U-Pb isotope dilution (ID-TIMS)

From the outset the EARTHTIME community has sought to improve the ability of the entire community through sharing of best practices.  The Labshare pages of the Boise State Isotope Geology Laboratory contains a wealth of information about U-Pb methods.

1. The issue of inter-laboratory bias was identified as a priority issue at the EARTHTIME I and EARTHTIME II workshops, at what level of uncertainty could we compare U-Pb ID-TIMS dates from different laboratories?

An initial U-Pb ID-TIMS inter-laboratory agreement exercise in 2004/5 demonstrated that bias existed between laboratories at a level that exceeded estimated calibration uncertainties, raising many questions about the calibration of the U-Pb ID system in each laboratory.

At the EARTHTIME II workshop it had been agreed that an effective way to effectively eliminate inter-laboratory bias related to the calibration of mixed U-Pb tracers used in each laboratory would be to mix and distribute a large quantity of U-Pb tracer that all labs engaged with ‘high-accuracy’ U-Pb geochronology for all to use.

This was carried out in 2005 and the tracer is now being used in XX U-Pb ID-TIMS laboratories worldwide.  Subsequent inter-laboratory agreement exercise indicate a significant reduction in bias (see below).

See ET353 and ET2355 Tracer information page for details of how to acquire them.

2. Calibration of the EARTHTIME U-Pb tracers provided an opportunity to assess and improve the accuracy of the U-Pb system.  A comprehensive calibration exercise was carried out on the EARTHTIME tracers, addressing many prior assumptions (e.g., ²³⁸U/²³⁵U = 137.88?) allowing for a calibration that can be traced back to SI units.  The results of the U-Pb calibration experiment can be found in this pair of papers:

• Condon, D.J., McLean, N., and Noble, S.R. 2010. Isotopic Composition (²³⁸U/²³⁵U) of some commonly used Uranium reference materials. Geochimica et Cosmochimica Acta, v. 74, p. 7127-7143.
• 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.

See U-Pb Gravimetric Calibration information page (to be written) and blogpost (to be written) for more information.

A number of experiments have been carried out to quantify

2004/5:  This experiment involved analyses of the Temora and R33 zircon standards which have been used for ion probe U-Pb geochronology.  Initial results were presented at the EARTHTIME II workshop in late 2004 and at the 2005 Goldshcmidt Meeting, where they demonstrated that inter-laboratory bias was present at a level greater than expected is due to estimated tracer calibration uncertainties.

2009: Results of an inter-laboratory agreement experiment using the then new synthetic U-Pb ‘age solutions’

Some text about the different packages being used…

Some text from Noah here.  Link to Cyber infrastructure page (to be written by Noah).

See Cyberinfrastructure page for further information.

We have been invited by GSA Bulletin to prepare a U-Pb ID-TIMS data reporting manuscript, similar to those published for ⁴⁰Ar/³⁹Ar, U-Pb microbeam and U-Th methods.

A draft manuscript is in development (May 2017), please email dcondon@bgs.ac.uk if you have any interest in contributing (please note that we intend to contact all active ID-TIMS groups once we have a working draft manuscript!).

Isotope dilution requires the use of either enriched or synthetic isotopes in order to accurately and precisely determine the amounts of U and Pb in samples.  For Pb measurements, where we want to measure the isotope composition and the amount simultaneously, only synthetic isotopes (those not appearing in natural samples) can be used, and for the best analyses, these need to be of high purity (contain very little natural Pb).  U-Pb ID-TIMS uses either ²⁰⁵Pb, ²⁰²Pb, or a combination of both.  Both isotopes are very difficult to produce and obtain. ²⁰⁵Pb has been produced either by proton irradiation of enriched ²⁰⁶Pb (naturally occurring Pb that has been artificially enriched in ²⁰⁶Pb), or by neutron irradiation of enriched ²⁰⁴Pb.  The small amount of ²⁰²Pb that is used was obtained from the byproducts of ²⁰¹Tl production (an isotope used for medical imaging).  Both ²⁰²Pb and ²⁰⁴Pb-derived ²⁰⁵Pb need to subsequently be enriched by electromagnetic separation to produce the required purity.

Current stocks of these isotopes are finite and running low, particularly ²⁰²Pb, which was only produced once.  The prospects for making more ²⁰⁵Pb are uncertain due to dwindling stocks of enriched ²⁰⁶Pb and ²⁰⁴Pb, and decreased capacity for isotope electromagnetic isotope enrichment.  Accelerators that produce ²⁰¹Tl are still extant but less abundant than they were decades ago, and regulatory tightening make it more challenging to obtain

This is a big issue and will require some coordination.  Please contact Ryan Ickert (Ryan.Ickert@glasgow.ac.uk) for more information.

Minutes of the U-Pb working group meeting 2016

2017 working group meeting – to be convened.

Bowring, J. F., N. M. McLean, and S. A. Bowring (2011), Engineering cyber infrastructure for U-Pb geochronology: Tripoli and U-Pb_Redux, Geochem. Geophys. Geosyst., 12, Q0AA19, doi:10.1029/2010GC003479.  Here Jim Bowring and colleagues outline the approach developed to …

Condon, D.J., McLean, N., and Noble, S.R. 2010. Isotopic Composition (²³⁸U/²³⁵U) of some commonly used Uranium reference materials. Geochimica et Cosmochimica Acta, v. 74, p. 7127-7143.  This paper provides some ²³⁸U/²³⁵U determinations for some commonly used reference materials.  Rather niche however prior to this the ratio for some reference materials was assumed, had no uncertainly and these determinations feed into the U-Pb tracer calibration exercise…

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.  This paper documents the calibration of the EARTHTIME U-Pb tracers (ET535 and ET2535), focusing on the metrology and principles that underpin the accuracy of U/Pb determinations by isotope dilution.  In addition to underpinning the EARTHTIME tracer calibrations the data and methods are applicable to the calibration of other tracers back to SI units.

Hiess, J., Condon, D.J., Noble, S.R., and McLean, N. (2012) Absolute ²³⁸U/²³⁵U determinations for use in U-daughter Geochronology. Science, v. 335, p. 1610-1614.  In this contribution Joe Hiess and colleagues determined ²³⁸U/²³⁵U values for a whole range of U-bearing accessory minerals and explores how our knowledge of this ‘constant’ impacts U-Pb geochronology.

Mattinson, J. (2005). Zircon U–Pb chemical abrasion (“CA-TIMS”) method: combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages. Chemical Geology, 220(1-2), 47–66.

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.  A companion paper to Condon et al (2016) tracer calibration paper which outlines the math and equations behind the calibration and its associated uncertainty estimation.

McLean, N. M., J. F. Bowring, and S. A. Bowring (2011), An algorithm for U-Pb isotope dilution data reduction and uncertainty propagation, Geochem. Geophys. Geosyst., 12, Q0AA18, doi:10.1029/2010GC003478.  This paper outlines…

Schaltegger, U., Schmidtt, A.K., and Horstwood, M.S.A. (2016) U–Th–Pb zircon geochronology by ID-TIMS, SIMS, and laser ablation ICP-MS: Recipes, interpretations, and opportunities. Chemical Geology.  DOI: 10.1016/j.chemgeo.2015.02.028.  This review paper provide an up to date (as of 2016) overview of U-Pb methods, ID and microbeam.

Schmitz, M.D., 2012. Radiogenic isotope geochronology. Geologic Time Scale, 115–126  In this chapter, Mark reviews the developments in radio-isotopic dating methods (esp. U-Pb ID-TIMS) from 2004 to 2012, covering many of the EARTHTIME and related innovations.

Schmitz, M.D. and Kuiper, K.F. Elements (2013) High-precision geochronology.  Elements.  Elements, v. 9, i. 1, p. 25-30.  In this contribution to the ‘One Hundred Years of Geochronology’ issue of Elements, Mark and Klaudia discuss…

Schmitz, M. D., and B. Schoene (2007), Derivation of isotope ratios, errors, and error correlations for U-Pb geochronology using ²⁰⁵Pb-²³⁵U-(²³³U)-spiked isotope dilution thermal ionization mass spectrometric data, Geochem. Geophys. Geosyst., 8, Q08006, doi:10.1029/2006GC001492.  Here Mark and Blair lay out some equations…

Schoene, B., Condon, D.J., Morgan, L., and McLean, N., (2013) Precision and Accuracy in Geochronology.  Elements, v. 9, i. 1, p. 19-24  This short note outlines and discusses issues around the precision and accuracy of geochronologic data and interpretations

Schoene, B., 2014, 4.10 – U–Th–Pb Geochronology, in Holland, H. D., and Turekian, K. K., eds., Treatise on Geochemistry (Second Edition): Oxford, Elsevier, p. 341-378  For everything you need to know about U-Th-Pb geochronology, but were afraid to ask…

Berkeley Geochronology Centre

Boise State University, Department of Geosciences

Copenhagen

Geochronology of ore formation, Earth sciences, ETH Zurich, Switzerland

Isotope geochemistry, geochronology and geochemistry labs, Earth Sciences, University of Geneva, Switzerland

Kansas

Massachusetts Institute of Technology (MIT), USA

NERC Isotope Geosciences Laboratory (NIGL), British Geological Survey, UK.

University of Oslo, Department of Geosciences

University of Princeton, Department of Geosciences

Santa Cruz

SUERC

University of Toronto, Department of Geology