A low-aluminum clinopyroxene-liquid geothermometer for high-silica magmatic systems

Several geothermobarometric tools have focused on clinopyroxene due to its prevalence in igneous rocks, however clinopyroxene produced in high-silica igneous systems is high in iron and low in aluminum, causing existing geothermometers that depend on aluminum exchange to fail or yield overestimated temperatures. Here we present a new clinopyroxene-liquid geothermometer recommended for use in natural igneous systems with bulk SiO (sub 2) > or = 70 wt%, which contain clinopyroxene with Mg# < or = 65 and Al (sub 2) O (sub 3) < or = 7 wt%. (1) T ( degrees C) = 300 [-1.89 - 0.601 (X (sub CaTs) (super Cpx) ) - 0.186 (X (sub DiHd2003) (super Cpx) ) + 4.71 (X (sub SiO2) (super liq) ) + 77.6 (X (sub TiO2) (super liq) ) + 10.9 (X (sub FeO) (super liq) ) + 33.6 (X (sub MgO) (super liq) ) + 15.5 (X (sub CaO) (super liq) ) + 15.6 (X (sub KO0.5) (super liq) )] The new geothermometer lowers calculated temperatures by approximately 85 degrees C on average relative to Putirka (2008, Eq. 33) and reduces the uncertainty by a factor of two (standard error of estimate + or -20 degrees C). When applied to natural systems, we find this new clinopyroxene-liquid geothermometer reconciles many inconsistencies between experimental phase equilibria and preexisting geothermometry results for silicic volcanism, including those from the Bishop Tuff and Yellowstone caldera-forming and post-caldera rhyolites. We also demonstrate that clinopyroxene is not restricted to near-liquidus temperatures in rhyolitic systems; clinopyroxene can be stable over a broad temperature range, often down to the solidus. An Excel spreadsheet and Python notebook for calculating temperature with this new geothermometer may be downloaded from GitHub at http://bit.ly/cpxrhyotherm.