Serpentinization reaction responsible for rodingite formation of the Ashidachi ultramafic complex, Southwest Japan

Abstract

The Ashidachi ultramafic complex, highly serpentinized, is emplaced into the prehnite-pumpellyite grade rocks in the Sangun metamorphic belt, Japan. A number of small gabbroic bodies enclosed in the serpentinite are ubiquitously rodingitized along their margins. Progressive chemical change of rodingite zone indicates introduction of CaO from ultramafic rocks. Selected serpentinites, derived from clinopyroxene-bearing harzburgitic peridotite, were nalyzed for CaO content and density. The relationship between the CaO content and density suggests that the serpentinization proceeded, at least, in two different reaction steps; an early step of serpentiniza-tion occurred without much loss of CaO, whereas in a later step CaO was depleted to a significant extent. The depletion rate of CaO in the later step of serpentinization is ascribed to the olivine-clinopyroxene decomposition reaction. Ca-OH type fluids roduced by the reaction are respon-sible for the rodingite formation. The Ca-OH type fluids, essentially incompatible with a CO₂ component, are in harmony with the fact that rodingite minerals typically are suggestive of CO₂-poor fluids. Such CO₂-poor metasomatic fluids might have been produced as well as maintained by the CO₂-buffering fluid regimes of the serpentinization with formation of talc-carbonate selvages under the regional metamorphism of the Sangun belt.