Identification of pumice derived from historic eruption in the same volcano: Case study for the Sakurajima-Bunmei tephra in the Miyazaki Plain

Refractive index measurements of volcanic glass shards with known hydration characteristics enable the discrimination of pumice samples from multiple historic eruption events of a single volcano. We analyzed mineral compositions, undertook a morphological classification of volcanic glass shards, and measured the refractive index of glass shards and orthopyroxene from products of the Bunmei, Anei, and Taisho eruptions of Sakurajima volcano. The refractive indices of hydrated and non-hydrated parts within each individual glass shard were measured separately. Concentrations of 58 elements in the glass shards were determined using a femtosecond laser ablation-inductively coupled plasma-mass spectrometry technique, to compare elemental concentrations of the sampled glass shards. The analyses successfully discriminated the Bunmei, An-ei, and Taisho eruption products. Pumice layers in core samples obtained from the southern part of the Miyazaki Plain were identified as the Sakurajima-Bunmei tephra, based on the analyses described above and radiocarbon age data for coaly materials in the pumice layers. The distribution of the pumice layer suggests that the Sakurajima-Bunmei tephra reached the southern part of the Miyazaki Plain as a pumice fall deposit.