University of Minnesota
School of Physics & Astronomy

Physics and Astronomy Colloquium

Thursday, December 1st 2016
3:35 pm:
Speaker: David Kohlstedt
Subject: From Reaction-driven to Stress-driven Melt Segregation – Formation of High-permeability Paths through Earth’s Mantle
Refreshments to be served outside Keller 3-210 after the colloquium.

Separation of small amounts of melt from residual solid and migration of that melt from deep beneath a mid-ocean ridge to its eruption at Earth’s surface require a transition from porous to channelized flow in order to preserve chemical and radiogenic disequilibrium. Chemically isolated, high-permeability melt conduits in Earth’s mantle develop as a consequence of instabilities in the deformable and dissolvable porous media. Models for the formation of such flow instabilities include stress-driven and reaction-driven melt channelization.

Melt rising from depth through mantle rocks, which are composed primarily of the minerals olivine and pyroxene, becomes under saturated in pyroxene with respect to the surrounding upper mantle. Thus, pyroxene preferentially dissolves into the melt as it migrates toward the surface. Tabular rocks rich in olivine and depleted in pyroxene found in peridotite massifs serve as channels for rapid melt extraction from partially molten regions of the mantle. Formation of such dunite channels involves dissolution-precipitation reactions between mantle rock and percolating reactive melt. Dunite channels also coincide with shear zones, indicating that deformation together with reaction plays an important role during melt channelization.

My talk focuses on results from laboratory investigations of the formation and evolution of melt-enriched channels in mantle rocks. The first part examines the formation of stress-driven, melt-enriched channels predicted by theory, while the second part considers the development of reaction infiltration instabilities in mantle rocks. These channelization processes significantly increase permeability and, hence, the flux of melt through the partially molten rocks.

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