Isotope-dilution thermal ionisation mass spectrometry (ID-TIMS) affords the ability to determine U-Pb dates with a precision of 0.1% or better. At NIGL we exploit this technique for a variety of problems where the highest age resolution is required (e.g. calibration of the stratigraphic record, rates of metamorphic and magmatic processes, and characterization of standard minerals and solutions).
Synthetic U-Pb 'age solutions' and U-Pb tracer calibration
As part of the EARTHTIME Initiative, NIGL has been involved in development and calibration the ET535 and ET2535 tracers, as well as development of new standard materials. In particular, we have prepared and calibrated a series of synthetic U-(Th)-Pb solutions that have 206Pb-238U and 207Pb-235U ratios that yield "concordant" 206Pb-238U and 207Pb-235U ages at 10 Ma, 100 Ma, 500 Ma and 2 Ga.
The synthetic solutions have been prepared in quantities sufficient that they can be used as long term standards, with the aim that they can be used for (1) assessment of long-term intra-laboratory reproducibility and (2) assessment of inter-laboratory agreement. We are currently using these solutions to assess the long-term reproducibility of U-Pb ID-TIMS data generated at NIGL and for direct comparison with similar data generated in other labs. We believe these solutions will augment natural mineral standards (such as R33 and Temora) for ID-TIMS, allowing for greater evaluation of mass-spectrometry as post-crystallisation Pb-loss and natural variation in zircon crystallisation will not be factors.
If you are interested in obtaining these materials please contact Dan Condon for more information.
238U/235U determinations for geochronology
Fundamental to U-daughter geochronology is the isotopic composition (238U/235U) of uranium in natural materials being analysed. For decades a consensus value (238U/235U = 137.88) has been used by the geochronology community, however recent studies on bulk-rock samples from low-temperature environments indicate that this ratio does vary considerably in nature. Our group has been investigating the isotopic composition (238U/235U) of uranium in materials used for U-daughter geochronology, including reference materials (Condon et al., 2010) and the minerals being dated (Hiess et al., 2012). Our data provides the first accurate 238U/235U values for use in terrestrial mineral geochronology, in particular we propose a new 238U/235U value of 137.818 ± 0.045 for use in U-Pb zircon geochronology based upon our study of a number of zircon samples of varying age, genesis and location
Condon, Daniel J.; McLean, Noah; Noble, Stephen R.; Bowring, Samuel A.. 2010. Isotopic composition (238U/235U) of some commonly used uranium reference materials. Geochimica et Cosmochimica Acta, 74 (24). 7127-7143. 10.1016/j.gca.2010.09.019.
Hiess, Joe; Condon, Daniel J.; McLean, Noah; Noble, Stephen R. 2012. 238U/235U systematics in terrestrial U-bearing minerals. Science (March 30th 2012)