U-Pb isotope dilution (ID-TIMS)


The U-Pb ID-TIMS working group is an informal collective of people from U-Pb ID-TIMS laboratories worldwide who have been working together since the start of the EARTHTIME Initiative (ca. 2003) to improve the U-Pb ID-TIMS method.  Below we provide a short summary of the topics being discussed and progressed by this working group.

Interested in joining the working group?  Email: earthtime4567@gmail.com with ‘U-Pb Working Group’ in the subject line.

[accordion multiopen=”true”]
[toggle title=”Best practices and U-Pb Labshare“]

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.

[/toggle]
[toggle title=”EARTHTIME U-Pb Tracers and Gravimetric Tracer Calibration“]

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:

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

[/toggle]
[toggle title=”EARTHTIME Synthetic U-Pb ‘Age Solutions’“]

 

[/toggle]
[toggle title=”EARTHTIME U-Pb Inter-laboratory Agreement Exercises“]

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’

[/toggle]
[toggle title=”EARTHTIME U-Pb ID-TIMS Data Reduction“]

Some text about the different packages being used…

[/toggle]

[/toggle]
[toggle title=”EARTHTIME U-Pb Cyber-infrastructure“]

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

See Cyberinfrastructure page for further information.

[/toggle]
[toggle title=”U-Pb ID-TIMS Data Reporting and Interpretation Guidelines and notes“]

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!).

[/toggle]

[toggle title=”Future provision of Pb synthetic tracer isotopes“]

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.

Correspondence between ORNL and CIT
Correspondence between ORNL and CIT regarding the provision of the so-called “Wasserburg 205Pb”. (Image courtesy D.A. Papanastassiou)

[/toggle]
[toggle title=”Minutes of U-Pb working Group meetings“]

Minutes of the U-Pb working group meeting 2016

2017 working group meeting – to be convened.

[/toggle]
[toggle title=”Key Papers arising from the Working Group (in alphabetical order)“]

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…

[/toggle]
[toggle title=”Directory of U-Pb ID-TIMS laboratories engaged in working group activities (in alphabetical order)“]

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

[/toggle]
[/accordion]