The application of laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) to in-situ trace element determinations in minerals (Keynote Address)

Jackson, S., GEMOC, Macquarie

This talk will chronicle the development of laser ablation microprobe (LAM)-ICP-MS for in situ trace element determinations in minerals at Memorial University of Newfoundland and will describe, with examples, the factors leading to current instrument configuration and operating protocols.

The current instrument configuration utilises a frequency quadrupled Nd:YAG laser (λ = 266 nm in the UV). Laser output energy is controlled using polarising optics before the beam is steered through a petrographic microscope, which provides the high quality optics required for viewing geological samples in petrographic sections. The LAM is coupled to an enhanced sensitivity ICP-MS with a standard solution sensitivity of ca. 500 million cps/ppm for the heavy mass range (La onwards). With minor hardware modifications, dry plasma instrumental backgrounds have been lowered to ca. 1 cps for the heavy mass range, providing detection limits of low ppb for a 30 µm ablation spot size. Signal intensity data are acquired in a time resolved manner, using a very fast peak hopping data acquisition sequence. This very powerful capability allows assessment of homogeneity of the ablation volume, recognition of analytical artifacts (elemental fractionation, signal spikes), selective integration of signals, when necessary, and production of elemental depth profiles.

For mineral analysis (silicates, oxides, phosphates, carbonates, fluorides), calibration is performed using well characterised glasses (synthetic silicates or fused rock powders), employing an internal standard to correct for differences in ablation yield. Excellent accuracy is obtained for elements which exhibit similar ablation behaviour to the internal standard. However, for elements which exhibit significant fractionation relative to the internal standard, significant errors can occur. Particular attention will be paid to the serious problem of elemental fractionation during laser sampling. The problem will be characterised and techniques, including spot cooling and novel focussing conditions, to minimise the effect will be described.

Several wide ranging examples, which highlight the enormous potential of LAM-ICP-MS in mineral studies, will be presented including: the study of trace element partitioning in an igneous melt, REE distribution in a hydrothermal fluorite deposit, analysis of single melt and fluid inclusions. Progress in developing the technique for analysis of sulphides will also be shown.

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