DIFFUSION OF CHLORINE IN GRANITIC MELTS

The chemical diffusivity of Cl in granitic and haplogranitic melts was determined as a function of temperature (650-1400-degrees-C), pressure (1 bar-4.6 kbar), H2O content, and NaCl concentration. Three series of experiments were made: (1) high temperature runs at 1 atm with a NaCl liquid, (2) runs at pressures to 2 kbar with a pure NaCl liquid or NaCl-rich brine, and (3) H2O-rich runs with NaCl/HCI solutions at pressures to 4.6 kbar. Chlorine concentrations were determined by electron microprobe. Chlorine diffusion follows the Arrhenius equation in both high temperature (log (D) = -4.5-4502/T (K)) and H2O-rich runs containing 10 wt% NaCl solution at 2 kbar (log (D) = -2.19-5780/ T (K)). The pressure effect at 850-degrees-C is moderate for both NaCl-rich (log (D) = -8.487-0.125 P) and H2O-rich runs (log (D) = -7.26-0.103 P). D(Cl) is related to the concentration of NaCl in the initial solutions for H2O-rich runs. At 850-degrees-C and 2 kbar, D(Cl) ranges from log (D) = -7.24 (5.8 wt% NaCl solution) to log (D) = -7.59 (20 wt% NaCl solution), where D is in cm2/s. D(Cl) in runs with a 10 wt% HCI solution is several times higher than with a 10 wt% NaCl solution at the same PT conditions. Furthermore, at higher concentration of NaCl, D(Cl) is lower. It was found that in the NaCl-rich series D(Cl) increases very sharply with the addition of H2O to the glass to 2-3 wt%, further addition of H2O has a significantly smaller effect. This difference is interpreted as a result of the change in the melt structure. The relationship of D(Cl) and viscosity does not follow the Eyring equation in the high temperature runs. The results of this study, combined with other investigations suggest that diffusion rates of volatiles decrease as: CO2>H2O>Cl>F. This indicates that during magma evolution differentiation of the volatile constituents may occur.