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What are refractory inclusions and why are they important for dating the solar system?

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Meteorite The Hoba meteorite, lying where it was discovered in 1920 near Grootfontein, Namibia. The largest meteorite on Earth and an iron meteorite by classification, it consists of a nickel-iron alloy and is estimated to weigh about 60 tons.

Refractory inclusions, often called calcium-aluminum-rich inclusions (CAIs), are minor but significant components found within chondrites. They are characterized by their high enrichment in the least-volatile, or refractory, elements—most notably calcium and aluminum. These inclusions range in size from tens of micrometres to over a centimetre and are thought to have formed either from a molten state or by condensing directly from a hot gas.1

Refractory inclusions are essential for dating the solar system because they are the oldest known solids to have formed in the solar nebula, with ages of approximately 4.567 billion years. Because they formed during early, short-lived heating events, they serve as the "time zero" reference point for the solar system. Scientists use the isochron method to measure the abundances of short-lived radionuclides within these inclusions, which allows for the determination of relative ages for other solar system materials. By comparing these relative ages to absolute ages derived from long-lived radionuclides, researchers have developed a precise timeline for the formation and evolution of the early solar system.23