TESTING THE POTENTIAL OF THERMOLUMINESCENCE DATING OF VOLCANIC GLASS WITH KNOWN-AGE SAMPLES

In luminescence dating, tephra is commonly dated indirectly by bracketing ages from sediment layers above and below the tephra deposit. A successful volcanic glass thermoluminescence (TL) dating approach would enable direct dating of tephras and allow for age determination of the eruption event producing it. The use of the fine-grained glass constituent in tephra would allow for both distal and proximal ash deposits to be dated, providing an excellent opportunity for tephrostratigraphic correlation over large distances. Moreover, unlike phenocrystic quartz, the glass component is ubiquitous throughout tephra deposits. Early attempts to date volcanic glass using luminescence had varying degrees of success but new technology and advances in technique development provides an opportunity to revisit the applicability of luminescence dating to volcanic glass.

Tephra samples were collected from pluvial shoreline deposits in the Great Basin physiographic province in the western United States. Samples from two rhyolitic tephra deposits with independently known age were collected: the Wono tephra (~29 ka) and the Turupah Flat tephra (~0.8 ka). The 4-11 µm volcanic glass fraction was separated from the bulk tephra for analysis. Results show that Blue TL glow curves have peaks centered near 250 °C with signals that respond systematically and reproducibly with a given beta dose. Results suggest a 25-30% reduction in the TL signal over a seven day storage interval at room temperature. Results on signal sensitivity to light exposure and long-term storage tests on the volcanic glass are also presented. TL ages are calculated using MAAD-TL and SAR-TL and compared with independent age control to assess their suitability.