Trace element partitioning in basaltic systems as a function of oxygen fugacity

Along with temperature, pressure and melt chemistry, magmatic oxygen fugacity (fO₂) has an important influence on liquid and solid differentiation trends and melt structure. To explore the effect of redox conditions on mineral stability and mineral-melt partitioning in basaltic systems we performed equilibrium, one-atmosphere experiments on a picrite at 1200-1110 °C with fO₂ ranging from NNO-4 log units to air. Clinopyroxene crystallizes from 1180 °C to near-solidus, along with plagioclase, olivine and spinel. Olivine Mg# increases with increasing fO₂, eventually reacting to pigeonite. Spinel is absent under strongly reducing conditions. Mineral-melt partition coefficients (D) of redox-sensitive elements (Cr, Eu, V, Fe) vary systematically with fO₂ and, in some cases, temperature (e.g. D_Cr in clinopyroxene). Clinopyroxene sector zoning is common; sectors along a- and b-axes have higher Al^IV, Al^VI, Cr and Ti and lower Mg than c-axis sectors. In terms of coupled substitutions, clinopyroxene CaTs (MgSi = Al^VIAl^IV) prevails under oxidized conditions (≥ NNO), where Fe³⁺ balances the charge, but is limited under reduced conditions. Overall, Al^IV is maximised under high temperature, oxidizing conditions and in slowly grown (a-b) sectors. High Al^IV facilitates incorporation of REE (REEAl^IV = CaSi), but D (except D_Eu) show no systematic dependence on fO₂ across the experimental suite. In sector zoned clinopyroxenes enrichment in REE³⁺ in Al-rich sectors is quantitatively consistent with the greater availability of suitably-charged M2 lattice sites and the electrostatic energy penalty required to insert REE³⁺ onto unsuitably-charged M2 sites. By combining our experimental results with published data, we explore the potential for trace element oxybarometry. We show that olivine-melt D_V, clinopyroxene-melt D_V/D_Sc and plagioclase-melt D_Eu/D_Sr all have potential as oxybarometers and we present expressions for these as a function of fO₂ relative to NNO. The crystal chemical sensitivity of heterovalent cation incorporation into clinopyroxene and the melt compositional sensitivity of the Eu²⁺-Eu³⁺ redox potential limit the use of clinopyroxene-melt and plagioclase-melt, however, olivine-melt D_V affords considerable precision and accuracy as an oxybarometer that is independent of temperature, and crystal and melt composition. Variation of D_V and D_V/D_Sc with fO₂ for olivine and clinopyroxene contains information on redox speciation of V in coexisting melt. By comparing the redox speciation constraints from partitioning to data from Fe-free synthetic systems and XANES spectroscopy of quenched glasses, we show that homogenous equilibria involving Fe and V species modify V speciation on quench, leading to a net overall reduction in the average vanadium valence. Mineral-melt partitioning of polyvalent species can be a useful probe of redox speciation in Fe-bearing systems that is unaffected by quench effects.

The online version contains supplementary material available at 10.1007/s00410-023-02069-x.