The most primitive mesosiderite Northwest Africa 1878, subgroup 0

1,2M. Kimura,3,4N. Sugiura,1,5A. Yamaguchi,3K. Ichimura
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13474]
1National Institute of Polar Research, Tokyo, 190‐8518 Japan
2Ibaraki University, Mito, 310‐8512 Japan
3University of Tokyo, Tokyo, 113‐0033 Japan
4Planetary Exploration Research Center, Chiba Institute of Technology, Chiba, 275‐0016 Japan
5Department of Polar Science, School of Multidisciplinary Science, SOKENDAI, The Graduate University for Advanced Studies, Tokyo, 190‐8518 Japan
Published by arrangement with John Wiley & Sons

All mesosiderites previously reported were subjected to thermal metamorphism and/or partial melting on the parent body. Therefore, their primordial features have been mostly lost. Here, we report detailed petrological and mineralogical features on a mesosiderite, Northwest Africa (NWA) 1878. This meteorite comprises silicate lithology and aggregates of small spheroidal Fe‐Ni metal grains. Silicate lithology typically shows igneous texture without recrystallization features, and mainly consists of low‐Ca pyroxene and plagioclase. Pyroxenes often show normal zoning. Exsolution lamella of augite is rarely noticed and very thin in width, compared with other mesosiderites. A few magnesian olivine grains are encountered without typical corona texture around them. They are not equilibrated with pyroxene on a large scale. Plagioclase shows a wide compositional range. These results show that NWA 1878 hardly experienced thermal metamorphism, distinguished from mesosiderites of subgroups 1–4. Therefore, we propose that this is classified as subgroup 0 mesosiderite. Nevertheless, NWA 1878 was locally subjected to secondary reactions, such as weak reduction of pyroxene and Fe‐Mg diffusion between olivine and pyroxene, on the parent body.

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