Rare earth element and Sr isotopic study of the Middle Permian limestone-dolostone sequence in Kuzuu area, central Japan: Seawater tetrad sffect and Sr isotopic signatures of seamount-type carbonate rocks

The Kuzuu limestone-dolostone complex, in central Japan, is a remnant of the Middle Permian carbonate succession formed on a basaltic seamount in an open-sea realm without inputs of terrigenous materials. REE and Sr isotopic data of the carbonate rocks are clues to understanding the diagenetic process of biogenic carbonates to form crystalline carbonate minerals. The ⁸⁷Sr/⁸⁶Sr ratios of bulk carbonate samples are 0.7073-0.7076, and their Sr isotopic stratigraphic age (260±5 Ma) coincides with their fusulinid fossil age (263±8 Ma). The carbonate rocks have much higher REE/Ca ratios than modern skeletal biogenic carbonates by factors of 10^2-10^4. Their REE/Ca ratios are about one order of magnitude higher than those of microbialite. The chondritenormalized REE patterns for the Kuzuu samples show large negative Ce anomalies, together with seawater-like tetrad effects and high Y/Ho ratios. The Ce anomalies are comparable to those of present-day ocean waters at depths of 600-800 m, except for the bottom part samples polluted with Fe(III) oxyhydroxide possibly by fluids derived from underlying basaltic volcanics. Their REE patterns normalized by Pacific seawater at a depth of 381 m exhibit smooth light REE enrichment trends similar to the experimental partition coefficients of REE between calcite-overgrowths and seawater solutions. There is no significant difference in REE and Y features between the limestone and dolostone members. The original biogenic carbonates deposited on a basaltic seamount in shallow water environment, but subsequently they were subsisted to the water depths suggested by their Ce anomalies. At the water depth, the biogenic carbonates were re-crystallized to calcite under a condition of sufficient supply of the seawater, and the seawater REE and Y were incorporated into calcite. The ⁸⁷Sr/⁸⁶Sr ratios of dolomite, however, are slightly lower than those of coexistent calcite by 0.0002, which is a time interval of about 2 Ma from the seawater ⁸⁷Sr/⁸⁶Sr curve for Middle to Late Permian. The dolomitization occurred almost immediately after the re-crystallization on biogenic carbonate to calcite in a moderately deep water, but did not alter the REE and Y features of limestone precursors. This is consistent with the recent diagenetic model for the carbonate sequences of Neogene atolls in the Pacific emphasizing the role of thermal convection of moderately deep water.