Water in Al-free Stishovite up to 65 GPa and 2000 K

Abstract:

Stishovite, found in some parts of subducted oceanic slabs and crustal fragments in the Earth's mantle, may contain small amounts of hydroxyl in conjunction with Al incorporation. Spektor et al. (2011, PNAS) showed that much larger amounts of water (1.3 wt%) can be incorporated into pure Al-free stishovite at 10 GPa and 723 K. However, solubility variations with pressure and temperature have not been measured in detail, and the effect of water on the physical properties of stishovite at mantle-related pressures and temperatures is still unknown.

We have synthesized different hydrous stishovite samples in the multi anvil press: two hydrous samples at 723 K and 10 GPa for different heating time and another hydrous sample at 723 K and 20 GPa. We also synthesized anhydrous stishovite at 1473 K and 10 GPa. X-ray diffraction (XRD) shows larger unit cell volume for all hydrous phases comparing to the anhydrous stishovite.

XRD patterns show that the samples synthesized at 10 GPa both have a tetragonal rutile type structure whereas the sample synthesized at 20 GPa shows peak splittings indicating a lower symmetry. It deviates from the rutile structure and it is closer to the CaCl₂-type structure but has additional splitting. From the unit cell parameters for those samples combined with the calibration by Specktor et al. the water content is determined: 1.6-4.9 wt% water, which increases with pressure and heating duration. The existence of water in stishovite was also confirmed by Raman, Infrared spectroscopy and Secondary Ion Mass Spectrometry.

We also synthesized hydrous stishovite in the diamond anvil cell at 26-60 GPa and 1300-2000 K. SiO₂ glass was loaded in an H₂O pressure medium and the sample was laser heated at high pressures and different temperatures. Synchrotron XRD patterns show that those phases are tetragonal stishovite. From unit-cell parameters, at 26-31 GPa, the water content of the samples synthesized at 1300 K is lower than the multi anvil sample. However, the samples synthesized at >50 GPa and 1300-2000 K have larger unit-cell volume suggesting that the pressure plays an important role in stabilizing OH in dense silica. The fact that hydrous stishovite is still stable and contains more water even at mantle geotherm temperatures suggests silica may transport a significant amount of water to the lower mantle.