Argon solubility in SiO2 melt under high pressures: A new experimental result using laser-heated diamond anvil cell

We have measured the solubility of argon in SiO2 melt under high pressure by melting a SiO2 crystal at the argon pressure as high as 19 GPa using laser-heated diamond anvil cell (LHDAC). SiO2 was successfully melted at the pressure of about 12 GPa and recovered with a completely glassy phase. Above 15 GPa, the sample contained a large amount of crystalline SiO2, although it was completely melted and spherical due to the surface tension. This may arise from the change in the coordination number of Si in SiO2 melt at around 12–15 GPa, which is close to the triple point of coesite, stishovite, and liquid SiO2. Argon content in the recovered SiO2 was measured by energy-dispersive spectroscopy and mass spectrometry. It increased at pressures up to approximately 5–7 GPa and exhibited a constant value up to at least 12 GPa; this implies that, at these conditions, the noble gas behaves as an incompatible element. The result is completely different from those of previous LHDAC studies, which claimed that the increased solubility then dramatically drops above the pressure of about 5 GPa. Here, we report the new details of experimentally determined pressure dependence of noble gas solubility by LHDAC, and discuss the experimental difficulties and the structure change of silicate melt under high pressures.