HR: 1340h
AN: MR43A-0880    [Abstracts]
TI: A suction mechanism for iron entrainment from the outer core into the lower mantle.
AU: * Kanda, R V
EM: rkanda@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, MC 252-21, Pasadena, CA 91125 United States
AU: Stevenson, D J
EM: djs@gps.caltech.edu
AF: Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd, MC 252-21, Pasadena, CA 91125 United States
AB: Variations in the earth's rotation rate (nutation data) strongly favors the existence of a thin high electrical conductivity layer, having a conductance of ~ 10$^8$ S, near the Core Mantle Boundary (CMB)(Buffett, JGR 1992). However, it is not clear if this layer is in the outer core-side or mantle-side of the CMB. Seismic data does not provide a clear-cut answer, but indicates that any core-side conducting layer must be very thin (Rost & Revenaugh, Science 2001). An earlier model of sediments containing lighter core elements at the top of the inner core can explain the nutation data only if very low permeabilities (and hence sub micron grain sizes) are assumed (Buffett et al., Science 2000). Here, we have investigated a simple 1-D, suction driven mechanism that can entrain significant quantities of iron into the lower mantle on time scales of ~ 10s of Ma - small compared to that of mantle convection. Our mechanical model is driven by the pressure difference between the liquid iron and the silicate mantle near the CMB. This "suction" drives compaction in a zone of several hundred meters of the lowermost mantle. Analytical solution to the linear problem with constant liquid (Fe) fraction and effective compaction viscosity indicates that sufficient iron can be entrained to explain the conductance required by nutation data. We then explored the 1-D nonlinear case, where the effective compaction viscosity, as well as the mantle viscosity near the CMB (hence the suction boundary condition at CMB) are strongly liquid fraction dependent - both decrease as the fraction increases. Compared to the linear problem, entrainment is harder in the non-linear problem. As the liquid fraction increases, suction at the CMB starts decreasing as the effective mantle viscosity drops. In addition, the effective compaction viscosity increases, reducing the liquid fraction entrainment rate. We solve the resulting coupled set of compaction and fluid continuity equations using an iterative finite difference scheme. We find that when the effective compaction viscosity is much smaller than mantle viscosity (by at least an order of magnitude), the maximum of the Fe liquid distribution is pushed deeper into the mantle. Sufficient iron gets entrained within a few hundred meters thick zone to yield the required conductance. Our results, although preliminary at this stage, would support the plausibility of a conducting layer on the mantle-side of CMB, thus obviating the need for complex assumptions regarding the chemistry and sedimentation dynamics in the outer core. It must be noted that the presence of partial melt as inferred from ULVZs, as well as recent research indicating the presence of Fe-rich silicate near the bottom of the mantle, will only enhance the conductance of our layer. We next propose to explore the effect of including the full two-phase physics of the problem (silicate matrix and Fe liquid) on our results.
DE: 8115 Core processes (1507)
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting