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3.4 c. Hydrous silicate spinelloids and the hydration state of the mantle (J. Smyth and D. J. Frost)

The hydrogen content of the Earth is one of the most poorly constrained compositional variables for the planet. Nominally anhydrous olivine and spinelloid phases thought to compose the bulk of the upper mantle and transition zone may contain many times the amount of H that resides in the hydrosphere. Exploratory synthesis experiments were carried out in order to better define the stability fields of the various hydrous spinelloid phases and to better constrain the effects of H incorporation into these phases on their physical properties. Specific objectives were to explore the olivine-wadsleyite and wadsleyite-ringwoodite transitions and synthesize single crystals of sufficient size for physical property measurements. Two different compositions were used: the first was close to that of a natural Fo90 hydrous olivine and the second, richer in trace elements and silica, approximated a natural peridotite.  Hydrogen is ten to twenty times more soluble in wadsleyite than in olivine, so it is possible that the olivine to wadsleyite transition may be displaced to lower pressures by the presence of water.

Three synthesis experiments were conducted on the hydrous Fo90 olivine composition at 12 GPa and 1100°C, 1300°C and 1700°C with the objective of producing hydrous olivine crystals for further study. Crystals were analyzed by single-crystal X-ray diffraction. The results of these experiments are shown in Figure 3.4-3 along with the phase relations of dry peridotite. The experiment at 1100ºC produced coexisting crystals of olivine and wadsleyite, whereas the two  experiments at higher  temperature produced olivine only. Although further study will be required to quantify the effect, these results suggest that the olivine to wadsleyite transition may be displaced to lower pressures by the presence of water. All experiments produced single crystals of sufficient size for X-ray diffraction study.

Three synthesis experiments were also conducted on the hydrous peridotite composition at 1400°C and at three different pressures of 15, 17.5 and 20 GPa. The objectives were to produce crystals for elastic property measurements and to see if other spinelloids phases might occur in this region. The experiment at 20 GPa produced single crystals of hydrous ringwoodite up to 500 µm in size, whereas the two runs at lower pressure produced crystals of wadsleyite of similar size. The wadsleyite II phase, which is a super-structure of wadsleyite similar to spinelloid IV observed in anhydrous nickel-aluminosilicate systems, was observed between the stability fields of hydrous wadsleyite and ringwoodite. Characterization of all of the run products by single-crystal X-ray diffraction is in progress.

Figure 3.4-3: Results from hydrous peridotite and hydrous olivine (Fo90) composition experiments are plotted on the dry peridotite phase diagram. Filled circles are olivine, grey squares are wadsleyite, the open square is wadsleyite II, and the open circle is ringwoodite. Although preliminary, it would appear that hydrous wadsleyite may be stable at lower pressures than dry wadsleyite and that the wadsleyite II spinelloid may have a stability field between hydrous wadsleyite and ringwoodite.

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