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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
Published for geochemistry community from Geochemical Society of Japan.

Significance of Cs/Rb ratios in volcanic rocks as exemplified by the Nohi Rhyolite complex, Central Japan

Kenji Okamoto, Susumu Nohda, Yasuyuki Masuda, Takashi Matsumoto
Geochemical Journal, Vol. 9, No. , P. 201-210, 1975


Cs/Rb ratio was examined on the Nohi rhyolite complex in Central Japan, which is composed mainly of rhyolitic ignimbrite. The variation in the Cs/Rb ratio of the magma through crystallization differentiation was estimated using a Rayleigh fractionation model and available distribution coefficients between phenocryst minerals and co-existing groundmass. It was found that the K/Rb ratio of this complex decreased from 273 to 125, but the Cs/Rb ratio remained constant through the differentiation process. The initial Cs/Rb ratio of this magma is 0.030 ± 0.005. The value is very close to the average Cs/Rb ratio of the ignimbrite in the Taupo region, Northern New Zealand, but is distinctly heigher than that of ocean floor basalts. To estimate the effect of contamination the Rb-Sr whole rock isochron method was used.

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