The role of Bells in the continuous accretion between the CM and CR chondrite reservoirs

Abstract

CM meteorites are dominant members of carbonaceous chondrites (CCs), which evidently accreted in a region separated from the terrestrial planets. These chondrites are key in determining the accretion regions of solar system materials, since in Mg and Cr isotope space, they intersect between what are identified as inner and outer solar system reservoirs. In this model, the outer reservoir is represented by metal-rich carbonaceous chondrites (MRCCs), including CR chondrites. An important question remains whether the barrier between MRCCs and CCs was a temporal or spatial one. CM chondrites and chondrules are used here to identify the nature of the barrier as well as the timescale of chondrite parent body accretion. We find based on high precision Mg and Cr isotope data of seven CM chondrites and 12 chondrules, that accretion in the CM chondrite reservoir was continuous lasting <3 Myr and showing late accretion of MRCC-like material reflected by the anomalous CM chondrite Bells. We further argue that although MRCCs likely accreted later than CM chondrites, CR chondrules must have initially formed from a reservoir spatially separated from CM chondrules. Finally, we hypothesize on the nature of the spatial barrier separating these reservoirs.

Chromium isotopic insights into the origin of chondrite parent bodies and the early terrestrial volatile depletion

“…The Cr isotope data for doping and external precision tests are reported in Table 2 and combined with literature data from Schiller et al (2014). In Table 3, we compare the 55 Mn/ 52 Cr and Cr isotope data for the same chondrites measured in this and previous studies (Go ¨pel et al, 2015;Jenniskens et al, 2012;Kadlag et al, 2019;Langbroek et al, 2019;Mougel et al, 2018;Moynier et al, 2007;Petitat et al, 2011;Qin et al, 2010;Sanborn et al, 2019;Schiller et al, 2014;Trinquier et al, 2007;2008b;van Kooten et al, 2020van Kooten et al, , 2016Williams et al, 2020;Zhu et al, 2020aZhu et al, , 2020b. The averaged group Cr isotope data and 55 Mn/ 52 Cr ratios of all the chondrites are reported in Table 4.…”

“…In Table 4, combining these new measurements with literature data (Go ¨pel et al, 2015;Qin et al, 2010;Trinquier et al, 2007;van Kooten et al, 2020;Zhu et al, 2021) results in an updated e 54 Cr sequence: CI ! CH !…”

“…Within the uncertainties, most analyses of the same meteorites are consistent. Two exceptions are the e 54 Cr values of Jbilet Winselwan (CM2) (van Kooten et al, 2020) and the e 53 Cr values of Lance ´(CO3) (Moynier et al, 2007;Trinquier et al, 2008b). The small-degree of e 54 Cr heterogeneity (0.82 ± 0.04-1.01 ± 0.12) in Jbilet Winselwan could also result from sample heterogeneity, given that CM chondrites also contain abundant refractory inclusions (Krot et al, 2014), and their chondrules possess variable e 54 Cr values (van Kooten et al, 2020).…”

“…Two exceptions are the e 54 Cr values of Jbilet Winselwan (CM2) (van Kooten et al, 2020) and the e 53 Cr values of Lance ´(CO3) (Moynier et al, 2007;Trinquier et al, 2008b). The small-degree of e 54 Cr heterogeneity (0.82 ± 0.04-1.01 ± 0.12) in Jbilet Winselwan could also result from sample heterogeneity, given that CM chondrites also contain abundant refractory inclusions (Krot et al, 2014), and their chondrules possess variable e 54 Cr values (van Kooten et al, 2020). Note that the e 53 Cr of Lance ´, À0.04 ± 0.07 in Trinquier et al (2008b), is lower than that of most other CO chondrites and even those of all other measured chondrites, and its e 54 Cr value is also lower than all the other CO chondrites.…”

The Aguas Zarcas (CM2) meteorite: New insights into early solar system organic chemistry

“…A recent study utilizing Mg and Cr isotopes from CM chondrites and chondrules concluded that many anomalous aspects of Bells could be explained by the prolonged accretion of outer protoplanetary disk material onto CM chondrite parent bodies (Van Kooten et al. 2020). However, the more sunward location (i.e., lower orbital distances) of the CMs would have precluded the accretion of larger amounts of the more volatile species such as NH 3 (needed for formation of amino acids and amines).…”