HR: 16:50h
AN: V24A-04    [Abstracts]
TI: An experimental study of the kinetics of lherzolite reactive dissolution: Implications for contrasting styles of melt transport in the mantle.
AU: * Liang, Y
EM: Yan_Liang@brown.edu
AF: Brown University, Dept. of Geo. Sci., Providence, RI 02912
AU: Morgan, Z T
EM: Zachary_T_Morgan@brown.edu
AF: Brown University, Dept. of Geo. Sci., Providence, RI 02912
AB: It has been suggested that dunite dikes or veins found in harzburgite and lherzolite hosts in the mantle sections of ophiolites are high porosity channels through which basaltic magmas were extracted from their source regions. The formation of such channels may involve pervasive melt flow and reactive dissolution. In order to better understand the kinetics of reactive dissolution we conducted two series of lherzolite dissolution experiments: one in an alkali basalt and the other in a basaltic andesite. Dissolution experiments were run at 1300$\deg$C and 1 GPa using lherzolite-melt reaction couple method. The lherzolite dissolution experiments produce a reactive boundary layer (RBL) that consists of distinct lithological units separated by sharp mineralogical interfaces. The details of the RBL depend on the relative stabilities of the lherzolite minerals with respect to the reacting melt. Dissolution of lherzolite in the basaltic andesite resulted in 2 distinct regions: harzburgite (45% ol, 45% opx, 10% melt) and lherzolite (45% ol, 35% opx, 12% cpx, 8% melt). In contrast, dissolution of lherzolite in the alkali basalt resulted in 3 distinct rock units: dunite (75% ol, 25% melt), harzburgite (60% ol, 30% opx, and 10% melt), and lherzolite (50% ol, 30% opx, 10% cpx, 10% melt). The average grain size of the dunite is greater than the average grain size of unreacted lherzolite, whereas the average grain size of the harzburgite in the two sets of dissolution experiments are nearly the same as the average grain size of the lherzolite. This implies that the permeability of the dunite is larger than either the newly created harzburgite or the unreacted lherzolite, and that the permeabilities of the harzburgite and lherzolite are about the same within the DHL sequence. Hence dunite dikes in the mantle are capable of serving as melt channels, whereas harzburgites may not. Systematic compositional variations in the interstitial melt, olivine, and to a lesser extent, pyroxenes in the RBL as functions of distance and time were also observed in the two sets of experiments. The Mg\# of olivine, for example, increases from 88 at the harzburgite-melt interface to 91 at the harzburgite-lherzolite interface in the basaltic andesite-lherzolite dissolution experiment. The systematic variations in mineralogy and mineral chemistry resulted from preferential dissolution of cpx and opx and precipitation of olivine in the alkali basalt dissolution experiments, and dissolution of cpx and precipitation of opx in the basaltic andesite dissolution runs. Results of our lherzolite dissolution experiments underscore the importance of reacting melt composition in determining the lithology and composition of the mineralogical regions developed during melt-rock reaction and in controlling the style of melt transport in the mantle. If a magma is only olivine saturated then a high permeability dunite channel will develop. If, on the other hand, the magma is more silica-rich or multi-saturated with olivine and opx, only harzburgite will be produced upon reaction with the host lherzolite. We suggest that reaction between slab-derived or mantle wedge-derived magmas with lherzolite may not form high permeability channels and therefore different styles of melt migration mechanism are required to transport such magmas through the mantle wedge.
DE: 8434 Magma migration
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3035 Midocean ridge processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting