Garnet with a complex compositional zoning was found from the northern edge of the Western Iratsu body in the Sanbagawa metamorphic belt of the Besshi district, southwest Japan. The studied garnet shows incipient Mn–reverse (increase) zoning from the center part and subsequent Mn–bell shape type (decrease) zoning towards the outer part, which is almost identical to the ‘Mn–caldera shaped zoning’ described by Xu et al. (1994) and Banno and Nakamura (2004). In order to display the chemical characteristics sterically, three–dimensional X–ray chemical mapping was performed for one very–coarse grain of garnet. The result clearly displays that there is a high–Mn layer with faceted euhedral shape of the garnet at the intermediate part, and that its composition continuously changes from the inside to the outside. Inclusion arrays in garnet getting across the high–Mn layer also suggest the continuous growth of the garnet. In the same sample, garnets without Mn–caldera shaped zoning are ubiquitously observed, which are relatively small in size up to 5 mm in diameter. Inclusion mineral assemblage in garnet and Raman barometry suggest the peak P–T conditions of the studied sample not having reached the eclogite facies, but being the epidote–amphibolite facies. The formation process of the ‘Mn–caldera shaped zoning’ can be best explained by the disequilibrium crystal growth under oversaturation of garnet in MnO–(MgO + FeO) binary system. Such oversaturation can be triggered by rapid heating. The finding of the remnant of rapid heating event would provide an important clue to discuss the exhumation history of the Sanbagawa metamorphic belt.
Argentian cryptomelane as a quite rare variety is determined during the investigation of Mn–Ag ore samples from Xiangguang deposit along the northern margin of North China craton. The mineral observed by a polarizing petrographic microscope involves concentric ring–band, pisolitic and veinlet structures and greyish white color. The scanning electron microscopy reveals a large number of elongated nanocrystals in the forms of nanofibers and nanorods in this densely natural argentian cryptomelane. The specifically chemical features in two samples of XG–C–1 and XG–C–2 of cryptomelane are: (1) (K0.55Na0.08Ca0.06Zn0.04Ag0.03Pb0.02Mg0.01)0.79(Mn7.21Fe0.52Al0.09Si0.09)7.91O16•nH2O; (2) (K0.37Ca0.28Ag0.13Na0.07Mg0.07Zn0.06Cu0.02)1.00(Mn7.01Fe0.40Al0.39Si0.03Ti0.01Cr0.01)7.85O16• nH2O. The silver content ranges from about 0.22–3.15 wt%, which is much higher than that of other manganese oxides including ranciéite, chalcophanite and coronodite found in this deposit as well. Both of two argentian cryptomelane samples feature two main Raman scattering contributions at about 580 cm−1 and 630 cm−1, belonging to the Mn–O lattice vibrations within the MnO6 octahedral double chains, which can distinguish from other three manganese oxides. The Ag+ prefers to locate in the tunnel sites substituting K+ of cryptomelane due to its large radius and the same monovalent state with K+. Some chain–width disorders characterized by transmission electron microscopy are probably caused by these cation substitutions.
We report newly obtained U–Pb SHRIMP ages of zircons from the granitoids in the vicinity of Chitradurga shear zone of Dharwar craton, South India. The analyses yielded two different ages, an older age of ca. 3300 Ma and younger ages of ~ 2600–2650 Ma, those suggest two major igneous activities in the study area. The former activity contemporaneous with the formation of Mesoarchean (~ 3.3 Ga) basement rocks (i.e., Peninsular Gneiss) and the latter reflects the Neoarchean (~ 2.6 Ga) regional plutonic activity in the Western Dharwar craton. Field and petrographic observations suggest that the intensity of deformation is high at the boundary between amphibolites and granitoids. Our new age dates imply that the formation of Chitradurga shear zone postdated the magmatic activity in the study area and the deformation structures possibly related to the development of Chitradurga shear zone. The area on either sides of the boundary between Eastern and Western Dharwar cratons experienced a Neoproterozoic event between 700–600 Ma indicated by prominent SHRIMP lower intercepts.
Thermal equation of state (EoS) of synthetic lawsonite [CaAl2Si2O7(OH)2·H2O] has been established using in–situ X–ray diffraction methods under high pressure and high temperature. Sodium chloride NaCl was used as the pressure standard in the experiments. The unit–cell volumes were measured up to 10 GPa and 973 K after the deviatric stress was released at high temperature. The P–V–T dataset was analyzed using a Birch–Murnaghan equation of state, yielding the room pressure volume V0 = 674.2(2) Å3 and the isothermal bulk modulus at room temperature K0 = 129(2) GPa (K′ set to 4). These values are comparable with the previous studies. When fitting the high temperature data, a second order temperature derivative of the bulk modulus was considered. Unlike Daneil et al. (1999) who reported a minimum value of bulk modulus at ~500 K, the bulk modulus decreases with increasing temperature at least up to 973 K. The dataset yields: V0 = 674.3(4) Å3, K0 = 128.7(15) GPa, (∂KT/∂T)P = −0.047(8) GPa K−1, (∂2KT/∂T2)P = 0.028(6) × 10−3 GPa K−1, α = 3.13(25) × 10−5 K−1, assuming K′ = 4. These data can be used to calculate the density and the stability of lawsonite under high pressure and high temperature conditions.
Enstatite in meteorite shows various emissions of cathodoluminescence (CL), and CL emission in terrestrial enstatite has been confirmed in this study. The CL spectra of these enstatite exhibit two broad emission bands at around 400 nm in a blue region and at around 670 nm in a red region. The emission components obtained by a spectral deconvolution can be assigned to impurity centers of Cr3+ (1.70–1.75 eV) and Mn2+ (1.85–1.90 eV) and to three defect centers (2.72–2.75, 3.18, and 3.87 eV). The emission component at 3.18 eV might be associated to the defect center derived from structural distortion by the substitution of Al for Si in a tetrahedral site. Extraterrestrial enstatite gives a characteristic emission at 3.87 eV in a blue to UV region, which is not detected in the terrestrial enstatite, suggesting radiation–induced defect by cosmic rays.
Vol.31 (1944) No. 5 and No.6 in the predecessor journal ″The Journal of the Japanese Association of
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