Calibration of high-temperature furnace assemblies for experiments between 200 and 600 MPa with end-loaded piston-cylinder apparatuses

We present pressure calibration results for piston-cylinder 3/4" and 1" high temperature furnace assemblies in the 200-600 MPa range aiming to expand its applicability to simulate upper crustal conditions. The furnace assemblies were made up by crushable MgO, graphite heater, pyrex glass spacer, and NaCl sleeve. Twenty-one experiments were carried out over a range of pressures (200-600 MPa) and temperatures (870-945 degrees C) to determine the liquidus curve of NaCl. The obtained curve slope fits that given by Siewert et al. (1998), but offsets by similar to-55 MPa, implying the need of a constant P upward correction in the 300-600 MPa and ca. 300-400 MPa pressure ranges for the 3/4" and 1" assemblies, respectively. However, if the NaCl melting points (918-920 degrees C) obtained by Bohlen (1984) at 500 MPa are used as reference, no pressure correction is needed. Larger offsets (to -120 MPa) were obtained under pressure conditions around and lower than 300 MPa with the 1" assembly. Our results confirm the applicability of Bristol-type end-loaded piston-cylinders coupled with appropriate furnace assemblies in assessing relatively low-pressure conditions (down to similar to 200 MPa, similar to 7 km depth) found in the Earth's upper crust, where important magmatic, hydrothermal and metamorphic processes occur.