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Technical advances in partial melting experiments

X. Liu, H.StC. O’Neill and W.O. Hibberson

Water is well known to have a large effect on the solidus temperature of mantle peridotite. It is therefore essential to exclude water as completely as possible from the experimental charge during investigation of mantle melting under anhydrous conditions. Historically, many melting experiments in the piston-cylinder apparatus have used traditional experimental pressure assemblies consisting of salt-pyrex or even talc-pyrex sleeves. Water from the incompletely dried assembly or other sources may react with the graphite heater to produce hydrogen. Diffusion of this hydrogen through the Pt capsule may reduce the experimental charge, and produce water at the same time.

Figure 1: New experimental assembly arrangement (salt-pyrex-Fe₂O₃). 1. salt sleeve; 2. pyrex tube; 3. graphite heater; 4. graphite; 5. MgO spacer; 6. Al₂O₃ spacer; 7. ruby disc; 8. Al₂O₃ sleeve; 9. Fe₂O₃ sleeve; 10. Capsule.In addition, even carefully dried experimental charges may possibly absorb some moisture from the air during the capsule-welding process.In order to solve these problems, a new experimental protocol has been developed: 1. New welding process. The Pt capsule is held in a steel block that is heated to ~700ºC during welding. Previously, the open capsule plus charge is dried at 150ºC for several hours.2. New experimental assembly arrangement (salt-pyrex-Fe₂O₃) –

Figure 1. The basic structure of the new assembly is the same as the old salt-pyrex assembly: a central part surrounded by a graphite heater, a pyrex tube and an outermost salt sleeve. The difference lies in the central part. In the central part of the new assembly, the Pt capsule is enclosed by a Fe₂O₃ sleeve, which is in turn surrounded by an alumina sleeve. At each end of the alumina sleeve, there is a ruby disc which completely separates the Fe₂O₃ sleeve from the rest of the assembly. This prevents reduction of the Fe₂O₃ sleeve by the graphite heater, or contamination of the thermocouple. Composite spacers of alumina and magnesia are positioned next to the ruby discs. This enhances the mechanical stability of the assembly. The Fe₂O₃ sleeve is made by cold pressing in a steel die and sintering at 850°C, 1 atm for 3 hours.Using these new experimental techniques, the water content of experimental charges is kept to insignificant levels. Another advantage of using the new cell assembly is that the hydrogen fugacity surrounding the capsule is kept extremely low by the Fe₂O₃ sleeve.