Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
Volume 101 , Issue 4
August
Showing 1-4 articles out of 4 articles from the selected issue
ORIGINAL ARTICLES
  • Mariko NAGASHIMA, Masahide AKASAKA, Tsuyoshi SAKURAI
    2006 Volume 101 Issue 4 Pages 157-169
    Published: 2006
    Released: August 08, 2006
    JOURNALS FREE ACCESS
    Chromian epidote was found in pebbles of omphacite rock derived from the Sambagawa metamorphic rocks in central Shikoku, Japan. The pebbles consist of chromian epidote, omphacite, amphibole, muscovite, phlogopite, chromite, albite, and zircon. The chromian epidote crystals are dark yellow to brown. Microscopically, they are subhedral and are pleochroic from yellowish orange to pale yellow-colorless. Chromian epidote has a zonal structure. Typically, cores of Sr-rich epidote are overgrown with Ca-epidote; alternatively Ca-epidotes are rimmed by and/or intergrown with REE-rich epidote. However, Cr distribution is not related to the zonal structure caused by Ca ↔ Sr and Ca + M3+ ↔ REE3+ + M2+ substitution since regions of higher Cr-concentration generally occur around chromite grains. The chromium content of epidote reaches 5.7 wt% Cr2O3 (0.36 Cr apfu/12.5 oxygens). In contrast, the Fe3+ content of the chromian epidote varies in a narrow range (5.1 to 8.9 wt% Fe2O3) irrespective of the chromium content. Associated minerals surrounding chromite (omphacite, amphibole, muscovite, and phlogopite) also tend to have a high and variable chromium content caused by Al ↔ Cr substitution, but with a nearly constant Fe content in individual minerals. Chromite is considered to be the source material of the chromian epidote and the associated minerals. The heterogeneous distribution of chromium may be attributed to the immobility of chromium under metamorphic conditions. The maximum Cr content of the chromian epidote in the present study is less than that of Fe3+-poor chromian epidote from other localities, whereas the Fe3+ content is greater. The substitution of Cr3+ for Al in the M3 site of the studied chromian epidote may be limited by the ferric iron occupying the M3 site to the extent of 0.52 apfu.
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  • Hidemichi HORI, Toshinori Kobayashi, Ritsuro MIYAWAKI, Satoshi MATSUBA ...
    2006 Volume 101 Issue 4 Pages 170-177
    Published: 2006
    Released: August 08, 2006
    JOURNALS FREE ACCESS
    Iwashiroite-(Y), YTaO4, was found from Suishoyama, Kawamata Town, Fukushima Prefecture, Japan. It is monoclinic, P2/a, a = 5.262(5) Å, b = 5.451(5) Å, c = 5.110(5) Å, β = 95.12(10)°, V = 146.0(2) Å3, Z = 2. The five strongest lines in the powder XRD pattern [d(Å), I/I0, hkl] are (3.13, 100, 111), (2.95, 94, 111), (2.73, 26, 020), (1.890, 29, 220) and (1.862, 29, 022). Electron microprobe analysis gave; Y2O3 29.10, Ce2O3 0.10, Nd2O3 0.10, Sm2O3 0.36, Gd2O3 1.06, Tb2O3 0.25, Dy2O3 2.38, Ho2O3 0.56, Er2O3 2.09, Tm2O3 0.37, Yb2O3 3.33, Lu2O3 0.85, ThO2 0.02, UO2 0.15, CaO 0.17, Ta2O5 40.64, Nb2O5 16.66, TiO2 0.41, total 98.60 wt%, and leads to the empirical formula, (Y0.81Yb0.05Dy0.04Er0.03Gd0.02Lu0.01Ho0.01Sm0.01Ca0.01Tm0.01)Σ1.00(Ta0.58Nb0.39Ti0.02)Σ0.99O4 on the basis of the 4 oxygen atoms per formula unit. Iwashiroite-(Y) is translucent and amber brown to brown in color with a glassy to subadamantine luster. The streak is pale brown and cleavage is {010} good, {001} poor. The Mohs' hardness is 6. The calculated density is 7.1 g/cm3. Iwashiroite-(Y) occurs as an aggregate of small euhedral platy crystals with a metamict substance in the main ore body of the granite pegmatite mine at the locality. Associated minerals are quartz, microcline and annite. The crystal structure was refined to R = 0.034 with single crystal XRD data. Although the structure consists of TaO6 octahedra and YO8 polyhedra, iwashiroite-(Y) is not isostructural with fergusonite-beta-(Y).
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  • Keisuke NAKAMURA
    2006 Volume 101 Issue 4 Pages 178-198
    Published: 2006
    Released: August 08, 2006
    JOURNALS FREE ACCESS
    We classify volcanic products of the 1914-1915 eruption of Sakurajima Volcano into four types according to vesicularity and the texture of plagioclase microlite. Type-1 is white Plinian fall pumice with high vesicularity (> 60 vol%) and low modal content (< 1.0 vol%) and number density (< 1014 m−3) of microlite. Type-2 is white Plinian fall pumice with high vesicularity (> 55 vol%) and intermediate modal content (1.0-11.0 vol%) and number density (1-10 × 1014 m−3) of microlite. Type-3 is slightly darker Plinian fall pumice with low vesicularity (25-50 vol%) and intermediate to high modal content (8-16 vol%) and number density (5 × 1014-2 × 1015 m−3) of microlite. The fourth type of volcanic product is lava flows with low vesicularity (< 25 vol%) and high modal content (> 16 vol%) and intermediate number density (1-5 × 1014 m−3) of plagioclase microlite. Type-1 and type-2 pumices were mainly deposited within the lower and middle parts of the Plinian fall deposit, whereas type-3 pumices were mainly deposited in the upper part. Homogeneous chemical features of preeruptive melts of all types of pumices indicate that the observed variations in number density and modal content of plagioclase microlite, as well as the decreasing vesicularity from type-1 to type-3 pumices, are related to a decrease in the decompression rate of magma from early to late stages of the Plinian phase. Residual water contents and degree of vesicularity of Plinian fall pumices suggest a change in the degassing process during the Plinian phase of eruption from minor degassing during the eruption of type-1 pumices to effective degassing during the eruption of type-3 pumices. Differences in microlite texture between type-1 and type-2 pumices of the early to middle stages of the Plinian phase and late-stage type-1, -2 and -3 pumices reflect changes in the decompression rate, perhaps related to differences between the central and marginal parts of conduits. A systematic increase in the An content of plagioclase microlites from Plinian fall pumice to Taisho Lava reflects a chemical change in the preeruptive melt from silicic to basic composition related to progressive magma mixing.
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  • Vivek P. MALVIYA, Makoto ARIMA, Jayanta K. PATI, Yoshiyuki KANEKO
    2006 Volume 101 Issue 4 Pages 199-217
    Published: 2006
    Released: August 08, 2006
    JOURNALS FREE ACCESS
    The occurrence of metamorphosed basaltic pillow lava in close association with serpentinized ultramafic rock, metamorphosed basaltic komatiite, volcaniclastic metasediment, and banded iron formation (BIF) in the Mauranipur area is the first explicit evidence for subduction related submarine volcanism in the Archean Bundelkhand craton, Central India. The Mauranipur pillow lava underwent greenschist to amphibolite facies metamorphism while retaining a geochemical signature of its igneous protolith. The pillow lava and associated massive volcanic rock is subalkalic, low-K tholeiitic basalt to basaltic andesite with SiO2 = 51.9-55.9 wt% and Mg/(Mg + Fetotal) × 100 = 63.4-67.2. The rock shows depletion in HFS elements with Nb/Nb* (0.07-0.17), and LIL elements are relatively enriched compared with present-day N-MORB compositions. Chondrite normalized REE patterns for the Mauranipur pillow lava are nearly flat with (La/Sm)N = 0.9-1.5, (Gd/Yb)N = 1.0-1.1, and Eu/Eu* = 0.81-1.10. The geochemical characteristics of the pillow lava are similar to those of low-K tholeiitic basalt reported from modern intra-oceanic arcs. The associated high-Mg andesite is compositionally similar to basaltic komatiite, enriched in LREE, with (La/Sm)N = 2.95-6.44, and depicts a nearly flat chondrite normalized HREE pattern with a low MREE/HREE ratio (Gd/Yb)N = 1.24-1.58. The basaltic komatiite displays remarkably similar geochemical characteristics to modern boninite. The present study, combined with available geological data, suggests that the supracrustal rocks of the Mauranipur area represent an Archean ophiolite sequence formed in a plate convergent setting.
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