Expanding the U-series disequilibrium dating to minerals beyond Zircon by LA-ICP-MS

In situ U-Th disequilibrium dating is applicable to Uranium bearing minerals crystallizing the past 300 ka and has long been the domain of ion microprobes. We introduced analytical protocols employing laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) to date igneous zircons in 2016. Using a high sensitivity sector field ICP-MS and applying corrections for interferences yields zircon U-Th isotope data comparable in precision to SIMS data.

When zircon is not available, other minerals may be used for U-Th geochronology. Thanks to the unmatched versatility of LA to vary the ablated amount by increased crater sizes and repetition rate, we showed that this technique is suitable for dating minerals with less than ~1 ppm U. A general approach to date new minerals is established and used successfully on garnet, ilmenite and vesuvianite. Details and challenges using the method, including an unexpected interference from contamination and some results from a quadrupole ICP-MS are discussed.

Examples of ilmenite ages are obtained on samples from the Aso-4 caldera-forming eruption and Nisyros. Magmatic garnets from eruptions at Somma-Vesuvius has been recently proven to give important insights into the lifetime and dynamics of phonolitic magma chambers. Additionally, U-Th disequilibrium dating of vesuvianite in the skarn lithic clasts from the same deposits provides additional information about the timing of interaction of the alkaline magmas from Vesuvius with the carbonate country rocks. The versatility of this technique opens a variety of new avenues for quantifying the timing of earth processes.