H. M. Gonnermann $^{a}$, M. Manga $^{a}$, D. R. Stegman $^{a}$, A. M. Jellinek $^{a}$
$^{a}$ University of California, Berkeley, California, USA
We performed laboratory experiments of thermochemical convection in order to determine the rate at which an initially density-stratified, and hence layered, mantle will be homogenized (Gonnermann et al., 2002). These experiments extend previous studies of thermochemical convection (Olson and Kincaid, 1991; Davaille, 1999) by quantifying the rate of entrainment across the density interface over the complete evolution of a layer. In addition, we provide a framework for predicting the temperature difference across the chemical boundary layer in a density stratified lower mantle. Working fluids in these experiments are corn syrup solutions or various polybutene oils. Two layers of fluid with similar viscosities are superposed and heated from below. As in previous work by Davaille (1999) two convective regimes are observed (Figure 1 $^{1}$): (1) a stratified and (2) a doming regime. Initially in the stratified regime, mechanical entrainment into both layers occurs by viscous coupling and theoretical models for the entrainment rate agree well with our experimental data. Over time, entrainment and mixing reduce the density difference between the layers until the doming regime is reached. The density interface becomes unstable, thermal plumes of the dense fluid penetrate into the overlying layer and the entrainment rate reaches a constant. Applying our results to the Earth's mantle, we confirm that it is possible for a compositionally distinct layer to persist over the age of the Earth, even if the initial density difference is less than 2\%. In our experiments a significant temperature difference exists between the two layers during the initial stratified regime (Figure 2 $^{1}$). Heat flux from the lower into the upper layer is in part by thermal conduction across this internal thermal boundary layer; however, there is also an advective component of heat flux associated with entrainment of fluid across the interface. The advective heat flux is proportional to the entrainment rate (inversely proportional to density difference). Owing to progressive entrainment and mixing between the two layers, the relative proportion of conductive to advective heat flux, as well as the temperature difference between both layers, decreases throughout the experiment (Figure 2). A number of previous studies, especially parameterized convection calculations (McNamara and van Keken, 2000 and references therein), have assumed the heat flux between the two layers to be entirely conductive (Figure 3 $^{1}$). As a consequence, temperature differences of up to several thousands of degrees have been suggested for a hypothetical layered mantle with a few percent density stratification. For such moderate density differences, however, the conductive heat flux is only a fraction of the total heat flux, and we predict the temperature difference associated with a hypothetically layered mantle to be only a few hundred degrees (Figure 3). $^{1}$ A PDF file with Figures 1,2 and 3 can be downloaded at: www.seismo.berkeley.edu/~manga/hmgsedi02.pdf. \begin{thebibliography}{0} Davaille, A., Simultaneous generation of hotspots and superswells by convection in a heterogeneous planetary mantle, {\it Nature} 402, 756-760, 1999. \\ \\ Gonnermann, H.M., M. Manga and A.M. Jellinek, Dynamics and longevity of an initially stratified mantle, {\it Geophys. Res. Lett.} vol. 29, \#10.1029/2002GL01485, 2002. Download PDF: www.seismo.berkeley.edu/~manga/paper53.pdf. \\ \\ McNamara, A.K and P.E. van Keken, Cooling of the Earth: A parameterized convection study of whole versus layered models, {\it G3} 1, \#2000GC000045. \\ \\ Olson, P. and C. Kincaid, Experiments on the interaction of thermal-convection and compositional layering at the base of the mantle, {\it J. Geophys. Res}, 96, 4347-4354, 1991. \end{thebibliography}