[S-CG49] Rheology, fracture and friction in Earth and planetary sciences

[SCG49-10] Small-angle X-ray scattering and wide-angle X-ray diffraction of magma under uniaxial tension

*Satoshi Okumura¹, Kentaro Uesugi², Akio Goto³, Tatsuya Sakamaki¹, Kazuhisa Matsumoto¹, Akihisa Takeuchi², Akira Miyake⁴ (1.Department of Earth Science, Graduate School of Science, Tohoku University, 2.SPring-8/JASRI, 3.Center for Northeast Asian Studies, Tohoku University, 4.Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University)

Keywords:magma, deformation, Small-angle X-ray scattering, X-ray diffraction, uniaxial tension test

Magma deformation and brittle failure control the dynamics of volcanic eruptions. However, the evolution of atomic to nanometer scale microstructure in magma under deformation has not been clarified, partially because of the difficulty of observing microstructure under deformation. Here, we report a new experimental system, developed at SPring-8 (BL20XU and BL47XU), designed to observe the microstructural evolution of magma under uniaxial tension. In this experimental system, we can obtain small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) of fibrous magma simultaneously that was heated and elongated in a furnace. In this presentation, we show two results obtained using the experimental system mentioned above, i.e., the evolution of the intermediate-order structure of dacite melt based on the observation of the first sharp diffraction peak (FSDP) and the effect of nanometer scale crystals on the viscosity of andesite magma. In the first result, we elongated haplodacite melt at a temperature of ~840°C and obtained WAXD during the elongation with a rate of ∼10 μm s^–1. The FSDP moved to the low Q range with deformation. This implies that the deformation resulted in an increase in the size of the ring structure in silicate network during tension. In the second result, the fiber of andesite magma was elongated at temperatures of ∼730–830°C. During heating and elongation, nanometer scale crystals formed, and their size, degree, and phase (magnetite) were monitored using SAXS and WAXD simultaneously. Based on the measured mechanical data, we investigated the effect of crystallization of the nanometer scale crystals on magma rheology and concluded that the formation of nanometer scale crystals causes only a slight increase in magma viscosity (less than one order of magnitude of melt viscosity). Additional information of atomic to nanometer scale microstructure of magma, such as the detailed structure and density fluctuation in silicate melts and the formation kinetics of nanometer scale crystals and bubbles, will be explored in future studies.