Trace element partitioning in basaltic systems as a function of oxygen fugacity
J Leuthold1,2, J Blundy3, P Ulmer1
1Department of Earth Sciences, ETH Zürich, Clausiusstrasse 25, 8092 Zurich, Switzerland.
Contributions to Mineralogy and Petrology. Beitrage Zur Mineralogie Und Petrologie
|April 15, 2024
Summary
Magmatic oxygen fugacity (fO2) significantly impacts basaltic systems. Olivine-melt V partition coefficients (DV) offer a precise, temperature-independent oxybarometer for determining redox conditions in magmas.
Area of Science:
- Geochemistry
- Petrology
- Experimental Mineralogy
Background:
- Magmatic oxygen fugacity (fO2) is a critical parameter influencing differentiation and melt structure alongside temperature, pressure, and melt chemistry.
- Understanding redox conditions is essential for interpreting mineral stability and mineral-melt partitioning in igneous systems.
Purpose of the Study:
- To investigate the influence of varying oxygen fugacity (fO2) on mineral stability and mineral-melt partitioning in basaltic systems.
- To evaluate the potential of trace element partitioning as an oxybarometer for magmatic systems.
Main Methods:
- Equilibrium, one-atmosphere experiments were conducted on picrite at 1200-1110 °C across a range of fO2 conditions (NNO-4 to air).
- Analysis of mineral-melt partition coefficients (D) for redox-sensitive elements (Cr, Eu, V, Fe).
- Evaluation of clinopyroxene sector zoning and coupled substitutions.
- Integration of experimental results with published data to develop oxybarometry expressions.
Main Results:
- Clinopyroxene, plagioclase, olivine, and spinel crystallize under varying conditions; spinel is absent under reducing conditions.
- Olivine Mg# increases with fO2, and mineral-melt partition coefficients for redox-sensitive elements systematically vary with fO2 and temperature.
- Clinopyroxene exhibits sector zoning, with distinct compositional differences related to crystallographic orientation and fO2.
- Olivine-melt DV, clinopyroxene-melt DV/DSc, and plagioclase-melt DEu/DSr show potential as oxybarometers.
- Olivine-melt DV is identified as a precise and accurate oxybarometer, independent of temperature, crystal, and melt composition.
- Vanadium speciation in melts is influenced by Fe-V equilibria, with quench effects leading to an overall reduction in average vanadium valence.
Conclusions:
- Mineral-melt partitioning of polyvalent species provides a valuable probe of redox speciation in Fe-bearing magmatic systems, unaffected by quench effects.
- Olivine-melt DV offers a robust method for determining magmatic oxygen fugacity (fO2) with high precision and accuracy.
- The study provides quantitative expressions for trace element oxybarometry, enhancing our understanding of redox processes in basaltic magmas.
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