Garnet and spinel oxybarometers: New internally consistent multi-equilibria models with applications to the oxidation state of the lithospheric mantle
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https://academic.oup.com/petrology/article/57/6/1199/2375399?searchresult=1
https://academic.oup.com/petrology/article/57/6/1199/2375399?searchresult=1
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New thermodynamic data for skiagite garnet (Fe 3 Fe 2 3+ Si 3 O 12 ) are derived from experimental phase-equilibrium data that extend to 10 GPa and are applied to oxybarometry of mantle peridotites using a revised six-component garnet mixing model. Skiagite is more stable by 12 kJ mol -1 than in a previous calibration of the equilibrium 2 skiagite = 4 fayalite + ferrosilite + O 2 , and this leads to calculated oxygen fugacities that are higher (more oxidized) by around 1-1.5 log fO 2 units. A new calculation method and computer program incorporates four independent oxybarometers (including 2 pyrope + 2 andradite + 2 ferrosilite = 2 grossular + 4 fayalite + 3 enstatite + O 2 ) for use on natural peridotite samples to yield optimum log fO 2 estimates by the method of least squares. These estimates should be more robust than those based on any single barometer and allow assessment of possible disequilibrium in assemblages. A new set of independent oxybarometers for spinel-bearing peridotites is also presented here, including a new reaction 2 magnetite + 3 enstatite = 3 fayalite + 3 forsterite + O 2 . These recalibrations combined with internally consistent PT determinations for published analyses of mantle peridotites with analysed Fe 2 O 3 data for garnets, from both cratonic (Kaapvaal, Siberia and Slave) and circumcratonic (Baikal Rift) regions, provide revised estimates of oxidation state in the lithospheric mantle. Estimates of log fO 2 for spinel assemblages are more reduced than those based on earlier calibrations, whereas garnet-bearing assemblages are more oxidized. Importantly, this lessens considerably the difference between garnet and spinel oxybarometry that was observed with previous published calibrations.
This work was supported by a Natural Environment Research Council studentship [NE/L501578/1] to W. Miller.
This work was supported by a Natural Environment Research Council studentship [NE/L501578/1] to W. Miller.