MODELING OF MELT SEGREGATION PROCESSES BY HIGH-TEMPERATURE CENTRIFUGING OF PARTIALLY MOLTEN GRANITES .1. MELT EXTRACTION BY COMPACTION AND DEFORMATION

The present study deals with the experimental modelling of two differe nt mechanisms of crystal-melt segregation in crustal rocks: (1) the bu oyancy-driven compaction of the crystal+melt matrix and (2) melt filte ring in a partially crystalline matrix due to differential stresses. T hese two segregation mechanisms have differing relative efficiencies i n the deformation of crustal rocks and result in different texture sca les depending on melt fraction, melt viscosity and tectonic stresses. A centrifuge furnace has been used in the present study for the modell ing of melt migration in partially molten granitic rocks. Samples of B eauvoir granite (Massif Central, France) with a grain size of 0.16-0.5 mm and dimensions of diameter similar to 5 mm, length similar to 16 m m were used. These samples had been pre-fused at temperatures of 1000- 1075 degrees C, yielding an initial average melt fraction of similar t o 45-50 volume per cent. The centrifuging of partially melted samples during similar to 6 hr at an acceleration of 1000g (g is gravity) resu lts in a linear vertical distribution of melt over the length of the s ample without the development of a compaction layer. The gradient of t he melt fraction (melt migrates to the top of samples) correlates with temperature: 1075 degrees C similar to 7 volume per cent mm(-1); 1050 degrees C similar to 4 volume per cent mm(-1); 1000 degrees C similar to 1.5 volume per cent mm(-1). The calculated rate of melt migration varies from 3 x 10(-5) cm s(-1) (1075 degrees C) to 2 x 10(-6) cm s(-1 ) (1000 degrees C). Differential stresses of similar to 0.7-1.4 MPa ha ve been generated in the centrifuge by putting a piston (weight simila r to 1.02-2.05 g, diameter similar to 4.5 mm) on the top of the partia lly melted sample, which is then centrifuged at similar to 1000g. The rate of melt squeezing from the sample in this case is about two order s of magnitude higher than that observed without the piston. After cen trifuging for 6 hr, a compaction layer below the piston is formed with a thickness of similar to 2.5 mm and a crystal fraction of similar to 70-65 volume per cent. Further centrifuging (similar to 15 hr) does n ot result in any increase of the compaction-layer thickness or volume percentage of crystals in it. The comparison of the two segregation me chanisms confirms the much greater efficiency of differential-stress i nduced melt segregation and accumulation in veins and pockets than the compaction mechanism.