A new method for analysing thin (>2um) zircon rims by LA-ICP-MS [abstract only]

Low temperature crustal melt granites (e.g. High Himalayan leucogranites) are often saturated in zirconium leading to the widespread prevalence of zircon crystals with inherited cores surrounded by very thin (often <10μm) and irregular magmatic rims. In these zircons only the igneous rims record the true crystallisation age of the leucogranite. Current in situ SIMS and ICP-MS zircon U-Pb analytical techniques generally employ static ‘spot’ analysis, such that it is difficult to position a spot so that only rim material is analysed. This in turn makes accurate and precise age determination of thin (<20μm) rim domains particularly difficult. In order to overcome this issue, we have developed a novel yet relatively simple analytical strategy that involves mounting whole crystals (with thin rims intact) on doublesided sticky tape and analysing them by LA-MC-ICP-MS. This presents the laser with a flat outer surface of the grain that thin rims can be ablated using dynamic ‘line’ (See Fig. 1) or ‘box’ rasters. With this method external growth zones as thin as 2μm can be confidently analysed without sampling deeper core material. Application of this technique to ‘standard’ zircons shows little, if any, degradation in overall data quality compared to conventional static spot analysis. This method is being further modified in an attempt to produce both trace element and U-Th-Pb isotopic depth profiles in zircon and other accessory phases. By matching the laser pulse rate to the isotopic integration rate and taking advantage of a short (<0.5s) transfer time of material from ablation cell to mass spectrometer (via a low volume ablation cell) a rapid depth profile can be generated similar to that currently generated by SIMS methods.