Coupling thermodynamic mineralogical models and mantle convection

In this thesis I advance the integration of mineral thermodynamics
into convection modeling. I have compiled a thermodynamic model of
mantle mineralogy in the five component CFMAS system
(CaO-FeO-MgO-Al2O3-SiO2), including mineral phases that occur close
to typical chemical models of the mantle and reasonable mantle
temperatures. In this system I have performed a system Gibbs free
energy minimization, including pure end-member phases and a
non-ideal formulation for solid solutions. Solid solutions were
subdivided into discrete pseudocompounds and treated as
stoichiometric phases during computation of chemical equilibrium by
the simplex method. I have complemented the thermodynamic model
with a model of shear wave properties [Stixrude and
Lithgow-Bertelloni, 2005] to obtain a full description of aggregate
elastic properties (density, bulk and shear moduli) that provide a
useful basis for the consideration of seismic and geodynamic models
of the Earth's mantle. By using this new thermodynamic database for
the mantle I have coupled the resulting density dynamically
(through the buoyancy term) with mantle convection models. I have
linked the database with a high-resolution 2-D convection code
(2DTERRA), dynamically coupling the thermodynamic model (density)
with the conservation equations of mantle flow. The coupled model
is run for different parameterisations of viscosity, initial
temperature conditions, and varying internal vs. external heating.
A common feature of all the models is that the convecting flow
creates a characteristic discontinuity of temperature around 660 km
depth in order to compensate for the entropy change due to the
phase transitions. I have studied the importance and the possible
consequences of such a thermal regime on the excess temperature of
plumes and on the transition zone thickness. The thermodynamic
mantle mineralogy model provides the conversion of the temperature
field into seismic velocities so that predictions from mantle
convection can be compared to seismic observations in terms of
radial profiles or lateral variations. This approach allows us to
predict a number of seismic observables from the convection model,
all of which agree remarkably well with observations from seismic
tomography.