⁵³Mn‐⁵³Cr dating of aqueously formed carbonates in the CM2 lithology of the Sutter's Mill carbonaceous chondrite

Abstract

Radiometric dating of secondary minerals can be used to constrain the timing of aqueous alteration on meteoritic parent bodies. Dolomite is a well-documented secondary mineral in CM chondrites, and is thought to have formed by precipitation from an aqueous fluid on the CM parent body within several million years of accretion. The petrographic context of crosscutting dolomite veins indicates that aqueous alteration occurred in situ, rather than in the nebular setting. Here, we present 53 Mn-53 Cr systematics for dolomite grains in Sutter's Mill section SM51-1. The Mn-Cr isotope data show well-resolved excesses of 53 Cr correlated with 55 Mn/ 52 Cr ratio, which we interpret as evidence for the in situ decay of radioactive 53 Mn. After correcting for the relative sensitivities of Mn and Cr using a synthetic Mn-and Cr-bearing calcite standard, the data yield an isochron with slope corresponding to an initial 53 Mn/ 55 Mn ratio of 3.42 AE 0.86 9 10 À6 . The reported error includes systematic uncertainty from the relative sensitivity factor. When calculated relative to the U-corrected Pb-Pb absolute age of the D'Orbigny angrite, Sutter's Mill dolomites give a formation age between 4564.8 and 4562.2 Ma (2.4-5.0 Myr after the birth of the solar system). This age is contemporaneous with previously reported ages for secondary carbonates in CM and CI chondrites. Consistent carbonate precipitation ages between the carbonaceous chondrite groups suggest that aqueous alteration was a common process during the early stages of parent body formation, probably occurring via heating from internal 26 Al decay. The high-precision isochron for Sutter's Mill dolomite indicates that late-stage processing did not reach temperatures that were high enough to further disturb the Mn-Cr isochron.

Chromium isotopic systematics of the Sutter's Mill carbonaceous chondrite: Implications for isotopic heterogeneities of the early solar system

“…Jilly et al. () showed that the 53 Mn‐ 53 Cr isotope systematics are well behaved in carbonate minerals in Sutter's Mill, SM 51, suggesting a uniform, homogeneous 53 Mn/ 55 Mn level at (3.42 ± 0.86) × 10 −6 when these carbonates were formed between 4,563.7 ( +1.1 / −1.5 ) Ma. Given that L1 and L2 steps would leach out carbonate fractions selectively (Table ), the observation of Jilly et al.…”

“…Given that L1 and L2 steps would leach out carbonate fractions selectively (Table ), the observation of Jilly et al. () would argue against scenario (3), i.e., 53 Mn was heterogeneously distributed at the leached mineral scales. Support for scenario (4) comes from the observation that L5 (Figs.…”

“…; Jilly et al. ) or in the nebula (e.g., Ciesla et al. ; Ciesla and Lauretta ), although there is a general agreement that aqueous alteration is kinetically prohibited in the solar nebula (Fegley ).…”

“…The major difference between Sutter's Mill and Murchison (subtype CM2.5) is the degree of aqueous metamorphism, which, for example, can be indicated by abundances of silicate phenocrysts in chondrules (i.e., silicate phenocrysts in Sutter's Mill are completely or mostly altered when those in Murchison are unaltered). Such alternation may have occurred either on the CM parent body (e.g., Grimm and McSween 1989;Hanowski and Brearley 2001;Jenniskens et al 2012;Jilly et al 2014) or in the nebula (e.g., Ciesla et al 2003;Ciesla and Lauretta 2005), although there is a general agreement that aqueous alteration is kinetically prohibited in the solar nebula (Fegley 2000). Therefore, isotopic comparison between different subtypes of CM chondrites may indicate the degree and nature of 54 Cr redistribution between mineral phases during such alteration process(es).…”

Diffusion kinetics of Cr in spinel: Experimental studies and implications for 53Mn–53Cr cosmochronology

“…It has, thus, emerged as an important tool to constrain the relative ages of events within the first 20 Ma of solar system evolution when 53 Mn was still extant. Excess 53 Cr relative to terrestrial values has been detected in different types of primitive material such as Ca-Al-rich inclusions or CAIs Allegre, 1985, 1988;Papanastassiou et al, 2005), chondrules (Nyquist et al, 2001a(Nyquist et al, , 2009Yin et al, 2007), ordinary chondrites (Lugmair and Shukolyukov, 1998;Nyquist et al, 2001bNyquist et al, , 2009), carbonaceous chondrites (Birck et al, 1999;Moynier et al, 2007;Gö pel et al, 2015); CI carbonates (Endress et al, 1996;Hutcheon and Phinney, 1996;Hutcheon et al, 1999;Jilly et al, 2014), enstatite chondrite sulfides (Wadhwa et al, 1997), and various achondrites (Lugmair and Shukolyukov, 1998;Nyquist et al, 2001b). Nyquist et al (2009) provided a compilation of 53 Cr ages, along with the ages determined by short-lived decay systems in igneous differentiated meteorites and chondrules from ordinary chondrites.…”