ABSTRACT

This report describes protodolomites found in Daito-jima to the east of the Okinawa Islands, Japan. To investigate the conditions for sedimentary protodolomite formation, the distribution and chemical composition of protodolomites were studied through field observations and laboratory analyses. From the thin section studies of carbonate rocks, it was observed that porous parts of coral were cemented by protodolomite crystals and that aragonite in hard tissue of coral was transformed into protodolomite. These observed facts may show that protodolomite was formed through the transformation from aragonite and magnesian calcite in magnesium rich solution. The distribution and chemical composition of protodolomite in Minami-daito-jima suggest that protodolomites may be formed in concentrated seawater.