Abstract
Melt transport across the ductile mantle is essential for oceanic crust
formation or intraplate volcanism. Metasomatic enrichment of the
lithospheric mantle demonstrates that melts chemically interact with the
lithosphere. However, mechanisms of melt migration and the coupling of
physical and chemical processes remain unclear. Here, we present a new
thermo-hydro-mechanical-chemical (THMC) model for melt migration coupled
to chemical differentiation. We study melt migration by porosity waves
and consider a simple chemical system of forsterite-fayalite-silica. We
solve the one-dimensional (1D) THMC model numerically using the finite
difference method. Variables, such as solid and melt densities or and
mass concentrations, are fully variable and functions of pressure (P),
temperature (T) and total silica mass fraction
(CTSiO2). These
variables are pre-computed with thermodynamic Gibbs energy minimisation,
which shows that dependencies of these variables to variations in P, T
and CTSiO2 are
considerably different. These
P-T-CTSiO2
dependencies are implemented in the THMC model via parameterized
equations. We consider P and T conditions relevant around the
lithosphere-asthenosphere boundary and employ adiabatic and conductive
geotherms. Variation of
CTSiO2 changes the
densities of solid and melt and has a strong impact on melt migration.
We perform systematic 1D simulations to quantify the impact of initial
distributions of porosity and
CTSiO2 on the melt
velocity. An adiabatic gradient generates higher melt velocities.
Reasonable values for porosity, permeability, melt and compaction
viscosities provide melt velocities between 10
[cm·yr-1] and 100 [m·yr-1].
We further discuss preliminary results of two 2D simulations showing
blob-like and channel-like porosity waves.