Official journal of Japan Association of Mineralogical Sciences (JAMS), focusing on mineralogical and petrological sciences and their related fields. Journal of Mineralogical and Petrological Sciences (JMPS) is the successor journal to both "Journal of Mineralogy, Petrology and Economic Geology" and "Mineralogical Journal". Journal of Mineralogical and Petrological Sciences (JMPS) is indexed in the ISI database (Thomson Reuters), the Science Citation Index-Expanded, Current Contents/Physical, Chemical & Earth Sciences, and ISI Alerting Services.
The Mogok metamorphic belt in central Myanmar is composed mainly of high–temperature paragneisses, marbles, calc–silicate rocks, and granitoids. The garnet–biotite–plagioclase–sillimanite–quartz and garnet–cordierite–sillimanite–biotite–quartz assemblages and their partial systems suggest pressure–temperature (P–T) conditions of 0.60–0.79 GPa/800–860 °C and 0.65 GPa/820 °C, respectively, for the peak metamorphic stage, and 0.40 GPa/620 °C for the exhumation stage. Ti–in–biotite and Zr–in–rutile geothermometers also indicate metamorphic equilibrium under upper amphibolite– and granulite facies conditions. Comparison of these estimates with previously described P–T conditions suggests that (1) the metamorphic conditions of the Mogok metamorphic belt vary from the lower amphibolite– to granulite facies, (2) metamorphic grade seems to increase from east to west perpendicular to the north–trending extensional direction of the Mogok belt, (3) granulite facies rocks are widespread in the middle segment of the Mogok belt, and (4) the granulite facies rocks were locally re–equilibrated at lower amphibolite facies conditions during the exhumation.
Priceite was found as a mass or a veinlet in crystalline limestone associated with gehlenite–spurrite skarns at the Fuka mine, Okayama Prefecture, Japan. Priceite occurs as grayish white or pale green aggregates of anhedral or prismatic crystals up to 0.5 mm in length in association with shimazakiite, sibirskite, uralborite and calcite. An electron microprobe analysis of priceite gave an empirical formula (Ca2.023Mn0.002Fe0.001Mg0.001Ni0.002Co0.001)∑2.030B5.033O7.160(OH)4.840·H2O based on O = 13. The unit cell parameters are a = 11.633(7), b = 6.977(3), c = 12.342(5) Å, β = 110.648(7)°. The calculated density is 2.486 g cm−3. It is likely that priceite from the Fuka mine was formed as a secondary mineral by a late–hydrothermal alteration of shimazakiite at temperature between 100 and 190 °C or less.
Microboudin paleopiezometry is an intensive endeavor that involves measurement of several hundred grains per sample to produce reliable estimations of far–field differential stress. This procedure is particularly time–consuming when conducting stress analysis for a large number of samples within a metamorphic belt. To improve and expedite the stress estimation procedure, we propose a numerical model that uses grain–shape data to calculate the relationship between the proportion of microboudinaged columnar grains (p) and the far–field differential stress (σ0). Our model combines the weakest link theory and the shear–lag model. The weakest link theory is used to derive the fracture strength of grains, whereas the shear–lag model is used to determine the relationship between the differential stress within a grain (σ) and σ0. An intact grain becomes a microboudinaged grain when σ is higher than its fracture strength at a specific point within the grain. Here, we make calculations of p for all intact grains under increasing σ0 from 0 to 20 MPa. Our calculations show that the modeled and observed distributions of p and the aspect ratio have similar patterns for both intact and microboudinaged grains. The value of p increases with increasing σ0, with 70% of the grains being microboudinaged when σ0 = 20 MPa. These results suggest that our model is capable of reproducing observed data for microboudinaged columnar grains and that the relationship between p and σ0 can be used to estimate the magnitude of differential stress without the need to measure grain–size data for several hundred grains with a wide range of aspect ratios.
High–pressure and high–temperature experiments were conducted to determine the stability field of phase Egg, AlSiO3OH, in the pressure range of 16.5–23.5 GPa and in the temperature range of 800–1500 °C. We found that phase Egg decomposes to δ–AlOOH + stishovite below ~ 1200 °C (16.5 GPa and 800 °C; 20.6 GPa and 1000 °C) and to Al–phase D + corundum + stishovite above ~ 1200 °C (21.8 GPa and 1500 °C) at P–T conditions corresponding to the mantle transition zone.
This indicates that phase Egg is unstable at the top of the lower mantle and can be a water reservoir only in the mantle transition zone. In addition, the present results imply that the superdeep diamonds that include phase Egg do not originate from the lower mantle but from the wet mantle transition zone.
The metagranite, that still preserves the igneous structure, is composed mainly of K–feldspar, plagioclase, quartz, biotite, white mica and accessory ilmenite. Quartz, K–feldspar, plagioclase and biotite preserve the original igneous shape, but are either re–equilibrated or replaced by new phases. Quartz occurs as fine–grained granoblastic aggregate, statically derived from inversion of coesite. Plagioclase is now composed of a fine–grained mineral aggregate including albite, zoisite, phengite, titanite and apatite. Biotite is surrounded and partly replaced by fine–grained white mica.
K–Ar analyses of K–feldspar from the metagranite were carried out, giving 42.6 ± 0.9 Ma. This age, significantly older than the SHRIMP zircon U–Pb age (35.4 ± 1.0 Ma) previously estimated for the metamorphic peak of the Brossasco–Isasca UHP unit, is apparent due to inherited excess argon (∼ 3.4%) from the host lithologies. This inherited excess argon is interpreted as related to the fact that K–feldspar has trapped the excess argon wave generated by the argon release from micas (that have large amount of radiogenic argon) during exhumation and cooling of the host lithologies.
We report the new finding of barroisite (Brs)–bearing metabasites within a metabasite layer from the Kebara Formation, a unit exposed between the Sanbagawa and the Chichibu belts in NW Kii Peninsula. The dominant lithotype of the metabasite layer shows pale–green colors and is mainly composed of sodium amphibole, actinolite, pumpellyite and epidote. It is in agreement with reported mineral assemblages in the Kebara Formation which document pumpellyite–actinolite (PA) or pumpellyite–blueschist (PBS) facies conditions (<340 °C and 0.8 GPa), and with geothermometry based on the Raman analysis of carbonaceous material from metapelite samples which give peak metamorphic temperatures of 300–340 °C. Brs grains are identified from metabasites with dark–green color in the layer, and are closely associated with epidote, chlorite, white mica, albite and quartz, but not with pumpellyite. Brs grains are replaced by sodium amphibole and/or winchite at the rim with a distinct compositional gap. Thermodynamic calculation suggests that the Brs + epidote + chlorite + albite + quartz assemblage is stable at P–T conditions higher than 450 °C and 0.4 GPa. The abovementioned data suggest that the Brs–bearing metabasites suffered an early higher temperature (>450 °C) metamorphism and then overprinted by PA or PBS facies metamorphism along with the main constituents of the Kebara Formation. In the Besshi area of the Sanbagawa belt, the earlier subducted higher grade rocks are considered to juxtapose to the newly subducted rocks and overprinted retrograde metamorphism during their exhumation stage. Our new finding suggests that the similar phenomenon was took place in the lower grade part of the Sanbagawa belt.
This study revealed for the first time the microtexture and crystallographic features of natural polycrystalline diamond, yakutite found in placer deposits in the Siberian Platform, Russia. Yakutite consists of well–sintered nanocrystalline (5–50 nm) diamond and small amount of lonsdaleite showing distinct preferred orientations. Micro–focus X–ray and electron diffractions showed a coaxial relationship between lonsdaleite 100 and diamond 111, suggesting the martensitic formation of yakutite from crystalline graphite. These textural and crystallographic features are well comparable to those of the impact diamonds from the Popigai crater located in the central Siberia and strongly support the idea that yakutite is a product of long–distance outburst from the Popigai crater, which has been inferred merely from the geochemical signatures.
We investigated the carbon solubility into silica under high temperature condition. Mixtures of amorphous silica and graphite powder sealed in silica glass tubes were heated at 1300 °C. The a lattice parameter and unit cell volume of α–cristobalite obtained are slightly increased compared with that heated without graphite. The c lattice parameter, on the other hand, almost unchanged. There is no clear dependency of the lattice parameter variations on carbon content mixed as the starting material. After re–heating the samples under atmospheric oxygen, the a lattice parameter and unit cell volume were reduced from the previous values. These changes indicate the possibility that carbon certainly incorporated into the α–cristobalite was oxidized with the atmospheric oxygen. The ab–initio calculation of disiloxane molecule showed that with carbon substitution for the bridging oxygen the Si–C bond distance increases whereas the Si–C–Si bond angle decreases compared with the Si–O bond distance and Si–O–Si bond angle. Consequently, the distance between Si atoms increases with the carbon substitution for the bridging oxygen. Since the expansion of Si–Si distance contributes twice as much to the a and b–axes than the c–axis in the unit cell of α–cristobalite, the ab–initio simulation result supports the observation that a lattice parameter increases with the carbon substitution relative to the c lattice parameter. The study strongly suggests that under reduced and high temperature conditions carbon is substituted not for silicon but for oxygen in α–cristobalite structure.
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