Kinetic aspects of major and trace element partitioning between olivine and melt during solidification of terrestrial basaltic materials

Olivine is an important mineral phase in naturally cooled basaltic rocks. The morphology and composition of olivine are controlled by the cooling kinetics, and the mechanisms controlling the crystal growth directly impact the partitioning of elements. Cations partitioning between two phases may document on the crystallization conditions of rocks. Indeed, the major elements composing a mineral usually affect the chemical equilibrium of a system, while trace elements are sensitive to thermodynamic conditions and record the chemical reactions occurring in the system without modifying the bulk reactions. Therefore, major and trace elements partitioning between olivine and melt is greatly considered by petrologists investigating terrestrial rocks through the use of phases exchange reactions. In this study I explore the partitioning of major and trace elements between olivine and basaltic melt under conditions encountered by magmas during natural solidification path, and the cation substitution and charge balance mechanisms controlling the cations entrance in the lattice site of olivine crystals. In this context, I have performed undercooling (-ΔT) and cooling rate (CR) experiments under relatively reduced environment (QFM-2) and atmospheric conditions using a tholeiitic basalt from Hawaii. Experiments started from the same superliquidus temperature of 1250 °C and was cooled at the rates of 4, 20, and 60 °C/h to the final temperature of 1175 and 1125 °C (-ΔT = 35 and 85 °C, respectively). The olivine textural results indicate equilibrium at -ΔT = 35 °C, whereas strong disequilibrium occur at -ΔT = 85 °C. It is verified by the determination of the Fe-Mg exchange between olivine and melt. Indeed, low -ΔT experiments show bulk chemical equilibrium, while local equilibrium occurs at higher -ΔT. The forsterite content decreases as CR increases and a diffusive boundary layer develops in the melt next to the crystal interface. However, the principal cations (Mg, Fe, Mn, and Ca) enter the crystal lattice (M-site) at near-equilibrium, which is compatible with the establishment of a local equilibrium. Ti, Al, and P cations are incorporated in olivine lattice by substituting Si in the T-site. Ti incorporation is controlled by a homovalent substitution [TSi4+] ↔ [TTi4+], while heterovalent substitutions occur for Al and P, following [MMg2+, TSi4+] ↔ [MAl3+, TAl3+], and [2 TSi4+] ↔ [TP5+, TAl3+]. Although Cr is an octahedrally coordinated cation, it shows the same behaviour as cations entering in the T-site. Indeed, Cr is incorporated in the olivine lattice by forming a coupled substitution with a tetrahedrally coordinated cation [MMg2+, TSi4+] ↔ [MCr3+, TAl3+], in order to maintain the charge balance. To preserve the charge balance, disequilibrium incorporation of minor elements is governed by the same mechanisms that occur under equilibrium crystallization, which is consistent with interface local equilibrium.