The join diopside—iron oxide—silica and its relation to the join diopside—forsterite—iron oxide—silica¹

Phase relations in the join (liopside-iron oxide-silica have been determined at oxvgcn fugacities corresponding to those associated with basic igneous rocks and defined by a constant room-temperature CO₂/H₂ ratio of 50 under one atmosphere total pressure. A small olivine primary phase field exists, together with three piercing [joints whose liquid compositions (given in weight percent with iron oxide calculated as Fe₃O₄), temperatures, and oxygen fugacities follow:

(1) 52 CaMgSi₂0₆, 40 iron oxide, 8 SiO₂, 1162°C, log fo₂ = −8.29, (2) 27 CaMgSi₂O₆, 47 iron oxide, 26 SiO₂, 1145°C, log fo₂ = −8.40, (3) 17 CaMgSi₂O₆, 57 iron oxide, 26 SiO₂, 1173°C, log fo₂= −7.97.

Crystalline phases in equilibrium with liquid are (1) olivine diopside—magnetite (magnesio-ferrite), (2) olivine silica—diopside, and (3) olivine—silica—magnetite. One pertinent invariant point (olivine—silica—diopside—magnetite) exists in the system CaMgSi₂O₆—FeO—Fe₂O₃—SOi₂. The present data permit qualitative delineation of liqui-dus phase relations in the petrologically important tetrahedron CaMgSi₂O₆—Mg₂SiO₄—iron oxide SiO₂ at oxygen fugacities defined previously. The sequence of crystallizing phases of simplified ultrabasic magmas is unchanged from that described in the literature at various constant oxygen fugacities.