Uranium Isotopes in the Oceans

Uranium Isotopes in the Oceans

L. Kinsley, G. Mortimer, T. Esat and M.T. McCulloch

 

We have determined the operating characteristics and the precision obtainable from the newly acquired Finnigan multi-collector inductively-coupled mass spectrometer 'NEPTUNE' in measuring uranium isotopes. Such a measurement places high demands on instrument performance as the isotope ratio of interest is small ~5x10-5 and susceptible to small amounts of interfering molecular beams. It involves combined use of Faraday-cups and an 'electron-multiplier' and the accounting for, or elimination of tailing from intense 238U beams. In addition, there appears to be a discrepancy in the U isotopic composition between modern corals and sea-water. We have determined the U isotopic composition of sea-water to help resolve the difference.

Data collection consisted of 30 2-second simultaneous integrations of 234U, 235U 238U, a peak jump, and 10 2-second integrations of 235U and 238U. Each experiment included 10 cycles of 6 blocks

and lasted for 1_ hours. The 235U/238U ratio with 235U measured in a Faraday cup and in the electron multiplier (SEM) provided a direct calibration for the SEM gain, including any non-linearities, except for a correction in dead-time between different count-rates of 234U and 235U. The use of 235U for SEM calibration restricts the usable signal intensity to about 2 volts 238U (1011 Ohm) so as not to overload the SEM.

The present results show that the isotopic composition of uranium can be measured to a precision better than 1 with Neptune SEM and retardation lense in a reproducible fashion. The instrument, and the particular measurement technique used, can achieve this independent of beam intensity. The measured 234U/238U ratio for the standard is identical to the secular equilibrium value (5.467x10-5). Indicating that standard 'ALH' is in secular equilibrium and also that the instrument can provide absolute values of this ratio when appropriately calibrated. Uranium isotopic composition of 'modern' sea-water is indistinguishable from uranium isotopic composition of 'modern' corals within the analytical precision.

The previously determined mean sea-water value of 144±2 (Chen et al., EPSL 80(1986)241) should be revised to ~148.4±0.5 in close agreement with the modern coral value of ~148-149 .

References
Chen, J.H., Edwards, R.L. and Wasserburg, G.J., 238U, 234U and 232Th in seawater, Earth and Planetary Science Letters, 80, 241-252

 

Figure 13: Results for the isotopic composition of uraninite standard 'ALH', assumed to be in secular equilibrium, are shown here. The 14 determinations over a period of two months yield a mean close to zero, relative to the secular equilibrium value of 5.467x10-5. This is a direct determination with Neptune without reference to any other standard.
Figure 14: Uranium from 50 to 100 ml seawater samples was extracted by co-precipitation with iron chloride and the Fe removed by ion-exchange. The results shown here (relative to 5.4709x10-5) are internally consistent and very similar to those measured in modern corals