We investigated the chronology and magma system of the Shiobara caldera-forming eruption at Takahara volcano in Tochigi Prefecture, using geologic investigation, chemical analyses of juvenile materials, and U-Pb dating of zircon grains. The Shiobara caldera-forming eruption deposits consist of three ignimbrite units: So-KT (KT), So-TN (TN), and So-OT (OT) (in ascending stratigraphic order). Two exotic tephra layers characteristically containing biotite are recognized in thick volcanic ash soils just below KT and TN, respectively. On the basis of their glass chemistry, Fe-Ti oxide compositions, and U-Pb ages, the lower tephra layer correlates with the Kaisho-Kamitakara Tephra (KMT: 620 ka), whereas the upper tephra layer correlates with one of the members of the Omachi APm Tephra Beds (A₁Pm to A₅Pm: 410-337 ka). In addition to these tephrochronologic results, loess chronometric calculations suggest that KT formed at ~596 ka, whereas TN and OT (OT/TN) are younger than 380 ka. Magma mixing was a common magmatic process in the production of the three ignimbrite units, with each ignimbrite having a different felsic end-member. The considerable geochemical gap between KT and OT/TN is interpreted to mean that crustal melting beneath Takahara volcano changed from wet to dry conditions after a long period of dormancy (> ca. 200 ky) following the KT eruption.

The Miocene Tottori Group in southwestern Japan was formed in association with the expansion of the Japan Sea as a back-arc basin along the margin of East Asia. Strata of this group record processes involved in the opening of this back-arc basin. However, despite previous stratigraphic studies, the geochronology of the Tottori Group is uncertain, and the sedimentology of conglomerate strata of the group exposed along the Hakuto Coast in eastern Tottori Prefecture has not been examined.

Here, we present results of geological investigation and K-Ar dating of andesite from the Kawabara Volcanic Member of the Yazu Formation, Tottori Group, exposed along Cape Keta, as well as sedimentary characteristics of unclassified conglomerate strata of the Iwami Formation, Tottori Group, exposed on Okinoshima Island off the Hakuto Coast. Plagioclase in the groundmass of two-pyroxene quartz andesite of the Kawabara Volcanic Member has a K-Ar age of 18.3±0.6 Ma, supporting the interpretation of previous studies that the Kawabara Volcanic Member is correlated with the Yoka Formation of the Yabu Subgroup, Hokutan Group. The unclassified conglomerate strata contain sediments that record a series of environmental changes in a fluvial system, including sedimentation due to intense volcanic activity and subsequent slope deformation caused by riverine downcutting, followed by Gilbert-type fan-delta deposition and finally alluvial-fan deposition. Paleo-current directions of the fan-delta and alluvial-fan sediments indicate a northeasterly to southwesterly orientation, implying the existence of land to the northeast of the study area during the sedimentary period. Findings of the study will be useful for reconstructing the paleogeography of the Sea of Japan during the Miocene.

We identified six Kuroko-type deposits in the Izu collision zone, central Japan, which formed as seafloor massive sulfides (SMS) in the Paleo-Izu Arc before its collision with the Honshū Arc during the middle Miocene. These deposits are found in the same stratigraphic horizon (~15 Ma) in the Nishiyatsushiro and Koma groups, between the basaltic volcanic sequence of the Furusekigawa Formation (or its equivalent) and the hanging-wall mudstone of the Tokiwa Formation of the Nishiyatsushiro Group (or the equivalent sedimentary unit in the Koma Group).

The most remarkable difference between the Kuroko-type deposits in this region and typical Kuroko deposits in the back-arc troughs of the Honshū Arc is the close association of the former with basalt, in contrast to the common association of the latter with rhyolite- or dacite-dominant bimodal volcanism. We interpret the deposits in this region as Kuroko-type deposits because they are the products of arc volcanism and related hydrothermal activity. This conclusion is supported by the sulfur isotopic compositions [δ34S vs. Canyon Diablo Troilite (‰)] of gypsum ores from the Takara and Mogura deposits, which fall within a narrow range of values (+21.9‰ to +22.5‰ and +20.1‰ to +22.0‰, respectively) that are consistent with those of middle Miocene seawater sulfate.

Recent exploration in the present-day Izu-Bonin arc indicates that SMS deposits occur exclusively at the summit or within the summit crater or caldera of submarine volcanoes where high-temperature hydrothermal activity could be focused. Therefore, it is highly likely that the footwall basaltic lava and pyroclastic units of these Kuroko deposits are components of the volcanic edifices that hosted the mineralization. The SMS deposits collided as parts of the thin-skinned uppermost crust of the Paleo-Izu Arc and accumulated near major faults, including the Itoigawa-Shizuoka Tectonic Line and Tonoki-Aikawa Tectonic Line.

Spherical concretions found in sedimentary rocks are fascinating natural objet trouvés because of their rounded shapes and distinct boundaries. They consist of several minerals, including carbonate minerals, silicate minerals, and iron oxides. Well-preserved fossils are often found in concretions, particularly those composed of calcium carbonate. Concretions are thought to form by diffusion and the development of a syn-depositional reaction front that travels rapidly from the center of the concretion toward its outer margins. Based on the examination of several hundred spherical calcium carbonate concretions, we developed a diffusion-based model to represent the generalized growth conditions of spherical concretions. This model shows that spherical concretions grow rapidly during the first few years of diagenesis. In particular, carbonate concretions consist mainly of CaCO₃, and their permeability is greatly reduced by cementation and sealing by calcite. As a result, any fossils inside the concretion are well preserved, as water is prevented from penetrating the concretion after its formation. This sealing can provide strong resistance to weathering for more than a million years. Based on this model, we have developed synthetic concretion-forming solvents. To test the effectiveness of these solvents in sealing groundwater flow paths, we conducted an in situ experiment in an underground laboratory in Horonobe, Hokkaido. In the experiment, groundwater flow paths in the excavation damaged zone around an underground gallery were successfully sealed. The experiment showed a decrease in permeability by a factor of 1/100 to 1/1,000 over one year. Here we present a detailed model of the concretion formation process and our conclusions about the sealing process. This sealing process can be applied to activities that require long-term containment of material underground; for example, the geological disposal of nuclear waste and underground carbon dioxide storage. These applications will become increasingly important in the near future.