Petrology and Pairing of Mesosiderites from Victoria Land,Antarctica

Abstract— The Allan Hills mesosiderites A77219, A81059 and A81098 are classified as subgroup 1B (Hewins, 1984), on the basis of very fine-grained silicate matrix, low plagioclase content and absence of highly ferroan pyroxenes. Since they are so similar petrologically, it is reasonable to pair them. ALHA 81208, a highly weathered orthopyroxenite, is probably a clast from one of the Allan Hills mesosiderites. Reckling Peak A80258, is a Floran 2B or Hewins 4B mesosiderite. It contains reversely zoned orthopyroxene clasts in a sparse matrix with interstitial/poikilitic plagioclase and highly magnesian (chadacryst) orthopyroxene (close to En₈₀). All pyroxene is much lower in FeO/MnO than Allan Hills mesosiderite pyroxene of similar FeO content. The other Reckling Peak mesosiderites A79015, A80229, A80246 and A80263 contain orthopyroxene and recrystallized orthopyroxenite clasts in a metal-troilite matrix. Orthopyroxenite clasts in ALH A81059 are very similar texturally and modally to RKPA 79015 orthopyroxenite, but differ in pyroxene composition. Orthopyroxene in RKPA 79015 (Prinz et al., 1982) is very similar in Fe/Mg and Fe/Mn to the cores of (reversely zoned) pyroxene clasts in RKPA 80258. On this basis, RKPA 80258 is related to the other Reckling Peak mesosiderites and they could all be paired, assuming that three components (metal, pyroxenite and silicate matrix) were very irregularly mixed in these breccias. Pairing is problematical, in that specimens of a polymict breccia can be so different that they would not be paired if they were not known to have fallen together. The silicate fraction of mesosiderites ranges from diogenitic (RKPA 79015) to analogous to polymict eucrite (Dyarrl Island) although the silicate fractions are not equivalent in detail. The mesosiderite subdivision scheme is amended recognizing this, permitting the classification of the formerly “anomalous” RKPA 79015.