Constraints on the Ability of Cl- and F- Bearing Aqueous Fluid to Dissolve and Transport Trace Elements (Y, Nb, Zr) in Subduction Zone Environments.

Abstract

Geologic source regions are often characterized by their trace element concentrations. For example, arc magmas show a depletion in high field strength elements (HFSE) relative to mid ocean ridge basalts. This depletion has been the topic of many debates, as arc petrologists largely attribute this to residual minerals (e.g., rutile) in the source sequestering these elements, while metamorphic petrologists studying exhumed terrains find evidence of trace element mobility. In addition, previously published experimental studies of the partitioning of trace elements involving aqueous fluids are often conducted at pressure and temperature conditions not appropriate for slab-mantle wedge conditions and are extrapolated to higher pressures (>2 GPa) and lower temperatures (<700C). Therefore, constraining mass transfer of the rare earth elements (REE) and HFSE from subducted oceanic crust and metasediments to the mantle wedge is fundamental toward interpreting and modeling processes that affect trace element mobility in subduction zone environments. This dissertation discusses the experimental techniques, results and application, to quantitatively determine trace element concentrations and mineral solubilities in aqueous fluids at geologically relevant conditions (0.5-6 GPa and 250-900C). The effect of multicomponent aqueous fluids (e.g., water + Cl, F, K, Na) on the solubility of xenotime and rutile are investigated. Experimental methods to achieve pressure and temperature include a hydrothermal diamond anvil cell in conjunction with synchrotron x-ray fluorescence to probe the fluid directly, and traditional mineral mass loss experiments using a piston cylinder apparatus. The results indicate that fluid composition exerts the greatest control on xenotime and rutile solubility in aqueous fluids, and that increasing temperature has a positive, albeit less pronounced, effect. The results are consistent with field studies of exhumed terranes, which demonstrate that normally fluid immobile minerals and elements (HFSE and REE) are mobile over short (e.g., mm to m), and possibly long (e.g., tens of m) distances. Given that each arc has a slightly different rate, angle, and sediment deposit during subduction; the order of magnitude variation of HFSE and REE from arc rocks may likely be a function the composition of fluids evolved during prograde metamorphism.