Demonstration of REE fractionation among individual chondrules from the Allende (CV3) chondrite

Abstract

Abundances of REE, Ba, Sr, Rb, K, Mg and Ca were determined by precise mass spectrometric isotope dilution techniques for 24 chondrules from the Allende (CV3) chondrite. The REE abundances are 2.5−10 × CI for barred olivine chondrules, 2−8 × CI for porphyritic and nonporphyritic pyroxene chondrules, 0.15−4 × CI for porphyritic olivine and porphyritic olivine-pyroxene chondrules and are more or less similarly fractionated. General REE fractionations and large (up to 170%) anomalies of Ce, Eu and Yb occur in all chondrule types, particularly for barred olivine and pyroxene-rich chondrules. Positive correlations of REE with the moderately volatile elements, K and Rb, as well as other refractory elements, Ca, Sr and Ba, are independent of textural type and major chemical compositions. Each type of chondrule has large and systematic abundance variations of K and Rb, but shows a constant K Rb ratio close to that of CIs. From these results, the following constraints on the chemical characteristics of precursors and chondrule-forming events are suggested: (1) vaporization loss of alkalis accompanied by K Rb fractionation did not occur during chondrule-formation melting events, (2) elemental abundances were basically established prior to melting events by accretion of alkali-free components) and alkali-bearing refractory precursors with fractionated REE, (3) gas/solid (or liquid) processes yielding REE fractionations took place during the formation of refractory precursors.