Trace-element abundances in hibonites from the Murchison carbonaceous chondrite: Constraints on high-temperature processes in the solar nebula

Abstract

Abundances of trace elements, including 14 rare earth elements (REE) and 8 other refractory trace elements (RTE) have been measured in 33 hibonite- and 3 perovskite-bearing grains from the Murchison carbonaceous chondrite. The normalized abundance patterns show a good correlation with three morphological and chemical groups of hibonites. Sixteen platy hibonite crystals (PLACs) typically have patterns which are depleted in the relatively volatile trace elements V, Nb, Ba, Eu, and Yb (Group III). Three PLACs also show large depletions in Ce; one PLAC is enriched in the ultrarefractory REE, while two other PLACs have fractionated patterns enriched in the light REE, with positive Tm anomalies. The PLACs commonly have a roll-off in the abundances of the HREE, which is consistent with solid-liquid partitioning. Three blue aggregates (BAGs) have patterns with low overall abundances and depletions in the ultrarefractory REE except Tm (Group II). Spinel-hibonite grains (SHIBs) show a variety of patterns mainly of Group II, but 4 SHIBs have relatively smooth patterns with anomalies in the volatile REE Ce, Eu, and Yb. The perovskites have patterns similar to Group II but with excesses in Eu, and Yb. The HREE roll-off in the PLACs and the igneous morphologies of the SHIBs indicate that these grains formed by the crystallization of melts. Distillation may be important for the BAGs and two of the PLACs which have large isotopic mass-fractionation effects. PLACs have the simplest trace element patterns and the largest Ti isotopic anomalies and probably were the first hibonites to form in the early solar system. The SHIBs have trace-element signatures that require multiple processing episodes. They are somewhat complementary to the PLACs in that they are enriched in the volatile elements which are depleted in the PLACs. On average, the SHIBs contain isotopically less-anomalous Ti than the PLACs. However, SHIBs contain 26Mg excesses that are consistent with the in situ decay of 26Al, whereas PLACs generally have only small, or no excesses of 26Mg despite high 27Al 24Mg . While an overall relationship exists between the morphology, chemistry, and Ti and Mg isotopic systematics of meteoritic hibonites, they do not, as yet, fit into a simple scenario for the formation of the solar system.