クロミタイト : 不思議なマントル構成岩 Chromitites : An Enigmatic Mantle Rock Type

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著者

    • 荒井 章司 ARAI Shoji
    • 金沢大学理工研究域自然システム学系地球学教室 Department of Earth Sciences, Kanazawa University

抄録

 Various characteristics of podiform chromitites, an enigmatic mantle rock member, are reviewed in this article. Chromitites are composed of chromian spinel, with the general formula (Mg, Fe<sup>2+</sup>)(Cr, Al, Fe<sup>3+</sup>)<sub>2</sub>O<sub>4</sub>, and silicates (mainly olivine). The Fe<sup>3+</sup> content is generally very low, being less than 0.1 to all trivalent cations, in mantle chromian spinels. The Mg/(Mg + Fe<sup>2+</sup>) ratio (= Mg#) changes inversely with the Cr/(Cr + Al) ratio (= Cr#), which increases with an increase of degree of partial melting of mantle peridotites. The Cr# of chromian spinel is generally higher than 0.4 (generally 0.6 to 0.8) in podiform chromitites, varying widely from 0.1 to 0.9 in the mantle peridotite. The podiform chromitite forms pod-like bodies (dimensions of up to 1.5 km × 150 m for an individual pod) with a dunite envelope, totally set within mantle harzburgite. In well-preserved ophiolites, they occur in the uppermost mantle, especially in and beneath the Moho transition zone, which is dominated by dunite. The Cr# of chromian spinel is relatively low (0.4 to 0.6) around the Moho transition zone, and high (>0.6) at deeper levels in the mantle section. Chromitites are denser and less anisotropic in <i>V</i><sub>p</sub> than peridotites, and the <i>V</i><sub>p</sub> is 8.5 to 9 km/sec depending on the proportion of chromian spinel, and higher in the former than in the latter. <br> The podiform chromitite has been interpreted to be one of melt/rock interaction products within the uppermost mantle harzburgite; hybridization of relatively Si-rich melt formed by the breakdown of orthopyroxenes of the wall harzburgite and subsequently supplied primitive melt cause oversaturation in chromian spinel, giving rise to formation of chromitite with a dunite envelope. The fractionated melt leaving high-Cr# podiform chromitite is possibly of arc-magma affinity. Chromitites with low-Cr# (0.4 to 0.6) chromian spinel can be in equilibrium with MORB. Recently found ultra-high pressure minerals, such as diamond, moissanite, Fe-silicides and Ni-Fe-Cr-C alloys, within chromian spinel of podiform chromitites make the genetical history of chromitites highly enigmatic. A new story, which incorporates the genesis and involvement of these highly reducing, ultra-high pressure minerals, is required.

収録刊行物

  • 地學雜誌

    地學雜誌 119(2), 392-410, 2010-04-25

    Tokyo Geographical Society

参考文献:  87件中 1-87件 を表示

被引用文献:  4件中 1-4件 を表示

各種コード

  • NII論文ID(NAID)
    10030369145
  • NII書誌ID(NCID)
    AN00322536
  • 本文言語コード
    JPN
  • 資料種別
    REV
  • ISSN
    0022135X
  • NDL 記事登録ID
    10705046
  • NDL 雑誌分類
    ZM41(科学技術--地球科学)
  • NDL 請求記号
    Z15-169
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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