Sub-epsilon natural 234U/238U measurements refine the 234U half-life and U-Th geochronology.

234U/238U ratios are used across various fields in Earth science, but their measurement is highly challenging due to the four orders of magnitude difference in isotope abundance between 234U and 238U. Here, we develop refined techniques for accurately and precisely measuring 234U/238U using multicollector inductively coupled plasma mass spectrometry, providing a reproducibility of ±0.08 per mil (2σ). At this sub-epsilon precision level, the half-life of 234U is refined to 245,670 ± 260 years. For U-Th geochronology, systematic uncertainties from decay constants largely cancel, effectively reducing the uncertainty in the 234U half-life to ~±25 years. This improved measurement precision consequently reduces total 2σ age uncertainties to ±3 thousand years (kyr) at ~600 thousand years ago (ka) and ±6 kyr at ~700 ka, a substantial improvement over earlier methods with uncertainties of tens of thousands of years.