Cr isotopes in physically separated components of the Allende CV3 and Murchison CM2 chondrites: Implications for isotopic heterogeneity in the solar nebula and parent body processes

Abstract

Chromium isotopic data of physically separated components (chondrules, CAIs, variably magnetic size fractions) of the carbonaceous chondrites Allende and Murchison and bulk rock data of Allende, Ivuna, and Orgueil are reported to evaluate the origin of isotopic heterogeneity in these meteorites. Allende components show ε53Cr and ε54Cr from −0.23 ± 0.07 to 0.37 ± 0.05 and from −0.43 ± 0.08 to 3.7 ± 0.1, respectively. In components of Murchison, ε53Cr and ε54Cr vary from −0.06 ± 0.08 to 0.5 ± 0.1 and from 0.7 ± 0.2 to 1.7 ± 0.1, respectively. The non‐systematic variations of ε53Cr and 55Mn/52Cr in the components of Allende and Murchison were likely caused by small‐scale, alteration‐related redistribution of Mn >20 Ma after formation of the solar system. Chondrule fractions show the lowest 55Mn/52Cr and ε54Cr values of all components, consistent with evaporation of Mn and ε54Cr‐rich carrier phases from chondrule precursors. Components other than the chondrules show higher Mn/Cr and ε54Cr, suggestive of chemical and isotopic complementarity between chondrules and matrix‐rich fractions. Bulk rock compositions calculated based on weighted compositions of components agree with measured Cr isotope data of bulk rocks, in spite of the Cr isotopic heterogeneity reported by the present and previous studies. This indicates that on a sampling scale comprising several hundred milligrams, these meteorites sampled isotopically and chemically homogeneous nebular reservoirs. The linear correlation of 55Mn/52Cr with ε53Cr in bulk rocks likely was caused by variable fractionation of Mn/Cr, subsequent mixing of phases in nebular domains, and radiogenic ingrowth of 53Cr.

Evaluating the O‐Cr‐Mo isotope signatures in various meteorites representing core–mantle–crust fragments: Implications for partially differentiated planetesimal(s) accreted in the early outer solar system

“…The bulk rock value calculated for Allende (+0.97 ± 0.21; Kadlag et al. 2019) is closely similar to those for the chondrules and matrix fractions.…”

“…2020) compared to those reported earlier (e.g., calculated whole rock = +0.97 ± 0.21, matrix = +0.94 ± 0.36; Kadlag et al. 2019). Despite the individual Allende chondrules showing highly variable ε 54 Cr (i.e., ~0.1 to ~1.4; average = +0.84 ± 0.95; Schneider et al.…”

“…2020), it seems that the average value does not overlap with average values of whole rock and matrix unlike the trend shown in Kadlag et al. (2019).…”

“…It is reported that ε 54 Cr values of separated components of Allende (e.g., chondrules = +0.91 ± 0.12, matrix = +0.94 ± 0.36; Kadlag et al. 2019) are indistinguishable within the uncertainty limits. They also found that the ε 54 Cr values of most of the Allende components (e.g., chondrules, matrix, magnetic, and slightly magnetic) range from +0.6 to +1.6 with the exception of nonmagnetic (−0.43 ± 0.03) and CAIs (+3.72 ± 0.06).…”

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.…”