1. Laser ablation: A new approach to U–Pb speleothem dating
25 October 2014
Laser ablation: A new approach to U–Pb speleothem dating
Laser ablation pits in speleothem calcite.
Christopher J.M. Smith
School of Geographical Sciences,
University of Bristol
Supervisors: D.A. Richards, P.L. Smart
Research collaborators: A.R. Farrant, D.J. Condon, M.S.A. Horstwood, N. McLean, S.R. Noble, R.R. Parrish, M.J. Simms, J. Woodhead, D.C. Ford
25th BCRA Cave Science Symposium

2. Why are speleothems important?
25 October 2014
Why are speleothems important?
Record of past chemical reactions
Globally distributed
High-resolution
Time-series
Absolute dating
Before we can begin to address the title of this talk and my research, we first need to understand why speleothems are important and what they can tell us about the world we live in and how it has changed over time.
First and foremost, they are physical records documenting past chemical reactions that have occurred as a result of the interactions between the atmosphere, hydrosphere and lithosphere. The chemical signatures from stable isotopes of Oxygen and Carbon, as well as trace and Rare Earth Elements, can act as proxies for palaeo- temperature, vegetation and hydrology during the growth period of the speleothem.
Speleothems are globally distributed records. Limestone karst covers approximately 1/5th of the Earth’s land surface, occurring on every continent and in almost every country. Consequently, speleothems have the potential to record vast amounts of climate and hydrological data form all over the world, at different times, and under changing climate regimes.
They often contain seasonal or annually banded layers, enabling scientists to establish prevailing conditions for very short and specific timeframes.
Unlike ice cores, speleothem records provide a true time-series record, rather than a depth-series records. The benefit of this is that we can conduct robust statistical analyses on the environmental proxy data that make up the time-series record.
Most importantly, speleothems can be precisely dated using by radiometric dating methods, which can be used to determine an absolute calendar age for a particular phase of growth or event in its growth history. Ultimately, this information when combined with environmental proxy data can help to understand how the local environment has changed over time.
Cheng et al. (2009) doi: /science
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3. How do we date speleothems?
25 October 2014
How do we date speleothems?
Radioactive decay dating
U–Th ≤ 500 ka
U–Pb > 1000 ka
Elemental separation
Mass spectrometry
Ages from ratios
Uranium-238 decay chain.
MC-ICP-MS schematic.
But how do we date speleothems?
Well, the most common forms of speleothem dating are based on radioactive decay dating, whereby an unstable parent isotope decays to a daughter isotope. The age of a speleothem can then by determined by measuring the ratio of the daughter to its parent, and the age of the material calculated from the half-lives of the measured isotopes.
This is epitomised by uranium-series dating, which utilises the decay of uranium-238 to its various daughter isotopes. Uranium-series dating can be divided into two main chronometers or “clocks”: the most frequently applied is uranium-thorium dating method, based on the decay of uranium-238 to uranium-234 and thorium-230; and, the other is the uranium-lead dating method, based decay of uranium-238 to stable lead The U–Th chronometer is used to date “geologically young” materials, up to half a million years in age, whilst the U–Pb chronometer is generally used to date materials over one million years old.
To date a speleothem, we first need to separate the elements of interest (i.e. uranium and thorium, or uranium and lead) from all the other elements that make up a speleothem. This is done by dissolving a sub-sample of the speleothem and chemically separating and purifying the elements of interest into solution form.
These solutions can then be run on mass spectrometer using either a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) or thermal ionisation mass spectrometer (TIMS) machine to determine the parent daughter isotope ratios. From these ratios it is then possible to calculate a sample age.
However, this process is incredibly labour intensive and time consuming, taking on average 7 days, from start to finish – including sample preparation, chemical separation and mass spectrometry, to produce around 8 U–Th ages, or just 2 U–Pb ages.
25th BCRA Cave Science Symposium

4. Yokoyama et al. (2011) doi:10.1021/ac2012448
25 October 2014
Research aims
Explore potential of laser ablation
Faster analyses
Increased throughput
Minimal sample preparation
Reduced sample consumption
Compare dating methods
Overlapping chronometers i.e. U–Th and U–Pb
Case studies
Yokoyama et al. (2011) doi: /ac
In this presentation I hope to demonstrate the unique potential of conducting U–Pb dating by laser ablation mass spectrometry. U–Pb dating by laser ablation has been widely applied to zircon dating, and in the right circumstances can be easily applied to speleothem dating, offering many potential benefits:
Reduced analytical time – faster analyses
Increased sample throughput – we can date more samples
Minimal sample preparation – cut, polish and clean, no chemical spiking or chemical separations required
Reduced sample consumption – greater sample conservation, which is important considering the history or U-series dating, 40 years we needed 10’s of grams for a sample age, where as now we can get a date on as little as 50 mg or 1/20th gram
To compare the results of U-Pb dating by laser ablation with conventional solution chemistry methods i.e. thermal ionisation mass spectrometry (TIMS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS)
Are the ages produced by different methods for the same material identical/comparable?
Does speleothem carbonate behave in a predictable manner during ablation - are age results biased i.e. consistently young/old?
To ascertain whether laser ablation offers any advantages over conventional dating methods for samples which fall between the two major U-series dating techniques i.e U–Th and U–Pb dating
Explore a variety of different case studies – that is optimal and sub-optimal samples for U–Pb dating
High U samples
High common Pb samples
Different age materials <500 ka, > ka and >>1000 ka
25th BCRA Cave Science Symposium

5. Challenges of U–Pb dating
25 October 2014
Challenges of U–Pb dating
Finding and identifying suitable samples
Closed system
High U
Low initial common Pb
Spread of isotopic ratios for U–Pb isochrons
Low Pb blank chemistry
Estimating initial 234U/238U ratio
So what are the major challenges of U–Pb speleothem dating in its current form?
By far the greatest challenge is finding suitable material for dating, as samples must meet a number of criteria in order for them to be successfully dated.
Firstly, and most importantly samples must have remained a closed system the time of formation. That is, no parent or daughter isotopes can have been lost or added to the system since the time of formation, otherwise this will create a false age. Uranium loss will result in a false old age, lead gain a false old age, uranium gain a false young age, and lead loss a false young age.
Secondly, in order to date a sample there must be sufficient uranium present. The more uranium present, the younger the speleothem you can date, as lead production is directly proportional to the number of uranium atoms present in the sample. Unfortunately, most speleothems only have around 1 ppm of uranium-238, effectively preventing them being from being dated until they reach an age of approximately 3 Ma.
Thirdly, samples must have low amounts of common Pb. Common Pb is Pb that is present in the sample at the time of formation, rather than radiogenic Pb, which is Pb that has occurred as a result of the decay of U in the sample. If the initial common Pb is too high it can swamp the detectors on a mass spectrometer, masking the radiogenic Pb signal.
Fourthly, we need a spread of U–Pb isotope ratios in order to define an U–Pb age. If the measured isotope ratios are confined to the upper part of this isochron we cannot precisely define the lower age intercept on the concordia line, and the true age of a speleothem. If we measure very radiogenic U–Pb ratios, then we are better able to define the lower intercept age and we get a more accurate age determination.
The next biggest challenge is the low Pb blank separation chemistry, as the addition of any external sources of Pb during the chemical separation and purification must be accounted and corrected. This is particularly onerous, as particulate anthropogenic Pb, principally from Pb in petrol to prevent engine knocking, is ubiquitous in the atmosphere. Therefore, separation chemistry must be conducted in a certified clean lab.
The final challenge is establishing the initial 234U/238U ratio, in order to correct the U–Pb ages for initial disequilibrium. For samples >10 Ma this correction can be ignored, but for younger samples this correction is essential. However, the after approximately Ma years the 234U/238U ratio reaches equilibrium, at which point it becomes impossible to determine its evolution path and the initial composition.
Six point U–Pb isochron and lower age intercept.
Evolution of 234U/238U ratio over time.
25th BCRA Cave Science Symposium

6. Challenges of U–Pb dating
25 October 2014
Challenges of U–Pb dating
16 years
14 papers
31 samples
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7. Current U–Pb dating procedure
25 October 2014
Current U–Pb dating procedure
Sample pre-screening
Sample unsuitable for U–Pb dating
Reject sample
Elemental analysis to measure areas of high common Pb.
Woodhead et al. (2012) doi: /j.quageo
Digital autoradiography to map radionuclide activity.
Chemical separation
Reject
analyses
Mass spectrometry
Monte carlo simulations of initial 234U/238U for disequilibrium corrections.
Dated speleothems. Woodhead et al. (2012) doi: /j.chemgeo
Thermo Triton (TIMS)
Thermo Neptune (MC-ICP-MS)
Sample pre-screening:
Is the sample >500 ka?
High U concentration, > 1 part per million?
Evidence of open system behaviour?
Digital autoradiography
If the samples don’t meet these criteria we reject the sample as unsuitable for U–Pb dating
Chemical separation:
Sample cleaning
Drilling
Weighing
Add tracer spike
Mass spectrometry
Radiogenic U–Pb ratios?
Corrections
Correcting for common Pb contributions
Disequilibrium corrections
U–Pb age
Are the ages in stratigraphic order?
Are the ages sensible?
If not, then it may be necessary to reject the sample, as unsuitable for U–Pb dating
Corrections
U–Pb age
25th BCRA Cave Science Symposium

8. Problems with current procedures
25 October 2014
Problems with current procedures
Pre-screening ≠ a usable U–Pb age
Labour intensive
Sample destruction
No a priori knowledge of U/Pb ratios or *Pb
Juts because we’ve pre-screened a sample for U–Pb dating doesn’t mean that it will produce a useable date. Essentially, until we measure the U and Pb ratios and calculate a date everything is pretty much up in the air in terms of usability.
Chemical separations and analysis are incredibly labour intensive, combine this with the potential for analysing samples that ultimately fail to provide a usable U–Pb date and this can lead to a lot of wasted time.
The next issue, is that isotopic analysis requires us to destroy a certain amount of sample. In the days of alpha-spectrometry this was grams of sample, typically grams, up to 14 cm3. Thankfully, thing have come on a long way since the 70’s and 80’s, as we can use just fractions of a gram. But as with all things, technology improves and who’s to say that in years time, the current generation won’t scorn us for destroying valuable records by removing milligrams of sample? So if we can find a way to reduce sample consumption in the name of conservation then we should take it.
For U–Pb dating to work we need a spread of radiogenic and non-radiogenic U/Pb ratios to define an isochron. Preference is obviously for radiogenic Pb, but until we measure the sample and determine the ratios, all the screening and looking at proxies is essentially pie in the sky.
Wee need another way.
25th BCRA Cave Science Symposium

9. A new approach to U-Pb speleothem dating
25 October 2014
A new approach to U-Pb speleothem dating
Experimental setup
NWR 193 nm ArF excimer laser
3 cm3, or 20 cm3 ablation cell
Nu Instruments HR MC-ICP-MS
Isochron ages from 40 spots
0.025 mg per 40 spots
1 isochron per hour
So we borrowed the experimental setup from the NERC Isotope Geosciences Laboratory (NIGL) at BGS Keyworth. This consisted of a gas powered laser with a very short wave-length to ensure maximum ablation power, and a mass spectrometer. By combining these two machines, we can put a cut and polished sample in the laser cell housing, which effectively becomes our laboratory, and fire a high energy laser beam at the sample and vapourise stuff.
During sample testing we soon found out that because of the low quantities of U and Pb in speleothems, compared to zircons, we had to use large spot sizes and as much power as possible.
25th BCRA Cave Science Symposium

10. WHC1 - Winnats Head Cave, UK
25 October 2014
WHC1 - Winnats Head Cave, UK
High U (11-21 ppm)
Sample datable by U–Th and U–Pb methods
248 ± 11 ka U–Pb age in Richards et al. (1998)
Known initial 234U/238U ratio – in disequilibrium
Stalactite WHC1
Richards et al. (2011) doi: /S (98)
25th BCRA Cave Science Symposium

11. WHC1 - LA versus TIMS ages
25 October 2014
WHC1 - LA versus TIMS ages
Laser ablation U–Pb age ± 4.9 ka 
TIMS U–Pb age ± 4.4 ka
25th BCRA Cave Science Symposium

12. 72030 - Grotte Valerie, Canada
25 October 2014
Grotte Valerie, Canada
High U (20-80 ppm)
Top ± 10.1 ka, suitable for U–Th and U–Pb dating
Base > 500 ka, U–Pb dating only
Variable initial 234U/238U over time, approaching secular equilibrium
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13. Laser ablation U–Pb age 687.1 ± 13.1 ka
25 October 2014
LA versus TIMS ages
Laser ablation U–Pb age ± 13.1 ka
TIMS U–Pb age ± 13.4 ka
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14. Simms & Farrant (2011) url: http://nora.nerc.ac.uk/
25 October 2014
OD Ogof Draenen, UK
Ogof Draenen over 1 Ma old?
Geomorphological questions… scarp retreat/incision rates
Samples beyond U–Th dating limit – in JQS soon!
Moderate to high U (1-50 ppm)
Simms & Farrant (2011) url:
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15. Laser ablation U–Pb age 1136.7 ± 15.7 ka
25 October 2014
OD LA age
Laser ablation U–Pb age ± 15.7 ka
25th BCRA Cave Science Symposium

16. New U–Pb dating procedure with laser ablation
25 October 2014
New U–Pb dating procedure with laser ablation
Pre-screening/ Laser ablation
Sample unsuitable for U–Pb dating
Reject sample
Reject
analyses
Chemical separation
A priori knowledge
Mass spectrometry
Corrections
Corrections
U–Pb age
25th BCRA Cave Science Symposium

17. Summary U–Pb LA dating is accurate, precise, comparable
25 October 2014
Summary
U–Pb LA dating is accurate, precise, comparable
Effective pre-screening tool
Standalone technique?
Faster analyses, larger datasets, more ages
“Deep-time” records more accessible for science
Initial disequilibrium makes dating < 10 Ma difficult
Ideal samples difficult to find, easier to identify
25th BCRA Cave Science Symposium