In situ X-ray diffraction study on pressure-induced structural changes in hydrous forsterite and enstatite melts

We investigated the pressure-induced structural changes in hydrous forsterite and enstatite melts using in situ synchrotron X-ray diffraction. Diffraction data was collected up to 6.9GPa at superliquidus temperatures. At pressures below 3GPa, the first sharp diffraction peak (FSDP), which reflects the silicate network ordering in the silicate melts that consist of ...SiOSi... linkages, is shifted notably toward higher Q (scattering vector [Å^-1]) in both melt compositions. This observation indicates that water has a depolymerizing effect on the silicate network below 3GPa, which means that ...SiOSi... linkages are partially disrupted by hydroxyl species (Si-OH units). In contrast, the peaks move to lower Q at pressures above 3GPa in spite of the compression, which indicates lengthening of the silicate network ordering (i.e., polymerization of silicate network). This observation indicates that water changes to have a polymerizing effect on the silicate network above 3GPa by a new free hydroxyl group such as Mg-OH, which was previously proposed in the study on hydrous silicate glasses structure. In fact, the structural changes in the present study are more pronounced in the hydrous Mg₂SiO₄ melt, suggesting that the MgO component has an important influence on the polymerization of hydrous melt structure at 3-5GPa. The present structural change, re-polymerization at high pressure, in hydrous silicate melt can influence the viscosity. Such a relatively high-viscosity hydrous magma may be able to stay (or be decreased in the rising velocity) at a depth of 100-180km, which can enhance the decrease in a seismological wave velocity in the Earth's asthenosphere as proposed in previous seismological observations.