The vicinity of the oceanic Mohorovicic discontinuity, transitional zone between the oceanic crust and mantle, is characterized by the common occurrence of dunite consisting mostly of olivine with small amounts of chromite. The most plausible formation mechanism of such dunite is believed to be an open-system reaction between pyroxene-bearing mantle peridotites, residues of partial melting, and basaltic silicate melts, partial melting products and the main ingredient of the oceanic crust. It is, therefore, important to specify the reaction stoichiometry and rates of influx and separation of basaltic melt involved in the reaction to better understand the formation mechanism of the transitional zone. Geological, petrological, and geochemical observations of ancient oceanic crust-mantle sections (ophiolites) and dredging and drilling of the current ocean floors provide key information for constraining the reaction processes. The status quo of studies on ophiolites and the ocean floor related to this subject is reviewed.
