We present detailed petrographic observations, modal compositions of phenocrysts, and whole-rock major and trace element data (Ba, Rb, Nb, Sr, Zr, and Y) for volcanic rocks of the Kuju Volcanic Group (KVG), northeast Kyushu Island, southwest Japan Arc. We classified these volcanic rocks into three groups based on the mineral assemblage and modal abundance of phenocrysts: (1) a group containing ≥2.9 vol% olivine and lacking hornblende (Ol-g); (2) a group containing hornblende (Hbl-g); and (3) a group containing ≥2.0 vol% orthopyroxene with a small amount of olivine (≤1.3 vol%) and lacking hornblende (Opx-g). In addition, based on the characteristics of plagioclase (Pl) and clinopyroxene (Cpx) phenocrysts, Hbl-g can be subdivided into three types: samples containing abundant Pl with dusty cores are defined as Hbl-g1, whereas those poor in Pl with dusty cores are divided into Cpx-rich Hbl-g2 and Cpx-poor Hbl-g3. The petrographic features of Opx-g, Hbl-g1, and Hbl-g2 indicate they experienced mixing between mafic and silicic magmas, as indicated by the occurrence of Pl phenocrysts with honeycomb and dusty cores, as well as the coexisting olivine and quartz. Fractional crystallization cannot explain the geochemical relationship between Ol-g and the other groups. In addition, the geochemical characteristics of each group show that Opx-g, Hbl-g1, and Hbl-g2 cannot be explained by simple binary mixing between the most mafic Ol-g and most silicic Hbl-g3 samples from the KVG. Furthermore, Opx-g, Hbl-g1, and Hbl-g2 are not related to each other by magma mixing. The variations in Y contents and Sr/Y ratios suggest that Hbl-g1 and Hbl-g3 were derived by partial melting of a subducting slab (i.e., slab melts), and that Hbl-2 may be petrogenetically related to a bajaitic magma that was generated by interactions between slab melt and mantle material. The variations in SiO₂, TiO₂, P₂O₅, and Nb contents, and the trend defined by Opx-g towards the compositional field of high-Nb basalts, similar to volcanic rocks erupted in areas around the KVG, are ascribed to interactions between slab melt and the mantle above the subducting slab.

Ferro-holmquistite with high Fe²⁺ content, which was previously predicted to be unlikely to occur, has been identified from a dyke composed of quartz and microcline in limestone at Mt. Hayamadake, Tamura City, Fukushima Prefecture, Japan. The specimen was collected in the 1980s and had been preserved in a private collection as an unnamed amphibole species; however, its name and features have now been revealed as ferro-holmquistite. The empirical formula, estimated on the basis of the constraints of Si = 8 is (□_0.93Na_0.07)_Σ1(Li_1.97Mg_0.03)_Σ2(Fe²⁺_2.62Mg_0.20Mn_0.18Al_1.75Fe³⁺_0.25)_Σ5Si₈O_22.05(OH_1.96F_0.04)_Σ2 for the platy-crystal sample. The unit-cell parameters obtained from single-crystal X-ray diffraction measurements are a = 18.4080(3) Å, b = 17.7942(3) Å, c = 5.29463(11) Å, and V = 1734.29(6) ų, with placement in the orthorhombic Pnma space group. Comparison of the single-crystal X-ray diffraction results with those for the holmquistite group reveals that the M1 and M3 octahedra expand with increasing Fe²⁺ content. In particular, the M3 octahedron exhibits marked distortion and anisotropic elongation, most notably along the b-axis. Conversely, an increase in the Fe³⁺ content enlarges and distorts the M2 octahedron, while mitigating deformation at the M1 and M3 sites. The tetrahedral chain demonstrates structural flexibility in response to these changes, either by adjustment of the tetrahedral volume and/or alteration in the degree of chain kinking. Through these changes, the low bond valence sums for O4A and O4B, which were once considered the starting point of structural limitation, remain consistently low. As a result, the previously held notion that geometric constraints inhibit the formation of Fe²⁺- and/or Fe³⁺-rich members should be reconsidered.

We report prehnite-forming reactions in scapolite-bearing calc-silicate granulites from the high-grade metamorphic terrane of Lützow-Holm Complex, East Antarctica. Based on textural observations, three possible reactions were identified: 1) Prehnite forming from andradite-grossular garnet, 2) Prehnite formation from anorthite, quartz, and calcite, and 3) Prehnite replacing meionite, quartz, and calcite. Phase equilibrium modeling indicates that prehnite formed under H₂O-rich conditions at T = 320-390 °C and X_CO2 < 0.004. The occurrence of Fe-rich prehnite reflects Al-Fe³⁺ partitioning that occurred during low-grade metamorphism and may serve as an indicator of oxygen fugacity conditions. The prehnite formation reactions exemplify the role of aqueous fluid infiltration during retrograde metamorphism and provide important constraints on changes in fluid conditions from fluid-absent to H₂O-rich.

Zircon U-Pb dating was conducted on granite, granodiorite, and gabbro from the central part of Yamaguchi Prefecture, Southwest Japan. The granodiorite and gabbro are recognized as components of the Susuma-Nagaho Plutonic Complex (SNPC). The zircon U-Pb dating yielded ages of 103 Ma for the granite and 92 Ma for the granodiorite and gabbro. These ages coincide with the magmatic flare-up event documented in Southwest Japan. A compiled dataset of Cretaceous gabbro-diorite from Southwest Japan shows that initial-εNd values (εNdI) decrease progressively to as low as −5.1 between 107 and 90 Ma. In addition, Th/La ratios exhibit a negative correlation with initial-Nd isotopic ratios. These geochemical trends, together with their temporal variation, suggest that subducted sediments increasingly interacted with the mantle wedge, producing progressively enriched mantle sources throughout the Cretaceous. This secular change may reflect variations in plate motion, such as change in the rate of subduction.

We investigated pyrite formation in modern stromatolite formed along the flow path of hot spring water in Fukiagesawa using detailed SEM and TEM analyses. The stromatolite consists of alternating white layers composed of silicified cyanobacteria, and dark layers composed mainly of clay minerals and other clastic particles. Pyrite is present in all layers, but shows considerable variation in frequency, morphology and occurrence depending on location. In the white layers, pyrite is generally scarce, occurring only in small amounts as isolated euhedral crystals and framboids near diatom shells, consistent with a relatively oxidizing environment maintained by cyanobacterial photosynthesis. By contrast, the dark layers are enriched in pyrite, mostly concentrated within pores between clastic particles and showing various morphologies including euhedral and framboidal forms, suggesting that localized sulfate reduction facilitated pyrite formation. The presence of jarosite within some framboids and euhedral crystals, along with the overgrowth of secondary pyrite indicates that reducing and oxidizing conditions alternated within the stromatolite over relatively short timescales.

Petrological and chemical analyses were conducted on garnet-bearing rhyolites of the Darumayama and Shiroyama volcanoes in the Hime-shima Volcanic Group (HVG) on Kyushu Island, southwest Japan arc. In the Darumayama rhyolite, garnet occurs at a frequency of approximately one grain per several thin-sections. It is mostly euhedral and dispersed within the groundmass, and locally exhibits resorption textures. Garnet in the Shiroyama rhyolite occurs as multiple euhedral grains that are dispersed within the groundmass and show no distinct resorption features. The almandine, spessartine, and pyrope proportions of the garnets in the Darumayama and Shiroyama rhyolites are nearly identical, indicative of a common origin. The Shiroyama rhyolite contains a plagioclase-rich aggregate containing garnet, sillimanite, and zircon, indicating a metamorphic origin. The garnet in this plagioclase-rich aggregate has a solid-solution composition identical to those outside the aggregate, indicating that all garnet in the HVG rhyolites has a common origin. The high MnO (5.6-7.0 wt%) and low CaO (0.5-1.2 wt%) contents of the garnet are indicative of metamorphic origin rather than a magmatic origin. These observations indicate that the garnets in the HVG rhyolites are xenocrysts derived from metapelites rather than magmatic phenocrysts.

The identification of dislocations and associated Burgers vectors in minerals is important to discuss the mechanisms of crystal plastic deformation of rocks. Observation of dislocations is normally carried out using transmission electron microscopy (TEM). However, the necessary sample preparation and observation can be challenging in particular for hydrous minerals that are sensitive to beam damage. In addition, the spatial scale at which TEM can be used to observe dislocations is limited, making it difficult to evaluate the process of bulk-rock deformation. Recently, the improved accuracy and indexing rate in Electron Backscatter Diffraction (EBSD) mapping and the ease of analyzing mapping data using a toolkit for analysis provided in programming software have made it possible to observe the microstructural features related to dislocations using EBSD measurements, such as small misorientations less than a few degrees of angular differences within a single grain. In this study, we examine how EBSD observations of natural antigorite-rich serpentinite samples can be used to derive information about the nature of dislocations and associated antigorite deformation mechanisms. In particular, we focused on the effect of different lengths in the crystal undulations, expressed as M- and m-values, of antigorite on the deformation processes and the resulting crystal preferred orientation (CPO) patterns of antigorite in antigorite schist samples. The resulting antigorite CPOs are all like the B-type CPO patterns that have been widely reported from natural antigorite schists regardless of the wavelength of the curved antigorite crystals. However, misorientation analyses using the EBSD maps suggest dislocations characteristic of the [100](001) slip system, responsible for A-type CPO formation, are more common in antigorite-rich serpentinites with shorter wavelengths, whereas dislocations characteristic of the [hk0](001) slip systems, responsible for G-type CPO formation, are more common in those with longer wavelengths. Therefore, the dislocation microstructures observed in this study do not provide evidence that the B-type CPO was formed by dislocation creep. However, these dislocation microstructures may preserve evidence of different deformation stages before and after the formation of the B-type CPO. This implies that, as M- and m-values tend to decrease under high-pressure conditions, A- and G-type CPOs are likely to form in deeper and relatively shallower domains, respectively. These results suggest the strength of schistosity, grain shape, and CPO strength in ductilely deformed antigorite-rich serpentinites may be affected by differences in the dominant deformation mechanisms and dislocation microstructures influenced by the wavelength of antigorite.

Arsenotučekite (Ni₁₈Sb₃AsS₁₆), a rare mineral of the hauchecornite group, is newly identified from carbonated serpentinite and schistose listvenite in the Median Tectonic Line (MTL) shear zone in eastern Ehime Prefecture, Japan. This is the second occurrence after the type locality and the first report from outcrop samples. At the Urayama River, arsenotučekite occurs in carbonated serpentinite and in schistose listvenite, and at the Ichiba River it occurs in schistose listvenite. Arsenotučekite is associated with millerite in antigorite, magnesite, and dolomite in carbonated serpentinite, and in quartz and dolomite in schistose listvenite. The chemical compositions are (Ni_16.08Fe_1.61Co_0.23)(Sb_3.47As_0.80)S_15.81 (carbonated serpentinite, Urayama River), (Ni_16.20Fe_1.14Co_0.57)(Sb_3.50As_0.60)S_15.98 (schistose listvenite, Urayama River), and (Ni_15.84Fe_1.73Co_0.28)(Sb_3.55As_0.59)S_16.02 (schistose listvenite, Ichiba River). The unit-cell parameters of arsenotučekite in carbonated serpentinite from the Urayama River are a = 10.184(4), c = 10.758(6) Å, and V = 1115.8(8) ų. The a-axis length is markedly larger than that reported from the type locality. The mineral paragenesis indicates that arsenotučekite formed in relation to the carbonation of serpentinite and pelitic schist by CO₂-rich slab-derived fluids.

Continental fragments with Australian or Gondwanan affinities are widely distributed in eastern Indonesia. Metamorphic rocks intruded by Permo-Triassic granitoids have long been considered remnants of Paleozoic basement. However, recent geochronological studies in eastern Indonesia have revealed Mesozoic metamorphism with Cenozoic overprints, challenging previous interpretations. The Banggai-Sula microcontinent (BSM), a prominent continental fragment at the eastern tip of Sulawesi, preserves such Paleozoic basement rocks. Although the sedimentary cover sequences of the BSM are well studied, the metamorphic history of its Paleozoic basement remains underexplored. This study integrates petrography and monazite U-Th-Pb geochronology to investigate the metamorphic evolution of the BSM. In contrast to the Cenozoic resetting observed in neighboring continental fragments, this study reveals multiple tectonothermal episodes from the Mesoproterozoic to the Permian. Garnet-sillimanite-biotite gneiss records the earliest metamorphism at ∼ 1500 and 1400 Ma, consistent with major tectonothermal events in the North Australian Craton, suggesting a Gondwanan affinity. Regional Ordovician-Silurian metamorphism (∼ 460-433 Ma) occurred under medium P-T conditions. This metamorphism is linked to the eastern continuation of the Proto-Tethyan oceanic subduction zone from south of the Tarim Block along the northern Gondwana margin. Devonian-Carboniferous metamorphism (∼ 360 Ma) correlates with the evolution of the Tasman Orogen in eastern Australia, followed by Permian-Triassic granitic intrusion (∼ 260 Ma) representing an extension of continental arc magmatism from eastern Australia into New Guinea and the Sula Spur. Comparable tectonothermal histories in the Bird’s Head region of New Guinea indicate a shared tectonic history since at least the Devonian. These results establish the BSM as a fragment of the North Australian Craton crust that underwent significant reworking during the Paleozoic-Triassic evolution of the Gondwana margin.

Fluid behavior during grain growth was observed in real time using a simple technique. Camphor as a rock analogue was ground in a mortar with ∼ 10 vol% ethanol (a fluid phase). A total of ∼ 5 mg of the mixture was pressed onto a glass slide, and the textual development of the dense aggregate was observed at room temperature under a polarizing optical microscope. Videos were taken, and snapshots were analyzed. The mean initial size of the camphor grains was 7.0 µm with 2.6 area% of ethanol. The sizes of fluid inclusions, expressed as the diameter of a circle with the same area, were ≤0.6-4.0 µm with a mode of 0.8 µm. During the growth of camphor grains, three distinct types of fluid behavior were observed: (1) the coalescence of neighboring fluid inclusions along a migrating grain boundary, (2) an inclusion originally at a grain boundary being left behind into the grain interior, and (3) the incorporation of a fluid inclusion from a grain interior into a migrating grain boundary. After ∼ 1 h of the experiment, the mean grain size had increased to 19.8 µm. The total area of the fluid inclusions in the viewing area did not change, whereas the total number of inclusions decreased. In addition, the size of the inclusions increased to ≤0.6-5.0 µm with a mode of 1.6 µm due to the coalescence of fluid inclusions. This study provides insights into the redistribution of fluid inclusions during textural development relating to grain growth, which can occur in the Earth’s crust where water is the dominant fluid phase.

We investigated the synthesis conditions of metastable α-cristobalite using amorphous silica (silica-gel) with and without V₂O₅ as a flux. Secondary electron microprobe observations for synthesized samples with flux revealed the formation of faceted α-cristobalite crystals (2-19 µm), whereas those synthesized without flux did not exhibit a clear facet. Powder X-ray diffraction analysis showed that the addition of flux lowered the formation temperature of metastable α-cristobalite to 1000 °C, which contrasts with the fact that the crystallization occurred at 1200 °C in the case without flux. These observations demonstrate that the V₂O₅ flux alone promotes crystallization by lowering the activation energy of the crystallization process of metastable cristobalite as well as other kinds of flux materials.

New allanite-group minerals, akasakaite-(Ce), akasakaite-(La), vanadoakasakaite-(Ce), and vanadoakasakaite-(La), associated with small quartz lenses within rhodonite-enriched rocks of the stratiform manganese deposits at the Mogurazawa mine, Gunma, Japan, were studied using electron microprobe analysis, Raman spectroscopy, and single-crystal X-ray diffraction methods. Akasakaite is defined by the ideal formula, ^A1Ca^A2REE^3+M1Me^3+M2Al^M3Mn²⁺(SiO₄)(Si₂O₇)O(OH) (Z = 2, space group P2₁/m). The four new minerals are distinguished by the dominant cation at the M1 site (Al or V³⁺) and the dominant REE³⁺ at the A2 site (Ce or La). The unit-cell parameters are a = 8.885-8.925, b = 5.694-5.744, c = 10.100-10.153 Å, β = 113.62-113.79°, and V = 468.0-476.4 ų. Structure refinements converged to R1 values of 2.20-2.86%. At the M1 site, Al content reaches 85% in akasakaite-(La), whereas V³⁺ reaches 82% in vanadoakasakaite-(La). The cation assignments for these specimens are as follows: ^A1Ca_0.54Mn²⁺_0.46, ^A2La_0.53Ce_0.18Nd_0.05Pr_0.02Sr_0.04Ca_0.18, ^M1Al_0.85V³⁺_0.13Ti_0.02, ^M2Al_1.00, and ^M3Mn²⁺_0.75Fe_0.01V³⁺_0.16Al_0.07 for akasakaite-(La) and ^A1Ca_0.60Mn²⁺_0.40, ^A2La_0.45Ce_0.17Nd_0.06Pr_0.03Sr_0.15Ca_0.14, ^M1V³⁺_0.82Al_0.18, ^M2Al_0.64V³⁺_0.33Ti⁴⁺_0.03, and ^M3Mn²⁺_0.71V³⁺_0.26Al_0.02 for vanadoakasakaite-(La). All studied specimens are enriched in Mn²⁺, which predominates at the M3 site and also occupies over 40% of the A1 site. The unit-cell dimensions of vanadoakasakaites are larger than those of akasakaites. This variation appears to be attributed mainly to the elongation of <M1-O> caused by the substitution of Al for V³⁺ at the M1 site. Moreover, the expansion of the M3 site by the substitution of divalent octahedral cations, such as Mn²⁺, elongates the M3-O8 distance, which shortens the A2-O8 distance despite no cation substitution at the A2 site. This suggests that the variation in the akasakaite structure is largely influenced by cation substitution at the M3 and M1 sites. The topological change of the A1 polyhedra due to Mn²⁺ substitution for Ca at the A1 site, as reported in previous studies, was also observed.

Quartz occurs ubiquitously in felsic plutons. The development of a methodology to reveal the quartz growth process in a granitic body provides essential insights into magma chamber processes. Cathodoluminescence (CL) characterization combined with Ti concentration of quartz crystals in granite is a prevalent tool for identifying the crystal growth and crystallization temperature of quartz in granitic magmas. This study focused on quartz crystal growth in the Kuki granite (KKG), northeast Japan, and Kurobegawa granite (KBG), central Japan. In the KKG quartz crystals, the difference in luminescence corresponds to Ti concentration: high luminescence corresponds to high Ti concentrations, which is consistent with our previous studies on the Toki and Okueyama granites. In the KBG quartz crystals, the Ti concentrations do not correspond to the luminescence level, whereas the Al concentrations are negatively correlated with the luminescence level. Therefore, quartz crystallizations in KKG and KBG are characterized by Ti- and Al-diffusion-controlled growth, respectively. Using the difference in the time-temperature history among the target rock bodies, this study provides insight into the nature of the relationship between quartz crystallization and magma chamber processes. Quartz crystallizations in the KKG were accompanied by gradual variations from oscillatory zonation through gradual zonation to homogeneous CL patterns with decreasing temperature in the cooling magma chambers. The quartz internal structure of the KBG was resulted from the rapid cooling of the KBG magma and the scarcity of Ti and enrichment of Al in the magma during quartz crystallization.