K-Ar dating of volcanic rocks from northern vicinity of Matsukawa geothermal field was carried out. Ages of 0.85±0.04 Ma and 0.58±0.10 Ma for Shimokura-Nakakura volcano and 0.72±0.03 Ma for 1470m peak lava were obtained. The ages of Shimokura-Nakakura volcano are correlated with the early stage of advanced argillic alteration around Matsukawa geothermal field. The contemporaneity of hydrothermal and magma activities in the area implies the volcanic-hydrothermal activity in the core of Shimokura-Nakakura stratocone. The new dating of this study, together with the previously reported K-Ar ages and the geological studies, suggests distinct temporal change in alignments of volcanic centers in Sengan geothermal field at 0.6 Ma. The change was caused by outspread of volcanic activity from upheaval zone, which is bounded by reverse faults, to the outward zone. The volcanic alignment older than 0.6 Ma is located on the upheaval zone and alignment direction is transverse with regional maximum horizontal stress direction. On the other hand, younger volcanic alignments are located outside the upheaval zone and parallel to the regional stress direction, except for Akita Komagatake which is younger than 0.6Ma but located on the upheaval zone.

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Volcanic rocks from Daito-dake area, Miyagi prefecture, represent early Pleistocene volcanism predating Zao and Funagata volcanoes. Stratigraphical and petrological study of these volcanic rocks revealed that they were derived from three sources. After the formation of late Tertiary Anadozawa lake deposits, but partly overlapping then, volcanism belonging to subaqueous lava domes became active. These lava domes accompany hyalloclastites. Banjiiwa Volcanics, consist of lavas, pyroclastic rocks, and dikes. The lower part shows evidence of subaqueous erupting and deposition, such as concentric pillows, pillow lobes, lava lobes. Toward the end of Banjiiwa activity, volcanism of Daito-dake, Kamuro-dake and Senohara-yama started, which consists of lavas and pyroclastic rocks. After the Banjiiwa volcanism, but during the same period as Kamuro-dake, Daito-dake volcanism started.
Volcanic rocks from subaqueous lava domes are made of dacite, whereas those from Banji-iwa, Daito-dake, Kamuro-dake and Senohara-yama change composition from basaltic andesite to dacite with time, and include both pigeonitic rock series and hypersthene rock series. Most volcanic rocks from Kamuro-dake are similar to Banjiiwa. These rocks of pigeonitic rock series and hypersthene rock series also show different chemical compositions in variation diagrams and FeO*/MgO-SiO₂ plot.

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Takatumayama, amakazari, hiuchiyama, Hokogatake and Nannbayama are distributed on the Nishikubiki uprift zone, northern Fossa Magna (Hayashi, 1997; Oba and Hayashi, 1998; Yahata and Oba, 2000; Sato and Oba, 2002; and Endo and Oba, 2002) The intrusive activities of these hypabyssal rocks are Pliocene-late Pleistocene. The Nishikubiki hypabyssal rocks are divided with three types as follows:Clinopyroxene-orthopyroxene-hornblende porphyrite, orthopyroxene-hornblende porphyrite or porphyry, and hornblende porphyrite or porphyry. In the Nishikubiki hypabyssal rocks, orthopyroxene-hornblende porphyrite or porphyry, and hornblende porphyrite or porphyry are common, Clinopyroxene- orthopyroxene-hornblende porphyrite are found in Nanbayama. Oba and Hayashi(1998, 2000) reported that the K-Ar age is 4.9Ma-3.08Ma. The K-Ar ages of Hokogatake and Takatumaya are 1.5Ma-1.0Ma, These from Amakazari and Hiuchi are 1.6Ma and 1.0Ma-2.0Ma, respectively. SiO2 of volcanic rocks from Nanbayama is 54-62%, on the other hand SiO2 of other hypabyssal rocks is 60-70%. K2O content of Nannbayama porphyrite is high, while K2O content of Hokogatake porphyry is low. The most rock of the Nishikubiki are calc-alkali rocks except for the part of Nanbayama.

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In order to understand the timescale of the evolution of shallow plumbing system beneath active volcanoes at subduction zones, we investigated U-series disequilibria of lavas erupted in the last 500 years (1469AD – 1983AD) at the Miyakejima volcano, Izu arc, Japan. Miyakejima lavas of this period have ²³⁸U-²³⁰Th-²²⁶Ra disequilibria enriched in ²³⁸U and ²²⁶Ra, which are due to the addition to the mantle wedge of slab-derived fluids (Yokoyama et al., 2003). These lavas show 1) a trend almost parallel to the equiline in a (²³⁰Th/²³²Th)-(²³⁸U/²³²Th) diagram, and 2) positive linear correlation in (²³⁸U/²³⁰Th)-1/Th and (²²⁶Ra/²³⁰Th) diagrams. Fractional crystallization cannot produce these trends. Instead, magma mixing between differentiated magma having lower (²³⁸U/²³⁰Th) and (²²⁶Ra/²³⁰Th)₀ ratios and higher Th content (andesitic end-member magma: AEM), with less-differentiated magma having higher (²³⁸U/²³⁰Th) and (²²⁶Ra/²³⁰Th)₀ ratios and lower Th content (basaltic end-member magma: BEM), is expected. However, it is not simple to determine the origin of individual end-components. The AEM is characterized by elevated (²³⁰Th/²³²Th) ratio (>1.40), while only old lavas erupted >7 kyr ago have such high values at Miyakejima. Though, this signature contradicts with the observation in the (²²⁶Ra/²³⁰Th)₀–1/Th diagram that the AEM should have ²²⁶Ra-²³⁰Th disequilibrium. The most plausible explanation is that, at least during the last 500 years, the AEM, probably originated from the magma of 7 kyr ago, has been evolved by intermittent replenishment of the BEM that had ²²⁶Ra-²³⁰Th disequilibrium higher than the AEM. This process can enhance the degree of ²²⁶Ra-²³⁰Th disequilibrium for the AEM, with keeping the (²³⁰Th/²³²Th) ratio higher than the BEM. Then, to explain the trend of lavas in the (²²⁶Ra/²³⁰Th)₀–1/Th diagram, (²²⁶Ra/²³⁰Th) ratio of the BEM would not be constant but decrease with the elapse of the time, suggesting that the magma chamber for the BEM is at closed system during the last 500 years. This result well matches with petrological observations that the temperature of the BEM decreases with the progress of fractional crystallization during this period.

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Miyake-jima has erupted about every 20 years (1962, 1983, 2000) since 1940. We have investigated pre-eruption processes for magmas erupted in 1983, and it was shown that the 1983 magma was a mixing product between basaltic and andesitic magmas (Kuritani et al., 2000). In this study, we examine the compositions of the two end-member magmas and the pressure conditions of the magma reservoirs, using constraints of multicomponent thermodynamics. By such information, recent magmatic processes beneath the volcano are also investigated. The whole-rock composition trends of the 1983 products are principally smooth and linear (SiO2 52.6-54.7 wt.%). Phenocryst contents of the products are 3-5 vol.%, and the mineral assemblage is olivine, plagioclase, augite, titanomagnetite, and minor orthopyroxene. Phenocrysts can be divided into two groups: those consisting of olivine, high-An plagioclase, high-Mg# augite, and low-Usp# titanomagnetite (referred to as A group), and those composed of low-An plagioclase, low-Mg# augite, orthopyroxene, and high-Usp# titanomagnetite (referred to as B group). Kuritani et al. (2000) discussed that the 1983 magma was produced by mixing between a homogeneous basaltic magma (referred to as BEM magma) containing the A group phenocryst and an heterogeneous andesite magma (referred to as AEM magma) with the B group phenocryst, just prior to eruption. Unlike the case of the mixing of two homogeneous magmas, estimation of the compositions of the end-member magmas is difficult, due to the heterogeneity of the AEM magma. The BEM compositions are, therefore, estimated using the constraint that the A-group phenocrysts were present in equilibrium in the BEM magma before the magma mixing. Thermodynamic models for both plagioclase-melt and olivine-melt pairs are applied to the calculated melt phase in the possible compositional area for the BEM magma, and the calculated equilibrium Mg# of olivine and An content of plagioclase are compared with the observed compositions, by which the BEM composition was constrained. The BEM magma is estimated to have a composition of SiO2 ~51 wt.% and a phenocryst content of ~15 %. The pressure condition of the magma chamber for the AEM magma is estimated using compositions of glass and plagioclase of the crystal aggregate of the B group. Application of the thermodynamic models of plagioclase-melt pair suggests that the magma chamber was present at the depth corresponding to about 1 kbar pressure. The estimated composition and phenocryst content of the BEM magma are similar to those of eruptive products of August 2000 eruption. This suggests that the products of the August 2000 eruption represent the one end-member magma in the recent activity of the volcano. The composition trend of the 1962 products is parallel to the composition trend of the AEM magma, and the 1962 products have similar phenocrysts to the A and B groups of the 1983 products. These observations suggest that the 1962 magma might also have been produced by mixing between the AEM and BEM magmas with different proportion from the case of the 1983 magmas.

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The 1991-1995 dacite of Unzen volcano contains mafic inclusions. They are usually ovoidal and 1-50 cm in diameter, constituting ca. 0.2-1 volume % of the host dacite. Nakada and Motomura (1997) showed that the bulk rock SiO2 contents of the mafic inclusions range from 50 to 60 wt%, and suggested that these mafic inclusions are derived from the marginal boundary layer of the magma chamber of Unzen volcano. In the present study, the author carried out petrographic work on the mafic inclusions, and concluded that the mafic inclusions represents quenched liquid drops of high-temperature mafic magmas in low-temperature felsic magma based on the following observations; i.e. (1) chilled margin in the mafic inclusion, (2) ubiquitous occurrence of skeletal plagioclase and hornblende in the mafic inclusions, (3) plagioclase microlites in the mafic inclusions tend to have higher Ca/(Ca+Na) ratio compared with the phenocryst plagioclase in the dacite, (4) pyroxenes in the mafic inclusions show high equilibration temperatures of ca. 1075°C. (1) Among 50 mafic inclusions, only one sample showed fined grained chilled margin against the host dacite. Grain width of plagioclase and hornblende are 0.02-0.05mm to 0.1-0.2 mm. (2) Generally, the mafic inclusions show diktytaxitic texture, consisting of elongated microlites of plagioclase and hornblende in vesicular glass. Even tabular plagioclase show skeletal distribution of Ca-rich framework with Na-rich plagioclase filling the hollow of the framework. (3) Although mineral compositions vary among mafic inclusions, Ca/(Ca+Na) ratio of plagioclase in mafic inclusions tends to be higher (up to 0.91) than that of phenocrysts in the dacite(up to 0.70). (4) One mafic inclusion contained pigeonite, augite, and orthopyroxene, giving equilibration temperature of 1075°C, where as only orthopyroxene appears in some of the mafic inclusions. A magma mixing model of Holtz et al.(submitted) indicate high-temperature end member magma has SiO2 content of 60-64 wt% with temperature of 1030°C, which is slightly high in SiO2 compared with the range of mafic inclusions. This discrepancy may be reconciled when we take the rheological behaviour of mafic magma during mafic-felsic magma interaction into consideration. Sparks and Marshall(1986) and Blake and Koyaguchi (1991) pointed out that mafic inclusion may form only if the temperature difference of two magmas is large, and the mass ratio of mafic to felsic magma is small, in which conditions the mafic magma would be congealed to form mafic inclusions. In other conditions the mafic magma remains fluidal and hybridization of the two magma proceeds, which actually took place to form the hybridized dacite of the 1991-1995 dacite of Unzen volcano.

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[in Japanese]

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LA lava flows of Izu Oshima 1986 eruption are observed in detail on the emplacement of the lava flows, and they still preserve their surface morphology. However, precise survey on surface morphology and modeling on its formation process are still lacking. Although we recognize the great influence of crystallinity on lava flow morphology through rheology change, there are few quantitative data on crystallinity change against flow distance. In the present study we discuss the factor controlled the surface morphology and crystallinity distribution of LA. LA lava flows (LA-I to LA-IV) are all classified in aa flow. Contrary to general aa flows whose surface clinkers are scoriacious, LA surface mainly consisted of dense platy or blocky clinkers with red oxidized surface. Scoriacious clinkers distributed only flow edges, i.e., near flow front and on outer flank of levee. These mean that the formation process of LA surface morphology was different from that of general aa flows. To reveal the origin of such clinkers, we examined their shape and size against flow distance. They became rounder and their average long axis sizes tended to decrease with distance (e.g., LA-II: about 140 cm at 10m and 80 cm at 700 m). They may be the results that solidified clinkers broke and rubbed on each other while advancing. With the facts that the distribution boundary between dense and scoriacious clinkers was unclear and no clinker had their transitional texture, we conclude that LA surface clinkers did not originate from usual scoriacious clinkers but from autobreccia formed on the summit lava lake.
Next, we examined crystallinity in LA lava groundmass. Both plagioclace and clinopyroxene tended to increase with flow distance (LA-II: plag 32.5% and cpx 24.5% at 10m, plag 35.5% and cpx 30.0% at 700m). If the temperature before overflow and the following cooling rate was the same, the crystallinity should be independent of flow distance, because the time available for nucleation and crystal growth was the same anywhere. We further classified the plagioclase in groundmass into microphenocysts and microlite by their shapes and sizes. Microlites tended to increase their number density and decrease their mean lengthlength length (e.g., LAII: 1.8*10^-3 N/μm² and 40μm at 10m, 2.3*10^-3 N/μm² and 30μm at 700m, respectively).These may reflect the difference of thermal history the lava had suffered before overflow, because when the degree of undercooling is high, nucleation rate becomes high and, resultantly, crystal number density becomes high and mean crystal size becomes small (Crisp et al., 1994). The present results imply that the front (i.e., the earlier overflowed) lava suffered higher degree of undercooling when microlites nucleated. If the microlites nucleated in the lava lake, the difference of their number density and size may correspond to the difference in the time spent near the surface; the later effused lava was not exposed at theshallower part of the lava lake longer, where the degree of undercooling was high.

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[in Japanese]

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At Shiowakka calcareous sinter dome, rare calcium carbonate minerals such as vaterite, monohydrocalcite and ikaite precipitate from cold spring water. Calcite is main precipitates in those minerals without in winter.
By the laboratory work in a cold chamber, the precipitation of ikaite from Shiowakka spring water at about 6 degrees C. water temperature was observed. Ikaite precipitate easily from melting water of ice formed by the spring water. Those crystals of ikaite floated on the surface of cold spring water in a beaker. Calcite precipitation following after that of ikaite and calcite aggregation showed just likes fractal texture. The order of crystallization from ikaite to calcite was common in the synthetic experiments and almost all calcite appeared following ikaite.
The inhibitor of calcite nucleation such as phosphates ions has been discussed for the formation of natural ikaite in previous papers. But, the contents of PO₄ ions in the Shiowakka water are the smallest in the natural water chemistry from which ikaite precipitated.
From observation of ikaite precipitation in a room and the field, it is supposed that natural dissolved organic matter is useful factor for the formation of ikaite from the cold spring water at low water temperature. That matter may effect on the inhibitation of calcite formation.

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Vaterite in association with calcite, small quantities of portlandite and ettringite group mineral was found from cuttings of MITI-borehole (Sado-oki) that was carried at offshore of Sado Island, the Sea of Japan. Stratigraphic horizon including vaterite correlates to the Uonuma Group of Pleistocene age.
Lumps containing vaterite are about 10 mm along with elongation. They are white in color and look like volcanic ash. This characteristic appearance is same to ones described in the previous papers that reported vaterite in cuttings. The studied lumps show flat and/or angular shape, and conchoidal fracture. Those appearances suggest that the lumps may be stripped from solid material.
An intensity of peak of vaterite in X-ray powder diffraction pattern has gradually increased by repeat experiments, which suggests that crushing of a lump and mixing with air may cause crystallization of vaterite.
The presence of portlandite and ettringite-group mineral suggests thatformation of vaterite may have relationship with cement material for drilling of oil well.

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Richelsdorfite was found from the Teine mine, Hokkaido, Japan. It occurs as very minute spherical aggregates of approximately 0.1 mm in diameter on druse of quartz vein. EDS analyses leads the empirical formula, Ca_1.97Cu_5.01Sb_1.03 (AsO₄)₄Cl_0.86 on the basis of As = 4 in anhydrous part. Richelsdorfite formed with the decomposition from luzonite, enargite and tetrahedrite in quartz vein.

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High-form pentlandite with Fe_4.94Ni_4.06S_8.00 first crystallizes by pseudoperitectic reaction between liquid and monosulfide solid solution (SS) at 870°C (865C for Fe=Ni in at. %) in the Fe-Ni-S system, and forms a limited SS including Fe_4.5Ni_4.5S_8.0 at 850°C. The SS grows rapidly to extend its field toward the Ni-rich side with decreasing temperature and connects with β2 in the Ni-S boundary at 806C. After the peritectic reaction, successive crystallization of most metal-rich high-form pentlandite from liquid directly continues to the pseudoeutectic temperature (746°C for Fe=Ni) with decreasing temperature. The high-form SS extended from the Ni-S boundary is maintained at 700 and 650°C and coexists with monosulfide SS, liquid (870 to 739°C), β1 SS (800 to 503°C) and/or γ(Fe,Ni) (762 to 579°C).
High-form pentlandite (Fe=Ni, 46.74 at. % S) transforms into pentlandite (low-form) at 615C. However, this transition temperature decreases continuously to 584°C (Fe=Ni, 46.10 at. % S) with decreasing S content. This inversion is reversible. The high form which is unquenchable has a primitive cubic cell with a=5.187(3)A (Fe=Ni, 46.74 at. % S) at 620C. Pentlandite belongs to cubic Fm3m with a=10.100(1)A ( Fe=Ni, 46.74 at. % S) at 25°C. The transition is thought to be an order-disorder inversion from the supercell (low form) to the subcell. The Fe-rich end of the high-form pentlandite SS breaks down to pentlandite and γ at the pseudoeutectoid (584°C for Fe=Ni). This reaction occurs successively using up the Fe-rich end of the SS with decreasing temperature. Consequently, the high-form SS retreats toward the Ni-rich side reducing its field and finally disappears at 503°C and Fe:6.47, Ni:50.00 and S:43.53 at %(ternary eutectoid) to break down into a mixture of pentlandite, high-form godlevskite and ternary β1. High-form pentlandite SS coexists with pentlandite SS at temperatures from 615 to 503°C. Pentlandite and heazlewoodite assemblage found in the Ni-Cu ores is first formed at 498°C. Ternary β1 decomposes into a mixture of pentlandite, heazlewoodite and γ at 484°C and Fe:5.07, Ni:55.94 and S:38.99 at. % (ternary eutectoid).
In the geological processes such as the formation of Ni-Cu ore deposits, it is thought as a result that pentlandite crystallized primarily as the high form by pseudoperitectic reaction between liquid (sulfide magma) and monosulfide SS, successively direct crystallization from liquid along solidus and pseudoeutectic at temperatures from 870 to 739°C (Fe-Ni-S system) and inverted to the low form (pentlandite) during cooling. Furthermore, it has also been ascertained experimentally that pentlandite appears due to exsolution and/or breakdown of monosulfide SS, high-form pentlandite SS and β1 SS below ∼600°C.

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The phase relations in the compositional range from 35 to 55 at. % S including Co₄S₃, Co₉S₈ and Co_1-xS in the Co-S system were investigated at temperatures from 700 to 1200°C using an evacuated silica glass-tube method. The products were examined by means of microscopy, high-temperature X-ray diffraction, DTA and EPMA. In this work the phases appeared are αCo, high-temperature solid solution (Co_4+xS3−Co_9-yS₈), cobalt pentlandite(Co₉S₈), Co monosulfide(Co_1-xS), cattierite(CoS₂) and liquid.
Cobalt pentlandite is stable below 831°C, and transforms into a high form at this temperature. This inversion is reversible. The high form makes a solid solution(SS) with Co₄S₃ above 831°C. This SS is provisionally named as high-temperature SS. It appears as a leaf-like form with the compositional range from 40.8(876°C) to 49.2(829°C) at. % S in maximum including Co₄S₃ and Co₉S₈ at temperatures from 806 to 930°C, and coexists with liquid, αCo, cobalt pentlandite and Co monosulfide. High-temperature SS breaks down into liquid and Co monosulfide at 930C and 46.5 at. % S(peritectic point) as its upper stability limit. It also breaks down into cobalt pentlandite and Co monosulfide at 829°C and 49.2 at. % S(S-rich eutectoid) and αCo and cobalt pentlandite at 806C and 42.9(∼Co₄S₃) at.% S(Co-rich eutectoid) as its lower stability limit. The SS is unquenchable. Cobalt pentlandite has a very limited SS from 46.4 to 47.1(∼Co₉S₈) at. % S at 800°C, but converges to stoichiometric Co₉S₈ at 700°C or below. It appears due to exsolutions from high-temperature SS and Co monosulfide and breakdown of its SS at two eutectoids besides inversion of the SS with Co₉S₈. It is quenchable. Co monosulfide forms a limited SS with the compositions from 50.0 to 53.0 at. % S at 1000C and from 51.7 to 53.8 at. % S at 700°C, and coexists with liquid, high-temperature SS, cobalt pentlandite and cattierite. It melts congruently at 1176°C and 51.8 at. % S.
High-temperature SS belongs to a primitive cubic cell and its cell edges with Co₄S₃ and Co₉S₈ are a = 5.178 and 5.171A, respectively, at 850°C, and a=5.183 and 5.175A, respectively, at 875°C. The value of the latter corresponds to about half of cobalt pentlandite(low form) cell edge a = 9.9287A at room temperature. This inversion is thought to be an order-disorder type from supercell(low form) to subcell and is analogous to high-form pentlandite.

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Impact and buried crater in active crustal regions are discussed by shocked quartz and Fe-Ni bearing grains at Takamatsu-Kagawa district in Japan. Crushed rocks at the buried crater can be mixed with foreign rocks which are not completely originated for impact crater. The previous reports on volcanic model for the buried structure cannot be fitted well with our collected data for impact structure. References:Miura et al. (2002): Geol. Soc. America (Denver),239-11; Miura et al.(2003) Large Meteorite Impact(Ries, Germany), #4122.; Miura et al.(2003) Submitted and in press

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Akiyoshi limetones blocks are fornmed by two stages of impact process at Equator area, and Takamatsu-Kagawa district impact.

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Cathodoluminescence (CL) is luminescence induced by electron bombardment. Detectable variations of CL wavelength and intensity, due to non-stoichiometry, structural imperfection or impurities in quartz and/or other crystals give insights into many geological processes. The aim of this study is to infer deformation and fracture characteristics in igneous quartz, by trace element and microdefect analysis, using a combination of SEM-CL, SIMS, EPMA, TEM and fluid inclusion microthermometry.Double polished (100mm thick) quartz sections were prepared for samples from the Takidani Granodiorite, for SEM-CL, SIMS, EDX and fluid inclusion analysis.Most quartz samples contain abundant liquid- and vapor-rich (secondary) fluid inclusions, with homogenization (Th) temperatures that range from 200 to 400°C. The salinity of most of the liquid-rich inclusions varies from 0 to 20wt% NaCl eq., and up to 60wt% NaCleq. for boiled fluids. High temperature (>600°C), hypersaline, halite-bearing fluid inclusions (∼40wt% NaCl) of magmatic origin were also trapped.CL observation of igneous quartz crystals from the Takidani Granodiorite reveals evidence of crystal zonation, dissolution, re-crystallization, fracturing and crack healing. The quartz characteristically shows euhedral crystal zonation, but also crystal rounding indicative of resorption. Most quartz crystals contain CL-dark luminescent lines, corresponding to healed cracks, which could not be readily detected using other observational tools (e.g. optical microscopy), with a well-defined alteration halo around the healed cracks.SIMS trace element analysis indicates that the alteration halo is enriched in sodium, potassium and magnesium. In addition, trace element analysis using EDX indicate that some of the healed cracks were sealed by aluminum-rich quartz. The deformation and fracturing history of the Takidani Granodiorite indicate quartz crystallization was a late-stage process, with crystal growth filling voids between plagioclase, orthoclase and accessory minerals. Fluid inclusions were trapped due to fracturing of the granodiorite at ∼400°C, based on the fluid inclusion (Th) data, which also marks the low temperature limit for brittle-ductile transition conditions in the Takidani Granodiorite. Chemical interactions occurred as a consequence of hydrothermal fluids moving through the cracks, leading to a sodium, potassium and magnesium-rich halo in the host rock, and crack healing. The occurrence of aluminium-rich quartz-healed microcracks, crosscutting alteration halos associated with other healed microcracks highlights the relative timing of microcrack generation in the igneous quartz.The application of CL observational techniques, complemented by SIMS/EDX analysis is useful for understanding hydrothermal quartz vein formation, and our experimental study shows the approach is also effective for deducing crystallization processes of igneous quartz and deformational history of granitoid intrusions.

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  The Kobushi skarn deposits are near the boundary of Nagano and Yamanashi Prefectures. It has developed as a gold mine for many years. Many pits are found along the ridgeline extending northward from the Kokusigatake.
   The Jurassic Kawakami Group distributes in the mining area, and consists mainly of sandstone, shale, slate, chert, lens-like limestone. It generally strikes NW-SE and inclines in the N direction, whereas near the deposits lens-like limestone strikes NE-SW and inclines steeply in the NW direction.
  The Kofu granitoids intrude to the south of this area, as a result, the contact metamorphic zone developed over about 2km, and small-scale dikes of quartz porphyry are often observed at the eastern slope of the ridgeline.
  The Kobushi skarn deposits are further divided into three ore deposits; Azusayama, Sanjin, and Kokushi from north, and two layers of chert are between each ore deposit. The Azusayama ore deposit is rich in magnetite, while the Kokushi ore deposit is rich in gold in the garnet skarn.
  This research mainly focuses the Kokushi ore deposit consisting of 24 skarn orebodies of origin of small-scale lens-like limestone. The main consituent minerals found from the ore deposit are garnet, hedenbergite, scapolite, adularia, chlorite, epidote, vesuvianite, quartz, calcite, scheelite, apatite, magnetite, gold, bismuth, Bi-Te minerals, pyrrhotite, chalcopyrite, arsenopyrite and others.
  Two big orebodies among 24 in the Kokushi ore deposit exist; one is No.2 orebody about 700m away from, another is No.9 orebody near the ridgeline of 1,000m away from the granitoids to north. No.2 orebody is in a garnet-hedenbergite skarn which mainly produced magnetite, and the size is 90m or more in the extended direction. No.9 orebody, about 30m of E-W, about 50m of N-S and about 50m of width, is lens-shaped and generally strikes NW-SE and inclines in the N direction. It is intruded by quartz porphyry with a diameter of about 10m. The skarn minerals have zonally arranged there; calcite zone (limestone), hedenbergite zone, and garnet zone. A quartz vein is observed near the boundary between the calcite and hedenbergite zones, and quartz and calcite veinlets and networks are in the garnet skarn.
  Gold occurs in the garnet skarn of No.9 orebody. Its size is generally about 100μm. It occurs interstitially among fine grains of garnet in the garnet skarn, coexisting with arsenopyrite, bismuth, and Bi-Te minerals. According to the analytical results of the gold, the inside of gold is comparatively homogeneous, and Ag content is about 7 % or about 11 % in many cases. Gold with calcite is also observed.

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Quadruple Mass Spectrometer (QMS) is widely used for detection of very little quantity of gases. As compared with magnetic field type mass spectrometer, it can be scanned continuously and very quickly from low mass-to-charge ratio (m/e) to high m/e. Use this characteristics, heat fluid inclusions in a mineral under vaccum, they explode one by one (decrepitation), enabled to measure for gas composition of fluid inclusions for every burst temperature by introducing the emitted gas into QMS immediately. The advantage of this method is as follows,1) It does not need to grind a sample, and adsorption of the gas on the surface of a sample is made as for it to the minimum. 2) Under continuously vaccum environment, measurement in the low background (less than 4x10^-6Pa) is possible. 3) Since measurement in the low background is possible, sample can be managed with small quantity (about 50mg). 4) To calculate mean ion current of each number of m/e detected by QMS during heating, it is possible to estimate for bulk gas composition. In this research, the quantitative gas analysis method of multi-composition gas established by [1] of having used QMS was applied, the new gas analysis system was built. In this system, fluid inclusions was gradually heated from 130-650 degrees C, over 30 minutes in the vaccum, and the emitted gases by decrepitation of fluid inclusions measured at intervals of 2 seconds by QMS. As a result of applying this method to gas analysis of fluid inclusion in natural pegmatitic quartz, it has checked that it was very effective to estimate quantity of water. Moreover, as for the ion current curve upon heating (gas release curve),the feature was seen for every area. Especially the area with CO₂-rich fluid inclusions (Ishikawa pegmatite area and Sakihama pegmatite area) found the release curve tends to incline toward a low temperature. In near 573 degree C, the sharp peak was detected on the release curve by all quartz samples. This peak is for fluid inclusion in which inner pressure increased to cause a explode in large quantities with change of the crystal structure at the time of alpha-beta transition of quartz. The weak peak was also detected for m/e=33,34 at the temperature of alpha-beta transition of quartz. It is the evidence of existence of H₂S in fluid inclusion. Reference:[1] Kamashima T.,Morikiyo T.,Jap. Mag. Min. Petr. Sci.,32,1-11(2003)

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Grain size is one of the most important factors that affects rheological properties of rocks. Grain size and differential stress determine whether deformation mechanism of rocks is diffusion creep or dislocation creep. In addition, grain size has a strong effect on diffusion creep rate. Therefore, knowledge of grain growth rate in peridotites is important to estimate grain size in the upper mantle.
We conducted grain growth experiments in wherlites with various forsterite(Fo)/diopside(Di) ratios. Starting materials of Ca-Mg-Si system, prepared from reagent by sol-gel method, were sintered with 1wt.%H2O in a Piston-cylinder apparatus at 1.2GPa and T=1200 for a week.
Mean grain size of Fo-rich wherlites whose Fo/Di ratios were Fo100-Fo80Di20 reached 50-70 micrometer. In contrast, Di-rich wherlites (Fo70Di30-Di100) had mean grain sizes of 5-15 microns. Therefore, grain growth rate of Fo-rich wherlites are 3-5 times larger than those of Di-rich wherlites. The threshold chemical composition is Fo70Di30. Considering the diffusion creep 1aw, it is deduced that strain rates of Di-rich wherlites (Fo70Di30-Di100) might be 9-125 times faster than those of Fo-rich wherlites (Fo100-Fo80Di20). This suggests that fartility has a large effect on mantle flow.
Mean grain size of Di in Fo-rich wherlites were as same as those in Di-rich wherlites, while Fo grains in Fo-rich wherlites were much coaser than in Di-rich wherlites. This means that grain boundaries of Fo in Fo-rich wherlites are mobile than in Di-rich wherlites. The grain boundary mobility of Fo depends on Di number density of Di grain on the grain boundaries. Fo grain growth rates of Di-rich wherlites are slowed down by larger Di grain size and higher number density of Di grain on grain boundaries.
Grain size of rocks in uppermantle are controled by grain growth and dynamic recrystallization. The results of theis study suggest the posssibility that clinopyroxene-rich fertile mantle has higher strain rate than olivine dominated depleted mantle.
Other factors that affect rheological properties of rocks than grain sizeinclude water weakening (e.g. Karato et.al,1986) and partial melting(e.g. Cooper & Kohlstedt,1986). These factors exert only on water or melt bearing systems. In contrast, the relationship between grain growth rate and fartility of rocks is important factor because it is applicable for whole the upper mantle.

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[in Japanese]

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[in Japanese]

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[in Japanese]

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Correspondence between various rock types and frequency of plagioclase twinning laws must be revised with the recent geological background for improving the criteria to discriminate geological settings by plagioclase twinning laws. For this revision, a rapid and reliable method to determine plagioclase twinning law types is necessary. Twinning laws can be recognized in a thin section with the aid of a universal stage. The plagioclase twinning law is determined from the composition plane (CP) and position of the symmetry axis (SA). The CP is determined with cleavages or optical elongation at the diagonal position. Normal, parallel or complex twin types can be understood by optical characters based upon the relationship of SA and CP. Based upon combinations of these characters, we can distinguish most plagioclase twinning laws in natural rocks.Plagioclase twinning laws of the granitic and quartz-feldspathic metamorphic rocks are presented with coordinates of frequencies of C twins (all twinning laws except albite and pericline laws) versus pericline twin for discriminative criteria of geological setting. Two contrast types of plagioclase twinning laws for granitoids are recognized, i.e., in the granitoids in Ryoke-Sanyo belts and in the granitoids in Sor Rondane Mountains. The former shows wide range of C twin frequency with moderate pericline twin frequency (for short, RS type), and the latter wide range of C twin frequency with obscure pericline twin frequency (SOR type). The RS type may be one example of plagioclase twinning laws for the granitic magmatism in subduction tectonics along the continental margin. On the other hand, the SOR type will be one representative for postorogenic or anorogenic magmatism within the Super-Continent. Difference of these plagioclase twinning law frequencies, especially difference of frequency of pericline twinning law, is explained by various shear stress due to various viscosity of magma. The viscosity of magma is defined by amounts of melt remaining during crystalization, which is caused from characteristic of magma related with various tectonics.Low-grade metamorphic rocks (greenschist facies to low grade amphibolite facies) contain small amounts of pericline twinning law. High-grade metamorphic rocks (high-grade amphibolite facies to granulite facies) show low to moderate frequency of pericline twinning law. Sheared high-grade metamorphic rocks have abundant pericline twinning law. These results are conformable to the previous experimental results which show formation of pericline twinning law under shear stress and high-temperature conditions.

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Fujimoto(1999a) found the antimony-ore deposit remains at the north slope of Mt. Amatsutsumi, Nishimera-mura, Koyu-gun, Miyazaki Prefecture. After that Fujimoto(1999b), Senba(2000), Yamanaka & Fujimoto (2001) and Hiramatsu et al. (2001) reported some minerals from that ore deposit remains. In 2002 we surveyed the area, took some ore samples and investigated them by microscopy, X-ray diffraction and electron probe micro analysis. Main ore minerals are stibnite, stibiconite, pyrite and sphalerite. In druse there exists a red colored transparent little mineral with diamond luster on stibnite crystal. It is decided pyrargyrite (Ag₃SbS₃) by X-ray diffraction, ore microscopy and electron probe micro analysis. As other druse mineral, a light yellow colored one with radial elongation associated with stibnite is found and is identified to be valentinite by the X-ray diffraction. By ore microscopy we found two silver minerals coexisted with stibnite. They are polybasite and miargyrite, which have the composition of (Cu,Ag,Fe)₁₂Sb₄S₁₃ and (Ag,Cu,Fe)SbS₂, respectively. Judging from the presence of only pyrite as for the Fe-S system mineral and the low iron content (1.5 atomic %) in sphalerite, it is considered that this antimony ore deposit was formed by the high sulfur fugacity ore solution.

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Chemical reaction between silicate and iron at high pressures and high temperatures was investigated, simulating the present core-mantle boundary and the core formation process in the early history of the Earth. Starting materials were synthetic pyrope garnet and MORB glass for the mantle material, and pure iron for the core material. High pressure and high temperature experiments were performed using laser heated diamond anvil cell up to about 60 GPa and above the melting point of pure iron. The temperature quenched samples were examined by in situ X-ray diffraction method at BL-13A, Photon Factory, KEK. The identified phases after heating for several dozens of minuets were basically consistent with the results in the literature and no extra phase was observed below the melting point of iron. However, a couple of samples treated above the melting point of iron indicate a possibility of the decomposition reaction of silicate phase. These observaions suggest a possible dissolution of silicon to the core.

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  The Upper Proterozoic Hantaishir ophiolite complex is located in the western part of Mongolia. This complex is 50 km long and 10-12 km wide, and its entire configuration is northwest to southeast trending. The Hantaishir ophiolite complex includes from bottom to top ultrabasic rocks, cumulate gabbros, sheeted dikes, pillow lavas and overlying siliceous sediments.
   Ultrabasic rocks of this complex are lightly serpentinised. Sheeted dike complexes are composed of boninites and diabases. Pillow lavas have andesitic composition. The gabbroic cumulates, dikes and andesitic lavas have comparable light REE-depleted and flat patterns.
   On the basis of the discriminant plots using immobile elements, gabbros and lavas from the Hantaishir complex are assigned an origin as island arc tholeiites. A range of petrological and geochemical parameters suggests that ultrabasic rock was generated in a supra subduction zone environment. A possible tectonic model for the ophiolite complex is a marginal basin behind an island arc.

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