Ll-chondrite Northwest Africa 6813: Sampling an Impact-cratered Asteroid

Introduction: Impact cratering is one of the dominant geologic processes affecting asteroids. It results in shock-induced melting and the formation of impactites, including impact melt breccias. Meteoritic samples of these breccias possess petrologic and geochemical signatures that can be used to deduce the collisional evolution of the respective parent body, such as the timing of the event, shock metamorphic conditions during the impact, subsequent two-stage cooling, and an estimate of the size of the impact crater itself. In this study we analyzed the petrography and mineral chemistry of Northwest Africa (NWA) 6813 in order to elucidate its thermal and impact history. NWA 6813 was classified as an LL6 impact melt breccia [1], which is part of a group of ordinary chondrites whose impact lithologies remain poorly described. Analytical Methods: We examined a thin-section (DKLPI–174) of NWA 6813. To determine modal abundances, a point count (n=4296) was conducted on a petrographic microscope at ×500 magnification, with a step size of 100 μm along lines with a separation of 300 μm. To determine chemical compositions, analyses were made with a JEOL 8530F electron microprobe at the NASA–JSC using an accelerating voltage 15 kV. Silicates were analysed using a 20 nA beam current and a 1 μm beam diameter. Feldspar was analysed using a 10 nA beam current and a 2-4 μm beam diameter. Metal and sulphide were analysed using a 30 nA beam current and a 1 μm beam diameter (metal) and 3 μm (sulphide). Some metal analyses were conducted along line scans with an analytical spacing of 2-4 μm. Petrography: The sample is an impact-brecciated LL-chondrite with melt veins and pockets, and is composed of two distinct lithotype domains: (i) a clast domain (CD), which is composed of 5 distinctive clasts, with two large clasts (1 and 5) 6.3×7.7 mm and 6.7×8.5 mm, and (ii) an optically dark, melt-bearing (DMB) domain that surrounds small entrained clast fragments and contains veins and pockets of impact melt (Fig. 1). The CD is ~68% of the sample. Clast 5 appears darker than the other clasts. Clast domain. This domain is composed of 84% silicates, oxides, 3% metals, 4% sulphides, and 7% undivided metal/sulphides (<4 μm in size), normalised to 100%, which is consistent with the modal composition of other LL-chondrites [2]. Chondrules have mostly porphyritic olivine, barred olivine, and radial pyroxene textures, and fewer porphyritic olivine-pyroxene textures. The average Figure 1. Thin section scan showing the clast domain (CD) and the dark, melt-bearing (DMB) domain.