Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.05.007]
1U.S. Naval Research Laboratory, Code 6366, Washington, DC 20375, USA
2Carnegie Institution of Washington, Washington, DC 20015, USA
3University of Hawai’i at Mānoa, Honolulu, HI, 96822, USA
4University of New Mexico, Albuquerque, NM, 87131, USA
We investigated six presolar grains from very primitive regions of the matrix in the unequilibrated ordinary chondrite Semarkona with transmission electron microscopy (TEM). These grains include one SiC, one oxide (Mg-Al spinel), and four silicates. This is the first TEM investigation of presolar grains within an ordinary chondrite host (in situ) and the first TEM study to report on any presolar silicates (in or ex situ) from an ordinary chondrite. Structural and elemental compositional studies of presolar grains located within their meteorite hosts have the potential to provide information on conditions and processes throughout the grains’ histories.
Our analyses show that the SiC and spinel grains are stoichiometric and well crystallized. In contrast, the majority of the silicate grains are non-stoichiometric and poorly crystallized. These findings are consistent with previous TEM studies of presolar grains from interplanetary dust particles and chondritic meteorites. The individual silicates have Mg#’s ranging from 15 to 98. Internal compositional heterogeneities were observed in several grains, including Al in the SiC, Mg and Al in the spinel, and Mg, Si, Al, and/or Cr in two silicates. We interpret the poorly crystalline nature, non-stoichiometry, more Fe- rather than Mg-rich compositions, and/or compositional heterogeneities as features of the formation by condensation under non-equilibrium conditions.
Evidence for parent body alteration includes rims with mobile elements (S or Fe) on the SiC grain and one silicate grain. Other features characteristic of secondary processing in the interstellar medium, the solar nebula, and/or on parent bodies, were not observed or are better explained by processes operating in circumstellar envelopes. In general, there was very little overprinting of primary features of the presolar grains by secondary processes (e.g., ion irradiation, grain-grain collisions, thermal metamorphism, aqueous alteration). This finding underlines the need for additional TEM studies of presolar grains located in the primitive matrix regions of Semarkona, to address gaps in our knowledge of presolar grain populations accreted to ordinary chondrites.