Study On The Method Of Re-Os Dating And Its Enrichment Mechanism For Changing(GSSP)Limestone

We demonstrate that the Re-Os Limestone geochronometer can yield precise age useful for time-scale research through study of the Permian-Triassic boundary within the Meishan Section,Changxing County,Zhejiang Province.The data provide three accurate isochron ages of 1 to 6 layers(257.3±4.0Ma),18 to 22 layers(253.3±6.0Ma)and 23 to 24 layers(251.8±4.7Ma),in accord with calibration interpolate from U-Pb zircon dates for magmatic rocks.Reasonable Re-Os isotopic ages are obtained showing the closure of Re-Os isotope system in Limestone is perfect.This successful application provides a new tool for the stratigraphic classification and correlation.Base on the research of sedimetrary environment which shows the method of Re-Os isotope dating can apply to limestone deposited in reducing environment.The major and trace elements,Re-Os isotopes and TOC of the limestone from the Permian-Triassic boundary interval of the global stratotype section and point(GSSP)at Meishan,Changxing,Zhejiang Province,have been used to contrain redox conditions of sedimentary environment.The study on the occurrence of Re and Os in the limestone demonstrates that Re occurs in the organic matter of limestone only under highly reducing conditions.Hence,Limestone can be targeted for Re-Os dating only under reducing conditions.In contrast,Os removal from seawater to limestone can be found under various redox conditions.Authigenic minerals under oxic environments can enrich some Os,Thus,a high 187Re/188Os ratio can occur only in reducing sediments.The Os isotope system of authigenic materials that occurs in an oxic matine environment can be used as a paleo-marine environmental tracer.