Laser Microprobe ICPMS: A Robust and Cost Effective Microbeam Technique for In Situ Quantitative Trace Element Analysis

M.D. Norman, N.J. Pearson, A. Sharma, and W.L. Griffin (GEMOC, School of Earth Sciences, Macquarie University, North Ryde NSW 2109 AU)

Laser ablation ICPMS is a relatively new microbeam technique that can provide rapid, precise determinations of trace element abundances at sub-ppm detection limits in a variety of geological materials. A laser microprobe system was installed at Macquarie University in December 1994. The laser is a Q-switched and frequency-quadrupled Nd:YAG laser that typically is operated at 4 Hz and 1-3 mJ per pulse. Laser repetition rates of 4 Hz have been found to produce a nearly steady state signal for over 4 minutes with much less inter-element fractionation per unit time compared to analyses performed at higher pulse rates (e.g., 10-20 Hz; Norman et al., 1996, Geostandards Newsletter, in press). Spot diameters are 50 microns or less, and average drill rates are about 1 micron per sec.

A typical spot analysis takes about 5 minutes, including backgrounds and wash-out, making the laser microprobe a highly cost-effective analytical tool. Up to 30 masses per analysis are determined, with relative element sensitivities calibrated against the NIST 610 and 612 glasses. Concentration values for trace elements in these silica-rich glasses have been established by calibration against other natural and synthetic material, including mantle-derived pyroxenes and garnets, and fused rock standards. Comparisons of laser microprobe analyses of natural garnets and pyroxenes using the NIST glasses as calibration standards, with data obtained by proton microprobe, solution ICPMS, INAA, and XRF show no matrix effects. Detection limits range from less than 2 ppm for Ni to less than 50 ppb for a diverse group of elements, including several of the REE, Th, and U. Replicate analyses of the NIST 610 and 612 glasses as unknowns indicate an analytical precision of 2-5% at the concentrations present in these standards. Similar precision (2-7%) has been obtained at the natural abundances present in the new glass standard BCR-2G. Error analysis shows that counting statistics and the external precision on the internal standard concentration are the most significant sources of analytical uncertainty in these determinations.

A typical analysis consists of 110 readings, with each reading consisting of one sweep of the mass range at a dwell time of 50-100 msec per mass. For each sample, 40 readings are counted on the carrier gas (high-purity Ar) alone to establish the background before beginning ablation, followed by 70 readings during ablation. Peak and background regions are selected graphically from the time-resolved spectra of each sample, and the selected readings averaged to determine the net count rate for each mass. Each analysis is normalized to a major element as an internal standard, e.g., 44Ca for clinopyroxene, basaltic glasses, garnets, and amphiboles.

We have applied the method to understanding processes of melting and metasomatism in the mantle and the distribution of heat-producing elements in the lithosphere through analysis of minerals from mantle xenoliths and granitic rocks. Trace element compositions of basalts and sediments have been determined rapidly and with a minimum of sample preparation through analysis of natural and fused glasses. Laser microprobe ICPMS is a flexible microanalytical approach applicable to a wide variety of geological problems.