Hafnium–tungsten dating
A dating method based on the extinct isotope hafnium-182, used to time metal–silicate separation and core formation in early planetesimals and planets.
Hafnium-182 decayed to tungsten-182 with a half-life of about 8.9 million years, so it was alive only during the first few tens of millions of years of the Solar System and is now extinct. Its chemistry makes it ideal for timing core formation. Hafnium is lithophile, staying in silicate rock, while tungsten is moderately siderophile, entering metal. When a body melts and its metal sinks to form a core, the core captures tungsten but almost no hafnium.
If this happens early, while ¹⁸²Hf still survives, the silicate mantle later develops excess ¹⁸²W relative to ¹⁸⁴W, while the core is left with the low ¹⁸²W/¹⁸⁴W of its separation time. Deficits are expressed in ε¹⁸²W units (parts per ten thousand). Comparing an iron meteorite's tungsten composition with that of CAIs gives a model age for metal segregation, after correcting for cosmic-ray effects with platinum or osmium isotopes.
Applied to magmatic iron meteorite groups such as the IIIAB irons, which include Cape York, the method shows their parent bodies formed cores within roughly one to three million years of CAIs, earlier than many chondrites accreted. On Earth and the Moon, hafnium–tungsten systematics constrain core formation and the timing of the giant impact. Results are reported as ages relative to CAI formation.
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