[S-MP29] Physics and Chemistry of Minerals

[SMP29-10] Stractural investigation on viscosity difference between albite and anorthite melts

*Nozomi Kondo¹, Yoshio Kono², Tatsuya Sakamaki³, Koji Ohara⁴, Fumiya Noritake⁵ (1.Institute for Planetary Materials, Okayama University, 2.Kwansei Gakuin University, 3.Tohoku University, 4.Shimane University, 5.University of Yamanashi)

Keywords:plagioclase, melt・glass, viscosity, melt structure

Plagioclase is a basic component of volcanic magma and albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈) are the end members. NaAlSi₃O₈ and CaAl₂Si₂O₈ melts have the same NBO/T (Number of non-bridging oxygen/tetrahedral atom, T is Si and Al) value, 0, though they have largely different viscosity (e.g., Li et al., 2021, Chem. Geol.). In previous studies, NBO/T has been considered as an important parameter which dominates viscosity of silicate melts (e.g., Mysen, 1983, AREPS). However, we cannot interpret the viscosity difference of NaAlSi₃O₈ and CaAl₂Si₂O₈ melts with the conventional understanding. Therefore, investigating the mechanism which causes the viscosity difference of NaAlSi₃O₈ and CaAl₂Si₂O₈ melts is essential to understand mobility of magma in the Earth’s interior and process of volcanic eruption.
In this study, we conducted viscosity measurements of CaAl₂Si₂O₈ melt, for which viscosity data are limited, from air pressure to around 4 GPa, and carried out pair distribution function analyses to investigate melt structures of NaAlSi₃O₈ and CaAl₂Si₂O₈ melts from air pressure to around 7 GPa, at 16-BM-B beamline of Advanced Photon Source. As a result, the CaAl₂Si₂O₈ melt has around two orders of magnitude lower viscosity than NaAlSi₃O₈ melt at air pressure, and the viscosity of CaAl₂Si₂O₈ melt does not change with increasing pressure up to around 4 GPa. For the melt structure, the FSDP (First Sharp Diffraction Peak) of S(Q) is clearly different between the NaAlSi₃O₈ and CaAl₂Si₂O₈ melts. In order to interpret the viscosity and structure differences between the NaAlSi₃O₈ and CaAl₂Si₂O₈ melts, we conducted high precision pair distribution function analyses of plagioclase glasses with different content of anorthite component. From structural data of the plagioclase glasses, we carried out Molecular Dynamics simulation and Reverse Monte-Carlo calculation, and constructed structural model of plagioclase melts. In this presentation, we discuss the structural occurrence of viscosity difference between NaAlSi₃O₈ and CaAl₂Si₂O₈ melts and mechanism of viscosity changes in NaAlSi₃O₈ and CaAl₂Si₂O₈ melts with increasing pressure.