HR: 13:40h
AN: T43E-01    [Abstracts]
TI: Lithospheric Delamination as a Process to Introduce Water Into the Mantle
AU: * Elkins-Tanton, L T
EM: Lindy@brown.edu
AF: Brown University, Dept. Geological Sciences 324 Brook St., Providence, RI 02912 United States
AB: Delamination of the lower continental lithosphere has been inferred geologically from crustal heat flow increase, rapid regional uplift, and the appearance of signature high-potassium magmas such as lamprophyres and leucitites. Seismic studies also support delamination in specific areas. Though delamination is the suggested process in many geologic settings, no previous quantified study of the dewatering and melting consequences of delamination have been made. We use numerical models to investigate delamination of the lower continental lithosphere, resulting upper mantle flow patterns, topographic expression, and most significantly, the potential for the delaminating material to dewater as it falls. A dense lower-lithospheric region may develop through melt injection and transformation into eclogitic phase assemblages, or through thickening and cooling of a lithospheric root, such as in an arc setting. Lower crustal and mantle compositions that result from arc magmatism may exceed asthenospheric density by 50 to 250 kg/m$^{3}$ (about 1 to 5% density contrast) (Kay and Kay, 1993; Jull and Kelemen, 2001). Density contrasts in this range are sufficient to drive gravitational Rayleigh-Taylor instabilities. As the dense lithosphere drops by Rayleigh-Taylor instability it pulls the topography down by tens to hundreds of meters below the undisturbed zero value. Asthenosphere moves upward and laterally to replace the mass of the instability, possibly resulting in initial dry adiabatic melt erupting during topographic subsidence. Numerical models indicate that this adiabatic melting may produce hundreds to thousands of cubic kilometers of primary melt. Eruption during subsidence, observed in some geologic regions, is inconsistent with simple melting in an upwelling. Eventually the lithosphere rebounds to topographic levels above its initial elevation because of the hot buoyant asthenosphere now filling the dome in the lithosphere. Any subsequent eruption will likely occur on an uplifted surface. In an extinct arc setting it is reasonable to assume that the lithosphere itself is hydrous. As the delaminating piece falls through the mantle and heats conductively water and other volatiles will be forced out of it, just as hydrous fluids rise from a subducting slabs as the slab moves out of the stability regimes of various low-pressure and low-temperature hydrous minerals. The phase relations of Schmidt and Poli (1998) indicate that the descending instability is heated more than sufficiently to dehydrate its outermost several kilometers even if it consists solely of hydrous peridotite. This dehydrating fluid will percolate upward and hydrate the asthenosphere. The temperatures in the heating, descending delaminated material are also sufficient to melt the outermost kilometer if the material consists of wet peridotite. The descending material itself would produce magma from wet peridotite, or about twice the volume of magma from wet basalt. This hydrous fluid therefore can have two effects: it can enter the overlying mantle and produce a laterally-heterogeneous hydrated mantle, which may melt, or it can allow parts of the delaminating material itself to melt. The process therefore provides a number of geochemically different source regions: the hydrated delaminating material itself; hydrated mantle material in the wake of the delamination; and dry mantle material upwelling beneath the delaminated lithosphere. Models indicate that, over a range of lithospheric thicknesses and mantle potential temperatures, these processes can create a wide range of primary melt volumes, up to tens of thousands of cubic kilometers.
DE: 8450 Planetary volcanism (5480)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 3210 Modeling
DE: 1025 Composition of the mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting