditions, according to laboratory fragmentation experiments.

Microlite textural characteristics are analyzed in the products of Vulcanian explosions, ash venting occurring synchronously with lava effusion (syn-extrusive ash venting), a precursory explosion, and lava dome effusion associated with Phases 3, and 5 of the current Soufriere Hills Volcano (SHV) eruption. 2D microlite population statistics and crystal size distributions are used to infer decompression pathways for each style of volcanism. In addition, magma ascent rates for each eruptive style were calculated using microlite number density (Toramaru et al. 2008). Strong differences between pyroclastic and effusive microlites are observed in 2D population statistics and crystal size distributions, indicating higher amounts of smaller microlites (<0.0054 mm) in the lava dome compared with pyroclastic products. Crystal size distributions show that the lava dome and Vulcanian explosion samples have similar amounts of large microlites (>0.0084 to 0.012 mm). However, syn-extrusive ash venting samples have comparatively greater amounts of large microlites. The similarity between lava dome and Vulcanian explosions at larger microlite sizes suggests comparable crystallization conditions in the deeper conduit system (>2 km depth). The contrast in eruption style despite similarity at depth suggests a shallow control to eruption processes at SHV. The greater amounts of large microlites in syn-extrusive ash venting compared to the lava dome indicates two distinct decompression pathways within a single conduit, inferred to result from differences in ascent rate. Sluggish conduit margin ascent leading to lower decompression rates is inferred to produce the increased degree of crystal growth observed in syn-extrusive ash venting compared with the lava dome. Calculated magma ascent rates for Vulcanian explosions and syn-extrusive ash venting have similar ranges, while lava effusion is up to a factor of 10 faster. Differences in magma ascent rate are inferred to result from variations in the depth at which ascent rate is calculated for each eruptive style. Ascent rate is calculated at microlite nucleation depth using the method of Toramaru et al. (2008). However, the average microlite nucleation depth will vary because of differences in microlite nucleation rate during ascent. More small microlites in the lava dome suggest greater nucleation rates in the shallow conduit, recording an on average shallower magma ascent rate (<1 km depth). Rapid decompression during pyroclastic eruptions restricts shallow crystallization, suggesting the microlite population is on average recording the deeper conduit system (>2 km depth). (C) 2019 Elsevier B.V. All rights reserved.

Submarine eruptions dominate volcanism on Earth, but few are observed or even identified. Knowledge of how they operate is largely based on inference from ancient deposits, lagging by a decade or more our understanding of subaerial eruptions. In 2012, the largest wholly deep-subaqueous silicic eruption with any observational record occurred 700–1220 m below sea level at Havre volcano, Kermadec Arc, New Zealand. Pre- and post-eruption shipboard bathymetry surveys, acquisition by autonomous underwater vehicle of meter-scale-resolution bathymetry, and sampling by remote-operated vehicle revealed 14 seafloor lavas and three major seafloor clastic deposits. Here we analyze one of these clastic deposits, an Ash with Lapilli (AL) unit, which drapes the Havre caldera, and interpret the fragmentation and dispersal processes that produced it. Seafloor images of the unit reveal multiple subunits, all ash-dominated. Sampling destroyed layering in all but two samples, but by combining seafloor imagery with granulometry and componentry, we were able to determine the subunits’ stratigraphy and spatial extents throughout the study area. Five subunits are distinguished; from the base these are Subunit 1, Subunit 2a, Subunit 3, Subunit 4 (comprising the coeval Subunit 4 west and Subunit 4 east), and Subunit 2b. The stratigraphic relationships of the four AL unit subunits to other seafloor products of the 2012 Havre eruption, coupled with the wealth of remote-operated vehicle observations and detailed AUV bathymetry, allow us to infer the overall order of events through the eruption. Ash formed by explosive fragmentation of a glassy vesicular magma and was dispersed by a buoyant thermal plume and dilute density currents from which Subunits 1 and 2 were deposited. Following a time break (days/weeks?), effusion of lava along the southern caldera rim led to additional ash generation; first by syn-extrusive ash venting, quenching, brecciation, and comminution (S3 and S4e) and then by gravitational collapse of a dome (S4w). Slow deposition of extremely fine ash sustained S2 deposition across the times of S3 and S4 emplacement, so that S2 ash was the last deposited. These thin ash deposits hold information critical for interpretation of the overall eruption, even though they are small in volume and bathymetrically unimpressive. Ash deposits formed during other submarine eruptions are similarly likely to offer new perspectives on associated lavas and coarse pumice beds, both modern and ancient, and on the eruptions that formed them. Submarine ash is widely dispersed prior to deposition, and tuff is likely to be the first product of eruption identified in reconnaissance exploration; it is the start of the trail to vent hydrothermal systems and associated mineralized deposits of submarine volcanoes, as well as a sensitive indicator of submarine eruptive processes.

Magma responds to applied stresses in either a viscous or elastic manner, depending on the time scales over which strain is accommodated. For silicic magmas, high strain rates of explosive volcanism cause brittle fragmentation and produce abundant small particles (ash). The A.D. 2012 Havre (Kermadec arc, southwestern Pacific Ocean) eruption at similar to 900 m water depth deposited a unit of silicic ash with features indicative of syn-and/or post-fragmentation viscous deformation. Viscously deformed ash makes up 3%-35% of the two main ash subunits, S1 and S2, with the remaining ash formed by brittle fragmentation. Viscous behavior of melt during production of fine ash is unexpected for the silicic Havre magma, and for the high strain rates typical of fine fragmentation. The occurrence together of viscous and brittle ash grains suggests local and/or short-term variations in eruption conditions. We infer an explosive eruption mechanism modified by magma-water interaction, during which multi-source steam-veiled fragmentation sites permitted rapid viscous deformation of ash prior to contact with water.

In volcanology, 2D morphometric analysis is a method often applied for quantitative characterization of eruptive products, used to compare tephra from different events or phases, infer eruptive styles and underlying clast generating mechanisms, or describe the aerodynamic behavior of tephra. Such particle shape analyses can be conducted using particle silhouettes or cross-sectional slices, obtained under by means of electron or optical microscope imagery. Over the course of the last years, a number of different morphometric systems have been used. Each of them uses its own nomenclature and mathematical definitions of shape-describing parameters, some of which can only be obtained using specific commercial software.With the PARTIcal Shape ANalyzer PARTISAN we present a freeware tool which parameterizes 2D shapes and provides a suite of shape descriptors, following the respective standards of the five most commonly used 2D morphometric systems. Use of PARTISAN will enable the user to study and archive the results of particle shape analysis in a format compatible with various published routines, thus increasing the potential for linking new work with results of work previously published by other groups. It will allow as well the cross-comparison of results obtained by these morphological routines. PARTISAN hence could be seen as a "Rosetta Stone" for volcanological particle morphometry, and opens the way towards an inter-group effort for a standardized 2D description of particle shapes.

The 2012 submarine eruption of Havre volcano in the Kermadec arc, New Zealand, is the largest deep-ocean eruption in history and one of very few recorded submarine eruptions involving rhyolite magma. It was recognized from a gigantic 400-km2 pumice raft seen in satellite imagery, but the complexity of this event was concealed beneath the sea surface. Mapping, observations, and sampling by submersibles have provided an exceptionally high fidelity record of the seafloor products, which included lava sourced from 14 vents at water depths of 900 to 1220 m, and fragmental deposits including giant pumice clasts up to 9 m in diameter. Most (>75%) of the total erupted volume was partitioned into the pumice raft and transported far from the volcano. The geological record on submarine volcanic edifices in volcanic arcs does not faithfully archive eruption size or magma production.