HR: 11:05h
AN: T32B-04    [Abstracts]
TI: Hydration of Olivine at 12 GPa
AU: * Smyth, J R
EM: smyth@colorado.edu
AF: University of Colorado, Department of Geological Sciences, Boulder, CO 80309 United States
AU: * Smyth, J R
EM: smyth@colorado.edu
AF: Bayerisches Geoinstitut, Universitaet Bayreuth Universitaetsstrasse 30, Bayreuth, 95440 Germany
AU: Frost, D J
EM: dan.frost@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth Universitaetsstrasse 30, Bayreuth, 95440 Germany
AU: Nestola, F
EM: fabrizio.nestola@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth Universitaetsstrasse 30, Bayreuth, 95440 Germany
AB: Although natural olivine of upper mantle origin generally contains less than 300 ppm H$_{2}$O by weight, olivine synthesized at temperature and pressure conditions near the 410 km discontinuity can contain up to 20 times this amount of H, enough to affect elastic properties, crystal structures, and unit cell volumes. Hydrous olivine has been synthesized with Fo95 composition at 12GPa and $1250\deg$ C in equilibrium with clinohumite in magnesia excess conditions and with clinoenstatite in silica excess conditions. Crystal structure refinements indicate that the hydration mechanisms are different in the two different conditions. The silica-excess sample appears to contain M2 cation vacancies, whereas the magnesia-excess sample shows tetrahedral site vacancies up to two percent, consistent with 5000 ppm by weight H$_{2}$O. Further, the partially vacant coordination polyhedra are significantly larger than their fully occupied counterparts. Cell parameter refinements indicate expansion of the unit cell by up to 0.3 percent with hydration in both samples, with tetrahedral vacancies causing an increase in both {\it b} and {\it c} axes and contraction of {\it a} relative to the crystal showing octahedral vacancies. If the excess volume were eliminated on compression to 12 GPa the resulting bulk modulus would be decreased by about 10 percent relative to anhydrous olivine. Static compression measurements are in progress. These results indicate that olivine can be a major host for H at depths approaching the Transition Zone in the upper mantle.
UR: http://ruby.colorado.edu/~smyth/Research/Talks/Talks.html
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3904 Defects
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 3954 X ray, neutron, and electron spectroscopy and diffraction
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting