MINERAL MICROANALYSIS BY LASER ABLATION INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETRY

The use of a modified Nd:YAG laser coupled to an inductively coupled plasma mass spectrometer is described for the direct analysis of carbonates, zircon, olivine and feldspars to give a variety of major and trace element data. The modification to the laser produces Q-switched ablation craters with a diameter between 20 and 30-mu-m compared with 150-200-mu-m in the un-modified version. This is suitable for direct mineral analysis in rocks. Calibration is achieved by ratioing analyte peak intensities to an internal standard and concentrations are produced by comparison with reference materials. The selection of internal standards is discussed for each mineral type and is dependant upon the assumption that the element chosen is present at a fixed molecular proportion in the mineral, despite other compositional changes. Spatial resolution of micro-analysis by laser ablation inductively coupled plasma mass spectrometry (ICP-MS) at 20-30-mu) is approximately ten times that of an electron probe micro-analysis (EPMA, 2-5-mu-m) but has the advantage of much lower detection limits and wider dynamic range (from ppm to 10-100% m/m) than routine EPMA analysis. Laser ablation ICP-MS can also offer isotopic information.