In Situ Th-230/U-238 Geochronology of Young Volcanic Rocks on Inclusion-Bearing Ilmenite

Abstract

Precise age determinations of volcanic deposits from the Late Pleistocene and Holocene are fundamental for hazard assessment. Retrieving accurate radiogenic ages typically relies on analysing specific minerals (such as zircon or sanidine), which are in many cases lacking in the deposits. We present a new Th-230-U-238 disequilibrium dating method by LA-ICP-MS for inclusion-bearing ilmenite, and discuss its details and challenges. The technique is tested on ilmenites from the 86.4 +/- 1.1 ka Aso-4 super-eruption, Japan, and from the Lower Pumice (63.1 +/- 4.7 ka) and Upper Pumice (58.4 +/- 2.7 ka) eruptions of Nisyros volcano, Greece. Ilmenites of the Aso-4 eruption produce well-defined global isochrons yielding ilmenite crystallisation ages of 89 +/- 10 ka (2s) and 81 +/- 15 ka (2s). Two-point isochron model ages from the Nisyros eruptions produce youngest population ages of 66.3 +/- 5.7 ka (1s) for the Lower Pumice and 61.2 +/- 4.4 ka (1s) for the Upper Pumice. The age determinations obtained during this study are compatible with data reported from conventional techniques. These results demonstrate the applicability of this new method to date young volcanic deposits in a quick, reproducible way, and without relying on zircon or K-bearing phases.