The mobility of W and Mo in subduction zone fluids and the Mo–W–Th–U systematics of island arc magmas

Abstract

We have studied the solubility of W- and Mo-oxide in aqueous fluids at 2.61GPa, 600 to 800°C at variable controlled oxygen fugacity conditions, relevant to subduction zones. We observed that the solubility of W-oxide is only slightly dependent on fO2 between conditions buffered by Co–CoO and Re–ReO2. In contrast, Mo-oxide solubility is strongly dependent on both oxygen fugacity and fluid salinity under the same conditions. At 2.61GPa, in the fO2 range between Co–CoO and Re–ReO2 their solubility can be described by the following equations:logW=0.07logfO2−4.72×1000/T+4.43andlogMo=0.44logfO2+0.42logNaCl−1.8×1000/T+4.80where W, Mo and NaCl are concentrations in molalities, fO2 is oxygen fugacity and T is temperature in Kelvin. We also studied the solubility of Mo and W in mantle minerals coexisting with Mo- and W-oxide and combined these data with the fluid solubilities to calculate fluid/mineral partition coefficients of W and Mo. Both W and Mo are incompatible in the structure of major eclogite minerals (garnet and clinopyroxene) in the presence of aqueous fluid, but they are compatible in rutile. Thus, the presence or absence of rutile strongly affects the mobility of these elements in subduction zones. From the partition coefficients we calculated the composition of aqueous fluid released from an N-MORB eclogite source. We found that the Mo–Ce–W–Th–U systematics of arc magmas can be modeled by the mixing of depleted mantle with an aqueous fluid phase that was released from a rutile-bearing N-MORB source in fO2-conditions between FMQ-1.6 and FMQ+3 having a maximum salinity of 20wt% NaClequiv. If rutile contents in residual eclogite are independently determined, redox conditions, salinity and amount of fluid added to the source region of melting can be tightly constrained by plotting Mo/Ce, W/Th, and Mo/W vs. U/Th. In particular, the W/Th ratio is a sensitive indicator of the amount of fluid, since W always very strongly partitions into the fluid, nearly independent of oxygen fugacity and salinity. The Mo/W ratio is particularly sensitive to redox conditions, while U/Th reflects both salinity and redox state.