FAYALITIC OLIVINE IN CV3 CHONDRITE MATRIX AND DARK INCLUSIONS - A NEBULAR ORIGIN

Fayalitic olivine (Fa(>32)) is the major component of the matrices and dark inclusions of CV3 and other unequilibrated chondrites. It occurs most commonly as rims, veins and halos in and around chondrule silica tes in the Allende-type (CV3(OxA) chondrites and, to a much lesser ext ent, in the reduced (CV3(R)) and Ball-type (CV3(OxB)) chondrites. The olivines have distinctive platy, tabular and lath- or irregular-shaped crystals, with the ratio of the two types varying widely. In CV3(OxB) chondrites, matrix fayalitic olivines range up to Fa(99.9); whereas, in the other CV3 chondrites, the range is much smaller. The platy and tabular anisotropic forms of the fayalitic olivines strongly suggest g rowth from a vapor, and the nature of the occurrences suggests that CV 3 matrices are unequilibrated mixtures of nebular materials. We argue that the parent body hydration/dehydration model has numerous inconsis tencies that make this hypothesis highly unlikely. These include: (1) There is no direct evidence linking fayalitic olivine to precursor phy llosilicates. (2) Dehydration of phyllosilicates cannot explain the wi de range of morphologies of the fayalitic olivines. (3) Fayalitic oliv ine clearly predates the formation of the hydrous phases in CV3 chondr ites and is one of the phases that breaks down to form phyllosilicates (Keller et al., 1994). (4) The unequilibrated nature of the matrix, i ncluding fine-scale zoning in 10 mu m sized fayalitic olivine crystals , would not survive the parent body metamorphism required in the dehyd ration model. (5) A dark inclusion in the Ningqiang chondrite contains fayalitic olivine rimmed by glassy and microcrystalline material (Zol ensky et al., 1997), which probably formed by radiation damage. This i ndicates that the fayalitic olivine was exposed to solar radiation in a nebular setting. (6) Some Allende chondrules contain unaltered prima ry, anhydrous glassy mesostasis in contact with the host matrix (e.g., Ikeda and Kimura, 1995). Chondrule mesostases would not have survived parent body hydration without becoming hydrated and would probably no t survive the metamorphic heating required in the dehydration scenario . (7) Single platy and barrel-shaped crystals of fayalitic olivine are present in accretionary rims in calcium-aluminum-rich inclusions (CAI s) (MacPherson and Davis, 1997), which developed in the nebula. (8) Ma trix lumps completely encased in chondrules in ordinary chondrites con tain mainly fayalitic olivine (Scott et al., 1984), which indicates a nebular origin. (9) Oxygen isotopic compositions of Allende matrix and dark inclusions strongly indicate little or no hydration for Allende and its components (Clayton, 1997), We favor a nebular vaporization/re condensation model in which vaporization of chondritic dust produced a fayalite-rich vapor, followed by formation of the fayalitic olivine b y direct recondensation from the vapor, epitactic growth on surfaces o f existing forsterite and enstatite in chondrules, and replacement of existing forsterite and enstatite by gas-solid exchange.