HR: 14:20h
AN: T33F-03    [Abstracts]
TI: A Two-Porosity Double-Lithology Model for Partial Melting, Melt Transport and Melt-Rock Reaction in the Mantle
AU: * Liang, Y
EM: Yan_Liang@Brown.edu
AF: Brown University, 324 Brook St., Providence, RI 02912 United States
AU: Parmentier, M E
EM: EM_parmentier@Brown.edu
AF: Brown University, 324 Brook St., Providence, RI 02912 United States
AB: Several lines of evidence suggest that the melting and melt extraction region of the mantle is heterogeneous consisting of interconnected networks of high porosity dunite channels in a low porosity harzburgite or lherzolite matrix. To better understand the dynamical processes of melting, melt migration, and melt-rock reaction in such a heterogeneous mantle, a two-porosity double lithology model has been developed. Here the region of interest is treated as two overlapping continua occupied by the low porosity matrix and high porosity channel system. Conservation equations for the matrix and channel continuum are coupled through interaction terms that take into account the mass, momentum and heat exchange between the two continua. Exchange terms for the mass conservation equations, for example, include reactive dissolution of the matrix, and diffusive and advective mixings between the melt in the channel and that in the matrix. The matrix dissolution rate is proportional to the extent of undersaturation of pyroxene with respect to the melt in the dunite channel. The diffusive exchange rate is proportional to the porosity of the matrix and the concentration difference between the melts in the channel and the matrix. The advective mixing rate is proportional to the permeability of the matrix and the pressure difference between the melts in the channel and the matrix. Both diffusive and advective mixing rates are inversely proportional to the square of the characteristic channel width (Xc). Essential features of the two-porosity double-lithology model have been investigated using simplified 1-D mass conservation equations. Key parameters include: element partition coefficients, porosity, channel volume fraction, rates of matrix dissolution, diffusive and advective mixing to the rate of matrix melting, and matrix-channel mass flux capacity ratio. The last parameter is a measure of the amount of material transport through the channel vs. that through the matrix. In general, the smaller these relative rates or ratios, the slower the rate at which the melt in the channel re-equilibrates with the matrix. For a given element of interest three important length scales are identified: matrix melting length (L_m); melt-rock reaction length (L_MR); and a critical length for channel-matrix partitioning (L_c). The geochemical signature of the melt developed in the deep part of the melting region can be preserved via channelized flow when L_MR > L_m, a condition that can be easily met for incompatible trace elements in the mantle when Xc > 10 m. For narrow channels (Xc < 1 m) significant reduction in matrix melt fraction (< 0.2%) and/or percolation of melt from the channel into the matrix continuum are needed to preserve the identity of the channelized melt. Given the chemical exchanges between the matrix and the channel continuum, the rate of matrix melting may be balanced by (or locked into) the rates of melt-rock reaction at some point in a 1-D mantle column. This dynamical rather than chemical equilibrium between the melt in the channel and that in the matrix is established when L_c > 5L_m for an incompatible element. In the limit of slow melt-rock reaction, incompatible trace elements are preferentially partitioned into the channel melt in a dynamically equilibrated double-lithology mantle. Implications of channel-matrix partitioning for partial melting, melt migration, and melt-rock reaction in a heterogeneous mantle will be discussed.
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8145 Physics of magma and magma bodies
DE: 8416 Mid-oceanic ridge processes (1032, 3614)
DE: 8434 Magma migration and fragmentation
SC: Tectonophysics [T]
MN: Fall Meeting 2005