Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
Volume 101, Issue 5
October
Displaying 1-6 of 6 articles from this issue
ORIGINAL ARTICLES
  • Takashi HOSHIDE, Masaaki OBATA, Takashi AKATSUKA
    2006 Volume 101 Issue 5 Pages 223-239
    Published: 2006
    Released on J-STAGE: October 14, 2006
    JOURNAL FREE ACCESS
    The Murotomisaki Gabbroic Complex is a sill-like layered intrusion of up to 220 m in thickness and is located at Cape Muroto, Kochi Prefecture, Japan. There are several olivine-rich zones within the intrusion, which may have been formed through accumulation of olivine crystals. However, up to now it has not been clear as to whether all of the olivine-rich zones formed in this way. To clarify this, we reinvestigated the layered structure by collecting a consistent data set of modal composition, crystal size, and crystal number density of olivine from throughout the intrusion. It was found out that nearly all of the olivine crystals, in terms of crystal numbers, occur in the basal olivine-rich zone (within 40 m of the base of the intrusion), and the average value of the crystal number density of olivine throughout the entire intrusion coincides with the crystal number density of olivine in the outermost parts of the lower and upper chilled margins. Hence, we conclude that most primary olivine phenocrysts within the magma settled under the influence of gravity and accumulated to form the basal olivine-rich zone. The crystal number density of olivine within the mid-level zones (40-100 m from the base of the intrusion) is much less than the initial values, as indicated by values recorded in the chilled margins. It is proved that the increase of the olivine mode within this zone is attributed not to the crystal accumulation of olivine but to the increase of the crystal size of olivine, i.e., the crystal growth. In this way, considering the mode, crystal size, and crystal number density of olivine throughout the intrusion, the olivine-rich zones within the intrusion can be classified, in terms of their origin, as either crystal accumulation zone (AC zone) or crystal growth zone (GR zone). The growth of olivine crystals in the GR zone was apparently accompanied by an increase in MgO, FeO, and MnO concentrations to levels well above initial (i.e., the chilled marginal) values. This enrichment suggests that crystal growth occurred within a chemically open system in the sense that the increase in MgO content within the GR zone arose from material transfer between the boundary layer (the GR zone) and the overlying magma.
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  • Yuki BEPPU, Takamoto OKUDAIRA
    2006 Volume 101 Issue 5 Pages 240-253
    Published: 2006
    Released on J-STAGE: October 14, 2006
    JOURNAL FREE ACCESS
    Supplementary material
    The Cretaceous low-pressure/high-temperature Ryoke Metamorphic Belt contains abundant Ryoke and San-yo Granitoids that intruded to various crustal depths during or after formation of the belt. To clarify the tectono-metamorphic history of the belt, it is necessary to evaluate the thermal effects of the intrusion of the different types of granitoids. The geology of Kasado-jima Island, SW Japan, consists of metasedimentary rocks, massive biotite granite, and migmatitic biotite tonalite. The granite and tonalite occur in the central and southern areas of the island, respectively. Within the central area, granite intrudes metasedimentary rocks and cuts across bedding-parallel schistosity within the metapelites; this indicates that intrusion of the granite postdates Ryoke metamorphism. In contrast, tonalite in the southern area was emplaced parallel to the schistosity. On the basis of the field occurrences and petrographic characteristics of these granitoids, we correlate the biotite tonalite and biotite granite with the Older Ryoke and San-yo Granitoids, respectively. Based on the paragenesis of schistosity-forming minerals within metapelites, we identify two mineral zones in the island: the biotite and cordierite zones. The biotite zone is characterized by a mineral assemblage of muscovite + biotite ± garnet, and the cordierite zone is characterized by the assemblage muscovite + biotite + K-feldspar + cordierite or garnet. On the basis of mineral chemistry, we estimate peak metamorphic conditions for the biotite and cordierite zones to be ∼ 470 °C (reference pressure: 200 MPa) and ∼ 550 °C at 250 MPa, respectively. In the central part of the island, a contact aureole adjacent to the biotite granite is identified by the occurrence of randomly oriented muscovite and very large dendritic cordierite porphyroblasts. These randomly oriented porphyroblasts may have formed under a high degree of supersaturation associated with the thermal perturbation of the shallow intrusion of the biotite granite into cooler crust. In the southern area, most metamorphic minerals are of similar size, and their long axes are aligned to form a schistosity; this indicates a small degree of thermal perturbation and/or a high strain rate. These observations indicate that the biotite tonalite on Kasado-jima Island, as a deep-seated granitoid, was related to regional low-pressure/high-temperature metamorphism.
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  • Kenichi HOSHINO, Ai NAGATOMI, Makoto WATANABE, Takamoto OKUDAIRA, Yuki ...
    2006 Volume 101 Issue 5 Pages 254-259
    Published: 2006
    Released on J-STAGE: October 14, 2006
    JOURNAL FREE ACCESS
    Nahcolite (NaHCO3) has been found in fluid inclusions in quartz veins hosted by the Ryoke metamorphic rocks in Kasado-Jima (Island), Yamaguchi Prefecture, Japan. The inclusions are classified into three types: type 1 containing vapor +/− liquid of the H2O-NaCl-CO2-CH4 system, type 2 of H2O-NaCl fluids and type 3 with nahcolite solid. Salinities of the nahcolite-bearing fluids are lower than 4 wt% NaCl eq. Extremely large volume fractions of nahcolite solids, occupying more than 50% of total inclusion volume, and various fluid compositions are indicative of their origin as accidentally trapped ones while the fluids were immiscible below the nahcolite melting temperature (270 °C). A thermodynamic calculation of the nahcolite stability and a phase analysis of the fluids show logaNa+ = 7.4 − pH as the minimum activity at P = 100 MPa and T = 270 °C where the neutral pH = 5.6. Hence, it can be concluded that the original (prior to immiscible state) liquids might have contained high Na+ and low Cl yielded by an interaction of a low-salinity fluid and Na-bearing minerals such as plagioclase in the host metamorphic rocks below the fractures now occupied by quartz if nahcolite precipitated from boiling fluids.
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LETTERS
  • Tomokazu HOKADA, Satoko SUZUKI
    2006 Volume 101 Issue 5 Pages 260-264
    Published: 2006
    Released on J-STAGE: October 14, 2006
    JOURNAL FREE ACCESS
    Felsic orthogneisses with tonalitic and granodioritic compositions are dominant in the ultrahigh-temperature (UHT) metamorphic terrane of the Archaean-early Proterozoic Napier Complex, East Antarctica. Antiperthitic or mesoperthitic ternary feldspar occurs in addition to Qtz, Opx with/without Cpx in these felsic orthogneisses. Such feldspar grains commonly display zonal structure with respect to the volume of exsolution lamella; lamellae-enriched core and lamellae-free rim. Pre-exsolution one-phase compositions have been recovered separately for the feldspar core (an:ab:or = 20-28:53-56:17-28 in tonalitic samples and an:ab:or = 15-17:37-48:36-48 in granodioritic samples) and for the whole feldspar grain (an:ab:or = 24-29:58-63:13 in tonalitic samples and an:ab:or = 18-20:45-56:24-37 in granodioritic samples). Application of feldspar thermometry for these recovered one-phase compositions and the feldspar cores yield a temperature range of 940-1100 °C, which is relatively higher temperatures than the whole feldspar grain giving > 850-1070 °C. These compositional differences between core and rim of single feldspar grains are probably due to either sub-solidus or melt-related processes. The compositional and textural information of ternary feldspars in felsic orthogneisses can be therefore potential tools for understanding UHT crustal processes.
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  • Yasuhiro KUDOH, Takahiro KURIBAYASHI, Hiroyuki KAGI, Toru INOUE
    2006 Volume 101 Issue 5 Pages 265-269
    Published: 2006
    Released on J-STAGE: October 14, 2006
    JOURNAL FREE ACCESS
    The crystal structure of a single crystal of synthetic hydrous forsterite, (Mg1.985Si0.993H0.06O4), synthesized at T = 1300 °C and P = 13.5 GPa has been investigated using X-ray diffractometry and unpolarized infrared absorption spectroscopy. The data show an Mg atom occupancy of 98.6(2)% at the M(1) site and 99.9(2)% at the M(2) site, indicating that the cation vacancies predominantly occur at the M(1) site. A comparison of the calculated bond distances shows that the most probable H atom locations are at the O(2) and O(3) sites, where H may be located in association with the vacancy at the M(1) or the T site, forming hydrogen bonds between two oxygen pairs, O(3)-O(1) and O(2)-O(1), which correspond to the OH stretching vibrations at 3566 cm−1 and 3612 cm−1 observed in the IR spectra.
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  • Takeshi SUGIMOTO, Tomoyuki SHIBATA, Masako YOSHIKAWA, Keiji TAKEMURA
    2006 Volume 101 Issue 5 Pages 270-275
    Published: 2006
    Released on J-STAGE: October 14, 2006
    JOURNAL FREE ACCESS
    We have characterized the Sr-Nd-Pb isotopes and major and trace element compositions of 10 Quaternary volcanic rocks from the Yufu and Tsurumi volcanoes in northeast Kyushu, Japan. The enriched incompatible elements, negative Nb, and positive Pb and Sr anomalies are generally interpreted to be from island arc affinities of the lavas. However, the LREE/HREE ratio of the lavas (La/Yb = 8.5 ± 1.3) is greater than that from the island arc intermediate volcanics from northeast Japan (La/Yb = 3.8 ± 0.6), suggesting a different origin. One dacite sample had a Sr/Y ratio > 40, and the SiO2, Al2O3, MgO, Y, Yb, Sr, and 87Sr/86Sr compositions fell within the range of typical adakites. Other lavas were classified as normal island arc-type magmas. These results suggest that a partial melting of the subducting Philippine Sea Plate played a role in the genesis of the Yufu-Tsurumi volcanic rocks. The Sr, Nd, and Pb isotopic compositions show evidence of mixing between melts, derived from oceanic basaltic crust and subducted terrigenous sediments for the adakite magma, whereas another source is required to explain the enrichment in Sr-Nd isotopes and depletion in Pb isotopes of the island arc-type magma. Although the island arc-type magma appears to have a different source, a mixing of the adakite magma with this magma is apparent from our observations of the trace elements and isotopes.
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