HR: 09:00h
AN: U11B-05    [Abstracts]
TI: Whole Mantle Thermo-Chemical Convection Models With Realistic Mineral Physics Naturally Develop Chemical Stratification
AU: * Tackley, P J
EM: ptackley@ethz.ch
AF: ETH Zurich, Institute for Geophysics, Schafmattstrasse 30, Zurich, 8093, Switzerland
AU: Nakagawa, T
EM: takashi@geo.kyushu-u.ac.jp
AF: Kyushu University, Department of Earth and Planetary Sciences, Kyushu, 12345, Japan
AU: Deschamps, F
EM: frederic.deschamps@erdw.ethz.ch
AF: ETH Zurich, Institute for Geophysics, Schafmattstrasse 30, Zurich, 8093, Switzerland
AU: Connolly, J A
EM: james.connolly@erdw.ethz.ch
AF: ETH Zurich, Institute for Mineralogy and Petrology Clausiusstrasse 25, Zurich, 8092, Switzerland
AB: Starting with [Christensen and Yuen, 1985 JGR], many isochemical convection models have demonstrated the existence of "intermittent" or "partial" layering enforced by the ringwoodite to perovskite+magnesiowustite phase transition over a certain range of Clapeyron slope values, which has often been cited as a possible mechanism for reconciling conflicting evidences for whole-mantle and layered convection. Current mineral physics constraints indicate, however, that the likely value of the Clapeyron slope is too low to enforce this mode, although studies have shown that a viscosity increase at 660 km depth might account for much of the observed variation in slab dynamics without appealing to a phase transition. When chemical variations are additionally taken into account, the dynamical effect of phase transitions can again become important. Firstly the additive effect of the '660' phase transition and chemical buoyancy can combine to keep denser than average material in the lower mantle and less dense than average material in the upper mantle, the so-called filter effect first identified by Weinstein [1992 EPSL]. Secondly, the pyroxene-garnet components transform to perovskite at a higher pressure than olivine components, giving positive buoyancy to MORB and negative buoyancy to harzburgite in the depth range 660-720 km, which has been shown to cause local chemical stratification around 660 km depth. Thirdly, MORB is likely denser than average mantle in the deep mantle, and some fraction of it settles into a layer above the CMB. These effects are here demonstrated and quantified in 3-D spherical convection calculations in which the mineralogy is calculated self-consistently as a function of temperature, pressure and composition (expressed as the ratios of 5 oxides) using free energy minimization. Compositional variations arise self-consistently from melting. These build on the earlier studies of Xie and Tackley [2004 PEPI, JGR], Nakagawa and Tackley [2005 Gcubed; 2006 GRL], and Tackley et al. [2005 AGU monograph]. In conclusion, as with most "great debates" the likely resolution lies inbetween the endmember scenarios, with circulation that extends through the entire mantle but a dynamically-maintained chemical stratification without a sharp boundary.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8125 Evolution of the Earth (0325)
SC: Union [U]
MN: 2007 Fall Meeting