HR: 08:00h
AN: V51E-01    [Abstracts]
TI: Permeability Constraints for the Sub-Ridge Mantle Inferred from Field Relations, Microstructures, and Geochemistry of Dunites in Oman
AU: * Braun, M G
EM: michael.g.braun@exxonmobil.com
AF: ExxonMobil Upstream Research Company, PO Box 2189, Houston, TX 77252
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Woods Hole Oceanographic Institution, McLean 210, MS#8, Woods Hole, MA 02543
AU: Kelemen, P
EM: peterk@ldeo.columbia.edu
AF: Lamont Doherty Earth Observatory, PO Box 1000, Palisades, NY 10964
AB: The segregation and transport of basaltic melt from its source deep beneath mid-ocean ridges is both efficient and rapid. Although mid-ocean ridge basalt (MORB) is an mixture of near-fractional melts derived from a range of pressures, the major element composition of primitive basalts, which form the bulk of the oceanic crust, implies that these primitive liquids traverse the uppermost 20-30 km of the mantle without significant chemical interaction with the rocks through which they pass. Additionally, $^{230}$Th excesses observed in recently erupted lavas indicate that some melts are segregated from their source at depths greater than 75 km and ascend to the surface with rates on order of a meter per year. Geochemical and petrologic data from abyssal and ophiolitic peridotites suggest that dunites accommodate the chemically-isolated transport of primitive melts to the surface via porous flow. Measured dunite size/frequency statistics coupled with observed systematic grain size variations in Oman peridotites provide a framework to determine the melt capacity of a network of porous dunite conduits and their ability to preserve major element disequilibrium with the shallow mantle as well as the excess radiogenic $^{230}$Th observed in MORB. Four permeability models are examined with respect to their ability to satisfy these chemical and volumetric constraints. A model in which both porosity and grain size vary systematically with dunite width produces both an unequilibrated melt flux sufficient to generate 6 km of oceanic crustal gabbro and the high melt velocities required to maintain $^{230}$Th excesses with sustainable maximum porosities consistent with independent microstructural constraints. The dunite width and grain size distributions observed in Oman imply that the permeability of dunites scales with their size and ranges from less than 10$^{-16}$ m$^{2}$ for dunites narrower than 100 cm to as high as 10$^{-11}$ m$^{2}$ in dunites 100 m wide. In an interconnected network of dunites which conserves flux, the bulk of the melt flux remains chemically unequilibrated and is accommodated in conduits with permeabilities sufficient to sustain average melt velocities on order of 0.1-1 m/yr. Although there is considerable uncertainty in the estimations of dunite abundance and grain size while in the melting region, the inferred variations in melt flux are consistent with melt-rock ratios calculated from the observed geochemical variations with dunite width and primitive lava compositions from Oman.
DE: 5114 Permeability and porosity
DE: 5139 Transport properties
DE: 3640 Igneous petrology
DE: 3035 Midocean ridge processes
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting