High precision U-(Th)-Pb chronology

Changing samples

Quantifying geological time is central to our understanding of the evolution of the earth system. By establishing the exact timing of geological events, such as climatic and evolutionary transitions, we can test hypotheses that invoke cause and effect relationships between different components of the earth system, and quantify rates of processes. Radioisotopic dating provides the means to obtain precise and accurate temporal constraints on nearly all types of geological archives, from the ocean sediments that record Cenozoic climate change, to xenoliths from the lower crust that record plate tectonic processes and detrital minerals that track landscape evolution.

We have comprehensive U-daughter (U-(Th)-Pb and U-Th) geochronology facilities with both high precision (ID-TIMS) and high-spatial resolution (LA-ICP-MS) facilities. These complementary techniques allow us to tackle a wide variety of geochronological problems on materials spanning nearly the entire age of the earth.

Our facilities include multiple modern mass spectrometers and lasers, imaging capabilities, and clean labs for low blank (high precision) dating. We are the only laboratory in the UK that undertakes high precision U-(Th)-Pb analysis of zircon and other accessory minerals by isotope dilution, and we maintain a strong international reputation in this field.

Isotope dilution thermal ionisation mass spectrometry (ID-TIMS) determines U-Pb dates with a precision of 0.1 per cent or better. We exploit this technique for a variety of problems where the highest age resolution is required (e.g. calibration of the stratigraphic record, rates of metamorphic and magmatic processes, and characterization of standard minerals and solutions).

Synthetic U-Pb 'age solutions' and U-Pb tracer calibration

As part of the EARTHTIME Initiative, the facility has been involved in development and calibration the ET535 and ET2535 tracers, as well as development of new standard materials. In particular, we have prepared and calibrated a series of synthetic U-(Th)-Pb solutions that have 206Pb–238U and 207Pb–235U ratios that yield 'concordant' 206Pb–238U and 207Pb–235U ages at 10 Ma, 100 Ma, 500 Ma and 2 Ga.

The synthetic solutions have been prepared in quantities sufficient for use as long term standards, with the aim that they can be used for assessment of long-term intra-laboratory reproducibility and inter-laboratory agreement. We are currently using these solutions to assess the long-term reproducibility of the U-Pb ID-TIMS data we generate and for direct comparison with similar data generated in other labs. We believe these solutions will augment natural mineral standards (such as R33 and Temora) for ID-TIMS, allowing for greater evaluation of mass spectrometry as post-crystallisation Pb-loss and natural variation in zircon crystallisation will not be factors.

If you are interested in obtaining these materials, please contact Dan Condon.

238U/235U determinations for geochronology

Fundamental to U-daughter geochronology is the isotopic composition (238U/235U) of uranium in the natural materials being analysed. For decades, a consensus value (238U/235U = 137.88) has been used by the geochronology community, however, recent studies on bulk rock samples from low temperature environments indicate that this ratio does vary considerably in nature. Our group has been investigating the isotopic composition (238U/235U) of uranium in materials used for U-daughter geochronology, including reference materials (Condon et al. 2010) and the minerals being dated (Hiess et al. 2012). Our data provides the first accurate 238U/235U values for use in terrestrial mineral geochronology. In particular we propose a new 238U/235U value of 137.818 ± 0.045 for use in U-Pb zircon geochronology, based upon our study of a number of zircon samples of varying age, genesis and location.

Key references

Condon, D J, McLean, N, Noble, S R, and Bowring, S A.  2010.  Isotopic composition (238U/235U) of some commonly used uranium reference materials. Geochimica et Cosmochimica Acta, 74 (24).  7127–7143.  10.1016/j.gca.2010.09.019.

Hiess, J, Condon, D J, McLean, N, and Noble, S R.  2012.  238U/235U systematics in terrestrial U-bearing minerals.  Science (March 30th 2012).

Stratigraphic record

A major focus of the high-precision U-(Th)-Pb facility is working with the earth science community to provide robust temporal constraints to underpin earth system evolution research. This is primarily focused upon 'critical transitions' in Earth's history, and, more recently, on Mesozoic and Cenozoic climate change. Critical transitions in Earth's history often involve large magnitude perturbations in global bio-geo-chemical cycles, such as those associated with the rise of atmospheric O2 at approximately 2.4 Ga, or the rise of animals at approximately 57 Ma. As such, they illustrate the interplay between geodynamics, ocean and atmospheric chemistry, and biology.

Geochronology provides the only independent means of integrating the various disparate records of these events, thereby allowing us to assess relationships/feedbacks between the components of the earth system.

Contact

Please contact Béatrice Bullock-von Moos for further information.