Standardising high-precision U-Pb geochronology
EARTHTIME was grown out of need to ‘standardise’ geochronology so that people who use such dates to understand a wide range of geological processes. at the turn of the millennium it was becoming clear that dates from different decay schemes, and different laboratories could not be compared at the level of uncertainty often reported. this was due to a combination of issues, such as systematic uncertainties related to calibration issues (i.e., tracer calibration for U-Pb, fluence monitor for 40Ar/39Ar) and ‘laboratory specific issues’.
In order to really bear down on these issues a plan of action was required, a series of connected experiments were needed to address the following: (1) U-Pb ID-TIMS dates could be better compared between laboratories, (2) to minimise bias between U-Pb ID-TIMS laboratories and develop means where that bias can be better quantified and reported; and (3) improve the absolute calibration of the U-Pb system. so it can be used to help calibrate other decay schemes, and integrate with astrochronologies.
The U-Pb ID-TIMS Plan of Action and activities undertaken:
- Carry out an inter-laboratory exercise to assess the scale of the problem. This was an action of the EARTHTIME workshop #1 and formed the basis of much discussion during workshop #2. The bottom line was – U-Pb ID-TIMS dates from different laboratories did not all agree, even when laboratory specific calibration uncertainties were considered.
- Make a common mixed U-Pb tracer. A common mixed U/Pb tracer calibration used in laboratories wanting to carry out ‘high-accuracy’ U-Pb geochronology would allow this source of inter-lab bias to be dialled out. So we got a large amount of high purity 202Pb, 205Pb, 233U and 235U and made a large amount of mixed U-Pb tracer which we call ET535. We took an aliquot and added some 202Pb to make a tracer where Pb fractionation can be quantified, we call this ET2535. We estimate there is enough tracer for lots of labs for quite a long time…
- Calibrate the common mixed U-Pb tracer solution. Now we have a large amount of high-quality mixed U-Pb tracer which can be used to effectively minimise inter-laboratory bias (although, see comments below…), the question is – how accurate are U/Pb dates? This question comes from a need to compare U-Pb dates to absolute rocks and minerals ages that are determined using other decay schemes (i.e., K-Ar and the derivative 40Ar/39Ar system) or using astrochronology, which is underpinned by our understanding of the physics of planets and other bodies in the solar system. With this new large batch tracer there was an opportunity to undertake a comprehensive tracer calibration exercise – and this is what we did! This experiment involved using gravimetric reference solutions, made by dissolving accurately weighed pieces of high-purity U and Pb metals, with determined isotopic compositions, so we have a solution where the concentration, U/Pb ratio is know through weighing, and the isotopic composition is determined relative to isotopic reference materials. The details for these experiments is outlined in two paper listed at the end of this blog.
- Make some ‘age solutions’ to send around to other labs. Above we suggested that a common tracer could effectively eliminate inter-laboratory bias, well that is the theory but how does it work in practice?
Get people to measure ‘age solutions’ and/or ‘standard’ U-bearing minerals (i.e., zircon reference materials). Now we have a common tracer, and a suite of U-Pb reference materials solutions for repeat analyses, we can start to verify the accuracy of data from each laboratory.
Condon, D.J., McLean, N., Schoene, B., Bowring, S.A., and Parrish, R.R. (2015). Metrology and Traceability of U-Pb Isotope Dilution Geochronology (EARTHTIME Tracer Calibration Part I). Geochimica et Cosmochimica Acta. DOI:10.1016/j.gca.2015.05.026.
McLean, N. Condon, D.J., Schoene, B., and Bowring, S.A. (2015) Evaluating Uncertainties in the Calibration of Isotopic Reference Materials and Multi-Element Isotopic Tracers (EARTHTIME Tracer Calibration Part II). Geochimica et Cosmochimica Acta. DOI: 10.1016/j.gca.2015.02.040.
U-Pb calibration schematic poster attempting to demonstrate calibration back to the the kilogram: