A new clinopyroxene-liquid barometer, and implications for magma storage pressures under Icelandic rift zones

Abstract

Pressure is one of the key variables that controls magmatic phase equilibria. However, estimating magma storage pressures from erupted products can be challenging. Various barometers have been developed over the past two decades that exploit the pressure-sensitive incorporation of jadeite (Jd) into clinopyroxene. These Jd-in-clinopyroxene barometers have been applied to rift zone magmas from Iceland, where published estimates of magma storage depths span the full thickness of the crust, and extend into the mantle. However, tests performed on commonly used clinopyroxene-liquid barometers with data from experiments on H2O-poor tholeiites in the 1 atm to 10 kbar range reveal substantial pressure-dependent inaccuracies, with some models overestimating pressures of experimental products equilibrated at 1 atm by up to 3 kbar. The pressures of closed-capsule experiments in the 1–5 kbar range are also overestimated, and such errors cannot be attributed to Na loss, as is the case in open furnace experiments. The following barometer was calibrated from experimental data in the 1 atm to 20 kbar range to improve the accuracy of Jd-in-clinopyroxene barometry at pressures relevant to magma storage in the crust: