The upper Kayaoka sheet in the Atsumi area, Northeast Japan, has an alkaline-basaltic composition. The modal olivine and Ni enrichment in the lower part of the sheet indicates the accumulation of ...early formed olivine crystals. Overall, the average aspect ratio of plagioclase increases from the central part of the sheet toward both the top and bottom margins. The average aspect ratios of plagioclase and calculated crystallisation times at each height of the sheet, except for the lowermost margin, are roughly plotted near the regression line drawn based on those from many sills in previous research. However, we observed that the average aspect ratios of plagioclase in some horizons of the central part of the sheet exhibit an offset from the regression line. The olivine analcite dolerite in the central part of the sheet is rich in zeolite amygdules and glassy mesostasis with a quench texture. These observations suggest that the cooling rate in the central part of the sheet was faster compared with assuming conductive cooling only. A plausible mechanism for the rapid cooling at the centre of the sheet is heat transport by the convection of seawater that has infiltrated the columnar joint.
Kelud is considered one of the deadliest volcanoes in Indonesia due to its proximity to three cities in East Java (Malang, Kediri, and Blitar) and violent eruptions over the last century. Regardless ...of the frequently witnessed eruptions, the volcanic stratigraphy for the historical ages has been poorly understood. Here, we provide the first documentation of volcanic stratigraphy for the last 1300 years of activity at Kelud. Componentry and 14C data are coupled with detailed field observations to develop a comprehensive stratigraphy and characterize the main eruption types. The volcanic succession can be subdivided into ten bed-sets composed of pyroclastic density current (PDC) and tephra fallout deposits. A total of twelve fallouts and ten individual PDC units were identified between ∼1.3 and 0.1 cal. ka B.P. One of these units may be attributed to the 1481C.E. eruption. The fallout layers are generally thin, derived from short-sustained eruption columns. By contrast, PDC deposits are massive to stratified, suggesting variable particle concentrations typically resulting from low-column collapses. This complex stratigraphy suggests that Kelud is capable of producing a wide spectrum of eruptions with an increasing eruptive frequency over time and shorter dormancy periods than those between witnessed eruptions (from once /∼100 years to once/∼10 years). This stratigraphic framework, which documents extended records from the oldest witnessed eruption in Kelud (1000C.E.), sheds new insights into Kelud's activity during historical time. Since the products share similar characteristics with those from the most recent activities at Kelud (e.g., 1990, 2007–2008, and 2014), similar eruptions may also occur in the future.
•Volcanic stratigraphy and sedimentology record of <1.3 ka Kelud's eruptive products•New stratigraphic record beyond the witnessed historical records•Characterization of major eruption patterns•Dominant explosive eruptions represent a major threat for surrounding populations.
The Miocene tectonics of Papua New Guinea, where subduction, arc-continent collision, and changes in subduction direction are considered to have occurred, is very complex and various tectonic models ...have been proposed. The Maramuni arc, active in the Miocene, is composed of a chain of granitoid bodies. As the chain-like distribution indicates the generation of igneous activities in a wide range of the same tectonic settings, the study of the Maramuni arc magmatism is important for elucidating the geologic events of the time. We provide data on the petrological and geochemical characteristics of the Morobe Granodiorite that form part of the Maramuni arc. The Morobe Granodiorite consists of metaluminous I-type granitoids, belonging to the medium-K to high-K series. The whole-rock major element variations in the granitoids can be explained by the fractionation of hornblende and plagioclase. They are generally within the composition range of experimental partial melts of amphibolites, and the whole-rock trace element compositions have characteristics of slab failure magma rather than arc. This suggests that the granitoids were generated by partial melting of the torn slab after slab failure. The mafic microgranular enclaves (MMEs) in the granitoids are classified as shoshonite, and their trace element compositions suggest that they were formed by partial melting of phlogopite-bearing mantle. The occurrences of native gold and barite within the MME show that MMEs transport Au from the mantle metasomatized by slab-derived sediment melt and/or fluid to the crustal magma chamber.
The problem of whether cumulate rocks were formed by crystal settling or by in situ crystallization after magma emplacement is an important issue concerning the mechanisms of magmatic ...differentiation. However, it is hard to distinguish these two processes for plutonic rocks because the primary texture and chemical composition have generally been modified by postcumulus processes. To contribute this problem, we studied the distribution and compositions of Cr-spinel inclusions hosted in olivine and plagioclase in the Murotomisaki Gabbroic Intrusion (MGI), SW Japan. It is shown that the olivine-hosted inclusions are restricted to specific horizons where accumulation of olivine phenocrysts is thought to have occurred and that the compositional variations of the Cr-spinel are explained by a secondary compositional modification that probably took place after the magma emplacement. It is also shown that the Cr-spinel inclusions in a chilled margin have suffered the least compositional modification and nearly retains the primary composition. Those in the interior of the intrusion, on the contrary, have been significantly modified by re-equilibration with residual melt driven by cation diffusions through the host phases. Those in plagioclase have been less modified. It is shown that all the spinel inclusions had primarily the same and common composition at the time of magma emplacement. This implies that all the inclusion-bearing crystals, olivine and plagioclase, represent primary phenocrysts that had already existed in the emplaced magma. In this way, spinel inclusion in the MGI may be regarded to be a useful petrographic “marker” for identifying intratelluric phenocrysts and also as a “tracer” to trace the motion of the primary phenocrysts after the magma emplacement.
The Lamongan Volcanic Field (LVF), East Java, Indonesia, has experienced numerous maar eruptions, producing varied properties and morphologies of ash particles. This study conducted textural, ...morphometric, and geochemical analyses of the juvenile particles to elucidate the factors governing their heterogeneous characteristics. Two distinct types of juvenile ash were identified: A (black and brown ash) and B (orange-brown ash), reflecting different fragmentation processes. The blocky to slightly elongate shapes of juvenile A across heterogenous basaltic compositions (resulting in variable textures, rheological properties, and/or cooling histories) highlight the phreatomagmatic process as the primary control of their shape. In contrast, the irregular-fluidal shapes of juvenile B particles indicate magmatic fragmentation of basaltic andesite magma. This study reveals that variable magma properties yield diverse ash components, yet fragmentation dynamics govern pyroclast shapes in the LVF maar complex. Our integrated approach emphasizes the importance of considering multiple variables when interpreting heterogeneous volcanic ash deposits.
This study presents a forward model to quantify the P‐wave velocity (VP), S‐wave velocity (VS), and electrical conductivity (σ) of the solid‐liquid mixtures for a given set of pressure, temperature, ...lithology, liquid phase (aqueous fluid or melt), liquid fraction, and geometrical parameters in relation to the aspect ratio and connectivity of the liquid phase. This is based on previous experimental and theoretical studies on seismic velocity and electrical conductivity of solid rocks and liquid phases. A total of 78 lithologies, an aqueous fluid with NaCl (∼0–10 wt.%), and mafic to felsic melt appropriate for the crust and the uppermost mantle conditions were described in terms of VP, VS, and σ, as per previous experimental measurements and molecular dynamics simulation. This forward model is provided as a Windows executable program, and generates synthetic VP, VS, and σ, referring to the seismic velocities and electrical conductivity observed in the northeast Japan arc. After generation of the synthetic VP, VS, and σ, the original lithology and liquid parameters (phase, fraction, aspect ratio, and connectivity) were searched by implementing the grid search algorithm to map the misfit over the broad parameter space. The mapping shows the presence of a global misfit minimum around the optimized solution and the possibility of resolving the lithology and the liquid phase parameters based on the observed VP, VS, and σ by using the forward model presented in this study.
Plain Language Summary
Liquid phases such as aqueous fluid and magma present within the solid Earth play crucial roles in various geodynamic processes including earthquake and volcanic eruption, as well as in the evolution of our planet. This study aims to improve the subsurface imaging of the potentially variable types of liquid phase and solid rock, based on the near‐surface observations concerning seismic velocity and electrical conductivity of the Earth's interior. We constructed a quantitative model to predict the physical properties of liquids and rocks. Then we used the model to perform “inversion,” which estimates the physical properties of liquid and rock based on the observed seismic velocity and electrical conductivity. The inversion test using synthetic data shows the utility of our model.
Key Points
A forward model is presented to quantify seismic velocity and electrical conductivity of solid‐liquid mixtures in crust‐uppermost mantle
The input parameters include pressure, temperature, lithology, liquid phase composition and fraction, and solid‐liquid geometry
For a given set of seismic velocity and electrical conductivity, the optimal parameter values can be found by inversion
The Genbudo lava, the late Pleistocene basaltic-andesitic lava flow in the southwestern part of Iwate Volcano, Japan, is a 70 m thick columnar jointed flow that can be divided into three parts from ...bottom to top: the colonnade, the entablature, and the partly-brecciated uppermost part. Two main types of fractures developed in the entablature: pseudopillow fractures that formed in a branching network-like pattern throughout the entablature, and sheet fractures with curved surfaces that are nearly parallel to each other. At the uppermost part of the flow, finger-like structures of lava extend upward from the coherent lava, and cogenetic autoclastic rocks form between the fingers. This occurrence suggests that hyaloclastites were generated during emplacement in the uppermost part of the flow, apparently when water from a dammed river valley covered the flow. The texture of the lava near the pseudopillow fractures in the entablature is commonly hypocrystalline, while the texture in other parts is holocrystalline. There are two types of pyroxene microlites, large prismatic (average size ~ 30 µm) and dendritic (< 10 µm in length) crystals in the lava near the pseudopillow fractures. These suggest that the cooling rate of the lava was greatest in the vicinity of the pseudopillow fractures. Networks of palagonite-filled micro-fractures (less than 10 µm in width) are found in this part of the flow, and many bubbles are observed along the fractures. This is clear evidence that the rapid cooling of the lava was caused by water infiltration through the pseudopillow fractures. From the measurement of Fe-rich droplet sizes that formed due to liquid immiscibility within the lava, we estimate the cooling rate within the colonnade as about 49 °C/h and within the entablature as 642 °C/h, consistent with much more rapid cooling by water infiltration from above.
Manguao Basalt is a Plio–Pleistocene basaltic lava flow located on the northeastern edge of Palawan Island, Philippines. The absence of active trenches surrounding the Palawan Continental Block (PCB) ...poses a challenge regarding the nature and origin of magmatism in the region. This study presents the petrographic and geochemical character of Manguao Basalt, as well as provides insights to the melt formation beneath the PCB. Manguao Basalt samples are olivine–phyric with minor pyroxenes, set in a plagioclase–dominated microcrystalline groundmass. Average bulk–rock major element composition of Manguao Basalt shows similarities to common olivine basalt tholeiite. Petrographic observations of the phenocrysts, however, show the unusual precedence of pyroxenes in the crystallization sequence. Calculated formation temperatures and pressures of the modal assemblage are consistent with this idea of early pyroxene formation. Simulations of mantle melting using the MELTS program show the formation of pyroxene–saturated primitive liquids. The evolution of these primitive liquids reaches similarities with Manguao Basalt composition at 1230–1260 °C. Simulations of equilibrium and fractional crystallization demonstrate the formation of olivine or orthopyroxene as the first crystals. However, the simulations done at equilibrium conditions are more consistent with the observed mineral chemistry of pyroxene phenocrysts in Manguao Basalt. Hence, maintaining the equilibrium between the source and melt is crucial for replicating the observed pyroxene chemistry. Magmatic underplating provides an excellent model for visualizing the melting and crystallization processes beneath the PCB. The model is also consistent with the narrative of other magmatic units in northern Palawan (e.g., Capoas Granite). The significant findings of this study contribute to the understanding of the tectonic evolution of the PCB.
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