CM carbonaceous chondrite falls and their terrestrial alteration

Abstract

The CM carbonaceous chondrites provide unique insights into the composition of the protoplanetary disk, and the accretion and geological history of their parent C‐complex asteroid(s). Of the hundreds of CMs that are available for study, the majority are finds and so may have been compromised by terrestrial weathering. Nineteen falls have been recovered between 1838 and 2020, and there is a hint of two temporal clusters: 1930–1942 and 2009–2020. Falls are considered preferable to finds to study because they should be near pristine, and here this assumption is tested by investigating their susceptibility to alteration before recovery and during curation. CMs falling on the land surface are prone to contamination by organic compounds from soil and vegetation. Where exposed to liquid water prior to collection, minerals including oldhamite can be dissolved and most fluid mobile elements leached. Within days of recovery, CMs adsorb water from the atmosphere and are commonly contaminated by airborne hydrocarbons. Interaction with atmospheric water and oxygen during curation over year to decadal timescales can produce Fe‐oxyhydroxides from Fe,Ni metal and gypsum from indigenous gypsum and oldhamite. Relationships between the petrologic (sub)types of pre‐1970 falls and their terrestrial age could be due to extensive but cryptic alteration during curation, but are more likely a sampling bias. The terrestrial history of a CM fall, including circumstances of its collection and conditions of its curation, must be taken into account before it is used to infer processes on C‐complex parent bodies such as Ryugu and Bennu.

Distinguishing between terrestrial and extraterrestrial organic compounds in the CM2 Aguas Zarcas carbonaceous chondrite: Implications for intrinsic organic matter

“…Here we employ the third criterion outlined by Lee et al. (2021) whereby if the compound is rare on Earth, or unlikely to have been present in the collection site where the meteorite was found, we concluded that it was intrinsic to the specimen. In addition, any compounds that are concluded to be intrinsic to the meteorite or determined to have come from the terrestrial surface must also be either absent from the procedural blanks, or present in higher abundance than the blanks.…”

“…This aligns with the three criteria that are commonly used help identify contamination versus intrinsic compounds that can be used together or individually. The criteria are (1) chemical and/or isotopic composition or mineralogy are related to post-fall characteristics, (2) chemical and/or isotopic composition or mineralogy from a collection of meteorites changes with respect to its terrestrial age, and (3) chemical and/ or isotopic composition or mineralogy of the meteorite is inconsistent compared to other CM chondrites (Lee et al, 2021). Here we employ the third criterion outlined by Lee et al (2021) whereby if the compound is rare on Earth, or unlikely to have been present in the collection site where the meteorite was found, we concluded that it was intrinsic to the specimen.…”

“…The criteria are (1) chemical and/or isotopic composition or mineralogy are related to post-fall characteristics, (2) chemical and/or isotopic composition or mineralogy from a collection of meteorites changes with respect to its terrestrial age, and (3) chemical and/ or isotopic composition or mineralogy of the meteorite is inconsistent compared to other CM chondrites (Lee et al, 2021). Here we employ the third criterion outlined by Lee et al (2021) whereby if the compound is rare on Earth, or unlikely to have been present in the collection site where the meteorite was found, we concluded that it was intrinsic to the specimen. In addition, any compounds that are concluded to be intrinsic to the meteorite or determined to have come from the terrestrial surface must also be either absent from the procedural blanks, or present in higher abundance than the blanks.…”

“…Due to geological and biological activity on Earth, all incoming extraterrestrial organic matter within meteorites is potentially subject to contamination, or alteration which may partially to completely replace or erase it (Sephton, 2002). This activity can result in the addition of organic matter, leaching and dissolution by water, hydrolysis, oxidation, terrestrial overprinting, and new mineral growth (Lee et al., 2021). Measures need to be taken to preserve meteorites in their most pristine state possible, including documenting their collection and curation histories in order to trace potential sources of terrestrial contamination (e.g., Tunney et al., 2020).…”

Organic compounds in the Tarda C2 ungrouped carbonaceous chondrite: Evaluating the sources of contamination in a desert fall

“…Upon reaching the Earth’s surface, meteoritic material may become contaminated, primarily due to the abundant life Earth hosts (Pizzarello & Shock, 2010). This terrestrial contamination and alteration have been found to occur rapidly on carbonaceous meteorite falls as soon as they enter Earth’s atmosphere (Burton et al., 2014; Glavin et al., 2021; Lee et al., 2021). The susceptibility of meteoritic materials to become contaminated makes the differentiation between extraterrestrial and terrestrial compounds crucial in order to make informed inferences about organic and/or prebiotic processes that occurred on the early Earth and in our solar system.…”