斑晶鉱物の元素拡散モデリングに基づく噴火準備タイムスケールの研究の現状

抄録

Diffusion timescale estimated from a chemical zoning profile in a phenocryst mineral indicates the time between formation of chemical heterogeneity in the crystal and eruption. Therefore, diffusion timescales offer important constraints for understanding pre-eruptive magmatic processes. In the last two decades, many studies have reported data of diffusion timescales estimated for phenocrysts of various mineral species in both pyroclasts and lavas from worldwide. In this study, we compiled published data of diffusion timescales with information of eruption characteristics such as eruption style, eruptive volume, and chemical composition of magma, and examine relationship between diffusion timescales and eruption characteristics. Our results suggest that (1) the estimated maximum and minimum diffusion timescales depend on species of mineral and element used for diffusion modeling, (2) estimation error of diffusion time is at least larger than ~±0.4 log unit, which should be considered into account when diffusion time is compared with geophysical observations, (3) each phenocryst records information about its own growth history, not a history of whole magma chamber, as chemical zoning, (4) diffusion timescales estimated for phenocrysts in pyroclasts of caldera-forming eruptions are systematically longer than those of non-caldera-forming eruptions, although number of data is small for non-caldera-forming rhyolitic eruption; this discrepancy may be attributed to differences of magma chamber volumes and/or mechanism of eruption triggering between caldera-forming and non-caldera-forming eruptions.