Geochemical Effects of Magmatic Solitary Waves .1. Numerical Results

Numerical studies have suggested that solitary waves may be a natural consequence of melt transport in the mantle. However, the likely physical properties of magma solitary waves mean that they are unlikely to be imaged by conventional geophysical techniques. This study aimed to determine whether solitary waves leave a detectable signature in the geochemistry of the melts they transport.The major effect seen during the interaction between a solitary wave and a concentration anomaly is a steepening of the concentration gradients by a factor S approximately (phi(max) + k(D))/(phi(b) + k(D)) where phi(max) is the maximum porosity in the solitary wave, phi(b) is the background porosity and k(D) is the partition coefficient. The solitary wave transports the tracer a distance proportional to S, a completely incompatible tracer (k(D) = 0) being transported a distance equal to the volume of the solitary wave. Considerable spreading of the concentration anomaly occurred in many experiments because the solitary waves enhance diffusion by steepening the concentration gradients. For a constant value of S, the increase in the width of the concentration anomaly is proportional to the square root of the diffusivity of the tracer, D1/2 . At constant D it depends on square-root D(eff)S2T where D(eff) is the effective diffusivity of the tracer and T is the length of time that the interaction takes.A single solitary wave passing through a set of tracers with different k(D) values will cause an apparent 'chromatigraphic' effect that causes the chemical peaks to separate much further than would be predicted using a constant porosity model. Approximately S times more diffusion than is predicted by the constant porosity model occurs. Moreover, because of enhanced steepening and dispersion, solitary waves may provide a mechanism for mixing melts normally separated by several kilometres.