MELT DYNAMICS OF A PARTIALLY MOLTEN MANTLE WITH RANDOMLY ORIENTED VEINS

Abstract

The statistical probability of a connected network of randomly oriented melt veins in partially molten mantle suggests that the critical melting model can account for observed (226Ra/230Th) isotope activity ratios. Similarly, if the reactive percolation model also includes such veins, then this modified model is also relevant. Critical melting models for the isotope activity ratios imply that material exchange between the melt and the residuum must be negligible. Hence, any diffusive exchange between the melt in the veins and the residuum must be limited. This condition suggests that the veins form a rooted system where several smaller veins supply larger veins. Further, the veins should be situated at least some decimetres apart, and the permeability constant of the residuum should be larger than ∼10−10 cm2. These conditions ensure that diffusion does not change the composition of the residuum significantly. Using isotope activity ratios alone, it is difficult to distinguish between the percolative and critical melting models, because both can account for the observed data. The concentrations of incompatible elements in residual harzburgites and other evidence appear to favour the critical melting model. It is suggested that veins can form as the result of overpressure caused by an interstitial melt, because of the volume expansion during melting.