Clinopyroxene–Liquid Equilibria and Geothermobarometry in Natural and Experimental Tholeiites: the 2014–2015 Holuhraun Eruption, Iceland

• Published in: Journal of Petrology
• Main Authors: Neave, David A, Bali, Enikő, Guðfinnsson, Guðmundur H, Halldórsson, Sæmundur A, Kahl, Maren, Schmidt, André-Sebastian, Holtz, François
• Other Authors: Alexander von Humboldt Foundation, the German Research Foundation, University of Manchester, European Commission
• Format: Article in Journal/Newspaper
• Language: English
• Published: Oxford University Press (OUP) 2019
• Subjects: Iceland
• Online Access: http://dx.doi.org/10.1093/petrology/egz042; http://academic.oup.com/petrology/advance-article-pdf/doi/10.1093/petrology/egz042/30312837/egz042.pdf; http://academic.oup.com/petrology/article-pdf/60/8/1653/31282230/egz042.pdf

Abstract Clinopyroxene–liquid geothermobarometry is a widely used tool for estimating the conditions under which mafic magmas are stored before they erupt. However, redox variability, sector zoning and disequilibrium crystallization present major challenges to the robust estimation of magma storage conditions. Moreover, most recent studies seeking to address these challenges have focused on clinopyroxenes from alkalic systems and are thus of limited use for understanding clinopyroxenes from the tholeiitic systems that dominate global magma budgets. Here we combine observations on natural clinopyroxenes from the 2014–2015 Holuhraun lava in Iceland with observations on experimental clinopyroxenes synthesized during high-pressure, high-temperature experiments on the same lava in order to investigate clinopyroxene–liquid equilibria in tholeiitic systems and optimize of geothermobarometric strategies. Natural clinopyroxenes from the 2014–2015 Holuhraun lava are sector zoned, with {1-11} hourglass sectors being enriched in the enstatite–ferrosillite component at the expense of all other components with respect to {hk0} prism sectors. In contrast with observations on clinopyroxenes from alkalic systems, sector zoning in clinopyroxenes from the 2014–2015 Holuhraun lava is characterized by differences in Ca and Na contents as well as in Ti and Al contents. The products of crystallization experiments performed at 100–600 MPa and 1140–1220 °C on a powdered starting glass at two sets of melt H2O content–oxygen fugacity conditions (∼0·1 wt % H2O and close to the graphite-oxygen redox buffer, and 0·5–1·0 wt % H2O and approximately one and half log units above the quartz–fayalite–magnetite redox buffer) demonstrate that clinopyroxene crystals from nominally equilibrium experiments can preserve strongly disequilibrium compositions. The compositional systematics of experimental clinopyroxenes are consistent with the presence of sector zoning. Furthermore, the magnitude of compositional variability increases with decreasing melt ...