“Sasamoriyama Volcanic Rocks” are distributed around the south-western part of Fukushima city in the volcanic front of Northeast Japan arc. The volcanic rocks consist of four geological members: Sasamoriyama Andesite Member, Chusakuyama Andesite Member, Ishiyama Andesite Member, and Kongosan Andesite Member. Lower units of Sasamoriyama Andesite Member and Chusakuyama Andesite Member are correlative to each other, and they were derived from a dissected volcanic edifice. Middle and upper units of Sasamoriyama Andesite Member are composed of anorthite megacryst-bearing andesite lavas and dacite lavas. Sasamoriyama lower units are composed of Icelandite-like tholeiite, whereas middle and upper units have the composition of island-arc tholeiite. Middle and upper units of Chusakuyama Andesite Member are composed of calc-alkaline rocks associated with pigeonitic and hypersthene rock series. Ishiyama Andesite Member is composed of calc-alkaline rocks with pigeonitic rock series, which can be interpreted as a lava dome effused from a subaerial vent. Kongosan Andesite Member is composed of calc-alkaline rocks with hypersthene rock series.

Above all, Icelandite-like tholeiitic lavas in lower units of Sasamoriyama Andesite Member and Chusakuyama Andesite Member contain few opaque minerals as phenocrysts, whereas lavas in middle and upper units of Sasamoriyama Andesite Member contain abundant phenocrysts of opaque minerals as glomerocrysts with other minerals. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios of the island-arc tholeiitic lavas increase and decrease, respectively, with the whole-rock SiO₂ contents, whereas these ratios of Icelandite-like tholeiitic lavas do not show systematic variation with the SiO₂ contents. This observation suggests that the Icelandite-like and island-arc tholeiitic magmas were derived from distinct magma plumbing systems, and they evolved in closed and open systems, respectively. We also suggest that the Icelandite-like tholeiitic magmas had higher temperature, lower water content, and lower oxygen fugacity than those of the island-arc tholeiitic magmas that were similar to those of Quaternary volcanic front tholeiitic magmas. Finally, calc-alkaline magmas occurred in the western part of volcanic front, not only Fukushima area but also some other areas in Northeast Japan arc.

Machine learning (ML) techniques are powerful tools for extracting continuous trends in geological processes from numerical datasets. To date, however, ML use has been largely restricted to classification tasks for geochemical datasets, and few studies have attempted regression tasks. In addition, the general scarcity of numerical data on rock observations—such as whole-rock mineral modes, mineral shape and distribution, and fracture-pattern statistics—has prevented broader ML applications in geology. Here, I propose two approaches to capture continuous geological processes as data-driven geology. First, I present a regression-based Protolith Reconstruction Model trained on a compositional dataset of basalt. Second, I introduce a methodology for the rapid numerical representation of whole-rock mineral information via powder X-ray diffraction. Finally, I propose the integrated approach of ML and numerical datasets to effectively and objectively capture the continuity of multidimensional geological processes.

Monazite crystals from a pegmatite within the Miyazu Granite, located at Shiraishiyama, Kobe, Kyotango, Kyoto Prefecture, Japan, were analyzed for U, Th, Pb, and rare earth element (REE) concentrations using electron microprobe analysis. Back-scattered electron imaging revealed both oscillatory and sector zoning within the monazite. The crystals exhibit a solid solution between monazite and huttonite components, with a maximum ThO₂ content of 30.81 wt%. The empirical formula of the Th-richest composition is (Ce_0.336La_0.151Nd_0.098Y_0.041Pr_0.033Sm_0.016Gd_0.008Dy_0.004Er_0.002Th_0.309U_0.009Ca_0.003)_Σ1.010 (P_0.660Si_0.338)_Σ0.998O₄. The observed compositional variations suggest that the (Th + U) content is primarily controlled by the coupled substitution (Th, U) + Si = REE + P. The CHIME age of the monazite grains is 61 ± 7 Ma (2σ), which is consistent with previously reported radiometric ages of the Miyazu Granite.

New Minerals and Occurrences in Japan (2024)

Hydrous asteroids formed from dust including ice of volatile species such as H₂O and CO₂ in outer cold regions of the early Solar System. Combined X-ray computed tomography and electron microscope analysis of samples from hydrous asteroids unveiled multiple events involved in evolutions of hydrous asteroids. Small and ultraporous lithology discovered in Acfer 094 carbonaceous chondrite was proven to be a fossil asteroidal ice originally formed as compact icy dust aggregates around the H₂O snow line in the solar nebula. The compact dust aggregates formed by sintering of fluffy ice-bearing dust. This finding further suggested the Acfer 094 parent body formation by agglomeration of fluffy dust with and without ice through its radial migration from the outer to the inner Solar System across the H₂O snow line. The combined analysis method was also applied for asteroid Ryugu samples collected by Hayabusa 2 sample return mission. The analysis revealed the presence of carbonated water preserved as a small fluid inclusion in a pyrrhotite grain. This provided evidence of Ryugu’s parent body formation in the outer Solar System beyond the CO₂ snow line. Analysis on space weathered Ryugu samples revealed modification processes of the asteroid Ryugu surface via cometary dust impacts. These findings provided important pieces to understand the long history of asteroid Ryugu.

Information collected from geological newsmagazines in 2024 is reviewed. First topic is mineral resources such as mineral commodity summary in USA, identified mineral resources in Australia, and African industrial minerals for renewable energy. Second topic is regional geology, that is, the volcanic landforms of the Jeju Island, Stark’s Knob Basalt near New York, Tezoantla Tuff in Mexico, and Pyterlite in Finland. The former two are authorized Geopark, the latter two are nominated Heritage Stone. Third topic is overview of Eclogites, Peridotites, and Syenites.

Back-arc spreading, rifting event in subduction zones, is an important geotectonic event to study magmatism through Earth’s history. Magmatism during back-arc spreading displays both geochemical features of convergent and divergent plate boundaries. In the Toyama Basin, which is one of the Cenozoic sedimentary basins on the side of the Sea of Japan, andesites and rhyolites related to the back-arc spreading in the Sea of Japan are widely distributed with large volumes (>1000 km³). I have studied them to reveal when, how, and why such large scaled volcanism had occurred in the basin during the back-arc spreading event. Firstly, I overviewed geology, chronology, and geochemistry of Cenozoic igneous rocks in and around the Toyama basin to know temporospatial changes of magmatism. Secondly, I examined detailed lithostratigraphy of the large volumed andesites to reveal eruption style of large scaled andesitic volcanism in the Toyama basin. Thirdly, I studied petrology and geochemistry of andesites and rhyolites to understand their magmatic processes and interactions in the crust. The geological and petrological studies in the Toyama basin will throw an arrow to geotectonic studies on magmatism in back-arc basin regions.

Since mineral dissolution and precipitation are important chemical reactions in geological processes such as metamorphism and hydrothermal alteration, they are considered to be significant for understanding the fundamental mechanisms of Earth scientific phenomena such as volcanic eruptions and earthquakes. The key points in Carbon dioxide Capture and Storage (CCS) and Carbon dioxide Capture, Utilization and Storage (CCUS) technologies can be thought of as the dissolution and precipitation of minerals. This reaction is not only a heterogeneous solid-liquid reaction through the surface layer between a mineral (solid) and water or carbon dioxide (fluid), but also a complex process involving a mixture of physical processes such as adsorption-desorption, chemical reactions, and transfer phenomena (diffusion, advection, etc.) across the interface between the mineral and fluid. When we consider mineral dissolution in terms of stoichiometry, it can be classified into congruent dissolution and incongruent dissolution. Congruent dissolution refers to dissolution that results in dissolved species consistent with the stoichiometry of the host mineral and rock, while incongruent dissolution is dissolution that shows non-stoichiometric behavior.

In many cases, the dissolution of silicate minerals is considered to be an incongruent dissolution. However, it has been suggested that the addition of chelating agents can greatly enhance the dissolution reaction, apparently exhibiting congruent dissolution.

Incongruent dissolution is one of the causes of heterogeneous solid-liquid reactions, but the results may have implications for a wide variety of geochemical processes. On the other hand, the coexistence of chelating substances such as organic acids may accelerate dissolution (weathering), which provides clues to understanding the origin of life and other processes through rock and fluid interactions.

Brilliant rainbow-colored scoriae have been recognized in deposits ejected from the B-fissure vents during the 1986 Izu-Oshima eruption. We hypothesize that the microtexture on the surface of the scoriae forms a structure that causes the colors. Based on microscopic observations, we found precipitates of approximately ∼ 1 µm thickness on the surface of the rainbow-colored scoriae. The precipitates consist of aggregates of spherulites, which are a composite mainly of polygonal Fe-oxide(s), dendritic Fe-silicate(s), and Mg-Ca-bearing mineral(s). The sizes of the spherulites range from 0.07-0.29 µm, 0.07-0.55 µm, and 0.16-0.81 µm for blue, yellow, and red areas, respectively. The areas showing metallic brilliance have larger Fe-oxide(s) areas, leading to a high reflectivity. The colors did not change for respective areas with different observation angles, indicating non-iridescent structural color from a randomly arranged component. Beneath the crystallized precipitates, Na-rich silicate glass (∼ 2 µm) was found to exist above the original silicate glass. These textural and compositional characteristics can be explained by the oxidation of basaltic glass at a high temperature in an SO₂-bearing gas and the subsequent rapid cooling of the glass. These results indicate that the structural colors are produced by randomly arranged spherulites with the size of visible wavelengths that crystallized near the glass surface. Thus, rainbow-colored scoriae record the high temperatures oxidation and rapid cooling, which can be attained inside an eruption column. The colors of scoriae can be an indicator for magma-air interactions during an eruption.

The Kawai-type multi-anvil press (KMA) is one of the powerful high-pressure appraratus to investigate the Earth’s interior. Because KMA can produce a stable pressure-temperature field in a relatively large sample volume, it provides reliable results. Here I introduce recent progress on studies of deep earth science, especially showing my research achievements. I have developed KMA techniques for ultra-high pressure generation and in-situ X-ray diffraction, which have been applied for studies on phase relations and crystal chemistry of mantle-constituent minerals up to mid-mantle conditions. I have studied crystal chemistry of bridgmanite up to 52 GPa in pyrolite and basaltic systems, and found that oxygen vacancy components in bridgmanite disappear around 40 GPa corresponding to 1000 km depth. This phenomenon can explain stagnation of subducting slabs around 1000 km depth. In-situ X-ray diffraction techniques to precisely and accurately determine phase stability have been developed and applied to determine bridgmanite-forimng reaction boundaries: Mg₂SiO₄ post-spinel, MgSiO₃ akimotoite-bridgmanite, and Mg₃Al₂Si₃O₁₂ post-garnet transitions. These studies provide insights into the structures and dynamics between 660 and 1000 km depths. In addition, I also introduce my recent studies on deep water cycle in the mantle: high-pressure phase relations and water partioning of mantle minerals under hydrous conditions. Water partitioning between nominally anhydrous minerals (NAMs) of olivine and its high-pressure polymorphs and hydrous phases has been studied. The results indicate that these NAMs are nearly dry when coexisting with hydrous minerals, resulting in factors of deep-forcus earthquakes and slab stagnation in a wet subducting plate. Phase relations of aluminous silica minerals under hydrous conditions have been also studied. I found that CaCl₂-type aluminous silica is stable even at top-lower mantle conditions and can accommodate more than 1 wt% water in the crystal structure, suggesting deep water cycle by silica minerals.