Microstructures and formation history of melilite-rich calcium–aluminum-rich inclusions from the ALHA77307 CO3.0 chondrite

The petrologic and oxygen isotopic characteristics of calcium-aluminum-rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0-3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite-bearing inclusions (spinel-hibonite spherules, monomineralic grains, hibonite-pyroxene microspherules, and irregular/nodular objects), grossite-bearing inclusions (monomineralic grains, grossite-melilite microspherules, and irregular/nodular objects), melilite-rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel-pyroxene inclusions (fluffy objects resembling fine-grained spinel-rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene-anorthite inclusions. They are typically small (98.4 AE 54.4 µm, 1SD) and comprise 1.54 AE 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca-Alsilicate during terrestrial weathering and converted melilite-rich inclusions into spinelpyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM-like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0-3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (Δ 17 O: À25 to À2&) but left those in CO3.0-3.05 chondrites mostly unchanged (Δ 17 O: À25 to À20&), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.

Section: Discussionsupporting

confidence: 84%

Petrology and oxygen isotopic compositions of calcium‐aluminum‐rich inclusions in primitive CO3.0‐3.1 chondrites

Zhang

Bonato

King

et al. 2020

“…However, their 3-D shapes have not been studied in detail. As the first solids in our solar system, they could have formed by coagulation of micro-sized minerals condensed from high-temperature gas or by the growth of new minerals on early condensed minerals via gas-solid reactions, or through a combination of both processes (Ebel and Grossman 2000;Charnoz et al 2015;Han and Brearley 2017). Thus, studying the shape and porosity of refractory inclusions could help us understand how dust coagulates in the protoplanetary disk.…”

Section: Introductionmentioning

confidence: 99%

Shape and porosity of refractory inclusions in CV3 chondrites: A micro‐computed tomography (µCT) study

Zhang

Clark

King

et al. 2021

Refractory calcium-aluminum-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs) in chondritic meteorites are the earliest solids of our solar system, bearing the information of nebular condensation as well as accretion and asteroidal shock and metasomatism processes. While the compositions of refractory inclusions have been intensely studied for~50 years, their physical properties such as shape and porosity are poorly constrained. Here, we present a microcomputed tomography (µCT) study on 16 refractory inclusions of condensate origin in five CV3 chondrites. We find that they are prolate or triaxial in shape with very rough morphologies. The CAIs have nodular textures and are thought to form by agglomerating individual nodules via collision-induced bouncing and/or fragmentation, where the nodules were grown by gas-solid reactions during condensation. On the parent body, refractory inclusions from the CV R meteorite Leoville experienced intense shocks that led to the flattening of their shapes and lowering of their porosities. High-temperature metasomatism in CV OxA meteorites and low-temperature metasomatism in CV OxB meteorites do not seem to have large effects on the porosities of their refractory inclusions, which have similar ranges and pore-size distributions. Instead, we infer that their pores are mostly inherited from the gas-solid condensation and subsequent agglomeration processes. The porosities of CAIs are higher than those of AOAs, which is mainly due to the high-temperature sintering process of AOAs.

“…2016), accounting for the structural controls on spinel formation reported by Han and Keller (2019; cf. Han and Brearley 2017). Few Allende CAIs are whole or fragmental remnants of spherical melted type B CAIs (Fig.…”

Section: Resultsmentioning

confidence: 99%

Comparison of the Murchison CM2 and Allende CV3 chondrites

Fendrich

Ebel

2021

The size distribution, abundance, and physical and chemical characteristics of chondritic inclusions are key features that define the chondrite groups. We present statistics on the size and abundance of the macroscopic components (inclusions) in the Murchison (CM2) and Allende (CV3) chondrites and measure their general chemical trends using established X‐ray mapping techniques. This study provides a fine‐scale assessment of the two meteorites and a semiquantitative evaluation of the relative abundances of elements and their distribution among meteorite components. Murchison contains 72% matrix and 28% inclusions; Allende contains 57% and 43%, respectively. A broad range of inclusion sizes and relative abundances has been reported for these meteorites, which demonstrates the necessity for a more standardized approach to measuring these characteristics. Nonetheless, the characteristic mean sizes of inclusions in Allende are consistently larger than those in Murchison. We draw two significant conclusions (1) these two meteorites sampled distinct populations of chondrules and refractory inclusions, and (2) complementary Mg/Si ratios between chondrules and matrix are observed in both Murchison and Allende. Both support the idea that chondrules and matrix within each chondrite group originated in single reservoirs of precursors with approximately solar Mg/Si ratios, providing a constraint on astrophysical models of the origin of chondrite parent bodies.