Noble gas signatures of magmatic sources and processes

抄録

Noble gas isotopic ratios and their abundance patterns reveal the characteristics of their sources and processes that have modified them. Although isotopic signatures reflect the characteristics of source materials, absolute abundances and their relative patterns are controlled by magmatic processes such as the partial melting, fractional crystallization, transportation and degassing. The degree of partial melting affects the concentration of noble gases in a lava. However, this effect is apparent only for heavier noble gases (Ar, Kr, Xe), in the case of Loihi samples, because kinetic processes work more effectively for lighter noble gases (He, Ne). Further, the amounts of atmospheric Ar observed in volcanic rocks are sometimes lower than those expected from the solubility data for Ar in the atmosphere at the surface. This suggests that the samples did not equilibrate with the atmospheric Ar, and the degassing of the magmatic Ar is incomplete. The ³He/⁴He-³He/³⁶Ar diagram indicates that the ³He/³⁶Ar ratios are quite variable even though the ³He/⁴He ratios are relatively uniform for MORBs and for some samples from hot spot areas. The higher ³He/³⁶Ar ratios observed for dunites relative to olivine phenocrysts could be a result of secondary introduction of He into dunites from surrounding magma.