HR: 13:40h
AN: V33D-01    [Abstracts]
TI: Abyssal Peridotites and Mantle Melting Beneath Ocean Ridges
AU: * Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. of Geology and Geophysics, Woods Hole, MA 02543 United States
AU: Snow, J E
EM: jesnow@mpch-mainz.mpg.de
AF: University of Houston, Department Of Geosciences, Houston, TX 77204 United States
AU: Hellebrand, E
EM: ehelle@mpch-mainz.mpg.de
AF: Max-Planck Institut Für Chemie, Postfach 3060, Mainz, D-55020 Germany
AU: Shimizu, N
EM: nshimizu@whoi.edu
AF: Woods Hole Oceanographic Institution, Dept. of Geology and Geophysics, Woods Hole, MA 02543 United States
AB: Studies of abyssal peridotite from ultraslow and slow spreading ridges show significant regional variability; with a strong correlation between the compositions of peridotite averaged by locality and spatially associated MORB reflecting higher degrees of mantle melting near mantle hot spots. Local variability of peridotite compositions, however, is often large, and may equal the regional variability along ocean ridges. The latter is attributed to local melting and melt transport processes such as melt channelization or late-stage melt impregnation in the lithosphere. The observed regional correlation appears only when many samples are averaged to eliminate local and outcrop scale variability. Almost all the peridotites used in these correlations are from transforms, and therefore represent similar thermal and mantle melting histories. Thus, regional differences in mantle composition are preserved. Until recently, little data were available for peridotites away from transforms representing the central mantle environment beneath magmatic segments. This is key, as geophysical and geologic evidence suggest focused melt flow beneath slow spreading ridges. If so, beneath individual magmatic segments there should be a corresponding mantle melting cell in which melt is focused from a broad melting region to a melt transport zone at its mid-point that feeds an overlying crustal magmatic center. High melt fluxes in the transport zone would produce very depleted peridotites stripped of pyroxene by melt-rock reaction during magma ascent. Studies of peridotites far from transforms at ultraslow Gakkel and SW Indian Ridges indicate this is the case: with near-Cpx free intergranular harzburgite and dunite locally abundant in contrast to transform peridotites. Recent mapping of the plutonic foundation of an ancient 35-km long slow spreading ridge segment at the Kane Core Complex also found a narrow 10-km wide zone of focused melt flow through the mantle marked by abundant dunite and spatially associated troctolites. Abyssal peridotite studies show that melts are produced in a geometrically complex melting regime. Not all portions of the source region contribute equally, and the mantle itself is likely multi component, with different components contributing to different degrees depending on the melting regime. A geochemical conundrum, then, is that mantle compositions estimated by inverting MORB provide only a fictive mantle 'source' composition - and not its true composition. Average abyssal peridotite compositions from ultra-slow spreading ridges where little crust is produced are only slightly different than those from slow spreading ridges with normal crustal thickness - suggesting that the average mantle composition could be more depleted than has been thought, and that mantle veins may contribute in greater proportion to melt production than has been believed.
DE: 1025 Composition of the mantle
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005