An in situ study of presolar grains and the fine‐grained matrices of the Meteorite Hills 00526 and Queen Alexandra Range 97008 unequilibrated ordinary chondrites

Abstract

Here we report in situ structural and chemical analyses of four presolar grains and the matrices of the Meteorite Hills (MET) 00526 L3.05 and Queen Alexandra Range (QUE) 97008 L3.05 unequilibrated ordinary chondrites (UOCs). The presolar grains in MET 00526 include one Fe‐rich single crystal olivine, and one olivine grain that contains both amorphous and polycrystalline material. The single crystal olivine likely has origins in the circumstellar envelope (CSE) of a red giant branch (RGB) or asymptotic giant branch (AGB) star, and the amorphous/polycrystalline olivine has an O‐isotopic composition consistent with origins in a type II supernova. The presolar grains from QUE 97008 are Fe rich and include one crystalline, stoichiometric olivine that contains a Ca‐rich core and one crystalline, stoichiometric pyroxene grain, both of which have O‐isotopic compositions consistent with origins in the CSEs of low‐mass AGB/RGB stars. The matrices of both UOCs are mineralogically diverse with evidence for unaltered material in the form of amorphous silicates and a C‐rich nanoglobule and altered material in the form of Ni‐rich sulfides, abundant Fe‐rich olivine, and Fe‐Mg zoning in matrix silicates. No phyllosilicates were observed. The Fe‐rich olivine grains are the dominant alteration phase in both UOCs and likely replaced primary amorphous silicates in the presence of an Fe‐rich fluid during parent body alteration. Our work suggests that the ordinary and carbonaceous chondrites received a similar inventory of dust with comparable structures and chemistries.