HR: 0800h
AN: U21B-0421    [Abstracts]
TI: Influence of major element disequilibrium on mantle structure
AU: * Lithgow-Bertelloni, C
EM: crlb@umich.edu
AF: Earth Sciences, University College London, Gower Street, London, WC1E6BY, United Kingdom
AU: Xu, W
EM: xuwenbo@umich.edu
AF: Geological Sciences, University of Michigan, 1100 N. University Ave., Ann Arbor, MI 48109, United States
AU: Stixrude, L
EM: stixrude@umich.edu
AF: Earth Sciences, University College London, Gower Street, London, WC1E6BY, United Kingdom
AU: Ritsema, J
EM: jritsema@umich.edu
AF: Geological Sciences, University of Michigan, 1100 N. University Ave., Ann Arbor, MI 48109, United States
AB: At mid-ocean ridges, partial melting of a presumably homogeneous and equilibrated pyrolitic source generates a basaltic crust and leaves behind its depleted complement, harzburgite. The oceanic lithosphere that subducts into the mantle is thus physically and chemically layered. During subduction we expect basalt to separate from harzburgite and the rest of the oceanic lithosphere. Hence, over convective and diffusion time-scales, do basalt and harzburgite re-equilibrate chemically as pyrolite? Most mineralogical models, upon which comparisons to seismology are based, view the mantle as homogeneous and pyrolitic or chemically stratified with homogeneous and equilibrated compositions in each layer. Petrological experiments have shown that a homogeneous pyrolite source region explains MORB and the seismic velocity profile of upper mantle and transition zone to first order. However, this view appears to violate the dynamical constraints given the low chemical diffusivity for mantle materials (10^-14-10^-16 cm²s^-1) in the solid-state, ignoring the effects of fluids and partial melting. Allègre and Turcotte (1986) suggested a mechanically mixed mantle, a marble cake structure in which subducted oceanic lithosphere is deformed into pervasive, narrow pyroxenite veins. Computer simulations suggest a heterogeneous mantle made of a mechanical mixture of basalt and harzburgite, in which pools of basalt may accumulate at the bottom. A stirring time of the mantle between 250 and 750 Myr limits the mixing, stretching and folding of heterogeneity in the mantle. Thus, it seems implausible to equilibrate basalt and harzburgite into pyrolite with a fine (0.1–10 m) stratification given typical chemical diffusivities. We demonstrate that, even with identical bulk compositions, an equilibrium assemblage (EA) along the basalt- harzburgite join and a mechanical mixture of basalt and harzburgite in perfect disequilibrium (MM) have different phase equilibria and therefore different seismic velocities. We compute the seismic velocities of EA and MM using a previously developed self-consistent thermodynamic model and explore the effects of bulk composition (in terms of basalt depletion) and temperature. For MM, V_S in the transition zone is higher and increases more rapidly with depth and is virtually insensitive to basalt fraction, while V_S decreases for EA. For both EA and MM increasing potential temperature from 1400K to 1800K yields a deeper 410-km and a shallower 660-km discontinuity. The radial gradient of the velocity between discontinuities decreases with increasing potential temperature. We find that the magnitude of the 520- km discontinuity depends strongly on temperature, which may explain lateral variations in its seismic detection. Both MM and EA feature "double-step" discontinuities in the range of 660-750 km due to the ringwoodite- perovskite transition and the gradual dissolution of garnet into perovskite between 665 km and ~725 km depth. Both MM and EA have lower velocities than published radial seismological models, a discrepancy that increases with increasing depth from 400 to 740 km depth. This suggests the presence of a radial gradient in bulk composition in the mantle, a sub-adiabatic geotherm, or both. We explore the dynamical consequences for Earth's gravitational field and plate motions by constructing a velocity-density scaling that takes into account compositional and thermal effects as well the potential for phase transformations to induce lateral heterogeneity in seismic velocities.
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
SC: Union [U]
MN: 2007 Fall Meeting