クロミタイト：クロム資源にして地球深部過程指示者

抄録

Chromitites (chromite ores) are reviewed for their importance in magmatism, hydrothermalism, geodynamics and production of resources. The key process for chromitite production is possibly the formation of magmas oversaturated with chromite (or chromian spinel).Production of relatively silica-rich magma during the formation of dunite envelope is coupled with the production of the podiform chromitite. The origin of stratiform chromitites is challenged based on a new idea on the origin of chromite-hosted mineral inclusions, commonly found in all types of chromitites. The chromite-hosted melt inclusions, now characterized by the assemblage pargasite+aspidolite+orthopyroxene, are possibly formed during reaction melting of orthopyroxene within the mantle. Chromite grains in the crustal stratiform chromitites, at least in part, are produced in the mantle stage and transported upward to the magma chamber. Origin of ultrahigh-pressure chromitites has been highly debated, but is still enigmatic. Chromitites of hydrothermal origin have been found and may contribute to redistribution of Cr both in the mantle and in the crust. Production of chromitite ores in Japan is also reviewed. Chromitites serve as a good petrologic marker in both mantle and crustal rocks, and will greatly contribute to our understanding of deep-seated structure and mantle processes in the mantle drilling.