HR: 15:25h
AN: T52E-08    [PDF]
TI: Permeability Constraints From Microstructural and Geochemical Correlations in Dunites
AU: * Braun, M G
EM: mbraun@whoi.edu
AF: Woods Hole Oceanographic Institution, MS#8, Woods Hole, MA 02543
AU: Kelemen, P B
EM: peterk@whoi.edu
AF: Woods Hole Oceanographic Institution, MS#8, Woods Hole, MA 02543
AU: Hirth, G
EM: ghirth@whoi.edu
AF: Woods Hole Oceanographic Institution, MS#8, Woods Hole, MA 02543
AB: Trace element compositions, field relationships, and numerical models indicate that dunites represent high porosity conduits for melt transport beneath mid-ocean ridges. In this study we exploit systematic correlations between microstructure, chemical composition, and dunite width in the Oman ophiolite to constrain melt transport processes in the shallow mantle. In detailed transects across dunites of different widths, we observe an increase in Ti content in spinel and decrease in Ni content in olivine, relative to the surrounding harzburgite, as dunite width increases. Mass balance calculations with increased melt-rock ratios in wider dunites fit the measured chemical variations. In the same rocks, we also observe an increase in olivine grain size with increasing dunite width. Grain growth kinetics are sufficiently rapid to allow for the observed coarsening on timescales shorter than the residence time of solid material in the melting region. Depending on the mechanism of coarsening, wider dunites either formed over a longer time than narrow dunites or had a higher melt flux to achieve the larger grain size. But in either case, the increased grain size implies an increased permeability in wider dunites. The observed correlations between chemistry and texture are consistent with dunite formation by reactive porous flow of ascending basalt with the adjacent harzburgite. If dunites grow by reaction at their boundaries, then wider dunites have evolved over longer periods of time, which, coupled with an increase in permeability due to grain coarsening, implies a higher time-integrated melt-rock ratio in wider dunites. Using the power-law size-frequency relationship for dunites in Oman, we can integrate the compositional and microstructural observations into porous flow models to quantify the variations in permeability required to satisfy the observed melt flux at mid-ocean ridges.
DE: 1025 Composition of the mantle
DE: 3035 Midocean ridge processes
DE: 5112 Microstructure
DE: 5114 Permeability and porosity
DE: 8120 Dynamics of lithosphere and mantle--general
SC: Tectonophysics [T]
MN: 2003 Fall Meeting