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Discussion papers
https://doi.org/10.5194/se-2019-120
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2019-120
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 08 Jul 2019

Research article | 08 Jul 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

The effect of effective rock viscosity on 2D magmatic porosity waves

Janik Dohmen, Harro Schmeling, and Jan Philipp Kruse Janik Dohmen et al.
  • Institute for Geoscience, Goethe University Frankfurt, Germany

Abstract. In source regions of magmatic systems the temperature is above solidus and melt ascent is assumed to occur predominantly by two-phase flow which includes a fluid phase (melt) and a porous deformable matrix. Since McKenzie (1984) introduced his equations for two-phase flow, numerous solutions have been studied one of which predicts the emergence of solitary porosity waves. By now most analytical and numerical solutions for these waves used strongly simplified models for the shear- and bulk viscosity of the matrix, significantly overestimating the viscosity or completely neglecting the porosity-dependence of the bulk viscosity. Schmeling et al. (2012) suggested viscosity laws in which the viscosity decreases very rapidly for small melt fractions. They are incorporated into a 2D finite difference mantle convection code with two-phase flow (FDCON) to study the ascent of solitary porosity waves. The models show that, starting with a Gaussian shaped wave, they rapidly evolve into a solitary wave with similar shape and a certain amplitude. Despite the strongly weaker rheologies compared to previous viscosity laws the effect on dispersion curves and wave shape are only moderate as long as the background porosity is fairly small. The models are still in good agreement with semi-analytic solutions which neglect the shear stress term in the melt segregation equation. However, for higher background porosities and wave amplitudes associated with a viscosity decrease of 50% or more, the phase velocity and the width of the waves are significantly decreased. Our models show that melt ascent by solitary waves is still a viable mechanism even for more realistic matrix viscosities.

Janik Dohmen et al.
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Short summary
In source regions of magmatic systems the temperature is above solidus and melt ascent is assumed to occur predominantly by two-phase flow. This two-phase flow allows for the emergence of solitary porosity waves. By now most solutions of these waves used strongly simplified viscosity laws, while in our laws the viscosity decreases rapidly for small melt fractions. The results show that for higher background porosities the phase velocities and the width of the wave are significantly decreased.
In source regions of magmatic systems the temperature is above solidus and melt ascent is...
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