Mixing and presevation of chemical signals during migration of localized, small melt fractions

Abstract:

The importance of heterogeneities in partial melting and melt migration is widely recognized, but the dynamics of melting in a heterogeneous mantle are still poorly understood. Here we show that one-dimensional models commonly used in geochemical studies of heterogeneous melting may lead to fundamentally different dynamics and may give misleading results.

In a low-porosity background the melting of a fertile heterogeneity leads to the formation of a porosity wave. The propagation velocity of this porosity wave is generally slower than the migration velocity of the melt. In one dimension this leads to a separation of the chemical signal transported by the melt and slower moving porosity wave. In addition the migration speed of a chemical signals depends inversely on its compatability in the solid matrix, so that chromatographic separations are expected where the most incompatible elments arrive first.

However, in higher dimensions, the porosity wave and the chemical signals travel together. Geochemical signals do not move linearly upward as in 1D, rather, they travel in a circular motion inside the rising porosity wave. This occurs when the background fluid velocity drops below the phase speed of the porosity wave. The phase speed of the porosity wave is controlled by its amplidude and is independent of the background porosity while the background fluid velocity declines as the cubic root of the background porosity.

In the reference frame of the moving porosity wave the melt streamlines are circular. In the center of the porosity wave the melt moves faster than the phase speed and rises to the top. Here the melt is deflected to the sides where porosity is low and the melt speed drops below the phase velocity of the porosity wave. After the porosity wave passes, the melt is reentrained and a circular motion develops. This flow slowly mixes the geochemical signal originating from the heterogeneity with that of the low-porosity ambient mantle that the wave traverses.

This circular pattern of melt migration allows the re-ordring of the chromatograpic sequence depending on the exact timing of the extration of the melt from the top of the porosity wave. This may explain the reverse chromatographic sequences observed in some monogenetic vents that are likely the product of the melting of heterogeneities in a otherwise low-porosity mantle.