26Al26Mg

26Al–26Mg is a short‑lived radionuclide system used in geochronology and cosmochemistry to date early Solar System processes. The parent nuclide, aluminium‑26 (^26Al), decays by positron emission or electron capture to the daughter nuclide magnesium‑26 (^26Mg) with a half‑life of about 717,000 years. Because ^26Al was produced in significant amounts by stellar nucleosynthesis and injected into the protosolar nebula, its decay provides a chronometer for events that occurred within the first few million years of Solar System history.

The system is most commonly applied to calcium–aluminium‑rich inclusions (CAIs) and chondrules in primitive meteorites. High initial ^26Al/^27Al ratios in CAIs (≈5 × 10⁻⁵) indicate formation shortly after the nucleosynthetic input, while lower ratios in later‑forming components record the passage of time. The ^26Mg excesses generated by decay are measured by mass spectrometry, and the initial ^26Al abundance is inferred by constructing isochrons that correlate ^26Mg/^24Mg with ^27Al/^24Mg.

Production of ^26Al occurs in massive stars, supernovae, and asymptotic giant branch stars through proton‑capture reactions on ^25Mg. Its presence in the early Solar System suggests a nearby stellar source, possibly a supernova or Wolf‑Rayet wind, that injected freshly synthesized nuclides into the nascent protoplanetary disk.

Limitations of the ^26Al–^26Mg chronometer include potential isotopic resetting by thermal metamorphism, alteration of the Al/Mg ratio during parent‑body processes, and uncertainties in the exact half‑life. Nevertheless, the system remains a powerful tool for reconstructing the timing and duration of planetary accretion, differentiation, and early Solar System evolution.