TEM analyses of in situ presolar grains from unequilibrated ordinary chondrite LL3.0 Semarkona

TL;DR

This study used TEM to analyze six presolar grains from the primitive Semarkona meteorite, revealing their formation conditions, mineralogy, and evidence of parent body alteration, with implications for understanding early solar system processes.

Contribution

First TEM analysis of presolar grains from Semarkona's primitive matrix, providing insights into their formation and alteration histories.

Findings

SiC and spinel grains are well crystallized and stoichiometric.

Most silicate grains are non-stoichiometric and poorly crystallized.

Limited secondary processing observed, highlighting preservation of primary features.

Abstract

We investigated six presolar grains from very primitive regions of the matrix in the unequilibrated ordinary chondrite Semarkona with TEM. These grains include one SiC, one oxide (Mg-Al spinel), and four silicates. 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. 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.