Insights into Magma Storage Beneath a Frequently Erupting Arc Volcano (Villarrica, Chile) from Unsupervised Machine Learning Analysis of Mineral Compositions

A key method to investigate magma dynamics is the analysis of the crystal cargoes carried by erupted magmas. These cargoes may comprise crystals that crystallize in different parts of the magmatic system (throughout the crust) and/or different times. While an individual eruption likely provides a partial view of the sub-volcanic plumbing system, compiling data from multiple eruptions builds a picture of the whole magmatic system. In this study we use machine learning techniques to analyze a large (>2000) compilation of mineral compositions from a highly active arc volcano: Villarrica, Chile. Villarrica's post-glacial eruptive activity (14 ka–present) displays large variation in eruptive style (mafic ignimbrites to Hawaiian effusive eruptions) yet its eruptive products have a near constant basalt-basaltic andesite bulk-rock composition. What, therefore, is driving explosive eruptions at Villarrica and can differences in storage dynamics be related to eruptive style? We used hierarchical cluster analysis to detect previously undetected structure in olivine, plagioclase and clinopyroxene compositions, revealing the presence of compositionally distinct clusters. Using rhyolite-MELTS thermodynamic modeling we related these clusters to intensive magmatic variables: temperature, pressure, water content and oxygen fugacity. Our results provide evidence for the existence of multiple discrete (spatial and temporal) magma reservoirs beneath Villarrica where melts differentiate and mix with incoming more primitive magma. The compositional diversity of an erupted crystal cargo strongly correlates with eruptive intensity, and we postulate that mixing between primitive and differentiated magma drives explosive activity at Villarrica.

Sub-arc mantle enrichment in the Sunda rear-arc inferred from HFSE systematics in high-K lavas from Java

Many terrestrial silicate reservoirs display a characteristic depletion in Nb, which has been explained in some studies by the presence of reservoirs on Earth with superchondritic Nb/Ta. As one classical example, K-rich lavas from the Sunda rear-arc, Indonesia, have been invoked to tap such a high-Nb/Ta reservoir. To elucidate the petrogenetic processes active beneath the Java rear-arc and the causes for the superchondritic Nb/Ta in some of these lavas, we studied samples from the somewhat enigmatic Javanese rear-arc volcano Muria, which allow conclusions regarding the across-arc variations in volcanic output, source mineralogy and subduction components. We additionally report some data for an along-arc sequence of lavas from the Indonesian part of the Sunda arc, extending from Krakatoa in the west to the islands of Bali and Lombok in the east. We present major and trace element concentrations, Sr–Nd–Hf–Pb isotope compositions, and high-field-strength element (HFSE: Nb, Ta, Zr, Hf, W) concentrations obtained via isotope dilution and MC-ICP-MS analyses. The geochemical data are complemented by melting models covering different source compositions with slab melts formed at variable P–T conditions. The radiogenic isotope compositions of the frontal arc lavas in combination with their trace element systematics confirm previously established regional variations of subduction components along the arc. Melting models show a clear contribution of a sediment-derived component to the HFSE budget of the frontal arc lavas, particularly affecting Zr–Hf and W. In contrast, the K-rich rear-arc lavas tap more hybrid and enriched mantle sources. The HFSE budget of the rear-arc lavas is in particular characterized by superchondritic Nb/Ta (up to 25) that are attributed to deep melting involving overprint by slab melts formed from an enriched garnet–rutile-bearing eclogitic residue. Sub-arc slab melting was potentially triggered along a slab tear beneath the Sunda arc, which is the result of the forced subduction of an oceanic basement relief ~ 8 Myr ago as confirmed by geophysical studies. The purported age of the slab tear coincides with a paucity in arc volcanism, widespread thrusting of the Javanese basement crust as well as the short-lived nature of the K-rich rear-arc volcanism at that time.

Erguveem Ore Region in the eastern Chukotka Peninsula: An effect of the tectonics of the ore-bearing volcano-structures on composition of the gold-silver mineralization

The geological and structural position of the Pepenveem and Korrida Au-Ag ore occurrences situated in the East-Chukotka segment of the Okhotsk-Chukotka Volcanogenic Belt (OCVB) was studied. The Pepenveem ore occurrence was characterized by one (volcanogenic) mineralization stage. It is localized within a graben-like monocline composed of Late Cretaceous volcanics. A relatively stable tectonic regime caused rather low temperature and pressure gradients during the ore formation and, consequently, simple mineral composition of the ores and absence of advanced argillic alteration. In contrast, the Korrida ore occurrence was characterized by two (volcanogenic and plutonogenic) mineralization stages. It is localized within a plutonogenic uplift complicated by a regional fault zone. Here, the basement of the volcano-structure, composed of island-arc volcano-sedimentary rocks, was uplifted to the surface by numerous high-angle faults. The here observed extensive development of zoned metasomatic haloes (including advanced argillic alterations), abundance of mineral species, and sharp temperature and pressure gradients could resulted from tectonic activity in a zone of interaction between the plutonic dome and deep-seated regional fault.

Machine learning thermobarometry and chemometry using amphibole and clinopyroxene: a window into the volcanic roots of an arc volcano (Mount Liamuiga, Saint Kitts)

The physical and chemical properties of magma govern the eruptive style and behaviour of volcanoes. Many of these parameters are linked to the storage pressure and temperature of the erupted magma, and melt chemistry. However, reliable single-phase thermobarometers and chemometers which can recover this information, particularly using amphibole chemistry, remain elusive. We present a suite of single-phase amphibole and clinopyroxene thermobarometers and chemometers, calibrated using machine learning. This approach allows us to intimately track the range of pre-eruptive conditions over the course of a millennial eruptive cycle on an island arc volcano (Saint Kitts, Eastern Caribbean). We unpick the story of Mount Liamuiga, a stratovolcano that pops its upper-crustal (2 kbar), dacitic cork at the beginning of the Lower Mansion Series eruptive sequence. This permits a progressive increase in the thermal maturity of the magma arriving at the surface from the middle to upper crust (2 – 5.5 kbar) through time. The temperature increase correlates well with matrix plagioclase chemistry, which itself displays a remarkable progression to less evolved (more anorthitic) composition in time. We find that amphibole is a reliable themobarometer (SEE = 1.4 kbar; 40 ˚C), at odds with previous studies. We suggest it is the regression strategy, as opposed to the abject insensitivity to pressure, that has hindered previous calibrations of amphibole only thermobarometers. By recognising this, we have constructed a high-resolution, quantitative picture of the magma plumbing system beneath an arc volcano.

Thermal squeezing of the seismogenic zone controlled rupture of the volcano-rooted Flores Thrust

Temperature plays a critical role in defining the seismogenic zone, the area of the crust where earthquakes most commonly occur; however, thermal controls on fault ruptures are rarely observed directly. We used a rapidly deployed seismic array to monitor an unusual earthquake cascade in 2018 at Lombok, Indonesia, during which two magnitude 6.9 earthquakes with surprisingly different rupture characteristics nucleated beneath an active arc volcano. The thermal imprint of the volcano on the fault elevated the base of the seismogenic zone beneath the volcanic edifice by 8 km, while also reducing its width. This thermal “squeezing” directly controlled the location, directivity, dynamics, and magnitude of the earthquake cascade. Earthquake segmentation due to thermal structure can occur where strong temperature gradients exist on a fault.

Complex subsurface hydrothermal fluid mixing at a submarine arc volcano supports distinct and highly diverse microbial communities

Hydrothermally active submarine volcanoes are mineral-rich biological oases contributing significantly to chemical fluxes in the deep sea, yet little is known about the microbial communities inhabiting these systems. Here we investigate the diversity of microbial life in hydrothermal deposits and their metagenomics-inferred physiology in light of the geological history and resulting hydrothermal fluid paths in the subsurface of Brothers submarine volcano north of New Zealand on the southern Kermadec arc. From metagenome-assembled genomes we identified over 90 putative bacterial and archaeal genomic families and nearly 300 previously unknown genera, many potentially endemic to this submarine volcanic environment. While magmatically influenced hydrothermal systems on the volcanic resurgent cones of Brothers volcano harbor communities of thermoacidophiles and diverse members of the superphylum “DPANN,” two distinct communities are associated with the caldera wall, likely shaped by two different types of hydrothermal circulation. The communities whose phylogenetic diversity primarily aligns with that of the cone sites and magmatically influenced hydrothermal systems elsewhere are characterized predominately by anaerobic metabolisms. These populations are probably maintained by fluids with greater magmatic inputs that have interacted with different (deeper) previously altered mineral assemblages. However, proximal (a few meters distant) communities with gene-inferred aerobic, microaerophilic, and anaerobic metabolisms are likely supported by shallower seawater-dominated circulation. Furthermore, mixing of fluids from these two distinct hydrothermal circulation systems may have an underlying imprint on the high microbial phylogenomic diversity. Collectively our results highlight the importance of considering geologic evolution and history of subsurface processes in studying microbial colonization and community dynamics in volcanic environments.

Bosphorus Volcano; Signs of an Ancient Volcano on an Ancient City

<p>In many ancient and active volcanic provinces dyke systems represent radial and concentric patterns. In İstanbul, NW Turkey, late Cretaceous dykes, which are emplaced in pre-Cretaceous basement rocks consisting of sedimentary rocks of Palaeozoic and Triassic ages, have both patterns. In the region, late Cretaceous volcanism is represented by three elements, (1) The Çavuşbaşı granitoid, (2) volcano-sedimentary units and (3) dykes.</p><p>Age of the Çavuşbaşı granitoid is given as 67.91±0.63 to 67.59±0.5 Ma. It is emplaced in shallow depth and has an indistinct contact aureole. Volcano sedimentary units were deposited in an intra-arc basin. Three types of dykes are reported in the region: lamprophyre, diabase and intermediate to felsic dykes (72.49±0.79 to 65.44±0.93 Ma). Different petrology and the crystallization depths of the crystals in the dykes and the Çavuşbaşı granitoid suggest two different magma chambers emplaced at two different depths, the Çavuşbaşı granitoid representing the shallower one.</p><p>Upper Cretaceous dykes are concentrated around the Çavuşbaşı granitoid and extend almost as far as 30 km away from the pluton. The intrusion of the plutonic body of the Çavuşbaşı granitoid caused a dome structure in the basement rocks. The formation of this dome structure may have controlled the stress field and the orientation of the dyke system. Similar patterns are observed in the British Tertiary igneous province, Galapagos volcanoes, Boa Vista (Cape Verde), Summer Coon volcano, Spanish Peak Mountain and Dike Mountain (Colorado), Vesuvio, Etna and Stromboli (Italy).</p><p>We suggest that Upper Cretaceous volcanic edifice in the İstanbul region is related to an arc volcano similar to the andesitic volcanoes in the Sumatra Island; we name it the Bosphorus Volcano.  </p>