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

Unlabelled: Along with temperature, pressure and melt chemistry, magmatic oxygen fugacity (O) 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 O 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 O, eventually reacting to pigeonite. Spinel is absent under strongly reducing conditions. Mineral-melt partition coefficients () of redox-sensitive elements (Cr, Eu, V, Fe) vary systematically with O and, in some cases, temperature (e.g. in clinopyroxene). Clinopyroxene sector zoning is common; sectors along a- and b-axes have higher Al, Al, Cr and Ti and lower Mg than c-axis sectors. In terms of coupled substitutions, clinopyroxene CaTs (MgSi = AlAl) prevails under oxidized conditions (≥ NNO), where Fe balances the charge, but is limited under reduced conditions. Overall, Al is maximised under high temperature, oxidizing conditions and in slowly grown (a-b) sectors. High Al facilitates incorporation of REE (REEAl = CaSi), but (except ) show no systematic dependence on O across the experimental suite. In sector zoned clinopyroxenes enrichment in REE 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 REE 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 , clinopyroxene-melt / and plagioclase-melt / all have potential as oxybarometers and we present expressions for these as a function of O relative to NNO. The crystal chemical sensitivity of heterovalent cation incorporation into clinopyroxene and the melt compositional sensitivity of the Eu-Eu redox potential limit the use of clinopyroxene-melt and plagioclase-melt, however, olivine-melt affords considerable precision and accuracy as an oxybarometer that is independent of temperature, and crystal and melt composition. Variation of and / with O 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.

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