Modelling of melt segregation processes by high-temperature centrifuging of partially molten granites-I. Melt extraction by compaction and deformation

Abstract

The present study deals with the experimental modelling of two different mechanisms of crystal-melt segregation in crustal rocks: (1) the buoyancy-driven compaction of the crystal + melt matrix and (2) melt filtering in a partially crystalline matrix due to differential stresses. These two segregation mechanisms have differing relative efficiencies in the deformation of crustal rocks and result in different texture scales depending on melt fraction, melt viscosity and tectonic stresses. A centrifuge furnace has been used in the present study for the modelling of melt migration in partially molten granitic rocks. Samples of Beauvoir granite (Massif Central, France) with a grain size of 0.16–0.5 mm and dimensions of diameter ∼5 mm, length ∼16 mm were used. These samples had been pre-fused at temperatures of 1000–1075 °C, yielding an initial average melt fraction of ∼45–50 volume per cent. The centrifuging of partially melted samples during ∼6 hr at an acceleration of 1000g (g is gravity) results in a linear vertical distribution of melt over the length of the sample without the development of a compaction layer. The gradient of the melt fraction (melt migrates to the top of samples) correlates with temperature: 1075°C ∼7 volume per cent mm-1; 1050°C ∼4 volume per cent mm-1; 1000°C ∼1.5 volume per cent mm-1. The calculated rate of melt migration varies from 3x10-5 cm s-1 (1075°C) to 2x10-6 cm s-1 (1000°C). Differential stresses of ∼0.7–1.4 MPa have been generated in the centrifuge by putting a piston (weight ∼1.02–2.05 g, diameter ∼4.5 mm) on the top of the partially melted sample, which is then centrifuged at ∼1000g. The rate of melt squeezing from the sample in this case is about two orders of magnitude higher than that observed without the piston. After centrifuging for 6 hr, a compaction layer below the piston is formed with a thickness of ∼2.5 mm and a crystal fraction of ∼70–65 volume per cent. Further centrifuging (∼15 hr) does not result in any increase of the compaction-layer thickness or volume percentage of crystals in it. The comparison of the two segregation mechanisms confirms the much greater efficiency of differential-stress-induced melt segregation and accumulation in veins and pockets than the compaction mechanism.