Home / Node / David Kohlstedt Colloquium Abstract (Feb 8, 2017)

David Kohlstedt Colloquium Abstract (Feb 8, 2017)

Stress- and reaction-driven melt segregation – formation of high-permeability paths in the mantle

Separation of small amounts of melt from the 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 upper mantle develop as a consequence of instabilities in the deformable, dissolvable porous media. Models for the formation of such flow instabilities include stress-driven as well as reaction-driven melt channelization.

As melt rises from depth, it 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 identified in peridotite massifs served as channels for rapid melt extraction from partially molten regions of the mantle. Formation of such dunite channels involves dissolution-precipitation reactions between the mantle rock and a percolating reactive melt. Dunite channels also coincide with shear zones, indicating that plastic deformation together with melt-solid reaction plays an important role during melt channelization.

In this context, my talk focuses on results from laboratory investigations of the formation and evolution of melt-enriched channels in deforming mantle rocks. In particular, it examines the formation of a network of stress-driven, melt-enriched channels during high-strain torsion experiments on partially molten rocks. The experimentally observed alignment of these channels at low angle to the shear plane motivated critical developments in the theory describing this phenomenon. In turn, theoretical predictions inspired a new generation of experiments to test important aspects of theory.