Journal of Mineralogical and Petrological Sciences
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Volume 112 , Issue 2
April
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ORIGINAL ARTICLES
  • Yann SONZOGNI, Allan H. TREIMAN, Susanne P. SCHWENZER
    Volume 112 (2017) Issue 2 Pages 59-76
    Released: May 12, 2017
    JOURNALS FREE ACCESS

    A partially serpentinized peridotite from the Josephine ophiolite has been studied in detail in order to characterize the chemical processes of its serpentinization. The original rock was harzburgite, and its olivine and orthopyroxene are partially replaced by veins and patches of lizardite serpentine and magnetite; brucite and talc are completely absent from the serpentinite, regardless of whether the precursor mineral was olivine or pyroxene. Petrographic and mineral–chemical data suggest at least two phases of serpentinization. Incipient serpentinization produced lizardite and magnetite veinlets, from preferential dissolution of orthopyroxene, and/or infiltration of a silica–rich fluid. No talc or brucite was produced, which suggests this serpentinization happened in a chemically open system. Later serpentinization was from a fluid closer to Fe–Mg–Si chemical equilibrium with the harzburgite, which should in theory favor formation of a brucite–bearing serpentinite. Brucite is absent from late serpentine veins, but they have some porosity which could represent former brucite that was dissolved out or was reacted out after serpentinization. Isocon modeling suggests that Si, Fe, and K were added during serpentinization and that Ca was lost; i.e., the serpentinization was not isochemical (except for H2O). Results of petrographic observations, thermodynamic modeling, and mass balance calculations were used to constrain the reactions for global serpentinization of the studied sample. These reactions indicate that water with a concentration of H2 up to two times that of deep sea vent fluids may have been produced during the serpentinization of the Josephine peridotite, which could then have been a potential host for significant biomass.

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  • Yumiko TSUBOKAWA, Masahiro ISHIKAWA, Tetsuo KAWAKAMI, Tomokazu HOKADA, ...
    Volume 112 (2017) Issue 2 Pages 77-87
    Released: May 12, 2017
    JOURNALS FREE ACCESS

    A metapelite preserving prograde and retrograde zoning in garnet from Mefjell, southern Sør Rondane Mountains (SRM), East Antarctica is described in detail and U–Th–Pb geochronological data are presented. Garnet, sillimanite, staurolite and biotite are in textural equilibrium in the matrix. The garnet shows three distinct compositional zones comprising core, mantle and rim. From core to the mantle, the spessartine content represents a bell–shaped profile. From mantle to the rim, spessartine content increases and pyrope decreases. Kyanite is present as tiny inclusions in the garnet core. The core also contains aggregates of sillimanite with radial cracks around them, interpreted to have resulted from the inversion of kyanite. The prograde P–T path recorded in the garnet is heating to approximately 700 °C at 5.6 kbar with a slight increase in pressure. U–Th–Pb dating of monazite grains yields a large range of age distribution between 700 and 540 Ma. The inferred prograde metamorphism of the metapelite sample in this study might be related to subduction and/or tectonic loading explained by a collision tectonic model of the SRM (Osanai et al., 2013).

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  • Mayuko SHIMIZU, Kenji SHIBATA, Kazuhiro SUZUKI, Shigeru SUEOKA, Masaka ...
    Volume 112 (2017) Issue 2 Pages 88-96
    Released: May 12, 2017
    JOURNALS FREE ACCESS

    This paper outlines an advanced procedure involving the chemical Th–U–total Pb isochron method (CHIME) dating of monazite using a field–emission electron probe microanalyzer (FE–EPMA) equipped with spectrometers of 100 mm Rowland circle (RR) radius. The higher count rate of RR = 100 mm wavelength dispersive spectrometer (WDS) compared to RR = 140 mm WDS enables measurements to be taken in a shorter time. CHIME dating using RR = 100 mm WDS had previously been difficult because of lower resolution compared to that of RR = 140 mm WDS. This problem has been overcome by a new interference correction method using natural monazites.

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LETTERS
  • Yurie NAKAMURA, Takahiro KURIBAYASHI, Toshiro NAGASE, Hiroyuki IMAI
    Volume 112 (2017) Issue 2 Pages 97-101
    Released: May 12, 2017
    JOURNALS FREE ACCESS

    The crystal structure of iridescent garnet from Tenkawa village, Nara prefecture, Japan was investigated by means of Single–crystal X–ray diffraction method. The garnet crystal is composed of {110}c and {211}c growth sectors and shows birefringence under the crossed polarizer. The BSE image of the cross section shows very fine lamellae texture consisting of andradite–rich and grossular–rich layers. The average chemical composition was determined as Ca3.01(Fe1.93Al0.09)Σ2.02Si2.99O12. The unit–cell parameters were obtained as follows; a = 12.0540(17), b = 12.0580(13), c = 12.0612(13) Å, α = 89.986(9), β = 89.994(10) and γ = 89.994 (10)°. Some reflections which violate the extinction rule for the space group Ia3d were observed and indicated that all glide planes are absent. Triclinic symmetry with the space group I1 should be suitable for the structure refinement. The refinement with a I1 model was convergent with R1 = 3.19%. The Fe3+ occupancies of the Y11, Y12, Y13, Y14, Y21, Y22, Y23, and Y24 were determined as 86.9(6), 96.1(6), 90.9(6), 98.0(6), 95.1(6), 91.3(6), 93.7(6), and 99.6(6)%, respectively. The Fe3+ occupancy of the Y11 site is relatively lower, and that of the Y24 site is higher. The iridescent garnet from Tenkawa shows very fine lamellae texture consisted of Fe–rich and Al–rich layers, therefore, the refined crystal structure of the Tenkawa garnet should be a superimposed structure of these chemically distinct layers. The Fe–rich layer have almost pure andradite composition, so cation ordering in the Fe–rich layer is negligible. The Al–rich layer should have more obvious ordered cation distribution than the Fe–rich layer. The ordered cation distribution in the Al–rich layer should mainly contribute the cation ordering in the refined crystal structure of the Tenkawa garnet.

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  • Takafumi YAMAMOTO, Jun–ichi ANDO, Naotaka TOMIOKA, Kaushik DAS, Gautam ...
    Volume 112 (2017) Issue 2 Pages 102-107
    Released: May 12, 2017
    JOURNALS FREE ACCESS

    Microstructural observations and chemical composition analyses of a fracture–filling goethite vein in quartzite bands adjacent to a terrain–boundary fault zone were carried out, primarily using electron microscopy, to determine its formation process. Two domains A and B were identified, based on microstructural and chemical characteristics. The domain A formed a layered structure characterized by goethite grains with higher Al contents, smaller grain size (several hundred nanometers to micrometers in size), with development of the strong shape– and lattice–preferred orientations (SPO and LPO) of [020] and [110] along the wall rock contact, whereas the inner region had lower Al contents with larger grain size (several micrometers in size). The domain B exhibited concentric zoning characterized by variation in chemical composition, grain–size grading approximately ten to several tens of nanometers in size, a change in the porosity, and the alignment of goethite [110] perpendicular to the zoning plane. The grain size distribution and development of SPO and LPO in domain A can be explained by the inhibition of crystal growth (due to the incorporation of Al3+ instead of Fe3+) and geometrical selection, respectively. Two possible formation processes for domain B can be proposed based on the analogy of chalcedony; precipitation of goethite colloidal particles with electrophoretic force or heterogeneous nucleation from the Fe–rich supersaturated fluid and subsequent crystal growth. The study results suggest that the goethite vein was formed by multiple stages of Fe–rich fluid infiltration, which may have been derived from the Banded Iron Formation in the Singhbhum cratonic crust related to the activation of the Kerajang Fault Zone.

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