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During recent years, a growing number of studies have focused on the solubility of water in nominally anhydrous minerals, as well as on the stability field of dense hydrous magnesium silicates. The results are of major importance since water can dramatically influence the physical properties of the Earth´s mantle such as melting, transport properties, and rheology. (Mg,Fe)SiO₃-perovskite, which constitutes up to 80% by volume of the lower mantle, is believed to be a potential canditate for water storage in the deep Earth. We conducted experiments on water partitionning in the system MgSiO₃ + 0.5wt% H₂O + excess silica at P,T conditions corresponding to the top of the lower mantle using a multi-anvil press. Water contents were calculated from FTIR absorption measurements on optically clear crystals using the calibration curve of Paterson. Below 24 GPa at 1600°C, the ilmenite polymorph of MgSiO₃ coexists with stishovite and a melt. The polarized spectra of ilmenite shows five pleochroic bands: three sharp and strong ones at 3390, 3320 and 3300 cm^-1 and two weak bands at 3260 and 3047 cm^-1 (Fig. 3.5-1). The calculated water content for ilmenite is 1750 ppm by weight. For stishovite coexisting with MgSiO₃-ilmenite, the measured absorbance yields a water content of 35 ppm (Fig. 3.5-2).
 

Fig. 3.5-1: Polarized FTIR spectra of MgSiO3 ilmenite synthesized at 22 GPa and 1600°C in the presence of a hydrous melt. Sample thickness is 50 µm.

Fig. 3.5-2: Unpolarized FTIR spectra of MgSiO3 ilmenite and stishovite coexisting in a sample synthesized at 22 GPa and 1600°C. Sample thickness is 50 µm.

Fig. 3.5-3: Unpolarized FTIR spectra of MgSiO3 ilmenite and MgSiO3 perovskite coexisting in a sample synthesized at 24 GPa and 1600°C. Sample thickness is 40 µm.

This yields a partition coefficient of water between MgSiO₃-ilmenite and stishovite of D^ilm/stish = 50. Large amounts of water were also found in the quenched melt; however, the absolute concentrations are difficult to quantify. With increasing pressure, perovskite becomes the stable polymorph of MgSiO₃. In a sample where ilmenite and perovskite coexisted, no water was detected in the perovskite while the ilmenite dissolved about 2000 ppm, indicating that D^ilm/per >>1 (Fig. 3.5-3).

This observation would imply that MgSiO₃ perovskite is not a major host for water in the lower mantle. However, it is conceivable that the solubility of water in this phase could greatly increase in the presence of chemical impurities. Accordingly, solubility experiments are currently being conducted in more complex systems containing aluminium, iron, chromium and other components.