Advancing micro-analytical isotopic and trace element ICP-MS techniques for future application to the geosciences

The desire to analyse smaller and smaller quantities of material in the geosciences has been a pursuit of geochemists for decades. Where total amounts of an element (or isotope) are exceptionally low within single crystals, there is currently an inability to analyse that material. The low total amount of element may be below instrument detection limits, and if higher than the detection limit, the achievable precision on the measurement will be catastrophically restricted. Small degrees of heterogeneity in the target system are therefore unresolvable.

Utilising ICP-MS and the Teledyne-CETAC MVX 7100 μl Workstation, this study maps the capabilities, optimises the outputs, and demonstrates the geological application of low-volume solution ICP-MS analysis. This method influences a step change in analytical geochemistry by facilitating the measurement of single- and sub-nanogram amounts of element at the level of counting statistics, adding to the ‘geochemists toolkit.’ Mapping the precisions achieved using conventional solution ICP-MS sample introduction, it is demonstrated that 2.5 times better precision is achievable on the same amount of element when utilising the low-volume method, as well as equivalent precision on four times less element. This improved measurement precision translates to either an increase in the spatial (and hence relative temporal) resolution of conventional elemental and isotopic analyses of conventionally-used minerals, or brings non-conventional minerals into the realms of analytical capability. Such minerals may have concentrations of analyte too low for conventional solution ICP-MS measurements. Integrating these analyses with petrographic and geochronological data better constrains the genetic history of geoenvironmental samples. Real geological problems interrogated through single crystal analysis are demonstrated in this thesis.

This new method of analytical geochemistry is applied to U-isotopes, the Lu-Hf isotopic system and split-aliquot analyses, including application to trace elements. The method for each of these applications has been developed and explained. For U-isotopes, the low-volume method highlights heterogeneity in reference materials previously thought to be homogenous, and the direct implication to U-Pb geochronology is shown. A comprehensive method of uncertainty quantification and propagation is developed, demonstrated and applied. Proof of concept for miniaturised Lu-Hf geochronology and single-crystal Hf-isotope analyses without the need for ion exchange chromatography is also shown. Finally, U-isotope, Hf-isotope and trace element analyses are applied to the same dissolution of a single zircon crystal, highlighting U-isotope heterogeneity from a single magmatic body for the first time. Supporting Hf-isotope and trace element analyses are used to interrogate the magmatic processes at play, elucidating the potential cause of this level of heterogeneity which until now, has only been theorised. These results would have been impossible to obtain without this step change in analytical geochemistry.