Other
Scientific paper
Jan 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58..113b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 1, pp.113-123
Other
6
Scientific paper
The chemical diffusivity of Cl in granitic and haplogranitic melts was determined as a function of temperature (650-1400°C), pressure (1 bar-4.6 kbar), H 2 O 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) H 2 O-rich runs with NaCl/HCl 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 H 2 O-rich runs containing 10 wt% NaCl solution at 2 kbar (log ( D ) = -2.19-5780/ T (K)). The pressure effect at 850°C is moderate for both NaCl-rich (log ( D ) = -8.487-0.125 P ) and H 2 O - rich runs (log ( D ) = -7.26-0.103 P ). D Cl is related to the concentration of NaCl in the initial solutions for H 2 O-rich runs. At 850°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 cm 2 /s. D Cl in runs with a 10 wt% HCl 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 H 2 O to the glass to 2-3 wt%, further addition of H 2 O 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: CO 2 >H 2 O>Cl>F. This indicates that during magma evolution differentiation of the volatile constituents may occur.
Bai T. B.
Koster van Groos A. F.
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