Laser Ablation ICPMS: Applications to Diamond Exploration

N.J. Pearson1, M. Norman1, A. Sharma1 and W.L. Griffin1,2

1. GEMOC, School of Earth Sciences, Macquarie University, Sydney, NSW 2109

2. CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 2113

Recent advances in understanding petrogenesis as dynamic, process-related phenomena have been achieved by the application of microbeam techniques to the determination of trace elements and isotopic ratios. Just as the high spatial resolution of the electron microprobe revolutionized mineralogy and contributed to the development of quantitative petrology, using major and minor elements, so the coupling of LAM (Laser Ablation Microprobe) to an ICPMS (Induced Coupled Plasma Mass Spectrometer) is now poised to do the same with trace elements.

The development of the LAM-ICPMS as a powerful new analytical technique in geochemistry is comparatively recent. Most simply, a focussed laser beam is used to ablate small amounts of solid material which are transported into an ICPMS for trace element and/or isotope analysis. Early laser ablation systems were designed primarily as solid sampling devices, designed to overcome the problems of getting rocks and minerals into solution. Recognition of the micro-analytical potential of the technique has been accompanied by rapid changes in laser technology, so that spot sizes of less than 10 µm are now achievable, depending on laser wavelength, energy and sample type. The sensitivity of the ICPMS is an additional factor that governs the minimum spot size for quantitative analysis.

The laser ablation system at Macquarie University was designed and installed by Drs Simon Jackson and Henry Longerich of Memorial University, Newfoundland. This system includes a Continuum Surelite I-20 Q-switched Nd-YAG laser with a fundamental wavelength of 1064 nm (IR) and frequency doubling crystals which produce 532 nm (visible) and 266 nm (UV) wavelengths. Ablation yield is improved for minerals with low abundances of transition elements using the frequency quadrupled UV wavelength because of higher absorption. The frequency range of the laser is 1-20 Hz (laser pulses per second) and maximum energy is approximately 7 mJ per pulse for 266 nm at 20 Hz. Typical operating conditions for the quantitative analysis of silicate minerals and glasses involve energies of 0.1 to 1 mJ per pulse, producing pit diameters of 20 to 50 µm. For grain mounts of infinite thickness, ablation times of 2-3 minutes are achievable at low repetition rates (< 5 Hz), before the analyte signal intensity decreases due to defocussing of the laser.

The ICPMS is a Perkin-Elmer ELAN 5100 and in our experience the instrument operating conditions for laser ablation analysis are comparable with those for analysis by solution nebulization. Parameters such as nebulizer gas flow and RF power are varied to optimize sensitivity, background intensities and oxide production. Routine analyses are performed with up to 30 analyte masses: this is not the maximum limit on the number of masses for an analysis but because the analyte signal is transient, precision will be reduced for larger numbers of analyte masses.

Calibration requires (1) an external standard (such as the NIST glasses) for relative element sensitivities and (2) an internal standard to correct for the ablation yield. The internal standard is used to normalize the isotope ratios of the unknown and standard to absolute abundances. Typically this is a major element, such as Ca, Ti, Fe or Mg which can be determined by electron microprobe or some other independent technique. Based on these operating conditions and calibration procedure, detection limits of 0.1 to 0.01 µg/g are achievable.

Proton-microprobe analysis of trace elements in garnets is now widely used in diamond exploration worldwide. Analysis of megacryst garnets and concentrate garnets derived from diamond exploration show excellent reproducibility of the proton-microprobe data, while the LAM analysis adds significant information, including rare earth elements and a number of other useful elements such as Sr, Sc, V, Co and Hf. The usefulness of this extra information in the exploration context is only beginning to be explored, but several important observations can be made. In particular, the shapes of garnet REE patterns carry information about metasomatic processes related to diamond preservation, and particular types of patterns have thus far only been observed in garnets from diamondiferous kimberlites.

The LAM-ICPMS represents a rapid, cost-effective method for in-situ trace element analysis. Compared to ion beam instruments it requires a smaller total capital investment (approx. $0.5 million) and can produce data for a wider range of elements with lower detection limits. This added information content will require a major interpretive effort, but will enhance the usefulness of trace-element data to explorationists.