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NERC Isotope Geosciences Laboratory



Nu Plasma ICP-MS

Chronology Methodology

At NIGL 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 complimentary 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. For more information on each of the techniques see sections below.


High-precision U-Th-Pb geochronology

Isotope-dilution thermal ionisation mass spectrometry (ID-TIMS) affords the ability to determine U-Pb dates with a precision of 0.1% or better. At NIGL 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, NIGL 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 that they can be used as long term standards, with the aim that they can be used for (1) assessment of long-term intra-laboratory reproducibility and (2) assessment of inter-laboratory agreement. We are currently using these solutions to assess the long-term reproducibility of U-Pb ID-TIMS data generated at NIGL 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 for more information.

 

238U/235U determinations for geochronology

Fundamental to U-daughter geochronology is the isotopic composition (238U/235U) of uranium in 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, Daniel J.; McLean, Noah; Noble, Stephen R.; Bowring, Samuel 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, Joe; Condon, Daniel J.; McLean, Noah; Noble, Stephen R. 2012. 238U/235U systematics in terrestrial U-bearing minerals. Science (March 30th 2012)


High-spatial resolution U-Th-Pb geochronology

We employ two laser ablation systems (both solid state (Nd:YAG) and excimer gas systems) and couple these to either a MC-ICP-MS, or SC-SF-ICP-MS. We can accurately determine the age of various U-Th-Pb mineral phases on the 10-50μm scale. Ablation pit depths average 15μm, using 6-140ng of zircon respectively for this spot size range. For a 25μm ablation spot, 35ng of material are utilized compared to 1-2ng for a SIMS analysis of equivalent spot size. Total combined uncertainties of c.1% 2SD on the 206Pb/238U age are directly comparable to that produced by SIMS. Recently we described a new approach to laser ablation geochronology, which involves single shot ablation so that thin (c.1μm) mineral overgrowths can be dated.

We produce a large volume of laser ablation U-Pb data on zircons (above right: detrital zircon from Svalbard). These are typically used to constrain the timing of magmatic and metamorphic events, and combined with other radiogenic isotope systems allow for detailed petrogenetic studies of crustal rocks. Recently, more emphasis has been directed to dating other accessory phases (monazite, titanite, allanite, xenotime, etc.). The use of multiple minerals is particularly useful to solve problems in metamorphic petrology, and dating of detrital U-bearing minerals (zircon, rutile) to inform studies of erosion and tectonics.

We take a comprehensive approach to such work, employing laser ablation ICP-MS and TIMS analyses where appropriate. We are constantly striving to improve our multiple mineral reference materials to be able to date almost any mineral that contains uranium, even at low concentration and young age.

 

Key references

John M Cottle, Matthew S A Horstwood & Randall R Parrish A new approach to single shot laser ablation analysis and its application to in situ Pb/U geochronology J. Anal. At. Spectrom., 2009 DOI: 10.1039/b821899d.

Horstwood, M S A. 2008. Data reduction strategies, uncertainty assessment and resolution of LA–(MC–)ICP–MS isotope data. In: Laser ablation ICP-MS in the Earth Sciences: current practices and outstanding issues. Ed. Paul Sylvester. Mineralogical Association of Canada Short Course Series Volume, 40, 283-300.


U-series geochronology

U-series and U-Pb capability for carbonate geochronology has been developed at NIGL to support NERC climate research, benefitting from extensive knowledge transfer from NIGL's U-Th-Pb geochronology facility. U-series (specifically 230Th/234U) geochronology projects currently in progress and recently completed include studies of speleothems, warm and cold water corals, terrestrial deposits (tufa), and lake sediments, as well as 234U/238U as a groundwater tracer work. Samples dated thus far have ranged in age from c. 450 ka to recent. Sample preparation takes place in NIGL'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 NIGL's 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 at NIGL and have been calibrated against gravimetric solutions prepared from high purity U and Th metals.

In addition to solution-mode U series analysis, NIGL has 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 NIGL's 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 at NIGL to carbonates to determine sample suitability and aid refined sample selection for U-series or U-Pb chronology.

NERC Standard Grant - Sharpening the U-Th chronometer through technical developments and community implementation

 
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© NERC 2014. This site is hosted by the British Geological Survey but responsibility for the content of the site lies with NERC Isotope Geosciences Laboratory (NIGL) not with the British Geological Survey. Questions, suggestions or comments regarding the contents of this site should be directed to Professor Randall R Parrish.