HR: 0830h
AN: V11E-0535    [PDF]
TI: Asymmetric Zoning in Peridotitic Spinels - Disequilibrium Melt/Rock Reaction in the Oceanic Mantle
AU: * von der Handt, A
EM: avdhandt@mpch-mainz.mpg.de
AF: Max-Planck Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Snow, J E
EM: jesnow@mpch-mainz.mpg.de
AF: Max-Planck Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Hellebrand, E
EM: ehelle@mpch-mainz.mpg.de
AF: Max-Planck Institut fuer Chemie, Postfach 3060, Mainz, 55020 Germany
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole, Oceanographic Institution, Woods Hole, MA 02543 United States
AB: Models of melt migration and melt/rock reactions in the mantle are based on the assumption of an instantaneous local equilibrium (Kelemen et al., 1992). These local equilibrium models have been thought to provide the most realistic outline for future investigations so far. Plagioclase lherzolites from Gakkel Ridge are characterised by strong compositional variations in all phases with an overall range of spinel Cr\# from 10-55 of the sample suite often in a single thin section. The spinels show distinct asymmetric zonation that is not in accordance with orientation, shape of the grains or other textural indicators. The zoning is best developed where the spinel grains are rimmed by plagioclase. Plagioclase trace element composition show that they originate by an intruding depleted melt. Symmetrical zoning with consistent orientation of Al-rich and Al-poor regions with respect to lineation and grain shape like in mylonitic peridotites from the Miyamori ophiolitic complex (Ozawa, 1988) was not observed. The Gakkel Ridge samples show only weak deformation; therefore a different explanation is required. Residual spinel peridotites from the same dredge are generally unzoned and have homogeneous compositions. The absence of crosscutting veins and the extreme chemical disequilibrium suggest melt injection conditions very close to the lherzolite solidus. The asymmetric zoning shows that diffuse reactive melt migration can take place at low melt/rock ratios in local disequilibrium conditions. If this is a general process then the assumption of local equilibrium cannot be made, at least in a major part of the melting system. Kelemen, P. et al. (1992) Nature, 358, p. 635-641; Ozawa, K. (1988) Nature, 338, p. 141-144.
DE: 1025 Composition of the mantle
DE: 3035 Midocean ridge processes
DE: 3640 Igneous petrology
DE: 3655 Major element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting