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

Abstract

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 6.9 GPa at superliquidus temperatures. At pressures below 3 GPa, the first sharp diffraction peak ( FSDP), which reflects the silicate network ordering in the silicate melts that consist of … Si O Si … 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 3 GPa, which means that … Si O Si … linkages are partially disrupted by hydroxyl species (Si-OH units). In contrast, the peaks move to lower Q at pressures above 3 GPa 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 3 GPa 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 Mg 2SiO 4 melt, suggesting that the MgO component has an important influence on the polymerization of hydrous melt structure at 3–5 GPa. 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 100–180 km, which can enhance the decrease in a seismological wave velocity in the Earth's asthenosphere as proposed in previous seismological observations.