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 crystallisation 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 synthesised during high-pressure, high-temperature experiments on the same lava in order to investigate clinopyroxene–liquid equilibria in tholeiitic systems and optimising geothermobarometric strategies. Natural clinopyroxenes from the 2014–2015 Holuhraun lava are sector zoned, with hourglass sectors being enriched in enstatite-ferrosillite component at the expense of { 111} all other components with respect to prism sectors. In contrast with observations on {ℎ푘0} clinopyroxenes from alkalic systems, sector zoning in clinopyroxenes from the 2014–2015 Holuhraun lava is characterised by differences in Ca and Na contents as well as in Ti and Al contents. The products of crystallisation experiments performed at 100–600 MPa and 1140–
1220 °C on a powdered starting glass at two suites 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 H2O contents and increasing deviations of experimental temperatures below clinopyroxene liquidus temperatures (i.e. degrees of undercooling sensu lato), indicating that kinetic processes play a key role in controlling clinopyroxene compositions under even notionally equilibrium conditions. Few published analyses of experimental clinopyroxene crystals may thus represent truly equilibrium compositions. Stoichiometric calculations on natural and
experimental clinopyroxenes show that Fe3+ is a major constituent of clinopyroxenes from tholeiitic magmas under naturally relevant oxygen fugacity conditions. They also show that Fe3+ is most likely incorporated as Ca- and Al- bearing Ca-Fe-Tschermak’s component rather than Na-bearing aegirine component at oxygen fugacities up to one and half log units above
the quartz–fayalite–magnetite buffer. Elevated oxygen fugacities are thus less likely to compromise clinopyroxene–liquid geothermobarometry than previously thought. Guided by our experimental results, we combined published descriptions of clinopyroxene–liquid equilibria with geothermobarometric equations to develop an internally consistent and widely applicable method for performing geothermobarometry on tholeiitic magmas that does not require equilibrium zones to be selected a priori. Applying this method to natural
clinopyroxene crystals from the 2014–2015 Holuhraun lava that formed under low but variable degrees of undercooling (perhaps 25 °C or less) returns values in excellent agreement with those from independent methods (232±86 MPa, 1161±11 °C). Robust estimates of magma storage conditions can thus be obtained by performing clinopyroxene–liquid geothermobarometry on tholeiitic magmas when disequilibrium is suitably accounted for.
1220 °C on a powdered starting glass at two suites 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 H2O contents and increasing deviations of experimental temperatures below clinopyroxene liquidus temperatures (i.e. degrees of undercooling sensu lato), indicating that kinetic processes play a key role in controlling clinopyroxene compositions under even notionally equilibrium conditions. Few published analyses of experimental clinopyroxene crystals may thus represent truly equilibrium compositions. Stoichiometric calculations on natural and
experimental clinopyroxenes show that Fe3+ is a major constituent of clinopyroxenes from tholeiitic magmas under naturally relevant oxygen fugacity conditions. They also show that Fe3+ is most likely incorporated as Ca- and Al- bearing Ca-Fe-Tschermak’s component rather than Na-bearing aegirine component at oxygen fugacities up to one and half log units above
the quartz–fayalite–magnetite buffer. Elevated oxygen fugacities are thus less likely to compromise clinopyroxene–liquid geothermobarometry than previously thought. Guided by our experimental results, we combined published descriptions of clinopyroxene–liquid equilibria with geothermobarometric equations to develop an internally consistent and widely applicable method for performing geothermobarometry on tholeiitic magmas that does not require equilibrium zones to be selected a priori. Applying this method to natural
clinopyroxene crystals from the 2014–2015 Holuhraun lava that formed under low but variable degrees of undercooling (perhaps 25 °C or less) returns values in excellent agreement with those from independent methods (232±86 MPa, 1161±11 °C). Robust estimates of magma storage conditions can thus be obtained by performing clinopyroxene–liquid geothermobarometry on tholeiitic magmas when disequilibrium is suitably accounted for.
Original language | English |
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Journal | Journal of Petrology |
Early online date | 10 Sept 2019 |
DOIs | |
Publication status | Published - 10 Sept 2019 |
Keywords
- geothermobarometry
- clinopyroxene
- tholeiite
- experimental petrology
- Iceland
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Petrology and volcanology
Burton, M. (PI), Hartley, M. (PI), Mccormick Kilbride, B. (PI), Mitchell, N. (PI), Neave, D. (PI), Pawley, A. (PI), Polacci, M. (PI), Biagioli, E. (Researcher), Bonechi, B. (Researcher), Buso, R. (Researcher), Davies, B. (Researcher), Esse, B. (Researcher), Bronziet, J. (PGR student), Delbrel, J. (PGR student), Höhn, M. (PGR student), Kember, A. (PGR student), Pardo Cofrades, A. (PGR student), Sen, R. (PGR student), Stewart, A. (PGR student) & Subbaraman, R. (PGR student)
Project: Research