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 |
| 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 |