U-series and U-Pb capability for carbonate geochronology has been developed in the geochronology and tracers facility to support NERC climate research, benefitting from extensive knowledge transfer from our U-(Th)-Pb geochronology facility. U-series (specifically 230Th/234U) geochronology projects currently in progress and recently completed include:
We are also carrying out work on the use of 234U/238U as a groundwater tracer.
Samples dated thus far have ranged in age from approximately 450 ka to recent. Sample preparation takes place in the geochronology and tracers facility's clean chemistry laboratories, which are equipped with an ESI/New Wave Research micromill for microsampling, Evapoclean unit for total sample dissolution involving perchloric acid if necessary, and in-house ultrapure acid production.
Samples are routinely analysed on a Neptune Plus MC-ICP-MS, although if required, U samples can be measured on our U-Pb geochronology Triton TIMS instrument. The Neptune is fitted with a variety of Faraday amplifier boards that would permit very high precision 234U/238U analysis if required by particular applications. Highly enriched mixed 229Th/236U tracers are used and have been calibrated against gravimetric solutions prepared from high purity U and Th metals.
Sea floor geochronology and tracers is a recently developed but rapidly growing area for the facility. This science area is focused on the chronology of sea floor deposits that can be dated by U-Th methods (e.g. deep water corals and methane derived authigenic carbonates (MDACs)). An issue with such projects is access to samples, and we are working with partners in Norway and the US to build collaboration and access to unique sample sets, and to include other UK interested parties.
In addition to solution-mode U series analysis, we have developed high spatial resolution laser ablation ICP-MS techniques and characterised in-house carbonate materials by both ICP-MS and TIMS as standards, all specifically suited for U-Pb carbonate dating and rapid carbonate U-(Th)-Pb reconnaissance screening. Carbonate dating is done using the very fast scanning AttoM (Nu instruments) SF-ICP-MS, while screening is performed using either our own AttoM or NuHR MC-ICP-MS, both employing an ESI-New Wave Instruments 193 nm short UV laser microprobe for sampling.
These techniques have been applied to a variety of projects including dating of > 1 Myr old hominid fossils and > 700 ka speleothems whose antiquity precludes analysis by the U-Th chronometer. Screening for U, Th and Pb concentrations and 207Pb/206Pb isotope ratio is routinely applied to carbonates to determine sample suitability and aid refined sample selection for U-series or U-Pb chronology.
Please contact Béatrice Bullock-von Moos for further information.