This paper presents the results of petrographical studies carried out to examine the development and variation of sub-solidus reactions occurring in the Toki granite, Central Japan. The results reveal the three-dimensional cooling pattern of this zoned pluton. Samples collected from 19 boreholes in the Toki granite indicate a spatial variation in the extent of sub-solidus reactions. Exsolution coarsening produced microperthite textures with albite-rich lamellae in this pluton, whereas deuteric coarsening resulted in patchperthite with albite-rich patches. The width and spacing of the albite-rich lamella in microperthite increase systematically and prominently with elevation in the pluton. This indicates that the Toki granite effectively cooled from the roof during the exsolution coarsening stage. Measurements obtained using the hornblende-plagioclase and ternary feldspar thermometers indicate that volume diffusion took place in the temperature range 780-690 °C; this diffusion was associated with exsolution coarsening.

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Dawsonite has been recently re-discovered in the Cretaceous Izumi Group, Southern Osaka Prefecture, the area where this mineral was first described in Japan. Dawsonite commonly occurs as thin veinlets that are strongly associated with aragonite. Marginal fringes of aragonite and dawsonite are found along mudstone walls of thick calcite-dominated veins that cut across the dawsonite-aragonite veins. The dawsonite-aragonite association is characterized by a large number of cavities of varying sizes, suggesting their precipitation in an open space formed by the forcible invasion of a CO₂-rich fluid. Thin-section observations reveal that the CO₂-rich fluid hardly reacts with mudstone. The order of formation of carbonate veins (from the dawsonite-dominated regime to the calcite-dominated regime) is generally concordant with the changes in the fluid chemistry and precipitated phases predicted on the basis of a geochemical simulation to the quartzofeldspathic CO₂ reservoir in CO₂ geological storage.

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It was reported by Meyer and Boyd (1972) that zincian chromite, having 2 wt% to 3 wt% ZnO and high (>0.9) Cr# (= Cr/(Cr + Al) atomic ratio), appears as inclusions in diamonds obtained from kimberlite. Zincian chromite is characterized by a low (<0.03) Mg# [= Mg/(Mg + ferrous Fe + Zn + Mn + Ni + Co) atomic ratio] and an appreciable amount of MnO (0.4-0.5 wt%). The chemistry of zincian chromite is very different from commonly found chromite inclusions (magnesiochromite) in diamonds, which contain low amounts of ZnO (<0.1 wt%) and MnO (<0.1 wt%). Zincian and manganoan chromites are also commonly found in meteorites and in altered/metamorphosed peridotites and related rocks. The chromites found in meteorites mostly have an intermediate Mg# (0.2-0.5) and a high Cr# (mostly >0.8); moreover, the major-element chemistry of these chromites is similar to that of the magnesiochromite inclusions in diamonds. Spinels obtained from altered rocks show a chemical range that comprises the zincian chromite inclusions found in diamonds. The origin of these inclusions possibly lies in deep recycling. They were initially formed at the Earth's surface in altered/metamorphosed peridotites; they then sank deep down into the mantle and were entrained to the surface again by kimberlite magmatism.

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Secondary microinclusions in olivine and orthopyroxene in a harzburgite sample obtained from northern Oman ophiolite were examined by micro-Raman spectroscopy. The microinclusions were a consequence of fluid inclusions trapped by the minerals during the healing of fluid-filled cracks. We can expect an almost closed-system reaction to have occurred between the fluid and minerals. We attempted to distinguish the individual contributions of olivine and orthopyroxene to the serpentinization of peridotite. The inclusions in olivine mainly consist of lizardite and brucite with small amounts of magnetite, H₂, and CH₄, while those in orthopyroxene mainly consist of talc and chromian spinel with or without CH₄ and graphite. The fluid involved was most probably a mixture of H₂O and a smaller amount of CO₂. We examined whether reduced gases were produced through reactions of the fluid with olivine and orthopyroxene. The role of magnetite (or magnetite component in chromian spinel) is of vital importance in the generation of these gases. Orthopyroxene facilitates the formation of talc in the serpentinization of peridotite.

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Several podiform chromitite samples from abyssal peridotite have been recovered from Site 1271 at the 15°20' N fracture zone (FZ) in the Mid-Atlantic Ridge (MAR) during an Ocean Drilling Program cruise, ODP Leg 209. It is observed that only chromian spinel is preserved as the primary mantle mineral and that all other primary minerals in the chromitite samples are completely altered. The primary chromian spinel has moderately high Cr#= Cr/(Cr + Al) atomic ratio; approximate value = 0.5, which is comparable with Cr# value of the chromitite “minipod” sampled from Hess Deep (Arai and Matsukage, 1996). Samples associated with chromitites consist of mainly dunite, some amphibole-bearing gabbros and troctolite, and a very small amount of harzburgite. Therefore, the chromitites from this site are categorized under the rock series that composes the Moho transition zone, similar to those in ophiolites. This implies that a mass of melt existed, but was consumed by melt-rock interactions in the uppermost mantle beneath this area. This in turn means that the area is not magma-starved, although it is unclear when and where the interactions occurred. The formation of the chromitites at Site 1271 is probably occurs through the series of hybridization processes which suggested by Takazawa et al. (2007).
     Some grains of the chromian spinel in the samples have thick rims of Cr-magnetite or of magnetites. Increase in the Cr content in the Cr-magnetite rim is accompanied by Fe enrichment. The chemical modification of the chromian spinel suggests that the chromitite from Site 1271 was metamorphosed at a high temperature up to the upper greenschist facies because the depletion of Al in the region extending from spinel cores to the rim takes place at high temperature as a result of equilibration of fluids in equilibrium with chlorite.

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Cr-rich allanite-(Ce) has been found in a reaction layer between the serpentinite lens and the host metapelite of the Sanbagawa metamorphic belt in Nushima, Hyogo Prefecture, Japan. The reaction layer is divided into talc, tremolite, and chlorite zones in order from the inner to the outer regions of the layer. The Cr-rich allanite-(Ce) occurs as fine and spindle-shaped grains of 30-60 μm in length in the tremolite zone, and coexists with tremolite and subordinate amounts of chlorite, titanite, graphite, and zircon. The Cr-rich allanite-(Ce) is enriched in Cr₂O₃ (0.2-9.0 wt%) and total iron as FeO (6.0-9.0 wt%), with the following Cr-richest formula:

(Ca_0.98Na_0.01K_0.01)_1.00(REE_0.70Ca_0.28Th_0.01)_0.99(Al_1.40Cr_0.69Fe²⁺_0.63Fe³⁺_0.11Ti_0.09Mg_0.07Mn_0.03)_3.02(Si_2.94Al_0.06)_3.00O₁₂(OH),

where REE includes La = 0.17, Ce = 0.33, Pr = 0.04, Nd = 0.13 and Sm = 0.03 per formula unit. The core part of the Cr-rich allanite-(Ce) is partially enriched in TiO₂ of up to 2.3 wt%, suggesting the substitution of (Fe²⁺,Mg)Ti(Fe³⁺,Al)_-2 in the octahedral sites.

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The sorption behavior and the mechanism of phosphate sorption on monohydrocalcite (CaCO₃·H₂O; MHC) were analyzed on the basis of sorption isotherms corresponding to three Mg concentrations. MHC is known to be metastable, and it quickly transforms to calcite or aragonite. However, a small amount of phosphate (30 μM) can significantly inhibit the transformation of MHC. At low phosphate loading, the sorption isotherms obtained are Langmuir adsorption isotherms. The mode of phosphate sorption is most likely to be adsorption on MHC surface at the low phosphate loadings. The Langmuir parameters indicate that the mass basis sorption capacity of MHC is significantly higher than those of aragonite and calcite. At higher phosphate loadings, the slopes of the obtained isotherms rapidly increase and deviate from that of a Langmuir isotherm. The slopes of the isotherms for low Mg concentrations are higher than those for high Mg concentrations. It is well documented that Mg inhibits the formation of calcium phosphate minerals. The saturation calculations performed in this study showed that the reacted solutions become supersaturated with respect to amorphous calcium phosphate at corresponding high phosphate loadings. The mode of phosphate sorption at higher phosphate loadings is considered to correspond to the formation of secondary calcium phosphate minerals.

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We analyzed the Sr isotope and trace element compositions of the post-caldera volcanic products of Aso volcano in central Kyushu, with the aim of investigating the genetic relationship between the last caldera-forming (Aso-4) magmas and post-caldera magmas. The ⁸⁷Sr/⁸⁶Sr ratio of the magmas drastically changed from the homogeneous (0.7041-0.7042) caldera-forming stage to the heterogeneous (0.7040-0.7044) post-caldera stage. In addition, the obtained geochemical data suggested that the Aso-4 magma did not contribute to the origin and compositional evolution of the post-caldera magmas. These observations indicated that the generation of post-caldera magmas was probably independent of the Aso-4 magma.

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Edited by the Abstracting Committee, Japan Association of Mineralogical Sciences

Crystal Growth
      Zeolites and Clay Minerals 106022—106025;
      Nano Crystals 106026—106032;
      Biomineralization 106033—106036

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