Acid alteration in the western Izu Peninsula reflects the process of the past hydrothermal activity in the district. For the purpose of considering the age of acid alteration in the western Izu Peninsula, K-Ar ages of separated alunite were measured. As a result, the ages of 3.3 ± 0.1 Ma from the Funabara deposits and 0.34 ± 0.05 Ma from the Nishina deposits were obtained. It is considered that the Funabara deposits formed in relation to the late extrusion of Kano andesites. The age of the Nishina deposits and surrounding acid alteration is comparable with intrusion at the northern part of Chokuro mountain.
    Before 2.0 Ma, hydrothermal attack occurred at Funabara district in Tertiary formation. The extensive acid alteration in Ugusu district formed about 2.0-1.0 Ma. At 1.0-0.3 Ma, subsequent hydrothermal system occurred in Nishina district.

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Orbital evolutions and spectral changes of small bodies in the solar system are discussed. Recent discovery of Kuiper belt objects changed the image of our solar system. The size of the largest body is close to that of Pluto. Orbital stability and simulation studies clarified a route from Kuiper belt objects to short period comets through Jupiter family comets. Recent dynamical studies including chaos and Yarkovsky effect clarified a route from main belt asteroids to near Earth asteroids and meteorites. Space weathering process not only solved spectral mismatches between asteroids and meteorites but also should provide age information of asteroids. Color variation of Kuiper belt objects may reflect ongoing surface alteration process.

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Studies of oxygen isotopes and rare earth elements in CAIs, the oldest rocks in the solar system, are briefly reviewed and inferences for their formation processes are discussed. Various models have been proposed for the origin of ¹⁶O-rich oxygen isotopic compositions in CAIs: (1) existence of an ¹⁶O-rich presolar component, (2) some kind of chemical reactions producing mass-independent isotopic anomalies, such as photo-dissociation of ozone to produce atomic oxygen, (3) self-shielding effect in the photo-dissociation of CO in the nebula. An ¹⁶O-rich presolar component origin is highly unlikely. Chemical reactions producing mass-independent isotopic effect have been suggested but not established by experiments nor theory and also no process has been proposed to fix such isotopic anomaly in CAIs. At present, self-shielding effect in photo-dissociation of CO in the nebula seems to be the most promising model to explain the observed oxygen isotopic variations in the solar system. Direct measurement of oxygen isotopic composition in the solar wind implanted in lunar soils or in metal foils recovered by GENESIS Mission would provide important constraints on the origin of oxygen isotopic variations in the solar system. Abundance patterns of rare earth elements (REEs) in CAIs are highly variable: unfractionated patterns (Groups I, III or V), Group II pattern (depleted in ultra-refractory, heavy REEs) and ultra-refractory pattern (complementary to Group II). The latter two patterns suggest that very effective gas/dust separation occurred at high temperatures (e.g., ∼1700 K) in the early solar system.

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A series of experiments involving interstellar organic materials, formation in interstellar clouds, evaporation in the solar nebula, collision and subsequent sticking, and metamorphism in the parent bodies of carbonaceous chondrite, were performed to investigate the role of organic materials in the formation and evolution of asteroids and meteorites. It was shown that diamond precursor nucleated as a result of UV photolysis of interstellar ice mixtures in molecular clouds, and grew with further UV irradiation in diffuse clouds. It was found that there would be molecular cloud and diffuse cloud organic materials in the solar nebula at heliocentric distances larger than 2.1 and 2.3 AU, respectively. Very reductive gas was evaporated at around 2.1 AU during the evaporation of diffuse cloud organic materials, and this causes very reduced condition in the solar nebula. The organic material was found to be stickiest at 2.3 and 3.0 AU, with a maximum sticking velocity of 5 m/s. This indicated a very rapid coagulation of the very sticky organic grain aggregates and the formation of planetesimals in the asteroid region, covering even the early stage of the turbulent solar nebula. The planetesimals formed in this region appear to represent achondrite parent bodies. In contrast, the formation of planetesimals at < 2.1 and > 3.0 AU begins with the establishment of a passive disk because silicate and ice are not as sticky as organic grains. The planetesimals formed in respective regions appear to represent ordinary and carbonaceous chondrite parent bodies. We found that both C and N contents and infrared spectra of insoluble organic materials in carbonaceous chondrites could be reproduced well by the aqueous alteration and the subsequent thermal metamorphism of organic materials formed in molecular cloud. The resulting sample shows strong evidence of diamond formation. The various characteristics of nano-diamonds in chondrites are well explained by the formation in interstellar clouds and parent bodies of carbonaceous chondrites.

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