Origin of C AIs, the oldest rocks in the solar system: A view from oxygen isotopes and rare earth elements

Abstract

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. © 2005, Japan Association of Mineralogical Sciences. All rights reserved.