Nebular thermal processing of accretionary fine-grained rims in the Paris CM chondrite

Abstract

Fine-grained rims (FGRs) are ubiquitous in chondrites. They consist of unequilibrated mineral assemblages that surround chondrules and refractory inclusions. As such, they carry information about the material that was accreted onto chondrules. To decipher the nature and the formation mechanism of FGRs and compare them to adjacent matrix material, we investigated their composition, mineralogy, density and texture in the pristine Paris CM chondrite. We coupled a new method (ACADEMY; Zanetta et al., 2019) at the scanning electron microscope (SEM) scale that allows high-resolution quantitative petrology and an analytical transmission electron microscope (TEM) study. Significant differences in modal abundance, grain size and porosity are observed between the FGRs and their adjacent matrix. Domains of amorphous silicates with embedded nanosulfides indicate a high degree of preservation. They are less abundant in the matrix than in the rims. In contrast, secondary alteration phases (phyllosilicates, carbonates and tochilinites) are more abundant in the matrix and associated with larger and fewer sulfide grains. The similar composition of the amorphous silicate in the rims and the matrix attests for a close relationship between the two reservoirs. However, matrix underwent more aqueous alteration. We interpret it as the result of the accretion of material with a higher amount of water in the matrix, leading to a more aqueously altered microenvironment. We also find that coarse-grained anhydrous silicates (olivine and pyroxene) are present in the matrix but not in the FGRs, likely as a result of a chondrule fragmentation episode that occurred after FGR but before matrix accretion. Most of the time, FGRs display distinct inner and outer layers. The inner part is compact and displays larger sulfide grains than the outer part, which is more porous (porosity ~ 45%) and altogether more pristine. These mineral and textural differences are not easily explained by differential aqueous alteration. Instead, a pre-accretion thermal process that preferentially affected the inner rim could have induced loss of porosity, compaction of the amorphous silicate domains as well as sulfides growth. We therefore suggest that FGRs acquired their characteristics in the nebula before matrix accretion to form Paris parent body and discuss possible mechanisms such as dust heating in the chondrule formation environment or secondary heating episode of the previously rimmed chondrule as well as gas friction in high-speed collision conditions.

Fine-grained rim formation via kinetic dust aggregation in shock waves around evaporating icy planetesimals

“…Our scenario also indicates that inner rims formed in a hotter environment than outer rims. This would be consistent with the observed characteristics of inner rims (e.g., silicate sintering, sulfides growth, and compaction; Zanetta et al 2021).…”

“…The majority of studies assumed that FGRs were formed via the accretion of dust particles onto the surfaces of chondrules/CAIs in the turbulent solar nebula (e.g., Metzler et al 1992;Morfill et al 1998;Cuzzi 2004;Xiang et al 2019Xiang et al , 2021Matsumoto et al 2021;Kaneko et al 2022). This nebular scenario naturally reproduces the positive correlation between the rim thickness and the chondrule radius, which is reported for FGRs around chondrules in CM chondrites (e.g., Metzler et al 1992;Hanna & Ketcham 2018;Zanetta et al 2021).…”

“…In addition, dust grains turned into fluffy aggregates prior to the accretion onto chondrules when the grain size is smaller than 1 µm (e.g., Arakawa 2017;Matsumoto et al 2019;Kaneko et al 2022). The typical grain size of FGRs in primitive chondrites is indeed submicron (e.g., Lauretta et al 2000;Chizmadia & Brearley 2008;Zanetta et al 2021), although grain size might be subsequently modified by aqueous/thermal alteration processes. Hence the structure of FGRs formed in the turbulent solar nebula would be highly porous; which seems to be inconsistent with the observed compact FGRs with low porosity of 10-20% (e.g., Trigo-Rodriguez et al 2006).…”

Formation of chondrule fine‐grained rims from local nebular reservoirs

“…Mounting evidence suggests that FGRs, or their immediate precursors, formed in relatively warm environments. Indeed, Zanetta et al (2021) reported that, despite their porosity, FGRs in Paris show low degree of aqueous alteration, which they attributed to low water/rock ratios, as expected if water was mostly gaseous at the time of FGR formation (and if water mobility was limited to a few tens of micrometers on the parent body after accretion; Brearley, 2014). Bigolski (2017) has found that most FGRs around UOC chondrules exhibit sintered textures and estimated a range of 350-800 °C for their formation.…”

Dependence of the initial internal structure of chondrule rim on dust size distribution

“…However, FGRs and the matrix exhibit different presolar grain abundance and alteration degree (e.g. Leitner et al, 2016;Haenecour et al, 2018;Zanetta et al, 2021). FGRs may have been formed soon after or during chondrule forming events (e.g.…”