Evolution and diagenetic implications of framboids in the methane-related carbonates of the northern Okinawa Trough

Authigenic carbonate samples were collected from the northern Okinawa Trough. Based on their carbon and oxygen isotopes, these samples were found to be methane-related carbonates precipitated by the anaerobic oxidation of methane (AOM). Petrological analysis revealed numerous framboidal pyrites that had been partly oxidized. In order to trace the variation and diagenetic information of these framboidal pyrites, their diameters and geochemical components were studied using an electron probe. The results showed that their diameters varied from 4 μm to 17 μm (n = 60; geometric mean of 9.9 μm) and were of a normal distribution. The diameters of single pyrite that formed the framboidal pyrites varied from 1 μm to 2 μm. The framboidal pyrites with diameters of 6–14 μm accounted for ～80％ of the total. The geometric mean of 9.9 μm indicates that they are probably diagenetic pyrites that were precipitated in a lower dysoxic environment (weakly oxygenated bottom waters). The S/Fe ratio of the framboidal minerals ranged from 0 to 1.67, and the pyrite content of single framboid varied between 0％ and 86.4％. Therefore, numerous pyrites were oxygenated to iron oxides or oxyhydroxides, and were retained as pseudomorphism pyrites. The size of framboidal pyrites precipitated in cold seeps can be used to trace the redox environment; however, acquisition of additional data via investigation of different cold seeps is necessary to obtain more persuasive results.