Investigation and reclassification of four mesosiderites—Implications for their formation

Abstract Mesosiderites are rare, differentiated meteorites, so‐called stony‐iron meteorites—they are impact breccias composed of an unusual mix of crustal basalt and pyroxenite, core‐derived metal, but no mantle materials. This odd mixture makes their origin enigmatic and has inspired many different formation theories over the last several decades. Some of the outstanding questions have regarded the origin of the metal, whether it came from another celestial body or from within the main parent body, and the puzzlingly low abundance, or absence, of mantle material in mesosiderites. The role of impacts has been central to most of the suggested theories, but mesosiderites show little to no evidence of shock metamorphism. The mystery of the origin of mesosiderites is further compounded by the relatively limited amount of published data, as well as the restricted number of samples available for research. With the detailed investigation and reclassification of the mesosiderites Lamont, Acfer 265, Queen Alexandra Range 86900 (QUE 86900), and MacAlpine Hills 88102 (MAC 88102) presented herein, our new observations shine some much‐needed light on this meteorite group. Based on their petrologic and metamorphic characteristics, Lamont is classified as a B3/4, Acfer 265 and QUE 86900 as A1, and MAC 88102 as an A4 mesosiderite. The observation of multiple sets of parallel thin lamellae in high‐Ca plagioclase and cristobalite in Lamont, and a silicate emulsion in QUE 86900 is proposed to be shock‐related features. In both Lamont and QUE 86900, these features are interpreted to be subsequent to the initial impact, which mixed crustal and core material, and prior to deep burial. No shock‐related features were noted in Acfer 265 and MAC 88102.