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A 2022 review of the abundances of radioactive isotopes at the time of Solar System formation [1] is updated, based on new observations and inferences over the past five years. The key isotopes for early Solar System chronology are $^{26}$Al , $^{53}$Mn, and $^{182}$Hf.
$^{26}$Al is very useful as a relative chronometer and is present in high enough abundance in the early Solar System to be a potent heat source for melting asteroids. A database of $^{26}$Al-$^{26}$Mg internal isochrons from calcium-, aluminum-rich inclusions (CAIs), the first objects to form in the Solar System, has recently become available [2]. It reinforces the early Solar System $^{26}$Al/$^{27}$Al value of 5.2×10$^{-5}$ and shows that most CAIs formed with $^{26}$Al/$^{27}$Al > 4.0×10$^{-5}$, or within the first 300,000 a of Solar System formation. The $^{26}$Al-$^{26}$Mg system seems to be robust as a relative chronometer for CAIs, chondrules, and early-formed differentiated meteorites. There remains a puzzle of why some hibonite-rich CAIs with large nucleosynthetic anomalies in $^{48}$Ca and $^{50}$Ti contained little $^{26}$Al when they formed, but these objects appear to have formed early and were exposed to a greatly enhanced solar cosmic ray flux [3].
$^{53}$Mn is a useful chronometer for chondrules and differentiated meteorites. The early Solar System $^{53}$Mn/$^{55}$Mn ratio must be established indirectly, as manganese is not a refractory element and CAIs have low Mn/Cr ratios; the measured half-life is also rather uncertain, 3.7±0.4 Ma. A statistical treatment of the $^{26}$Al-$^{26}$Mg, $^{53}$Mn-$^{53}$Cr, $^{182}$Hf-$^{182}$W, and Pb-Pb systems in achondritic meteorites of volcanic origin inferred that the most probable early Solar System $^{53}$Mn/$^{55}$Mn ratio is (8.09±0.65)×10$^{-6}$ and the $^{53}$Mn half-life is 3.80±0.23 Ma [4].
$^{60}$Fe was once thought to have been abundant enough to serve as a heat source for asteroidal melting, with $^{60}$Fe/$^{56}$Fe ratio as high as 1.5×10$^{-6}$ [5], but this was based on a measured half-life shorter by nearly a factor of 2 than the currently accepted value and on a large decay correction. A decade ago, the $^{60}$Fe/$^{56}$Fe ratio was inferred to be as high as 7×10$^{-7}$, based on secondary ion mass spectrometry of chondrule silicates [6]. Resonance ionization mass spectrometry of these silicates showed that the high $^{60}$Fe/$^{56}$Fe ratio was likely an artifact of mass fractionation corrections during data reduction [7]. High precision nickel isotopic measurements of meteorites showed that the early Solar System ratio was ~1×10$^{-8}$ [8]. Recent measurement of the unique achondrite Erg Chech 002, which crystallized <2 Ma after CAI formation, show that the early Solar System $^{60}$Fe/$^{56}$Fe ratio was (7.71±0.47)×10$^{-9}$ [9]. The low $^{60}$Fe/$^{56}$Fe and high $^{26}$Al/$^{27}$Al in the early Solar System remain a challenge, as both $^{60}$Fe and $^{26}$Al are made in core-collapse supernova. Decoupling them by carrying $^{26}$Al in winds of Wolf-Rayet stars may be a way out of this conundrum [10].
[1] Davis A. M. (2022) ARNPS 72, 339–363. [2] Dunham E. T. et al. (2026) ApJS 282, 11. [3] Kööp L. et al. (2018) Nature Astron. 2, 709–713. [4] Desch S. J. et al. (2023) Icarus 402, 115611. [5] Shukolyukov A. & Lugmair G. W. (1993) Science 259, 1138–1142. [6] Telus M. et al. (2018) Geochim. Cosmochim. Acta 221, 342–357. [7] Trappitsch et al. (2018) ApJL 857, L15. [8] Tang H. & Dauphas N. (2015) ApJ 802, 22. [9] Fang L. et al. (2025) Science Adv. 11, eadp9381. [10] Dwarkadas V. V. et al. (2017) ApJ 851, 147.
| Career stage | Tenured mid-to-late-career researcher |
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