From stress-driven to reaction-driven melt segregation – the frog’s eye view (1)

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• David Kohlstedt (University of Minnesota)
• Monday 11 April 2016, 10:00-11:00
• Seminar Room 1, Newton Institute.

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Co-authors: Matej Pec (University of Minnesota), Ben Holtzman (Columbia University)

Separation of melt from residual solid and its migration from beneath a mid-ocean ridge to 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 becomes under saturated in pyroxene with respect to the surrounding upper mantle; thus, pyroxene 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 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.

Understanding stress-driven and reaction-driven melt segregation processes requires a close coupling of experiment with theory. My talk focuses on results from laboratory investigations of the formation and evolution of melt-enriched channels in mantle rocks. (i) The first part examines the formation of stress-driven, melt-enriched channels predicted by theory. Here, the experimentally observed alignment of channels motivated further development in theory. (ii) The second part considers experimental investigations of reaction infiltration instabilities in mantle rocks. In partially molten rock samples composed of olivine and pyroxene sandwiched between a source of reactive (pyroxene under-saturated) melt and a porous sink, finger-like melt-enriched channels composed of olivine + melt propagated into and often through the rock in response to a gradient in fluid pressure.

This talk is part of the Isaac Newton Institute Seminar Series series.

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