Volcanic Centers Research Articles

The Waiteariki Ignimbrite: Eruption of A Large-Volume, Monotonous Intermediate Ignimbrite At The Dawn Of The Taupō Volcanic Zone, New Zealand

Abstract Monotonous intermediate eruptions typically eject large volumes of pyroclastic material generally thought to represent material from the underlying magma mush systems. Their geological occurrence represents an upper end-member in terms of eruption size and styles, providing key information to understand the construction of large magma reservoirs. Here we constrain dispersal and geochemical aspects of the ~2.1 Ma Waiteariki supereruption that erupted ~870 ± 270 km3 dense rock equivalent (DRE) crystal-rich (~37 %), rhyodacite (~71 wt. % SiO2) magma from the newly defined Omanawa Caldera of the Tauranga Volcanic Centre (TgaVC), Aotearoa New Zealand. The Omanawa caldera is identified here using ignimbrite thickness variations, textural features and the presence of numerous silicic lava domes. Our geologically constrained vent location correlates with a prominent gravity anomaly situated at the southern end of an asymmetrical rifted graben underlying the northern Mamaku Plateau. The monotonous Waiteariki Ignimbrite is characterised by: 1) a mineral assemblage comprising plagioclase, hornblende, orthopyroxene and quartz, 2) restricted ranges in whole-rock geochemistry, and 3) absence of both compositional gradients through stratigraphy and a precursory Plinian fallout phase. In contrast, changes in fiamme mineralogy, large ranges in mineral chemistry and subtle variations in glass compositions provide evidence for open system magma processes and a highly heterogeneous and vertically extensive mush-source zone. As volcanism associated with the Omanawa Caldera and the broader TgaVC occurred within the defined structural boundaries of the Taupō Rift, volcanism of the TgaVC is regarded here to represent the first silicic system of the Taupō Volcanic Zone, one of the most productive silicic systems on Earth. This places the Waiteariki Ignimbrite within an important transitional period between the older Coromandel Volcanic Zone and the currently active Taupō Volcanic Zone.

Recurrence rates of explosive volcanism in paleo-equatorial Pangaea, and implications for climate near the peak late Paleozoic ice age

The continental Brousse-Broquiès basin (Massif Central, France) preserves a well-exposed section of upper Carboniferous pyroclastic-volcaniclastic lacustrine strata. We present high-precision geochronology and detailed sedimentology that together define a Kasimovian (305.77–306.10 Ma) volcanic recurrence interval of < 10 ky. This is the first quantification of volcanic recurrence from deep (pre-Mesozoic) time, and importantly, represents an unique opportunity to gain insight into the nature of frequent and highly explosive silicic volcanism occurring across eastern equatorial Pangaea during the peak of the late Paleozoic ice age (LPIA). These data emanate from a single, unusually well-exposed and well-constrained site. However, this site is representative of many documented but poorly preserved volcanic centers of coeval or similar ages across west-central Europe. The concentration in both time and space of such highly explosive volcanism in paleoequatorial Pangaea has large implications for the climate system during Earth’s penultimate icehouse. The recurrence rates established here support ongoing climate modeling efforts to test the hypothesis that this volcanism imposed a significant cooling effect near and during peak LPIA conditions prior to Permian icehouse collapse. In the wake of enhanced precision in geochronological tools, the integrated approach we apply for quantifying volcanic recurrence rates has broad utility in Earth’s deep-time record.

A late Pleistocene-Holocene record of explosive eruptions from central Sumatra (Indonesia) in the western Sunda volcanic arc

The island of Sumatra within the Indonesian archipelago is home to over 130 active or potentially active volcanoes with a history of explosive eruptions. Highly explosive eruptions with volcanic explosivity index (VEI) ≥ 6 in Sumatra, such as those originating from the massive Toba caldera, have been well-documented in the literature. However, moderately explosive eruptions with VEI 3–5 have received inadequate attention due to their limited preservation within the proximal stratigraphic record. This gap in knowledge hinders existing attempts to conduct hazard assessments for these potentially impactful eruptions. In this study, we address this knowledge gap by presenting a combination of geochemical, geochronological and tephrochronological datasets associated with distal tephra layers sampled from deep-sea cores collected off the coast of West Sumatra, as well as proximal pyroclastic deposits throughout central Sumatra. Our datasets reveal geochemical and stratigraphic correlations between seven distal tephra layers and their proximal sources, allowing for the quantification of their eruption ages and volumes. Notably, we identified the ~ 1.53 ka Lubuk King Tephra (LKT) eruption from Malintang volcano that discharged ≥ 1.4 km3 dense-rock equivalent (DRE) of magma, representing the youngest known VEI 5 eruption in Sumatra. In addition, we determined Tandikat volcano as the proximal source for a pair of temporally proximate (~ 580 yr apart) VEI 5 eruptions (Tandikat II and I Tephra, TDK II and I; ~ 4.36 and ~ 4.94 ka) that produced ≥ 1.1 and ≥ 2.7 km3 DRE of magma, respectively. We also ascertained that at least two VEI 4 eruptions occurring within the last ~ 36 kyr can be correlated to the active Marapi volcano. Furthermore, we traced distal tephra layers AB4 (~ 36.8 ka) and AB5 (~ 41.0 ka) to two distinct VEI ≥ 5 eruptions at volcanic centres in neighbouring provinces (Ranau Tuff, RAN from South Sumatra; Djudjun Tephra, DJT from Jambi). Volcanic source provenances for another six distal tephra layers remain unknown due to the lack of known proximal correlatives. Overall, our study provides an improved tephrochronological framework for late Pleistocene-Holocene explosive volcanism in central Sumatra that will help refine existing volcanic hazard assessments and enhance the integration of terrestrial and marine palaeoenvironmental archives regionally.

Persistent High-Pressure Magma Storage beneath a Near-Ridge Ocean Island Volcano (Isla Floreana, Galápagos)

Abstract Volcanic evolution in ocean island settings is often controlled by variations in the chemistry and volumetric flux of magma from an underlying mantle plume. In locations such as Hawaiʻi or Réunion, this results in predictable variations in magma chemistry, the rate of volcanic activity, and the depth of magma storage with volcanic age and/or distance from the centre of plume upwelling. These systems, however, represent outliers in global plume volcanism due to their high buoyancy flux, frequent eruptions, and large distance from any plate boundary. Most mantle plumes display clear interaction with nearby plate boundaries, influencing the dynamics of solid plume material in the upper mantle and the distribution of melt across regions of active volcanism. Yet, the influence of plume–ridge interaction and plume–ridge distance on the structure, characteristics, and evolution of magma storage beneath ocean island volcanoes remains under constrained. In this study, we consider the evolution of magmatic systems in the Galápagos Archipelago, a region of mantle plume volcanism located 150–250 km south of the Galápagos Spreading Centre (GSC), focusing on the depth of magma storage during the eastward transport of volcanic systems away from the centre of plume upwelling. Geochemical analysis of gabbro xenoliths from Isla Floreana in the southeastern Galápagos suggest that they formed at ~2–2.5 Ma, when the island was located close to the centre of plume upwelling. These nodules, therefore, provide rare insights into the evolution of volcanic systems in the Galápagos Archipelago, tracking variations in the magma system architecture as the Nazca plate carried Isla Floreana eastwards, away from the plume centre. Mineral thermobarometry, thermodynamic modelling, and CO2 fluid inclusion barometry reveal that Isla Floreana’s plume-proximal stage of volcanic activity—recorded in the gabbro xenoliths—was characterized by the presence of high-pressure magma storage (&amp;gt;25 km), below the base of the crust. In fact, we find no petrological evidence that sustained, crustal-level magma storage ever occurred beneath Isla Floreana. Our results contrast with the characteristics of volcanic systems in the western Galápagos above the current centre of plume upwelling, where mid-crust magma storage has been identified. We propose that this change in magmatic architecture of plume-proximal volcanic centres in the Galápagos—from high-pressure mantle storage at 2.5 Ma to mid-crustal storage at the present day—is controlled by the variations in plume–ridge distance. Owing to the northward migration of the GSC, the distance separating the plume stem and GSC is not constant, and was likely &amp;lt;100 km at 2.5 Ma, significantly less than the current plume–ridge distance of 150–250 km. We propose that smaller plume–ridge distances result in greater diversion of plume-material to the GSC, ‘starving’ the eastern Galápagos islands of magma during their initial formation and restricting the ability for these systems to develop long-lived crustal magma reservoirs.

Evidence for Partial Melting in Mantle Xenoliths of Spinel Lherzolites of Zhokhov Island, De Long Archipelago, Eastern Arctic

—Zhokhov Island belongs to the De Long Archipelago located in the Eastern sector of the Russian continental shelf within the Arctic Basin. The island is a young volcanic center and is composed of lava flows of alkaline olivine-porphyry basalts and subordinate limburgites. The study was aimed at identifying the possible regional and geodynamic factors influencing the specifics of the partial melting and mineral transformations in mantle xenoliths of Zhokhov Island. Five xenoliths selected from alkali basalt samples on Zhokhov Island were studied using a scanning electron microscope. The data obtained allowed us to conclude that the formation of high-sodium glasses in the mantle xenoliths of Zhokhov Island is associated with the interaction between spinel lherzolites and parental melts of host olivine basalts. At the same time, high-potassium glasses inside mantle xenoliths were formed in situ during the melting of a primary potassium-bearing phase, likely phlogopite. The formation of two distinct compositionally contrasting recrystallization zones in contact between the mantle xenoliths and the host basalt is caused by the evolution of host alkaline silicate melt from sodic to potassic composition. Signs of activation of young intraplate magmatism that brought up the fragments of metasomatized shallow mantle to the surface are established over a large area of the Arctic Basin within the HALIP large igneous province.

Nature, origin and significance of a clayey cave deposit: Relationship between karst and Oligocene/Miocene tephra (NW Dinarides)

AbstractA limestone cave filled with montmorillonite clay was found in a hill above the karst plateau on the north‐eastern edge of the Adriatic Sea. The cave fill shows no correlation with sediments found in the surrounding caves or other known caves of the Classical Karst, where montmorillonite content is negligible. In other caves, the primary deposited sediments originate from weathered Eocene flysch and were transported into caves by sinking streams. Powder X‐ray diffraction analyses indicate a pure montmorillonite composition with a few coarser quartz grains and heavy minerals. Based on these results and comparisons with tephras from volcanic centres in the wider region, we conclude that the geochemical characteristics of the montmorillonite clay (rare earth element distribution, enriched patterns of light rare earth elements, LaN/YbN ratios and significantly negative Eu/Eu* ratio) are similar to weathered volcanoclastic material of the Smrekovec Volcanic Complex (Oligocene to Miocene eruptions in north‐eastern Slovenia) and transported to the depositional centre by north‐eastern winds. Dating the clay using apatite fission‐track thermochronology yielded 22 ± 7 Ma, while K‐Ar dating gave 23.4 ± 1.7 Ma. Both ages are consistent with the activity of the Smrekovec Volcanic Centre, previously dated to 28–23 Ma. The presence of weathered volcanic ash in the studied cave indicates considerable explosive activity of the stratovolcano, previously interpreted only as a submarine edifice, and confirms the existence of a contemporary karst landscape at the time of volcanic activity where volcanic ash was deposited and subsequently in situ weathered in a levelled corrosion‐tectonic plane. Consequently, it can be concluded that a karst landscape in the north‐western Dinarides has existed since the Late Oligocene, with the studied cave being the only known remnant identified so far. This study presents a rare sedimentary record and highlights the role of karst landscapes as valuable archives of past geological events and environmental changes.

Magmatic activity at the slowest spreading rates: insights from a high-resolution earthquake catalog obtained from Gakkel Ridge Deep (Arctic Ocean)

At the eastern end of Gakkel Ridge, Arctic Ocean, spreading rates drop below 5 mm/y near the termination of the active mid-ocean ridge in the Laptev Sea. A small-scale ocean bottom seismometer network deployed for one year at a volcanic center near Gakkel Ridge Deep in sea ice covered waters revealed abundant microseismicity despite the low spreading rate. In order to reveal spreading processes, we analyze a manually picked earthquake catalog refined by low-magnitude events detected by template matching. We attribute seismicity occurring randomly in time and space to tectonic stress release along the ridge. During short time periods of hours to days, seismicity is organized in time and densely clustered in space with signs of migration away from an aseismic area. In analogy to volcanic centers at Knipovich Ridge and in Iceland, we interpret the seismicity as signs of ongoing localized magmatism occurring even at the slowest spreading rates.

Origin and Evolution of Rhyolitic Intraplate Magmatism: A Case Study from the Peak Ranges Volcanics, Central Queensland, Australia

Abstract The Peak Ranges Volcanics represent one of the most extensive and compositionally diverse Cenozoic volcanic centres of eastern Australia, and hence can provide crucial insights into the evolution of continental intraplate magmatic systems. Trachytic to rhyolitic volcanic suites are well preserved as a series of eroded plugs, domes and lava flow stacks, and can be divided into three zones based on spatial and temporal associations, mineralogy and geochemistry. The Southern Volcanic Zone features peralkaline silicic volcanic rocks with highly enriched incompatible element (e.g. REE, Zr, Nb, Ta) contents and isotopic compositions (ɛNdi ~ +3 to +4; 87Sr/86Sri ~ 0.704 to 0.7045) that overlap with the mafic volcanic rocks (ɛNdi ~ +3.5 to +6; 87Sr/86Sri ~ 0.703 to 0.7045) that dominate Peak Ranges. The Northern Volcanic Zone largely comprises peraluminous rhyolites, with relatively unradiogenic Nd isotope (ɛNdi ~ −1 to +1) and radiogenic 87Sr/86Sri (~0.7045 to 0.7065) compositions. The Central Volcanic Zone has chemical and isotopic affinities that are intermediate between the Northern and Southern Volcanic Zones. We interpret the rhyolites of the Northern Volcanic Zone (and most of the Central Volcanic Zone) to represent erupted products of highly fractionated mantle-derived magmas that had experienced approximately 10 to 20% crustal assimilation at lower crustal depths. This magmatic evolution was favoured by an overall N-S compressional regime at this time (ca. 30 to 32 Ma). Subsequent relaxation of compressional stress by ca. 28 Ma allowed effective tapping of alkaline, mantle-derived magmas to upper crustal chambers, where they underwent extensive crystal fractionation, largely or completely free of crustal contamination, to produce the enriched peralkaline southern rhyolites. Despite the dominant mantle origin for Peak Ranges magmatism, our study highlights the major control that the physical state and structure of the overlying continental lithosphere has on the compositional evolution of silicic volcanic rocks, and in turn their potential to host critical metal mineralization. Lithospheric geodynamics is, therefore, considered integral to understanding continental intraplate magma evolution.

LITHOLOGY UNITS WANAJAYA AREA AND SURROUNDING, BUAHDUA SUBDISTRICT, SUMEDANG DISTRICT, WEST JAVA PROVINCE

The research site is located in Wanajaya Village, Buahdua Subdistrict, Gunung Kidul Regency, West Java Province. Accessibility to the research location using a motorbike. The purpose of this study is to determine the geological conditions of the study area in terms of lithological units and stratigraphic order. The lithological unit of the study area is divided into 4 lithological units, namely the Mudstone Unit, the Andesite Intrusion Unit, the Volcanic Breccia Unit and the Andesite Lava Unit, namely the naming of rock units based on observable physical characteristics, including rock type, uniformity of lithological symptoms and stratigraphic position. The geological history of the first research site in the Late Miocene to Early Pliocene time was deposited by the mudstone unit. In the late Pliocene to Pleistocene volcanism activity occurred due to the displacement of the volcanic centre from the south to the centre of Java (Martodjojo, 1984) which caused the formation of the wanajaya fold and continued with the formation of the wanajaya ascending fault. As a result of the weak fault zone, the andesite intrusion unit appeared, which was the beginning of the activity of the old volcano. In Pleistocene time, namely deposition of Volcanic Breccia Unit and continued by the deposition of andesite lava unit due to volcanism activity of Tampomas volcano issued pyroclastic material.

Trans-crustal magmatic processes revealed by amphibole breakdown textures at the Quillacas monogenetic volcanic center, Bolivia

Trans-crustal magmatic processes revealed by amphibole breakdown textures at the Quillacas monogenetic volcanic center, Bolivia

Offshore evidence for volcanic landslide post Last Glacial Maximum at sub-Antarctic Heard Island, southern Indian Ocean

Heard Island, an active sub-Antarctic intraplate volcanic island on the Kerguelen Plateau, is mostly covered by glaciers. The amphitheatre shaped summit of the active volcanic centre, Big Ben (2813 m), has been interpreted to be the product of a significant volcanic landslide. Here we present the first offshore geomorphological and geological evidence supporting a volcanic landslide on Big Ben, including: (1) the seafloor to the southwest of Heard resembling a landslide deposit, covering at least 467 km2, (2) the spatial correlation between the onshore landslide scar and the offshore deposit and (3) the consistency in lithologies and compositions of rocks sampled from the deposit with the onshore in situ lithologies. 40Ar/39Ar geochronology constrains the maximum age of the volcanic landslide to 18.0 ± 1.4 ka, post the Last Glacial Maximum. Finally, we assess the risk of volcanic landslide at Heard Island in the future.

Variations in the slope of the earthquakes recurrence curve in the tonga subduction zone in 2005–2022

The Tonga‒Kermadec subduction zone is located between the Pacific and Australian plates and is the site of the highest rates of Pacific plate subduction and dominant extension. In 2006 and 2009 in this region, two strong earthquakes occurred with magnitudes Mw = 8.0 and 8.1. There are about 170 islands in the Tonga region. They are volcanic centers that have erupted regularly over the past few decades. The paper presents the results of determining temporal variations in the slope of the earthquakes recurrence curve (b-value) in the Tonga subduction zone for 2005–2022 and variations in b depending on depth. Temporal variations in the b-value reflect the general tendency for the most powerful earthquakes to occur against the background of a decrease in b-value only in the surface layer at depths of 0–100 km. By comparing the variation of b-value with depth with a tectonic model of the Tonga subduction zone, it suggested that lower b-value might reflect greater stress at the top of the subducted slab due to its bending. Elevated b-value can apparently be associated with stretching mechanisms. For the Tonga subduction zone, as for other subduction zones, the increased b-value identified at a depth of 90‒100 km, which may be due to the presence at this depth of a magmatic front, which is associated with active volcanism.

Microseismicity Around Loki's Castle Hydrothermal Vent Field Reveals the Early Stages of Detachment Faulting at the Mohns‐Knipovich Ridge Intersection

AbstractAt slow to ultraslow spreading ridges, the limited melt supply results in tectonic accretion and the exhumation of mantle rocks. Melt supply is focused toward volcanic centers where magmatic accretion dominates. In areas where the ridges reorientate, both types of accretion can occur across the ridge axis with detachment faults developing on the inside corners and hydrothermal vent fields located in close proximity. Microseismicity studies improve the understanding of the tectonic processes at detachment faults and their interplay with hydrothermal vent systems, but are mostly limited to mature detachment faults or short deployment times. This study presents results from a ∼11 months ocean bottom seismometer deployment around the Loki's Castle hydrothermal vent field at the intersection of the slow to ultraslow spreading Mohns and Knipovich Ridge. We observe seismicity to be highly asymmetric with the majority of the plate divergence being accommodated by an emerging detachment fault at the inside corner of the intersection west of Loki's Castle. Seismic activity related to the detachment fault displays a distinct contrast, with continuous low‐magnitude events occurring at depth and episodic large‐magnitude events concentrated in clusters within the footwall. The detachment fault shows no significant roll‐over at shallow depths and the locus of spreading is located east of the detachment. These results suggest that the detachment fault west of Loki’s Castle is at an early development stage.

Potentially Toxic Elements in the Surface Waters of the Elbrus Volcanic Center

Potentially Toxic Elements in the Surface Waters of the Elbrus Volcanic Center

Volcanic risk ranking and regional mapping of the Central Volcanic Zone of the Andes

Abstract. The Central Volcanic Zone of the Andes (CVZA) extends from southern Peru, through the Altiplano of Bolivia, to the Puna of northern Chile and Argentina, between latitudes 14–28° S of the Andean cordillera, with altitudes rising up to more than 4000 m above sea level. Given the large number of active volcanoes in this area, which are often located close to both urban areas and critical infrastructure, prioritization of volcanic risk reduction strategies is crucial. The identification of hazardous active volcanoes is challenging due to the limited accessibility, the scarce historical record, and the difficulty in identifying relative or absolute ages due to the extreme arid climate. Here, we identify the highest-risk volcanoes combining complementary strategies: (i) a regional mapping based on volcanic hazard parameters and surrounding density of elements at risk and (ii) the application of the recently developed volcanic risk ranking (VRR) methodology that integrates hazard, exposure, and vulnerability as factors that increase risk and resilience as a factor that reduces risk. We identified 59 active and potentially active volcanoes that not only include the volcanic centres with the most intense and frequent volcanic eruptions (e.g. the El Misti and Ubinas volcanoes, Peru) but also the highest density of exposed elements (e.g. the cities of Arequipa and Moquegua, Peru). VRR was carried out for 19 out of the 59 volcanoes, active within the last 1000 years or with unrest signs, highlighting those with the highest potential impact (i.e. Cerro Blanco in Argentina and Yucamane, Huaynaputina, Tutupaca, and Ticsani in Peru) and requiring risk mitigation actions to improve the capacity to face or overcome a disaster (e.g. volcanic hazard and risk/impact assessments, monitoring systems, educational activities, and implementation of early warning systems).

Heterogeneous provenance and intracontinental rift evidence in the early–middle Jurassic La Boca Formation in the Huizachal-Peregrina Anticlinorium, Mexico

ABSTRACT The Early–Middle Jurassic Nazas continental rift province in Mexico has been debated as either an extensional continental arc due to eastward subduction beneath North America or an intracontinental rift linked to Pangea break-up. New petrographic and U-Pb geochronological data from the La Boca Formation near Ciudad Victoria – a key locality within this province – reveal diverse clastic sources, forming three provenance groups. Group 1 (Juan Capitán Member) crops out in the Huizachal Valley and consists of volcaniclastic fluvial deposits derived from a local volcanic centre. Group 2 (Agua de Las Minas Member), also found in the Huizachal Valley, overlies Group 1 and has mixed provenance from local volcanic and surrounding basement uplift sources. Both groups transition laterally to deposits of Group 3 (San Pedro Member) to the north, which consists of metamorphiclastic fluvial deposits from the eastern basement uplift and is not influenced by the volcanic centre. The rapid dilution of volcanic detritus within ~35 km highlights the limited extent of the volcanic centre as a source of sediment. Our results suggest that the Nazas province was more likely formed in a continental rift basin with sparse volcanic activity linked to Pangea break-up rather than as an extensive volcanic arc.

Mantle conduits of the K-Pg Reunion mantle plume rise beneath the Indian subcontinent revealed by 3D magnetotelluric imaging

Mantle conduits of the K-Pg Reunion mantle plume rise beneath the Indian subcontinent revealed by 3D magnetotelluric imaging

Geochemical Signature of Basalts of the MAR Rift Valley at 20°31′ N: Origin Conditions of the Anomalous Volcanic Center of Puy des Folles in the Axial Zone of the Mid-Atlantic Ridge

The results of a study of chilled glasses sampled during the 45th cruise of the R/V Professor Logachev at the top of the submarine volcano Puy des Folles are presented. The Puy des Folles volcano is located in the axial part of the rift valley of the Mid-Atlantic Ridge (MAR) at 20°31′ N. Unlike typical volcanic axial highs which usually does not exceed several hundred meters the summit of the Puy des Folles volcano is located a depth of 1950 m and rises 1800 m above the bottom of the rift valley. The data on geochemistry and isotope composition of chilled glasses examined allow us to come to a number of conclusions that expanded existing ideas about magmatic and tectonic processes conducted in the rift valley of the slow spreading ridges. Chilled glasses sampled at the top of the Puy des Folles volcano are originated from a very depleted melt formed by partial melting of the DM reservoir. Puy des Folles volcano was formed as result of the activity of a long-lived magma chamber located below the rift valley axis. It is possible that, in addition to the DM reservoir, a mantle source enriched in incompatible elements may have participated in the formation of the parental melts for the studied chilled glasses. A weak geochemical signal of contamination of the parental melt with a hydrothermal component in chilled glasses was established. Signs of stagnation in the spreading of the oceanic crust in the rift valley segment studied in this work have been established.

Strain Localization at Volcanoes Undergoing Extension: Investigation of Long‐Term Deformation at Krafla and Askja Volcanic Systems in North Iceland

AbstractVolcanoes in extensional environments may show gradual subsidence over decades during quiescent periods, due to various processes such as magma withdrawal, cooling, contraction, plate spreading and viscoelastic response. If significant rheological anomalies reside in volcano roots, due to the presence of magma and hot rock, they can influence the style of deformation. We use Finite Element Method (FEM) models to explore how strain localization due to extension can lead to volcano deflation. We apply rheological models comprising an elastic layer overlying a viscoelastic domain and include local up‐doming regions of low viscosity material beneath volcanic centers. The models reveal a localized subsidence above the rheological anomaly, influenced by the tectonic extension, and by the up‐doming volume and its viscosity. The models suggest that plate divergence may account for 4–5 mm/yr of observed subsidence at Krafla and Askja volcanic systems (KVS and AVS, respectively) in North Iceland.

Boron isotopes identify deep-slab serpentinite in the source of Aleutian arc magma

Abstract Seafloor lavas of the Western Aleutian arc have isotopically heavy boron (δ11B to +13.4‰) that is negatively correlated with B content (ppm). Endmember samples are primitive dacites and rhyodacites (δ11B &amp;gt; +10‰, SiO2 = 63%–70%, Mg# &amp;gt; 0.60) with adakitic trace-element and isotopic characteristics that require roles for residual garnet and rutile in their formation. The source of isotopically heavy B is likely serpentinite in the mantle section of the subducting plate, which dewaters into an inverted geothermal gradient and drives melting within the overlying volcanic section at depths where prior effects of seawater alteration were minimal. Most volcanic rocks from the Aleutian Island locations have 10–30 ppm B with an average δ11B of ~+1.0‰ ± 1.3‰, reflecting a mixed source dominated by subducted sediment. A subset of island samples has B that is isotopically light (δ11B &amp;lt; –2.4‰) and at low concentrations (&amp;lt;11.0 ppm), which is typical of arc lavas globally from rear-arc settings where depth-to-slab is high, and where δ11B may be interpreted to reflect a source in dehydrated (isotopically light) altered oceanic crust. Mass balance modeling indicates that isotopically heavy B from deep-slab serpentinite is present in the Aleutian source arc-wide but is typically masked by sediment-derived B at volcanic centers outside of the westernmost segment of the arc.