Meteorite petrology versus genetics: Toward a unified binominal classification

Abstract

The current meteorite taxonomy, a result of two centuries of meteorite research and tradition, entangles textural and genetic terms in a less than consistent fashion, with some taxa (like "shergottites") representing varied lithologies from a single putative parent body while others (like "pallasites") subsume texturally similar objects of multifarious solar system origins. The familiar concept of "group" as representative of one primary parent body is also difficult to define empirically. It is proposed that the classification becomes explicitly binominal throughout the meteorite spectrum, with classes referring to petrographically defined primary rock types, whereas groups retain a genetic meaning, but no longer tied to any assumption on the number of represented parent bodies. The classification of a meteorite would thus involve both a class and a group, in a twodimensional fashion analogous to the way Van Schmus and Wood decoupled primary and secondary properties in chondrites. Since groups would not substantially differ, at first, from those in current use de facto, the taxonomic treatment of "normal" meteorites, whose class would bring no new information, would hardly change. Yet classes combined with high-or low-level groups would provide a standardized grid to characterize petrographically and/or isotopically unusual or anomalous meteorites-which make up the majority of represented meteorite parent bodies-for example, in relation to the carbonaceous/ noncarbonaceous dichotomy. In the longer term, the mergers of genetically related groups, a more systematic treatment of lithology mixtures, and the chondrite/achondrite transition can further simplify the nomenclature.

Reconciling fast and slow cooling during planetary formation as recorded in the main group pallasites

“…Framed in this way, perhaps the unusual feature of pallasite meteorites is that they were excavated in such a way that preserved them and allowed them to be delivered to Earth, as opposed to their formation being a unique event. This is supported by the evidence for planetesimal growth in two distinct reservoirs in our Solar System (Morbidelli et al, 2022), both of which are sampled by pallasitic material: while we specifically discuss and model the parent body of the Main Group Pallasite meteorites, the umbrella group of pallasites including the Eagle Station Pallasites, the Pyroxene Pallasites, and anomalous ungrouped samples, must sample multiple parent bodies sourced from both the carbonaceous and non-carbonaceous reservoirs (Jacquet, 2022). These similar lithologies, samples from different regions of the early Solar System, from isolated planetesimals, suggests that this process was repeated on multiple bodies.…”

From whom Bells tolls: Reclassifying Bells among CR chondrites and implications for the formation conditions of CR parent bodies

“…Although the empirical concept of a chondrite “group” does not mandate a single primary parent body (Jacquet, 2022), it is worthwhile to investigate whether matrix‐rich CRs derived from the same type of parent body as the more common matrix‐poor CRs. On its own, the range of the degree of aqueous alteration across the whole CR group (i.e., from type 3 to 1) does not preclude the existence of a single primary parent body.…”

“…Although planetary accretion and celestial mechanics have biased meteorite delivery to the Earth (e.g., Meibom & Clark, 1999), and thus limited our sampling of the C superclan planetesimals, some structure can still be discerned: i. Most CCs (e.g., CM, CO, CV, CK, CL) contain >1 vol% refractory inclusions (RIs), and thus define an "RI-rich" chondrite class (Jacquet, 2022).…”

“…iii. CR, CH, and CB chondrites define a distinctive clan, classically known as the "CR clan" (Weisberg et al, 1995), but recently proposed to be termed the "CRHB clan" to avoid confusion with their eponymous chemical group (Jacquet, 2022); we will follow this convention herein. Compared to their RIrich counterparts, CRHB chondrites have distinctive isotopic properties: isotopically heavy hydrogen and nitrogen compositions (Aleon, 2010;Piani et al, 2021), a deficit in 26 Mg (derived from 26 Al decay; Schrader et al, 2017;Tenner et al, 2019;van Kooten et al, 2016), and fewer 16 O-rich chondrules (Marrocchi et al, 2022;Schrader et al, 2013;Tenner et al, 2015).…”

“…Nontraditional isotopic systematics have been used to partition meteorites into two superclans: carbonaceous (C) and noncarbonaceous (NC) meteorites (e.g., Kleine et al., 2020) Carbonaceous chondrites (CCs) are widely considered to represent the outer regions of the solar protoplanetary disk due to their (i) spectroscopic affinities to C, P, and D asteroids (e.g., DeMeo et al., 2022) and known cometary samples and interplanetary dust particles (e.g., Zolensky et al., 2008) and (ii) high concentrations of volatile elements (Alexander et al., 2012; Marrocchi et al., 2021; Vacher et al., 2020). Although planetary accretion and celestial mechanics have biased meteorite delivery to the Earth (e.g., Meibom & Clark, 1999), and thus limited our sampling of the C superclan planetesimals, some structure can still be discerned: Most CCs (e.g., CM, CO, CV, CK, CL) contain >1 vol% refractory inclusions (RIs), and thus define an “RI‐rich” chondrite class (Jacquet, 2022). CI chondrites are nearly 100 vol% matrix, although isolated olivine and pyroxene fragments isotopically reminiscent of RIs and chondrules are known to occur (Leshin et al., 1997; Morin et al., 2022; Piralla et al., 2020).…”