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Geochemical Journal
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Noble gases in six GSJ igneous rock samples

Yayoi Miura, Keisuke Nagao
Geochemical Journal, Vol. 25, No. 3, P. 163-171, 1991

ABSTRACT

Six igneous rocks of the GSJ reference samples were analyzed mass spectrometrically to determine the elemental abundances and isotopic compositions of noble gases. The concentrations of noble gases except for radiogenic 4He and 40Ar are low and in the range reported for various types of igneous rocks. In JR-1 obsidian a significant enrichment in Ne was found, which might be caused by diffusion of atmospheric Ne into the sample. K-Ar ages of JR-1, JGb-1 and JA-2 calculated using the 40Ar contents measured in this study and K in literature are 0.94 ± 0.09, 96 ± 10 and 13 ± 1 Ma, respectively. They agree with the ages previously reported. JG-1 has excesses of 21Ne and heavy Xe isotopes, which can be attributed to the nuclear reaction 18O(α, n)21Ne and 238U spontaneous fission, respectively. Since the 3He/4He ratio of JP-1 peridotite is similar to the ratio of MORB, which is higher than the ratio associated with arc volcanism, this sample may provide information about the noble gas state in the wedge mantle beneath the Japanese Islands. The low 40Ar/36Ar ratio of 384 for this sample implies that 99.8% of total 36Ar came from atmosphere. If the contaminative Ar was transported from sea water to the subducting oceanic plate, 0.2 mg of water for each gram of peridotite is sufficient to account for the contamination. The 3He/4He ratio, however, is not changed by this contamination.

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