Abstract
The equilibrium hedenbergite + ilmenite + oxygen = titanite + magnetite + quartz is important in distinguishing relatively oxidized from relatively reduced granitic rocks. The equilibrium expression is
where T is the temperature (in kelvins) and P is the pressure (in bars). Occurrences of hedenbergite-rich clinopyroxene and ilmenite in granitic plutons imply oxygen fugacities similar to those required for the stability of fayalite. When the assemblage titanite + magnetite + quartz occurs with clinopyroxene or amphibole with intermediate or higher Mg/(Mg + Fe) ratios, relatively high oxygen fugacities are implied. Mesozoic batholiths of the circum-Pacific, such as the Sierra Nevada batholith, crystallized under much higher oxygen fugacities than did alkaline batholiths of continental affinities, such as the Pikes Peak batholith.