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摘要
By crushing olivine and pyroxene phenocrysts in volcanic rocks from Kyushu Island, Japan, we determined (3)He/(4)He of 3-7 Ra and (40)Ar/(36)Ar of up to 1750. These values are lower than those of MORB. (4)He/(40)Ar* (down to 0.1) is much lower than the production ratio of (4)He/(40)Ar* (1-5), where an asterisk denotes correction for the atmospheric contribution. Such values are typical of mantle-derived samples from the island arcs and active continental margins. Although the origin of the low (3)He/(4)He and (40)Ar/(36)Ar of subcontinental mantle has been widely discussed, low (4)He/(40)Ar* has been given little attention. Actually, (3)He/(4) He and (4)He/(40)Ar* of phenocrysts overlap with those of subcontinental mantle xenoliths. Although noble gas compositions of phenocrysts are affected considerably by diffusive fractionation in ascending magma, they have little effect on the noble gases in the mantle xenoliths because it takes 100 years for He/Ar fractionation of ca. 15% for a mantle xenolith with 5 cm diameter. Therefore, the low (4)He/(40)Ar* of the mantle xenoliths is inferred to result from another kinetic fractionation in the mantle. During generation and migration of magma in the mantle, lighter noble gases diffuse rapidly out into the magma. This diffusive fractionation can explain low (4)He/(40)Ar* and somewhat low (3)He/(4)He in the residual mantle. Furthermore, the combination of the diffusive fractionation and subsequent radiogenic ingrowth explain the fact that data from subcontinental mantle xenoliths have extremely low (3)He/(4)He and various (4)He/(40)Ar*. Consequently, (4)He/(40)Ar* and (3)He/(4) He in mantle-derived materials are proposed as indicators of the degree of noble gas depletion of the source mantle.
- 出版日期2009-4-15
