地球化学
Online ISSN : 2188-5923
Print ISSN : 0386-4073
企画総説「地球化学の最前線」
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
野崎 達生加藤 泰浩鈴木 勝彦
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2014 年 48 巻 4 号 p. 279-305

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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.

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