2013/02/06 by Guilherme Mallmann, Hugh O’Neill, Hugh St.C. O’Neill · 159 citations
Chemistry · Earth and Planetary Sciences · #Basalt #Chemistry #Fayalite #Fugacity #Geochemistry #Geological and Geochemical Analysis #Geology #High-pressure geophysics and materials #Inorganic chemistry #Magma #Mantle (geology) #Melt inclusions #Mineral redox buffer #Mineralogy #Olivine #Oxygen #Phenocryst #Physical chemistry #Redox #Silicate #Volcanic rock #Volcano #earthquake and tectonic studies
paper · pdf · doi:10.1093/petrology/egt001
published in Journal of Petrology 54(5), 933-949 (Oxford University Press)
openalex publication_date 2013/02/06 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
The evolution of basaltic magmas depends on their redox state, hence oxygen fugacity, but there is increasing evidence that this intensive thermodynamic variable may be less well understood in basalts than commonly supposed. The redox state of terrestrial basalts has to a large extent been inferred from the Fe3+/Fe2+ ratios of their quenched glasses. However, this quantity appears to be significantly affected during late and post-eruptive processes in magmatic systems (e.g. by degassing, charge-transfer reactions of redox-variable species, and alteration), so that the degree to which the Fe3+/Fe2+ ratios preserved in basaltic glasses reflect the oxidation state of the magma at high temperature is unclear. Because olivine is the first silicate mineral to crystallize from primitive basaltic liquids on cooling following decompression, the equilibrium partitioning relations preserved in olivine phenocrysts in basalts are, in principle, less disturbed by these late and post-eruptive processes and, therefore, may better reflect the high-temperature (pre-eruptive) conditions of the magma. Here we calibrate an oxybarometer based on the strong sensitivity of the partitioning of vanadium between olivine and silicate melt to oxygen fugacity. Our empirical parameterization, calibrated over a range of redox conditions between four log10 units above and below the quartz–fayalite–magnetite (QFM) oxygen buffer, takes into account the effects of temperature, olivine composition (i.e. Mg/Fe ratios) and melt composition (namely the activities of CaO, SiO2, AlO1·5, NaO0·5 and KO0·5), and allows oxygen fugacity determinations to within ∼0·25 log10 units. We also explore the sensitivity of the exchange partitioning of Sc and Y between olivine and melt to temperature as a geothermometer. Our calibration indicates that this geothermometer allows temperature to be estimated to within 15°C, but precision is strongly dependent on the Sc and Y measurements in olivine and melt.