Comparative geochemistry of rhenium in oxidized arc magmas and MORB and rhenium partitioning during magmatic differentiation

The geochemical behavior of Re in oxidized arc magmas (Eastern Manus basin) and MORB has been reexamined. In the early differentiation stage of oxidized arc magmas, Yb, Cu, Au, Ag, and Re have a bulk partition coefficient (D) between the crystallized mineral assemblage (olivine, clinopyroxene, and plagioclase) and the residual melt on the order of 0.33 = DYb ~ DAu > DRe > DAg > DCu, while in the late stage they have a bulk D between the crystallized mineral assemblage (clinopyroxene, plagioclase, magnetite, and monosulfide solid solution — MSS) and the residual melt on the order DRe > DCu > DAu ≥ 1 > DYb = 0.29 > DAg. In oxidized arc magmas, Cu, Au, and Ag are primarily controlled by crystalline MSS, whereas Re is primarily controlled by magnetite and it has a MSS/silicate melt partition coefficient significantly lower than 20. Unlike oxidized arc magmas, most MORB are saturated with sulfide liquid. Based on the positive correlation of log (DCu / DRe) and ∆FMQ regardless of fS2, log (DCu / DRe) = 0.997ΔFMQ + 0.181 (R2 = 0.99) (D = partition coefficient between sulfide liquid and silicate melt; FMQ = fayalite–magnetite–quartz oxygen buffer), the partition coefficient of Re between sulfide liquid and silicate melt for MORB is estimated to be in the range of 600–10,000 at fO2 of FMQ to FMQ − 1, increasing with decreasing fO2. These partitioning data constrain that during MORB differentiation Re may be as compatible as Cu or Ag at relatively oxidized conditions (FMQ − 0.5 < fO2 < FMQ) or as Au at relatively reduced conditions (FMQ − 1 < fO2 < FMQ − 0.5), and its concentration in MORB maybe expected to decrease with decreasing MgO in a similar manner as Cu, Ag, or Au. Yb is an incompatible element during MORB differentiation. Consequently, during MORB differentiation Yb/Re ratio may be expected to increase with decreasing MgO. However, the previously observed nearly constant Yb/Re ratio in a suite of MORB indicates similar incompatibility of Yb and Re and thus a minor role of sulfide liquid in controlling the geochemical behavior of Re during MORB differentiation. More work is required to resolve this paradox.