¹S. A. Singerling,²A. J. Brearley
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70061]
¹Schwiete Cosmochemistry Laboratory, Goethe University, Frankfurt, Germany
²Department of Earth and Planetary Sciences, MSC-03 2040, 1 University of New Mexico, Albuquerque, New Mexico, USA
Published by arrangement with John Wiley & Sons

We conducted a scanning electron microscopy (SEM) and transmission electron microscopy (TEM) study of sulfide–metal assemblages (SMAs) in minimally to moderately altered CR2 chondrites. The assemblages occur on chondrule rims and consist of kamacite cores rimmed by pyrrhotite. The kamacite and pyrrhotite share orientation relationships, arguing for a genetic link. The SMAs contain secondary alteration products, including nanoscale magnetite at the sulfide–metal interface (minimally altered SMAs) and magnetite, serpentine, nanoscale Ni-rich metal at metal–magnetite interfaces, and Ni,S-bearing reaction fronts within magnetite (moderately altered SMAs). We argue the SMAs initially formed in the solar nebula from the separation of immiscible metal and silicate melts followed by sulfidization of the metal. Aqueous alteration on the asteroidal parent body caused the kamacite to transform into magnetite and the magnetite to transform into serpentine. Alteration of kamacite to magnetite occurred under oxidizing and alkaline conditions, whereas alteration of magnetite to serpentine occurred under reducing, alkaline, and higher aSiO2 conditions. Serpentinization of magnetite appears to be a relatively common process in some carbonaceous chondrites. Additionally, theoretical and experimental studies are needed that simulate the oxidation of metal by H2O gas and water and also serpentinization of magnetite to form serpentine with variable Mg-Fe contents.

¹Gabriel Zachén,¹Carl Alwmark,¹Sanna Alwmark,^2,3Ludovic Ferrière,^4,5Roger H. Hewins
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.70051]
¹Department of Geology, Lund University, Lund, Sweden
²Natural History Museum Vienna, Vienna, Austria
³Natural History Museum Abu Dhabi, Abu Dhabi, United Arab Emirates
⁴IMPMC, MNHN, UMR CNRS 7590, Sorbonne Université, Paris, France
⁵Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
Published by arrangement with John Wiley & Sons

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.