“Low pressure” (P≥150 MPa) experiments in piston cylinder apparatus

The piston cylinder apparatus is one of the most versatile instruments operating in laboratories of experimental petrology because it provides a safe, inexpensive, and easy-to-use technique for accessing high-pressure and high-temperature phases and processes. The operating pressure of the piston cylinder usually ranges between 0.5 and 5 GPa, which allows Earth scientists to investigate materials and processes occurring at deep crustal to upper mantle levels. Incomplete compaction of the assembly and thermocouple failure are typical problems in experiments performed at pressures lower than 0.5 GPa. In this work, we present new calibration data that demonstrates that the piston cylinder apparatus is suitable for experiments at pressure as low as to 150 MPa. Two new designs for the 25 mm assembly have been developed and calibrated using two different calibration methods: the NaCl melting curve and the solubility of H2O in albite and rhyolite melts. NaCl calibration data shows that positive friction corrections of 45, 55, and 60 MPa applied to the force of the piston are necessary to establish sample pressures of 300, 200, and 150 MPa respectively. The H2O solubility experiments in albite and rhyolite not only confirm the corrections determined using the NaCl calibration method, but also indicate that the friction correction is the same even when operating at temperatures higher than 800°C and for 24-hour durations. The accuracy of the pressure estimate associated with the calibration methods is ±25 MPa. The major advantage of using the new design assemblies in the piston cylinder apparatus is that experiments for which low-pressure conditions as well as very fast heating and quenching rates are required (e.g. volcanic and hydrothermal systems), can be performed with the same ease and precision as for the pressure ranges for which piston cylinders are routinely employed.