Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
Volume 102, Issue 6
December
Displaying 1-4 of 4 articles from this issue
ORIGINAL ARTICLES
  • Hiroyuki OHYAMA, Toshiaki TSUNOGAE
    2007 Volume 102 Issue 6 Pages 325-336
    Published: 2007
    Released on J-STAGE: January 10, 2008
    Advance online publication: September 22, 2007
    JOURNAL FREE ACCESS
    Supplementary material
    A systematic fluid inclusion study on pelitic and granitic gneisses of the Cretaceous Abukuma metamorphic rocks, exposed in the Hanazono district, northeast Japan, identified three categories of fluid inclusions: primary aqueous inclusions in porphyroblastic garnet, primary to pseudosecondary carbonic inclusions in quartz, and secondary aqueous inclusions in quartz. The primary aqueous inclusions in garnet exhibit melting and homogenization temperatures of −2.5 to −0.3 °C and +168.2 to +360.0 °C, respectively. The dominant primary to pseudosecondary fluid inclusions in undeformed quartz display melting temperatures of −64.2 to −56.6 °C, indicating a CO2-rich composition with additional CH4 and/or N2. The carbonic inclusions can be divided into high-density (1.121-1.153 g/cm3) and medium-density (0.683-1.111 g/cm3) types. The fluid densities of the latter inclusions, when translated into isochores, indicate entrapment of CO2 at the peak P-T conditions experienced by rocks in the study area (2.8-5.2 kbar at 770-850 °C). In contrast, the isochores of high-density category in quartz and aqueous inclusions in garnet do not intersect the expected P-T trajectory, yet are consistent with the prograde high-pressure condition of the Abukuma metamorphic rocks in the Takanuki district (8-10 kbar at 700-750 °C). Isochores calculated for carbonic inclusions in granitic gneiss intersect the P-T path at 2-3 kbar at 650 °C, suggesting that the fluids were captured during the retrograde stage. It is inferred that H2O was the dominant fluid species during the prograde stage, while CO2 was more abundant during the peak to retrograde stages of the Abukuma metamorphic belt. The prograde H2O-bearing fluid was probably derived from the break down of hydrous minerals. Although the origin of carbonic fluid is not known, the presence of CO2-rich fluid during peak metamorphism probably lowers the H2O activity in the rocks. The isochores computed for secondary aqueous inclusions in quartz give very low P-T conditions (300 °C and 1.7-3.5 kbar), suggesting that the fluids are of retrograde origin.
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  • Rei SHIRAISHI, Eiji OHTANI
    2007 Volume 102 Issue 6 Pages 337-345
    Published: 2007
    Released on J-STAGE: January 10, 2008
    Advance online publication: September 22, 2007
    JOURNAL FREE ACCESS
    A garnetite xenolith, KC-99-M5, from Malaita Island reported to contain minerals from the mantle transition zone was investigated using micro-Raman spectroscopy and electron probe microanalysis. The xenolith is mainly composed of Ti-bearing pyropic garnet. It contains many amoeboid areas with radiating cracks and veins that suggest fracturing during decompression. The phases in the veins and amoeboid areas are identified as orthopyroxene, clinopyroxene, amphibole, spinel, and plagioclase. High-pressure minerals such as majorite and silicate perovskite were not confirmed in this work. Texture, mineral chemistry, and Raman spectroscopic data indicate that the minerals in the veins and amoeboid areas are the reaction products of primary minerals with the melt from the hosting alnöite magma that penetrated along the cracks.
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  • Ichiro OHNISHI, Kazushige TOMEOKA, Noriko ISHIZAKI
    2007 Volume 102 Issue 6 Pages 346-351
    Published: 2007
    Released on J-STAGE: January 10, 2008
    JOURNAL FREE ACCESS
    The Karoonda CK4 chondrite exhibits pronounced silicate darkening of matrix and chondrule mesostases. Our scanning electron microscope study showed that the Karoonda matrix contains unusual olivine, with a high density of vesicles and inclusions of mainly magnetite and pentlandite, all between 0.1 and 3 μm in diameter. The vesicular and nonvesicular olivines occur as an intermixture, and vesicularity is particularly abundant in regions that exhibit a high degree of silicate darkening. Our transmission electron microscope study revealed that the vesicular olivine is entirely crystalline and contains a high density of much smaller (< 10 to 100 nm) vesicles and inclusions of mostly pentlandite. In contrast, nonvesicular olivine contains no such inclusions. The observations indicate that the silicate darkening is caused by the dispersion of not only opaque mineral inclusions but also vesicles, and the microinclusion-rich vesicular olivine is the principal cause of the silicate darkening. The texture and mineralogy of the vesicular olivine suggest that it was formed by crystallization of melt that was produced from nonvesicular olivine by shock at a high temperature.
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  • Atsushi MIYAMOTO, Masaki ENAMI, Motohiro TSUBOI, Kazumi YOKOYAMA
    2007 Volume 102 Issue 6 Pages 352-367
    Published: 2007
    Released on J-STAGE: January 10, 2008
    JOURNAL FREE ACCESS
    Supplementary material
    Kyanite-bearing eclogitic assemblages occur in the highest-grade zone of the Sanbagawa metamorphic belt, central Shikoku, Japan. The eclogites consist mainly of garnet, omphacite, phengite, kyanite, epidote, quartz and rutile. Compositionally variable amphibole (glaucophane/barroisite/pargasite), phengite and paragonite occur as inclusions in garnet and other eclogite facies phases. Careful examination of garnet zoning in kyanite-eclogites suggests that (i) most garnet grains show complex zoning consisting of relatively Ca-rich/Mg-poor inner and Ca-poor/Mg-rich outer segments, (ii) the inner segment of the zoned garnet formed at the eclogite facies stage, and (iii) the Mg-rich outermost rim of garnet does not always represents a composition at peak eclogite stage but could form at lower-pressure conditions of subsequent epidote-amphibolite facies. The assemblage of inner segment of garnet, omphacite, phengite, kyanite and quartz points to equilibrium conditions of 2.3-2.4 GPa/675-740 °C. The metamorphic P-T conditions of the eclogite facies stage reported in literature have been estimated assuming that the outermost rim of garnet with Mg-rich composition was in equilibrium with other eclogite facies phases. Therefore, P-T estimations of the eclogite facies stage in the Sanbagawa metamorphic belt should be re-examined carefully on the basis of textural and compositional heterogeneities of constituent minerals.
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