Skip to main content

234U/238U activity ratios

We are currently utilizing 234U/238U and 87Sr/86Sr measurements in isotope hydrology studies of surface and groundwater mixing and residence time. Whereas 87Sr/86Sr can be used as a tracer of both silicate and carbonate rock sources, 234U/238U tracks the flowpath and residence time of water during subsurface flow. Both U and Sr can be sequentially isolated from the same acidified water aliquot for high-precision isotope ratio measurement by thermal ionization mass spectrometry.

For larger samples, measurement of the 234U/238U ratio is accomplished by a two sequence dynamic analysis, peak-switching the 235U beam between Faraday cup and Daly detector to establish a real time gain, and fractionation correcting relative to the known 235U/238U ratio. Alternatively, for small samples, the same measurement is made by peak jumping all isotopes (including 238U) into the Daly detector due to its large dynamic range. Using the latter method and maintaining a 1 Mcps 238U beam, the external reproducibility of the 234U/238U ratio in an in-house secular equilibrium U standard is 0.00005493 ± 10 (0.18%; 1-sigma, n=37; accepted value=0.00005489).

Isotope diagram