  The Soret effect, temperature gradient driven diffusion, in silicate melts
has been investigated intensively in the earth sciences from the 1980s. The
SiO2 component is generally concentrated in the hotter region of silicate melts
under a temperature gradient. Here, we report that at ultra-high temperatures
above approximately 3000 K, SiO2 becomes concentrated in the colder region of
the silicate melts under a temperature gradient. The interior of an
aluminosilicate glass (63.3SiO2-16.3Al2O3-20.4CaO(mol%)) was irradiated with a
250 kHz femtosecond laser pulse for local heating. SiO2 migrated to the colder
region during irradiation with an 800 pulse (3.2 ms irradiation). The
temperature analysis indicated that migration to the colder region occurred
above 3060 K. In the non-equilibrium molecular dynamics (NEMD) simulation, SiO2
migrated to the colder region under a temperature gradient, which had an
average temperature of 4000 K; this result supports the experimental result. On
the other hand, SiO2 exhibited a tendency to migrate to the hotter region at
around 2400 K in both the NEMD and experimental study. The second-order like
phase transition was observed at ~2000-3400 K when calculated using MD without
a temperature gradient. Therefore, the second-order phase transition could be
related to the migration of SiO2 to colder region. However, the detailed
mechanism has not been elucidated.
