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), pressur e (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 Na Cl 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 pres sures 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 contai ning 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(C l) is related to the concentration of NaCl in the initial solutions fo r 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 solu tion), where D is in cm2/s. D(Cl) in runs with a 10 wt% HCI solution i s several times higher than with a 10 wt% NaCl solution at the same PT conditions. Furthermore, at higher concentration of NaCl, D(Cl) is lo wer. It was found that in the NaCl-rich series D(Cl) increases very sh arply with the addition of H2O to the glass to 2-3 wt%, further additi on of H2O has a significantly smaller effect. This difference is inter preted as a result of the change in the melt structure. The relationsh ip of D(Cl) and viscosity does not follow the Eyring equation in the h igh temperature runs. The results of this study, combined with other i nvestigations suggest that diffusion rates of volatiles decrease as: C O2>H2O>Cl>F. This indicates that during magma evolution differentiatio n of the volatile constituents may occur.