HR: 0800h
AN: DI41A-1256    [Abstracts]
TI: The Formation of Calcium Perovskite From Majorite Garnet: Implications for Splitting of the 520 km Seismic Discontinuity.
AU: * Saikia, A
EM: ashima.saikia@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, D-95447 Germany
AU: Frost, D J
EM: dan.frost@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, D-95447 Germany
AU: Rubie, D C
EM: Dave.Rubie@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universitaet Bayreuth, Bayreuth, D-95447 Germany
AB: A weak seismic discontinuity in the transition zone region of the Earth's mantle at approximately 520 km depth has been recognized in a number of studies. Recently it has been reported that the 520 appears to split into two discontinuities in some regions of the mantle. The "520" is generally attributed to the wadsleyite to ringwoodite transformation, however, a plausible explanation for a split discontinuity is that the formation of calcium perovskite from majorite garnet, that occurs at a similar depth, also reflects seismic waves. We have studied the CaSiO₃ perovskite formation reaction from majorite garnet at mid- transition zone to lower mantle conditions The formation of CaSiO₃ perovskite is dependent on the CaSiO₃ saturation limit of garnet. The solubility of CaSiO₃ in majorite garnet has been studied as a function of composition between 17 and 24 GPa and 1200-1600^oC using a multianvil apparatus. Ca free garnet glasses mixed with CaSiO₃ wollastonite are used as starting compositions. We have performed reversal experiments starting with CaO-rich presynthesized majorite and we have used B₂O₃ as a flux in some experiments. Pressure is determined in each experiment from the divariant phase relations of a (Mg,Fe)₂SiO₄ calibrant. We observed that the solubility of CaSiO₃ in majorite garnet increases at a given pressure and temperature with decreasing majorite component. Results of this study show that for a peridotite mantle composition with ~ 3.5 wt % CaO the exsolution of CaSiO₃ perovskite from majorite occurs over a 1 GPa pressure interval, which corresponds to a depth of about 30 km. This is a similar depth interval to the wadsleyite-ringwoodite transformation. As opposed to the 410 and 660 km discontinuities, the CaSiO₃-forming reaction is potentially a probe for the fertility of the mantle whereby refractory regions that have been involved in extensive near-surface melt extraction, and are therefore CaO depleted, should exhibit little or no splitting of the 520.
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3621 Mantle processes (1038)
DE: 3625 Petrography, microstructures, and textures
DE: 3924 High-pressure behavior
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005