Amorphous silicates as a record of solar nebular and parent body processes—A transmission electron microscope study of fine‐grained rims and matrix in three Antarctic CR chondrites

Abstract

Renazzo‐type (CR) carbonaceous chondrites belong to one of the most pristine meteorite groups containing various early solar system components such as matrix and fine‐grained rims (FGRs), whose formation mechanisms are still debated. Here, we have investigated FGRs of three Antarctic CR chondrites (GRA 95229, MIL 07525, and EET 92161) by electron microscopy techniques. We specifically focused on the abundances and chemical compositions of the amorphous silicates within the rims and matrix by analytical transmission electron microscopy. Comparison of the amorphous silicate composition to a matrix area of GRA 95229 clearly shows a compositional relationship between the matrix and the fine‐grained rim, such as similar Mg/Si and Fe/Si ratios. This relationship and the abundance of the amorphous silicates in the rims strengthen a solar nebular origin and rule out a primary formation mechanism by parent body processes such as chondrule erosion. Moreover, our chemical analyses of the amorphous silicates and their abundance indicate that the CR rims experienced progressive alteration stages. According to our analyses, the GRA 95229 sample is the least altered one based on its high modal abundance of amorphous silicates (31%) and close‐to‐chondritic Fe/Si ratios, followed by MIL 07525 and finally EET 92161 with lesser amounts of amorphous silicates (12% and 5%, respectively) and higher Fe/Si ratios. Abundances and chemical compositions of amorphous silicates within matrix and rims are therefore suitable recorders to track different alteration stages on a submicron scale within variably altered CR chondrites.

Extraterrestrial hydroxy amino acids in CM and CR carbonaceous chondrites

“… g Although Antarctic Meteorite Newsletter, 34 and NASA Antarctic Meteorite Petrographic Description (https://curator.jsc.nasa.gov/antmet/samples/petdes.cfm?sample=MIL07525, accessed on 4/22/2021) classified MIL 07525 as a CR3, subsequent reporting (e.g., Vollmer et al. 2020) has regarded MIL 07525 as a CR2 because of abundant secondary phases. …”

Earth’s carbon deficit caused by early loss through irreversible sublimation

“…The calculated sublimation temperatures are in general agreement with literature values based on heating experiments (e.g., 4, 35, Table S2). A number of pyrolysis studies found that some IOM may survive heating in the laboratory at temperature above 500 K in time scales ranging from near-instantaneous to several hours (e.g., [36][37][38]. However, these studies were not designed to investigate the kinetics of thermal decomposition.…”

Processes and temperatures of FGR formation in chondrites

“…Zanetta et al (2021) recently proposed an alternative nebular scenario based on the study of the Paris (CM) chondrite. They observed, in FGRs but not in the matrix, a characteristic micro-texture made of sulfide-rich and sulfide-poor compacted domains of amorphous silicate (as also observed in the CM2 chondrite Y-791198 (Chizmadia and Brearley, 2008), and two CR2 chondrites MIL 07525, and GRA 95229 (Vollmer et al, 2020)). They interpreted this micro-texture as the result of a thermal process, which occurred in the nebula during FGR accretion onto the chondrule and before incorporation into the matrix (i.e.…”

“…They consist of an unequilibrated assemblage of submicrometric phases but in comparison to the nearby matrix, large (>5 mm) crystalline anhydrous phases such as Mg-rich olivine, pyroxenes and sulfides are absent (Brearley, 1993;Zolensky et al, 1993;Zega and Buseck, 2003;Metzler, 2004;Chizmadia and Brearley, 2008;Zanetta et al, 2021). Rims are chemically comparable to the matrix but are more compact (Metzler et al, 1992;Zolensky et al, 1993;Lauretta et al, 2000;Zega and Buseck, 2003;Greshake et al, 2005;Hanna and Ketcham, 2018;Vollmer et al, 2020;Zanetta et al, 2021).…”

“…The favored scenario to explain the origin of FGRs is that dust first accreted onto chondrules in a nebular environment before they were all assembled with the matrix to form parent bodies. Chondrules and their attached FGRs then evolved in a primary parent-body as a function of hydrothermal and metamorphic conditions (Brearley and Geiger, 1991;Metzler et al, 1992;Zolensky et al, 1993;Hanowski and Brearley, 2000;Hua et al, 2002;Leitner et al, 2016;Haenecour et al, 2018;Vollmer et al, 2020). Another scenario invoking a nebular environment is the Kinetic Dust Aggregation (KDA) model, where FGRs are formed through relatively high-speed (order of meters per second to kilometers per second) collisions (Liffman and Toscano, 2000;Liffman, 2019).…”