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Generally all orogenic ultrapotassic rocks are formed after melting of metasomatized sub-continental lithospheric mantle via subducted crustal mica-bearing lithologies. Here we present another possible model, based on the study of the small Stomanovo ultrapotassic monzonite porphyry intrusion in the Central Rhodope Massif, Bulgaria. The monzonite dated at 30.50 ± 0.46 Ma is intruded into the voluminous Oligocene (31.63 ± 0.40 Ma) Bratsigovo–Dospat ignimbrite. The monzonite hosts both normally and reversely zoned clinopyroxene phenocrysts. The normally zoned clinopyroxene is characterized by gradually diminishing core-to-rim Mg no. (89–74), whereas the reversely zoned clinopyroxene has green Fe-rich cores (Mg no. 71–55) mantled by normally zoned clinopyroxene (Mg no. 87–74). Neither the core of the normally zoned clinopyroxene nor the Fe-rich green cores are in equilibrium with the host monzonite. This ultrapotassic monzonite shows more radiogenic Sr isotopes ((87Sr/86Sr)i= 0.71066) and ϵNd(t) = −7.8 to −8.0 that are distinct from the host ignimbrites with (87Sr/86Sr)i= 0.70917–0.70927 and ϵNd(t) = −4.6 to −6.5. The Sr–Nd isotopic data and the presence of copious zircon xenocrysts from the underlying metamorphic basement suggest extensive crustal assimilation. Our observations indicate that the Stomanovo ultrapotassic monzonite formed after extensive lower or middle crustal fractional crystallization from an evolved magma producing cumulates. The process was followed by hybridization with primitive mantle-derived magma and subsequent continuous crustal contamination. We suggest that instead of inheriting their high K2O and large-ion lithophile element enrichments from slab-derived/metasomatic fluids, the Stomanovo ultrapotassic monzonite may owe some of its unusually high alkalinity to the assimilation of potassium-rich phases from the Rhodope Massif basement rocks.

The remnant rear-arc segment of the early Izu-Bonin arc, known as the Kyushu-Palau Ridge (KPR), is a key location where magmatic outputs can be constrained during the lifetime of an island arc. We present new geochemical data for coarse-grained basaltic to andesitic volcaniclastic sandstones derived from the KPR and deposited in the Amami Sankaku Basin (IODP Site U1438, Unit III rocks) in the time period 40–30 Ma. Bulk disaggregated and cleaned volcaniclastic sandstones of Unit III at Site U1438 retain primary magmatic signatures and can be used to infer the evolution of magmatic sources of the juvenile Izu-Bonin island arc through time. A sharp increase of slab-derived components to the source of KPR magmatism developed at about 35 Ma, indicated by increasing Th/La and decreasing Sm/La, Yb/Hf and Nb/Nd. Systematic variations in trace element ratios and increasing trace element abundances in younger samples through the 40–30 Ma time window are decoupled from Hf-Nd isotope ratios, which are measurably more depleted (eHf = 16.5–15, eNd = 9.6–8.2) than boninites produced during the preceding magmatic phase and sampled in the modern Izu-Bonin forearc. Hafnium isotopic compositions in the Unit III sandstones remain little-changed and similar to the subducting Pacific Plate after 40 Ma and do not revert to highly radiogenic compositions of the Indian-type MORB mantle wedge which is reflected in highly-depleted basalts produced at Site U1438 and in the forearc (commonly eHf ≥ 18.0). The overall pattern recorded in Unit III sandstones indicates that the Pacific-type MORB slab-melt component, which was present in the preceding boninite phase of magmatism, persisted after 40 Ma, while the subducted sediment component in the boninite source was lost or significantly reduced. Variations in trace element ratios (at constant eNd and near-constant and radiogenic eHf) and in high field strength element abundances of the early Izu Bonin arc are controlled by the addition of a subducted Pacific MORB melt or supercritical fluid to the mantle wedge. A subducted MORB (slab melt) component is thus sampled throughout the early life of the Izu-Bonin arc and in the currently active Izu-Bonin arc-backarc system.

Subduction zones are active sites of chemical exchange between the Earth’s surface and deep interior and play a fundamental role in regulating planet habitability. However, the mechanisms by which redox sensitive elements (e.g., iron, carbon and sulfur) are cycled during subduction remains unclear. Here we use Fe stable isotopes (δ56Fe), which are sensitive to redox-related processes, to examine forearc serpentinite clasts recovered from deep sea drilling of mud volcanoes formed above the Mariana subduction zone in the Western Pacific. We show that serpentinisation of the forearc by slab-derived fluids produces dramatic δ56Fe variation. Unexpected negative correlations between serpentinite bulk δ56Fe, fluid-mobile element concentrations (e.g., B, As) and Fe3+/ƩFe suggest a concomitant oxidation of the mantle wedge through the transfer of isotopically light iron by slab-derived fluids. This process must reflect the transfer of either sulfate- or carbonate-bearing fluids that preferentially complex isotopically light Fe.

The oldest known, intact sedimentary record of a nascent intraoceanic arc was recovered in a ∼100-m-thick unit (IV) above ca. 49 Ma basaltic basement at International Ocean Discovery Program Site U1438 in the Amami Sankaku Basin. During deposition of Unit IV the site was located ∼250 km from the plate edge, where Izu-Bonin-Mariana subduction initiated at 52 Ma. Basement basalts are overlain by a mudstone-dominated subunit (IVC) with a thin basal layer of dark brown metalliferous mudstone followed by mudstone with sparse, graded laminae of amphibole- and biotite-bearing tuffaceous sandstone and siltstone. Amphibole and zircon ages from these laminae suggest that the intermediate subduction-related magmatism that sourced them initiated at ca. 47 Ma soon after basement formation. Overlying volcaniclastic, sandy, gravity-flow deposits (subunit IVB) have a different provenance; shallow water fauna and tachylitic glass fragments indicate a source volcanic edifice that rose above the carbonate compensation depth and may have been emergent. Basaltic andesite intervals in upper subunit IVB have textures suggesting emplacement as intrusions into unconsolidated sediment on a volcanic center with geochemical and petrological characteristics of mafic, differentiated island arc magmatism. Distinctive Hf-Nd isotope characteristics similar to the least-radiogenic Izu-Bonin-Mariana boninites support a relatively old age for the basaltic andesites similar to detrital amphibole dated at 47 Ma. The absence of boninites at that time may have resulted from the position of Site U1438 at a greater distance from the plate edge. The upper interval of mudstone with tuffaceous beds (subunit IVA) progresses upsection into Unit III, part of a wedge of sediment fed by growing arc-axis volcanoes to the east. At Site U1438, in what was to become a reararc position, the succession of early extensional basaltic magmatism associated with spontaneous subduction initiation is followed by a rapid transition into potentially widespread subduction-related magmatism and sedimentation prior to the onset of focused magmatism and major arc building.

Radiogenic and noble gas isotopes have been integral for demonstrating the existence, source, and age of (geo) chemical reservoirs in the mantle, yet, the volatile element composition of the Earth’s interior remains poorly characterized. Boron isotopes have the potential to constrain the processes that generate distinct mantle reservoirs, as they fractionate strongly at the surface of the Earth and during subduction but are little perturbed during high-temperature mantle processes, and so can inform our understanding of mantle volatile cycling. Here, we present the largest, internally consistent, high-precision B isotope dataset from ocean island basalt (OIB) glasses and olivine-hosted melt inclusions measured by Secondary Ion Mass Spectrometry (SIMS) to date, including new data derived from the Pitcairn Islands, Tristan da Cunha, St. Helena, Ascension Island, the MacDonald (Ra) Seamount, and Fogo (Cape Verde Islands) in addition to previously published data from La Reunion, La Palma (Canary Islands), Iceland, and Hawai’i. This dataset allows a comparison of ocean island basalts that contain heterogeneous recycled components (e.g., Pitcairn Islands) to those with primordial components (e.g., La Reunion) in their sources. We focus on basaltic glass and melt inclusions (>6 wt% MgO) as they are least affected by shallow differentiation and assimilation processes. We find that our new OIB data show a limited spread in average δ11B (−5.9 ± 3.0‰ to −10.8 ± 0.7‰), which is smaller compared to previous OIB studies. These data generally overlap with mid-ocean ridge basalts (MORB; −7.1 ± 0.9‰) and display lighter values when compared to mafic arc magmas (∼−9 to +20‰). Importantly, some trace element enriched OIB endmembers display lighter δ11B values and lower B/P and B/Zr, indicative of a source with lower B concentrations relative to the primordial mantle. This suggests that the deeper mantle is becoming increasingly B-depleted over time because boron is effectively stripped from recycled lithologies during subduction and slab dehydration. In addition, the results highlight the decoupling of B isotopes from radiogenic (Sr, Pb) isotopes providing a new perspective on volatile recycling.

rom ca. 50 Ma to present, the western Pacific plate has been subducting under the Philippine Sea plate, forming the oceanic Izu-Bonin-Mariana (IBM) subduction system. It is the only known location where subduction zone products are presently being transported to the surface by serpentinite-mud volcanoes. A large serpentine mud “volcano” forms the South Chamorro Seamount and was successfully drilled by ODP during Leg 195. This returned mostly partially serpentinized harzburgites enclosed in serpentinite muds. In addition, limited numbers of small (1 mm–1 cm) fragments of rare blueschists were also discovered. U–Pb dating of zircon and rutile from one of these blueschist clasts give ages of 51.1 ± 1.2 Ma and 47.5 ± 2.0 Ma, respectively. These are interpreted to date prograde high-pressure metamorphism. Mineral equilibria modelling of the blueschist clast suggests the mineral assemblage formed at conditions of ∼1.6 GPa and ∼590 °C. We interpret that this high-pressure assemblage formed at a depth of ∼50 km within the subduction channel and was subsequently exhumed and entrained into the South Chamorro serpentinite volcano system at depths of ∼27 km. Consequently, we propose that the material erupted from the South Chamarro Seamount may be sampling far greater depths within the Mariana subduction system than previously thought. The apparent thermal gradient implied by the pressure–temperature modelling (∼370 °C/GPa) is slightly warmer than that predicted by typical subduction channel numerical models and other blueschists worldwide. The age of the blueschist suggests it formed during the arc initiation stages of the proto-Izu-Bonin-Mariana arc, with the P–T conditions recording thermally elevated conditions during initial stages of western Pacific plate subduction. This indicates the blueschist had prolonged residence time in the stable forearc as the system underwent east-directed rollback. The Mariana blueschist shows that subduction products can remain entrained in subduction channels for many millions of years prior to exhumation. Refereed/Peer-reviewed

Recycling of the lithosphere via subduction drives the trace element and isotopic heterogeneity of the mantle, yet, the inventory of volatile elements in the diverse array of mantle reservoirs sampled at ocean islands remains uncertain. Boron is an ideal tracer of volatile recycling because it behaves similarly to volatiles during high-temperature geochemical reactions and carries a distinctive isotope signature into the mantle, but is subsequently little-influenced by degassing on return to the surface. Furthermore, B-rich recycled lithologies will have a strong influence on typical upper mantle compositions characterized by low B concentrations (11 B −7.1 ± 0.9‰). Here, we present and compare the B abundances and isotope compositions, together with the volatile element contents (H 2 O, CO 2 , and Cl) of basaltic glasses and olivine-hosted melt inclusions from two different ocean island localities (La Palma, Canary Islands, and Piton de Caille, La Réunion Island). Our results suggest that olivine hosted melt inclusions are protected from contamination during ascent and provide more robust estimates of primary mantle source δ 11 B than previous bulk rock studies. We find that the δ 11 B of the La Réunion samples (−7.9 ± 0.5‰ (2σ)) overlaps with the recently defined MORB datum, indicating that the depleted upper-mantle and ‘primitive mantle’ reservoirs are indistinguishable with respect to δ 11 B, or that B concentrations are sufficiently low that they are diluted by partial melting in the uppermost mantle. In contrast, the La Palma samples, notable for their radiogenic Pb isotope ratios, are characterized by δ 11 B values that are distinctly isotopically lighter (−10.5 ± 0.7‰ (2σ)) than La Réunion or MORB. We suggest these isotopically light values are derived from significantly dehydrated recycled materials preserved in the La Palma mantle source region, in keeping with their lower B/Zr and H 2 O/Ce. This work therefore provides strong new support for subduction zone processing as a mechanism for generating radiogenic Pb isotopic signatures and volatiles heterogeneities in the mantle.

The magmatic character of early subduction zone and arc development is unlike mature systems. Low-Ti-K tholeiitic basalts and boninites dominate the early Izu-Bonin-Mariana (IBM) system. Basalts recovered from the Amami Sankaku Basin (ASB), underlying and located west of the IBM’s oldest remnant arc, erupted at ~49 Ma. This was 3 million years after subduction inception (51-52 Ma) represented by forearc basalt (FAB), at the tipping point between FAB-boninite and typical arc magmatism. We show ASB basalts are low-Ti-K, aluminous spinel-bearing tholeiites, distinct compared to mid-ocean ridge (MOR), backarc basin, island arc or ocean island basalts. Their upper mantle source was hot, reduced, refractory peridotite, indicating prior melt extraction. ASB basalts transferred rapidly from pressures (~0.7-2 GPa) at the plagioclase-spinel peridotite facies boundary to the surface. Vestiges of a polybaric-polythermal mineralogy are preserved in this basalt, and were not obliterated during persistent recharge-mix-tap-fractionate regimes typical of MOR or mature arcs., Magmatism associated with early growth of subduction zones is unlike that of mature island arc systems. Here, the authors find basalts with distinct mineralogical and geochemical characteristics were erupted during this early stage, and derived from extremely refractory, hot mantle sources.

Large (VEI= 4-6) Quaternary explosive eruptions have repeatedly occurred in Armenia and the neighboring territories. Worth noting are the Plinian eruptions of Aragats stratovolcano (4096m), located in the vicinity of the Armenian capital city Yerevan (pop. >1 million) and producing lava flows variable in composition and size, pyroclastic density currents (PDCs) and fallout deposits (Connor et al., 2011; Gevorgyan et al., 2020). The youngest lavas from Aragats are 0.52 million years (myr) old and the youngest ignimbrites are 0.65 myr old. (Connor et al., 2011, Gevorgyan et al., 2020).Here we present some features of a violent explosive Plinian eruption (VEI=4) from the relatively small, subsidiary Irind vent on the slopes of Aragats stratovolcano. We report results from newly mapped thick pumice fall deposits and pumice-rich welded lapilli-tuff and vitrophyres. Formation of up to ~10 m thick pumice fall deposits is related to a sustained Plinian eruption, while the formation of overlaying pumice tuffs (age= 0.490±0.028 M.yrs, Connor et al., 2011) and vitrophyre cover is interpreted as result of collapse of the eruption column due to a decrease of the magma supply.Following the pyroclastic eruption, a voluminous (2.9-3.6 km3) effusive eruption of Irind created up to 120 m thick trachydacite lava flows that extended 18 km from the vent. Such long and thick lava flows are not typical for viscous felsic lavas. The Irind eruption products are characterized by a plagioclase-two pyroxene mineral association that is atypical for Aragats. The Irind magmas are trachydacitic (SiO2= 66 wt; MgO= 0.7 wt%) with high- K2O contents (5.2 wt%) and enrichments in U, Th, LILE and LREE compared to Aragats. Geothermobarometry and hygrometry based on detailed textural analysis and mineral chemistry (Cpx, Opx, plagioclase, glass) reveals that Irind magmas also have elevated H2O, increased alkalinity and high T (~970 °C)- all features capable to generate magmas with much lower viscosity (4.2–4.5 log η Pa·s) in respect to typical dacites.Our results support the view that often small eruptive vents (Irind) on the slopes of large coeval stratovolcanoes (Aragats) are not necessarily tapping their voluminous magma mushes underneath and are capable to deliver independent Plinian eruptions. We speculate that these are triggered by intrusions of hot, volatile-rich, alkaline felsic magmas, presumably emplaced fast, similar to the Chaiten eruption in 2008, and did not mix well with the otherwise dominant and older magmatic system under Aragats.ReferencesConnor C., Connor L., Halama, R., Meliksetian, K., Savov, I., 2011. Volcanic Hazard Assessment of the Armenia Nuclear Power Plant Site, Final Report, 278 pp.Gevorgyan, H., Breitkreuz, C., Meliksetian, K, et al., 2020. Quaternary ring plain- and valley-confined pyroclastic deposits of Aragats stratovolcano (Lesser Caucasus): Lithofacies, geochronology and eruption history, JVGR 401, 1-22.

Exposed plumbing systems provide important insight into crystallization and differentiation in shallow sills beneath volcanic fields. We use whole rock major element, trace element and radiogenic isotopic compositions, along with mineral geochemical data on 125 samples to examine the conditions of melt differentiation in shallow sills from the exposed 4-Ma-old San Rafael subvolcanic field (SRVF), Utah. The field consists of ∼2000 dikes, 12 sills and 63 well preserved volcanic conduits. Intrusive rocks consist of mainly fine-grained trachybasalts and coarse-grained syenites, which are alkaline, comagmatic and enriched in Ba, Sr and LREE. Within sills, syenite is found as veins, lenses, and sheets totally enveloped by the basalt. The SRVF intrusions have geochemical signatures of both enriched sub-continental lithospheric and asthenospheric mantle sources. We estimate partial melting occurred between 1·2 and 1·9 GPa (50–70 km), with mantle potential temperatures in the range 1260–1326 ± 25°C, consistent with those estimated for volcanic rocks erupted on the Colorado Plateau. Geobarometry results based on clinopyroxene chemistry indicate that (1) basalt crystallized during ascent from at least 40 km deep with limited lithospheric storage, and (2) syenites crystallized only in the sills, ∼1 km below the surface. San Rafael mafic magma was emplaced in sills and started to crystallize inward from the sill margins. Densities of basalt and syenite at solidus temperatures are 2·6 and 2·4 g/cc, respectively, with similar viscosities of ∼150 Pa s. Petrographic observations and physical properties suggest that syenite can be physically separated from basalt by crystal compaction and segregation of the tephrophonolitic residual liquid out of the basaltic crystal mush after reaching 30–45% of crystallization. Each individual sill is 10–50 m thick and would have solidified fairly rapidly (1–30 years), the same order of magnitude as the duration of common monogenetic eruptions. Our estimates imply that differentiation in individual shallow sills may occur during the course of an eruption whose style may vary from effusive to explosive by tapping different magma compositions. Our study shows that basaltic magmas have the potential to differentiate to volatile-rich magma in shallow intrusive systems, which may increase explosivity.

Located in the heart of the Lesser Caucasus mountains, where the Arabian and Eurasian tectonic plates collide, Armenia occupies an exceptional geological position shaped through millions of years of subduction and collision. It is a unique place on the Earth recording extensive intrusive and volcanic activity related to the long‐standing continental convergence. The volcanoes of Armenia provide a rare opportunity to study the sources and processes involved in this unusual type of magmatism. More than 500 Quaternary volcanoes have been mapped in Armenia, most of them formed from single eruptive episodes. Among several large composite volcanoes, the mighty Aragats stands out as the largest volcano in Armenia and the region altogether. Volcanic deposits testify to the range of eruptive styles—from the ignimbrites formed in eruptions as explosive and voluminous as any seen globally in the modern era to the enormous fissure‐fed lava flows that form the Southern Caucasus flood basalt province, the smallest and youngest Large Igneous Province in the world. Several pre‐historical and historical eruptions have been documented, highlighting the potential for future volcanic activity in the region. In recent years, research has focused on the volcanic hazards associated with the Armenian Nuclear Power Plant, located in the foothills of Aragats volcano. This article highlights some of the extraordinary volcanic and intrusive features observed in Armenia and summarizes aspects of recent volcanological and petrological research.

The evidence for periods of increased volcanic activity following deglaciation, such as following ice sheet retreat after the Last Glacial Maximum, has been examined in several formerly glaciated areas, including Iceland, Alaska, and the Andean Southern Volcanic Zone. Here we present new evidence supporting the theory that during episodes of cooling in the Holocene, Icelandic volcanic activity decreased. By examining proximal and distal tephra records from Iceland spanning the last 12,500 years, we link two observed tephra minima to documented periods of climatic cooling and glacial advance, at 8.3 to 8 and 5.2 to 4.9 cal kyr BP. We simulate these periods in atmosphere-ocean and ice sheet models to assess the potential validity of the postglacial ‘unloading effect’ on Icelandic volcanic systems. We conclude that an increase in glacial cover may have decreased shallow magma ascent rates, thus limiting eruption potential and producing apparent quiescent periods in proximal and distal tephra records. However, several major uncertainties remain regarding the theory, including geographical and temporal preservation biases and the importance of any unloading effects against other factors, and these will require more prolonged investigation in future research.

International Ocean Discovery Program (IODP) Expedition 351 ‘Izu–Bonin–Mariana (IBM) Arc Origins’ drilled Site U1438, situated in the northwestern region of the Philippine Sea. Here volcaniclastic sediments and the igneous basement of the proto-IBM volcanic arc were recovered. To gain a better understanding of the magmatic processes and evolution of the proto-IBM arc, we studied melt inclusions hosted in fresh igneous minerals and sampled from 30–40 Myr old deposits, reflecting the maturation of arc volcanism following subduction initiation at 52 Ma. We performed a novel statistical analysis on the major element composition of 237 representative melt inclusions selected from a previously published dataset, covering the full age range between 30 and 40 Ma. In addition, we analysed volatiles (H2O, S, F and Cl) and P2O5 by secondary ion mass spectrometry for a subset of 47 melt inclusions selected from the dataset. Based on statistical analysis of the major element composition of melt inclusions and by considering their trace and volatile element compositions, we distinguished five main clusters of melt inclusions, which can be further separated into a total of eight subclusters. Among the eight subclusters, we identified three major magma types: (1) enriched medium-K magmas, which form a tholeiitic trend (30–38 Ma); (2) enriched medium-K magmas, which form a calc-alkaline trend (30–39 Ma); (3) depleted low-K magmas, which form a calc-alkaline trend (35–40 Ma). We demonstrate the following: (1) the eruption of depleted low-K calc-alkaline magmas occurred prior to 40 Ma and ceased sharply at 35 Ma; (2) the eruption of depleted low-K calc-alkaline magmas, enriched medium-K calc-alkaline magmas and enriched medium-K tholeiitic magmas overlapped between 35 and 38–39 Ma; (3) the eruption of enriched medium-K tholeiitic and enriched medium-K calc-alkaline magmas became predominant thereafter at the proto-IBM arc. Identification of three major magma types is distinct from the previous work, in which enriched medium-K calc-alkaline magmas and depleted low-K calc-alkaline magmas were not identified. This indicates the usefulness of our statistical analysis as a powerful tool to partition a mixture of multivariable geochemical datasets, such as the composition of melt inclusions in this case. Our data suggest that a depleted mantle source had been replaced by an enriched mantle source owing to convection beneath the proto-IBM arc from >40 to 35 Ma. Finally, thermodynamic modelling indicates that the overall geochemical variation of melt inclusions assigned to each cluster can be broadly reproduced either by crystallization differentiation assuming P = 50 MPa (∼2 km deep) and ∼2 wt% H2O (almost saturated H2O content at 50 MPa) or P = 300 MPa (∼15 km deep) and ∼6 wt% H2O (almost saturated H2O content at 300 MPa). Assuming oxygen fugacity (fO2) of log fO2 equal to +1 relative to the nickel–nickel oxide (NNO) buffer best reproduces the overall geochemical variation of melt inclusions, but assuming more oxidizing conditions (log fO2 = +1 to +2 NNO) probably reproduces the geochemical variation of enriched medium-K and calc-alkaline melt inclusions (30–39 Ma).

Alkaline igneous rocks are relatively rare in settings of tectonic convergence and little is known about their petrogenesis in these settings. This study aims to contribute to a better understanding of the formation of alkaline igneous rocks by an investigation of the Tezhsar volcano-intrusive alkaline ring complex (TAC) in the Armenian Lesser Caucasus, which is located between the converging Eurasian and Arabian plates. We present new petrological, geochemical and Sr-Nd isotope data for the TAC to constrain magma genesis and magma source characteristics. Moreover, we provide a new 40Ar/39Ar age of 41.0±0.5 Ma on amphibole from a nepheline syenite that is integrated into the regional context of ongoing regional convergence and widespread magmatism. The TAC is spatially concentric and measures ~10 km in diameter representing the relatively shallow plumbing system of a major stratovolcano juxtaposed by ring faulting with its extrusive products. The plutonic units comprise syenites and nepheline syenites, whereas the extrusive units are dominated by trachytic-phonolitic rocks. The characteristic feature of the TAC is the development of pseudomorphs after leucite in all types of the volcanic, subvolcanic and intrusive alkaline rocks.\ud 40 Whole-rock major element data show a metaluminous (Alkalinity Index = 0-0.1), alkalic and silica-undersaturated (Feldspathoid Silica-Saturation Index

Boron has become the principle proxy for the release of seawater-derived fluids into arc volcanics, linked to cross-arc variations in boron content and isotopic ratio. Because all ocean floor serpentinites so far analysed are strongly enriched in boron, it is generally assumed that if the uppermost slab mantle is hydrated, it will also be enriched in boron. Here we present the first measurements of boron and boron isotopes in fast-spread oceanic gabbros in the Pacific, showing strong take-up of seawater-derived boron during alteration. We show that in one-pass hydration of the upper mantle, as proposed for bend fault serpentinisation, boron will not reach the hydrated slab mantle. Only prolonged hydrothermal circulation, for example in a long-lived transform fault, can add significant boron to the slab mantle. We conclude that hydrated mantle in subducting slabs will only rarely contribute to boron enrichment in arc volcanics, or to deep mantle recycling., Boron is one of the main proxies for seawater-derived fluids in subduction zone volcanics and it is vital to characterise the location and concentration of boron in the oceanic lithosphere. Here the authors show that boron concentration in the mantle of downgoing slabs has been overestimated, because boron is strongly decoupled from water in the hydration process.

This research was undertaken while E.W. held a NERC-funded Doctoral Training grant (NE/K500847/1). This research was also supported by a Young Research Workers award to E.W. from the Quaternary Research Association.

The Early Toarcian was characterised by the eruption of the Karoo-Ferrar large igneous province (LIP), rapid global warming, significant perturbations in the global carbon cycle, the development of widespread anoxia known as the Early Toarcian Oceanic Anoxic Event (T-OAE) and a biotic crisis in the marine realm known as the Early Toarcian Mass Extinction (ETME). Despite the purported global nature of these environmental and biotic changes, the majority of records come from Western European sections, and remain particularly focused on settings in which the T-OAE was clearly expressed. Fewer studies focus on sections where the manifestation of ocean deoxygenation appears to have been considerably weaker, or even absent. We herein focus on Lower Jurassic successions of the Moesian Basin in the Balkan Mountains of the Balkan Mountains (Bulgaria) that were deposited on an open-ocean facing carbonate shelf. The Bulgarian δ13C and δ18O profiles show similar trends through the Lower Jurassic to coeval European sections, suggesting that seawater in the Moesian Basin was recording global palaeoclimatic and palaeoceanographic conditions during this time. Analysis of the carbon isotope record reveals a broad positive carbon isotope excursion (CIE) of 3‰ in δ13Cbel through the Early Toarcian, interrupted by a negative CIE of 3.5‰ recorded in organic carbon (Tenuicostatum–Falciferum zones). Progressive warming of seawater and increased influx of freshwater into the Moesian Basin through the Early Toarcian is recorded in δ18Obel isotopes. Such changes are attributed to the eruption of the Karoo-Ferrar LIP, reflected in the Moesian Basin by an enrichment in Mercury (Hg) recorded as a shift in sedimentary Hg/TOC values synchronous with the negative CIE. A biotic crisis is recorded amongst bivalves and considered to be part of the ETME, which is here recorded in Bulgaria for the first time. Although a significant loss amongst bivalves during the ETME often coincides with the spread of anoxia, this link is not clearly seen in Bulgaria as geochemical and sedimentological records do not support the prevalence of anoxic conditions. As such, oxygen deficiency cannot be considered a key driving mechanism for the ETME in this part of the ocean and other factors such as rapid warming may have been more important in this central Tethyan region.

Metasomatized subarc mantle is often regarded as one of the mantle reservoirs enriched in fluid-mobile elements (FMEs; e.g., B, Li, Cs, As, Sb, Ba, Rb, Pb), which, when subject to wet melting, will contribute to the characteristic FME-rich signature of arc volcanic rocks. Evidence of wet melts in the subarc mantle wedge is recorded in metasomatic amphibole-, phlogopite-, and pyroxene-bearing veins in ultramafic xenoliths recovered from arc volcanoes. Our new B and δ11B study of such veins in mantle xenoliths from Avachinsky and Shiveluch volcanoes, Kamchatka arc, indicates that slab-derived FMEs, including B and its characteristically high δ11B, are delivered directly to a melt that experiences limited interaction with the surrounding mantle before eruption. The exceptionally low B contents (from 0.2 to 3.1 μg g–1) and low δ11B (from –16.6‰ to +0.9‰) of mantle xenolith vein minerals are, instead, products of fluids and melts released from the isotopically light subducted and dehydrated altered oceanic crust and, to a lesser extent, from isotopically heavy serpentinite. Therefore, melting of amphibole- and phlogopite-bearing veins in a metasomatized mantle wedge cannot alone produce the characteristic FME geochemistry of arc volcanic rocks, which require a comparatively large, isotopically heavy and B-rich serpentinite-derived fluid component in their source. peerReviewed

Through decades of field studies and laboratory analyses, Volcán de Colima, Mexico has one of the best known proximal eruption stratigraphies of any volcano, yet the frequency and magnitudes of previous eruptions are still poorly resolved. Hazard assessments based on models of well-known, well-mapped recent eruptions may appear to have low uncertainty, but may be biased by the nature of those events. We present a comprehensive stratigraphy of explosive eruption deposits combining new data collected as part of this study together with published and unpublished data. For the first time we have been able to model five of the best exposed and cross-correlated pre-historical Holocene explosive events at Volcán de Colima. By modelling the volumes and magnitudes of Holocene eruptions at Volcán de Colima, we are able to improve estimations of the potential range of magnitudes of future explosive eruptions, which can be incorporated into hazard assessments for nearby communities. Based on recent studies we demonstrate that these volumes may be underestimated by at least an order of magnitude, and show that even with an exceptionally well-defined stratigraphic record our understanding of the full range of explosive eruptions may still be biased.

The Lesser Caucasus mountains sit on a transition within the Arabia-Eurasia collision zone between very thin lithosphere (< 100 km) to the west, under Eastern Anatolia, and a very thick lithospheric root (up to 200 km) in the east, under western Iran. A transect of volcanic highlands running from north-west to south-east in the Lesser Caucasus allows us to look at the effects of lithosphere thickness variations on the geochemistry of volcanic rocks in this continental collision zone. Volcanic rocks from across the region show a wide compositional range from basanites to rhyolites, and have arc-like geochemical characteristics, typified by ubiquitous negative Nb-Ta anomalies. Magmatic rocks from the south-east, where the lithosphere is thought to be thicker, are more enriched in incompatible trace elements, especially the light rare earth elements, Sr and P. They also have more radiogenic 87Sr/86Sr, and less radiogenic 143Nd/144Nd. Across the region, there is no correlation between SiO2 content and Sr-Nd isotope ratios, revealing a lack of crustal contamination. Instead, “spiky” mid-ocean-ridge basalt normalised trace element patterns are the result of derivation from a subduction-modified mantle source, which likely inherited its subduction component from subduction of the Tethys Ocean prior to the onset of continent-continent collision in the late Miocene. In addition to the more isotopically enriched mantle source, modelling of non-modal batch melting suggests lower degrees of melting and the involvement of garnet as a residual phase in the south-east. Melt thermobarometry calculations based on bulk-rock major elements confirm that melting in the south-east must occur at greater depths in the mantle. Temperatures of melting below 1200 °C, along with the subduction-modified source, suggest that melting occurred within the lithosphere. It is proposed that in the Northern Lesser Caucasus this melting occurs close to the base of the very thin lithosphere (at a depth of ∼45 km) as a result of small-scale delamination. A striking similarity between the conditions of melting in north-west Iran and the southern Lesser Caucasus (two regions between which the difference in lithosphere thickness is ∼ 100 km) suggests a common mechanism of melt generation in the mid-lithosphere (∼ 75 km). The southern Lesser Caucasus magmas result from mixing between partial melts of deep lithosphere (∼ 120 km in the south) and mid-lithosphere sources to give a composition intermediate between magmas from the northern Lesser Caucasus and north-west Iran. The mid-lithosphere magma source has a distinct composition compared to the base of the lithosphere, which is argued to be the result of the increased retention of metasomatic components in phases such as apatite and amphibole, which are stabilized by lower temperatures prior to magma generation.

A crucial problem at most volcanoes is reconstructing past eruptions from the geological record. The rapid erosion of many volcanic terrains results in geologically recent eruptions leaving a relatively sparse record of the event. Here we consider the tephra-stratigraphic record of the 1913 eruption of Volcán de Colima, a recent but greatly eroded tephra fallout deposit. A total of 38 stratigraphic sections of the 1913 deposit have been analysed for thickness, granulometry and geochemistry. The 1913 scoria are hornblende and two-pyroxene andesites with approximately 58 wt% SiO2, providing a geochemical and petrographic signature that is distinct from earlier (1818) and later tephra fallout deposits. Tephra2, a tephra dispersion computer code based on the advection-diffusion equation, is used to model thickness variation and particle size distribution of the pyroclasts for the 1913 deposit. Based on computer simulations, the observed tephra stratigraphy is best fit with a total eruption mass of ~5.5 × 1010 kg. Computer simulations including reports of tephra accumulation from the historical record produces an alternative deposit model with a finer median particle size (~1.77 ϕ), a higher eruption column (~25 km above mean sea level, amsl), and a greater total eruption mass (~1.4 × 1011 kg). This larger eruption magnitude is supported by modelling the granulometry of the 38 stratigraphic sections. The models suggest a median deposit particle size of at least 2ϕ, a deposit mass of 1–5 × 1011 kg (VEI 4), and that significant segregation by particle size as a function of height occurred in the 1913 eruption column. This analysis highlights potential bias in eruption magnitude estimates that use only thickness of proximal deposits, and the advantage of modelling the granulometry of the deposit in such circumstances.

Argon isotopic analyses were funded by NERC grant to I. Savov, IP-1690-1116. PJS was funded through a NERC studentship as part of the Leeds York Spheres Doctoral Training Partnership (DTP) (grant number NE/L002574/1). Part of the field work and research were supported by base funding of the Institute of Geological Sciences (IGS) and a thematic project by the Science Committee of the Armenian Ministry of Education and Science (project #18 T-1E368). The post-collisional Syunik and Vardenis volcanic highlands, located in the southern Lesser Caucasus mountains (part of the Arabia-Eurasia collision zone) are host to over 200 monogenetic volcanoes, as well as 2 large Quaternary polygenetic volcanoes in the Syunik highland. The latter are overlain by lavas from the monogenetic volcanoes, suggesting there was a transition in the style of volcanic activity from large-volume central vent eruptions to dispersed small-volume eruptions. 12 new high quality 40Ar/39Ar ages are presented here, with 11 ages calculated by step-heating experiments on groundmass separates, and the final age obtained from total fusions of a population of sanidines. All the ages were younger than 1.5 Ma, except for one ignimbrite deposit whose sanidines gave an age of 6 Ma. While the bulk of the exposed products of post-collisional volcanism relate to Pleistocene activity, it is clear there has been active volcanism in the region since at least the late Miocene. All ages for monogenetic volcanoes in the Syunik highland are younger than 1 Ma, but to the north in Vardenis there is geochronological evidence of monogenetic volcanism at 1.4 and 1.3 Ma. An age of 1.3 Ma is determined for a lava flow from one of the polygenetic volcanoes- Tskhouk, and when combined with other ages helps constrain the timing of the polygenetic to monogenetic transition to around 1 Ma. The new ages illustrate a degree of spatio-temporal coupling in the formation of new vents, which could be related to pull-apart basins focussing ascending magmas. This coupling means that future eruptions are particularly likely to occur close to the sites of the most recent Holocene activity. The polygenetic to monogenetic transition is argued to be the result of a decreasing magma supply based on: (i) volume estimates for Holocene eruptions and for all monogenetic volcanoes and their lava flows in Syunik; and (ii) the volcanic stratigraphy of the Lesser Caucasus region which shows late Pliocene- early Pleistocene continental flood basalts being succeeded by a few large andesite-dacite volcanoes and then the most recent deposits consisting of small-volume scoria cones. The Syunik highland has the highest density of monogenetic centres in the Lesser Caucasus, which is taken to indicate this region has the highest magma flux, and was therefore the last location to transition to monogenetic volcanism, which is why the transition is most clearly seen there. There is no evidence from Sr-Nd-B isotope measurements for the exhaustion of fusible slab components in the mantle source, showing that an inherited slab signature can survive for millions of years after the end of subduction. Although volcanism in the Lesser Caucasus is currently waning, a future pulse of activity is possible. Publisher PDF

Fine ash produced during volcanic eruptions can be dispersed over a vast area, where it poses a threat to aviation, human health and infrastructure. We analyse the particle size distributions, geochemistry and glass shard morphology of 19 distal (>1000 km from source) volcanic ash deposits distributed across northern Europe, many geochemically linked to a specific volcanic eruption. The largest glass shards in the cryptotephra deposits were 250 µm (longest axis basis). For the first time, we examine the replicability and reliability of glass shard size measurements from peatland and lake archives. We identify no consistent trend in the vertical sorting of glass shards by size within lake and peat sediments. Measuring the sizes of 100 shards from the vertical sample of peak shard concentration is generally sufficient to ascertain the median shard size for a cryptotephra deposit. Lakes and peatlands in close proximity contain cryptotephras with significantly different median shard size in four out of five instances. The trend toward a greater amount of larger shards in lakes may have implications for the selection of distal sites to constrain the maximum glass shard size for modelling studies. Although the 95th percentile values for shard size generally indicate a loss of larger shards from deposits at sites farther from the volcano, due to the dynamic nature of the controls on tephra transport even during the course of one eruption there is no simple relationship between median shard size and transport distance.

Tephrochronology (the dating of sedimentary sequences using volcanic ash layers) is an important tool for the dating and correlation of sedimentary sequences containing archives and proxies of past environmental change. In addition, tephra layers provide valuable information on the frequency and nature of ash fallout from volcanic activity. Successful tephrochronology is usually reliant on the correct geochemical identification of the tephra which has, until now, been based primarily on the analysis of major element oxide composition of glass shards using electron probe microanalysis (EPMA). However, it is often impossible to differentiate key tephra layers using EPMA alone. For example, the Hekla AD 1947 and 1510 tephras (which are found as visible layers in Iceland and also as ‘crypto-tephra’ microscopic layers in NW Europe) are currently indistinguishable using EPMA. Therefore, other stratigraphic or chronological information is needed for their reliable identification. Raman spectroscopy is commonly used in chemistry, since vibrational information is specific to the chemical bonds and symmetry of molecules, and can provide a fingerprint by which these can be identified. Here, we demonstrate how Raman spectroscopy can be used for the successful discrimination of mineral species in tephra through the analysis of individual glass shards. In this study, we obtained spectra from minerals within the glass shards – we analysed the microlites and intratelluric mineral phases that can definitely be attributed to the tephra shards and the glass itself. Phenocrysts were not analysed as they could be sourced locally from near-site erosion. Raman spectroscopy can therefore be considered a valuable tool for both proximal and distal tephrochronology because of its non-destructive nature and can be used to discriminate Hekla 1510 from Hekla 1947.

This work was supported as part of Patrick Sugden's PhD studentship funded through the Leeds-York SPHERES NERC doctoral training partnership (NE/L002754/1). Part of the fieldwork and research was funded by the University of Leeds, the Carnegie Institution of Washington, the ERASMUS exchange programme (for IS) and the Science Committee of the Armenian Ministry of Education and Science (project #18T-1E368). The majority of the B isotope analytical work was supported by IGG-CNR funds P1600514. We present the first boron abundance and δ11B data for young (1.5-0 Ma) volcanic rocks formed in an active continent-continent collision zone. The δ11B of post-collisional volcanic rocks (−5 to +2‰) from the Armenian sector of the Arabia-Eurasia collision zone are heavier than mid-ocean ridge basalts (MORB), confirming trace element and isotope evidence for their derivation from a subduction-modified mantle source. Based on the low B/Nb (0.03-0.25 vs 0.2-90 in arc magmas), as well as low Ba/Th and Pb/Ce, this source records a subduction signature which is presently fluid-mobile element depleted relative to most arc settings. The heavier than MORB δ11B of post-collision volcanic rocks argues against derivation of their subduction signature from a stalled slab, which would be expected to produce a component with a lighter than MORB B, due to previous fluid depletion. Instead, the similarity of δ11B in Plio-Pleistocene post-collision to 41 Ma alkaline igneous rocks also from Armenia (and also presented in this study), suggests that the subduction signature is inherited from Mesozoic-Paleogene subduction of Neotethys oceanic slabs. The slab component is then stored in the mantle lithosphere in amphibole, which is consistent with the low [B] in both Armenian volcanic rocks and metasomatic amphibole in mantle xenoliths. Based on trace element and radiogenic isotope systematics, this slab component is thought to be dominated by sediment melts (or supercritical fluids). Previously published δ11B of metasediments suggests a sediment-derived metasomatic agent could produce the B isotope composition observed in Armenian volcanic rocks. The lack of evidence for aqueous fluids preserved over the 40 Myr since initial collision supports observations that this latter component is transitory, while the lifetime of sediment melts/supercritical fluids can be extended to >40 Myr. Publisher PDF

International audience; Negative Cerium (Ce) anomalies are observed in chondrite-normalized Rare Earth Element patterns from various volcanic arc suites. These anomalies are well defined in volcanic rocks from the Mariana arc and have been interpreted as the result of addition of subducted sediments to the arc magma sources. This study combines 143Nd/144Nd and 138Ce/142Ce isotope measurements in Mariana volcanic rocks that have Ce anomalies ranging from 0.97 to 0.90. The dataset includes sediments sampled immediately before subduction at the Mariana Trench (Sites 801 and 802 of ODP Leg 129) and primitive basalts from the Southern Mariana Trough (back-arc basin). Binary mixing models between the local depleted mantle and an enriched end-member using both types of sediment (biosiliceous and volcaniclastic) found in the sedimentary column in front of the arc are calculated. Marianas arc lavas have Ce and Nd isotopic compositions that require

Whole-rock isotope ratio (Hf, Nd, Pb, Sr) and trace element data for basement rocks at ocean drilling Sites U1438, 1201 and 447 immediately west of the KPR (Kyushu-Palau Ridge) are compared to those of FAB (forearc basalts) previously interpreted to be the initial products of IBM subduction volcanism. West-of-KPR basement basalts (drill sites U1438, 1201, 447) and FAB occupy the same Hf-Nd and Pb-Pb isotopic space and share distinctive source characteristics with εHf mostly > 16.5 and up to εHf = 19.8, which is more radiogenic than most Indian mid-ocean ridge basalts (MORB). Lead isotopic ratios are depleted, with 206Pb/204Pb = 17.8–18.8 accompanying relatively high 208Pb/204Pb, indicating an Indian-MORB source unlike that of West Philippine Basin plume basalts. Some Sr isotopes show affects of seawater alteration, but samples with 87Sr/86Sr < 0.7034 and εNd > 8.0 appear to preserve magmatic compositions and also indicate a common source for west-of-KPR basement and FAB. Trace element ratios resistant to seawater alteration (La/Yb, Lu/Hf, Zr/Nb, Sm/Nd) in west-of-KPR basement are generally more depleted than normal MORB and so also appear similar to FAB. At Site U1438, only andesite sills intruding sedimentary rocks overlying the basement have subduction-influenced geochemical characteristics (εNd ∼ 6.6, εHf ∼ 13.8, La/Yb > 2.5, Nd/Hf ∼ 9). The key characteristic that unites drill site basement rocks west of KPR and FAB is the nature of their source, which is more depleted in lithophile trace elements than average MORB but with Hf, Nd, and Pb isotope ratios that are common in MORB. The lithophile element-depleted nature of FAB has been linked to initiation of IBM subduction in the Eocene, but Sm-Nd model ages and errorchron relationships in Site U1438 basement indicate that the depleted character of the rocks is a regional characteristic that was produced well prior to the time of subduction initiation and persists today in the source of modern IBM arc volcanic rocks with Sm/Nd > 0.34 and εNd ∼ 9.0.

Developing a comprehensive understanding of the interactions between the atmosphere and the geosphere is an ever-more pertinent issue as global average temperatures continue to rise. The possibility of more frequent volcanic eruptions and more therefore more frequent volcanic ash clouds raises potential concerns for the general public and the aviation industry. This review describes the major processes involved in short- and long-term volcano–climate interactions with a focus on Iceland and northern Europe, illustrating a complex interconnected system, wherein volcanoes directly affect the climate and climate change may indirectly affect volcanic systems. In this paper we examine both the effect of volcanic inputs into the atmosphere on climate conditions, in addition to the reverse relationship – that is, how global temperature fluctuations may influence the occurrence of volcanic eruptions. Explosive volcanic eruptions can cause surface cooling on regional and global scales through stratospheric injection of aerosols and fine ash particles, as documented in many historic eruptions, such as the Pinatubo eruption in 1991. The atmospheric effects of large-magnitude explosive eruptions are more pronounced when the eruptions occur in the tropics due to increased aerosol dispersal and effects on the meridional temperature gradient. Additionally, on a multi-centennial scale, global temperature increase may affect the frequency of large-magnitude eruptions through deglaciation. Many conceptional models use the example of Iceland to suggest that post-glacial isostatic rebound will significantly increase decompression melting, and may already be increasing the amount of melt stored beneath Vatnajokull and several smaller Icelandic glaciers. Evidence for such a relationship existing in the past may be found in cryptotephra records from peat and lake sediments across northern Europe. At present, such records are incomplete, containing spatial gaps. As a significant increase in volcanic activity in Iceland would result in more frequent ash clouds over Europe, disrupting aviation and transport, developing an understanding of the relationship between the global climate and volcanism will greatly improve our ability to forecast and prepare for future events.

This chapter reviews recycling of boron (B) and its isotopes in subduction zones. It discusses the profound changes in B concentrations and B isotope ratios of various materials involved in convergent margin evolution, in particular highlighting the fate and evolution of progressively dehydrating subducting slabs and the behavior of B during burial and subsequent metamorphism. We review various models used to parameterize these complex and often poorly understood processes and critically evaluate the available data from the literature. We proceed by reviewing B isotope data from mafic arc volcanic rocks and explore systematic variations with subduction zone geometry as well as familiar geochemical tracers of subduction processes. Finally, the role of serpentinisation in the mantle wedge is discussed in the light of new geochemical and petrological insights on the importance of serpentinites and subduction erosion. We provide recommendations for further research on B isotopes in subduction zones and directions where we think this exciting stable isotope tracer may make the biggest impact.

The last Plinian-type eruption of Volcán de Colima, Mexico, occurred in 1913; this resulted in the removal of the top 100 m of the edifice and the deposition of a tephra layer that blanketed the slopes of the Colima Volcanic Complex (CVC). Road-cuts on the flanks of the nearby Nevado de Colima edifice expose pre-1913 air-fall tephra, pyroclastic flow and ash-rich surge deposits resulting from numerous highly explosive events throughout the Holocene. The majority of the pumice and scoria fallout deposits are medium-K subalkaline basaltic andesite and andesite in composition, defining a clear major element differentiation trend. In contrast, three newly discovered scoria fallout deposits are high-K subalkaline, transitional to alkaline, basaltic andesite in composition and are characterized by the presence of phlogopite; these deposits have high MgO (up to 7·9 wt %), K2O (up to 2·6 wt %) and P2O5 (up to 0·67 wt %) contents. They are also strongly enriched in fluid- and melt-mobile large ion lithophile elements (LILE; Rb, Ba, K, Sr and Th) and light REE (LREE; La, Ce, Pr and Nd) relative to the majority of the Colima tephra fallout deposits. Strontium and Nd isotope systematics reveal that the high-K mafic scoria have more radiogenic Sr (87Sr/86Sr = 0·70365–0·70408) and less radiogenic Nd (143Nd/144Nd = 0·51279–0·51294) compared with the majority of the subalkaline tephras (87Sr/86Sr = 0·70338–0·70371 and 143Nd/144Nd = 0·51290–0·51295). Two-component mixing models, using whole-rock geochemical data, indicate the importance of magma mixing in the petrogenesis of the Colima magmas, with addition of up to 50% by volume of an alkaline mafic magma component in the most potassic magmas. This is supported by mineral chemistry and textural data, which reveal multiple episodes of decompression and magma mingling within a shallow crustal magma storage region. The presence of these potassic tephra fall deposits among the otherwise prevailing medium-K subalkaline stratigraphy indicates that pulses of K-rich alkaline mafic magmas periodically enter the CVC plumbing system on timescales of a few thousand years and may trigger Plinian explosive eruptions.

The late Pliensbachian–Toarcian (Jurassic) sedimentological, paleontological and geochemical (belemnite 87Sr/86Sr, δ13C and δ18O) record is examined in two Eastern Tethyan (Bulgarian) locations. This interval contains the well-known early Toarcian ocean anoxic event (T-OAE) and its manifestation and temporal context is examined in Bulgaria. Many of the features seen in south-western Europe are identified: collapse of carbonate platform productivity at the Pliensbachian/Toarcian boundary, the T-OAE (a short pulse of euxinic deposition in the Falciferum Zone), an early Toarcian rapid warming event seen in the belemnite δ18O record that peaked around the Falciferum/Bifrons Zone boundary. The long-recognized positive δ13C excursion in the late Falciferum Zone is also seen but a precursor, sharp δ13C negative excursion seen around the Tenuicostatum/Falciferum Zone boundary in most organic carbon records is not seen in the belemnite data, a curious absence noted from other belemnite records. Subsequent perturbations in 87Sr/86Sr, δ13C and δ18O suggest that there may be more global isotopic excursions in the Early Jurassic. On the other hand, belemnite Sr isotope values from Bulgaria are in accord with those recorded in Western Europe and hence, demonstrating its value as a chronostratigraphic tool.

Highlights: • Whole-rock, mineral and isotope chemical data of Cerro Machin Volcano. • Mixing of distinct mafic and silic melts produced hybrid magma. • Amphibole thermobarometry indicates resident magma reservoir at 13 ± 2 km depth. • Dacites have adakitic geochemical signature. The last known eruption at Cerro Machin Volcano (CMV) in the Central Cordillera of Colombia occurred ∼900 years BP and ended with the formation of a dacitic lava dome. The dome rocks contain both normally and reversely zoned plagioclase (An24–54), unzoned and reversely zoned amphiboles of dominantly tschermakite and pargasite/magnesio-hastingsite composition and olivine xenocrysts (Fo = 85–88) with amphibole/clinopyroxene overgrowth, all suggesting interaction with mafic magma at depth. Plagioclase additionally exhibits complex oscillatory zoning patterns reflecting repeated replenishment, fractionation and changes in intrinsic conditions in the magma reservoir. Unzoned amphiboles and cores of the reversely zoned amphiboles give identical crystallization conditions of 910 ± 30 °C and 360 ± 70 MPa, corresponding to a depth of about 13 ± 2 km, at moderately oxidized conditions (fO2 = +0.5 ± 0.2 ΔNNO). The water content in the melt, calculated based on amphibole chemistry, is 7.1 ± 0.4 wt.%. Rims of the reversely zoned amphiboles are relatively enriched in MgO and yield higher crystallization temperatures (T = 970 ± 25 °C), slightly lower melt H2O contents (6.1 ± 0.7 wt.%) and overlapping pressures (410 ± 100 MPa). We suggest that these rims crystallized following an influx of mafic melt into a resident magma reservoir at mid-crustal depths, further supported by the occurrence of xenocrystic olivine. Crystallization of biotite, albite-rich plagioclase and quartz occurred at comparatively low temperatures (probably

The volcanic ash or ‘tephra’ cloud resulting from the relatively small (volume and VEI) eruption of the Icelandic volcano Eyjafjallajokull in 2010 caused major air travel disruption, at substantial global economic cost. On several occasions in the past few centuries, Icelandic eruptions have created ash and/or sulphur dioxide clouds which were detected over Europe (e.g. Hekla in 1947, Askja in 1875, and Laki in 1783). However, these historical observations do not represent a complete record of events serious enough to disrupt aviation in Europe. The only feasible evidence for this is within the geological tephra record. Ash layers are preserved in bogs and lakes where tephra deposited from the atmosphere is incorporated in the peat/mud. In this article we: 1, introduce the analysis of the Northern European sedimentary tephra record; 2, discuss our findings and modelling results; 3, highlight how these were misinterpreted by the popular media; and 4, use this experience to outline several existing problems with current tephra studies and suggest agendas for future research.

Metamorphosed, vesuvianite-bearing dykes occur in serpentinised peridotites of the Raspas Complex (Ecuador), which represents a piece of oceanic lithosphere that has experienced high-pressure, subduction-related metamorphism. The serpentinite mantle protoliths are geochemically indistinguishable from modern oceanic lithosphere entering subduction zones. Positive Eu anomalies (Eu/Eu* = 1.3-7.2) and relative LREE enrichments (LaN/SmN = 1.2-5.5) point to hydrothermal alteration of the peridotite precursor rocks at or near the seafloor. Major mineral phases in the metamorphosed dykes include chlorite, diopside, amphibole and vesuvianite. In each dyke, only two of these phases − either amphibole + vesuvianite, diopside + chlorite, or amphibole + chlorite dominate the modal mineralogy with >∼90 vol.%, suggesting metasomatic replacement at elevated P-T conditions during subduction, controlled by an external fluid. This fluid caused the decrease in coexisting mineral phases and overprinting of initial Sr isotope ratios (0.7025-0.7031). Preserved geochemical signatures from the dyke protoliths, including positive Eu anomalies (Eu/Eu* = 1.2-2.0) and Na enrichment due to spilitisation, reveal that the dykes originated as oceanic olivine gabbros and troctolites.Vesuvianite in the Raspas Complex formed by hydration and silica removal from gabbroic mineral assemblages during subduction. It has a wide stability in P-T space for hydrated and silica deficient bulk compositions so that it potentially represents a significant repository for the cycling of elements during subduction. In addition to Ca, Mg and Al, incorporation of significant amounts of Ti, Fe and Na (up to 2.4, 1.7 and 1.6 atoms per formula unit, respectively) in vesuvianite bears evidence for the potential of vesuvianite as petrogenetic indicator, although lack of relevant thermodynamic and experimental data precludes the extraction of quantitative information. For cold subduction zones in particular, vesuvianite appears to be able to carry significant amounts of water to mantle depths. Preferential incorporation of HREE (up to 2.2 ppm Yb), Sr (up to ∼ 300 ppm) and Pb (up to 4.5 ppm) in vesuvianite underlines its potentially important role for the storage, transport and release of these key elements in radiogenic isotope geochemistry during subduction zone cycling

Arsenic (As) is naturally enriched in most terrestrial and marine hydrothermal systems. Its discharge from shallow-sea hydrothermal vents into coastal marine environments can dramatically affect the surrounding ecosystems. Shallow-sea hydrothermal venting of acidic (pH ~ 5), hot (40–116 °C), and highly sulfidic (up to 3.1 mM H2S) fluids occurs at Palaeochori and Spathi Bays, off the southeast coast of Milos Island, Greece. Two distinct types of fluids, both extremely As enriched, were collected from the submarine vents often within a few meters of each other: (1) a high-Cl fluid (enriched in Cl by up to 47% relative to seawater) depleted in Mg and SO4 and enriched in major (Na, Ca, K, B, Sr, and Br) and minor/trace elements (Si, Li, Rb, Ce, As, Fe, Mn, and Ba), and (2) a low-Cl fluid (depleted in Cl by up to 66% relative to seawater) also depleted in Na, Mg, SO4, and Br. The maximum concentration of As reached 39 μM in the high-Cl fluids and up to 78 μM in the low-Cl fluids. This is approximately 3000 times the concentration of As in seawater and far exceeds the concentrations typically found in mid-ocean ridge (MOR) and back-arc basin (BAB) fluids, which are typically We used a combination of elemental analyses and stable and radiogenic isotope data (oxygen, deuterium, and strontium) to understand the primary controls on As enrichment. Those parameters all indicated that the fluids were a mixture of seawater and a hydrothermal fluid originally derived from seawater but altered by water–rock interaction, subcritical phase separation (boiling), and vapor/brine segregation. Since As was extremely enriched in both the high-Cl and the low-Cl thermal fluids, we conclude that this element readily partitions into the vapor phase in the Milos hydrothermal system. The original source of As is most likely the leaching of the greenschist facies metamorphic basement rocks transpierced by an abundance of secondary quartz and calcite veins rich in pyrite. It is possible that these veins were deposited from As-rich magmatic fluids, similar to some epithermal Au deposits. A poorly-crystalline orpiment precipitates as hydrothermal fluids mix with overlying seawater, and removes much of the dissolved As prior to discharge.

This research was undertaken while Elizabeth Watson held a NERC-funded Doctoral Training Grant (NE/K500847/1). GTS acknowledges support from the Dutch Foundation for the Conservation of Irish Bogs. IS and EJW thank CGS for generous support of the fieldwork in Sweden. Despite the widespread application of tephra studies for dating and correlation of stratigraphic sequences (‘tephrochronology’), questions remain over the reliability and replicability of tephra records from lake sediments and peats, particularly in sites >1000 km from source volcanoes. To address this, we examine the tephrostratigraphy of four pairs of lake and peatland sites in close proximity to one another (

In this study, we report the first discovery of microdiamond inclusions in kyanite–garnet schists from the Central Rhodope Mountains in Bulgaria. These inclusions occur in garnets from metapelites that are part of a meta-igneous and meta-sedimentary mélange hosted by Variscan (Hercynian) orthogneiss. Ultra-high-pressure (UHP) conditions are further supported by the presence of exsolved needles of quartz and rutile in the garnet and by geothermobarometry estimates that suggest peak metamorphic temperatures of 750–800°C and pressures in excess of 4GPa. The discovery of UHP conditions in the Central Rhodopes of Bulgaria compliments the well-documented evidence for such conditions in the southernmost (Greek) part of the Rhodope Massif. Dating of garnets from these UHP metapelites (Chepelare Shear Zone) using Sm–Nd geochronology indicates a Late Cretaceous age (70.5–92.7Ma) for the UHP metamorphic event. This is significantly younger than previously reported ages and suggests that the UHP conditions are associated with the Late Mesozoic subduction of the Vardar Ocean northward beneath the Moesian platform (Europe). The present-day structure of the RM is the result of a series of subduction–exhumation events that span the Cenozoic, alongside subsequent post-orogenic extension and metamorphic core complex formation.

Most tephra fallout models rely on the advection–diffusion equation to forecast sedimentation and hence volcanic hazards. Here, we test the application of the advection–diffusion equation to tephra sedimentation using data collected on the proximal (350 to ~1,200 m from the vent) to medial (greater than ~1,200 m from the vent) tephra blanket of a basaltic cinder cone, Cerro Negro volcano, located in Nicaragua. Our understanding of tephra depositional processes at this volcano is significantly improved by combination of sample pit data in the medial zone and high-resolution ground-penetrating radar (GPR) data collected in the near vent and proximal zones. If the advection–diffusion equation applies, then the thickness of individual tephra deposits should have Gaussian crosswind profiles and exponential decay with distance away from the vent. At Cerro Negro, steady trade winds coupled with brief eruptions of relatively low energy (VEI 2–3) create relatively simple deposits. GPR data were collected along three crosswind profiles at distances of 700–1,600 m from the vent; sample pits were used to estimate thickness of the 1992 tephra deposit up to 13 km from the vent. Horizons identified in proximal GPR profiles exhibit Gaussian distributions with a high degree of statistical confidence, with diffusion coefficients of ~500 m2 s−1 estimated for the deposits, confirming that the advection–diffusion equation is capable of modeling sedimentation in the proximal zone. The thinning trend downwind of the vent decreases exponentially from the cone base (350 m) to ~1,200 m from the vent. Beyond this distance, deposit overthickening occurs, identified in both GPR and sample pit datasets. The combined data reveal three depositional regimes: (1) a near-vent region on the cone itself, where fallout remobilizes in granular flows upon deposition; (2) a proximal zone in which particles fall from a height of less than ~2 km; and (3) a medial zone, in which particles fall from ~4 to 7 km and the deposit is thicker than expected based on thinning trends observed in the proximal zone of the deposit. This overthickening of the tephra blanket, defining the transition from proximal to medial depositional facies, is indicative of transition from sedimentation dominated by fallout from plume margins to that dominated by fallout from the buoyant eruption cloud—a feature of deposits previously identified in larger-volume eruptions. We interpret this change to represent a change in diffusion law, occurring at total particle fall times (the fall time threshold of numerical models) of ~400 s. Thus, the detailed GPR profiles and pit data collected at Cerro Negro help to validate current numerical models of tephra sedimentation.

The ash cloud resulting from the A.D. 2010 eruption of Eyjafjallajokull in Iceland caused severe disruption to air travel across Europe, but as a geological event it is not unprecedented. Analysis of peats and lake sediments from northern Europe has revealed the presence of microscopic layers of Icelandic volcanic ash (tephra). These sedimentary records, together with historical records of Holocene ash falls, demonstrate that Icelandic volcanoes have generated substantial ash clouds that reached northern Europe many times. Here we present the first comprehensive compilation of sedimentary and historical records of ash-fall events in northern Europe, spanning the past 7000 yr. Ash-fall events appear to have been more frequent in the past 1500 yr. It is unclear whether this reflects a true increase in eruption frequency or dispersal, or is an artifact of the records or the way in which they have been generated. In the past 1000 yr, volcanic ash clouds reached northern Europe with a mean return interval of 56 ± 9 yr (the range of return intervals is between 6 and 115 yr). Probabilistic modeling using the ash records for the last millennium indicates that for any 10 yr period there is a 16% probability of a tephra fallout event in northern Europe. These values must be considered as conservative estimates due to the nature of tephra capture and preservation in the sedimentary record.

Volcan de Colima in Western Mexican Volcanic Belt is the most active volcano in North America. We present a detailed tephrochronological study of its Holocene proximal tephra-fall, pyroclastic surge and pyroclastic flow deposits based on 98 new stratigraphic sections, 183 new dendrocalibrated radiocarbon [ 14 C] age determinations, 209 new geochemical analyses of pumice and ash, > 50 texture and mineral (point-counting) mode determinations and 230 grain-size analyses. These results provide a much clearer understanding of the explosive eruptive behavior of Volcan de Colima during the past 10,000 years. Our data reveals that only one of the many eruptions described in historic documents has been large enough to be preserved. We discuss the major and trace element geochemistry and petrology of the Holocene tephra and compare it with the currently erupting calc-alkaline andesites and alkaline minettes and basanites from the adjacent cinder cones. It appears that several of the oldest (∼ 12,000–6000 yrs BP) samples of the Holocene tephra may record the last episodes of calc-alkaline and alkaline magma mixing as first described in the pioneering work of Luhr and Carmichael (1981). Finally, the tephrochronological record from the northern, downwind side of the Colima Volcanic Complex is compared with the extensive, radiocarbon-constrained, record of Holocene debris–avalanche deposits on the south side of the complex, to build a more integrated picture of its past volcanic activity.

Magma is transported through Earth9s solid crust by two different processes, diking and diapirism, although other mechanisms, such as porous and channeled flow, can transport melt through partially molten crustal areas. Dikes are ubiquitous indicators of the transport of magma in the shallow crust by brittle fracture, and there is ample geological and geophysical evidence supporting diking as a magma-ascent mechanism through the crust. On the other hand, igneous diapirism, involving magma ascent by gravitational instability and requiring viscous or plastic flow of country rock (“hot Stokes” diapirs), is often invoked as a magma-transport mechanism restricted to the ductile upper mantle or lower crust. However, unequivocal geological field evidence for igneous diapirism has proven elusive and has been a matter of considerable debate. We report geological and geophysical evidence showing that Pliocene sills emplaced in the upper levels of brittle continental crust of the Colorado Plateau in the San Rafael subvolcanic field (Utah) became gravitationally unstable by mechanically altering the overlying sedimentary rocks. These sills grew into structures that we recognize as domes and plugs at the current level of exposure. Some of these plugs continued to transport magma to shallower levels of the continental crust and eventually acted as conduits feeding volcanic eruptions. Our geological and geophysical findings indicate that gravitational instability is a viable mechanism for the initiation of magma ascent in the upper continental crust for small volumes of basaltic magma under specific conditions.

The geochemistry of NW USA rhyolites correlates strongly with geography and the nature of the underlying basement terranes. Rhyolites from the Snake River Plain-Yellowstone (SRPY) province have higher 87 Sr/ 86 Sr, 207 Pb/ 206 Pb, and lower 143 Nd/ 144 Nd than those from the Oregon High Lava Plains (HLP) province, reflecting a dominant influence of Precambrian cratonic crust east of the western Idaho suture zone versus accreted oceanic terranes of Phanerozoic age to the west. Rhyolites from the cratonic domain show significant enrichments of Th, U, and LREE/HREE, whereas B concentration and especially B/Nb and B/Rb are systematically higher west of the tectonic boundary. This decoupling of B from the other incompatible elements is best explained in terms of distinctive magmatic sources east and west of the suture zone. B isotopic composition [δ 11 B] was measured for natural and synthetic glasses via multiple multiplier laser ablation-ICP-MS. δ 11 B values are systematically lighter in SRPY rhyolites (-5.6 to ―8.9 ‰) compared to those from the HLP (-0.8 to -3.1 ‰). These data are consistent with strongly fluid-depleted and/or metamorphosed sources for SRPY rhyolites, whereas HLP sources resemble those of typical oceanic basalts, and could reflect melting of juvenile basalt-derived protoliths in the crust. B isotope ratios of low-δ 18 O rhyolites are indistinct from those with normal δ 8 O, suggesting that δ 11 B values are not strongly affected by hydrothermal processes that alter source materials with meteoric water. Considering all data, it is likely that B compositions of the rhyolites are inherited from their sources in the crust. Although low δ 11 B (

Low temperature vent fluids (

Two large pegmatitic crystals of sodic pyroxene (aegirine) and sodic amphibole (arfvedsonite) from the agpaitic igneous Ilimaussaq Complex, south Greenland were found to be suitable as reference materials for in situ Li isotope determinations. Lithium concentrations determined by SIMS and micro-drilled material analysed by MC-ICP-MS generally agreed within analytical uncertainty. The arfvedsonite crystal was homogeneous with [Li] = 639 ± 51 μg g−1 (2s, n = 69, MC-ICP-MS and SIMS results). The aegirine crystal shows strongly developed sector zoning, which is a common feature of aegirines. Using qualitative element mapping techniques (EPMA), the homogeneous core of the crystal was easily distinguished from the outermost sectors of the crystals. The core had a mean [Li] of 47.6 ± 3.6 μg g−1 (2s, n = 33) as determined by SIMS. The seven micro-drilled regions measured by solution MC-ICP-MS returned slightly lower concentrations (41–46 μg g−1), but still overlap with the SIMS data within uncertainty. Based on MC-ICP-MS and SIMS analyses, the variation in δ7Li was about 1‰ in each of the two crystals, which is smaller than that in widely used glass reference materials, making these two samples suitable to serve as reference materials. There was, however, a significant offset between the results of MC-ICP-MS and SIMS. The latter deviated from the MC-ICP-MS results by −6.0 ± 1.9‰ (2s) for the amphibole and by −3.9 ± 1.9‰ (2s) for the aegirine. This indicates the presence of a significant matrix effect in SIMS determinations of Li isotopes for amphibole and pyroxene relative to the basalt glasses used for calibration. Based on the MC-ICP-MS results, mean δ7Li values of +0.7 ± 1.2‰ (2s, n = 10) for the arfvedsonite crystal and of −3.7 ± 1.2‰ (2s, n = 7) for the core of the aegirine crystal were calculated. Adopting these values, SIMS users can correct for the specific IMF (instrumental mass fractionation) of the ion probe used. We propose that these two crystals serve as reference materials for in situ Li isotope determinations by SIMS and pieces of these two crystals are available from the first author upon request. Deux grands cristaux pegmatitiques, un pyroxene sodique (aegerine) et une amphibole sodique (arfvedsonite) provenant du complexe agpaitique Ilimaussaq (sud du Groenland) ont ete selectionnes pour etre des materiaux de reference lors d'analyses in situ des isotopes de Li. Les concentrations en lithium ont ete determinees par SIMS et par MC-ICP-MS sur du materiel preleve par micro-forage. Les deux determinations sont en accord aux erreurs analytiques pres. Le cristal de arfvedsonite est homogene avec [Li] = 639 ± 51 μg g−1 (2s, n = 69, donnees MC-ICP-MS et SIMS). Le cristal d'aegerine presente des zonations tres franches, comme il est classique dans les aegerines. En utilisant les techniques de cartographie chimique qualitative des elements (EMPA) il a ete possible de clairement distinguer le cœur du cristal, homogene, de ses bordures zonees. Le cœur a un [Li] moyen de 47.6 ± 3.6 μg g−1 (2s, n = 33) d'apres l'analyse par SIMS. Les sept regions micro forees et analysees par MC-ICP-MS en solution presentent des concentrations legerement inferieures (41–46 μg g−1) mais qui neanmoins sont dans la meme gamme, aux erreurs analytiques pres, que celles determinees par SIMS. Les donnees obtenues par MC-ICP-MS et SIMS montrent que la variation de δ7Li dans chaque cristal est de 1‰, ce qui est bien inferieur a celle mesuree dans les verres de reference generalement utilises. Ces deux echantillons sont donc tres adaptes a une utilisation comme materiaux de reference. Il y a neanmoins un decalage significatif entre les donnees de MC-ICP-MS et les donnees obtenues par SIMS, ces dernieres different des premieres de −6.0 ± 1.9‰ (2s) pour l'amphibole et de −3.9 ± 1.9‰ (2s) pour l'aegerine. Ceci demontre l'existence d'un effet de matrice non negligeable lors de l'analyse par SIMS des isotopes de Li dans des amphiboles et des pyroxenes, par rapport au verre basaltique utilise pour la calibration. Les valeurs moyennes de δ7Li de +0.7 ± 1.2‰ (2s, n = 10) pour le cristal d'arfvedsonite et de −3.7 ± 1.2‰ (2s, n = 7) pour le cœur du cristal d'aegerine ont ete calculees a partir des donnees obtenues par MC-ICP-MS. Les utilisateurs de SIMS peuvent utiliser ces valeurs pour corriger leurs resultats du fractionnement de masse instrumental (IMF) specifique a la sonde ionique utilisee. Nous proposons d'utiliser ces deux cristaux comme materiaux de reference lors de determinations in situ des isotopes de Li par SIMS et des fragments de ces deux cristaux peuvent etre demandes au premier auteur.

Lava (n = 8) and bulk ash samples (n = 6) erupted between July 1999 and June 2005 were investigated to extend time-series compositional and textural studies of the products erupted from Volcan Colima since 1869. In particular, we seek to evaluate the possibility that the current activity will culminate in major explosive Plinian-style event similar to that in 1913. Lava samples continue to show relatively heterogeneous whole-rock compositions with some significant mafic spikes (1999, 2001) as have prevailed since 1976. Groundmass SiO2 contents continue trends to lower levels that have prevailed since 1961, in the direction of the still lower groundmass SiO2 contents found in 1913 scoriae. Importantly, ash samples from investigated Vulcanian-style explosive eruptions in 2005 are devoid of particles with micro-vesiculated groundmass textures; such textures characterized the 1913 scoriae, signifying expansion of in-situ magmatic gas as the propellant of the 1913 eruption. All magmas erupted since 1913 appear to have arrived in the upper volcanic conduit system in a degassed state. The small to moderate Vulcanian-style explosive eruptions, which have been common since 1999 (> 16,000 events), have blasted ash clouds as high as 11 km a.s.l. and sent pyroclastic flows out to distances of 5 km. These eruptions do not appear to be powered by expansion of in-situ magmatic gas. New small lava domes have been observed in the crater prior to many explosive eruptions. These plugs of degassed lava may temporarily seal the conduit and allow the build-up of magmatic gases streaming upward from below ahead of rising and degassing magma. In this interpretation, when gas pressure exceeds the strength of the plug seal in the upper conduit, an explosive Vulcanian-style eruption occurs. Alternatively these explosive eruptions may represent interactions of hot rock and groundwater (phreato-magmatic).