The measurement and estimation of high pressure in experimental igneous petrology

Abstract

Uncertainty regarding the pressure applied to the sample has been a major limitation to the effectiveness of the piston-cylinder apparatus for high pressure phase equilibrium studies. Previously, the price of simplified generation of high pressure by using solid pressure media was dissipation of some of the ram thrust in internal friction and the application of non-hydrostatic pressure on the sample. The present study describes the development of a high-pressure cell that exhibits low sheer strength at temperatures between 500° and 1000° and prevents the build-up of localised pressure differentials around the sample capsule. The high-pressure cell was calibrated by locating the equilibrium boundary of the reaction albite = jadeite + quartz. Equilibrium pressures for the transition at 500°, 600° and 700°C were 15.2, 16.9 and 19.0 Kb, respectively. The observed pressures were sufficiently close to all previous determinations of the albite breakdown reaction to suggest that non-symmetrical friction within the high-pressure cell was negligible. The absence of symmetrical friction components was confirmed by the coincidence of a piston-in and a piston-out determination of the transition. A potential limitation to previous thermodynamic calculations of Ptotal of igneous rocks has been uncertainty as to the formation temperature and to the equilibrium mineral compositions. The thermodynamic treatment of Nicholls et al (1971) was examined by assessing the assumptions and errors inherent in the calculation method by applying the requisite equations to mineral assemblages produced under controlled experimental pressure and temperature conditions. Pressure estimates for the synthetic mineral assemblages were found to be sensitive to uncertainty in both temperature and mineral composition. An uncertainty of ±10°C produced a pressure error of ±0.15 Kb in 10.7 Kb and compositional variation of XMgSiO3± percent introduced an error of ±0.5 Kb at the same total pressure. The calculations were shown, however, to constitute a reliable check for equilibrium in synthetic mineral assemblages.