Compositional variation of talc in metamorphosed serpentinites from Southwest Japan

Compositional variations of talc in peridotites and serpentinites could have significant implications for modeling of geochemical cycles involving the upper mantle but have been scarcely studied. We analyzed chemical compositions of prograde and retrograde talc and associated minerals in thermally metamorphosed serpentinites from Southwest Japan. The analyzed talc has variations of Si, Al, Mg, Fe, and Na contents. Most of the Si, Al, Mg, and Fe variations indicate mechanical mixing with serpentine and chlorite at a submicroscopic scale. Spatial distribution of talc-chlorite mixtures suggests their prograde metamorphic origin. Talc-serpentine mixtures could be formed by retrograde decomposition of talc-olivine assemblage and orthopyroxene at conditions of higher temperature and/or higher Si activity than serpentine-brucite mixtures, which are the typical products of serpentinization of olivine. Talc itself, regardless of prograde or retrograde origin, has compositional variations with Na enrichment as a likely result of solid solution or Na-mica mixing. The Na enrichment suggests that talc could be the most capable reservoir of Na in metamorphosed peridotites and serpentinites.