Skip to content

Assigning an eruption age: The challenges of dispersed single crystal Ar-Ar ages and insights from geochemistry

^^Hayden Dalton^1^, Saini Samim1, Ashley Savelkouls1, David Phillips1

Abstract

The increasing precision of modern noble gas mass spectrometers has revealed significant dispersion in datasets of single crystal 40Ar/39Ar eruption ages, ranging from tens to hundreds of thousands of years. This widespread observation suggests that the dispersion is unlikely to be an analytical artefact but instead results, at least in part, from geological or volcanological processes. The geochronology and volcanology community faces two main challenges in light of these observations: 1) interpreting such dispersed datasets to assign a meaningful 'eruption' age, and 2) understanding the magma storage and/or eruption mechanisms that allow such a wide range of single-crystal ages to be preserved. We present data from four silicic volcanic eruptions in the Turkana Basin, within the Kenyan branch of the East African Rift System: the Silbo (~750 ka), Nariokotome (~1.26 Ma), and Koobi Fora (~1.5 Ma) tuffs from the Plio-Pleistocene, and the Miocene Buluk Tuff (~16 Ma). In these cases, single sanidine crystal 40Ar/39Ar apparent ages span 80,000 to 300,000 years for a single eruption. Notably, this age variance persists both within and between individual pumice clasts of a given eruptive sequence. In this study, we combine major and trace element geochemical analyses with radiogenic isotope (Sr-Nd-Hf-Pb) geochemistry to investigate the possible magmatic controls on the observed age variations. Additionally, these are the first radiogenic isotope geochemistry data for the extensive Plio-Pleistocene silicic volcanic deposits in the Turkana Basin and reveal important genetic affinities to other volcanic activity in the rift system.