Chromitites: An Enigmatic Mantle Rock Type

Abstract

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 2＋ ） （Cr, Al, Fe 3＋ ） 2O4, and silicates （mainly olivine）. The Fe 3＋ content is generally very low, being less than 0.1 to all trivalent cations, in mantle chromian spinels. The Mg / （Mg＋Fe 2＋ ） 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 V p than peridotites, and the V p 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. Key words： chromitite, podiform chromitite, upper mantle, Moho transition zone, melt-perido-tite interaction, physical properties, island arc ： クロミタイトポディフォーム・クロミタイト上部マントルモホ遷移帯メルト / かんらん岩相互反応物性島弧 ＊ 金沢大学理工研究域自然システム学系地球学教室