Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICPMS), a Microanalytical Tool for Heavy Minerals Characterization: Principles and Applications

摘要

Chemical characterization of detrital minerals is an important tool to determine provenance and estimate maximum depositional age of clastic sequences. The fast evolution of such technique in the last decade is driven by the availability of laser ablation probes, coupled with plasma mass spectrometers which are fast and provide accurate results without significant loss in accuracy, and are moderately inexpensive. Although several heavy minerals can be studied for their chemical composition, zircon is best suited because of its robustness to chemical and mechanical abrasion. A combination of geochemical studies can be performed for the characterization of zircon in detrital rocks: U-Pb geochronology, elemental composition, and isotope fingerprinting. For UPb zircon dating by LA-ICPMS many laboratories routinely perform 1.5 to 2 minutes long analyses, with ablation spots ranging between 30-40 μm in diameter, a depth of 20-25 μm, for a zircon ablated mass between ~70-150 ng. These spots can generally resolve different growth episodes in the zircon, but can be reduced even further to a range of 10-15 μm, if needed. Ages can then be matched with the elemental composition of the analyzed domains, to model the pedogenesis of their rock sources. The rare-earth elements pattern yields important information related to enrichment, or depletion of a zircon domains during igneous crystallization or high-grade metamorphic overprint. Further information can be extracted by the implementation of other isotopic systems, such as Hf, which can be used to trace the genesis of the parental magma.