11:30 〜 11:45
[SIT21-04] Potential source materials of terrestrial noble gases inferred from mantle noble gas isotopic compositions
★招待講演
キーワード：希ガス、コンドライト、マグマオーシャン、拡散分別
The noble gas compositions of oceanic basalts provide valuable constraints on potential primordial sources of mantle noble gases and allow us to explore large-scale events in the Earth's formation, such as fractionation processes in magma ocean. Focusing on the solar wind-like 20Ne/22Ne and 3He/22Ne in mantle, a model of nebula gas incorporation by equilibrium dissolution into magma ocean has been discussed by many studies as a scenario for the formation of noble gases in the Earth's mantle. Nevertheless, the nebula gas seems to have disappeared before the magma oceans emerged on Earth, requiring an alternative scenario to explain the solar wind-like noble gases in mantle. Here we report potential sources of mantle noble gases based on the noble gas compositions of ocean island basalts (OIB) and mid-oceanic ridge basalts (MORB). Also, we present calculated results of the fractionation processes of noble gas compositions in magma ocean. Nebula gas, solar wind-implanted meteoritic materials, and CI chondritic meteorites have been regarded as the potential sources of mantle noble gases. Because the nebula gas cannot be directly dissolved into the Earth as described above, we assumed a mixture of the other two as the source of mantle noble gases. Given a mixture with approximately 87% contribution from solar wind in 22Ne equivalents, the 20Ne/22Ne and noble gas elemental ratios of the mixture are consistent with those of OIB mantle. Moreover, the calculation results for noble gas fractionation of disequilibrium outgassing on magma ocean followed by diffusive ingassing of noble gases from a deep mantle into magma ocean well explain the difference in noble gas elemental and isotopic ratios between the OIB and MORB mantles. These results indicate that the source and formation process of mantle noble gases can be explained with no use of nebula gas.