HR: 1340h
AN: T13A-0440    [Abstracts]
TI: The Effects of Melt Depeltion on Mantle Density and the Formation of Subcratonic Lithospheric Mantle
AU: * Schutt, D L
EM: schutt@uwyo.edu
AF: Derek L. Schutt, Dept. of Geology and Geophysics, University of Wyoming, Dept. 3006, 1000 University Ave., Laramie, WY 82071 United States
AU: Lesher, C E
EM: lesher@geology.ucdavis.edu
AF: Charles E. Lesher, Department of Geology, University of California, One Shields Ave., Davis, CA 95616
AB: The extraction of partial melt in the Earth's mantle leaves behind a residue depleted in incompatible elements and modified in mineral compositions and mode. This phenomenon, referred to as melt depletion, is geodynamically important because a melt-depleted residuum is expected to be more buoyant than its fertile parent material. This process can lead to the formation of buoyant upper mantle ''tectosphere'' beneath cratons. We examine the effects of melt depletion at pressures from 1-7 GPa, and find mantle densities vary significantly with pressure. At 20% melt removal, residue density changes are -0.42%, -0.46%, -0.90%, -1.14%, -0.95%, -0.66%, and -0.57%, for pressures of 1, 3, 3.5, 4, 6, and 7 GPa, respectively. We note that at adiabatic temperatures, realistic composition upper mantle has a higher thermal expansivity than olivine, ranging from α = 4.91-3.47 x 10^-5/° between 1-7 GPa. This implies that 1% melt depletion leads to a density change equivalent to the increase in mantle temperature of 3-15° , depending on pressure. Under Archean cratons, where cold melt-depleted mantle is generally considered to have the same density as fertile adiabatic mantle (i.e. subcratonic mantle is isopycnic), we find subcratonic mantle above ~110 km is negatively buoyant with respect to adiabatic mantle. This suggests vertical transport of residues initially formed above 110 km may play a role in the stabilization of subcratonic mantle. Furthermore, the volume of basalt necessary to form isopycnic mantle below 110 km is greater than the total volume of existing cratonic crust, supporting geochemical evidence for the recycling of mafic lower crust . We also find that melt depletion has little, if any, effect on seismic velocities for garnet peridotite residues, and leads to only a 0.5% decrease in V_p with 20% melt removal from fertile spinel peridotite. The major element effects of melt depletion are thus insufficient to produce the high mantle velocities imaged under cratons or cause significant lateral velocity variations below about 40 km depth.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3909 Elasticity and anelasticity
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8103 Continental cratons
SC: Tectonophysics [T]
MN: Fall Meeting 2005