Project IV.3: Intercalibrating 238U, 235U and 40K derived ages for geochronology (Joe Hiess supervised by Dan Condon)

Objective: To address the fundamental calibrations which underpin the accuracy of high-precision U-Pb dates and quantify the (relative) inaccuracies in the 238U, 235U and 40K decay constants used in geochronology, and to prepare reference materials for geochronological applications worldwide.

Background: Determination of the radio-active decay rate for the isotopes 238U, 235U and 40K used in geochronology underpins all age determinations. These 'decay constants' have been empirically determined during the last century primarily through counting experiments. These determinations have uncertainties (e.g., ca. 0.1% for 238U and ca. 1% for 40K) that must be propagated when compared with dates generated using different chronometers (i.e., 238U-206Pb and 40Ar/39Ar dates), resulting in a loss of resolution. Repeating the decay constant counting experiments would be very expensive and it is not clear that we could improve upon the previous determinations. We propose a series of systematic experiments (building upon previous studies) to assess and improve the accuracy (relative to the 238U decay constant) of the U/Pb dates to intercalibrate those with 40K derived dates. We wish to go further and prepare precisely calibrated reference materials (solutions of U and Pb) that when used will improve the performance of all U-Pb laboratories worldwide.

Research strategy and methodology: We plan a two phase approach. Phase 1 will address the fundamental calibrations which underpin the accuracy of U/Pb dates and prepare reference materials useful for U-Pb measurements; Phase 2 will build upon phase 1 and tackle the inter-calibration of 238U, 235U and 40K derived ages through the analyses of isochronous minerals.
Phase 1: At present tracer calibration for U/Pb ID-TIMS analyses is accurate to ca. 1‰ to 0.5‰ for modern laboratories worldwide. The EARTHTIME initiative has developed a large amount of 205Pb-233U-235U tracer for community use in order to eliminate tracer calibration as a source of systematic bias between labs. The accuracy of this tracer would then underpin all ages produced using it, and therefore its precise calibration is paramount. This calibration will be undertaken using multiple reference solutions with contrasting initial materials (metals, solutions) to arrive at a precise and consistent Pb/U calibration for the tracer, hopefully to better than 0.3‰. A rigorous metrological approach will be undertaken in cooperation the EU Institute of Reference Materials and Metrology in Geel Belgium in this process. As these calibrations depend upon very precise and accurate TIMS mass spectrometry measurements, the team will assess every single performance aspect of these measurements, using multiple instruments and detector types in order to quantify all biases and uncertainties, in order to produce a realistic but much improved precision of the Pb/U value of this valuable tracer, upon which all user labs worldwide will depend. Finally we will prepare radiogenic Pb-natural U mixed reference solutions that will help laboratories improve their quality assurance but which will depend upon the measurement of mineral standards of uncertain homogeneity. Thus phase 1 will underpin a vast amount of earth science research globally for at least a period of 10-20 years.
Phase 2: This aspect will focus on dating several key samples spanning a range of ages from ca. 50 Ma to > 2 Ga by both U-Pb and 40Ar/39Ar.  Previous attempts to generate such a dataset have relied in part upon published literature and/or a very limited sample size, and tracers of less certain calibration than described above. The U-Pb data will employ the EARTHTIME tracers and calibrations (Phase 1) and the 40Ar/39Ar dates will be obtained in conjunction with researchers executing task IV.2.

Feasibility: This task will build upon many recent and ongoing advances in mass spectrometry, metallurgy and isotope geochemistry, many of which the partners in this proposal are leading along with instrument manufacturers and metrological institutes.  Previous studies have demonstrated the feasibility of this approach however they have never been executed in a fully integrated fashion.

Innovative aspects and relevance: Like pivotal advances in U-Pb geochronology made in ~1973 (simplification of chemistry and lowering of blanks by Krogh and Mattinson), and in the ~1980s (widespread availability of 205Pb tracer, air abrasion and use of multicollector mass spectrometry by Krogh, Parrish, and Roddick) our proposed EARTHTIME -related advances in tracer calibration and availability will lead directly to more accurate U-Pb dating and a much better inter-calibration of 238U,235U and 40K decay constants that will finally permit very high precision (<= 0.3‰) inter-calibration of chronometers leading to much more robust testing of synchroneity of short term geological events worldwide and determination of much more precise rates of short terms geological and climatic events in the geological past.

Link with other projects:  This task will provide fundamental constraints which will underpin all of GTSnext that involve, directly or indirectly, radio-isotopic dating.  Much earth science research worldwide addresses the pace of environmental and climatic change in the recent and geological past the understanding of which underpins the usefulness and robustness of global climate and environmental modelling of the earth. Our proposed improvements to the science of chronology are fundamental to better understanding of how Earth has operated in the geological past. Our proposed work will improve the state of the art in geochronology facilities all over the globe for the next 20 years, a contribution that multiplies the benefit of the investment that this proposal puts forward.