Evaluation of clinopyroxene-based thermobarometers for tholeiitic arc systems using experimental data and application to Mutnovsky volcano (Kamchatka)

To date, reconstructing temperature and pressure conditions in magmatic systems remains challenging. Commonly used tools, such as thermobarometers, often suffer from imprecise underlying calibration datasets and erroneous application to natural samples resulting in significant uncertainties on obtained results. This study uses phase equilibria experiments on a primitive arc tholeiitic basalt from the Mutnovsky volcano in Kamchatka to evaluate the performance of different clinopyroxene-based thermobarometers in calc-alkaline and tholeiitic arc systems. Common application errors were evaluated using an improved analytical protocol for EPMA measurements (e.g. improving statistical errors by accounting for the number of individual measurement points which are averaged for P-T calculations, selecting matrix-matching calibration and reference standards, and investigating the occurrence of beam damage). Errors introduced during data processing were accounted for via clinopyroxene component calculations and measurements of H₂O contents of coexisting silicate glasses. Our results demonstrate that the performance of commonly-used clinopyroxene-based thermobarometers can substantially be improved by thoroughly addressing analytical and application-related uncertainties.

Subsequently, we applied our refined analytical approach to natural samples from Mutnovsky volcano. Calculated temperatures of clinopyroxene crystallization during pre-eruptive magma storage range from 1310 °C to 1440 °C and pressures from 0 to 500 MPa. Pressure estimates are in alignment with previous studies postulating a shallow magma chamber at ~1.5-3 km depth (≈ 40-80 MPa), although showing a distinctively higher variability. In contrast, temperatures are overestimated by the clinopyroxene-based thermobarometers as previous studies report onset of clinopyroxene crystallization around 1000 °C indicating a potential systematical error of the applied thermobarometers.