Solubility of carbonate in aqueous fluids at HP-HT: a combined X-ray and Raman spectroscopic study

Isabelle Daniel

Universite Lyon1

The increase of P and T in the subducting oceanic lithosphere leads to devolatilization reactions releasing C-O-H-Si bearing fluids. The composition of natural HP fluid inclusions indicates that H2O is the main solvent phase. The increase of their CO2 content as a function of depth is interpreted as a result of the progressive decarbonation of the slab. Consequently, the solubility of carbonate minerals at P-T conditions of subduction is a key for understanding the fate of incoming C in the deep carbon cycle. We report here in situ measurements of the solubility of strontianite SrCO3 as a carbonate model, at P up to 8 GPa and T up to 500 °C. A single crystal of strontianite and milli-Q water were loaded in an externally heated diamond anvil cell, in a sample chamber lined with platinum. The total amount of dissolved Sr was measured in situ after the intensity of the Kα X-ray fluorescence line of Sr. Experiments were carried out at the ESRF (ID22 beamline) using a bright monochromatic beam (19 keV, ca. 1011ph/s). The relative amount of dissolved carbonate and bicarbonate was measured by Raman spectroscopy in our lab. with a confocal LabRamHR800 vis Jobin Yvon Horiba. The latter new Raman data show that bicarbonate is the most abundant species in low-pressure fluids whereas carbonate becomes progressively dominant at high pressure. Taking into account activity coefficients, measured concentrations of dissolved Sr and measured carbonate-bicarbonate ratios in the aqueous fluid at equilibrium with a crystal, the solubility constant of SrCO3 could be assessed at pressure conditions relevant to the upper mantle.