Carbonaceous achondrites Northwest Africa 6704/6693: Milestones for early Solar System chronology and genealogy

Abstract Northwest Africa (NWA) 6704/6693 are medium- to coarse-grained achondriteswith unique petrologicand geochemical traits that are distinct from the currently established meteorite groups. Here, we report on the extinct 26 Al- 26 Mg and 53 Mn- 53 Cr systems to establish fine-scale chronology of its formation and Cr and Ti isotopic anomalies to constrain the composition of the source reservoir of NWA 6704/6693. Excesses in the neutron-rich 54 Cr and 50 Ti isotopes, due to nucleosynthetic anomalies, separate NWA 6704/6693 from the vast majority of established achondritesand instead resemble the excesses seen among the carbonaceous chondrites; specifically, the CR-type chondrites. The excesses in these isotopes indicate a common feeding zone during accretion in the protoplanetary diskbetween the source of NWA 6704/6693 and that of the carbonaceous chondrites. The 26 Al- 26 Mg data for pyroxene and plagioclase from NWA 6704 yield a ( 26 Al/ 27 Al) 0  = (3.15 ± 0.38)×10 −7 (MSWD = 0.49) and an initial δ 26 Mg ∗  = −0.004 ± 0.005 at the time of isotopic closure. This initial ( 26 Al/ 27 Al) 0 translates to an absolute age of 4563.14 ± 0.38 Ma, relative to the D’Orbigny angrite. However, given the potential heterogeneity of 26 Al, the D’Orbigny angrite might not be a good age anchor for the purpose of calculating 26 Al- 26 Mg ages. The 26 Al- 26 Mg age relative to another carbonaceous achondrite, NWA 2976, is 4562.66 ± 0.60 Ma. The 53 Mn- 53 Cr systematics of NWA 6704/6693 indicate a ( 53 Mn/ 55 Mn) 0 of (2.59 ± 0.34) × 10 −6 (MSWD = 1.2) with an evolved initial e 53 Cr of +0.14 ± 0.03. The ( 53 Mn/ 55 Mn) 0 yields an 53 Mn- 53 Cr age of 4562.17 ± 0.76 Ma relative to the D’Orbigny angrite. Concordant ages determined using the short-lived 26 Al- 26 Mg and 53 Mn- 53 Cr systems and extant 207 Pb- 206 Pb system (4562.60 ± 0.30 Ma for NWA 6704/6693; Amelin et al., 2019) indicate rapid cooling and nearly contemporaneous closing of multiple isotope systems. The ancient crystallization ages and positive 54 Cr and 50 Ti anomalies of NWA 6704/6693 indicate widespread melting and differentiation processes occurring in both non-carbonaceous (NC) and carbonaceous chondrite(CC) regions of the protoplanetary disk. Additionally, we report the Cr and Ti isotopic composition for a petrologicrange of CR-type materials (CR2, CR6, and achondrites). The additional Cr and Ti isotopic data for these CR-type materials indicates a range in isotopic composition not previously observed based on CR2 chondrites alone.