Barometers behaving badly II: A critical evaluation of Cpx-only and Cpx-Liq thermobarometry in variably-hydrous arc magmas

The chemistry of erupted clinopyroxene crystals (±equilibrium liquids) have been widely used to deduce the pressures and temperatures of magma storage in volcanic arcs. However, the large number of different equations parametrizing the relationship between mineral and melt compositions and intensive variables such as pressure and temperature yield vastly different results, with implications for our interpretation of magma storage conditions. We use a new test dataset comprised of the average Cpx-Liq composition from N=543 variably-hydrous experiments at crustal conditions (1 bar to 17 kbar) to assess the performance of different thermo...  more

The chemistry of erupted clinopyroxene crystals (±equilibrium liquids) have been widely used to deduce the pressures and temperatures of magma storage in volcanic arcs. However, the large number of different equations parametrizing the relationship between mineral and melt compositions and intensive variables such as pressure and temperature yield vastly different results, with implications for our interpretation of magma storage conditions. We use a new test dataset comprised of the average Cpx-Liq composition from N=543 variably-hydrous experiments at crustal conditions (1 bar to 17 kbar) to assess the performance of different thermobarometers, and identify the most accurate and precise expressions for application to subduction zone magmas. First, we assess different equilibrium tests, finding that comparing the measured and predicted EnFs and KD (using Fet in both phases) are the most useful tests in arc magmas, while CaTs, CaTi and Jd tests have limited utility. We then apply further quality filters based on cation sums (3.95-4.05), number of analyses (N>5), and the presence of reported H2O data in the quenched experimental glass (hereafter ‘liquid’) to obtain a filtered dataset (N=214). We use this filtered dataset to compare calculated versus experimental pressures and temperatures for different combinations of thermobarometers. A number of Cpx-Liq thermometers perform very well when liquid H2O contents are known, although the Cpx composition contributes relatively little to the calculated temperature. Most Cpx-only thermometers perform very badly, greatly overestimating temperatures for hydrous experiments. These two observations indicate that the Cpx chemistry alone holds very little temperature information in hydrous systems. Cpx-Liq and Cpx-only barometers show similar performance to one another, with most expressions yielding RMSEs of 2-3.5 kbar. We also assess the sensitivity of different equations to melt H2O contents, which are poorly constrained in many natural systems. Overall, this work demonstrates Cpx-based barometry on individual Cpx only provides sufficient resolution to distinguish broad storage regions (e.g., upper, mid, lower crust). However, after significant amounts of averaging of Cpx compositions from experiments reported at similar pressures, RMSEs can reduce to ~1.3-1.9 kbar for the best-behaving expressions. We hope our findings motivate the substantial amount of experimental and analytical work that is required to obtain precise and accurate estimates of magma storage depths from Cpx±Liq equilibrium in volcanic arcs.