REE and Hf distribution between pyrope and MgCO3- or NaCl-bearing aqueous fluid at 800°C and 3 GPa

The effect of dissolved NaCl (96 g/l) and MgCO3 (~ 100 g/l) on the partitioning of REE and Hf between synthetic pyrope and aqueous fluid has been investigated at 3 GPa and 800°C using the diamond trap technique. HREE incorporation in pyrope is achieved according to a YAG-type substitution: HREE + Al = Mg + Si. Detailed inspection of the diamond aggregate using FE-SEM as well as TEM, indicates transient crystallization of Al-bearing silicates including törnebohmite, LREE2Al(OH)[SiO4]2. However, some crystals of törnebohmite persist in the capsule with MgCO3 solution. The persistance of a LREE-rich mineral in MgCO3 solution was independently inferred on the basis of a mass balance calculation on the trace elements. Due to the presence of törnebohmite, the derivation of meaningful partition coefficients for LREE between garnet and CO2-bearing fluid could not be achieved. Whereas Hf partitioning between aqueous fluid and garnet is similar whatever the fluid composition (NaCl or MgCO3), HREE are favorably partitioned into garnet in NaCl-bearing solution and into the HP-HT fluid for the MgCO3 solution. This result demonstrates that CO2 tends to form stable complexes with LREE at high pressure and temperature and that CO2-bearing fluid circulation can potentially decouple HREE and Hf.