HR: 08:36h
AN: DI51B-04    [Abstracts]
TI: Calcium perovskite exsolution from majorite garnet and splitting of the 520 km seismic discontinuity: insights into mantle heterogeneity.
AU: Frost, D J
EM: Dan.Frost@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, D-95440, Germany
AU: * Saikia, A
EM: ashima.saikia@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, D-95440, Germany
AU: Rubie, D C
EM: Dave.Rubie@uni-bayreuth.de
AF: Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, D-95440, Germany
AB: Seismic observations have identified a weak seismic discontinuity in the mantle at approximately 520 km depth which has been attributed to the isochemical phase transformation of (Mg,Fe)₂SiO₄ wadsleyite to the higher pressure polymorph ringwoodite. Recent seismic observations have shown that in some regions of the mantle two separate discontinuities can be observed within a similar mantle depth range of 500 km to 560 km. A possible cause of the second discontinuity is the exsolution of calcium perovskite from majorite garnet at mid transition zone conditions. An important question to address, therefore, is why this doubling of the discontinuity is only observed in certain regions of the mantle. We performed multianvil experiments to investigate the depth interval of the calcium perovskite forming reaction as a function of temperature and garnet majorite content. Results show the exsolution to be a non-linear function of pressure resulting in formation of a significant proportion of calcium perovskite over a narrow depth interval. The calculated sound velocity impedance contrast for this reaction in a fertile peridotite is comparable to that of the wadsleyite to ringwoodite transition and is in good agreement with seismic observations. The effective Clapyeron slope of the reaction indicates that at high temperatures its depth would converge with that of the wadsleyite to ringwoodite transition, potentially explaining observations of a single discontinuity. However, this depth is much deeper than 520 km, where a single discontinuity is generally observed. Temperature variation alone cannot, therefore, explain the variability in 520 splitting, which is more likely to result from variability in the Ca content of the mantle, either due to varying mantle fertility or due to a varying proportion of recycled oceanic crust. The split in the 520 km discontinuity is therefore a sensitive indicator of mantle heterogeneity
DE: 1212 Earth's interior: composition and state (7207, 7208, 8105, 8124)
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 7200 SEISMOLOGY
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting