MODELING OF MELT SEGREGATION PROCESSES BY HIGH-TEMPERATURE CENTRIFUGING OF PARTIALLY MOLTEN GRANITES .2. RAYLEIGH-TAYLOR INSTABILITY AND SEDIMENTATION

The present experimental study deals with the laboratory modelling of two different mechanisms of gravitational percolation in partially mel ted rocks: (1) diapiric percolation of heavy material and (2) the sedi mentation of heavy particles. These two mechanisms of mass transport i n 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 parti ally molten granite at temperatures of up to 1000 degrees C. Samples o f Beauvoir granite (Massif Central, France, grain size 0.16-0.5 mm wit h an initial degree of partial melting similar to 45 per cent) were us ed as a matrix. A layer of Pt powder suspended in a melt of the same c omposition as the partially melted matrix was placed on the top of the granite sample. After centrifuging for various times (up to 2 x 10(4) s), X-ray images of samples were obtained and the evolution of the pe rcolation process of heavy suspension in the partially molten granite was monitored from the Pt particle distribution. The diapiric or finge r regime of percolation starts when the growth rate of a Raleigh-Taylo r instability of the heavy layer is faster than the Stokes sedimentati on velocity of individual particles in the upper layer. This relations hip is a complex function of the size and initial concentration of hea vy particles, as well as the ratio of particle to crystal size, the pe rmeability of the matrix, and the heterogeneity scale in the partially melted matrix. At small concentrations (several per cent) and at larg e concentrations (where close packing of heavy particles results in an anomalous viscosity increase in the upper heavy layer) Stokes sedimen tation is dominant in the vertical percolation of the heavy material. The sinking velocity of the diapir decreases when the size of heavy pa rticles in it becomes comparable with the size of crystals in the part ially melted granite. In this situation the vertical sinking of the di apir is not stable and the horizontal instability of the vertical mass transport starts to become important. Mass transport via diapiric per colation results in more efficient crystal-melt segregation of partial ly 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 partia l melt segregation with a length-scale comparable with the diapir size .