HR: 10:50h
AN: T32B-03 INVITED     [Abstracts]
TI: High-Pressure Raman Spectroscopic Studies of Hydrated Transition Zone Spinelloid and Spinel Phases
AU: * Kleppe, A K
EM: annettek@earth.ox.ac.uk
AF: Oxford University, Earth Sciences, Parks Road, Oxford, OX1 3PR United Kingdom
AU: Jephcoat, A P
EM: andrew@earth.ox.ac.uk
AF: Oxford University, Earth Sciences, Parks Road, Oxford, OX1 3PR United Kingdom
AU: Jephcoat, A P
EM: andrew@earth.ox.ac.uk
AF: Diamond Light Source, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX United Kingdom
AU: Smyth, J R
EM: joseph.smyth@colorado.edu
AF: University of Colorado, Department of Geological Sciences, Boulder, CO 80309 United States
AB: Wadsleyite (\beta-(Mg,Fe)$_{2}$SiO$_{4}$, spinelloid III) and ringwoodite (\gamma-(Mg,Fe) $_{2}$SiO$_{4}$,spinel) are thought to be the most abundant minerals in the Earth's mantle transition zone between 410 and 660 km depth. Though nominally anhydrous their crystal structures can accommodate significant amounts of OH: wadsleyite up to 3.3 wt% H$_{2}$O and ringwoodite up to 2.3 wt%. Wadsleyite II (spinelloid IV) is a hydrous, Fe-bearing silicate phase that might occur between the stability regions of wadsleyite and ringwoodite. We present Raman spectra from 50 to 4000 cm$^{-1}$ of high-quality single-crystals of hydrous Mg end-member and hydrous Fe-bearing wadsleyite, wadsleyite II, and ringwoodite up to 60 GPa. The experiments were performed with a diamond-anvil cell and rare-gas pressure-transmitting media. The Raman spectra of wadsleyite and wadsleyite II exhibit subtle differences. In the OH stretching region the spectrum of wadsleyite II appears more complex than the spectrum of wadsleyite consisting of at least 6 modes. The high-pressure behaviour of hydrous iron-bearing ringwoodite is unique: near 40 GPa additional Raman bands emerge in the region 550-580 cm$^{-1}$. Comparisons of Raman spectra of hydrated transition zone phases with varying iron and hydrogen content reveal that pure protonation has only a minor effect on the lattice dynamics whereas coupled iron and proton substitution can lead to additional levels of transformation complexity.
DE: 3620 Crystal chemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3924 High-pressure behavior
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting