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Viscosity is one of the most important properties of a melt. It is essential for the quantitative modelling of geological processes in the Earth's interior such as magma ascent and eruption. In particular the H₂O content can strongly influence the rheology. Measurements under room-pressure are restricted to investigations of samples with small amounts of water. This work presents a novel method to measure the viscosity of water-bearing melts in the range
10⁸ < < 10¹¹ Pa s under high pressure.

The apparatus has been developed to fit into a vertical furnace which is placed in an internally heated pressure vessel. This set-up is designed to be used at high temperature (to 1000°C), and high pressure, up to 10 kbar. It is built up on a high pressure closure head and is based on the micropenetration method. It consists mainly of a penetration-rod, a linear variable differential transformer (LVDT), a relay and a glass-tube which supports the sample as illustrated in Fig. 3.8-5.

Fig. 3.8-5: The penetration viscosimeter.

The principle of measurement is simple. After the apparatus is placed into the autoclave, a relay which raises the penetration rod about 1-2 mm above the sample is switched on. When the conditions of measurement are achieved the relay is switched off and the rod with an iridium-hemisphere on the top penetrates the surface of the sample. The penetration distance is measured by a LVDT. Three thermocouples inside the furnace are necessary to control the temperature and minimise the temperature gradient.

Distance, time and temperature are recorded by a computer, and the viscosity can be calculated according to

In this equation is F the force on the rod which penetrates the sample, t the time, S the penetration distance and r the radius of the iridium-hemisphere.

This project is still in the preparation phase. Tests under atmosphere pressure are currently in progress to improve and calibrate this set-up for experiments under high pressure.