Calibration of an Empirical Thermometer and Oxybarometer based on the Partitioning of Sc, Y and V between Olivine and Silicate Melt

Author： Mallmann Guilherme

Publisher： Oxford University Press

ISSN： 1460-2415

Source： Journal of Petrology, Vol.54, Iss.5, 2013-05, pp. : 933-949

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Abstract

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 quartzfayalitemagnetite (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, AlO₁₅, NaO₀₅ and KO₀₅), and allows oxygen fugacity determinations to within 025 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 15C, but precision is strongly dependent on the Sc and Y measurements in olivine and melt.