Chromium mobility in hydrous fluids at upper mantle conditions

Chromium is a minor component in the earthʹs mantle and a trace component in the continental crust and is considered to be immobile in aqueous fluid under crustal and most mantle conditions. Exceptions to this are K-rich diamond forming fluids, that can be shown to contain up to 1 wt.% Cr2O3 (on a volatile free basis). A series of high-P experiments on the solubility of Cr2O3 in KCl-bearing water were carried out using rocking multi anvil diamond trap techniques (1000 and 1200 °C; 4 and 6 GPa) combined with cryogenic LA-ICP-MS in order to determine Cr mobility in saline fluids (e.g., diamond forming fluids) under mantle conditions and the Cr bearing fluid possible involvement in deep metasomatism. Chromium solubility in KCl-bearing water increases with salinity (KCl). At 250 μg/g KCl, Cr solubility in water is between 100 and 200 μg/g, independent of pressure. At 4 GPa, an order of magnitude increase in Cr solubility is observed only for the 3 wt.% KCl solution, while at 6 GPa a similar increase is observed at 1.3 wt.% KCl. No significant effect of temperature is observed. Metasomatic Cr-rich mineral assemblages are encountered in both subduction zone and deep lithospheric mantle environments, where alkali-rich hydrous fluids, similar to those examined in this research, are regarded as major metasomatic agents. In some cases, unique minerals found as micro- and macro-inclusions within diamonds (e.g., chromite and phlogopite) have significantly different compositions than common mantle minerals and are much richer in Cr. Thus, saline hydrous fluids can be considered an important metasomatic agent at deep lithospheric mantle conditions and appear to be efficient in transporting elements such as Cr during rock water interaction.