Temporally distributed accretion of chondritic and differentiated meteorite parent bodies in the C reservoir of the early solar system

Accretion processes in protoplanetary disks produce a diversity of small bodies. In our solar system, such bodies played a crucial role in potentially multiple reshuffling events and in both early and late accretion of planets. Application of thermo-chronometers to meteorites provides precise dating of the formation or cooling ages of various mineralogical components. Nucleosynthetic isotopic anomalies that indicate a dichotomy between non-carbonaceous (NC) and carbonaceous (C) meteorites and precise parent body (PB) chronology can be combined with planetesimal thermal evolution models to constrain the timescale of accretion and dynamical processes in the early solar system.

Achondrite parent bodies are considered to have accreted early and mostly in the NC region. By contrast, late accretion in the C region produced mostly undifferentiated objects, such as the parent body of CR1-3 chondrites that could have formed as late as 4 Ma after solar system formation (Schrader et al., 2011). However, presence of more evolved CR-like meteorites suggests also an earlier accretion timing. Observations of C-type NEA and laboratory investigations of CC meteorites indicate a high porosity of C-type asteroids. The boulder microporosity derived for Ryugu (Grott et al., 2019) is substantially higher than for water-rich carbonaceous chondrites, such as CI and CM meteorites, and could indicate distinct thermal evolution paths. Aqueous alteration of Ryugu’s and CI and CM samples suggests accretion times not dramatically different from that of the aqueously altered CR parent body.

In a previous study, we constrained size and accretion time of Ryugu’s parent body using a numerical model for the evolution of the temperature and porosity (Neumann et al., 2021). These calculations indicate a size of only a few km in radius and an early accretion within ≲2-3 Ma after CAIs. By contrast, calculated properties for CI and CM parent bodies obtained by fitting carbonate formation ages indicate radii of ≈20-25 km and accretion times of ≈3.75 Ma after CAIs. In the present study, we fitted thermo-chronological data available for the C chondrite Flensburg and for CR-related meteorites (CR1-3, NWA011, NWA 6704, and Tafassites, see Ma et al., 2021) that range from altered chondrites to basaltic achondrites to constrain the accretion times of their respective parent bodies. We present modeling evidence for a temporally distributed accretion of parent bodies of CR-like meteorite groups that originate from a C reservoir and range from aqueously altered chondrites to highly equilibrated chondrites and partially differentiated primitive achondrites. The parent body formation times derived range from <1 Ma to ≈4 Ma after solar system formation, with ≈3.7 Ma, ≈1.5-2.75 Ma, ≲0.6 Ma, and ≲0.7 Ma for CR1-3, Flensburg, NWA 6704, and NWA 011, respectively. This implies that accretion processes in the C reservoir started as early as in the NC reservoir and produced differentiated parent bodies with carbonaceous compositions in addition to undifferentiated C chondrite parent bodies. The accretion times correlate inversely with the degree of the meteorites' alteration, metamorphism, or differentiation. The accretion times for the CI/CM, Ryugu, and Tafassites parent bodies of ≈3.75 Ma, ≈1-3 Ma, and ≈1.1 Ma, respectively (Ma et al., 2022, Neumann et al., 2021), fit well into this correlation in agreement with the thermal and alteration conditions suggested by the meteorites (Fig. 1).

Figure 1: Parent body accretion times (colored patches) and meteorite metamorphic ages (data points) for the C reservoir modeled incl. Tafassites, Ryugu, and CI/CM PB (Ma et al., 2021, Neumann et al., 2021). Younger Mn-Cr carbonate ages (circles) of CR1-3 (blue) and CI/CM (yellow) than for Flensburg (grey) result in moderately younger accretion age for the CR1-3 and CI/CM PBs. NWA 6704 (cyan) and NWA 011 (green) have older Pb-Pb whole-rock ages (squares) than Tafassites (light blue), resulting in earlier accretion than the Tafassite PB. The NWA 011 and NWA 6704 Al-Mg (triangles) and Mn-Cr (circles) data support an early PB formation. While Tafassites also include a very late, i.e., young, phosphate Pb-Pb age, their older iron-silicate Hf-W (light blue triangle) and Mn-Cr ages cause a shift to an intermediate PB formation time between NWA 011 and NWA 6704 on one hand, and Flensburg, CR1-3, and CI/CM PBs on the other. The youngest Pb-Pb age has its major effect on the large Tafassites PB size. Younger CR1-3 chondrule formation age (blue diamond) supports the PB accretion at ≈3.7 Ma. All accretion times correlate inversely with the meteorite petrologic type and the degree of metamorphism or melting.

 

Grott M. et al. (2019) Nature Astronomy, 3, 971-976.

Ma N. et al. (2021) Goldschmidt 2021, https://doi.org/10.7185/gold2021.3852.

Neumann W. et al. (2021) Icarus, 358, 114166.

Schrader D. L. et al. (2011) GCA, 75, 308-325.