Genesis of the Calcium-Rich a Chondrites in Light of Rare-Earth and Barium Concentrations

Abstract

Rare-earth and barium concentrations have been determined by mass-spectrometric isotope-dilution for a chondrite composite, nine calcium-rich achondrites, two plagioclase separates, and two pyroxene separates. The brecciated basaltic achondrites have higher (3–17 times) absolute rare-earth and barium concentrations than those in the average chondrite, but similar relative concentrations. Rare-earth and barium data, petrography, and chemistry indicate that the brecciated achondrites cannot be cumulates, to any great extent, of pigeonite or plagioclase, which have fractionated relative rare-earth and barium concentrations. The brecciated achondrites apparently represent liquids, perhaps with different amounts of phenocrystic hypersthene; the closed-system crystallization of the Juvinas achondrite, which is induced from the whole-rock and mineral europium concentrations, strongly supports this interpretation. Hypabyssal or extrusive (rather than plutonic) crystallization is indicated. Measured plagioclase and clinopyroxene partition coefficients for the rare earths and barium place limits on maximum amounts of plagioclase and pigeonite of approximately 20% each that could have been removed from the achondritic magmas.

Plagioclase compositions, and rare-earth and barium concentrations considered in terms of measured plagioclase partition coefficients, indicate that the Serra de Mage unbrecciated achondrite is a cumulate formed from a liquid of normal brecciated achondrite composition. The fractionated whole-rock and pigeonite rare-earth and barium concentrations of the Moore County unbrecciated achondrite indicate that this meteorite is also such a cumulate; at the same time, the europium anomalies in the pigeonite and plagioclase, and the rare-earth pattern of the plagioclase, indicate considerable post-accumulation closed-system competition. It is of interest that the available rare- earth and barium data distinguish the brecciated and unbrecciated achondrites as magma and cumulates respectively; this suggests different environments. The Shergotty unbrecciated achondrite has rare-earth and barium concentrations roughly similar to those in the brecciated achondrites, but unexplained irregularities exist. The rare-earth and barium concentrations of Angra dos Reis (stone) and its composition (more than 90% augite), argue against this meteorite being a cumulate from a normal achondritic liquid; the Angra dos Reis (stone) rare-earth and barium concentrations are surprisingly similar to those observed in terrestrial sub-alkaline oceanic tholeiites.