Resolve the initial 53Mn/55Mn ratio at CAI formation

Determine the initial 53Mn/55Mn ratio at the time of calcium–aluminum-rich inclusion formation in order to reduce the uncertainty in the Mn–Cr precipitation ages of dolomite from the Ryugu samples and the resulting constraints on the accretion age and pebble radius of Ryugu’s parent planetesimal.

Background

The paper calculates dolomite precipitation ages using a canonical initial 53Mn/55Mn ratio of 6.8 × 10−6, while another study proposed a higher value of 8.1 × 10−6. Because the calculated precipitation ages depend directly on this initial ratio, the choice of ratio affects the inferred accretion age and pebble-size parameter space for Ryugu’s parent planetesimal.

The authors’ sensitivity analysis finds that the overall modeling trend is similar under the alternative ratio, but the threshold accretion age shifts. Establishing the initial ratio would therefore improve the absolute chronology underlying the thermal-evolution constraints.

References

We acknowledge that the initial ${{53}{\rm Mn} / {55}{\rm Mn}$ ratio at the time of CAI formation remains a subject of debate, and the calculated precipitation ages directly reflect the associated uncertainty.

— Formation of Ryugu's parent planetesimal beyond the CO$_{2}$ snow line from small pebbles: insights from thermal evolution modeling  (2609.20608 - Arakawa et al., 17 Sep 2026) in Section 4.4, "Initial 53Mn/55Mn ratio at the time of CAI formation"