The Effect of Chlorine on the Rheology of Na2O-Fe2O3-Al2O3-SiO2 Melts

Because of the high fluid mobility of chlorine, the mantle wedge in subduction zone settings may be enriched in chlorine by the fluids released by the subducted altered oceanic crust plus sediments. The presence of chlorine (and other halogens) will affect the solidus temperature and the rheology of melts, thus influencing the magma evolution, eruption and degassing. Very little is known about the solubility mechanism and diffusivity of Cl in silicate melts and even less is known about the effect of Cl on viscosity. The present study addresses the effect of the halogen chlorine on the viscosity of silicate melts. The shear viscosities of Cl-bearing melts in the system Na₂O-Fe₂O₃-Al₂O₃-SiO₂ were determined over the temperature range 550-950C at room pressure in air. Viscosities were determined using the micropenetration technique in the range of 10^8.5 to 10^12.0 Pa s. The compositions are based on addition of Fe₂O₃ or FeCl₃ to aluminosilicate glasses with a fixed amount of SiO₂ (67 mol %). Although there was loss of Cl^- during the glass syntheses, no loss occurred during the viscometry experiments. It is to be expected that Cl^- takes the structural position of an oxygen, and thus reduces the polymerization of the melt structure, and therefore the viscosity of the melt; as F^- does. Our measurements show that, depending upon the melt composition, the addition of Cl^- will either increase or decrease the viscosity of the melt. In the present melts at least 20% of the iron exists as network modifying, viscosity reducing Fe²⁺; while the rest exists as network forming Fe³⁺. It is proposed here that the different effects of chlorine on viscosity are due to the preferred Cl^--Fe²⁺_NBO bonding together with the different structure of peralkaline and peraluminous melts. In peralkaline aluminosilicate melts, the addition of Cl₂O_-1 will destroy 2 NBOs and create one BO if Cl^- bonds primarily to the Fe²⁺ creating non-bridging oxygens. This would result in an increase in viscosity. In peraluminous melts, the addition of Cl₂O_-1 may result in Cl^- bonds to the charge-balancing Fe²⁺, creating 2 new tri-clusters (assuming (Al³⁺, Fe³⁺)Si₂O₅ tri-clusters exist). The viscosity should then decrease due to the weaker bonds of the charge balancing Fe²⁺-Cl^- units. The preference of Cl^- to form bonds to the NBO-forming Fe²⁺ is indicated by the small amount of Cl^- soluble in the peraluminous melt structure in comparison to that soluble in the peralkaline structure.