Dacitic Magma Research Articles

Geochronology and Origin of Quaternary Dacites from the Daliuchong Volcano in the Tengchong Volcanic Field (TVF), SE Tibetan Plateau

Quaternary volcanoes from the southeastern Tibetan Plateau occur at the Tengchong volcanic field (TVF). The Daliuchong volcano is the largest volcano in the TVF, which has the most felsic compositions with explosive eruptions. The eruption history and origin of the Daliuchong volcano are a matter of debate. In the present paper, we report the groundmass K-Ar ages, whole-rock Sr-Nd-Pb-Hf isotopes, zircon U-Pb ages, and Hf-O isotopic compositions for the Daliuchong volcano to constrain its eruption history and petrogenesis. The groundmass K-Ar ages and zircon U-Pb ages indicate mid-Pleistocene (0.6 Ma to 0.3 Ma) eruptions. The presence of zircon phenocrysts with enriched mantle-like O-Hf isotopes (δ18O < 6‰, and εHf about −2) suggests the involvement of mantle-derived basaltic magmas. The whole-rock Pb isotope compositions and Sr-Nd isotope modeling reveal the involvement of magma from the lower crust. The zircon xenocrysts reveal previously unrecognized 20-Ma magmatic activity at the TVF and contamination of late Cretaceous (66–80 Ma) S-type granites during the formation of the Daliuchong dacites. The dacite magma at Daliuchong was formed by mixing of the mantle-derived magma and lower-crust-derived magma and subsequently contaminated by upper crustal materials, including late Cretaceous S-type granitic rocks.

Crystals and melt inclusions record deep storage of superhydrous magma prior to the largest known eruption of Cerro Machín volcano, Colombia

Abstract Cerro Machín, a volcano located in the northern segment of the Andes, is considered one of the most dangerous volcanoes in Colombia with an explosive record that involves at least five plinian events. Prior studies focused on the last dome-building eruption have suggested the presence of a water-rich mid-crustal magma reservoir. However, no direct volatile measurements have been published and little work has been completed on the explosive products of the volcano. Here, we study the largest known eruption of Cerro Machín volcano which occurred 3600 yr BP producing dacitic pyroclastic fall deposits that can be traced up to 40 km from the vent. Lapilli pumice clasts have a mineral assemblage of plagioclase, amphibole, quartz, and biotite phenocrysts, with accessory olivine, Fe-Ti oxides, and apatite. The occurrence of Fo89-92 olivine rimmed by high Mg# amphibole and the established high-water contents in the magma imply the presence of magma near or at water saturation at pressures >~500 MPa. Measurements of up to 10.7 wt% H2O in melt inclusions hosted in plagioclase and quartz in the 3600 years BP eruption products support the idea that Cerro Machín is a remarkably water-rich volcanic system. Moreover, this is supported by measurements of ~103 – 161 ppm H2O in plagioclase phenocrysts. The application of two parameterizations of water partitioning between plagioclase and silicate melt allows us to use our water in plagioclase measurements to estimate equilibrium melt water contents of 5 ± 1 – 11 ± 2 wt% H2O, which are in good agreement with the water contents we measured in melt inclusions. Results of amphibole geobarometry are consistent with a magma reservoir stored in the mid-to-lower crust at a modal pressure of 700 ± 250 MPa, corresponding to a depth of ~25 km. Minor crystallization in the shallow crust is also recorded by amphibole barometry and calculated entrapment pressures in melt inclusions. Amphibole is present as unzoned and zoned crystals. Two populations of unzoned amphibole crystals are present, the most abundant indicate crystallization conditions of 853 ± 26 °C (1 se; standard error), and the less abundant crystallized at an average temperature of 944 ± 24 °C (1 se). Approximately 18% of the amphibole crystals are normally or reversely zoned, providing evidence for a minor recharge event that could have been the trigger mechanism for the explosive eruption. Plagioclase crystals also show normal and reverse zoning. The moderate Ni concentrations (<1600 μg/g) in the high-Fo olivine xenocrysts suggest that Cerro Machín primary magmas are generated by inefficient interaction of mantle peridotite with a high-silica melt produced by slab melting of basaltic material. Some sediment input is also suggested by the high Pb/Th (>2.2), Th/La (0.3 – 0.4), and low La/Th (<13; relative to mantle array) ratios. Whole rock chemistry reveals heavy rare earth element (HREE) depletion and Sr enrichment that likely formed during the crystallization of garnet and amphibole in the upper part of the mantle or lower portion of the crust, promoting the formation of water-rich dacitic magma that was then injected into the middle-to-lower crust. Textural and compositional differences in the crystal cargo that erupted during dome-building and plinian events support the idea that large volumes of magma recharge lead to effusive eruptions, while only small recharge events are needed to trigger plinian eruptions at Cerro Machín.

The 2019 pumice raft forming eruption of Volcano-F (Volcano 0403–091) and implications for hazards posed by submerged calderas

Low volcanic explosivity index (VEI) eruptions are common occurrences in the Southwest Pacific but, as demonstrated by the 2021/2022 eruption of Hunga Volcano, submerged calderas in the region are also capable of producing much larger and more hazardous eruptions. As such, characterising smaller events from potentially hazardous systems is essential. The 2019 eruption of Volcano-F, a submerged caldera, would likely have gone totally undetected had it not produced a pumice raft that inundated beaches in Fiji and eventually washed up in Australia. New data, acquired 5 months after the eruption, reveal the development of a new vent and the accumulation of at least 3.1*107 m3 bulk volume (dense rock equivalent of 5.6*106 m3) of material on the seafloor. Between 30 and 70% of erupted material entered the raft, while the rest remained near to or was dispersed down-current of the vent. This previously unaccounted for material increases the volume estimate for the eruption, confirming it as a VEI 3 event and highlights the importance of considering not just the floating component of a pumice raft forming eruption for VEI estimation. Geochemical analysis reveals the eruption comprised a homogenous batch of dacitic magma, with compositional characteristics similar to that erupted from the same volcano in 2001, and an until-now-unidentified pumice raft in the Coral Sea in 1964. Volcano-F therefore appears to have had at least three explosive eruptions in the last 60 years, indicating it is significantly at unrest. Repeated eruptions of similar composition and low crystal content magma over decadal to centennial scales indicate the existence of a melt-dominant magma body beneath the volcano. Submerged calderas, like Volcano-F, are common in the wider Southwest Pacific region, with many such calderas producing regular eruptions, implicating active magmatic recharge. Our findings motivate a need to carefully monitor and characterise even apparently small eruptions at this volcano, and others along the Tonga-Kermadec Arc. This is because such eruptions have the potential to subsequently prime or trigger more explosive eruptions and provide critical geochemical evidence about the plumbing system and evolution of the volcano, essential for understanding the diverse hazards they pose.

Chaotic Mixing Experiments: Unravelling Contamination Processes on Dacitic Melts

Magmas are the ultimate source of volcanism on Earth. At high-temperatures these complex silicate melts may easily chaotically mix, on their way towards the surface of the planet.The Paraná-Etendeka Magmatic Province (PEMP), one of the largest volcanic provinces of our planet, serves as the focus of this experimental study. The investigation simulates the contamination of high-Ti dacites passing through the continental crust. Our final aim is to contribute to the origin of the high-Ti Chapecó dacites from PEMP, with particular attention to the genesis of Chapecó-Ourinhos dacites (COD).The experimental apparatus is based on the Journal Bearing System. All experimental runs have been operated at atmospheric pressure and a temperature of 1500 °C. A dacitic magma chamber under the assimilation of crustal material has been simulated. The resulting experimental products (dry/bubble free glasses) exhibit concentric folded and stretched filaments of contrasting compositions. The calculated fractal dimension for a representative section is Dbox = 1.60(3), confirming the achievement of chaotic dynamics. Electron Microprobe Analysis (EMPA) from the experimental glasses clearly shows SiO2-contamination, while Al2O3-data indicate lower contamination levels. As a consequence, different elements depict dissimilar curves in chemical transects. We used the parameter σn2 (normalized variance) to quantify the mixing efficiency and differential elemental mobilities. In addition, our data reproduced the chemical behaviour of some elements in COD- dacites, especially the network formers such as Fe, Al, Ti and Si. EMPA reveals clear deviations from the linear mixing model. Our experiments and data replicate a selective contamination process. Differential elemental mobilities could be calculated. Comparisons with natural data depict how the mixing involving Chapecó-Guarapuava (CGD) and Chapecó-Ourinhos (COD) dacitic end-members may have contributed to the generation of more evolved COD from the interaction of CGD pre-existing melts with crustal rocks from the PEMP basement via chaotic dynamics.

Geology, chronology, and temporal evolution of basaltic to dacitic magma system in Raung volcano, East Java, Indonesia

Raung volcano, located within the Ijen UNESCO Global Geopark in East Java, poses a significant risk of volcanic hazard for nearby residents and visitors. Our study provides a framework to understand Raung long-term behavior and potential hazards by examining its stratigraphy, petrology, and temporal magma evolution. The erupted products of Raung vary from lava flow, pyroclastic density current (ignimbrite and block and ash flow), scoria fall, and pumice fall. Radiocarbon dating of charcoal samples within pyroclastic deposits and weathered sediments beneath tephra fall layers yield the age of 840 ± 30 BP to 370 ± 30 BP. It provides an important chronological marker that confirms the past VEI-4 to VEI-5 eruption around 1200 to 1600 CE. Petrological and geochemical data reveal that Raung magma composition ranges from basalt to dacite (48–64 wt% SiO2) and can be classified into two distinct magma types. Type 1 magma has med-K series, low Rb/Nb, and no Eu anomaly. Type 2 magma has high-K series, high Rb/Nb, and negative Eu anomaly. Evidence of disequilibrium features (e.g., reverse zoning, sieve texture, resorption texture, orthopyroxene mantled by clinopyroxene) and mingling texture, along with geochemical features, indicate magma mixing and many episodes of mafic magma replenishment. While the current volcanic activity is dominated by andesitic Strombolian eruption, the characteristics of Raung eruptive products suggest that past major Plinian eruptions (VEI 4–5) had occurred in both andesitic and dacitic magmatic systems, with greater VEI associated with dacitic composition. The study of Raung temporal evolution documented various eruptive behaviors related to its wide range of magma composition, thus providing an essential database for hazard assessment and mitigation.

Pleistocene to recent evolution of Mocho-Choshuenco volcano during growth and retreat of the Patagonian Ice Sheet

Abstract Mocho-Choshuenco volcano (39.9°S, 72.0°W) produced ~75 explosive eruptions following retreat of the >1.5-km-thick Patagonian Ice Sheet associated with the local Last Glacial Maximum (LGM, from 35 to 18 ka). Here, we extend this record of volcanic evolution to include pre- and syn-LGM lavas that erupted during the Pleistocene. We establish a long-term chronology of magmatic and volcanic evolution and evaluate the relationship between volcanism and loading/unloading of the Patagonian Ice Sheet via twenty-four 40Ar/39Ar and two 3He age determinations integrated with stratigraphy and whole-rock compositions of lava flows and glass compositions of tephra. Our findings reveal that the edifice is much younger than previously thought and preserves 106 km3 of eruptive products, of which 50% were emplaced immediately following the end of the penultimate glaciation and 20% after the end of the LGM. A period of volcanic inactivity between 37 and 26 ka, when glaciers expanded, was followed by the eruption of incompatible element-rich basaltic andesites. Several of these syn-LGM lavas dated between 26 and 16 ka, which crop out at 1500–1700 m above sea level, show ice contact features that are consistent with emplacement against a 1400- to 1600-m-thick Patagonian Ice Sheet. Small volume dacitic eruptions and two explosive rhyolitic eruptions dominate the volcanic output from 18 to 8 ka, when the Patagonian Ice Sheet began to retreat rapidly. We hypothesize that increased lithostatic loading as the Patagonian Ice Sheet grew prohibited dike propagation, thus stalling the ascent of magma, promoting growth of at least three discrete magma reservoirs, and enhancing minor crustal assimilation to generate incompatible element-rich basaltic andesitic to dacitic magmas that erupted between 26 and 17 ka. From an adjacent reservoir, incompatible element-poor dacites erupted from 17 to 12 ka. These lava flows were followed by the caldera-forming eruption at 11.5 ka of 5.3 km3 of rhyolite from a deeper reservoir atop which a silicic melt lens had formed and expanded. Subsequent eruptions of oxidized dacitic magmas from the Choshuenco cone from 11.5 to 8 ka were followed by andesitic to dacitic eruptions at the more southerly Mocho cone, as well as small flank vent eruptions of basaltic andesite at 2.5 and 0.5 ka. This complex history reflects a multi-reservoir plumbing system beneath Mocho-Choshuenco, which is characterized by depths of magma storage, oxidation states, and trace element compositions that vary over short periods of time (<2 k.y.).

Petrological and geochemical insights into the magma plumbing system of the Daliuchong dacite eruption, Tengchong Volcanic Field, SW China

The formation of highly evolved, dacitic magmas has been attributed to various processes, including crystal fractionation, partial melting of overlying crust, and/or assimilation of crustal material into an evolving magma chamber. These processes are undoubtedly primary processes involved in the formation of dacites, but they may not be the only mechanism involved in the formation of high-silica dacites. For instance, mafic magma replenishment has been proposed as an additional mechanism but has not been assessed, and thus, its role has not been well-constrained. The Daliuchong volcano is the result of one of the largest eruptive events within the Tengchong Volcanic Field (TVF) in southwest China during the Early-Middle Pleistocene. Here, we conducted detailed mineral textures, mineral chemistry, and geochemical studies on Daliuchong pyroclastic rocks to explore the pre-eruptive storage conditions and evolution processes of the magma. The Daliuchong pyroclastic rocks are dacitic in composition. The samples show porphyritic textures characterized by phenocrysts of plagioclase, amphibole, clinopyroxene, orthopyroxene, and Fe-Ti oxides. Additionally, two distinct types of glomerocryst are identified: a gabbroic glomerocryst containing plagioclase + clinopyroxene + orthopyroxene ± Fe-Ti oxides assemblage and a dioritic glomerocryst containing plagioclase + amphibole ± pyroxene ± Fe-Ti oxides assemblage. Both phenocryst and glomerocryst show rich micro-textures. The Daliuchong dacite exhibits bulk compositional heterogeneity. Analysis of bulk-rock data suggests that this heterogeneity may arise from both the differentiation of the dacite itself and the injection of mafic magma. The compositional similarity between the Daliuchong dacite and experimentally produced partial melts of metamorphic basalt supports that the Daliuchong dacite was predominantly formed through the partial melting of the mafic lower crust. Thermobarometry estimation indicates that clinopyroxenes with high Mg# crystallized at 560–870 MPa, whereas amphibole and clinopyroxenes with low Mg# crystallized at 185–300 MPa. Based on the observed phase relations and the calculated crystallization conditions, we propose that during the differentiation of the Daliuchong dacite, heterogeneous dacitic magma formed by partial melting accumulated in a deep magma reservoir (21–32 km) before subsequently ascending toward shallower depths. Crystallization of plagioclase, amphibole, Fe-Ti oxides, and small amounts of pyroxene and apatite occurred at a shallower depth (7–10 km). The presence of coarse-sieve texture, fine-sieve texture, and oscillatory zoning with high amplitude in plagioclase suggests intermittent injection of mafic magma into the shallow magma reservoir, with the eruption of dacitic magma occurring after the final mafic magma replenishment. The petrological evidence above advocates that primitive magma replenishment could have been involved in the formation and triggered the eruption of dacite in the Daliuchong volcano.

Paleoproterozoic Dacite Dikes of the Vorontsovka Terrane, Volga–Don Orogen: Geochemistry, Age, and Petrogenesis

Metamorphosed dacitic porphyry dikes were first found in the western part of the Vorontsovka terrane, which is located in the Paleoproterozoic Volga–Don orogen at the margin of Archean Sarmatia and Volga–Ural cratons. The magmatic protolith age for the metadacites is ca. 2.07 Ga. These are ferrous, metaluminous calc-alkali I-type granitoids. The sodium specialization of the rocks and their low concentrations of Mg, Cr, Ni, and incompatible elements, with significant REE fractionation, the absence of Eu* anomalies, high Sr/Y ratio, remarkably high (Gd/Yb)n values (>10), and the radiogenic Nd isotopic composition indicate that the dacitic melts were derived from a juvenile mafic source. According to petrogenetic estimations, such conditions could be caused by the partial melting of depleted N-MORB basites in equilibrium with an eclogitic residue. The dacitic magmas were likely generated by the partial melting of mafic rocks at lower levels of the significantly thickened crust (>60 km) in relation to collision processes.

Heterogeneous dacitic magmas and the role of crustal assimilation in the Sierra Chichinautzin Volcanic Field, central Mexico: a perspective from crustal xenoliths

ABSTRACT The Sierra Chichinautzin Volcanic Field (SCVF) is constituted by diverse monogenetic volcanic structures with heterogeneous chemical composition. Dacitic magmas are widely distributed in this volcanic field and are the most felsic products. Tabaquillo lava is an example of such felsic rocks, it is porphyritic and highly crystalline, constituted by large phenocrysts of plagioclase and quartz (>1 mm long) + ortho and clinopyroxene + biotite and minor amphibole. Coarse-grained and equigranular clasts classified as ‘crystal-rich enclaves’ are also common in the lava flow, with variable compositions (basaltic andesite, andesite, and dacite) and ages of 0.35–0.36 Ma. The presence of metasandstone crustal xenoliths in the Tabaquillo lava flow, with a maximum sedimentation age of 150 Ma, attests to crustal assimilation of the Guerrero terrain rocks. Isotopic data of Tabaquillo lava and other volcanic rocks from the SCVF show notable variations (87Sr/86Sr = 0.70379–0.70474; 143Nd/144Nd = 0.51272–0.51288). Most of them lie in the mantle array but trending to the more radiogenic metasandstone xenolith and Guerrero terrain rocks (87Sr/86Sr = 0.71271 and 0.71627), suggesting that SCVF magmas have partially assimilated these rocks. Interaction between SCVF magmas with metasandstone, quartzite, and skarn rocks took place at depths > 1.3 km, imprinting geochemical and isotopic characteristics on the felsic magmas and produced the diverse compositional lithologies in the entire SCVF. Additionally, crystal-rich enclaves produced during previous magmatic pulses stalled at different depths, were later mixed with the ascending mafic magma, and subsequently incorporated into the crystal cargo that reached the surface. The common presence of quartz crystals (sometimes with reaction rims of clinopyroxene) found in many SCVF lavas could derive from the crystal-rich magmatic enclaves and/or from the metasandstone basement rocks.

Crystal mush rejuvenation in the formation of Triassic dacites in Mengku, SE Tibetan Plateau

The tectono-magmatic event of the Changning-Menglian Paleo-Tethys orogen is an important process for understanding the tectonic evolution of the SE Tibetan Plateau. This orogen is composed of Changning-Menglian suture, Lancang group, Lincang batholith, and Yunxian volcanics. The Lincang batholith and its volcanic counterparts are part of Triassic magmatism in this orogen. The botholith is composed of biotite monzogranite, granodiorite and diorite. The Middle to Late Triassic volcanics include Manghuai rhyolite, Xiaodingxi basalt and Mengku dacite. The Mengku dacite is volcanic dome in west of the batholith. Lincang biotite monzogranite and Mengku dacite have similar geochemistry and are characterized by negative εHf (t) values (–13.2 to –5.2) and εNd (t) values (–14.7 to –8.8), whereas diorite dikes within the Lincang batholith have slightly high εHf (t) values (–8.3 to –4.9) and εNd (t) values (–6.9 to –6.7). Manifestations of resorption textures in quartz and reverse compositional zoning in feldspars are recognized from Mengku dacite. We consider that Mengku dacites resemble the erupted Lincang biotite monzogranite as they occur in the same tectonic settings and have similar geochemical features and mineral assemblages dominated by plagioclase, alkali-feldspar, quartz, and biotite. The evolution of the Mengku dacite magma is characterized by the rejuvenation of crystal mush represented by the Lincang batholith. The required thermal input was supplied by mafic magma, represented by diorite dikes and enclaves within the Lincang batholith. The Middle to Late Triassic magmatic event in this orogen likely resulted from lithospheric delamination and upwelling of the asthenosphere in a post-collisional environment.

Evolution of a large Quaternary monogenetic field; the multifaceted volcanism of the Serdán-oriental basin, México

The intermontane Serdán-Oriental basin (SOB), located in the Trans-Mexican Volcanic Belt, records a highly contrasting volcanic field with nearly 166 monogenetic volcanoes, including cones, maars, tuff rings, domes, and lava flows, with basaltic to rhyolitic compositions and from effusive to explosive, and magmatic to phreatomagmatic eruptive styles. Even when this volcanic field has been the focus of several volcanological studies, an evolutive model of its volcanism and its implications has not been reported. This is a geological study of SOB volcanism based on the spatio-temporal evolution and characteristics of its volcanoes. For this, new and published data is integrated, including chronostratigraphic, geochemical, and petrological observations, which were further processed and analyzed by GIS tools. The resulting model indicates that volcanism has been continuous from the middle Pleistocene to the Holocene. Based on parameters such as erupted magma chemistry and volcanic landforms, it can be classified into the following volcanic groups: Group 1 is mafic in composition, forming cones and lava flows during the middle Pleistocene (490 to 190 ka). Group 2 comprises predominantly andesitic volcanism characterized by cones and dome clusters erupted during the middle Pleistocene (250 to 200 ka), partially coeval with Group 1. Group 3 is characterized by rhyolitic volcanism associated with the emplacement of domes in the central part of the SOB during the middle to upper Pleistocene (220 to 24.5 ka). Group 4 includes a predominantly basaltic to andesitic volcanism with minor rhyolitic composition, with a characteristic occurrence of phreatomagmatic and magmatic activity, forming maars and tuff rings, scoria cones, and lava flows during the middle Holocene (12.2 to 6.25 ka). The youngest Group occurs in the upper Holocene (<7.5 ka), and it is marked by mafic volcanism in the form of cones and lava flows, as well as by the emplacement of the voluminous Las Derrumbadas rhyolitic twin domes. Overall, the geochemical data shows a bimodal compositional character with a gap of dacitic magmas, compatible with a general scenario dominated by felsic melt extraction from highly crystalline magma reservoirs. Eruptive rate calculations indicate a general increase with time, which goes from <0.18 km3/ky in the first three groups to 1 and 2.9 km3/ky in groups 4 and 5, respectively. Lastly, density maps and lineaments of the eruptive vents, together with crater elongations, allowed the identification of the spatial trends of the SOB volcanism and a comparison with the existing regional major tectonic structures characterizing the local basement. These findings are relevant to understanding the eruptive nature of this active volcanic field and its highly contrasting and multifaceted volcanism.

Magma storage conditions over the past 4 Ma on Martinique Island, Lesser Antilles

We constrain, for the first time, the storage conditions beneath recent (<5 Ma) volcanic edifices in Martinique Island, combining petrography, geochemistry, and thermobarometry. Specifically, we investigate Morne Jacob volcano (5.2–1.5 Ma), Pitons du Carbet complex (998 ka – 322 ka), Trois Îlets Volcanic field (2.35 Ma – 346 ka), and Mt. Conil – Mt. Pelée system (<550 ka). Lava samples range in composition from basalt to dacite, and in age from 4.1 Ma (Morne Jacob) to 1929 CE (Mount Pelée). For the last ∼5 Ma, the volcanoes in Martinique were fed by andesitic to dacitic magma stored in the upper 6 km of the crust, at temperatures between 900 and 1100 °C. These shallow reservoirs were frequently replenished and remobilized by basaltic and basaltic-andesite magmas originating from the lower crust with temperatures as high as 1200 °C. Erupted products contain simple to more complex mineral assemblages, and plagioclase phenocrysts exhibit textures and compositional profiles typical of antecrysts, identified as (1) normal, (2) oscillatory, (3) sieved, (4) resorbed core, and (5) resorbed rim. All complexes also show a crystallization gap in the middle crust (10–18 km). Trois Îlets, a distributed volcanic field, is the only complex that shows continuous crystallization through the entire crust, distinct magma batches and storage regions for each individual volcano. More importantly, we see drastic changes in terms of rock composition, storage conditions, and crystal populations in pre - and post – collapse lavas. Large flank collapse (> 5 km3) can induce decompression as large as 5 MPa in the upper crust and as low as 0.1 MPa down to 10–12 km. Such decompression magnitudes might be sufficient to enhance ascent of basaltic magma from depth, initiate dike intrusions in the mid to upper crust, and remobilize shallow silicic reservoirs. Such events are likely to initiate magma mixing in shallow reservoirs. This noteworthy eruption-triggering mechanism may lead to the incorporation of distinct crystal populations and subsequently contribute to heightened rates of eruption. In summary, the magma storage conditions and plumbing system architecture for the recent volcanoes in Martinique closely resemble those observed in other volcanoes within the Lesser Antilles Island Arc, such as Soufriere Hills Volcano or La Soufriere de Guadeloupe. These newly established correlations offer valuable insights that can aid in the assessment of potential future volcanic eruptions.

Mafic magma recharge triggered eruption of a long-lived shallow silicic magma reservoir beneath a submarine volcano from the southwestern Okinawa Trough

Abundant submarine volcanoes have been identified in the southwestern Okinawa Trough, yet the duration of shallow magma reservoirs and processes driving their eruptions remains unclear. Here, the texture and in situ element compositions of orthopyroxene and magnetite in the dacite and mafic magmatic enclaves (MMEs) from the FDV-1 submarine volcano were investigated to decipher the duration of pre-eruption shallow silicic magma storage and eruption triggering processes. Both unzoned and zoned orthopyroxene phenocrysts were observed in the host dacite. The rims of zoned and unzoned orthopyroxene have similar compositions with Mg# from 49 to 55, Cr = 2.31–9.26 μg/g, V = 54.10–137 μg/g, and Ni = 16.44–21.19 μg/g, and they are in equilibrium with the rhyolitic matrix, which is consistent with growth in the shallow silicic magma reservoir. In contrast, the high Mg# cores (Mg# = 63–79) in the zoned orthopyroxenes are in equilibrium with basalt, with significant enrichment in Cr (1092–1737 μg/g), Ni (106–181 μg/g), and V (183–295 μg/g), which indicates a recycled (antecryst) origin by mafic magma recharge and magma mixing. Furthermore, the timescales from FeMg interdiffusion in orthopyroxene phenocrysts suggest that the magma mixing is approximately 600 years before eruption. Correspondingly, the shallow silicic magma reservoir beneath the submarine volcano is longevity. The presence of bubble and V-depleted magnetite aggregates in the MMEs suggests that volatile-saturated and oxidized mafic magma recharges into the shallow silicic magma chamber. Moreover, the distinct contents of relatively fast-diffusing Ni in magnetite between the MMEs and host dacite suggest that the mafic magma injection into the silicic magma reservoir and formation of the MMEs must have occurred over very short timescales (< 24 h) before the eruption to inhibit complete re-equilibration. Therefore, this last mafic magma recharge induces destabilization of the long-lived shallow silicic magma reservoir and leads to the dacitic magma eruption.

Architecture of the Lesser Antilles Arc Illustrated by Melt Inclusions

Abstract Volatiles are an essential aspect of subduction zones and constraining their cycling through subduction zones is of prime importance to better understand the genesis, transport, storage and eruption of arc magmas. Here we performed an along-arc investigation of the chemical composition of melt inclusions trapped in minerals representative of ten volcanic centers and 23 key explosive eruptions along the presently active Lesser Antilles arc, from Montserrat in the North to St. Vincent in the South. We use the melt inclusion compositions to reconstruct pre-eruptive conditions, especially pre-eruptive magma storage and degassing levels that highlight how the magma plumbing system is organized and works and to discuss magma source characteristics. All major and selected trace elements and volatiles (H2O, CO2, S, halogens (F, Cl, Br)) have been measured on the same melt inclusions when possible. Eruptions dominantly involved andesitic to dacitic magmas (Montserrat, Guadeloupe, Dominica, Martinique, St. Lucia) and basaltic andesite magmas from St. Vincent. Melt inclusions have been used as pressure probes for magmas, for inferring crustal equilibration pressures. We shed light on the systematic occurrence and lateral complexity of a vertical transcrustal magmatic systems feeding active volcanoes. The geochemical view of the architecture of the plumbing system and in particular the Moho's depth is more variable than the view obtained by seismic data along the Lesser Antilles arc. We propose that the discontinuity between the upper and the middle crust is a major magma ponding zone beneath most of the Lesser Antilles islands and that the crustal structure thus imparts a control on the geochemical signature of arc lavas. Melt inclusions are highly differentiated (dacitic to rhyolitic in composition), so they are distant in composition from the primary, mantle-derived magmas, but they provide indirect information about the magmatic sources. The along-arc variability in Y and heavy rare earth element contents of melt inclusions is consistent with the presence of garnet in the mantle source. Our results also indicate an important contribution of oxidized and saline slab-derived fluids to the magma source predominantly in the southern Lesser Antilles that may have implication on the accretionary system behavior. In addition, a high geographical gradient between sediment melt and slab-derived fluid contributions, illustrating high variability in magmas erupted in close spatial association is reported for some islands, such as Martinique and Dominica. Volatile contents are variable for MIs across the arc: the highest H2O (&amp;lt;8 wt%), Cl (up to 3800 ppm) and Br (up to 20 ppm) concentrations occur in MIs from Dominica. However, there is no systematic correlation between MI volatile content and position along the arc. Halogen Cl/F and Cl/Br ratios vary from one island to another, even between the different eruptions, but without any along arc zoning, indicating that halogen fractionation occurred by fluid transfer (variable assimilation rate of fluids derived from seawater) or by heterogeneities of mantle origin inherited from the initial differentiation of the mantle.

Devonian ignimbrites of central Victoria: explosive magmas from multiple sources, and deep crustal structure in the Selwyn Block

The magmas that formed each Devonian, S-type, volcanic complex in central Victoria were generated through partial melting of a distinct assemblage of rock types in the crustal source region. The more silicic rhyolites were formed independently of the rhyodacitic to dacitic magmas, rather than as differentiates. Within every complex, each ignimbritic unit is also heterogeneous in its chemical and isotopic characteristics. Thus, in this part of Paleozoic Australia, magma heterogeneity occurred on at least three scales—the individual rock unit, the complex and across the region. This means that the Selwyn Block, the logical source of the ignimbrite magmas, is highly heterogeneous, probably in three dimensions. The statistically significant differences in the ages of the various complexes suggest that the heat sources for partial melting of the deep crust were themselves rather local in extent, and probably reflected numerous asynchronous intrusions of mantle-derived mafic magmas, most likely in relatively small batches. Thus, there should be no thick underplated or intraplated layer of gabbroic rock in the deep crust here. Thinner bodies of hot mafic magmas were probably emplaced in different places and at different levels, inducing local high-grade metamorphic conditions. Published radiometric dates for various volcanic units suggest that the magmatic heating of the crust occurred over a period of at least 17 Myr, although maximum intensity occurred between ca 375 and ca 371 Ma. Thus, a model of the deep crust here should involve numerous, perhaps thin metasedimentary units with highly variable ages and original clay contents, as well as sills of Late Devonian mafic rocks scattered both vertically and horizontally and intruded episodically. The dominantly meta-igneous sources of crustally derived I-type magmas generally lie at higher structural levels, where they did not experience metamorphic temperatures sufficient to cause the higher-temperature melting reactions needed to form I-type volcanic magmas. KEY POINTS Each Late Devonian, S-type, volcanic magma in central Victoria formed through melting of a distinct assemblage of crustal source rocks, most likely within the Selwyn Block. The rhyolites formed independently of the rhyodacitic to dacitic magmas, not as differentiates. The mafic magma heat sources for partial melting of the deep crust were emplaced as numerous, relatively small batches, in sheets distributed horizontally and vertically. The dominantly meta-igneous and shallower crustal sources of I-type magmas did not experience the higher temperatures required to form I-type volcanic magmas.

Plaster and magnets: Modelling magnetic fabric development in magma intrusions using scaled analogue experiments

Understanding magma behaviour during emplacement within the crust is vital for understanding the dynamic processes occurring in volcanic systems. However, linking the static record of magma flow to its dynamic origin is challenging, particularly as macroscopic indicators of magma flow are often not observed, absent and/or have been modified after emplacement. Anisotropy of magnetic susceptibility (AMS) has been used as an important tool in field studies to infer magma flow direction, using the magnetic fabric as a record of the magma intrusion dynamics and to identify magma source regions. Here, we describe a new method to explore magnetic fabric development in magma intrusions and lava flows using scaled analogue laboratory experiments. Coloured mixtures of Plaster of Paris (the magma analogue) seeded with magnetite particles were loaded concentrically into a piston and injected through a central port in the base of a box filled with compacted fine-grained wheat flour (the crust analogue). This created a series of interconnected sheet and tube-like ‘magma’ intrusions which eventually breached the surface to feed a model ‘lava flow’. Once solidified, the intrusions were excavated and sampled for AMS, with the results showing that magnetic fabrics were preserved. A new dynamic scaling analysis shows the plaster mixture represents the intrusion of dacite magma into the shallow crust. These models provide proof-of-concept that this new methodology and scaling analysis can be used to explore AMS development in viscous (dacite) magma intrusions in nature, with the potential for direct comparison with field-based indicators of magma flow dynamics.

Interconnection of magma sources beneath the Katmai volcanic system inferred from seismic tomography and petrology

The Katmai volcanic cluster in Alaska hosts eight volcanoes, most of which are presently active, including Novarupta, whose eruption in 1912 is considered as the largest in the World during the 20th and 21st centuries. We present a new seismic tomographic model for the upper crust beneath the Katmai Group calculated with the use of ∼173,000 arrival times of the P and S waves from ∼11,000 local earthquakes. The resulting distribution of the Vp/Vs ratio reveals a common deep magma source for the entire system below the depth of 5 km centered in the area of Katmai Pass. The tomographic model also demonstrates interconnections of this source with most of the active volcanoes of the group. Based on this model, we propose a possible mechanism for the Novarupta eruption in 1912. The high-Vp/Vs anomaly beneath Novarupta at depths below 5 km may represent a trace of intrusion of low-density volatile-rich rhyolite magma that was made additionally buoyant by vesiculation. This hypothesis is supported by our petrological analysis showing that amphibole is a stable phase in 1912 rhyolite, which require a deeper source than previously suggested. This ascent was accompanied by draining of denser andesite and dacite magmas from under Mount Katmai, which caused the Katmai edifice to collapse. A shallow anomaly of low Vp/Vs ratio right below Novarupta may trace the conduit filled with pumice-type highly porous rocks remained after the eruption.

Genetic pattern of the Albian volcanic rocks in the Ziruco area, Northern Tibet: Implications for A-type granites

Genetic pattern of a continuous volcanic series of andesites, dacites, and rhyolites from the Ziruco area, northern Tibet was debated in this paper to evaluate the later evolution of A-type granites. We present new zircon UPb ages, whole-rock geochemical, zircon Hf isotopic, and whole-rock SrNd isotopic data for these magmatic rocks. Our analysis shows that the zircon UPb ages of the andesites, dacites, and rhyolites are ca. 105.4 Ma, ca.105.3 Ma, and ca.106.8 Ma, respectively. The andesites have high Nb contents (16.3–24.9 ppm) and Nb/U ratios (22.7–32.4), and chondrite normalized REE and primitive-mantle normalized trace element patterns similar to Nb-rich basalts. They were most likely derived from the mantle that had been metasomatized by melts derived from subducted slab and sediment, and then underwent fractional crystallization. The dacites and rhyolites have high Ga/Al ratios (3.05–3.70 and 2.93–3.43, respectively), REE (226–256 and 195–285 ppm, respectively), Nb (30.0–34.4 and 20.3–24.4 ppm, respectively) and Zr contents (443–513 and 147–163 ppm, respectively), and zircon saturation temperatures (856–886 °C and 823–837 °C, respectively), which are similar to A-type granites. The dacites were most likely formed by partial melting of underplated basaltic lower crust with ilmenite and zircon, followed by slight fractional crystallization. The rhyolites were most likely formed by fractional crystallization of the dacite magmas. The minerals of fractional crystallization probably include feldspar, apatite, zircon, and titanite. This continuous volcanic series in Ziruco area may be related to break-off of the Meso-Tethyan slab in northern Tibet during the late Early Cretaceous. Our studies indicate that fractional crystallization is important later evolutionary process for A-type granites from the crustal source area, and it mainly controls the geochemical characteristics of significant negative Ba, Sr, P, Ti, and Eu anomalies of A-type granites from the crustal source area.

Contrasting magmatic evolutions of the Three Sister Volcanoes reflect increased heat flow, crustal melting and silicic magmatism in the Central Oregon Cascade Arc

The Three Sisters stretch, one of the most magmatically active segments of the Oregon Cascade Arc, includes the Three Sisters volcanoes proper (North, Middle and South) as well as the shields and cinder cones of the Mount Bachelor Chain and McKenzie Pass. The Three Sisters record a magmatic transition from fundamentally basaltic andesitic activity in older North Sister products to more diverse activity at Middle Sister, and unusually silicic activity in a Cascade volcano, including rhyolite lava, at South Sister.North Sister magmas reflect magmatic evolution of basaltic andesite through crystal fractionation, mostly of olivine, and incorporation of crustal melts. Middle Sister, although mostly composed of basaltic andesite and andesite, produced dacitic magma through assimilation fractional crystallization processes involving incorporation of silicic melts. South Sister developed a subjacent magma chamber under the southern half of the volcano, where andesitic magmas evolved, dominantly through fractional crystallization, to dacite and rhyolite. A prominent compositional gap at South Sister between dacite and rhyolite and depletion of middle and heavy rare earth elements in rhyolite suggest that amphibole was prominently involved in fractionation processes involving separation of silicic magma from a gabbroic or amphibolitic crystal mush. Silicic magma eventually formed a stagnant cap in the system, where it evolved through small scale fractional crystallization. Rhyolite magma was finally heated to elevated temperatures prior to eruption, presumably through influx of less evolved magma into the base of the stagnant silicic cap and was erupted in late rhyolite lava of Rock Mesa and the Devil's Hill Chain.South Sister lies along strike of the Northwest Rift of Newberry Volcano, which forms the northwestern terminus of the High Lava Plains trend of south-central Oregon. The increased magmatic activity of the Three Sisters stretch and unusual abundance of silicic products, including rhyolite, of South Sister may reflect the arrival and intersection of the High Plains magmatic anomaly with the Cascade Arc.

Dynamics of Two Caldera-Forming Eruptions (Banda Besar and Naira) in the Marine Conservation Zone of Banda, Maluku, Indonesia

This is the first study discussing the dynamics of two caldera-forming eruptions in the Banda volcanic complex (BVC) in the marine conservation zone of Banda, Maluku, Indonesia. The first and second caldera episodes are, hereafter, termed as Banda Besar and Naira, respectively. The formation of Banda Besar caldera (ca. 8 × 7 km) ejected homogeneous rhyolitic magmas (bulk-rock, 73.1–73.8 wt.% SiO2) in the following three stages: (1) sub-Plinian (BB-5a), (2) intra-sub-Plinian flow (BB-5b), and (3) caldera collapse (BB-5c and BB-5d). The BB-5a stage produced a reversely graded white pumice fall layer with moderate lithics (2–11%), which originated from a sub-Plinian eruption with an estimated plume height of 22–23 km. Subsequently, intensive erosion of wall rock (13–25%) causes conduit enlargement, leading to the partial collapse of the eruption columns, forming intra-sub-Plinian flow deposits (BB-5b). It is likely that conduit size surpassed the minimum threshold value for a buoyant plume during the final phase of the second stage, causing the complete formation of a pumice-rich pyroclastic density current (PDC) during the early-third stage (BB-5c). Finally, the evacuation of voluminous magma from the reservoir yields the first caldera collapse during the late-third stage, producing a lithic-dominated PDC with minor pumices (BB-5d). The formation of the Naira caldera (ca. 3 × 3 km) ejected homogeneous dacitic magmas (bulk-rock, 66.2–67.2 wt.% SiO2) in the following three stages: (1) early sub-Plinian (N-2a and 2b), (2) late sub-Plinian (N-2c, 2d, 2e), and (3) caldera collapse (N-2f). This research distinguishes the sub-Plinian into two stages on the basis of different vent locations (assumed from the isopach map). In particular, this research suggests that the early sub-Plinian stage (N-2a and 2b) erupted from the northern vent, producing 14 and 8 km eruption plume heights, respectively. Additionally, the late sub-Plinian stage (N-2c, 2d, 2e) was generated from a newly-formed conduit located in the relatively southern position, producing 12–17, 9, and 6 km eruption plume heights, respectively. Conduit enlargement is expected to occur during at both sub-Plinian stages, as lithic portions are considerably high (10–72%) and ultimately generate PDCs during the third stage (caldera collapse; N-2f). Because most of the erupted materials (for both caldera-forming eruptions) are emplaced in the ocean, estimating the erupted volume becomes difficult. However, with the assumption that the caldera dimension represents the erupted volume of magma (Vmagma), and that the total erupted volume (Vtotal) is a summation of Vmagma and the now-vanished pre-caldera island (Vvanished, represented by average lithic fractions), the first and second caldera might produce (at least) 35.2 and 2.4 km3 of erupted materials, scaling them as VEI (volcano explosivity index) 6 and 5, respectively. That VEI is more than enough to initiate a secondary hazard in the form of tsunamis triggered by volcanic activities.