HR: 0800h
AN: T41E-1353    [Abstracts]
TI: Contrasting Trace Element Signatures in Abyssal Peridotites: High Temperature Refertilization Versus Cooling-Controlled Trace Element Zoning in Orthopyroxenes
AU: * von der Handt, A
EM: avdhandt@mpch-mainz.mpg.de
AF: Max Planck Institute for Chemistry, Geochemistry Division, Postbox 3060, Mainz, 55020 Germany
AU: Hellebrand, E
EM: ehelle@mpch-mainz.mpg.de
AF: Max Planck Institute for Chemistry, Geochemistry Division, Postbox 3060, Mainz, 55020 Germany
AU: Snow, J E
EM: jesnow@uh.edu
AF: Max Planck Institute for Chemistry, Geochemistry Division, Postbox 3060, Mainz, 55020 Germany
AU: Snow, J E
EM: jesnow@uh.edu
AF: University of Houston, Dept. of Geosciences, 4800 Calhoun Road, Houston, TX 77204-5007 United States
AB: Under equilibrium conditions, lithophile trace element exchange between clinopyroxene (cpx) and orthopyroxene (opx) is strongly temperature dependent [1] and can vary over more of an order of magnitude for highly incompatible elements. Considering fast uplift rates, the cooling and uplift history of abyssal peridotites should be recorded by this process. Studies on mantle peridotites have focussed on cpx so far because of its relatively high trace element (TE) abundances. However, its high TE abundance and buffering effect is at the same time its disadvantage as relative changes are small. Despite lower TE abundances, opx should be very sensitive to interpyroxene TE-redistribution due to cooling. Two plagioclase-free spinel peridotites from Gakkel Ridge were chosen because of their high TE concentrations in the cpx to ensure good analytical accuracy in associated opx. Both samples are very heterogeneous in their pyroxene content, grading from almost lherzolitic to dunitic composition on decimetre scale. Pyroxene textures seen in thin section are indicative of late formation by reaction with a melt. The two spinel peridotites are homogeneous in terms of major elements in their silicates but show a significant variation in spinel Cr-numbers ranging between 18-28 and 16-29, respectively, with Ti-contents lower than 0.15 wt%. Cpx shows high Na-concentration of up to 1.1 wt%, inconsistent with its occurence in harzburgites. Cpx measured by ion probe shows a flat REE-pattern at about 7xCI, homogeneous within analytical certainty. It is in equilibrium with an LREE-enriched melt most likely derived by low degrees of melting. In contrast, opx is very heterogeneous in composition ranging over a magnitude in its LREE concentrations. Large opx grains show zoning with highest REE-concentrations in the core and lowest at the rim. Opx/cpx interpartitioning data suggest high temperature equilibration between cpx and opx cores. Diffusive exchange between an enriched infiltrating melt and a preexisting opx would initially produce a U-shaped zoning pattern in opx. This is not consistent with the observation that opx has bell-shaped trace element zoning instead. The dependence of the Kdopx/cpx on temperature has been recently studied [1] and could account for the observed core-rim trace element variation in opx. Nevertheless, the presence of such a pronounced variation on grain scale in opx has never been reported before. Overall, pyroxenes in the studied samples record two subsequent processes, high temperature equilibration after refertilization by an enriched low degree melt followed by rapid cooling preserved in bell- shaped REE profiles in opx. [1] G. Witt-Eickschen & H. O'Neil (2005): Chemical Geology, 65 - 101.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
DE: 3600 MINERALOGY AND PETROLOGY
DE: 8000 STRUCTURAL GEOLOGY
DE: 8400 VOLCANOLOGY
SC: Tectonophysics [T]
MN: Fall Meeting 2005