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Halogens and trace elements in subduction zones
(2011)
- This thesis concentrates on solubilities and incorporation mechanisms of halogens and trace elements in minerals and aqueous fluids at high temperatures and pressures. The solubility of fluorine and chlorine in upper mantle minerals (forsterite, enstatite and pyrope) and halogen partitioning between aqueous fluids and these minerals were investigated by piston-cylinder experiments at 1100 °C and 2.6 GPa. Chlorine solubility in forsterite, enstatite and pyrope is below the ppm level, and it is independent of fluid salinity. The fluid-mineral partition coefficient of chlorine is 103-106, indicating extreme incompatibility of chlorine in nominally anhydrous silicates. The fluorine solubility in enstatite and pyrope is two orders of magnitude higher than for Cl, with no dependence on fluid salinity. Forsterite dissolves 246-267 ppm up to a fluid salinity of 1.6 wt. % F. At higher fluorine contents in the system, forsterite is replaced by the minerals of the humite group, which host fluorine in the hydroxyl site. The fluid-mineral partition coefficient of fluorine ranges from 101 to 103. Due to the extreme incompatibility of Cl in a peridotite mineral assemblage, fluid flow from a subducting slab through the mantle wedge will lead to more efficient sequestration of H2O (when compared to Cl) into minerals, thus inducing a gradual increase in the fluid salinity. Mass balance calculations reveal that rock-fluid ratios of (1.3-4)∙103 are required to produce the characteristic Cl/H2O signature of primitive arc magmas. This indicates that fluid flow from subducting slabs into the melting regions in the overlying mantle is not confined to narrow channels but it is sufficient to pervasively metasomatize the bulk wedge. Energetics of fluorine incorporation in forsterite and forsterite-humite chemical equilibria were explored in the system Mg2SiO4-MgF2 by first principles computations. The pressure-volume equations of state and ground-state energies were determined for orthorhombic Mg2SiO4-Mg2F4 solutions, fluorine-bearing end-members of the humite group, and sellaite (MgF2). Humite group minerals and sellaite are energetically more stable than their equivalent solid solution compounds, hence they can act as buffers of fluorine solubility in forsterite. Compressibility increases systematically with the F content for both solid solution compounds and stable minerals. Nevertheless, end member solids are systematically less compressible than the respective solid solution compounds. The pressure-volume equations of state, internal energies, configurational and excess properties were used to set up a thermodynamic model of fluorine solubility in forsterite buffered by humite-group minerals up to 1900 K and 12 GPa. Humite is the stable F buffer in the investigated pressure and temperature range. The fluorine solubility in forsterite increases with temperature, from 0.01 ppm F at 500 K up to 0.33 wt. % F at 1900 K and 0 GPa. By contrast, the effect of pressure on the fluorine solubility is small, leading to its minor decrease as pressure rises to 12 GPa. These results demonstrate that partition coefficients of fluorine between forsterite and aqueous fluid (or silicate melt) are expected to increase with increasing temperature and decreasing pressure. When fluids or melts pass through the mantle wedge, fluorine will most efficiently be stored in the high-temperature portions of the wedge, promoting mantle metasomatism beneath the arc, and it will be released when the metasomatized mantle is advected to colder regions or to higher pressures. The mobility of high field strength elements in aqueous fluids in subduction zones was addressed by in-situ zircon solubility measurements in a hydrothermal diamond anvil cell. The zircon solubilities in aqueous fluids at 865-1025 oC and 6-20 kbar buffered by quartz are very low, ranging from 1.0 to 3.3 ppm Zr, and solubilities weakly increase with temperature and pressure. Experimental results were fitted to a density model: , where c is the Zr concentration in the fluid (ppm), T is temperature (K) and rho is the fluid density (g cm-3). Additional experiments have shown that Zr solubility increases with a decrease in silica activity and with the presence of NaCl and albite due to Zr-Cl or alkali-Zr complexing but it still remains very low. Therefore, the low Zr content observed in arc magmas is due to a very low mobility of Zr in aqueous fluid.
