HR: 15:25h
AN: DI43A-08    [Abstracts]
TI: Reaction between lherzolite and eclogite-derived melts in the upper mantle
AU: * Lo Cascio, M
EM: mauro@brown.edu
AF: Brown University, Dept. of Geological Sciences, Box 1846, Providence, RI 02912, United States
AU: Liang, Y
EM: Yan_Liang@brown.edu
AF: Brown University, Dept. of Geological Sciences, Box 1846, Providence, RI 02912, United States
AB: During mantle upwelling, pyroxenite-rich regions are likely to start melting at greater depths than peridotite. As a result, we can anticipate the existence of at least two regimes of chemical and mechanical interaction between peridotites and pyroxenites: one in which pyroxenite is partially molten while peridotite is subsolidus, and the other in which both lithologies are partially molten. In this study we explored the nature of such interactions in both regimes by conducting lherzolite\--pyroxenite-derived melt dissolution experiments. All experiments were performed at 1300°C - 1375°C and 2 GPa in a piston cylinder apparatus using the reaction couple method. At lherzolite subsolidus conditions, the reaction involves negligible amounts dissolution of the lherzolite with crystallization of garnet at the rock-melt interface, shifting the melt towards a more qz-normative composition. The lherzolite is chemically unaffected by the reaction, suggesting that dissolution is rate-limited by the slowest diffusing component in the melt. In contrast, when the lherzolite is partially molten we observed large dissolution rates and the formation of an opx-rich harzburgite + melt layer, sandwiched between the original partially molten lherzolite and pyroxenite-derived melt. The composition of the minerals across the capsule shows the existence of chemical gradients that extend beyond the boundaries of the newly formed lithology. The fast dissolution rates and chemical gradients indicate that melt is interconnected, as confirmed by BSE images. Therefore the opx-rich region does not represent an impermeable barrier as suggested in previous studies. The critical difference between the two regimes is the physical state of the surrounding lherzolite: subsolidus or partially molten. The P-T conditions and the composition of the pyroxenite-derived liquid are important additional factors that determine, for example, the mineralogy of the reaction boundary layer, the dissolution rates, and the time-scale of equilibration. Depending on the style of melt-rock reaction taking place in the mantle, the trace element and isotopic contribution of pyroxenite-derived melts will vary dramatically. During the highP/lowT regime, dissolution of lherzolite will be extremely slow and the pyroxenite-derived melt will react very little with the surrounding mantle, keeping most of its original geochemical signature. As melting of the pyroxenite proceeds, the lack of reaction and infiltration into the peridotite will cause the melt fraction to increase, which could lead to local brittle failure. In order for the pyroxenite-derived melt to reach the base of the lithosphere unaffected by the transport process, a combination of fracturing at highP and flow through pre-existing high-porosity conduits is necessary. Alternatively, extensive re-equilibration will take place, as expected by the lowP/highT regime, and the melt composition will inherit a hybrid signature of both the lherzolite and pyroxenite-derived melts. Using two-phase flow theory applied to a 1\-D mantle column, we calculated the chromatographic effect that an enriched pyroxenite-derived melt would undergo by reacting with a porous mantle. The model shows that the melt composition leaving the porous column evolves from an initial DMM-derived composition towards a variable U- shape trace element pattern in a spider diagram, depending on depth. Although the exact melt compositions produced by this melting model depends on the source composition, transport properties, and P-T path, we believe these initial results show some of the essential features of melt-rock reaction in the mantle.
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1038 Mantle processes (3621)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting