Application of Laser Ablation ICP-MS to the Microanalysis of Volcanic Glasses and Minerals

Nicholas J. G. Pearce Institute of Earth Studies, University of Wales, Aberystwyth, SY23 3DB, UK

William T. Perkins Institute of Earth Studies, University of Wales, Aberystwyth, SY23 3DB, UK

John A. Westgate Department of Geology, University of Toronto, Toronto, Canada M5S 3B1

Modifications to a pulsed Nd:YAG laser ablation (LA) system to operate at 266 nm (ultra-violet) when coupled to an enhanced sensitivity inductively coupled plasma mass spectrometer (ICP-MS) offers the geochemist a versatile and highly sensitive analytical tool which can achieve spatial resolutions as low as 5 micrometers and sub-part per million detection limits. Unmodified lasers (operating at 1064 nm) can be configured to produce 20-30 micrometer craters in the infra-red but only in strong IR absorbers (generally Fe-bearing minerals). Ablation in colourless materials is less predictable in IR and proceeds by erosion of the sample by a suprajacent plasma, formed at the point of focus of the laser. At UV wavelengths, most minerals and natural glasses absorb strongly and ablation proceeds by direct absorption of laser energy, giving more predictable behaviour.

Whilst laser hardware has developed rapidly to produce smaller craters, it has been necessary to modify the ICP-MS and the analytical rationale to maximise the sensitivity of the instrument to capitalise on the reduced signal generated from the better spatial resolution. Increasing the pumping rate of the first stage vacuum system can produce an order of magnitude increase in sensitivity, and careful selection of scan parameters and instrument settings can further lower detection limits. Ultimately, analysis of normal thickness polished thin sections, previously used for EPMA analysis to determine the concentration of an internal standard, is possible by UV LA-ICP-MS and provides a wealth of additional geochemical data.

UV-LA-ICP-MS is ideal for the trace element analysis of small samples. For the analysis of glass shards from tephra deposits (particularly thin, distal tephra deposits where
material is sparse), block mounted shards, already anlaysed by EPMA to determine Si or Ca for internal standardisation, can be analysed by LA-ICP-MS to enable correlation and/or discrimination of individual deposits. In addition, the range of internal variation between individual shards can be determined, which may relate to the eruptive process and
fractionation within the parent magma body, variation which would not be detected by a single bulk-sample analysis.

The table below provides and indication of the accuracy of the technique for the analysis of volcanic glasses by either fully quantitative methods (each element calibrated individually) or by a single internal standard - multi-element method (often referred to as semi-quantitative), which can both produce data within ±10% of accepted values. In addition, the effects of varying operating conditions (laser power, spatial resolution, vacuum conditions, element menu and scan parameters) will be described and assessed for analysis of geological materials presented for analysis in various ways.

Table 1.

Fully quantitative analysis, 30Si internal std. "Semi-quantitative" analysis, 57Fe internal std.

Lost Chicken tephra, late Pleistocene, Alaska Santorini tephra, Gohlisar, Turkey

UT771 UT771 UT86 UT86 UT86 >150µm >150µm 85-150µm 85-150µm

LA-ICP solution LA-ICP solution INAA LA-ICP solution LA-ICP solution

(n=3) ICP-MS (n=1) ICP-MS (n=3) ICP-MS (n=3) ICP-MS

La 12.34 11.95 11.60 11.30 14.95 28.74 28.82 28.66 28.95

Ce 24.10 22.95 22.31 22.34 31.8 65.25 55.06 65.11 55.33

Pr 2.82 2.80 2.39 2.66 7.69 6.55 7.85 6.74

Nd 10.51 9.74 9.08 9.69 29.67 25.80 28.72 25.20

Sm 2.01 1.87 1.99 1.71 2.66 6.16 5.71 6.08 5.44

Eu 0.39 0.41 0.38 0.38 0.51 1.15 0.97 1.15 0.99

Gd 1.84 2.00 1.74 1.89 7.49 6.55 7.33 6.60

Tb 0.31 0.30 0.30 0.28 0.43 1.08 1.05 1.15 1.06

Dy 1.84 1.88 1.83 1.77 7.63 6.89 7.66 7.03

Ho 0.42 0.39 0.39 0.36 1.67 1.58 1.64 1.53

Er 1.30 1.18 1.12 1.10 4.76 4.79 5.43 4.58

Tm 0.22 0.19 0.20 0.17 0.86 0.74 0.84 0.71

Yb 1.43 1.41 1.24 1.34 1.74 5.72 5.09 5.45 5.12

Lu 0.24 0.20 0.23 0.19 0.31 0.87 0.77 0.88 0.82