Chromium is a minor component in the earth's 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 1wt.% 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 6GPa) 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.
Bibliographical noteFunding Information:
This work was supported by Israel Science Foundation grants ( 320/05 and 324/05 ) and the Swiss National Science Foundation ( PP002-106569 ). We want to thank Omri Dvir with his help in setting the experiments, Dr. Nadya Teutsch for her help with the analytical work and advice and Prof. Oded Navon for constructive discussions. Vitali Gutkin and Dr. Vladimir Uvarov from the nano-characterization center at the Hebrew University are thanked for their help with the SEM and XRD work. Robert Luth and Ben Harte have provided very constructive reviews and helped to clarify several points and are greatly acknowledged. The Editor-in-chief, Dr Andrew Kerr, is thanked for his contrastive comments and linguistic editing.
- Multi anvil
- Saline fluids