Decoding pre-eruptive process using crystal clots: Unzen 1792 and 1991-95 eruption

Monday, 14 December 2020: 15:15
Virtual
Kurumi Iwahashi, The University of Tokyo, Tokyo, United States, Atsushi Yasuda, University of Tokyo, Bunkyo-ku, Japan and Hidemi Ishibashi, Shizuoka University, Shizuoka, Japan
Recorded Presentation
Abstract:
Magma mixing induced by magma plumping to mushy magma reservoir is one of the major causes of volcanic eruption in differentiated arc volcanoes. However, it is unknown that how mushy magma reservoir having high viscosity involved in the mixing and how it affects the eruption styles. So, revealing pre-eruptive processes at mushy magma reservoir is necessary for understanding mechanism and cause of volcanic eruption at arc volcanoes.

On this purpose, we focus on crystal clots. Crystal clots are assembladges of multiple minerals. According to the occurrence of crystal clots, we judge that most of them are part of the mushy magma reservoir. So, they are likely to hold melts of pre-mixing magma reservoir.

We use crystal clots in products of Unzen 1991-95 and 1792 eruptions. Previous studies revealed that both eruptions showed different eruption styles although they had experienced magma mixing at stagnant mushy magma reservoir just before the eruptions.

We used 13 samples erupted at different period of the 1991–1995 eruption in this study. For 1792 samples, we used 6 samples of lava flows. Crystal clots in produsts of these eruptions are mainly composed of plagioclases, amphiboles, pyroxenes and Fe-Ti oxides. Between crystals, there exist interstitial melts. We analyzed the interstitial melts, amphiboles, and plagioclases in crystal clots. For comparison, we analyzed some phenocrysts of Fe-Ti oxides and amphiboles.

As a result, we constrain that crystal clots are useful tools to see the process at mushy magma reservoir. For example, from crystal clots, we constrain the temperatures and melt compositions of each end member magma and reveal different heating effect in mushy magma reservoir. Some interstitial melts clots show the heterogeneity of mushy magma reservoir.

In this AGU innovative session, I will propose the implication for pre eruptive process at mushy magma reservoir in 1991-95 and 1792 eruption and cause of eruptive process.

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