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In situ X-ray absorption spectroscopy measurement of vapour-brine fractionation of antimony at hydrothermal conditions

2008; Cambridge University Press; Volume: 72; Issue: 2 Linguagem: Inglês

10.1180/minmag.2008.072.2.667

1471-8022

Gleb S. Pokrovski, J. Roux, Jean‐Louis Hazemann, Anastassia Y. Borisova, Anastasia Gonchar, Mikhail Lemeshko,

Geochemistry and Geologic Mapping

Abstract Despite the growing geological evidence that fluid boiling and vapour-liquid separation affect the distribution of metals in magmatic-hydrothermal systems significantly, there are few experimental data on the chemical status and partitioning of metals in the vapour and liquid phases. Here we report on an in situ measurement, using X-ray absorption fine structure (XAFS) spectroscopy, of antimony speciation and partitioning in the system Sb 2 O 3 -H 2 O-NaCl-HCl at 400°C and pressures 270—300 bar corresponding to the vapour-liquid equilibrium. Experiments were performed using a spectroscopic cell which allows simultaneous determination of the total concentration and atomic environment of the absorbing element (Sb) in each phase. Results show that quantitative vapour-brine separation of a supercritical aqueous salt fluid can be achieved by a controlled decompression and monitoring the X-ray absorbance of the fluid phase. Antimony concentrations in equilibrium with Sb 2 O 3 (cubic, senarmontite) in the coexisting vapour and liquid phases and corresponding Sb III vapour-liquid partitioning coefficients are in agreement with recent data obtained using batch-reactor solubility techniques. The XAFS spectra analysis shows that hydroxy-chloride complexes, probably Sb(OH) 2 Cl 0 , are dominant both in the vapour and liquid phase in a salt-water system at acidic conditions. This first in situ XAFS study of element fractionation between coexisting volatile and dense phases opens new possibilities for systematic investigations of vapour-brine and fluid-melt immiscibility phenomena, avoiding many experimental artifacts common in less direct techniques.