Modelling of melt segregation processes by high-temperature centrifuging of partially molten granites-II. Rayleigh-Taylor instability and sedimentation

Abstract

SUMMARY The present experimental study deals with the laboratory modelling of two different mechanisms of gravitational percolation in partially melted rocks: ( 1) diapiric percolation of heavy material and (2) the sedimentation of heavy particles. These two mechanisms of mass transport in partially melted rocks result in different scales of the segregation process in the melt-crystal matrix. A centrifuge furnace was used to simulate the percolation of the heavy particle layer through the partially molten granite at temperatures of up to 1000 C. Samples of Beauvoir granite (Massif Central, France, grain size 0.16-0.5 mm with an initial degree of partial melting -45 per cent) were used as a matrix. A layer of Pt powder suspended in a melt of the same composition as the partially melted matrix was placed on the top of the granite sample. After centrifuging for various times (up to 2 x lo4 s), X-ray images of samples were obtained and the evolution of the percolation process of heavy suspension in the partially molten granite was monitored from the Pt particle distribution. The diapiric or finger regime of percolation starts when the growth rate of a Raleigh-Taylor instability of the heavy layer is faster than the Stokes sedimentation velocity of individual particles in the upper layer. This relationship is a complex function of the size and initial concentration of heavy particles, as well as the ratio of particle to crystal size, the permeability of the matrix, and the heterogeneity scale in the partially melted matrix. At small concentrations (several per cent) and at large concentrations (where close packing of heavy particles results in an anomalous viscosity increase in the upper heavy layer) Stokes sedimentation is dominant in the vertical percolation of the heavy material. The sinking velocity of the diapir decreases when the size of heavy particles in it becomes comparable with the size of crystals in the partially melted granite. In this situation the vertical sinking of the diapir is not stable and the horizontal instability of the vertical mass transport starts to become important. Mass transport via diapiric percolation results in more efficient crystal-melt segregation of partially melted rocks. The percolation of individual particles provides only local melt-crystal flow on a scale comparable with the heavy particle size. The diapiric percolation provides a much larger scale of partial melt segregation with a length-scale comparable with the diapir size.