HR: 0800h
AN: DI41A-1263    [Abstracts]
TI: The Influence of Temperature Dependent Clapeyron Slope at the Base of the Transition Region on Mantle Mixing
AU: * Liu, Y
EM: ygliu@atmosp.physics.utoronto.ca
AF: Department of Physics, University of Toronto, 60 St. George Street, Toronto, ON M5S 1A7 Canada
AU: Peltier, W R
EM: peltier@atmosp.physics.utoronto.ca
AF: Department of Physics, University of Toronto, 60 St. George Street, Toronto, ON M5S 1A7 Canada
AB: Although it was initially believed that the γ-spinel to Perovskite plus Magnesiowüstite phase transition had a significantly negative Clapeyrone slope, more recent experimental studies (e.g. Fei et al., JGR 109, 2305-2312, 2004) have led to the suggestion that the magnitude of the Clapeyron slope for this transition was initially overestimated. This raises the question as to whether this interface could provide the strong barrier to mixing between the upper and lower mantle circulation that is apparently required by trace element geochemical data. It needs to be understood, however, that in the cold slabs there exist a pressure coincident additional phase transition, namely that from Ilmenite to Perovskite, which also has a very negative Clapeyron slope. Even if the new experimental data on the nature of the primary transition were correct, the seismic discontinuity at 660 km depth may still provide a strong barrier to vertical mass flux. As discussed in detail by Vacher et al. (PEPI 106, 275-298, 1998), however, the volume fraction of Ilmenite in the upper mantle is highly dependent on the temperature of the upper mantle material, meaning that the effective Clapeyron slope at the 660 km interface must be considered to be temperature dependent. Our purpose in this paper is to describe the impact that this physical effect will have upon the mixing process. The axi-symmetric model that we have elected to employ for this initial investigation is that previously developed by the Toronto group (Solheim and Peltier, JGR 99, 6997-7018, 1994; Butler and Peltier, JGR 105, 3175-3208, 2000). A number of simulations have been performed with various temperature dependent strength of the effective Clapeyron slope of 660 km phase boundary, for both high and low Rayleigh number models and for constant viscosity and depth dependent viscosity models. Our results show that for high Rayleigh number (O(10⁷)) models, even in the extreme situations, where only the subducting slabs are blocked by 660 km phase boundary, the mixing between the upper and lower mantle is significantly reduced compared to that predicted by a whole mantle convection model; however, for the low Rayleigh number (O(10⁶)) models, there is no significant inhibition of mixing. Compared to the standard models, in which the Clapeyron slope has a constant value of -2.8 MPa K^-1, all the models with temperature dependent Clapeyron slope produce higher mixing levels even when the Clapeyrone slope for the subducting slabs is as negative as -4.0 MPa K^-1. Our results suggest that when the 660 km phase boundary serves as a ­ñhot pass filter­ñ, it may provide a weaker barrier to vertical mass flux than previously thought, but still a strong barrier especially for high Rayleigh number convection as is presently characteristic of the mantle general circulation. Early in Earth history the inhibition of vertical mixing would have been considerably enhanced.
DE: 0545 Modeling (4255)
DE: 8125 Evolution of the Earth (0325)
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005