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[PPS07-03] What kinds of the meteorite are CM chondrites?
★Invited Papers
Keywords:Meteorite, Carbonaceous chondrite, CM chondrite
CM chondrites are the most abundant group of carbonaceous chondrites and are likely to be related to samples returned by the Hayabusa2 and OSIRIS-REx missions. They are characterized by abundant water and organic materials. However, they show a wide variation of petrological and oxygen isotopic features. CMs experienced varying degrees of aqueous alteration and partly secondary heating. Here I review the variation and formation conditions of these CMs.
CM chondrites have been commonly classified into subtypes 2.7-2.0 after the criteria, such as the alteration features and metal abundance, proposed by Rubin [1]. Kimura et al. [2] recently reported two CMs, Asuka (A) 12169 and A 12236, that experienced very low degrees of aqueous alteration and heating. Chondrules and refractory inclusions show no evidence for alteration, and the matrices hardly contain phyllosilicates. Kimura et al. [2] newly proposed the subtypes 3.0-2.8 of CMs. A 12169 and 12236 are classified into subtypes 3.0 and 2.9, respectively. These chondrites are the most primitive CM chondrites so far reported. This conclusion is supported by recent discoveries, such as abundant organic materials [3] and presolar grains [4,5], the predominant occurrence of amorphous materials in the matrix [6], and primitive features of chondrules in these CMs [7].
Because of the secondary alteration, the primitive features of CMs were not yet enough clarified. However, the discovery of CM3.0 gives a significant hint for the formation conditions of CMs. Although the bulk oxygen isotopic compositions of CMs were reported to be different from those of CO chondrites, the oxygen isotopic compositions of CM3.0-2.9 are overlapped to COs. The precursor material to CMs appears to have been isotopically nearly identical to that of the COs. However, the characteristic features of CMs had primarily been different from those of CO3.0, not only in petrologic features but the bulk chemical compositions. Therefore, CMs and COs are probably not both derived from a single parent body.
The precursor materials of CMs later experienced the aqueous alteration in the parent body by the decay of 26Al [e.g., 8]. The subtypes reflect the degree of such alteration. Some CMs were later subjected to secondary heating to decompose phyllosilicates. Many CMs are breccias and contain various kinds of clasts in them. These clasts are important to give constraints for the evolution and internal structure of the CM parent body [9]. CMs show the complicated formation processes, including alteration, heating, and brecciation. In addition to these CMs, many anomalous CMs or related chondrites have been reported [e.g., 10]. Their oxygen isotopic compositions, bulk chemical compositions, or mineralogical features distinguish them from other CMs. However, the genetic relationships between anomalous and other CMs are open questions.
References
[1] Rubin 2015 MAPS 50, 1595-1612. [2] Kimura et al. 2020 Polar Science 26 100565. [3] Glavin et al. 2020 MAPS 55,1979-2006. [4] Nittler L. R. et al. 2020 MAPS in press. [5] Xu et al. 2020 11th Symposium on Polar Science, OAo05.pdf. [6] Noguchi et al. 2020 51st LPSC, 1666.pdf. [7] Fukuda et al. 2020 11th Symposium on Polar Science, OAo06.pdf. [8] Fujiya et al. 2012 Nat.Comms. 3,627. [9] Kerraouch et al. 2019 Geochemistry 79, 125518. [10] Brearley 1995 GCA 59, 2291-2317.
CM chondrites have been commonly classified into subtypes 2.7-2.0 after the criteria, such as the alteration features and metal abundance, proposed by Rubin [1]. Kimura et al. [2] recently reported two CMs, Asuka (A) 12169 and A 12236, that experienced very low degrees of aqueous alteration and heating. Chondrules and refractory inclusions show no evidence for alteration, and the matrices hardly contain phyllosilicates. Kimura et al. [2] newly proposed the subtypes 3.0-2.8 of CMs. A 12169 and 12236 are classified into subtypes 3.0 and 2.9, respectively. These chondrites are the most primitive CM chondrites so far reported. This conclusion is supported by recent discoveries, such as abundant organic materials [3] and presolar grains [4,5], the predominant occurrence of amorphous materials in the matrix [6], and primitive features of chondrules in these CMs [7].
Because of the secondary alteration, the primitive features of CMs were not yet enough clarified. However, the discovery of CM3.0 gives a significant hint for the formation conditions of CMs. Although the bulk oxygen isotopic compositions of CMs were reported to be different from those of CO chondrites, the oxygen isotopic compositions of CM3.0-2.9 are overlapped to COs. The precursor material to CMs appears to have been isotopically nearly identical to that of the COs. However, the characteristic features of CMs had primarily been different from those of CO3.0, not only in petrologic features but the bulk chemical compositions. Therefore, CMs and COs are probably not both derived from a single parent body.
The precursor materials of CMs later experienced the aqueous alteration in the parent body by the decay of 26Al [e.g., 8]. The subtypes reflect the degree of such alteration. Some CMs were later subjected to secondary heating to decompose phyllosilicates. Many CMs are breccias and contain various kinds of clasts in them. These clasts are important to give constraints for the evolution and internal structure of the CM parent body [9]. CMs show the complicated formation processes, including alteration, heating, and brecciation. In addition to these CMs, many anomalous CMs or related chondrites have been reported [e.g., 10]. Their oxygen isotopic compositions, bulk chemical compositions, or mineralogical features distinguish them from other CMs. However, the genetic relationships between anomalous and other CMs are open questions.
References
[1] Rubin 2015 MAPS 50, 1595-1612. [2] Kimura et al. 2020 Polar Science 26 100565. [3] Glavin et al. 2020 MAPS 55,1979-2006. [4] Nittler L. R. et al. 2020 MAPS in press. [5] Xu et al. 2020 11th Symposium on Polar Science, OAo05.pdf. [6] Noguchi et al. 2020 51st LPSC, 1666.pdf. [7] Fukuda et al. 2020 11th Symposium on Polar Science, OAo06.pdf. [8] Fujiya et al. 2012 Nat.Comms. 3,627. [9] Kerraouch et al. 2019 Geochemistry 79, 125518. [10] Brearley 1995 GCA 59, 2291-2317.