Editorial Message

Herein, an in situ high–temperature synchrotron X–ray diffraction study of gypsum is performed in the temperature range of 30–200 °C to investigate the continuous structural change from gypsum to soluble anhydrite through hemihydrate. Thermogravimetric and differential thermal analysis curves reveal that dehydration occurs in two stages. The diffraction peaks of β–hemihydrate with the trigonal space group P 3₁21 gradually become sharper above 90 °C, whereas those of gypsum become less intense and cannot be distinctly observed at 160 °C. The CaO₈ dodecahedra and SO₄ tetrahedra in gypsum expand negligibly with temperature. The site occupancy parameter of the water oxygen (Ow) atom in gypsum remains at approximately 1.0, within the experimental error. When water molecules are lost from gypsum, it immediately transforms into β–hemihydrate, without maintaining its structure. The volumetric thermal expansion coefficient of gypsum is 1.31 × 10⁻⁴ K⁻¹. The site occupancy of Ow in β–hemihydrate continuously decreases from 0.8 and reaches approximately 0.5 at temperatures of 130–140 °C, where soluble anhydrite with a hexagonal space group P 6₂22 begins to form. Therefore, β–hemihydrate dehydration can be translated by the chemical formula CaSO₄·xH₂O (0.5 ≤ x ≤ 0.8). The volumetric thermal expansion coefficient of β–hemihydrate, determined at temperatures between 90 and 140 °C is 1.54 × 10⁻⁴ K⁻¹. β–Hemihydrate coexists with soluble anhydrite above 140 °C; however, the amount of β–hemihydrate decreases with temperature. In β–hemihydrate, water molecules are continuously released from the CaO₉ tetradecahedra, thereby resulting in its contraction. Consequently, the structural change to a smaller CaO₈ dodecahedron triggers its transformation into soluble anhydrite without the collapse of its one–dimensional linear chains. With further heating, β–hemihydrate completely transforms into soluble anhydrite at 170 °C. The volumetric thermal expansion coefficient of soluble anhydrite determined in the temperature range of 170–200 °C is 1.69 × 10⁻⁵ K⁻¹, which is an order of magnitude smaller than the values of gypsum and β–hemihydrate.

Granitic and gabbroic xenoliths have been found within dacitic lavas in the Hime–shima volcanic group (HVG) of northeastern Kyushu, Japan. The HVG is located near the boundary between the Ryoke and Sangun belts, suggesting that the HVG and associated crustal xenoliths may provide insights into the subsurface distribution of the Ryoke and Sangun belts in Kyushu. This study focuses on xenoliths obtained from the coastal boulders near the Kane Lava of the HVG. The HVG xenoliths consist of gabbro, gabbroic diorite, amphibolite, gneiss, basaltic andesite, and tuffaceous sandstone, with the latter two types resembling those found in the basement rocks of the HVG. The gabbroic xenoliths are geochemically similar to gabbros in the Ryoke belt. The U–Pb dating for zircon in the gneiss xenoliths yielded a metamorphic age of ~ 111 Ma with Th/U values <0.1, similar to the age obtained for metamorphic rocks in or of the Ryoke belt. The new data presented in this study indicate that the xenoliths in the HVG were derived from basement units associated with the Ryoke belt, which in turn, means that the HVG is tectonically underlain by the Ryoke belt. This also suggests that the Ryoke belt extends further north in Kyushu than was previously considered, as implied by the presence of this belt directly below the HVG.

Oxyyttrobetafite–(Y) is the first member in the betafite group of the pyrochlore supergroup found in albite–rich pegmatite from Souri Valley, Komono, Mie Prefecture, Japan. This new mineral occurs as small anhedral grains with sizes of 20 to 200 µm in cylinder–shaped aggregates with a substrate of thalénite–(Y) and synchysite–(Y). Small amounts of aeschynite–(Y), thorianite, and thorite are also associated in the same occurrence with oxyyttrobetafite–(Y), and gadolinite–(Y) is also included at the boundary between the aggregate and albite. The physical properties are: brown in color, brittle, transparent, non–fluorescent, vitreous luster, white streak with a Mohs hardness of 5, and a calculated density of 5.54 g·cm⁻³. Oxyyttrobetafite–(Y) is an optically isometric material with brown color under the microscope with a refractive index of n = 2.3 calculated using the Gladstone–Dale relationship. The empirical formula of oxyyttrobetafite–(Y) calculated on the basis of B = 2 with A₂B₂X₆Y composition is (Y_1.58Dy_0.13Yb_0.07Er_0.06Tm_0.05Gd_0.04Ho_0.03Sm_0.02Tb_0.02Eu_0.01Lu_0.01)_Σ2.02
(Ti_1.85Ta_0.09Fe_0.05Sn_0.02Nb_<0.01)_Σ2O_7.05 and leads to the ideal formula of Y₂Ti₂O₆O, which requires TiO₂ 41.44 wt% and Y₂O₃ 58.56 wt%, total 100 wt%. The structure is isometric cubic with the space group Fd3m and unit cell parameters of a = 10.11090(10) Å, V = 1033.64(3) ų, and Z = 8 by single crystal X–ray diffraction measurements. The seven strongest peaks in the powder X–ray diffraction pattern [d in Å (I/I₀) hkl ] were 2.918(100) 222, 2.527(18) 400, 2.321(13) 331, 1.788(53) 440, 1.525(46) 622, 1.162(13) 662, and 1.033(9) 844 with unit cell parameters of a = 10.121(3) Å, V = 1036.6(9) ų, and Z = 8. The crystal structure was refined to R₁ = 0.018 for 159 observed reflections with the criteria of I > 2σ (I ). Oxyyttrobetafite–(Y) is characterized by Y dominance at the A sites, Ti dominance at the B sites, and O dominance at the X and Y sites in the A₂B₂X₆Y pyrochlore–type formula.

The origins of eclogite associated with garnet peridotite in continent–continent collision belts are still debated. We performed petrological studies of eclogites collected from a garnet peridotite block from Nové Dvory in the Gföhl Unit of the Moldanubian Zone in the Variscan orogenic belt, Czech Republic. The eclogite was divided into three types: one kyanite (Ky)–bearing and two Ky–free types. Garnet and omphacite in the Ky–bearing eclogite have lower Fe contents than those in the Ky–free eclogite. Furthermore, the Ky–free eclogite was divided into two types on the basis of Ca content in garnet: Ca–rich (X_grs > 0.32) and Ca–poor (X_grs < 0.32) types, except for Ca–poor rim compositions. Application of conventional geothermobarometers to the Ky–bearing type and the Ky–free type with Ca–rich garnet yielded similar pressure–temperature (P–T) conditions (3.2–4.8 GPa and 920–1160 °C) to those of previous studies, whereas the Ky–free type with Ca–poor garnet yielded slightly lower P–T conditions (3.1–3.4 GPa and 950–990 °C) than the other two types. The observed chemical variation of garnet is probably due to the difference in origins, whereby, according to our new results and previous findings, the Ky–bearing eclogite was derived from plagioclase–bearing crustal gabbro, whereas the Ky–free eclogite with Ca–rich garnet was derived from a crystal cumulate possibly in the mantle wedge. In the Ky–free eclogite samples with Ca–poor garnet, chemical compositions of garnet and omphacite are different from those in the other Ky–free samples, and those samples with Ca–poor garnet would have a different origin from the others. One of the Ky–free type with Ca–poor garnet retains garnet grains with chemical zonings probably created during the prograde history, although the eclogite underwent the extremely high temperature (~ 1000 °C) metamorphism. Calculated diffusion distances in garnet reach 0.5 mm during 2 million years, even if we adopted a low value of diffusion coefficient data. Thus, the residence time of the eclogite at the peak meatamorphic conditions would have been shorter than at least 2 million years. The subsequent decompression and cooling after the peak metamorphism were also probably very fast to avoid the chemical homogenization of garnet.

The compositional dependence of the intensity and electric field gradient (EFG) tensors for Fe²⁺ at the M1 sites, Fe²⁺ at the M2 sites, and Fe³⁺ at the M1 sites in Ca–rich pyroxene were obtained from Mössbauer spectra of crystallographically oriented single–crystal thin sections of four Ca–rich natural pyroxenes. Components of the intensity tensor (I_XX, I_YY, I_XY, and I_ZZ) for Fe²⁺ at the M1 sites in Wo₅₀ Ca–rich pyroxene were almost the same; the averages of the I_XX, I_YY, I_XY, and I_ZZ components were 0.342, 0.477, 0.039, and 0.681, respectively. Intensity tensor for Fe²⁺ at the M1 sites of Wo₄₀ Ca–rich pyroxene was different from the Wo₅₀. Whereas principal axes of EFG tensors for Fe²⁺ at the M1 sites of Wo₅₀ pyroxenes had the same direction, principal axes of V_XX and V_YY of EFG of Wo₄₀ had different directions from that from Wo₅₀. The difference of intensity and EFG tensors between Wo₅₀ and Wo₄₀ suggests that the intensity and EFG tensors for Fe²⁺ at M1 sites are dependent on the Ca contents and are independent of Fe contents. Some intensity and EFG tensors for Fe²⁺ at the M2 sites and Fe³⁺ at the M1 sites in Ca–rich pyroxene were also obtained. However, the compositional dependence of the intensity and EFG tensors has yet to be clarified, because the number of examples is insufficient.

This paper reports the first outcrop occurrence of an ultrahigh–temperature (UHT) metamorphic rock from the Sør Rondane Mountains (SRM), East Antarctica. A pelitic gneiss from Balchenfjella, eastern SRM, contains mesoperthite that gave UHT condition (>900 °C) by ternary feldspar thermometry. The UHT mesoperthite is present both in the matrix and as an inclusion in garnet. The garnet also has nanogranitoid inclusions next to the mesoperthite, which are interpreted to be an UHT melt. The re–integrated nanogranitoid composition is plotted in the primary phase region of quartz and classified as granite. Even crystallized nanogranitoids can provide appropriate original melt composition in the An–Ab–Or and Qz–Ab–Or spaces, whereas Mg concentration is enriched due to local retrograde Fe–Mg exchange reaction between the nanogranitoid inclusions and the host garnet. Although metamorphic rocks in the SRM are highly retrogressed, this study revealed that the microstructural evidence of UHT condition is partially preserved. Further investigation of timing and areal extent of UHT metamorphism helps us to understand the tectonic model of the SRM.

Mineralogical investigation of platinum–group minerals (PGM) from the placer deposit in northwestern Hokkaido, Japan, was conducted at six rivers and two coasts covering an area of 70 km north–south and 30 km east–west: the Moshosanbetsu River (M), the Shosanbetsu River (S), the Ainusawa River (A), the Tomamae coast (T), the Obira coast (OC), the Kamikinenbetsusawa River (K), the Obirashibe River (OR), and the Numatapon River (N) from north to south. Details of the major PGM grain and the occurrence of small but diverse PGM inclusions were revealed in this study. Among diverse PGM inclusions, Cu₃Pt mineral was discovered in PGM placer from the coast of Tomamae town, and it has been approved as a new mineral, tomamaeite, named after the type locality, by the International Mineralogical Association, the Commission on New Minerals, Nomenclature and Classification (IMA–CNMNC). Later, tomamaeite was also discovered in five localities. Tomamaeite occurs in Pt–Fe(Cu) alloys such as tulameenite, ferronickelplatinum, tetraferroplatinum, and hongshiite as an anhedral particle with a size of less than 20 µm. Tomamaeite is an opaque mineral and has metallic luster with a pale mist white color in reflected light. The hardness of tomamaeite has yet to be determined, although it is estimated to be ~ 3½ from comparison with compositionally related minerals and the calculated density is 12.4 g·cm⁻³ using the empirical formula and powder X–ray diffraction data. The empirical formula of tomamaeite calculated on the basis of 4 apfu is (Cu_3.01Fe_0.06Ni_0.01)_Σ3.08(Pt_0.90Ru_0.01Rh_<0.01Pd_<0.01Os_<0.01Ir_<0.01)_Σ0.93, (Cu_2.92Fe_0.05Ni_0.04)_Σ3.01(Pt_0.97Pd_0.01Rh_0.01)_Σ0.99, (Cu_2.86Sb_0.13Fe_0.03Ni_0.02)_Σ3.03(Pt_0.92Ir_0.02Pd_0.01Os_0.01)_Σ0.97, (Cu_2.71Sb_0.19Fe_0.02Ni_0.02)_Σ2.94(Pt_1.01Ir_0.05)_Σ1.06, (Cu_2.93Fe_0.02)_Σ2.94(Pt_1.04Rh_0.01)_Σ1.06, and (Cu_2.82Fe_0.04Ni_0.04)_Σ2.90(Pt_1.07Ir_0.03Pd_<0.01)_Σ1.10 from the Tomamae coast, the Moshosanbetsu River, the Shosanbetsu River, the Ainusawa River, the Kamikinenbetsusawa River, and the Numatapon River, respectively. Crystal chemistry was investigated using tomamaeite from the Tomamae coast. Tomamaeite is cubic, Pm3m, with lattice parameters a = 3.683(2) Å and V = 49.97(7) ų (Z = 1) of Cu₃Au–type structure, in which Pt occupies the position of origin, and Cu occupies the face–centered positions on a face–centered cubic lattice. PGM from northwestern Hokkaido probably have a mostly common origin and are characterized by depleted ultramafic rocks, and tomamaeite is a non–unique mineral that is formed during the universal post–magmatic process with alteration of such ultramafic rocks to serpentine.

We have found corundum in direct contact with quartz and biotite and as inclusions in garnet in the pelitic gneisses of northern Austkampane in the Northeastern (NE) Terrane of the Sør Rondane Mountains (SRM), East Antarctica. Our samples, which include corundum–bearing gneisses, show petrographic features such as staurolite inclusions in garnet, compositional zoning of orthoamphibole with Al decreasing toward the rims, and late–stage cordierite formation, and these features are characteristic of a clockwise P–T trajectory. The observations are consistent with the proposed regional clockwise P–T evolution of the NE Terrane in the SRM. The corundum and other inclusions observed in the garnet porphyroblasts are interpreted to have formed owing to either staurolite breakdown or metastable crystallization relative to kyanite prior to the peak metamorphism. The close association of biotite and quartz surrounding corundum inclusions suggests fluid– or melt–related processes. These petrographic features imply that the corundum and quartz (rarely observed in high–grade metamorphic rocks) formed as a result of metastable crystallization during the prograde stage of the clockwise P–T evolution of a continental collision zone.

A new type of compact deep–UV micro–Raman spectroscopy system was developed with a single monochromator, front–illuminated cooled charge–coupled device, and 266 nm nanosecond pulsed laser to overcome laser–induced fluorescence from surrounding minerals and organic material. Deep–UV micro–Raman spectroscopy is particularly useful in analyzing the fluorescence–free Raman spectra of dispersed low–maturity carbonaceous material and coal, although deep–UV excitation lasers may cause serious degradation and laser–induced heating of the sample surface, especially in microanalysis. The laser–induced damage threshold for fully ordered graphite and coal (VR_r = ~ 0.5%) was assessed to facilitate the acquisition of accurate Raman spectra with a spot size of ~ 1 µm. For fully ordered graphite, there was no serious degradation of the sample surface with an energy fluence of 0.10–2.50 J cm⁻². Some sample surfaces became black at higher fluences of 1.96–2.50 J cm⁻², suggesting irreversible damage by deep–UV lasers. The Raman shift of the G band after measurement involves a downshift of 1.7–7.4 cm⁻¹ relative to other spectra obtained at low laser fluences of <0.34 J cm⁻². The G band full width at half maximum (FWHM) also increased with increasing laser fluence. Serious degradation of polished coal surfaces occurs at even lower laser fluences of 0.34–2.50 J cm⁻². The degree of change in Raman parameters such as the D and G band FWHM depends on the laser fluence during measurements. Heating and damage by a deep–UV laser is greater than that by visible lasers. Laser fluences of <0.16 and 0.34 J cm⁻² are required for accurate Raman analyses of dispersed carbonaceous material in sedimentary rocks and fully ordered graphite in metasediment, respectively.

This study presents the use of petrographic plagioclase alteration indicators as a new method for quantitatively evaluating the extent of plagioclase alteration within granites, using the Toki granite, central Japan, as an example. The new petrological indicator enables us to discuss the similarities/differences in the extent of alteration within a rock body and between rock bodies. Alteration indicators and areal fractions of microvoids in the plagioclase grains were obtained through the analysis of backscattered electron images. The volume of the micropores in the altered plagioclase was estimated using the areal fraction of microvoids in the grains. The plagioclase alteration indicators were obtained as the ratio between the alteration product domain and the original plagioclase domain. We found positive correlations between the plagioclase alteration and biotite chloritization indicators presented in Yuguchi et al. (2021), indicating that each alteration indicator can be used independently as a representative value for the sample. The positive correlations between the areal fraction of microvoids in the altered plagioclase and the alteration indicator in the samples and petrographic observations indicated the following: 1) the altered plagioclase contains the incipient micropores and the alteration micropores, 2) the incipient micropores, which were caused by the dissolution of plagioclase during the incipient stage of plagioclase alteration, acted as a pathway of hydrothermal fluid within the plagioclase, resulting in alteration progress, and 3) the hydrothermal alteration resulted in the production of new alteration micropores. In the Toki granite, the progress of plagioclase alteration is essentially dominated by the progress of biotite chloritization. The progress of biotite chloritization essentially influenced the progress of plagioclase alteration.

The Permo–Triassic was a crucial interval for the Changchun–Yanji suture zone in NE China since it witnessed the late–stage tectonic evolution of the Paleo–Asian Ocean (PAO). Thus, provenance analysis of Triassic sedimentary rocks in this region can shed light on the eventual closure process of the PAO. In this study, we carried out petrological, geochemical, and detrital zircon U–Pb geochronological research on two (meta)–sedimentary rocks from the Yitong–Gongzhuling area located in the westernmost section of the Changchun–Yanji suture zone. LA–ICP–MS U–Pb ages of detrital zircons from the two samples indicate a middle Triassic maximum depositional age (~ 240 Ma). The two samples present a most significant Permo–Triassic (299–240 Ma) detrital zircon population that corresponds to the multi–phase subduction and collision–related magmatism along the Changchun–Yanji suture zone. In addition, a late Silurian–early Devonian (429–390 Ma) detrital zircon population is also present and presumably fed from a pre–existing accreted arc terrane along the northern margin of the North China Craton (NCC). In contrast to some previously reported Permo–Triassic sedimentary rocks in adjacent areas, our new results show an absence of ~ 2.5 and ~ 1.8 Ga ages characteristic of NCC magmatism or metamorphism. The results imply that the Yitong–Gongzhuling area probably has a higher relief than the adjacent area during the middle Triassic, preventing the deposition of materials from the NCC to the south. This is further evidenced by the proximal orogenic source, short–distance transportation, and rapid accumulation for the studied samples as indicated by the subhedral to anhedral and poor roundness morphology of detrital zircons. The inferred regional uplift was hypothesized to be caused by the continued transpression during the PAO final closure based on the widely distribute early–middle Triassic syn–collisional granitic rocks along the Changchun–Yanji suture zone.

The metamorphic complex from the Mt. Tenzan area in northern Kyushu consists mainly of mafic rocks with small amounts of siliceous, calc–silicate, and ultramafic rocks. These lithofacies can be recognized as an ophiolitic complex. Metamorphosed mafic rocks are divided into two types, amphibolites I and II, which are probably derived from supracrustal and intrusive rocks, respectively. The geochemical data of both amphibolites plotted within the field between mid–ocean ridge and island arc basalts; such geochemical features resemble those of back–arc basin basalts. As the metamorphic complex was intruded by Cretaceous granitoids, protoliths of the complex could have been formed prior to the Cretaceous. The protolith lithofacies assemblage and geochemical constraints of the Tenzan metamorphic complex indicate the correlation with the Yakuno ophiolite rather than the Oeyama ophiolite.

Groundmass pyroxene crystals in pumice from the 1914 eruption of Sakurajima in Japan show varied combinations of crystallographic faces (i.e., ‘tracht’). To investigate whether the groundmass pyroxene tracht depends on magma decompression conditions, we performed isothermal single–step decompression experiments on hydrous Sakurajima dacite magma. The magma was held under water–saturated conditions at 920 °C, 120 MPa, and oxygen fugacity conditions no more oxidizing than one log unit above Ni–NiO equilibrium for 24 h. Then, a control experiment was immediately quenched, whereas others were decompressed to final pressures of 20, 10, or 5 MPa and held for 3 h before quenching. Groundmass pyroxenes in the control experiment and that quenched at 20 MPa showed octagonal shapes, whereas those decompressed to lower pressures characteristically had hexagonal shapes. Some pyroxenes in the 20 MPa experiment were hexagonal near plagioclase crystals because plagioclase crystallization locally increased the supersaturation of pyroxene in the melt. We conclude that the tracht of groundmass pyroxenes changes from octagonal to hexagonal as the degree of effective undercooling increases and thus reflects the decompression history of a magma during its ascent in a volcanic conduit.

To determine the phase boundary between diaspore (α–AlOOH) and δ–AlOOH, in situ X–ray diffraction experiments were carried out using a multi–anvil high–pressure apparatus and synchrotron X–ray. The stability of each phase was determined by observing the change in powder X–ray diffraction patterns. The equilibrium phase boundary is described by the formula P (GPa) = 12.2 (±4.9) + 0.0027 (±0.0044) × T (K). The boundary determined in this study is located at a lower pressure than that estimated by previous quenching experimental studies.

Here we first report the in situ U–Pb dating of metamorphic grossular garnet (Grs) with distinction between internal zonation textures. The studied Grs occurs in an eclogite–facies marble collected from the eastern Iratsu body of the Sanbagawa metamorphic terrane, Japan. The Grs has a patchy texture, predominantly with pure Grs cores and andradite (Adr)–rich rims formed during eclogite–facies and exhumation stages, respectively. The U–Pb ages for the Grs core and Adr–rich rim were 97 ± 10 and 106 ± 16 Ma (95% confidence level), respectively. Despite the compositional zoning formed under different P–T conditions, the U–Pb ages of the core and rim were in similar values within analytical uncertainties. This decoupling of chemical zonation and U–Pb ages implies that the U–Pb chronological signatures of rims were inherited from cores owing to the redistribution of radiogenic Pb in cores during the rim formation through fluid–mediated dissolution and reprecipitation. The Grs U–Pb age (97 ± 10 Ma) thus directly corresponds to previously reported P–T conditions of the core formation during the eclogite–facies metamorphism. This advantage of Grt petrochronology as the combination of radiometric ages obtained by in situ analysis and P–T conditions deduced from paragenesis can contribute to reconstruct reliable metamorphic histories.