Re–Os同位体を用いた地球化学:年代決定から古環境解読まで

抄録

Along with the progresses in analytical procedure and mass spectrometry, the number of papers using a Re–Os isotope system has rapidly increased, although Re and Os are trace elements whose typical concentrations in geological materials are ppt～ppb levels. Rhenium has two isotopes and ¹⁸⁷Re is a radio isotope which generates ¹⁸⁷Os through β- decay in the half-life of 41.6 Gyr. Since both Re and Os are highly siderophile and chalcophile elements, they are concentrated into sulfide minerals. Besides the ¹⁸⁷Re–¹⁸⁷Os decay system, Re is more incompatible than Os during a magma differentiation process, producing a large variety of the ¹⁸⁷Re/¹⁸⁸Os and ¹⁸⁷Os/¹⁸⁸Os ratios among various reservoirs on the Earth. The seawater ¹⁸⁷Os/¹⁸⁸Os ratio is mainly controlled by riverine flux having a high ¹⁸⁷Os/¹⁸⁸Os ratio (～1.4) and hydrothermal fluid/cosmic dust fluxes with low values (0.12～0.13). Thus, the Re–Os isotope system is a powerful tool for (1) geochronology of sulfide deposit, black shale and petroleum deposit using an isochron method, (2) geochronology of ferromanganese crust whose sedimentary age can be determined by fitting their ¹⁸⁷Os/¹⁸⁸Os ratios with the secular variation curve of the marine Os-isotope ratio, (3) decoding the trigger and processes of global climate change and impact event, and (4) unraveling the magma source and formation processes of volcanic rocks. Here, we review the recent geochemical study using the Re–Os isotope system and especially focus on a geochronology of the sulfide deposit.