EarthRef.org Reference Database (ERR) -- Yokochi et al. 2005

In isotope studies of magmatic systems, magmatic He is preferentially extracted by crushing minerals under vacuum, whereas cosmogenic and/or radiogenic He isotopes are released by mineral melting as they are produced and remain in the mineral's matrix. Interpretation of magma source composition or surface residence time depends on the assumption that vacuum crushing releases insignificant cosmogenic 3He. We reinvestigated the helium isotopic composition of olivines in xenoliths from Takashima and Kurose volcanoes, Japan, which were reported to have high 3He/4He, interpreted as mantle plume contributions . Combining vacuum crushing and heating protocols, we show that samples from both localities contain considerable concentrations of cosmogenic 3He. Significantly, up to 25% of the matrix-sited helium was extracted by prolonged crushing. It seems possible that previous high 3He/4He ratios measured in samples from Kurose and Takashima were the result of the high cosmogenic 3He concentrations, and do not imply a mantle plume beneath Japan. For Kurose lavas sampled about 1 m above present-day sea level, exposure ages computed from cosmogenic 3He accumulation are similar to K-Ar eruption ages of 1.13 ¿ 0.12 Ma, implying that erosion of the sampling site has been limited and that the samples have not been below sea level for prolonged periods during the last Myr. Helium extraction from the matrix by crushing cannot be explained by volume diffusion because the diffusivity of helium in olivine is slow at the crushing temperature and therefore requires an additional mechanism to enhance the release. A fracture-related extraction mechanism may be responsible for this phenomenon, and a semiquantitative model is developed assuming that the increase in specific surface area during crushing represents newly created fractures. According to this model, He released from about 10 unit cells of olivine from the fracture results in helium extraction of only 1%, but the spallation damage tracks along fractures may provide sufficient pathway for observed helium extraction of up to 25%. In isotope studies of magmatic systems, magmatic He is preferentially extracted by crushing minerals under vacuum, whereas cosmogenic and/or radiogenic He isotopes are released by mineral melting as they are produced and remain in the mineral's matrix. Interpretation of magma source composition or surface residence time depends on the assumption that vacuum crushing releases insignificant cosmogenic 3He. We reinvestigated the helium isotopic composition of olivines in xenoliths from Takashima and Kurose volcanoes, Japan, which were reported to have high 3He/4He, interpreted as mantle plume contributions . Combining vacuum crushing and heating protocols, we show that samples from both localities contain considerable concentrations of cosmogenic 3He. Significantly, up to 25% of the matrix-sited helium was extracted by prolonged crushing. It seems possible that previous high 3He/4He ratios measured in samples from Kurose and Takashima were the result of the high cosmogenic 3He concentrations, and do not imply a mantle plume beneath Japan. For Kurose lavas sampled about 1 m above present-day sea level, exposure ages computed from cosmogenic 3He accumulation are similar to K-Ar eruption ages of 1.13 ¿ 0.12 Ma, implying that erosion of the sampling site has been limited and that the samples have not been below sea level for prolonged periods during the last Myr. Helium extraction from the matrix by crushing cannot be explained by volume diffusion because the diffusivity of helium in olivine is slow at the crushing temperature and therefore requires an additional mechanism to enhance the release. A fracture-related extraction mechanism may be responsible for this phenomenon, and a semiquantitative model is developed assuming that the increase in specific surface area during crushing represents newly created fractures. According to this model, He released from about 10 unit cells of olivine from the fracture results in helium extraction of only 1%, but the spallation damage tracks along fractures may provide sufficient pathway for observed helium extraction of up to 25%.