Uranium and thorium solubilities in subduction zone fluids

Abstract

Uranium is enriched in depleted island arc magmas more than would be anticipated, due to the overall enrichment in large ion lithophile elements. To attempt to understand this enrichment calculations were performed to establish concentrations of uranium and thorium in fluids under the pressure and temperatures experienced in a subduction zone. The solubility and speciation of uranium(VI), uranium(IV) and thorium(IV) were determined along the pressure-temperature profile of a subduction slab, using a density model for water. The presence of hydroxide, carbonate, fluoride and phosphate ligands were investigated. Results suggest that only hydroxide and carbonate complexing are important. The uranium(VI) carbonate species has a higher solubility, by over 10 orders of magnitude, than all other hydroxide and carbonate species. Thorium(IV) solubility is extremely low at all pressures and temperatures. With increasing pressure and temperature the uranium(IV) hydroxide species become more soluble than the uranium(VI) carbonate species, when only molecular species are considered. Calculations assuming mantle wedge enrichment of a 1 km high column over 10 7 yr, indicate that enrichment from pure H 2O fluids could be of the order of 1 part per billion (ppb) for uranium(IV) and 1000 ppb for uranium(VI). Average primitive mantle contains 18 ppb uranium and 64 ppb thorium. Fluid addition of uranium can, therefore, increase U/Th ratios into the range observed in island arcs. It therefore appears most probable that uranium is enriched by the action of H 2O-rich fluids, derived from the dehydration of minerals in the subduction slab. Carbonate-rich fluids can enrich uranium over thorium but the enrichment produced in the fluids by molecular species is too small to account for the observed uranium concentrations in island arc magmas, unless ionic species or very high water/rock ratios are anticipated.