| 概要 |
Biotite crystals separated from a rock sample of the Haki granodiorite were split into eight classes of different grain size and then analyzed for major elements to see the relationship between the ch...emistry and the grain size. Rb and Sr contents, and Sr isotopic composition were also determined before and after the elimination of mineral inclusions to see their effect on Rb-Sr radiometric dating of biotite. Biotite samples free from the mineral inclusions show, independently of their grain size, the same age of 95 ±2m. y. This age is younger by about 10 to 43 m. y. than those of biotite samples with the mineral inclusions. The relative amount to the present Rb87 of radiogenic *Sr^87 lost during a period of cooling after the solidification of the rock is constant and independent on grain sizes of biotite. This is due to the fact that the radiogenic *Sr^87 was trapped by the inclusions in biotite. The inclusions are apatite, chrolite, epidote and zircon. The proportion, K, of the radiogenic *Sr^87 captured by the inclusions to the whole radiogenic *Sr^87 generated during the cooling period is dependent on the grain size of biotite. This proportion increases with the increase of the grain size and is tentatively expressed to be K=K_0{1+3.69exp (-6.94xl0^6/r^3)}. The grain size dependent variation of K allows us to estimate the minimum cooling period during which the radiogenic *Sr^87 in biotite was subjected to diffusion. The minimum period is about 120 m. y., which corresponds to a time of 80 % of the whole rock age (156 m. y.).続きを見る
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Mendeley出力
