Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
Volume 113 , Issue 2
April
Showing 1-6 articles out of 6 articles from the selected issue
ORIGINAL ARTICLES
  • Anatoly I. CHEPUROV, Valeri M. SONIN, Egor I. ZHIMULEV, Aleksei A. CHE ...
    2018 Volume 113 Issue 2 Pages 59-67
    Published: 2018
    Released: May 11, 2018
    [Advance publication] Released: March 06, 2018
    JOURNALS FREE ACCESS

    As a result of artificial dissolution of diamonds at 4 GPa and 1400 °C in a heterogeneous solvent (Fe–S melt with the addition of natural kimberlite in an amount of 5 wt%), it is established that, in the dissolution process the diamond crystals of octahedron form with flat faces and sharp edges are transformed into rounded octahedroids. The role of silicate additives is to local screen the surface of diamonds with the formation of etch hillocks, which gives crystals a visually complex external morphology. Thus morphologically complex natural diamonds with irregular shapes may form by dissolution by heterogeneous solvents in the mantle. Metal–sulfide–silicate melts consisting of immiscible components with different carbon solubility are the most likely candidates for such solvents.

    Download PDF (4147K)
  • Yuki MORI, Takeshi IKEDA
    2018 Volume 113 Issue 2 Pages 68-81
    Published: 2018
    Released: May 11, 2018
    JOURNALS FREE ACCESS

    Triple–layer coronas around corundum occur in the Lützow–Holm Complex at Akarui Point, East Antarctica. The monomineralic layers of green spinel, sapphirine, and plagioclase are arranged in this order from corundum to the matrix hornblende, indicative of the reaction

     corundum + hornblende = green spinel + sapphirine + plagioclase + H2O–fluid.

    Singular value decomposition analysis in the simplified Na2O–CaO–MgO–Al2O3–SiO2–H2O system implies that either Na2O and CaO, Na2O and MgO, or CaO and SiO2 were open during the corona formation. The position of the initial boundary between corundum and hornblende coincided with the present sapphirine–plagioclase boundary, as judged from the spatial position of the brownish–green spinel in the coronas which is also present in the matrix and hornblende. The flow of Al2O3 decreased outward and completely ceased at the plagioclase–hornblende boundary. This feature together with the monotonic decrease in Al content from corundum to hornblende as well as within sapphirine and plagioclase suggests that diffusion of Al2O3 controlled the corona formation. The net reaction showed an increase in volume, indicating that the products were stable at lower pressures. Therefore, the corona–forming reaction took place during the decompression stage of the Lützow–Holm Complex.

    Download PDF (12466K)
  • Tatsu KUWATANI, Kenji NAGATA, Kenta YOSHIDA, Masato OKADA, Mitsuhiro T ...
    2018 Volume 113 Issue 2 Pages 82-95
    Published: 2018
    Released: May 11, 2018
    [Advance publication] Released: April 19, 2018
    JOURNALS FREE ACCESS

    Geothermometry and geobarometry are used to study the equilibration of mineral inclusions and their zoned host minerals, which provide information on the PT conditions of inclusions at the time of their entrapment. However, reconstructing detailed PT paths remains difficult, owing to the sparsity of inclusions suitable for geothermometry and geobarometry. We developed a stochastic inversion method for reconstructing precise PT paths from chemically zoned structures and inclusions using the Markov random field (MRF) model, a type of Bayesian stochastic method often used in image restoration. As baseline information for PT path inversion, we introduce the concepts of pressure and temperature continuity during mineral growth into the MRF model. To evaluate the proposed model, it was applied to a PT inversion problem using the garnet–biotite geothermometer and the garnet–Al2SiO5–plagioclase–quartz geobarometer for mineral compositions from published datasets of host garnets and mineral inclusions in pelitic schist. Our method successfully reconstructed the PT path, even after removing a large part of the inclusion dataset. In addition, we found that by using a probability distribution of the most probable PT path, rather than a single solution, an objective discussion of the validity of the thermodynamic analysis is possible.

    Download PDF (1472K)
  • H.P.T. Sasanka HEWATHILAKE, N.W.B. BALASOORIYA, Yoshihiro NAKAMURA, H. ...
    2018 Volume 113 Issue 2 Pages 96-105
    Published: 2018
    Released: May 11, 2018
    JOURNALS FREE ACCESS

    Sri Lanka is endowed with high purity vein graphite deposits with extensive mineralization in the tectonically weakened zones of the basement high–grade rocks. Distinctly different crystal shapes of graphite are found even within a single vein and it is controversial in interpreting prevailed fluid activities and crystallization process to form such a variation. Therefore, this study was carried out to interpret the origin of vein graphite using geochemistry, crystal–morphology, and structure of the crystals. Sampling was conducted on four different depths at the Kahatagaha–Kolongaha mine, Sri Lanka. Characterizations of graphite were carried out by micro–Raman spectroscopy, X–ray diffraction spectroscopy, scanning electron microscopy, and inductive couple plasma mass spectroscopy. The results indicate that the genesis of the vein graphite is related to a single phase of fluid activity and the fluid was mostly pure with possible trace impurities. Further, it was revealed that the thickness of the veins, interaction with host rocks and mobility of the impurity elements have influenced the formation of different morphologies of graphite in a single vein.

    Download PDF (7280K)
LETTERS
  • Ryota ABE, Yuki SHIBAZAKI, Shin OZAWA, Itaru OHIRA, Hiromu TOBE, Akio ...
    2018 Volume 113 Issue 2 Pages 106-111
    Published: 2018
    Released: May 11, 2018
    [Advance publication] Released: April 19, 2018
    JOURNALS FREE ACCESS

    In situ X–ray diffraction studies of the phase relation of Al2SiO4(OH)2 were conducted within a pressure range of 19.7–32.2 GPa and a temperature range of 800–1600 °C. We observed the coexistence of δ–AlOOH and stishovite at 31.0 GPa and 1500 °C and the formation of phase Egg together with corundum at 30.6 GPa and 1600 °C. Our results indicate that phase Egg is stable at least up to 31 GPa and 1600 °C and should be the important water carrier after the avalanche of the stagnant slab to a depth of approximately 900 km in the lower mantle.

    Download PDF (624K)
  • Satoshi SAITO, Nikolai S. BAGDASSAROV
    2018 Volume 113 Issue 2 Pages 112-117
    Published: 2018
    Released: May 11, 2018
    JOURNALS FREE ACCESS

    We performed laboratory measurements of the electrical conductivity of a gabbro sample of the Oman ophiolite under high–pressures (0.6 and 0.8 GPa), high–temperatures (250–908 °C), and dry conditions using a piston–cylinder type high–pressure apparatus. The studied gabbro can be regarded as a representative sample of the lower oceanic crustal rocks based on its previously reported elastic wave velocities. The gabbro sample was powdered and hot pressed, and then sintered prior to the electrical conductivity measurements. The measured electrical conductivity of the sample decreased with decreasing temperature, which is consistent with the Arrhenius relationship. Compared with the electrical conductivity values of lower oceanic crust derived from a recent electromagnetic survey in offshore Nicaragua, the measured electrical conductivity of the sample was considerably lower in the temperature ranges expected for lower oceanic crust. The discrepancy can be explained by the presence of conductive pore–fluids in the lower oceanic crust.

    Download PDF (1980K)
feedback
Top