Abstract
The Dufek intrusion is a stratiform mafic body 8 to 9 km thick underlying 24,000 to 34,000 km2 in the Pensacola Mountains, Antarctica. Textures, structures, magmatic stratigraphy, and chemical variation indicate that layered gabbros and related rocks of this body developed by crystal accumulation on a magma chamber floor. Major cumulus phases in the exposed part of the intrusion are plagioclase, Ca-rich pyroxene, Ca-poor pyroxene, and iron-titanium oxide minerals. Modal amounts of cumulus oxide minerals, ferrian ilmenite and titaniferous magnetite, generally range from 2 to 12 percent.
Existing textures and compositions of the oxide minerals largely reflect subsolidus recrys-tallization and equilibration. Nevertheless, the amounts of vanadium and aluminum in ilmeno-magnetite are reliable indicators of fractionation trends in the magma during crystallization of the oxide minerals. Application of experimental T-f(Or)-X relations suggests that, for most of the intrusion with cumulus iron-titanium oxide minerals, conditions of crystallization and cooling were close to the conditions ol the quartz-fayalite-magnetite buffer essentially parallel to ilmenite isopleths. An oxygen fugacity of l0-10.1 bar is inferred for primary crystallization at 1090°C.
