^1,2P-M.Zanetta,¹H.Leroux,¹C.Le Guillou,^2,4B.Zanda,^2,3R.H.Hewins
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.015]
¹Univ. Lille, CNRS, INRAE, Centrale Lille, UMR 8207 – UMET – Unité Matériaux et Transformations, F-59000 Lille, France
²IMPMC, Sorbonne Université, MNHN, UPMC Paris 06, UMR CNRS 7590, 75005 Paris, France
³EPS, Rutgers Univ., Piscataway, NJ 08854, USA
⁴Observatoire de Paris, IMCCE, 75014 Paris, France
Copyright Elsevier

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 at the SEM scale (ACADEMY) that allows high-resolution quantitative petrology and an analytical TEM study.

Significant differences in modal abundance, grain size and porosity are observed between the FGRs and their adjacent matrix. Amorphous silicates domains embedding nanosulfides are indicative of a high preservation degree. 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 sulfides 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 water/rock ratio 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 and discuss possible mechanisms such as dust heating in the chondrule formation environment or secondary heating episode of the previously rimmed chondrule.

¹A.T.Hertwig,¹M.-C.Liu,²A.J.Brearley,¹S.B.Simon
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.12.014]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angeles, CA 90095, USA
²Department of Earth and Planetary Sciences, MSC03-2040, University of New Mexico, Albuquerque, NM 87131, USA
³Institute of Meteoritics, Department of Earth and Planetary Sciences, MSC03-2040, University of New Mexico, Albuquerque, NM 87131, USA
Copyright Elsevier

We performed high-precision SIMS (secondary ion mass spectrometry) 26Al-26Mg and oxygen isotope analyses of two unique CAIs, “Mesquite” and “Y24”, found in the CO3.05 chondrites Northwest Africa 7892 and Yamato-81020, respectively. Mesquite is unusually large (∼5×3 mm) for a CAI from any CO chondrite and exhibits a layered texture comprising a melilite-rich core surrounded by hibonite- and spinel-rich mantle layers and a semi-continuous spinel-dominated rim. The CAI Y24 stands out because of its distinct mineralogy: grossite, hibonite, and spinel are accompanied by abundant ultra-refractory-element-rich phases such as warkite, kangite, and perovskite. Silicates are absent in Y24.

Negatively fractionated δ25Mg values of phases in the core and mantle layers of Mesquite suggest that the inclusion as a whole was never molten and, hence, represents an aggregate of condensates. The relatively large grain sizes of melilite in the core (up to ∼300 µm) most likely are the result of solid-state recrystallization and coarsening of melilite in the course of a heating event occurring in the solar nebula. This heating event, however, did not disturb the Al-Mg systematics of Mesquite. Regardless of their position within Mesquite and the phases analyzed, spots analyzed for Al-Mg plot on a single isochron characterized by an initial 26Al/27Al of (4.95 ±0.08) ×10–5 and a δ26Mg*0 of –0.14 ±0.05‰. We suggest that this initial 26Al/27Al ratio corresponds to the formation of Mesquite in the solar nebula that was slightly heterogeneous with respect to Mg isotopes. Spinel in the rim is uniform in Δ17O (∼ –25‰); in contrast, hibonite in the core and mantle layers, albeit also 16O-rich, show variable oxygen isotope ratios (Δ17O ∼ –15‰ – –23‰), which would be consistent with hibonite condensation in a gas with quickly-changing oxygen isotope compositions. The 16O-poor composition of melilite (Δ17O ∼ –1‰ – 0‰) in the core could be the result of isotope exchange with an 16O-poor gas, perhaps during the heating event that caused the solid-state recrystallization and coarsening of melilite or the result of oxygen isotope exchange with a fluid on the parent body. Abundant calcite, phyllosilicates, and sodalite are witnesses to late-stage and low-temperature alteration of the Mesquite CAI; calcite and phyllosilicates most likely are of terrestrial origin, but sodalite could have formed in the parent body.

Inclusion Y24 is irregularly-shaped, indicating a condensation origin. Completely enclosing other phases, warkite forms the matrix of Y24, which could be the result of simultaneous condensation and growth of warkite, grossite, and hibonite. Possibly, spinel formed by replacing grossite or hibonite or both minerals in a gradually cooling gas before any silicates condensed. SIMS analyses indicate that condensation occurred in an 16O-rich gas when 26Al/27Al was (5.4 ±1.0) ×10–5. Oxygen isotope exchange with an 16O-poor fluid in the parent body or with an 16O-poor gas in a nebular setting caused the 16O-poor compositions in grossite and kangite.