Reinvestigation and application of olivine-quartz-orthopyroxene barometry

Experiments in a piston-cylinder apparatus have been carried out at 700–1050°C, 10–16 kbar to determine the stability of ferrosilite (FeSiO₃) relative to fayalite + quartz. Reaction reversals within 0.1-kbar intervals locate the equilibrium at 10.5, 11.0, 11.5, 12.0, 12.6, 13.3, 14.1 and 14.8 kbar at 700, 750, 800, 850, 900, 950, 1000, and 1050°C, respectively, reflecting the intercept with the α-β quartz transition at about 880°C. The tight reversals severely constrain the reaction slope, providing a basis for limited extrapolation and calculations. However, the lack of accurate activity and cation-distribution data for orthopyroxene and olivine generates substantial uncertainties when considering the effects of large proportions of additional components such as MgO. Experiments and calculations indicate that additional components dramatically extend the pyroxene stability field and that pressures that have been inferred from ferrosilite-rich pyroxenes in natural assemblages are 1–3 kbar too high.