Neils Colloquia on Innovative Science (NCIS)

Friday, February 24, at 3:30 PM in NSC 234. Join us for refreshments in the NSC lobby at 3:10 PM.

"Shearing Melt Out of the Earth: The Role of Deformation on Extraction of Melt from Great Depths"

 

Dr. David Kohlstedt (VU '65), Department of Geophysics, University of Minnesota

 

Melting in Earth’s mantle occurs primarily at the boundaries between tectonic plates, which are also regions of intense deformation.  Because melt formation and rock deformation coexist in time and space on a large scale, the importance of melt on the viscosity of partially molten rocks has long been appreciated.  Initial laboratory investigations focused on the influence of a melt phase on the strength of partially molten mantle rocks, with emphasis on the dependence of rock viscosity on the amount of melt present.  More recent experimental studies concentrated on the intimately related/coupled problem of the effect of viscous deformation on melt distribution, demonstrating the occurance of spontaneous segregation and self-organization of melt into melt-rich channels.  These melt-rich channels provide high-permeability paths for rapid extraction of melt from deep within Earth’s mantle; such paths are required to preserve chemical disequilibrium between the erupted lavas and the mantle rocks through which the magma ascends.   In this talk, progress in understanding melt extraction from the mantle is reviewed, starting with an historical perspective on physical/mechanical mechanisms for channelizing the flow of melt in partially molten rocks.  In particular, experimental investigations of the influence of stress on melt distribution are examined, both at the grain scale and at the rock scale.  Finally, the application of stress-driven melt segregation to the Earth in the context of melt extraction at plate boundaries is examined, and the process of extrapolating results of laboratory experiments to the longer temporal and spatial scales of geological processes is discussed.