URANINUM ISOTOPIC VARIBILITY AND PB-PB DATING OF METEORITES

The recently reported variability of ²³⁸U/²³⁵U in early solar system objects [1] casts doubt on all published Pb-Pb ages for meteorites. We have confirmed this variability by analyzing Ca-Al-rich Inclusions (CAIs) from the CV chondrite Efremovka that are known from a companion Al-Mg study [2] to have formed within a time span of 5,000 years. Assuming a fixed ²³⁸U/²³⁵U ratio of 137.88, we have determined that Pb-Pb apparent ages for CAIs in Efremovka vary from at least 4570.0±0.2 Ma to 4568.2±0.3 Ma. This makes clear the need for precise and accurate U isotopic compositions for materials being dated by the Pb-Pb method in the future.

Routinely producing Pb-Pb ages of meteorites and their inclusions with errors of ca. 0.3 Myr will require analyses of nanogram quantities of U at ca. 150-200 ppm precision. Recognizing the low abundances of ²³⁵U, we are developing analytical protocols for a Thermo Neptune multi-collector inductively-coupled plasma mass spectrometer that has been upgraded with the Plus option. This configuration returns 2500V/ppm for U or ca 1.0V of ²³⁸U and 0.007V of ²³⁵U for 1 ng of U dissolved in 2.5 mls of 2% HNO₃. On peak zeros are measured before each analyses for the same duration as samples and standards. We add IRMM3636 (equal-atom ²³³U-²³⁶U) to samples and standards to internally correct for instrumental mass fractionation. ²³⁸U, ²³³U and ²³⁶U are measured by faraday detectors (FD) whereas ²³⁵U is simultaneously measured in an axial Electron Muliplier (EM) ion counting system. The FD-EM conversion factor is determined by running three bracketing U standards before and after the sample. The EM-FC calibration is sufficiently stable to allow interpolation of ca. 100 ppm errors to the analyses of the intervening analyses of unknowns.

[1] Brennecka G. et al. 2010. Science. 327: 449-451. [2] Larsen K. et al. 2010. 73^rd Annual Meteorlogical Society Meeting Abstract Volume.