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3.4 c. Salty supercritical fluids in subduction zones (H. Bureau)

A wide range of silicate melt-water systems show complete miscibility at the conditions corresponding to the deeper part of the upper mantle. This means that with increasing pressure and temperature, the solubility of water in silicate melts and the solubility of silicate in a coexisting hydrous fluid increase, until the compositions of the two phases become equal. This has major consequences for the phase relationships in subduction zones where aqueous fluids are released in large amounts by slab dehydration. It is generally accepted that most of the slab water is returned to the atmosphere by island arc magmas. A second important constituent of slab-released fluids is chlorine. Since Cl substitutes for OH groups in minerals, it is probably also released from subducted oceanic crust as pressure and temperature increase.

The purpose of this study was to evaluate the potential effect of chlorine on the stability field of supercritical fluids in the pressure and temperature conditions of the upper mantle. Experiments were carried using a Bassett-type externally heated diamond-anvil cell. Supercritical phenomena were observed "in-situ" at high pressure and temperature using a microscope. Starting materials were synthetic compositions of iron-free silicate melts (Cl-free albite, Cl-bearing albite and Cl-free haplogranite). For each experiment, the sample chamber was filled with pieces of glass, a solution (pure H2O or H2O-NaCl mixtures), and an air bubble. The aqueous solutions were the pressure media.

The results show good agreement between experiments where chlorine was originally added to the glass and others where it was originally dissolved in the fluid. It appears that adding Cl to the system affects the stability field of supercritical fluids. For a given temperature, the supercritical behavior occurs at higher pressure, which means deeper in the upper mantle. Even small quantities of Cl (1 g/kg of NaCl in the aqueous fluid, or about 700 ppm Cl in the melt) significantly change the critical curve. It appears therefore, that even small amounts of chloride in the system may have large effects on the phase relationships and the processes of magma generation in subduction zones.

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