METHODOLOGY FOR DETAILED TRACE ELEMENT MAPPING OF GARNET BY LASER ABLATION ICP-MS: A LOOK AT UNRAVELING ZONING AND INCLUSIONS

Chemical heterogeneity within garnet can have pronounced effects (some good and some bad) on successful unraveling of tectonometamorphic history. Chemical heterogeneity in garnet can be present in the form of inclusions and chemical zoning. For the rare earth element (REE) involved geochronometers (Sm-Nd and Lu-Hf) the presence of REE rich inclusions within the garnet are known to have a detrimental effect. Chemical zoning within garnet may record geologically useful differences in age, growth rate and/or other metamorphic processes (e.g. Pollington and Baxter, 2010). These new high precision age measurements within garnet benefit from detailed studies of the nature, distribution and concentration of the REE zoning and inclusion phases. Laser ablation ICP-MS has been used to provide reconnaissance trace element and REE data and has been suggested as a screening tool prior to geochronology. Here detailed LA-ICP-MS methodology for capturing information for all of the trace element zoning, inclusions and the overall concentration of trace elements for garnet are presented.

Using detailed LA-ICP-MS spot mapping of garnet cross sections, full elemental maps can be generated at resolutions of <100 microns for concentrations as low as 1-10 parts-per-billion. Trace element mapping as opposed to limited spot or line analyses provides significantly more information and the ability to produce false color maps of elemental information. A comparison of multiple discrete spots vs. line scanning mapping is presented. While spot mapping is slower (sometimes >20 hours per map) than line scan mapping, spot mapping allows the recovery of useful information even in the presence of inclusions. For line scan mapping, if a high REE inclusion were encountered along the line the signal washout would blur over a considerable distance rendering that portion of the grain unrecoverable in terms of true REE concentration in the garnet. Whereas with spot mapping and detailed data mining for each analyses, useful garnet may still be recoverable from that spot. With subsequent TIMS analyses of the garnet, detailed comparisons between the LA-ICP-MS and TIMS data are obtained. Calibration, automation and data processing methods will be presented in this comprehensive overview of the use of LA-ICP-MS for garnet analyses.