This is a proposal to investigate the stability, crystal chemistry and equation of state of hydrous spinel and spinelloid phases that could occur in the Transition Zone of the mantle (410-670km depth). In collaboration with colleagues at the Bavarian Geological Institute (Bayerisches Geoinstitut), the investigators will perform synthesis experiments to see if spinelloid V may exist in mantle compositions and to better refine the pressure-temperature-composition field of spinelloid IV (wadsleyite II). Preliminary experiments in hydrous systems have produced crystals of sufficient size for single-crystal diffraction methods, so run products will be analyzed by this method. They will use single-crystal X-ray diffraction measurements in the diamond anvil cell (DAC) to measure compressibilities (isothermal bulk moduli) and thermal expansion of the hydrous spinelloid observed in these experiments. Because the bulk modulus is directly related to seismic velocity, if the compressibilities of these phases differ greatly from the anhydrous modifications, it may be possible to place some constraint on the amount of OH actually present in the minerals of the Transition Zone. Samples will also be made available for collaborative studies by various spectroscopic and ultrasonic methods.
