Induced thermoluminescence as a method for dating recent volcanism: The Blue Dragon flow, Idaho, USA and the factors affecting induced thermoluminescence

Abstract

In order to continue our investigation into the possible utility of induced thermoluminescence as a chronometer for volcanic activity we have measured the induced thermoluminescence properties of twenty-eight samples collected from the almost 3 km long Blue Dragon flow at Craters of the Moon National Monument and Preserve, Idaho. The properties of the samples are consistent with previous conclusions that the major mineral causing the luminescence is feldspar and that variation in the glow curves (light emitted as a function of heating temperature) is caused by variations in the thermal history of the samples. However, in nine of the 28 samples a there is a significant contribution to the TL signal from apatite. The induced thermoluminescence level along the flow is essentially constant, showing at most a factor of four decrease from beginning to the end (from ∼16 to ∼4, on a scale of the Dhajala meteorite = 1000), and this is not statistically significant. Thus sampling distance from the vent should not be a problem for the proposed TL dating method. However, there are details that might pose a problem for dating. (1) There is considerable (factor of 10 or so) scatter in the TL data due to the high heterogeneity of the samples. (2) There are abundant highly luminescent spherules which could explain some of the scatter; however, they are restricted to samples taken very close to the vent and could be screened from TL samples using cathodoluminescence, a microscopic means of identifying components producing the TL. (3) There is a factor of three increase in crystallization along the core (from ∼20 vol% to ∼60 vol%) but no factor of three increase in induced TL value. This is additionally problematic because crystallization is accompanied by a small decrease in anorthite content of the feldspar which should further increase induced TL values. While empirical data suggest that there is a weak correlation between induced TL and age for volcanics from Idaho and Hawaii, our detailed study of the Blue Dragon flow removes one complication for the dating method but reveals many details that need to be kept in mind as this possible dating method is explored. Most importantly, we need a method for reducing the scatter in induced TL displayed by samples from a single volcanic eruption.