An in situ investigation of the preservation and alteration of presolar silicates in the Miller Range 07687 chondrite

Abstract

Dust grains that formed around ancient stars and in stellar explosions seeded the early solar protoplanetary disk. While most of such presolar grains were destroyed during solar system formation, a fraction of such grains were preserved in primitive materials such as meteorites. These grains can provide constraints on stellar origins and secondary processing such as aqueous alteration and thermal metamorphism on their parent asteroids. Here, we report on the nature of aqueous alteration in the Miller Range (MIL) 07687 chondrite through the analysis of four presolar silicates and their surrounding material. The grains occur in the Fe‐rich and Fe‐poor lithologies, reflecting relatively altered and unaltered material, respectively. The O‐isotopic compositions of two grains, one each from the Fe‐rich and Fe‐poor matrix, are consistent with formation in the circumstellar envelopes of low‐mass Asymptotic Giant Branch (AGB)/Red Giant Branch (RGB) stars. The other two grains, also one each from the Fe‐rich and Fe‐poor matrix, have O‐isotopic compositions consistent with formation in the ejecta of type‐II supernovae (SNe). The grains derived from AGB/RGB stars include two polycrystalline pyroxene grains that contain Fe‐rich rims. The SNe grains include a polycrystalline Ca‐bearing pyroxene and a polycrystalline assemblage consistent with a mixture of olivine and pyroxene. Ferrihydrite is observed in all focused ion beam sections, consistent with parent‐body aqueous alteration of the fine‐grained matrix under oxidizing conditions. The Fe‐rich rims around presolar silicates in this study are consistent with Fe‐diffusion into the grains resulting from early‐stage hydrothermal alteration, but such alteration was not extensive enough to lead to isotopic equilibration with the surrounding matrix.

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

“…In addition to the Fe-Mg zoning of several larger matrix grains, the presolar silicates in this study are notably Fe rich. Fe-rich presolar silicate grains were reported in previous studies from both carbonaceous and ordinary chondrites (Bose et al, 2010;Floss & Stadermann, 2009;Nguyen et al, 2007Nguyen et al, , 2010Nguyen et al, , 2016Ong & Floss, 2015;Seifert et al, 2022Seifert et al, , 2023Vollmer et al, 2009). Observations of crystalline Fe-rich presolar silicates in planetary materials are inconsistent with astronomical observations that suggest abundant Mg-rich silicates in circumstellar environments (Kemper et al, 2004;Molster et al, 2002).…”

“…Electron-transparent sections (<100 nm in projected thickness) of five grains, three from MET 00526 and two from QUE 97008 and three matrix sections were prepared using the Thermo Fisher Scientific (formerly FEI) Helios G 3 focused ion beam-scanning electron microscope (FIB-SEM) located in the Kuiper Materials Imaging and Characterization Facility (KMICF) at the Lunar and Planetary Laboratory (LPL), University of Arizona. We followed previously described procedures for producing electron-transparent cross sections of presolar grains, which is described in detail in the literature (Haenecour et al, 2019(Haenecour et al, , 2020Holzapfel et al, 2009;Leitner et al, 2012Leitner et al, , 2016Nguyen et al, 2007Nguyen et al, , 2016Seifert et al, 2022Seifert et al, , 2023Stroud et al, 2004;Vollmer et al, 2007Vollmer et al, , 2009Vollmer et al, , 2013Zega et al, 2007Zega et al, , 2011Zega et al, , 2014Zega et al, , 2015Zega et al, , 2020.…”

“…Although Fe-rich amorphous silicates can be formed through nonequilibrium condensation processes (Ferrarotti & Gail, 2001;Gail & Sedlmayr, 1999). The origins of crystalline Fe-rich presolar silicates are largely attributed to nebular or parent body processing, however, the observation of crystalline Fe-rich presolar silicates in the most pristine chondrites that have experienced little to no secondary processing suggests that they could be primary in origin (Bose et al, 2010;Floss & Stadermann, 2009;Nguyen et al, 2007Nguyen et al, , 2010Nguyen et al, , 2016Ong & Floss, 2015;Seifert et al, 2022Seifert et al, , 2023Vollmer et al, 2009). In the present study, evidence for hydrothermal processing is observed in the FIB sections containing presolar silicates MET 3a2-24 o1, QUE b1-3 o1, and QUE c4-4 o1 as discussed above and all three grains have crystalline rather than amorphous structures.…”