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Mantle Melting: the achievements, promise, and limits of thermodynamics

Presented by: 
Paul Asimow CALTECH (California Institute of Technology)
Date: 
Monday 15th February 2016 - 11:30 to 12:30
Venue: 
INI Seminar Room 1
Abstract: 
This workshop is an opportunity to consider the state of the art in application of thermodynamics to mantle melting. This approach constructs and calibrates Gibbs free energy models for relevant phases, enabling predictions of equilibrium states including phase proportions, compositions, and some physical properties. We will consider the achievements of this approach, its unfulfilled promise, and its theoretical limits.

Under achievements, a body of work has applied the MELTS and pMELTS models to upper mantle magmatism in ocean ridge, back-arc, intraplate, and subduction environments. From 0-3 GPa pressure, models of multicomponent minerals and silicate liquid appear good enough to recover most experimental results, to provide an internally-consistent forward model from source peridotite through melting to the observed volume and composition of magmas, to constrain the potential temperature of source regions, to elucidate the role of water, and more. There have been a few forays into integrating this approach with magma dynamics at various scales.

Under promise, we expect near-future, improved versions that incorporate carbon species, extend to substantially higher pressure, and account for variable speciation in the melt to achieve higher fidelity. Calibration strategies based on a flexible, evolving, community framework will liberate the enterprise from the slow progress of models based on fixed, internally-consistent datasets. Fully coupled, large-scale dynamical simulations incorporating MELTS thermodynamics are practical and close to implementation.

Under limits, we must recall the limitations of the equilibrium assumptions underlying thermodynamics. At some time and length scales these assumptions break down. In a heterogeneous mantle containing species with widely divergent mobility, there must be important roles for kinetics. Progress in incorporating kinetics in major element models has been nearly nil and must be a major focus of our efforts.

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