Modeling melting of the Martian mantle and crust-mantle differentiation
with global thermochemical evolution models
Abstract
Basaltic melts are produced when convection adiabatically brings deep
and hot mantle to lower pressures. Such primary melts were extracted
from the mantle of Mars, crystallized near the surface and progressively
built the Martian crust. This process displaced a large fraction of the
heat producing elements from the mantle to the crust and created an
insulating layer that slowed down further cooling of the mantle. The
complex crust-mantle system controlled many aspects of the geologic
history of Mars, including the development of an atmosphere and whether
conditions favorable to life could have existed. Our knowledge of the
mineralogy, chemical composition and physical properties of the crust of
Mars is rapidly expanding. Global geodynamical models can be used to
interpret the available data and constrain the processes of crust-mantle
differentiation. However, existing models still treat melting in a
simplified way. For example, the degree of melting is often assumed to
increase linearly above the solidus temperature, while the density of
the residue is assumed to decrease linearly. Calculating the density of
the residual mantle more accurately is critical because the
compositional buoyancy that develops during partial melting
fundamentally modifies mantle dynamics. Here, we present an improved
parametrization of partial melting of the Martian mantle, which will be
combined with the convection code Gaia. We created a new empirical model
of melting that calculates the composition of the extracted melts and,
when combined to thermodynamic models (e.g., Perple_X), the density of
the corresponding residual mantle. Another advantage of the new melting
parametrization is that the major-element composition of partial melts
can be tracked and used to constrain the petrogenesis of surface rocks.
Preliminary results will be compared to available Martian rocks believed
to represent primary mantle melts or melts affected by minor fractional
crystallization.