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

Abstract

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 wide variety 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 of N=505 Cpx-Liq pairs from variably-hydrous experiments at crustal conditions (0-13 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 liquid to obtain a filtered dataset (N=194). 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. Cpx-Liq and Cpx-only barometers show similar performance to one another, all showing low precision and systematic offsets (overestimating pressure for low P experiments, and underestimating pressure for High P expressions). We also assess the sensitivity of different equations to melt H2O contents, which are poorly constrained in many natural systems. Overall, this work demonstrates that substantial work is needed to obtain precise and accurate estimates of magma storage depths from Cpx±Liq equilibrium in volcanic arcs. At present, Cpx-based barometry only provides sufficient resolution to distinguish broad storage regions (e.g., upper, mid, lower crust), rather than ability to precisely and accurately locate magma reservoirs to compare to geophysical records.

Geophysical and Geochemical Constraints on Magma Storage Depths Along the Cascade Arc: Knowns and Unknowns

“…Analytical uncertainty related to the measurement of Na 2 O in pyroxene can easily yield errors spanning 3-5 kbar on each individual Cpx measurement (Wieser et al, 2023c). Averaging multiple Cpx compositions at each volcano can help to mitigate these random analytical errors (Putirka et al, 1996), and results in a substantial improvement when applied to the experimental data investigated by Wieser et al (2023b). Thus, we predominantly focus on median pressures calculated for each volcano.…”

“…The relatively thick crust in the Cascades (∼40–50 km, Jiang et al., 2023; Kiser et al., 2016; Parsons et al., 1998; Shehata & Mizunaga, 2022) means that Cpx‐based barometry can roughly distinguish between storage in the upper, mid and lower crust at best. Another advantage of these two barometers is that they are independent of temperature and H 2 O content, which are difficult to estimate from Cpx compositions alone (Wieser et al., 2023b). An additional complication with literature compilations is that the quality of mineral analyses in our compilation is not known, as publications do not quote the analytical precision of each measurement.…”

“…The general agreement within the ±2–3 kbar uncertainty of Amp and Cpx‐based barometers gives us confidence in this result, along with the fact that geodetic and seismic constraints on magma storage generally have depths equivalent to ∼0–5 kbar. With the limitations of the reported information, it is difficult to interpret whether the spread of individual calculated pressures for a given volcanic center represents true transcrustal storage or analytical uncertainty (Wieser et al., 2023b).…”

“…(2017) used the Cpx‐Liq barometer from Putirka (2008, Equation 32c) with an unspecified thermometer. However, applying different Cpx‐Liq barometry equations to the same Cpx‐Liq pairs yield pressures that can differ by 3–12 kbar (∼10–45 km, see Wieser et al., 2023b), which is an offset equivalent to the thickness of the Cascade crust (Das & Nolet, 1998). Additionally, Cpx‐Liq barometry relies on identifying or reconstructing a liquid composition in equilibrium with each Cpx composition (Scruggs & Putirka, 2018), which is challenging in arcs where whole‐rock compositions may not necessary represent true liquids (Kent et al., 2010) and many equilibrium tests perform badly (Wieser et al., 2023b).…”

“…However, applying different Cpx-Liq barometry equations to the same Cpx-Liq pairs yield pressures that can differ by 3-12 kbar (∼10-45 km, see Wieser et al, 2023b), which is an offset equivalent to the thickness of the Cascade crust (Das & Nolet, 1998). Additionally, Cpx-Liq barometry relies on identifying or reconstructing a liquid composition in equilibrium with each Cpx composition (Scruggs & Putirka, 2018), which is challenging in arcs where whole-rock compositions may not necessary represent true liquids (Kent et al, 2010) and many equilibrium tests perform badly (Wieser et al, 2023b). Given that Cpx-only and Cpx-Liq barometers show similar performance for a data set of arc-like experimental products (Wieser et al, 2023b), we avoid melt-matching complications by using the Cpx-only barometers of Jorgenson et al (2022 and Wang et al (2021.…”