T282 EARTHTIME: In Honor of the Contributions of James Mattinson to High-Precision U-Pb Geochronology

Wednesday, 3 November 2010
1:30 PM - 5:30 PM
Room 207

URANINUM ISOTOPIC VARIBILITY AND PB-PB DATING OF METEORITES

Paper number: T282-5
Program: Technical Sessions
Time: 2:30 PM – 2:45 PM

Authors

Abstract

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. 73rd Annual Meteorlogical Society Meeting Abstract Volume.