The mobility of U and Th in subduction zone fluids: An indicator of oxygen fugacity and fluid salinity

The solubility of U and Th in aqueous solutions at P-T-conditions relevant for subduction zones was studied by trapping uraninite or thorite saturated fluids as synthetic fluid inclusions in quartz and analyzing their composition by Laser Ablation-ICPMS. Uranium is virtually insoluble in aqueous fluids at Fe-FeO buffer conditions, whereas its solubility increases both with oxygen fugacity and with salinity to 960 ppm at 26.1 kbar, Re-ReO₂ buffer conditions and 14.1 wt% NaCl in the fluid. At 26.1 kbar and 800°C, uranium solubility can be reproduced by the equation: log U = 2.681 + 0.1433logfO₂ + 0.594C1, where fO₂ is the oxygen fugacity, and Cl is the chlorine content of the fluid in molality. In contrast, Th solubility is generally low (<10 ppm) and independent of oxygen fugacity or fluid salinity. The solubility of U and Th in clinopyroxene in equilibrium with uraninite and thorite was found to be in the order of 10 ppm. Calculated fluid/cpx partition coefficients of Th are close to unity for all conditions. In contrast, D^fluid/cpx for uranium increases strongly both with oxygen fugacity and with salinity. We show that reducing or NaCl-free fluids cannot produce primitive arc magmas with U/Th ratio higher than MORB. However, the dissolution of several wt% of oxidized, saline fluids in arc melts can produce U/Th ratios several times higher than in MORB. We suggest that observed U/Th ratios in arc magmas provide tight constraints on both the salinity and the oxidation state of subduction zone fluids.