論文の公開元へNoble gas studies in meteorites: Constraints on the origin of trapped noble gases in primitive CR chondrites and past solar wind fluxes on solar-gas-rich meteorite parent bodies
概要
Noble gases are very rare in meteorites because they are the most volatile elements, they behave as the most incompatible elements, and they hardly take part in any chemical interactions. Because of the scarcity in meteorites, noble gases have been used as one of the most sensitive tracers in cosmochemistry. In general, noble gases in meteorites occur as a mixture of discrete “components”, where a component is defined by having a certain isotopic and elemental composition. Primordial noble gas components that had been originally trapped in meteoritic materials serve as tracers for the accreted materials and the post accretionary processes in the parent body. Solar wind noble gases that had been implanted onto the surface materials of the parent bodies serve as a tracer for the solar activity. In this study, I determined the inventories of primordial noble gas components in primitive CR chondrites and experimentally examined their resistance to aqueous alteration in chapter 2. In chapter 3, I estimated solar wind fluxes in the past that are recorded in solar-gas-rich meteorites using a newly developed estimation model.
In chapter 2, concentrations and isotopic ratios of all the noble gases (He, Ne, Ar, Kr, and Xe) in the primitive Renazzo-type (CR) chondrites Elephant Moraine (EET) 92048, Miller Range (MIL) 090657, Northwest Africa (NWA) 801, and hydrothermally treated MIL 090657 were measured by stepwise heating methods and total melting methods. On the basis of a petrographic classification scheme, the CR chondrites are classified as petrological type-2.7 for EET 92048, type-2.8 for MIL 090657, and type-2.8 for NWA 801, indicating minimal aqueous alteration. NWA 801 contains abundant solar noble gases while EET 92048 and MIL 090657 are solar-gas-free. Major primordial noble gas components in the solar-gas-free EET 92048 and MIL 090657 are Q, HL, Ar-rich, and the water-susceptive He and Ne component. The last one was lost during an aqueous alteration experiment. In terms of the Q-like isotopic compositions and low release temperatures (400ºC – 600ºC), the water-susceptive He and Ne component is similar to that observed in a refractory amorphous interplanetary dust particle (IDP) that may be of cometary origin. I argue that similar materials that host Q-like gases accreted on both the CR chondrite parent body and comets, and CR chondrites may have formed at a greater heliocentric distance. Cosmic-ray exposure ages for the solar-gas-free EET 92048 and MIL 090657 are estimated to be 6.5 ± 0.4 Ma and 6.8 ± 0.4 Ma, respectively, consistent with most CR chondrites that cluster around ~5 - 7 Ma.
The theoretical solar wind evolution models and the observations of young stars resembling our Sun suggest much larger solar wind flux in the past, while the abundance of implanted solar wind noble gases in lunar regolith implies that the mean average solar wind flux in the past up to ~4 Ga has been similar to the present-day flux. In chapter 3, solar wind fluxes in the past are estimated using solar and cosmogenic noble gas compositions in seven solar-gas-rich meteorites based on a newly developed estimation model. Any hint for the higher solar wind flux in the past is not found. I argue that the theoretical models overestimate the past solar wind flux in our solar system. Assuming present-day solar wind flux at the time of solar wind irradiation for the solar-gas-rich meteorites, the past heliocentric distances of the meteorite parent bodies were estimated. Except for the Rumuruti chondrite Mount Prestrud (PRE) 95410, the distances are almost consistent with the present-day distributions for the analogous asteroids and the predicted formation locations, suggesting that the heliocentric distances of the parent bodies of the most solar-gas-rich meteorites have never changed largely after their formation. The estimated short heliocentric distance for PRE 95410 is consistent with the previous study, suggesting inward migration from the asteroid belt regions where the parent body formed. Alternatively, the solar wind flux at the time of solar wind irradiation for PRE 95410 was a few to several times higher than the present-day flux.
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