The Highly Unequilibrated Eh Chondrite, Sahara 97072, May Be a Primitive Breccia

Introduction: Breccias are common among ordinary and enstatite chondrites [1]. Some type-3 ordinary chondrites have been described as primitive breccias because they contain fragments of types-3 to-6 chon-drites, carbonaceous, and impact-melted material [2]. Sahara 97072, one of the most primitive ECs [3], contains fragments of chondrules, shock-melt veins of Fe metal and troilite in eutectoid intergrowths, and lumps that appear to be impact-melted. We investigated several spheroidal shocked lumps containing fragments of metal, sulfide, and silicates. The lumps are similar in size to the chondrules and metal-sulfide nodules. (Fig. 1). Our goal is to describe one of these spheroidal lumps and to interpret the implications of their occurrence for the formation of Sahara 97072. Methods: Element maps and high-resolution images of a 500-µm lump were made with two field-emission-source scanning electron microscopes (SEMs) equipped with energy-dispersive spectrometers. Quantitative analysis was performed with a JEOL JXA-8600 electron microprobe using wavelength-dispersive spectrometry. Results: The spheroidal lump consists of fragments ranging from 5 to 40 µm in diameter set in fine-grained matrix that is crosscut by thin melt veins of Fe metal and sulfide (Fig. 1, 2). Minerals composing the fragments are Fe-Ni metal with perryite inclusions, troilite, oldhamite, niningerite, pyroxene, SiO 2 , albite, and sparse schreibersite. The mineralogy reflects bulk Sahara 97072, although differences exist. Most pyrox-ene in the lump is low in Ca. The Fe contents of sili-cates inside the lump are higher than the surrounding chondrules (Fig. 3). High-magnification SEM images of the lump reveal 10-µm intergrown Fe metal and troilite blebs similar to the large eutectoid melt veins in other regions of the meteorite, as well as micron-sized veins and spherules of metal and sulfide within and between the silicates. Many of the silicate fragments are angular and appear to be fractured (Fig. 3). Discussion: Pyroxene in chondrules surrounding the lump and near the edge has low FeO (ferrosilite = 1 to 2 wt %). Some of the pyroxene in the lump has high FeO content (ferrosilite 10 to 20 wt %; Fig. 3). Melt veins, metal spherules and the eutectic melt texture of mixed Fe metal and troilite fragments suggest this lump was formed in an impact episode. Shock heating may explain the Fe enrichment in the silicates perhaps through formation of Fe-filled fractures too fine to be excluded from the electron microprobe