The Abukuma Plateau is one of the major granitic terranes in the Japan Arc. Older granitic rocks (foliated and intermediate), younger granitic rocks (massive and felsic), and minor gabbroic rocks are complexly distributed in this region. In order to examine the cooling histories of the plutons, we have determined four 40Ar-39Ar hornblende ages and five K-Ar biotite ages of the plutonic rocks in the central Abukuma Plateau. The results are as follows:
Utsushiga-take gabbroic body: 103.8 ± 0.5 Ma (hbl) Nagaya body (older): 97.3 ± 0.6 Ma (hbl), 93.4 ± 3.1 Ma (bt) Shikayama body (older): 103.0 ± 0.4 Ma (hbl), 86.1 ± 3.8 Ma (bt) Ishimori body (older): 99.0 ± 0.5 Ma (hbl), 94.2 ± 2.6 Ma (bt) Miharu body (younger): 91.3 ± 2.3 Ma (bt) Hatsumori body (younger): 90.5 ± 2.6 Ma (bt) (hbl, 40Ar-39Ar hornblende age; bt, K-Ar biotite age).
On the basis of these analytical results and the petrography of the samples, we made the following observations: (1) The cooling rates of the Nagaya and Ishimori bodies (56 and 46 °C/m.y.) are equivalent to the common cooling rates of the Japanese granitic batholiths. The cooling rate of the Shikayama body (13 °C/m.y.) is slow because its volume is large. (2) The excellent 40Ar-39Ar plateau age of and the scarcity of metamorphic minerals in the Utsushiga-take gabbroic body suggest that the hornblende age is not a rejuvenated age but the original cooling age. (3) We were unable to detect any distinct geochronological boundary between the older and younger granitic rocks in the central Abukuma Plateau, in terms of the K-Ar biotite ages.
The study of chemical zoning, color and pleochroism of cassiterites aids in understanding of the metallogeny and exploration of the primary Sn source. These characteristics assist in discriminating the host granitoids into two associations: metasomatized granites and lithium albite granites. The cassiterite of the metasomatized alkali feldspar granites (i.e., apogranites) is characterized by enhanced to moderate Nb, Ta, (with high Nb/Ta ratios), Ti, FeO* and lower Ga2O3 (<0.01 wt%). Also, it is characterized by the development of deep brown to dark brown pleochroic color zones which oscillate or progressively alternate with lighter color zones. On the other hand, the cassiterite in the lithium albite granite is enriched in Ta, Nb (with low Nb/Ta ratio), Ti, FeO*, and Ga2O3 0.01-0.04%. It is also characterized by deep-orange to reddish brown pleochroic core or bands which are alternate with lighter color bands. It is noteworthy that the conspicuous variation in the colors, pleochroism, and chemistry of cassiterite from metasomatized apogranites and lithium albite granites can be considered as a valuable exploration tool when prospecting for primary cassiterite mineralization. In other words, during the panning survey, which is largely applied to rare metal mineralizations in general and cassiterite deposits in particular (where cassiterite is essentially dispersed in the mechanical aureoles), the pleochroism of any cassiterite present indicates the nature of the primary mineralizations (i.e., magmatic or metasomatic).
Munakataite, Pb2Cu2(Se4+O3)(SO4)(OH)4, occurs as a thin coating on a fracture in a quartz vein containing Cu-Zn-Pb-Ag-Au ore minerals in the Kato mine, Munakata City, Fukuoka Prefecture, Japan. It is monoclinic with the space group P21/m and lattice parameters a = 9.766(8), b = 5.666(5), c = 9.291(10) Å, β = 102.40(8)°, V = 502.1(8) Å3, and Z = 2. An electron microprobe analysis gave the empirical formula as Pb2.03(Cu1.94Ca0.01)Σ1.95(Se4+O3)1.00(SO4)1.02(OH)3.92 on the basis of Pb + Cu + Ca + Se + S = 6 apfu and the calculated (OH) with a charge balance. Munakataite is a member of the linarite-chenite group, and it is the first selenite mineral in Japan, corresponding to a (SO4)-dominant analogue of schmiederite. The mineral occurs as light-bluish aggregates composed of minute fibrous crystals up to 30 μm long. The calculated density is 5.526 g/cm3, and Mohs hardness is less than 2.
The Genina Gharbia mafic-ultramafic complex consisting of peridotite, pyroxenite, and gabbronorite represents a remnant of a hydrous magma chamber that was crystallized at the mantle-crust boundary. The hydrous nature of the magma is expressed in the high modal (up to 60%) of amphibole in cumulates and is reflected in variable textural and chemical features of the complex. One of the important manifestations of the hydrous nature is the development of a corona structure between olivine and plagioclase. The corona texture and mineralogy varied according to the activity of the interstitial liquid, temperature, and PH2O. The corona in peridotite is limited to thin (<100 microns) orthopyroxene and/or amphibole shells. Thick orthopyroxene-(orthopyroxene-spinel symplectites)-clinopyroxene-amphibole coronas are common in norite. Extensive analysis of different minerals of various textures by electron microprobe and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) revealed distinct chemical difference between the interstitial and cumulus phases of the corona. The corona pyroxenes and amphibole are depleted in V, Ti, Zr, and Nb and enriched in LREE, reflecting the LILE-rich and HFSE-depleted nature of the evolved hydrous liquid. The textural and chemical features of the corona are consistent with the formation of corona minerals at the late-magmatic stage. The orthopyroxene corona in peridotite crystallized directly from the evolved melt, while the amphibole corona developed at the expense of plagioclase. Variable corona minerals in norite are the result of direct crystallization from progressively evolved magma during cooling and continuous increase in PH2O.
Monohydrocalcite (CaCO3·H2O; MHC) is a rare mineral in geological settings. It is metastable with respect to calcite and aragonite. This metastability of MHC is considered to make it a rare mineral in geological settings. Alteration experiments of MHC in aqueous solutions in a closed system were conducted at temperatures between 10 and 50 °C in order to measure its metastability quantitatively. In the present study, monohydrocalcite transformed to aragonite with time. There are two rate-limiting steps in the transformation of monohydrocalcite to aragonite: the nucleation and crystal growth of aragonite. On the other hand, the dissolution of monohydrocalcite is a faster process than the nucleation and crystal growth of aragonite. The amounts of aragonite were calculated from the X-ray diffraction (XRD) intensity to evaluate the rate of both the processes at different temperatures. The induction times for the nucleation of aragonite were estimated to be 2.7 ± 0.9×103, 5.4 ± 1.8×103, 3.2 ± 0.4×104, and 7.3 ± 0.4×105 s at 50, 40, 25, and 10 °C, respectively. The conventional rate constants by assuming a zero-order reaction of aragonite crystal growth were estimated to be 1.0 ± 0.3×10−5, 6.1 ± 0.8×10−5, 1.0 ± 0.3×10−5, and 1.6 ± 0.3×10−6 mmol·s−1 at 10, 25, 40, and 50 °C, respectively. From Arrhenius plots, the apparent activation energies were estimated to be 108.1 kJ·mol−1 and 80.7 kJ·mol−1 for the nucleation and crystal growth steps, respectively.
Chromium K-edge X-ray absorption near-edge structure (XANES) spectra of natural ferropericlase were measured using an X-ray microbeam from a synchrotron radiation source to estimate the valence state of Cr. Measured samples were inclusions collected from lower-mantle diamonds. The obtained results revealed that divalent chromium is present in the materials in the lower mantle and that the proportion of Cr2+ in the total Cr content varied among the samples. The valence state of chromium in the inclusions in diamonds is a potential indicator of the redox conditions in the lower mantle.
We report petrological and fluid inclusion data on garnet-clinopyroxene rocks from Paramati in the Palghat-Cauvery Shear Zone System, southern India. The rocks are composed of garnet, clinopyroxene, and plagioclase with or without orthopyroxene and retrograde pargasite. The peak P-T conditions of 9-12 kbar and 830-880 °C were obtained from early garnet-clinopyroxene-plagioclase-quartz assemblage. Available petrological data from this region suggest that the high-pressure event was followed by heating to the peak ultrahigh-temperature condition and decompression as supported by the occurrence of orthopyroxene + plagioclase symplectite between garnet + clinopyroxene and plagioclase lamella in clinopyroxene. High Ca-Eskola molecule in the integrated clinopyroxene (∼ 8.6%) is also comparable with its high-pressure origin. Primary and secondary fluid inclusions in garnet are dominantly CO2-rich. They were homogenized into the liquid phase in the temperature range of −11.1 to +14.2 °C, corresponding to low-CO2 densities of 0.82 to 0.99 g/cm3. The fluid densities, when computed into isochores, indicate lower pressures (∼ 6-7 kbar at 900 °C) than the P-T conditions estimated by geothermobarometry. The low-density values and the primary nature of the inclusions trapped in garnet, as well as wide variation in homogenization temperatures, indicate density modification of prograde metamorphic fluid during exhumation. The results of this study support prograde high-pressure metamorphism and subsequent decompression along a clockwise P-T path for the evolution of the rocks in this Gondwana suture zone probably related to subduction of crustal materials to the mantle depth (>40 km) and following continent-continent collision during the amalgamation of the Gondwana Supercontinent.
Stable isotope compositions (δ13C, δ18O) of calcite within the veins hosted by the Sanbagawa metamorphic rocks in the Nagatoro area, Kanto mountains, Japan gave clues on movement of fluids during exhumation. The host rock calcite shows the nearly uniform δ18O values ranging from +14.5 to +16.5‰ (V-SMOW), and wide δ13C variation from −12.1 to +1.3‰ (V-PDB). The δ13C values of calcite within the exhumation-related veins also have wide variation, and the Δhost-vein (= δ13Chost rock-δ13Cvein) value vary depending on vein thickness and textures. Thin stretched crystal veins contain calcite with δ13C value close to that of the host rock calcite (Δhost-vein < ±2‰). In contrast, calcite grains within the thick blocky veins have homogenous δ13C values with large variations with host rock (Δhost-vein = +3.5 - −3.0‰). The latter types of sealed cracks may have played as dominant pathways of regional fluid flow.
Howieite (Hw) was recently found in meta-manganese siliceous rocks intercalated with lawsonite blueschists in the Hakoishi serpentinite mélange of the Kurosegawa belt, Yatsushiro, Kyushu, Japan; the peak pressure and temperature conditions were around 0.7-0.9 GPa and <350 °C, respectively. Hw is closely associated with Mn-rich stilpnomelane (Mn-stp), riebeckite, lawsonite, aegirine, calcite, quartz, and opaque minerals. Both Hw and Mn-stp occur as acicular to fine-grained aggregates and show yellow to pale yellow pleochroism. Electron microprobe analysis suggests that the distribution of Hw is relatively homogeneous in each grain, with an average composition of (Na1.19K0.01)1.21(Fe2+4.87Mn4.07Mg0.71)9.65(Fe3+1.11Al0.89)2.00(Si12.06Ti0.07)12.12O31(OH)13. Taneyamalite (Tn), a Mn-dominant member of Hw, has been reported from a Mn ore bed in the study area by Aoki et al. (1981). The difference in Mn content between Hw and Tn in the study area may be a consequence of the variety of host rock compositions present. The literature data and this study suggest that chlorite and white mica, which are representative hydrous phases in low-grade metamorphic rocks, are scarce or not present in meta-manganese siliceous rocks containing Hw or Tn. Therefore, we emphasize that Hw-Tn series minerals (along with Mn-stp) act as major water storage in meta-manganese siliceous rocks under low-temperature (<350 °C) conditions.
A qualitative understanding of the incorporation of hydrogen atoms in forsterite and corresponding octahedral vacancies are given from the inspection of the crystal structure with the view point of minimum strain configuration. In the case of Fe-free hydrous forsterite, the structural strain from the ideal size is larger in the M1 site than in the M2 site. Two H atoms may replace the Mg atom predominantly at the M1 site to reduce the structural strain, giving the vacancy at the M1 site. In the case of Fe-bearing hydrous forsterite, two H atoms may replace the Mg atom predominantly at the M2 site to avoid the combination of Fe in M1 site and Mg in M2 site for the reduction of the structural strain, giving the vacancy at the M2 site. In both cases, the most probable H atom locations are near to the edges of MO6 octahedra. Variation of the unit cell dimensions (∂b/b0 > ∂c/c0 > ∂a/a0) with the incorporation of hydrogen atom is interpreted in terms of retractable and unretractable chains in the part of olivine structure.