Weathering Effects on Luminescence Dating—An Example of the Toya Tephra in Japan—

Abstract

The dating of volcanic quartz from the same volcanic source, namely the Toya pyroclastic flow deposits and the ash-fall deposits, was conducted using the red thermoluminescence method (RTL). The results yielded the anticipated ages for the pyroclastic flow deposit (99–103 ka) and considerably older ages for the ash-fall deposits (51%–83% older, 150–188 ka). This discordance is attributed to changes in the annual dose rate due to elemental migration resulting from the weathering of the ash-fall deposit. X-ray diffraction (XRD) and solid ²⁹Si nuclear magnetic resonance (NMR) analysis of the ash-fall deposits indicate that allophane/imogolite was newly generated. Furthermore, solid ²⁷Al NMR measurements indicate that the fresh glass with tetrahedral Al has undergone a transformation to the octahedral Al of allophane/imogolite. A comparison of the Ti-normalized values of elements between the pyroclastic flow deposit and the ash-fall deposit, conducted using LA-ICP-MS measurements, revealed a significant reduction in alkali and rare earth elements (REEs) and an enlargement in aluminum in the ash-fall deposit. However, the Ti-normalized values of uranium (U) and thorium (Th) showed different migration trends depending on the sample. The following weathering factors are correlated with elemental migration: (1) The release of positive ions by the weathering of volcanic glass, (2) The adsorption and desorption of ions on the surface functional groups of clay (allophane/imogolite) and iron hydroxide, (3) The high hydrophilicity of the allophane/imogolite, and (4) Non-equilibration of the U and Th decay series due to Rn release. The annual dose rate of the ash-fall deposit has been subject to fluctuations as a consequence of the weathering process. Consequently, the adoption of the present annual dose rate for the dating may result in an unexpected age. It is therefore crucial to select sediments that can ensure a closed system of element transfer.