Most chondritic materials experienced diverse styles of secondary alteration resulting in formation of hydrous and anhydrous oxygen-bearing minerals—silicates, oxides, and carbonates (phyllosilicates, magnetite, calcite, dolomite, breunnerite, ferrous olivine, hedenbergite, wollastonite, grossular, monticellite, forsterite, andradite, nepheline, sodalite) (Brearley 2003, 2005 and references therein). Petrographic, mineralogic, oxygen and short-lived isotope systematics (26Al-26Mg, 53Mn-53Cr, 129I-129Xe) suggest that alteration occurred in the presence of aqueous solutions under variable physico-chemical conditions (temperature, water:rock ratio, pH, fO2, and fluid compositions) in an asteroidal setting; it started within 1–2 m.y. after formation of the CV CAIs, was multistage, and lasted up to 15 m.y. (Krot et al. 2006 and references therein). Here we review bulk O-isotopic compositions of chondritic materials, O-isotopic compositions of secondary minerals produced during asteroidal alteration, and possible effects of fluid-assisted thermal metamorphism on O-isotopic exchange in primary, high-temperature minerals (melilite and anorthite) in the CV and CO CAIs. The implications of these data for understanding temperatures of alteration, water:rock ratios, and oxygen isotopic composition of water, that probably accreted into chondrite parent bodies in the form of ice, are discussed. The inferred or measured O-isotopic composition of the meteoritic water is close to the terrestrial fractionation line (Δ17O = ±2‰); it is very different from the inferred O-isotopic compositions of the Sun [Δ17O ~ −25‰] (Clayton 2002; Yurimoto and Kuramoto 2004; Hashizume and Chaussidon 2005; Lyons and Young 2005), suggesting significant evolution of O-isotopic composition of the gaseous reservoir in the inner protoplanetary disk over its short (~3–5 m.y.) life time.

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