クロミタイト: 不思議なマントル構成岩

Arai, Shoji; 荒井, 章司

doi:10.5026/jgeography.119.392

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クロミタイト: 不思議なマントル構成岩

http://hdl.handle.net/2297/35951

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SC-PR-ARAI-S-392.pdf (3.5 MB)

Item type

学術雑誌論文 / Journal Article(1)

公開日

2017-10-03

タイトル

クロミタイト: 不思議なマントル構成岩

タイトル

Chromitites: An Enigmatic Mantle Rock Type

言語

jpn

資源タイプ

資源タイプ識別子

http://purl.org/coar/resource_type/c_6501

資源タイプ

journal article

著者

荒井, 章司

WEKO 51
e-Rad 20107684
金沢大学研究者情報 20107684
研究者番号 20107684

	荒井, 章司

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著者別表示

Arai, Shoji

書誌情報

地學雜誌 = Journal of geography

巻 119, 号 2, p. 392-410, 発行日 2010-01-01

ISSN

収録物識別子タイプ

ISSN

収録物識別子

0022-135X

NCID

収録物識別子タイプ

NCID

収録物識別子

AN00322536

DOI

関連識別子

10.5026/jgeography.119.392

出版者

東京地学協会 = Geographical Society

抄録

内容記述タイプ

Abstract

内容記述

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, Fe2+)(Cr, Al, Fe3+)2O4, and silicates (mainly olivine). The Fe3+ content is generally very low, being less than 0.1 to all trivalent cations, in mantle chromian spinels. The Mg/(Mg + Fe2+) 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 Vp than peridotites, and the Vp is 8.5 to 9 km/sec depending on the proportion of chromian spinel, and higher in the former than in the latter. 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.

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出版タイプ

VoR

出版タイプResource

http://purl.org/coar/version/c_970fb48d4fbd8a85