Melt-rock reaction in the asthenospheric mantle: Perspectives from laboratory dissolution experiments (Invited)
Abstract
The primary motivation for melt-rock interaction in the mantle is chemical disequilibrium that arises when melt generated in the deep mantle percolates through the overlying mantle. This is a continuous process involving both thermal and chemical exchange between the melt and the solid matrix. It occurs in all major active tectonic regimes within the upper mantle and is responsible for a range of petrologic and geochemical observations. Factors that are important in controlling the kinetics of melt-rock interaction include temperature, pressure, and melt composition. The present study focuses on the effect of reacting melting composition, especially water and silica content, on reaction kinetics. In terms of melt composition, we can broadly divide melt-rock interaction into three groups: (1) reaction between peridotite and silica-undersaturated (anhydrous) melt; (2) reaction between peridotite and silica-saturated (anhydrous) melt; and (3) reaction between peridotite and hydrous melt. (1) is important to melt migration beneath mid-ocean ridges, (2) is relevant to magma genesis involving pyroxenite, and (3) is important to melt generation in the mantle wedge and back-arc basin. Reaction between peridotites and silica-undersaturated melts such as alkali basalt and MORB at moderate to low pressures can produce a melt-bearing dunite-harzburgite or dunite-harzburgite-lherzolite sequence. Reaction between peridotites and siliceous melts derived from melting of eclogite or garnet pyroxenite do not form dunite at moderate to high pressures (1-3.5 GPa and 1200-1550°C). Instead, they produce melt-bearing harzburgite at moderate pressure and opx-rich harzburgite or orthopyroxenite at high pressure. The opx is produced at the expense of olivine and the siliceous melt, which may hinder further reaction because of porosity reduction in these reactions. The rate of peridotite dissolution is considerably lower when the peridotite is subsolidus, and a high-porosity harzburgite or orthopyroxenite channel is unlikely to form under the (anhydrous) conditions examined. Reaction outcomes between peridotites and hydrous basaltic melts depend strongly on temperature, pressure and water content in the melt. For example, reaction between a ferrous basalt and lherzolite at 1300°C and 1 GPa produced a dunite-harzburgite sequence at low water content in the melt but an orthopyroxenite-harzburgite sequence at a higher water content. Furthermore, the same reactions at 1385°C and 2 GPa produced a harzburgite-lherzolite sequence at low melt water content but an orthopyroxenite-harzburgite sequence at a higher water content. These contrasting behaviors in reaction kinetics are related to the effect of water on the stabilities of olivine and opx in the reacting melt and may have important bearing on melt migration and melt-rock reaction in the mantle wedge and backarc spreading centers. Presence of orthopyroxenite dikes and veins in the mantle sections of ophiolites and mantle xenoliths may serve as a tell-tale sign of hydrous basalt-peridotite interaction. High-porosity orthopyroxenite channels instead of dunite channels may develop in the presence of water during melt migration and melt-rock interaction in the mantle, facilitating the transport of SiO2-rich melts through the mantle wedge.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V23D..03L
- Keywords:
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- 8412 VOLCANOLOGY Reactions and phase equilibria;
- 8416 VOLCANOLOGY Mid-oceanic ridge processes;
- 8413 VOLCANOLOGY Subduction zone processes