Compositional evolution of slab-derived fluids during ascent: implications from trace-element partition between hydrous melts and Cl-free or Cl-rich aqueous fluids

Slab-derived supercritical liquids separate into aqueous fluids and hydrous melts during their migration. Separated aqueous fluids further release melt components that cannot be dissolved during ascent. During these processes, elemental partitioning occurs, which may contribute to the geochemical evolution of subduction-zone fluids. Here, we report new experimental results of partition coefficients between a hydrous dacitic melt and Cl-free or Cl-rich aqueous fluids (D^fluid/melt) for 26 elements at a temperature of 1100°C and pressures of 0.3 and 0.7 GPa using internally-heated pressure vessels. Our results reveal that high-field strength elements (HFSE), except Th, are hardly partitioned into aqueous fluids, regardless of their salinity and pressure conditions. In contrast, the partitioning of other elements varies depending on the fluid salinity. D^fluid/melt of large-ion lithophile elements (LILE) and U increases with salinity, whereas that of rare earth elements (REE) and Th decreases. These results predict that slab-derived aqueous fluids can evolve to become richer in LILE and U and poorer in HFSE and REE by separating melt components, which explains the LILE- and U-rich and HFSE- and REE-poor characteristics of subduction-zone magmas. This also explains the higher LILE/HFSE and LILE/REE ratios in frontal-arc basalts than in rear-arc basalts: frontal-arc basalts can be generated by the addition of aqueous fluids that sufficiently separate the melt components at shallower depths, whereas rear-arc basalts are generated by the addition of supercritical liquids or aqueous fluid that insufficiently separate the melt components at greater depths. Such separation of melt components from ascending slab-derived fluid can determine the geochemical signature and across-arc compositional variation of subduction-zone magmas.