Chemical, isotopic and mineralogical evidence for the origin of matrix in ordinary chondrites

Abstract

We report the first combined chemical, isotopic and mineralogical study of fine-grained opaque matrix material from a type 3 ordinary chondrite, Allan Hills A77299 (H3.7). Electron microprobe and instrumental neutron activation analysis of a large matrix lump show that it has a major element composition typical of matrix material in type 3 chondrites. Unlike chondrules, it shows no siderophile element depletions and is remarkably unfractionated relative to CI chondrites, suggesting that it is primitive solar system material. The matrix lump has an unique oxygen isotopic composition which lies below the terrestrial fractionation line and differs markedly from the composition of fine-grained matrix lumps and chondrule rims from Semarkona ( Grossman et al., 1987). Transmission electron microscope (TEM) studies of microtomed samples of the fine-grained fraction (<5 μm) of the matrix lump show that it consists of rare, angular, clastic grains (>1 μm) of pyroxene and olivine, embedded within a groundmass of rounded, fine-grained (<0.5 μm) olivines. The mineralogy and textures of the matrix lump are typical of other occurrences of fine-grained matrix studied by TEM in other type 3 ordinary chondrites. We conclude that the matrix lump in ALHA 77299 consists largely of material that formed by annealing of amorphous presolar dust or nebular condensates and could not have been derived from chondrules by any reasonable mechanism. However, the observed compositional differences between the matrix lump and chondrules in ordinary chondrites are consistent with the idea that some chondrules formed by melting of matrix-like material, accompanied by loss of siderophile and volatile elements.