Carbonaceous

Carbonaceous

Carbonaceous chondrites represent the most primitive rock samples of our solar system. This rare (less than 5% of all meteorite falls) class of meteorites are a time capsule from the earliest days in the formation of our solar system. They are divided into the following compositional groups that, other than CH (High Metal) and C-UNG (Ungrouped), are named after the first or most prominent meteorite of that type: CB (Bencubbin), CI (Ivuna), CK (Karoonda),CM (Mighei), CO (Ornans), CR (Renazzo) and CV (Vigarano). CI and CM meteorites (and comets) are rich in organic compounds and water that, as precursors to life, played a critical role in the development of life on Earth. The term “carbonaceous” is a misnomer in that they contain relatively little carbon.Expand/Collapse

 

Further discriminating characteristics are distinct abundances of high-temperature components such as chondrules and Calcium Aluminum Inclusions (CAIs), opaque phases and the fine-grained matrix, as well as the degree of bulk chondrite alteration. Chondrules are products of transient heating events of unknown origin and depleted in volatile elements relative to their host bulk chondrite. In contrast, the matrix is enriched in volatile elements relative to the bulk chondrite. CI chondrites almost entirely consist of matrix and no chondrules. Their relative volatile elemental abundances, compared to all other meteorite classes, most closely matches those of the solar photosphere, attesting to their primitive chemical composition. Virtually all other known planetary materials, i.e., other groups of meteorites, the terrestrial planets and the Moon are significantly depleted in volatile elements (e.g. Na, K, Zn, S, Cd) relative to CI.

The definition above is a highly modified version of the introductory paragraph from “The Chemical Composition of Carbonaceous Chondrites” by Braukmüller et al., 2018.

In addition to silicates, oxides, and sulfides, a subset of carbonaceous chondrites also contain water and/or aqueously altered minerals such as serpentine, together with broad range of organic compounds, such as amino acids, that are precursors and building blocks to life.

Petrologic type indicates the degree of either aqueous or thermal alteration experienced by chondritic meteorites. Aqueous alteration occurred in the parent body under temperatures probably in the range of 20 ° C to 50 ° C in a water-rich environment. This contrasts to the thermal metamorphism of ordinary chondrites that occurred in the range of 600 to 900 ° C under very dry conditions. Carbonaceous chondrites may contain up to 20 weight % water. Different groups of carbonaceous chondrites have been identified that came from parent bodies in different parts of the solar nebula.

Content above used with permission from J. H. Wittke.

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