Water incorporation in olivine at upper mantle pressures and water-undersaturated conditions

Experiments on water incorporation in olivine are exclusively conducted under water-saturated conditions. Water saturation is ensured by a buffer assembly. Geochemical observations show however that MORB-source upper mantle is far from water saturation. We performed experiments to investigate water incorporation in olivine at water-undersaturated conditions and pressures up to 8 GPa. The samples consisted of Ti-doped olivine encapsulated in Pt or Ni in both hydrous and anhydrous piston cylinder as well as anhydrous multi-anvil assemblies. No water was added, but the powders were not dried prior to encapsulation. The Pt encapsulated samples are close to, or are water-saturated at higher pressure, as indicated by connected porosity at three-grain edge intersections. The spectra become increasingly complex with increasing pressure, but among the absorption bands are always those due to the titanium-clinohumite-type point defect. At the relatively oxydizing environment in Pt capsules, hydroxyl associated with trivalent defects (presumably related to Fe3+) becomes more abundant with increasing pressure. The Ni encapsulated samples contain significantly less hydroxyl, the spectra are dominated by the hydroxylated Ti-defect. The experiments therefore broadly confirm that at water-undersaturated conditions typical for the upper mantle the Ti-defect is also the first one to be hydrated at higher pressure. Quantification of the water content is made difficult by the complexity of the patterns. Work to deconvolve the spectra into possible end members is currently in progress.