クロミタイト

―不思議なマントル構成岩―

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, 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 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.