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3.5 h. An experimental study of the partitioning of halogens (Cl, Br, I) between hydrous fluid and silicate melt (H. Bureau and H. Keppler, in collaboration with N. Métrich/Saclay)

Chlorine is a useful tracer to monitor degassing processes in magmatic systems. Chlorine partitions strongly into hydrous fluids evolving from water-saturated magmas. However, the behaviour of the heavy halogens bromine and iodine in these systems is virtually unknown. This is partly due to their extremely low contents in natural rocks and volcanic glasses, which make their detection difficult, whereas chlorine (100 to 10000 times more abundant than Br and I) is much easier to measure with classic analytical methods such as the electron microprobe. The influence of magmatic degassing or hydrothermal fluid-magma interaction on the fractionation of Cl, Br, and I is still unknown. In order to constrain the behaviour of heavy halogens in magmatic-hydrothermal systems, we have experimentally investigated the partition coefficients of Cl, Br, and I between silicate melts and aqueous fluids. The starting materials were a glass of albitic composition (NaAlSi3O8) and solutions of NaCl, NaI, and NaBr with defined concentrations (from 0.1 to 20 g/l). Powders of glass and equal amounts of solution containing one of the halogens were sealed in Pt capsules. The experiments were run in rapid quench autoclaves at 900°C and 2 kbars for a few days in order to reach equilibrium. Chlorine contents in the quenched glasses were measured by electron microprobe. However, the concentrations of Br and I turned out to be below the detection limit under the same measurement conditions. Therefore, Br and I are measured using a nuclear microprobe by PIXE (Proton-Induced X-ray Emission) in Lab. P. Süe, Saclay, France. Preliminary tests demonstrated that it is possible to detect a few ppm of Br with an incident proton energy of 3.5 MeV. The concentrations in the fluid were obtained by mass balance and fluid/melt partition coefficients D were calculated according to the equation D = cfluid/cmelt, where c is the concentration of Cl, Br and I. First preliminary results give a Br glass content in the range 20-30 ppm in equilibrium with an aqueous fluid containing 0.5 g/l of NaBr. The resulting partition coefficient is about 18.8 ± 5.

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