Daniel J. Condon
NERC Isotope Geoscience Laboratories, British Geological Survey, Keyworth, NG12 5GS, UK (email@example.com)
The EARTHTIME Initiative (www.earth-time.org) has been working towards a step-wise improvement in the application of radio-isotopic dating techniques to the calibration of geologic time. Since its inception in 2003, the efforts of EARTHTIME have mainly centred on the chronometers used for calibration of the majority of the fossiliferous stratigraphic record (U-Pb, 40Ar/39Ar and astronomical tuning) with an emphasis on the following: (1) identifying analytical limitations (inter- and intra-chronometer biases, capacity etc.) and actions for their reduction/elimination; (2) identifying science drivers that require enhanced temporal frameworks, and (3) developing outreach and in-reach and programs.
As part of the EARTHTIME Initiative the international U-Pb isotope-dilution thermal ionisation (ID-TIMS) community has made a concerted effort to quantify and eliminate interlaboratory bias, to fully assess the accuracy of the chronometer, and to ensure benefits are made available to as many laboratories as possible. Initial attempts (2004/5) to assess inter-laboratory bias involved analyses of zircon mineral standards. Results from this experiment indicated significant interlaboratory scatter (>0.3%) however the nature of the experiment (too many variables), combined with the variation in quality and quantity of data submitted, made it difficult to discern which factor(s) (tracer calibration, mass spectrometry, sample dissolution etc.) were responsible for the observed scatter. Following community post-mortem analysis of this first experiment, further endeavours were initiated: (a) preparation and calibration of a large quantity of a mixed U-Pb tracer for community use; and (b) preparation and calibration of a large quantity of synthetic (matrix free) U-Pb ‘age standard’ solutions of varying apparent age that can be used as standards to quantify both intra- and inter-laboratory agreement. These initiatives involved multiple collaborating laboratories and were complete in 2007/2008, with some >20 laboratories now using these materials. After the distribution of the materials a second (more prescriptive) interlaboratory exercise was undertaken. Initial data from 13 laboratories indicate improved agreement between laboratories with the majority of labs agreeing at the <0.1% level (U-Pb and Pb-Pb dates). The tracer calibration exercise has been comprehensive, including 238U/235U determinations on uranium reference materials (CRM U500 and CRM 112a) that underpin the tracer calibration, and provides the basis for estimating the total uncertainty of the U-Pb system. Finally, these analytical improvements have also fed into efforts in developing new open source and transparent data reduction software packages and considerations of limiting uncertainties.