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

Abstract

The Ashidachi ultramafic complex, being highly serpentinized, was emplaced into the prehnite-pumpellyite grade rocks in the Sangun metamorphic belt. Many small gabbroic bodies enclosed in the serpentinite are ubiquitously rodingitized along the margins. Progressive chemical change in the rodingite zone indicates introduction of CaO from ultramafic rocks. CaO content and density of selected serpentinites derived from clinopyroxene-bearing harzburgitic peridotite were determined. The relations between CaO content and density suggest that the serpentinization proceeded through two different reaction steps; an early step of serpentinization occurred without much loss of CaO, whereas a later step is ascribed to the olivine-clinopyroxene decomposition reaction. CaO-OH type fluids, essentially incompatible with CO2, are in harmony with a view that rodingite minerals are suggestive of CO2-poor fluids. Such CO2-buffering fluid regimes of the serpentinization with formation of talc-carbonate selvages were maintained under the regional metamorphism of the Sangun belt.-K.O.