Arc Volcanoes Research Articles

Composite Granitic Plutonism in the Southern Part of the Wadi Hodein Shear Zone, South Eastern Desert, Egypt: Implications for Neoproterozoic Dioritic and Highly Evolved Magma Mingling during Volcanic Arc Assembly

The Abu Farayed Granite (AFG), located in the southeastern desert of Egypt, was intruded during the early to late stages of Pan-African orogeny that prevailed within the Arabian–Nubian Shield. The AFG intrudes an association of gneisses, island arc volcano–sedimentary rocks, and serpentinite masses. Field observations, supported by remote sensing and geochemical data, reveal a composite granitic intrusion that is differentiated into two magmatic phases. The early granitic phase comprises weakly deformed subduction-related calc–alkaline rocks ranging from diorite to tonalite, while the later encloses undeformed granodiorite and granite. Landsat-8 (OLI) remote sensing data have shown to be highly effective in discriminating among the different varieties of granites present in the area. Furthermore, the data have provided important insights into the structural characteristics of the AFG region. Specifically, the data indicate the presence of major tectonic trends with ENE–WSW and NW–SE directions transecting the AFG area. Geochemically, the AFG generally has a calc–alkaline metaluminous affinity with relatively high values of Cs, Rb, K, Sr, Nd, and Hf but low contents of Nb, Ta, P, and Y. The early magmatic phase has lower alkalis and REEs, while the later phases have higher alkalis and REEs with distinctly negative Eu anomalies. The AFG is structurally controlled, forming a N–S arch, which may be due to the influence of the wadi Hodein major shear zone. The diorite and tonalite are believed to have been originally derived from subduction-related magmatism during regional compression. This began with the dehydration of the descending oceanic crust with differential melting of the metasomatized mantle wedge. Magma ascent was long enough to react with the thickened crust and therefore suffered fractional crystallization and assimilation (AFC) to produce the calc–alkaline diorite–tonalite association. The granodiorite and granites were produced due to partial melting, assimilation, and fractionation of lower crustal rocks (mainly diorite–tonalite of the early stage) after subduction and arc volcanism during a late orogenic relaxation–rebound event associated with uplift transitioning to extension.

Deep Low‐Frequency Earthquake Reveals Unsteady Fluid Flow Beneath Tengchong Volcano Field in Southeast Tibet

AbstractDeep low‐frequency earthquakes (DLFE) are observed beneath volcanoes worldwide but are limited to island arc volcanoes, hotspot volcanoes, and rift zones. Here we show DLFEs in the Tengchong Volcano Field, southeast Tibet, located ∼300 km from the Indo‐Burma volcanic arc, by analyzing a 12‐year continuous seismic data set. The earthquakes were at a depth of ∼12 km, near the sidewall of the magma body detected by the magnetotelluric survey. The features of isotropic focal mechanism, episodic occurrence, and possible non‐power‐law scaling, with no detectable geodetic deformation, as well as the petrological signatures of the Holocene eruption product, suggest that the earthquakes were likely associated with the weak intermittent magma flows near the magma body. This finding may demonstrate the existence of unsteady magmatic processes in the margin of the Indo‐Eurasia collision zone, which could indicate unneglectable volcanic hazards, underestimated geothermal resources, and mineralization processes in similar regions.

Crystal Resorption as a Driver for Mush Maturation: an Experimental Investigation

Abstract The thermal state of a magma reservoir controls its physical and rheological properties: at storage temperatures close to the liquidus, magmas are dominated by melt and therefore mobile, while at lower temperatures, magmas are stored as a rheologically locked crystal network with interstitial melt (crystal mush). Throughout the lifetime of a magmatic system, temperature fluctuations drive transitions between mush-dominated and melt-dominated conditions. For example, magma underplating or magma recharge into a crystal mush supplies heat, leading to mush disaggregation and an increase in melt fraction via crystal resorption, before subsequent cooling reinstates a crystal mush via crystal accumulation and recrystallisation. Here, we examine the textural effects of such temperature-driven mush reprocessing cycles on the crystal cargo. We conducted high-P-T resorption experiments during which we nucleated, grew, resorbed, and recrystallised plagioclase crystals in a rhyolitic melt, imposing temperature fluctuations typical for plumbing systems in intermediate arc volcanoes (20–40 °C). The experiments reproduce common resorption textures and show that plagioclase dissolution irreversibly reduces 3D crystal aspect ratios, leading to more equant shapes. Comparison of our experimental results with morphologies of resorbed and unresorbed plagioclase crystals from Mount St. Helens (MSH) (USA) reveals a consistent trend in natural rocks: unresorbed plagioclase crystals (found in MSH dacite, basalt and quenched magmatic inclusions [QMIs]) have tabular shapes, while plagioclase crystals with one or more resorption horizons (found in MSH dacite, QMIs, and mush inclusions) show more equant shapes. Plagioclase crystals showing pervasive resorption (found in the dacite and mush inclusions) have even lower aspect ratios. We therefore suggest that crystal mush maturation results in progressively more equant crystal shapes: the shapes of plagioclase crystals in a magma reservoir will become less tabular every time they are remobilised and resorbed. This has implications for magma rheology and, ultimately, eruptibility, as crystal shape controls the maximum packing fraction and permeability of a crystal mush. We hypothesise that a mature mush with more equant crystals due to multiple resorption–recrystallisation events will be more readily remobilised than an immature mush comprising unresorbed, tabular crystals. This implies that volcanic behaviour and pre-eruptive magmatic timescales may vary systematically during thermal maturation of a crustal magmatic system, with large eruptions due to rapid wholesale remobilisation of mushy reservoirs being more likely in thermally mature systems.

Pre-eruptive magmatic processes at Taranaki volcano from an amphibole perspective

Amphibole phenocrysts in post-1 ka lava domes deposits of Taranaki volcano, an andesite arc volcano in New Zealand, provide an opportunity to explore late-stage changes in melt composition and volatiles in the run up to eruption. Amphibole is a subordinate phase (0–7% of modal phenocrysts) and the phenocrysts display three types of growth history. Type 1 are large compositionally and texturally uniform phenocrysts that have low MgO (<13 wt%) and high K2O (∼1–1.4 wt%) contents. In contrast, Type 2 have similar cores but have been partially resorbed and overgrown by a mafic rim distinguished by high MgO (up to 15 wt%) and low K2O (<1.0 wt%) contents. Type 3 are the least common, and have either normal zoned or concentrically zoned interiors with respect to MgO and FeO. Overall, elemental substitutions and zonation patterns are related to periods of resorption and regrowth that are best explained by changes in melt composition. Based on calculated equilibrium melt compositions, the amphibole crystallisation is consistent with an incrementally grown andesitic to dacitic (SiO2 58–67 wt%) crystal mush that was periodically recharged with more mafic magma (SiO2 ∼ 55 wt%). The melts span a compositional gap (SiO2 ∼ 60–65 wt%) that is not represented in eruption products from the last 8 ka. Such melts were likely brief stages during ongoing fractional crystallisation and magma mixing. Each recharge event disrupted an evolving system and mixed crystals from different parts of it. This concept supports triggering of eruptions by recharge of mafic melts, as also inferred from plagioclase zonation for some eruptions from Taranaki. The outermost rims (0–5 μm) of many amphibole phenocrysts are enriched in fluorine (up to 1.9 wt%) and surrounded by a thick opacitic decomposition growths. We interpret this as the result of slow magmatic ascent and extensive crystallisation that produced a late-stage halogen-rich interstitial melt. This likely extended the conditions for amphibole stability to lower pressure. If F enrichment was accompanied by comparable enrichments in other halogens that preferentially partition into an aqueous phase, then extensive degassing would have occurred during the dome extrusions.

Abundance of submarine volcanism in arcs

Explosive submarine arc volcanoes can cause tsunamis, affect climate, and pose hazards to airplanes and ships. Although 70 % of the Earth is submarine, only 15 % of Holocene arc volcanoes in the Smithsonian Global Volcanism database are submarine. Merging locations of active submarine hydrothermal vents in arcs and the above database, we found 71 unlisted submarine arc volcanoes. Using Baker [2017, doi: 10.1016/j.oregeorev.2017.02.006], only 44 % of hydrothermal vents in arcs are known. Only 77 of the vent fields are on volcanoes in the Smithsonian database. Assuming that unknown submarine arc volcanoes are present in the same proportions as unknown vents, there are ~160 as-yet undiscovered Holocene age submarine arc volcanoes. Using geophysical data, we located 291 unsurveyed seamounts &lt;30 km from arc axes, the distance within which active vents occur. We estimate 119±16 have active vents. Because more unknown seamounts exist, this means that &gt;32 % of Holocene arc volcanoes are submarine.

Coulomb Stress Analysis in Nemrut Caldera (East Anatolia, Türkiye)

In volcanic areas, seismic events with low energy occur before seismic activity or due to the movement observed in the magma. These earthquakes, which are caused by the expansion-contraction movement that has been revealed in different studies and is mostly observed in the magma chamber, can be recorded with continuous observations. On the other hand, it is not easy to distinguish between tectonic and volcanic origins of earthquakes occurring in volcanic areas. In this study, Coulomb stress analysis was carried out using earthquakes in the Nemrut Stratovolcano, which is located in the eastern Türkiye and is at the westernmost end of a volcano arc, and it was concluded that the stress is related to the Nemrut Caldera, therefore the positive stress caused by the expansion of the magma chamber of the Nemrut Volcano creates tremors.

Decoding degassing modes of magma chamber of arc volcanoes: Insights from CO2/Cl and Cl/H2O ratios of magmatic fluids in groundwater

The H2O-CO2-Cl composition of the fluids released from the magma chambers for arc volcanoes was calculated based on the solubilities of H2O and CO2 in the silicate melt and the partition coefficient of Cl between the aqueous fluid and melt for the stages defined by a simple evolution model of the magma chamber. The degassing modes consisted of mafic bubbling (MB: first boiling) and felsic bubbling (FB: second boiling) releasing fluids as separate bubbles due to the mafic magma supply and felsic magma formation by differentiation, respectively, and felsic solidification (FS) releasing fluid due to felsic pluton formation. The variations in the CO2/Cl and Cl/H2O ratios of the fluids were examined for each degassing mode at pressures ranging from 150 to 400 MPa and were found to have very large variations at 0.00024–845 for CO2/Cl and 0.00043–0.041 for Cl/H2O. The CO2/Cl is concluded to be a good indicator of magma chamber conditions because each mode has a specific ratio distinguished from that of the other modes; i.e., CO2/Cl > 43 for MB, 0.027–23 for FB, and < 0.62 for FS.The alteration of magmatic composition of fluid during ascent was discussed considering the phase separation of fluid, CO2 bubbling in groundwater, dissolution, and deposition of minerals; furthermore, the limitation of the method when applied to natural groundwater systems were outlined. This method was tested for a natural volcanic system, the Kuju volcano in central Kyusyu, Japan, which is an active volcano complex composed of various magma types from rhyolitic to basaltic, replacing the eruption centers from west to east. Groundwater samples from springs and boreholes were collected and analyzed for chemical and isotopic compositions to determine the magmatic H2O, dissolved inorganic carbon (DIC), and Cl concentrations. The new approach to determine the magmatic CO2 concentration in groundwater using 3He concentration proposed herein cancelled the effect of carbonate deposition/dissolution, addition of DIC from organic matter, and bubbling of CO2. The estimated magmatic CO2/Cl ratios were 0.0009–14, which overlapped consistently with the modeled variation in the CO2/Cl ratio. The spatial variation of magmatic CO2/Cl in groundwater showed that higher-CO2/Cl are found near the central part of the volcanic complex which composed of felsic to mafic volcanoes, and low-CO2/Cl are only found around old felsic volcanoes. The CO2 bubbling springs were found to be associated with younger mafic volcanoes. The condition of the magma chambers, as shown by the spatial variation in CO2/Cl, were consistent with the record of the eruptive activities of the Kuju volcano. The CO2/Cl method proposed in this study using groundwater is useful for estimating magmatic conditions in chambers.

The 15 January 2022 Hunga (Tonga) eruption: A gas-driven climactic explosion

An extraordinarily powerful, explosive eruption occurred from Hunga volcano in the Tonga island arc on 15 January 2022 and generated an eruption column 58 km high. The explosive eruption also generated atmospheric gravity waves, extreme runup tsunamis and quite unusual and destructive meteotsunamis. Together these place this VEI 6 eruption as, globally, one of the largest of the past 300 years.Based on the oceanic context of Hunga volcano, it has previously been assumed that the eruption was phreatomagmatic through a fuel-coolant Surtseyan-type interaction, but this is not supported by satellite imagery. Similarly, it has been suggested that a caldera-collapse was the eruption trigger, but this is not supported by bathymetric data or the seismicity recorded during the eruption. Here we develop a new model based on the observed energetics and time sequence of the eruption integrated with understanding of the internal structure of active volcanoes and their characteristic high flux discharges of volcanic gas.It has been shown elsewhere that magma-derived reactive gases (H2O, CO2, SO2, HCl, etc) aggressively alter the volcanic rocks in the core of a volcano leading to self-sealing of gas flow to the surface and consequent changes to deviatoric stress in the structure. Common minerals developed by these reactions include anhydrite (CaSO4), sulphides and silica (quartz), all of which have been recorded in volcanic ejecta including at Hunga.We here develop a first order numerical model that quantifies how the free discharge of such gas to the surface may progressively become choked by these sealing reactions leading to increased internal gas pressure. Hydraulic fracture of the seal occurs when the transmitted pressure of the compressed magmatic gas beneath the seal increases to a value greater than the lithostatic pressure plus the tensile strength of the sealed rock. This initiates the explosive release of compressed gas whose high-power discharge progressively develops and enlarges a crater. At the same time, the explosion feeds upon itself by generating larger pressure gradients in the pressurized gas within the fractured porous rock mass of the core of the volcano. Excavation of the crater may intersect high level intrusions and produce the pumice rafts that were observed after the eruption. The eruption itself diminished in intensity as the gas pressure in the reservoir declined.At Hunga, the eruption excavated an 850 m deep, 2-3 km diameter steep-walled crater. This volume may be assumed to approximate the volume of fractured porous rock (the control volume of the eruption) whose trapped gas was mined by the eruption until surrounding gas pressure was depleted. Our numerical model shows that the calculated potential energy of the trapped compressed gas matches the independent observations of the scale of the eruption. Sensor data have since shown that gas bubble flares continued for at least 6 months after the eruption indicating continued depletion of the gas reservoir of rocks surrounding the new crater. The systems-based, gas-driven model for the Hunga climactic eruption developed here also applies to Plinean-type eruptions on subaerial arc volcanoes such as at Pinatubo (Philippines) 1991.

Deciphering Contribution of Recycled Altered Oceanic Crust to Arc Magmas Using Ba‐Sr‐Nd Isotopes

AbstractAltered oceanic crust (AOC) plays a critical role in geochemical recycling in subduction zones. However, identifying contributions of subducted AOC to arc magmas remains a conundrum due to the lack of effective tracers. Here, we investigate the Ba‐Sr‐Nd isotopic compositions of lavas from the Mariana arc and back‐arc. Based on a statistical analysis of the Sr‐Nd isotopes for global arc volcanoes, we confirm that AOC‐derived fluid (or hydrous melt), rather than sediment‐derived melt or fluid, is responsible for the Sr‐Nd isotope decoupling (i.e., 87Sr/86Sr is “excessively” enriched relative to 143Nd/144Nd when compared to the “normal” mantle derivates) observed in island arc lavas. We show that the arc lavas with increasingly decoupled Sr‐Nd isotopes generally have heavier Ba isotope ratios, which is also a characteristic feature of AOC‐derived fluids. Thus, these results establish an intimate link between subducted AOC, heavy Ba isotope compositions, and Sr‐Nd isotope decoupling signature in island arcs, which provides a powerful tool to trace the AOC recycling in subduction zones. Furthermore, a similar correlation is observed between Sr‐Nd isotope decoupling and heavy B isotope ratios for global arc lavas, implying that the recycling of AOC component is generally linked to serpentinite dehydration in subduction zones.

Identifying Distinct Pre‐Eruptive Composition‐H2O‐Time Trends Using Plagioclase

AbstractMacrocrysts (large crystals) in magmas offer a premier record of pre‐eruptive magma storage conditions encoded in their chemistry and texture. Careful study of macrocryst zoning can deconvolve the conditions of crystal growth and the relative time a magma spends in a given physical‐chemical state prior to eruption. Importantly, identifying discrete macrocryst zones requires consideration of both chemistry and texture simultaneously. Here, we employ a novel image segmentation approach to characterize zoning from 2D chemical maps of plagioclase macrocrysts. We apply the method to 15 volcanic eruptions, across three stratigraphic sections, to track statistical differences in crystal zoning through relative time at an arc volcano (Mount Liamuiga, Saint Kitts). Plagioclase from the 15 eruptions are described by 7 unique textural‐chemical zoning populations, which we term “zoning groups,” each of which has a unique An# fingerprint. Two of the studied stratigraphic sections overwhelmingly record low‐An# zoning groups (ZGs), whereas the other section records mostly high‐An# ZGs, suggestive of two distinct storage conditions. Using observations from equilibrium experiments relevant to Saint Kitts bulk magma compositions, we show that differences in melt H2O are the primary drivers of the An# variability. Negative whole rock K2O versus predicted H2O trends are suggestive of ubiquitous H2O‐saturated conditions throughout the middle and upper crust, with a correlation between H2O‐saturated storage pressure (Psat) and eruptive dynamics; magmas stored in the upper‐crust (0.48 ± 0.28–1.08 ± 0.45 kbar) produce larger‐volume, pumice‐rich eruptions compared to magmas stored in the middle crust (3.29 ± 0.87–3.88 ± 1.05 kbar) which generally produce smaller‐volume, centimeter to decimeter‐thick fall deposits.

Exceptionally low mercury concentrations and fluxes from the 2021 and 2022 eruptions of Fagradalsfjall volcano, Iceland

Mercury (Hg) is naturally released by volcanoes and geothermal systems, but the global flux from these natural sources is highly uncertain due to a lack of direct measurements and uncertainties with upscaling Hg/SO2 mass ratios to estimate Hg fluxes. The 2021 and 2022 eruptions of Fagradalsfjall volcano, southwest Iceland, provided an opportunity to measure Hg concentrations and fluxes from a hotspot/rift system using modern analytical techniques. We measured gaseous Hg and SO2 concentrations in the volcanic plume by near-source drone-based sampling and simultaneous downwind ground-based sampling. Mean Hg/SO2 was an order of magnitude higher at the downwind locations relative to near-source data. This was attributed to the elevated local background Hg at ground level (4.0 ng m−3) likely due to emissions from outgassing lava fields. The background-corrected plume Hg/SO2 mass ratio (5.6 × 10−8) therefore appeared conservative from the near-source to several hundred meters distant, which has important implications for the upscaling of volcanic Hg fluxes based on SO2 measurements. Using this ratio and the total SO2 flux from both eruptions, we estimate the total mass of gaseous Hg released from the 2021 and 2022 Fagradalsfjall eruptions was 46 ± 33 kg, equivalent to a flux of 0.23 ± 0.17 kg d−1. This is the lowest Hg flux estimate in the literature for active open-conduit volcanoes, which range from 0.6 to 12 kg d−1 for other hotspot/rift volcanoes, and 0.5–110 kg d−1 for arc volcanoes. Our results suggest that Icelandic volcanic systems are fed from an especially Hg-poor mantle. Furthermore, we demonstrate that the aerial near-source plume Hg measurement is feasible with a drone-based active sampling configuration that captures all gaseous and particulate Hg species, and recommend this as the preferred method for quantifying volcanic Hg emissions going forward.

Variable oxidizing capacity of slab-derived fluids: Insights from Fe and S speciation in glasses from the Troodos Ophiolite

Oxygen fugacity (ƒO2) varies systematically across tectonic environments. The typically high ƒO2 recorded in arc settings relative to oceanic ridges is attributed to recycling of oxidized materials derived from subducting slabs into regions of the mantle that undergo partial melting. To evaluate further the relationship between ƒO2 and mantle metasomatism, Fe and S X-ray Absorption Near-Edge Structure measurements were conducted on volcanic glass wafers sampled across the extrusive section of the Troodos Ophiolite, Cyprus, which is a type locality for studying the influence of subduction on mantle melting processes and the generation of oceanic crust. The glasses record ƒO2 values relative to the quartz-fayalite-magnetite (FMQ) buffer of FMQ+0.13±0.16(1σ) to FMQ+0.74±0.23(1σ) upon quenching at the seafloor. At a given MgO content, the ƒO2 values recorded by the Troodos glasses do not vary systematically based on type (i.e., boninitic versus tholeiitic) nor by sampling location, indicating that the lavas were derived from primary melts and mantle sources that were indistinguishable in their initial ƒO2 levels. The glass ƒO2 values do not vary with dissolved H2O contents, nor with trace element ratios (e.g., Ba/La, Ce/Pb) and Pb and Sr isotopic compositions, all used to monitor interaction of slab-derived materials with the Troodos mantle source. The decoupling of ƒO2 and H2O and fluid-mobile elements renders the Troodos an important endmember for interpreting global variations in the redox state of mantle melts formed in subduction-influenced settings. For example, the Troodos glasses are reduced relative to Izu-Bonin boninites and melt inclusions from Mariana and Cascades arc volcanoes, despite overlapping and elevated H2O contents, Ba/La, and S/Dy relative to mid-ocean ridge basalts. The lack of correlation between ƒO2 and volatile contents and with proxies for subduction influence in the Troodos glasses is similar to basalts formed in back-arc environments and in near-trench settings in Izu-Bonin-Mariana. Such comparisons suggest that the slab-derived fluids that infiltrated the Troodos melting region were derived from a shallow slab and had low oxidizing capacity relative to fluids or melts which interacted with the mantle in the Izu-Bonin-Mariana arc and in the Cascades, which represent arc environments with comparable glass Fe XANES measurements. A simple framework accounts for local and global variability in ƒO2 in fractionation-corrected glasses, where fluids of variable oxidizing capacity interact with depleted mantle to produce hydrous melts. The inheritance of volatile and fluid-mobile elements in regions of the mantle that undergo partial melting does not necessarily lead to higher ƒO2 if the oxidizing capacity of the infiltrating hydrous fluids or melts is low. In back-arcs and near-trench environments, partial melts formed by interaction with fluids of low oxidizing capacity are similar to arc melts in their volatile contents, trace element, and isotopic signatures, but modestly oxidized relative to mid-ocean ridge basalts.

Oceanic Plateau and Spreading Ridge Subduction Accompanying Arc Reversal in the Solomon Islands

AbstractWe study the collision of the Ontong Java Plateau, reversal of the Solomon Islands arc, and subduction of the Woodlark Basin spreading ridge in the southwest Pacific. Double‐sided subduction and intra‐arc deformation, not just polarity reversal, characterize the rapid Australia‐Pacific plate convergence. The rifted margins of the 0–6.2‐Ma Woodlark Basin separate its unflexed young lithosphere without a trench from the older flexed lithosphere with &gt;8‐km‐deep trenches subducting to either side. A gap evident in the seismicity and tomography between the NE‐subducted slabs of the Solomon Sea and Australia plates indicates that the Woodlark lithosphere likely melts and/or is quickly (&lt;1 m.y.) resorbed into the mantle. A section of the Pacific Plate remains attached within the gap and provides the slab pull to form the Malaita accretionary prism. Quaternary arc‐composition volcanoes that occur on both sides of the Woodlark Basin subduction front are not sourced from a mantle wedge overlying a Wadati‐Benioff zone, but come from and through breaks in the young incoming and subducting lithosphere and the overturning mantle beneath. Though the basin is too weak to sustain plate flexure, arc volcanoes on the subducting crust result in locally strong coupling to the forearc, manifest by tsunamigenic earthquakes. At 150–400‐km depth, the subducted slabs under the Solomon Islands are near vertical. They are near‐horizontal between the 410 and 660‐km mantle discontinuities beneath the Woodlark Basin and further south, whereas beneath the Solomon Sea Basin they dip southwest and penetrate the 660‐km discontinuity, having been detached by the north‐dipping Solomon Sea slab.

Chalcophile element degassing at an active continental arc volcano

Arc volcanoes are significant natural sources of trace chalcophile elements to the atmosphere via gas and aerosol plumes. Villarrica volcano, part of the Andean arc, erupts basaltic magmas and is characterised by a persistent volcanic gas plume and therefore presents an opportunity to quantify volcanic chalcophile processing in a subduction zone from slab to surface. Here we present geochemical data for olivine-hosted melt inclusions, as well as for the gas and aerosol plume. We show that melts erupted at Villarrica are enriched (over mid-ocean ridge basalts) in a suite of fluid-mobile elements comprising the large ion lithophiles, including Cs, and chalcophile elements W, Tl, Pb and Sb. Volcanic gas and aerosol samples show that the chalcophile elements, and Cs, are strongly enriched in the gas phase over the silicate melt, 103 to 106 times more so than the non-volatile Rare Earth Elements. Volatilities (the percentage of an element that degasses from a melt on eruption) reach ∼ 45 % for Tl, with Pb, Sn and Mo exhibiting volatilities of up to 0.3 % and Cu up to 0.08 %. Many of the chalcophile elements (e.g. Cu, Ag, Zn) have an affinity for chloride in the gas phase and we observe that the volatility of chloride-speciating trace metals is linked strongly to the availability of chlorine in volcanic plumes globally. Overall, we show that the trace element composition of the volcanic gas—and hence probably also the deeper, denser and more saline fluids in the subsurface—is sensitive to both the availability of chloride in the gas phase and the composition of the melt, which is controlled by the slab flux and may be variable between subduction zones.

Seismic Imaging of the Westward Transition From Yakutat to Pacific Subduction in Southern Alaska

AbstractAlaska is located at the northernmost point of the interface between the Pacific plate and the North American continent. The subduction of the Pacific plate generates arc volcanoes along the Aleutian trench, which stops to the east at the Denali Volcanic Gap. This volcanic gap has been linked to the underthrusting of the Yakutat terrane, which might alter the thermal state of the mantle wedge and prevent melt formation. This implies that the limits of the volcanic activity should mirror the extent of the Yakutat subduction. However, the transition from the Pacific slab to the Yakutat terrane at depth is not fully understood. To investigate this issue, we processed a new composite seismic data set from six arrays deployed in the region from 2000 to 2018. We apply a multi‐mode 3D Kirchhoff migration to obtain high‐resolution 3D scattering images of the region. Our results highlight a sharp lateral boundary in the slab structure, with a 10 km Moho step, just offshore Anchorage, and a more gradual slab transition beneath the southern part of the Kenai peninsula. Our images from the Yakutat slab plunge down to 150 km depth are consistent with previous estimates of the Yakutat slab extent below the Alaska Range. Although the steeply dipping boundaries of the subducting Pacific lithospheres are not fully recovered, deep coherent signals from the Pacific slab are observed down to 150 km depth. These observations suggest that the crust is still partially uneclogitized at these depths in both slabs.

Ongoing fragmentation of the subducting Cocos slab, Central America

Abstract Fundamental to plate tectonics is the subduction of cold and mechanically strong oceanic plates. While the subducted plates are conventionally regarded to be impermeable to mantle flow and separate the mantle wedge and the subslab region, isolated openings have been proposed. By combining new shear wave splitting measurements with results from geodynamic modeling and recent seismic tomography and geochemical observations, we show that the upper ~200 km of the Cocos slab in northern Central America is intensively fractured. The slab there is strong enough to produce typical arc volcanoes and Benioff Zone earthquakes but allows mantle flow to traverse from the subslab region to the mantle wedge. Upwelling of hot subslab mantle flow through the slab provides a viable explanation for the behind-the-volcanic-front volcanoes that are geochemically distinct from typical arc volcanoes, and for the puzzling high heat flow, high elevation, and low Bouguer gravity anomalies observed in northern Central America.

Role of subduction dynamics on the unevenly distributed volcanism at the Middle American subduction system

A typical subduction of an oceanic plate beneath a continent is expected to be accompanied by arc volcanoes along the convergent margin. However, subduction of the Cocos plate at the Middle American subduction system has resulted in an uneven distribution of magmatism/volcanism along strike. Here we construct a new three-dimensional shear-wave velocity model of the entire Middle American subduction system, using full-wave ambient noise tomography. Our model reveals significant variations of the oceanic plates along strike and down dip, in correspondence with either weakened or broken slabs after subduction. The northern and southern segments of the Cocos plate, including the Mexican flat slab subduction, are well imaged as high-velocity features, where a low-velocity mantle wedge exists and demonstrate a strong correlation with the arc volcanoes. Subduction of the central Cocos plate encounters a thick high-velocity feature beneath North America, which hinders the formation of a typical low-velocity mantle wedge and arc volcanoes. We suggest that the presence of slab tearing at both edges of the Mexican flat slab has been modifying the mantle flows, resulting in the unusual arc volcanism.

3D Body-Wave Velocity Structure of the Southern Aegean, Greece

This study delves into the southern Aegean regionwhere the subduction of the oceanic Mediterranean lithosphere under the Aegean continental one takes place. This region is considered one of the most active ones in the eastern Mediterranean Sea due to intense tectonic movements in the Late Quaternary. More than 1200 manually revised events from 2018 to 2023 have been used in order to obtain the 3D structure of body-wave velocity and VP/VS ratioto 80 km depth through earthquaketomography. A series of resolution tests have been performed and demonstrated fair resolution of the derived velocity structures in the area of interest. The derived anomalies of body-waves (dVP, dVS) and VP/VS ratio provided important information about the southern Aegean regional tectonics and secondarily active faults of smaller scale (&gt;20 km). The region is marked by significant low-velocity anomalies in the crust and uppermost mantle, beneath the active arc volcanoes. The seismicity related to the Hellenic Subduction Zone (HSZ) is connected to a low-angle positive anomaly of VP and VS, correlated withthe observed intermediate-depth seismicity (H ≥ 40 km) in this part of the study area. This result could be related to the diving HSZ slab.

Formation of columnar/tabular and vermicular kaolinite in the crystal tonsteins within the Late Carboniferous Coal in the Datong Coalfield, Shanxi Province, North China

The Datong Coalfield, Shanxi Province, North China, is rich in high-grade coal-measure kaolin, primarily occurring as partings within the Late Carboniferous–Early Permian coal seams. There is a type of kaolinite-dominant coarse-grained parting similar to sandstone in appearance, whereas the phanerocrystalline minerals are columnar/tabular and vermicular kaolinite rather than terrigenous detrital minerals. The origins of the coarse-grained partings and the formation mechanism of the columnar/tabular and vermicular kaolinite remain ambiguous. Herein, petrographical, mineralogical, whole-rock geochemical, and zircon U–Pb–Hf isotopic studies of the coarse-grained partings were conducted to explore these issues. The zircons in the coarse-grained partings show typical features of magmatic origin and yield a single age group of very latest Carboniferous at ca. 300 Ma, indicating they were transformed from air-fall volcanic ash. Therefore, the coarse-grained partings are essentially crystal tonsteins. The wide range of negative εHf(t) values and arc-related tectonic affinity of the zircons genetically link the primary volcanic ash to the Late Paleozoic continental arc volcanoes that existed in the Inner Mongolia Paleo-Uplift, related to the southward subduction of the Paleo-Asian oceanic plate beneath the North China Block. Two orthogonal cleavages are generally observed on the columnar/tabular and vermicular kaolinite in the crystal tonsteins, contradicting the crystalline structure of kaolinite that belongs to a 1:1 type phyllosilicate with only one cleavage along its (001) lattice plane. These columnar/tabular and vermicular kaolinite are considered to be pseudomorphic kaolinite after plagioclase in the crystal-rich volcanic ash. When the crystal-rich volcanic ash fell into open and acidic peat-forming environments (mires), the low-pH water containing abundant organic acids and good water circulation conditions facilitate dissolution of plagioclase along the chemically unstable (001) lattice planes and leaching out of Na+, Ca2+, and much of silica. Simultaneously, the Al3+ or hydroxides of aluminum and silica recrystallized along the (001) lattice planes of plagioclase as the substrate, constraining layer-by-layer growth of large kaolinite platelets to eventually form pseudomorphic kaolinite. The cleavages perpendicular to the exclusive (001) cleavage of pseudomorphic kaolinite should be inherited from the twin planes and (010) cleavage cracks (if present) of plagioclase.

Subduction of Submarine Arc Volcanoes Beneath the Solomon Islands Arc

In the Solomon Islands, arc magmas are erupting on the subducting Australia Plate. These island (Simbo) and submarine arc volcanoes (Kana Keoki, Coleman and Pavuvu) are about to be recycled by rapid subduction. We identify eight of their former equivalents beneath the forearc by the morphologies and deformation structures that are characteristic of seamount subduction. Tsunamigenic earthquakes recently nucleated just ahead of two of the subducting seamounts. A third (Pavuvu), that has indented the subduction front and uplifted the lower forearc, is associated with a historic earthquake gap. It is positioned such that a rupture there has the potential for tsunami waves to impact the capital, Honiara.