Al-Cu-Fe alloys in the solar system: Going inside a Khatyrka-like micrometeorite(KT01) from the Nubian Desert, Sudan

A recently described micrometeorite from the Nubian desert (Sudan) contains anexotic Al-Cu-Fe assemblage closely resembling that observed in the Khatyrka chondrite(Suttle et al., 2019;Science Reports9:12426). We here extend previous investigations of thegeochemical, mineralogical, and petrographic characteristics of the Sudan spherule bymeasuring oxygen isotope ratios in the silicate components and by nano-scale transmissionelectron microscopy study of a focused ion beam foil that samples the contact between Al-Cu alloys and silicates. O-isotope work indicates an affinity to either OC or CR chondrites,while ruling out a CO or CM precursor. When combined with petrographic evidence weconclude that a CR chondrite parentage is the most likely origin for this micrometeorite.SEM and TEM studies reveal that the Al-Cu alloys mainly consist of Al metal, stolperite(CuAl), and khatyrkite (CuAl2) together with inclusions in stolperite of a new nanometric,still unknown Al-Cu phase with a likely nominal Cu3Al2stoichiometry. At the interfacebetween the alloy assemblage and the surrounding silicate, there is a thin layer (200 nm) ofalmost pure MgAl2O4spinel along with well-defined and almost perfectly spherical metallicdroplets, predominantly iron in composition. The study yields additional evidence that Al-Cu alloys, the likely precursors to quasicrystals in Khatyrka, occur naturally. Moreover, itimplies the existence of multiple pathways leading to the association in reduced form ofthese two elements, one highly lithophile and the other strongly chalcophile.