Chondrules: chemical, mineralogical and isotopic constraints on theories of their origin

Abstract

Chondritic meteorites have not been involved in planetary melting for the past 4.5 Ga. They contain millimetre-sized subspherical objects, ‘ chondrules ’, dominantly of silicate, but occasionally grading to rare, metal-sulphide chondrules. Some meteorites comprise 70% or more of chondrules by volume. About 30% of chondrules, or less, originated as molten droplets; the remainder formed from solid or plastic rocks. No rock known from the Earth or Moon contains enough glassy spherules to be termed a chondrite, so volcanism and impact on brittle planetary surfaces were not chondrule-forming mechanisms. Chondrites have chemical compositions close to that of the volatile-free Sun, so a nebular origin for chondrules is often favoured. However, the mineralogical and chemical diversity among chondrules indicates that some had planetary origins. Furthermore, chondrules co-existed in space with disrupted fragments of planetary igneous rocks. Some chondrules may be nebular but others are not. Alternatively, all may be planetary. In either case the mechanism (s) of formation is (are) unknown.