Noble gases are the most sensitive tracer which indicates various sources of noble gas and nuclear interactions induced by high energy particles because of their unique isotopic compositions as well as ultra-high sensitive analytical method. I-Xe and K-Ar (Ar-Ar) systems are also applicable to chronological studies of the early solar system. Objectives of my PhD study were focused on the chronological study for the surface materials of the asteroids which interacted with solar wind (SW) emitted from the ancient Sun and galactic cosmic-rays (GCR). Brecciated meteorites had been located on its parent body surface. The meteorites can trace behaviors of SW, GCR, and the surface of the irradiated body in the early solar system because some of them were exposed to SW and GCR. Thus, history of the interactions can be made clear by combination of noble gas isotopic compositions from various origins and isotopic chronometers. 1) The important event which must have occurred in the early solar system was dissipation of gas and dust from the protoplanetary disk. Timing of the gas dissipation was estimated mainly by theoretically and/or from observation of the protoplanetary disk at the period of pre-main-sequence. I present a new method to determine the timing of the dispersal by combination of I-Xe chronometer and solar-noble-gas concentrations in brecciated meteorites. 2) In the ancient Sun deuterium burning is thought to have occurred in the T Tauri stage, which produced 3He from D and H. The deuterium burning produced the 3He/4He of ca. 3.8×10(-4) in the outer convective zone of the present Sun, which is higher than the primordial 3He/4He ratios of ca. (1.4-1.7)×10(-4) observed in primitive meteorites and in atmosphere of Jupiter. I tried to find out a transitional change in 3He/4He ratios in ancient solar gases trapped in solar-gas-rich brecciated meteorites. 3) I also discovered a clear evidence of two-stage irradiation of meteorites by GCR; i.e., an eucritic inclusion in the Vaca Muerta mesosiderite was exposed to GCR for more than 60 Myr before incorporation into the bulk material.

1) Timing of the clearing of proto-solar nebula: The aim of this study was to determine the age when SW started to irradiate planetary materials after dissipation of protoplanetary gas between the Sun and the planetesimals in asteroidal belt. Concentrations and isotopic ratios of light noble gases derived from SW were precisely determined for several solar-gas-rich meteorites. The SW noble gases in Willard (b) were preserved well. The gases in Zag might have been partially degassed during the formation of light/dark structure. In contrast to these meteorites, Cook 011 had mostly degassed by thermal metamorphism at 1 Ga, which was determined by (39)Ar-(40)Ar method. I-Xe dating method was applied to these meteorites to measure formation age of brecciated structures or trapping age of the SW gases. The gas-rich portions of the meteorites revealed younger I-Xe age than the gas-poor ones. The boundary between the different I-Xe ages implies the timing when nebular gas dispersed from the inner solar system (<3 AU), because SW could not reach to the asteroidal belt until the gas density was reduced to about ten orders of magnitude lower than the initial condition of the proto-solar disk. The I-Xe ages obtained for the meteorites indicate that the nebular gas was totally dispersed between 4550±2 and 4555±3 Ma.

2) Ancient 3He/4He ratio of solar wind: The SW He and Ne compositions were analyzed for the gas-rich portions of the regolithic breccias. The SW antiquities of the samples ranged from 4.51 to 4.55 Ga, which were obtained by the I-Xe dating method. Variable SW 3He/4He ratios which were lower than that in the present Sun were obtained. The low 3He/4He ratios suggest that the old SW reflected a direct evidence for the deuterium burning in a core of the Sun or scarce mixing between the core and outer convective zone after deuterium burning was over. However, SW 20Ne/22Ne ratios corresponding to the 3He/4He were also lower than the present one, which cannot be explained by nuclear fusion in the Sun. Thus, the low 3He/4He and 20Ne/22Ne ratios observed in the gas-rich meteorites could be produced by a partial removal of outermost coat of the grains with high isotopic ratios during the gardening, compaction, and impact heating processes. The result of this work suggests that the SW 3He/4He was unchanged for 4.55 Gyr keep the present-day value. Consequently, the deuteron burning and a mixing process between the core and the surface layer of the Sun had totally finished before 4.55 Ga.

3) Cosmic-ray exposure history of the Vaca Muerta mesosiderite: Noble gas isotopic compositions were measured for a eucritic pebble and bulk material of a silicate-metal mixture from the Vaca Muerta mesosiderite as well as pyroxene and plagioclase separated from the eucritic pebble by total melting and stepwise heating methods. Trapped noble gases were degassed completely by a high-temperature thermal event, probably at the formation of Vaca Muerta parent body (VMPB). The presence of fissiogenic Xe isotopes from extinct 244Pu in the bulk samples might be a result of rapid cooling from an early high-temperature metamorphism. High concentrations of cosmogenic noble gases enabled us to determine precise isotopic ratios of cosmogenic Kr and Xe. Spallogenic Ne from Na and unique Ar isotopic compositions were observed. The 81Kr-Kr exposure age of 168±8 Myr for the silicate pebble is distinctly longer than the age of 139±8 Myr for the bulk samples. The precursor of the pebble had been irradiated on the surface of the VMPB for more than 60 Myr (first stage irradiation), with subsequent incorporation into bulk materials approximately 4 Ga. The Vaca Muerta meteorite was excavated from the VMPB 140 Ma (second stage irradiation). Relative diffusion rates among the cosmogenic Ar, Kr, and Xe based on data obtained by stepwise heating indicate that Kr and Xe can be partially retained in pyroxene and plagioclase under the condition which resets the K-Ar system. This result supports the presence of fission Xe and of excess concentration of cosmogenic Kr, which could have survived the thermal event ca. 3.8 Ga.

本論文は5章からなる.

第1章は序論である.角礫岩質隕石の特徴とその宇宙科学的な意味づけが述べられ,その中で揮発性物質に富む角礫岩質コンドライトとメソシデライトの研究背景が述べられている.それらの隕石が太陽風起源の希ガスにさらされるようになった時代を決定することにより,原始太陽系星雲の晴れ上がりの時期や初期太陽のヘリウム同位体比変化を解明しようというこの研究の目的が述べられている.後半では太陽風起源核種,銀河宇宙線生成核種,放射壊変起源核種,始原的希ガス,地球の大気希ガスなど,この論文に関係する希ガスの各コンポーネントについて同位体の特徴などがレビューされている.

第2章は希ガスの分析方法について述べられている.129I-129Xe年代測定のための中性子放射化した試料を分析する方法と,放射化せずに隕石中の試料の極微量希ガスを分析する方法について述べられている.なお,申請者は本論文のデータを取得するのに用いた装置を含めた希ガス質量分析装置の改良と低ブランク化や、放射化した試料の全希ガス分析を可能にする上で大きな貢献をしてきた.このような高度な分析技術を背景に、ハヤブサ試料の初期分析チームの正式なメンバーとして希ガス分析に携わっている。

第3章では,第2章で開発された分析法を用いた,太陽風起源の希ガスに富む角礫岩質コンドライト隕石の分析結果が述べられている.Cook 011,Willard (b),Zag隕石を研究に使用している.申請者は,129I-129Xe系はガーデニングプロセス(小天体の表面に微小物質が衝突し表面が掘り返される過程)が継続することによってリセットされ,この年代はガーデニング終了の時期を記録していることを示すことを,これまで発表されている文献と申請者のデータから明らかにした.Zag隕石とWillard隕石について129I-129Xeが古い部分から新しい部分になるにつれて太陽風起源のガスの捕獲量が増加していることに注目し,初期太陽系星雲のガスの晴れ上がりの年代を,太陽系の誕生から約1700万年後の45.5億年と推定している.Zag隕石については,明るい色の部分は初期太陽系星雲ガスが分散する以前に固化したことを,暗色部については,星雲ガスが分散した後で形成され,太陽風が打ち込まれたことを明らかにした.これは、古くから成因が謎であった、太陽風希ガスの乏しい明るい部分と太陽風希ガスに富んだ暗い部分からなる隕石が、原始太陽系星雲ガス散逸の時期を秘めていることを初めて明らかにしたものである。また、角礫岩質隕石の暗色部分に残されている3He/4He,20Ne/22Ne同位体比の情報から,ガーデニングプロセスや圧縮過程でおこるヘリウム同位体比の二次的な変化を補正し,隕石に打ち込まれた太陽風ヘリウム同位体比を復元する方法を考案した.復元された結果は,太陽風のヘリウム同位体比が45.5億年前からほとんど一定であることを示した.この結果は,太陽活動の初期の重水素燃焼は,それ以前に終了していたことを示している.この年代は、これまで報告されていた40億年前までの太陽風のヘリウム同位体比の推定を5億年遡るものである。

第4章では,メソシデライトに分類されるVaca Muerta隕石の希ガス分析結果が述べられている.まず,太陽風希ガスやトラップされた始原的希ガス成分がないことから,母天体の変成作用で,揮発性元素が失われたことを示した.244Pu起源のキセノンが認められることから高温の変成作用からの冷却は全岩規模では速かったことを示している.いくつかの希ガス同位体から宇宙線照射年代を求めている.このうち最も信頼度の高い81Kr-Kr法で求めた年代は岩石相-金属相の混ざった全岩試料とユークライト的円礫岩では,それぞれ1億4千万年と1億7千万年程度であった.この照射年代の差から,円礫岩は母天体の表面で6千万年以上の間宇宙線照射を受けたものが,母天体の内部に取り込まれて1億4千万年前に母天体を脱出したことを明らかにした.

第5章では,本研究で得られた成果をまとめ結論を示している.

以上のように,申請者は129I-129Xe年代測定と,希ガス同位体比測定の高精度化に成功した.開発した方法を種々の隕石試料に適用し,初期太陽系星雲の晴れ上がり時期について制約を与え得ることなどを明らかにした.これらの成果は同位体宇宙化学への大きな貢献と評価することができ,学位授与にふさわしいと判断できる.

なお本論文の第3,4章は,長尾敬介との共同研究であるが,論文提出者が主体となって分析及び検証を行ったもので,論文提出者の寄与が十分であると判断する.

したがって,博士(理学)の学位を授与できると認める.