Abstract
AbstractCM chondrites are complex impact (mostly regolith) breccias, in which lithic clasts show various degrees of aqueous alteration. Here, we investigated the degree of alteration of individual clasts within 19 different CM chondrites and CM‐like clasts in three achondrites by chemical analysis of the tochilinite‐cronstedtite‐intergrowths (TCIs; formerly named “poorly characterized phases”). To identify TCIs in various chondritic lithologies, we used backscattered electron (BSE) overview images of polished thin sections, after which appropriate samples underwent electron microprobe measurements. Thus, 75 lithic clasts were classified. In general, the excellent work and specific criteria of Rubin et al. (2007) were used and considered to classify CM breccias in a similar way as ordinary chondrite breccias (e.g., CM2.2‐2.7). In BSE images, TCIs in strongly altered fragments in CM chondrites (CM2.0‐CM2.2) appear dark grayish and show a low contrast to the surrounding material (typically clastic matrix), and can be distinguished from TCIs in moderately (CM2.4‐CM2.6) or less altered fragments (CM2.7‐CM2.9); the latter are bright and have high contrast to the surroundings. We found that an accurate subclassification can be obtained by considering only the “FeO”/SiO2 ratio of the TCI chemistry. One could also consider the TCIs’ S/SiO2 ratio and the metal abundance, but these were not used for classification due to several disadvantages. Most of the CM chondrites are finds that have suffered terrestrial weathering in hot and cold deserts. Thus, the observed abundance of metal is susceptible to weathering and may not be a reliable indicator of subtype classification. This study proposes an extended classification scheme based on Rubin’s scale from subtypes CM2.0‐CM2.9 that takes the brecciation into account and includes the minimum to maximum degree of alteration of individual clasts. The range of aqueous alteration in CM chondrites and small spatial scale of mixing of clasts with different alteration histories will be important for interpreting returned samples from the OSIRIS‐REx and Hayabusa 2 missions in the future.
Highlights
Santa Cruz, Y-791198, and Maribo only show very weak or unclear evidence of brecciation, and the degree of aqueous alteration was obtained by random analyses of TCIs in several areas of the bulk sample (Table 2)
A large number of TCI-rich fragments/areas from 27 thin sections out of 19 different CM chondrites and three brecciated HEDs were examined in order to reveal information on their brecciation processes and on the degree of aqueous alteration of their individual clasts
According to the scheme of Rubin et al (2007), 80 fragments, main lithologies, and bulk rocks were classified by determination of the “FeO”/SiO2 ratios of the TCIs
Summary
Meteorites classified as CI and CM chondrites have the highest degree of aqueous alteration (e.g., Fredriksson and Kerridge 1988; Tomeoka and Buseck 1988; Endress and Bischoff 1993; Johnson and Prinz 1993; Browning et al 1996; Zolensky et al 1997, 2002; Bischoff 1998; Brearley 2006; Rubin et al 2007; Howard et al 2009; Alexander et al 2012, 2013; Garenne et al 2014; Tonui et al 2014; Visser et al 2018). Almost all of these carbonaceous chondrites are heavily brecciated and contain clasts of different mineralogy and chemistry (e.g., Metzler et al 1992; Bischoff et al 2006, 2017; Morlok et al 2006; Lindgren et al 2013; Zolensky et al 2015, 2017; Alfing et al 2019). Some of these rocks even contain fragments that were formed in water-free environments or that have completely lost their volatiles due to heating. As TCIs and other secondary phases in CM chondrites formed by different degrees of aqueous alteration (e.g., Fuchs et al 1973; McSween 1979a; Rubin and Wasson 1986; Grimm and McSween 1989; Brearley [2006] and references therein), different classification schemes for these phases have been proposed to quantify the degree of aqueous alteration of CM chondrites (e.g., McSween 1979b; Browning et al 1996; Rubin et al 2007; Howard et al 2009; Alexander et al 2012, 2013; Garenne et al 2014)
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