Research Themes

We are interested in the use of isotope and trace element data to address a wide range of questions in the Earth (and other) Sciences. In addition we share a passion for the development of new analytical techniques and methodologies to assist in solving intractable problems. We have currently active research projects in areas of mantle evolution, Quaternary environmental change, granite magmatism, subduction zone processes, kimberlite petrogenesis, analytical technique development, and archaeometry, in addition to frequent projects from Biological and Zoological Science colleagues. Four areas have been identified, however, representing a significant proportion of our research effort, and into which additional resources have been allocated:

1. 'C3 Lab' (Palaeoclimate research)

2. The Future of ICPMS (technique development)

3. Extreme subduction: a return to New Britain (subduction zone processes)

4. Data integrity

Recent research news

U-Pb dating of speloethems

Zircon Hf-isotope analysis with an excimer laser

SEM images reveal ablation processes

Laser-ablation U/Th dating of Quaternary carbonates

In situ Sr isotope analysis of carbonates

In situ Pb isotope analysis of melt inclusions

U-Pb dating of speleothems

Speleothems—stalagmites, flowstones and other cave deposits—are widely used archives of palaeoclimate palaeoclimate variation and are producing some detailed insights into climatic oscillations in the recent past. A major contributor to their success is their remarkable suitability for dating by U-series analysis. The U-Th decay scheme provides a highly robust dating method for materials up to around 600 ka in age but beyond this the value of speleothems has been limited by a lack of appropriate chronometers. The U-Pb decay scheme, in contrast, offers the possibility to significantly extend the use of speleothems back many millions of years. Since the pioneering study by Richards et al. (1998) of a Quaternary speleothem from the U.K, however, very few groups have reported success with the U-Pb technique as applied to speleothem geochronology.

Using low-blank Pb chemistry, coupled to MC-ICPMS technologies, we have developed a method for U-Pb dating of speleothems which appears to be relatively robust. Although sample selection will still be of critical importance, we have succeeded in producing high quality isochrons for samples from caves beneath the Nullarbor plain (western Australia) and from the Alpi Apuane massif of Tuscany.

A manuscript describing this new technique is in press in Quaternary Geochronology. Contact: Jon Woodhead (jdwood@unimelb.edu.au)

Zircon Hf-isotope analysis with an excimer laser

The mineral zircon constitutes a geochemical data repository of unparalleled quality, forming a highly robust phase in most geological environments, and being readily amenable to radiometric dating methods. Furthermore zircon preserves a high quality record of near-initial Hf-isotope ratios, which can be used both in provenance studies, and as a petrogenetic indicator.

An analytical system, coupling an ArF excimer laser to a Nu Plasma MC-ICPMS, has been used to explore potential methods for in-situ Hf isotope analysis of zircons, and the levels of precision and accuracy obtainable in ‘real world’ situations. The system is characterised by high sensitivity and rapid response and thus allows analysis using relatively small spot sizes and slow drill rates. An analytical protocol for correction of Yb interference has also been implemented which provides potentially significant improvements in accuracy over previously reported studies.

Novel solutions have been explored for the analysis of complex zircon growth zones, including the ablation of non-standard pit geometries, ablation along pre-digitised tracks, and depth profiling. An experimental simulation of the latter using a two-layer zircon composite suggests that the depth-profiling protocol should allow determination of the isotopic composition of deep layers for ablation pit aspect ratios as high as 2:1 (depth: width) – see figure.

Theoretical calculations of the limiting precision possible for in situ Hf isotope analysis of zircons demonstrate a strong dependence on ion beam size and counting time, and also emphasise the importance of background: peak count ratio. Using typical analytical conditions, 2s precision ~100ppm can be achieved.

These results are documented in:

Woodhead, J., Hergt, J., Shelley, M., Eggins, S., and Kemp, R. (2004) Zircon Hf-isotope analysis with an excimer laser, depth profiling, ablation of complex geometries, and concomitant age estimation. Chemical Geology 209, 121-135

In addition we have undertaken a detailed study of a variety of zircons standards in order to establish their usefulness as in situ Hf-isotope reference materials. The results of this study can be seen in

Woodhead, J.D. & Hergt, J.M.  (2005) A preliminary appraisal of seven natural zircon reference materials for in situ Hf-isotope analysis. Geostandards and Geoanalytical Research, 29, p.183-195.

Contact: Jon Woodhead (jdwood@unimelb.edu.au)

SEM images reveal ablation processes

Scanning electron microscope (SEM) images of excimer laser ablation pits and products obtained in zircon with the University of Melbourne 193nm excimer laser ablation system.

1. a typical ablation pit produced in zircon by 300 pulses at a laser fluence of <10mJ cm-Note ‘top hat’ profile and lack of evidence for sample melting beyond the immediate vicinity of the ablation site (sub micron thick frozen melt ‘waves’ can be seen on the pit walls, with a micron thick rim).

2. detail near the ablation pit rim shows apparently large ejecta fragments with diameters of around 1 micron. Closer inspection (3 and 4), however, reveals that these are aggregates of much smaller particles with average diameters of only a few tens of nanometres. These are vapour condensates from the ablation plume rather than simple ejecta and suggest highly efficient coupling of the laser beam with the sample.

Contact: Jon Woodhead (jdwood@unimelb.edu.au)

Laser-ablation U/Th dating of Quaternary carbonates

Uranium-series disequilibrium dating of carbonate materials has become of great importance in establishing accurate chronologies for the late Quaternary, particularly since the development of mass spectrometric methods for measuring the required isotope ratios. In recent years multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) has begun to replace thermal ionisation mass spectrometry as the best analytical technique for uranium-series analysis, with the promise of significant increases in both sensitivity and sample throughput.

An extremely sensitive parallel ion-counting 238U-234U-230Th method already in use at The University of Melbourne for digested carbonate samples, has been adapted to in situ analysis using a HelEx 193 nm laser-ablation system developed at the Australian National University. The sensitivity of the system is such that percent-level precision can be obtained simultaneously on the 230Th/238U and 234U/238U ratios for two-minute analyses of speleothem samples containing as little as 1 ppm uranium.

The well-recognised principal obstacle to accurate laser-ablation 238U-234U-230Th dating is the problem of elemental fractionation between Th and U in the ablation-transport-ionisation process. This fractionation is not constant, but varies slightly according to the material being ablated and the laser ablation parameters used. There are two simple approaches to constraining this fractionation factor for a given sample such that the apparent 230Th/238U ratio can be normalised to give a true value. The first approach is to bracket unknowns with analyses of samples of the same type for which the age (and hence 230Th/234U ratio) is already known.

The second approach to constraining U/Th fractionation is to treat laser-ablation analyses as a means of producing high-resolution profiles or maps of apparent 238U/234U/230Th ratios, which can be anchored at one or more tie points to their absolute values using conventional analyses of powder drilled or scraped from the sample surface. This is made easy in that any sample containing enough uranium for laser-ablation analysis also contains enough uranium that it can be dissolved and introduced directly to the MC-ICP-MS after addition of a mixed spike solution, allowing simple, rapid and accurate analysis.

Laser-ablation U/Th age analyses of a flowstone-like carbonate rind from a shallow cave in the Flinders Ranges, indicating apparent laser-ablation 230Th/238U ratios to be consistently ca. 70% of the true values determined by conventional solution-mode analyses, regardless of ablation parameters used. An image of the sample is shown at the same distance scale, showing ablation pits and a scan tracks.

A short Quicktime movie of a rectangular laser spot scanning over the flowstone surface can be found here (950 kb)

Contact: John Hellstrom (j.hellstrom@unimelb.edu.au)

In situ Sr isotope analysis of carbonates

In situ Sr-isotope analysis by laser ablation multi-collector ICPMS is a potentially powerful tracer technique with widespread application to many fields of study. The utility of the method, however, depends very strongly upon the quality of data that can be obtained (compared to conventional ‘solution-based’ analyses), and the spatial resolution, particularly in samples with strong compositional zonation or fine-scale growth banding. In this study we demonstrated that highly accurate (~50 ppm) and precise (external precision ~125 ppm) analyses of carbonate materials can be obtained in situ and that, by utilising the aperture-imaging optics of an excimer laser system with appropriate time-resolved software, isotopic variations on the scale of tens of micrometres can be resolved.

The example shown below is from a relatively small (~500 mm diameter) otolith from a diadromous fish species, Galaxias maculatus, and demonstrates how such analyses can be used to track migration between salt and fresh water environments.

These results can be found in:

Woodhead, J.D., Swearer, S., Hergt, J, & Maas, R. (2005) In situ Sr-isotope analysis of carbonates by LA-MC-ICP-MS: interference corrections, high spatial resolution and an example from otolith studies. Journal of Analytical Atomic Spectrometry 20, 22-27.

Contact: Jon Woodhead (jdwood@unimelb.edu.au)

In situ Pb isotope analysis of melt inclusions

Building on the pioneering work of Saal et al. (Science 282, 1481) we have developed an analytical technique for the in situ analysis of low level (1-20ppm) Pb materials, but extended to incorporate measuerement of the low abundance isotope 204Pb. As this work was intended primarily for the analysis of glassy melt inclusions, preliminary studies and validation were undertaken on a variety of glass standards with known Pb-isotopic compositions (see figure). The analytical procedure utilises the multi-ion counting potential of the Nu Plasma MC-ICPMS with real time correction for Hg interference on mass 204, and produces Pb-isotope ratios which are both accurate and have precision which approaches theoretical limits.

The results of this study can be found in Journal of Analytical Atomic Spectrometry 20 (2005), 1350-1357. For more information contact Bence Paul (b.paul@pgrad.unimelb.edu.au) or Jon Woodhead (jdwood@unimelb.edu.au).