Search

As volcanoes are closely related to the living environment of humans, in particular via natural hazards and geothermal energy, scientific studies on volcanic edifice structures are required. Ulleung Island is a Quaternary volcanic island located in the mid-western East Sea (Sea of Japan). In this study, we conducted an audio-frequency magnetotelluric (AMT) survey to image the substructure of the Ulleung volcanic edifice. In addition, the electrical structure was interpreted from the geothermal system perspective, geochemical compositions of volcanic rocks, and the possibility of the presence of a magma reservoir. AMT data were obtained from 25 stations and processed using the remote reference technique. Then, the three-dimensional (3-D) approach was reasonably adopted according to dimensionality analysis. Before conducting the 3-D inversion, the effects on topography and ocean were analyzed using a simplified 3-D synthetic model, because Ulleung Island is surrounded by sea and the topography is undulating. Most AMT stations on Ulleung Island are distorted by topographical and oceanic effects; in particular, oceanic effects are significant at frequencies lower than 10 Hz. The 3-D inversion was conducted with full impedance components and vertical magnetic transfer functions in a frequency range of 10,000–1 Hz. The results show that the Ulleung volcanic edifice is characterized by two layers of electrical structures as follows: near-surface resistive anomalies and an underlying conductive layer. Considering the high geothermal gradient on Ulleung Island, we suggest that the conductive layer of the volcanic edifice is due to hydrothermal alteration of basaltic rocks, with a potential heat source underneath. Based on the geochemical characteristics of the Ulleung volcanic rocks, the possibility of heat transfer from a trachytic magma reservoir within the shallow crustal is suggested. In summary, this study presents a geological interpretation of shallow volcanic structures beneath Ulleung Island and the possibility of an active magma reservoir as a potential heat source for the volcanic edifice. [ABSTRACT FROM AUTHOR]

Carbon flux metasomatism in the subduction environment is an important process, but it remains poorly understood. The paucity of exposed lower crust and upper mantle rocks in continental arcs renders xenoliths a major target for studying the slab-derived carbon cycle. This study of the carbonate phases in volcanic rocks from three drill cores in Ulleung Island, South Korea, sheds light on the interaction of carbon flux in the upper mantle and lower crust in a back-arc setting. The volcanic rocks from Ulleung Island range in composition from trachybasalt to trachyte and contain abundant euhedral pseudomorphic carbonate grains, ulvöspinel-hosted and biotite-hosted carbonate-silicate melt inclusions, and irregular carbonate globules. Integrated petrographic and geochemical studies of a variety of phenocrysts, carbonate phases, and carbonate-silicate inclusions in biotite and ulvöspinel indicate that recharging of carbon flux affected magma evolution. Carbon and oxygen isotopes of the pseudomorphic carbonate grains overlap with mantle values, indicating a carbonatite-like origin of the carbonate phases. The (MgO,FeO,CaO)-rich silicates in ulvöspinel-hosted silicate inclusions and pseudomorphic carbonate grains likely represent a primary melt, which formed from the partial melting of carbonated eclogite of the subducted slab within the mantle wedge beneath Ulleung Island. A petrogenetic model is proposed to illustrate that the crystal mush in the magma chamber was intruded by carbonate-rich liquids and caused alteration of cumulate crystals to generate the euhedral pseudomorphic carbonate grains. The extrusive magma captured those pseudomorphic grains and erupted to form the trachybasalt-trachyte units. The observed carbonate phases and their geochemical characteristics indicate that carbon flux metasomatism played a fundamental role in this back-arc magmatism. [ABSTRACT FROM AUTHOR]

The multi-stage magmatic plumbing system beneath the Ulleung, Dokdo, and Oki-Dogo Island has attracted considerable attention but has seldom been investigated in detail. Herein, we present a comprehensive geochemical analysis of volcanic rocks sampled at the Ulleung Island, including whole–rock major and trace elements as well as Sr and Nd isotopic compositions. Late Cenozoic alkaline volcanic rocks from the Ulleung Island are commonly divided into basaltic rocks (tephrite basanite and basalt) and acidic rocks (trachyte and rhyolite) based on the large petrologic and geochemical variations. All rocks are significantly enriched in light rare-earth elements (REEs) and large-ion lithophile elements and relatively depleted in heavy REEs. The acidic rocks have slightly enriched 87 Sr/ 86 Sr and ε Nd ratios (0.704895–0.705173; −2.48 to −1.91) and distinctly negative Ba, Sr, P, Eu, and Ti anomalies relative to the basaltic rocks (0.704733–0.705006, -2.61– -1.17). A multi-stage magma evolution model was finally developed beneath the Ulleung, Dokdo, and Oki-Dogo Island, characterized by at least two magma reservoirs at different depths approximately ranging between 20 and 100 km. These ascending magmas were retained in a shallower reservoir for ~1.0, ~0.5, and ~4.5 Ma, respectively, prior to eruptions with significant crystal fractionation. Mantle-derived basaltic melts either directly travelled to the surface to erupt as tephrite-basanite and basalt, or were stagnated in shallower reservoirs where they differentiated into acidic magma to erupt as trachyte and rhyolite. The Late Cenozoic Korean Peninsula basalts are mainly divided into two groups, i.e., the depleted mantle (DM) and enriched mantle 1 (EM1) mixing trend beneath the Ulleung and Dokdo Island, and the DM-EM2 mixing connection beneath the Jeju Island, which likely indicates the traversed boundary beneath Korean Peninsula that represents a simple lateral continuation of the boundary between the southeast and northeast China Blocks. [ABSTRACT FROM AUTHOR]

Ulleung Island is a Quaternary volcanic island located in the mid-western part of the East Sea (Sea of Japan) back-arc basin, which has erupted from the Pliocene until the late Holocene. This study focuses on reconstructing the latest eruptive history of the island by describing the sedimentological and stratigraphic characteristics of the most recent, trachytic/phonolitic pyroclastic sequence, named the Nari Tephra Formation. This formation is preserved as a succession of unwelded pyroclastic and epiclastic deposits within an embayed margin of the Nari Caldera. The embayment acted as a topographic trap for proximal pyroclastic deposits, and contains a complete record of the past 19,000 years of eruption history. The formation includes evidence for five separate eruptive episodes (Member N-1 to N-5), with intervening weathered and/or soil horizons indicating hundreds to thousands of years of repose between each eruption. Eruption styles and depositional mechanisms varied between and during individual episodes, reflecting changing dynamics of the magma plumbing system, magmatic gas coupling, and a variable role of external water. Extra-caldera sequences show that only a few of these eruptions generated sustained eruption columns or pyroclastic density currents (PDCs) large enough to overtop the caldera wall. Thus tephra sequences outside the caldera provide an underestimate of eruption frequency, and care needs to be taken in the interpretation and correlation to distal tephra sequences recognized in marine and terrestrial records. In addition, topographic effects of caldera structures should be considered for the assessment of PDC-related hazards in such moderately sized pyroclastic eruptions. [ABSTRACT FROM AUTHOR]

The volcanic activity of Ulleung Island generated considerable amount of pyroclastic materials composed of volcanic ash, lapilli tuff, pumice, and breccia, during the period ranging from the Pleistocene to the Holocene. The Nari Formation exposed in the southern part of the Nari caldera can be divided into four units; Nari-I, Nari-II, Nari-III and Nari-IV. Nari-I, Nari-II, and Nari-III are characterized by dense gray pumice, white and gray pumices, and brown and black pumices, respectively. In this study, we dated radiocarbon ages and identified species of four charcoal fragments collected from the pumiceous deposits of Nari-I and Nari-II. Radiocarbon ages for Nari-I and Nari-II are 15,880 ± 60∼15,560 ± 100 BP and 7,660 ± 40∼7,600 ± 40 BP, respectively. Calibrated ages from these ages are 19,055 ± 95∼18,840 ± 150 cal BP for Nari-I and 8,465 ± 75∼8,400 ± 40 cal BP for Nari-II. This result indicates that the interval between Nari-I and Nari-II might be about 10,000 years. Nari-III was formed after 5,600 cal BP that records the youngest age data correlated with Nari-II. Therefore, it can be concluded that Albong lava dome, the youngest volcanic activity of the island, is more likely to cease a few thousands of years ago. On the basis of the anatomical characteristics of the charcoal fragments, they were identified to be Picea spp. and Fagus crenata. To summarize, the paleovegetation around the Nari caldera consisted of Picea spp. until 19,055∼18,840 cal BP and Fagus crenata, representing a broadleaf forest of Ulleung Island at present, flourished after about 8,465 cal BP. [ABSTRACT FROM AUTHOR]

We take two typical Northeast Asia bimodal volcanoes as examples to explain the general features of Cenozoic bimodal ocean island basalt (OIB)-type volcanism in Northeast Asia. We present mineralogical, petrological, geochemical, isotopic, and full-waveform seismic tomographic evidence for the existence of two-layer magma chambers of Late Cenozoic volcanic activity beneath Ulleung Island and Mt. Changbai (Paektu). Ulleung Island volcanic rocks, which are composed of alkaline basalt, phonotephrite, trachyte, and phonolite, belong to the alkaline magma series and display enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs), slight depletion of heavy rare earth elements (HREEs), enriched 87Sr/86Sr (0.70475–0.70507) and 143Nd/144Nd (0.51250–0.51255) isotopic values, and enriched 207Pb/204Pb (15.544–15.626) and 208Pb/204Pb (38.750–38.954) values, similar to the geochemistry of OIB. Ulleung Island felsic volcanic rocks are characterized by significant negative Ba, Sr, P, Eu, and Ti anomalies (δEu = 0.14–0.35) and positive Pb anomalies, slightly higher 87Sr/86Sr isotopic ratios relative to those of mafic volcanic rocks, although the mafic and felsic samples have similar Sr, Nd, and Pb isotope compositions without significant differences. Ulleung Island and Mt. Changbai volcanic activities are likely related to the involvement of subduction-related compositions, but parts of the Mt. Changbai samples have been contaminated by crustal components to a certain extent. Mafic volcanic rocks of Ulleung Island are segregated from a deeper mantle source within a pressure range of 10.1–21.2 kbar compared with felsic volcanic rocks, which exhibit fractional crystallization of clinopyxene, spinel, plagioclase, and olivine. To explain this, a two-layer magma chamber beneath Ulleung Island with depths of 30–40 and 60–80 km is proposed, which is supported by mineral crystallization pressure and a three-dimensional full-waveform seismic tomography model. We also suggest that similar magma eruption processes and a two-layer magma chamber with depths of ~10 and 40–60 km also exist beneath Mt. Changbai. Taking the typical Cenozoic bimodal samples from Ulleung Island and Mt. Changbai as examples, we argue that two-layer magma chambers exist beneath Cenozoic bimodal OIB-type volcanic activities in Northeast Asia. [Display omitted] • A two-layer magma chamber beneath the Ulleung Island is proposed. • Fractional crystallization might play a key role for Ulleung Island volcanic rocks. • Northeast Asia volcanism is related to the involvement of subduction-related compositions. • The general features of Cenozoic bimodal OIB-type volcanism in Northeast Asia are proposed. [ABSTRACT FROM AUTHOR]

Ulleung Island is a Quaternary intraplate alkaline volcano located in the central-western part of the East Sea (Sea of Japan) back-arc basin. Previous studies have divided the geological history of the volcano into five stages: Stage-1 trachybasalt, Stage-2 and -3 trachytes, Stage-4 phonolite, and Stage-5 leucite-bearing trachyandesite. This study presents geochemical compositions, including Sr–Nd–Pb–Li isotopes, for the volcanic rocks. The trachytes and phonolite were formed by fractional crystallization of trachyandesitic magma at different depths. The trachybasalts have highly fractionated REE patterns with (La/Yb) N values of 23.6–30.7 and lack Eu anomalies. In a primitive-mantle-normalized multi-element distribution diagram, the trachybasalts show OIB-like LILE enrichment patterns without HFSE depletion. However, they show negative K anomalies, suggesting hydrous residual potassic phases in the source. The lithology and physical conditions of the mantle source for the Ulleung basalts are estimated to have been a pyroxenite fraction of 4 ± 2%, a mantle potential temperature of 1303 ± 8 °C, a minimum pressure of basalt segregation (P mt) of 2.5 ± 0.0 GPa, a temperature of the melt at P mt of 1327 ± 8 °C, and a degree of melting of 1.9 ± 0.6 wt%. The basalts have EM1-type OIB-like Sr–Nd–Pb isotopic compositions (87Sr/86Sr = 0.70472–0.70507; ε Nd = −4.5 to −1.8; 206Pb/204Pb = 17.95–18.09). The Stage-5 trachyandesites lack pronounced negative K anomalies and are characterized by slightly more depleted Sr–Nd–Pb isotopic compositions (87Sr/86Sr = 0.70445–0.70453; ε Nd = −1.3 to −1.2; 206Pb/204Pb = 18.06) compared with the Stage-1 trachybasalts, implying their origin from a relatively dry source. δ7Li values of the basalts range from −8.8‰ to +1.9‰, but the trachyandesites are relatively heavy, having δ7Li values ranging from +3.5‰ to +6.3‰, suggesting differential loss of low-temperature 7Li from a recycled oceanic slab in the source. The differentiated Stage-2 to −4 trachytes/phonolite have Sr–Nd–Pb–Li isotopic systematics resembling those of the trachyandesites, but the effects of assimilation of thinned lower continental crust during magma evolution are observed in some samples. The Ulleung magmatism was likely generated by wet upwelling from the stagnant slab in the mantle transition zone in combination with small-scale/edge-driven convection in the asthenosphere. Unlabelled Image • We report Sr–Nd–Pb–Li isotope compositions for volcanic rocks from Ulleung Is. • They show the EM1-type OIB-like Sr–Nd–Pb isotopic systematics. • They were from a hydrous phase-bearing pyroxenite-garnet peridotite source. • Variably dehydrated recycled slab is responsible for the δ7Li value (−8.8–8.1‰). • Interplay of edge-driven convection and wet plume from MTZ caused the volcanism. [ABSTRACT FROM AUTHOR]

Ulleung Island is an alkaline volcano that consists of Pleistocene basalts (Stage I), effusive trachytes (Stages II–III), and Holocene explosive phonolitic volcanics (Stage IV). Syenites are found as volcanic ejecta in Stage III and IV, hence, acting as a window for pre-eruptive shallow alkaline magma dynamics. We integrate geochemical data from Ulleung Island to track the magma batch interconnection underneath this island and discuss the petrological effect of magma mixing on the resultant eruption style through shallow preconditioning. The magma mixing characteristics of syenite with melanocratic enclaves are reported in this study from a textural and geochemical context. The textural assemblage from syenite is likely to indicate a couple of magmatic conditions including quenching of hotter magma, overgrowth of orthoclase-rich feldspar, and disequilibrium of kaersutite. The magma mixing conditions were traced through the predicting models from clinopyroxene, amphibole, biotite and alkali-feldspar chemistry, and constrained by ~690–740 °C at ~77–180 MPa. In addition, the Zr/Ce versus Zr/Y plot indicates that the syenites can be subdivided into the Group I as felsic cumulate from Stage IV, and the Group II as mixing product of Stage IV and II. Consequently, the magma mixing of the Stage IV and II produced the Stage III. The syenite ejecta from Ulleung Island indicate the petrological significance of the shallow alkaline magma dynamics: the explosive eruption of Stage IV is induced by the volatile (especially, magmatic water) transportation from Stage II and subsequent saturation in Stage IV. Furthermore, a discussion on the manifested eruption style through magma mixing from comparative perspective implies that the explosive eruption of a volcano requires the transportation and saturation of volatile components and a short stagnant period, avoiding stabilization of magma chamber prior to its eruption. • Magma mixing event from Ulleung Island was discovered during this study. • Syenites as a plutonic equivalent record the mixing-induced petrological evidence. • The mixing conditions from syenites were constrained by ~690–740 °C at ~77–180 MPa. • Syenites represent the pre-eruptive shallow alkaline magmatism at shallow depth. • Explosive eruption requires the volatile saturation and a short stagnant period. [ABSTRACT FROM AUTHOR]