Hydroxylbastnäsite-(Sm) under high pressure and high temperature Insights into REEs, carbon, and water cycling into the deep Earth

Carbonatite is one of the major archives of rare earth elements (REEs), and in some cases, its formation is linked to the deep subduction of carbonated ocean crust and REE-rich sediments.The formation conditions of hydroxyl rare earth carbonate, hydroxylbastnsite-(Sm) Sm(CO 3 )OH, in a subduction zone were simulated at 3 GPa and 1073 K.The crystal structure of Sm(CO 3 )OH was determined using single crystal X-ray diffraction (XRD), which shows the crystal to be hexagonal with cell parameters a = b = 12.2143 6 (17 3 , and space group P .The high-pressure properties of synthesized Sm(CO 3 )OH were investigated using in-situ synchrotron powder XRD and Raman spectroscopy at pressures up to 20.9 and 20.6 GPa at ambient temperature, respectively.Additionally, the structural stability of Sm(CO 3 )OH under pressure and temperature conditions up to 5.9 GPa and 473 K was also investigated using Raman spectroscopy.A third-order Birch-Murnaghan equation of state fitted to the ambient temperature and high-pressure data points yielded = 76 (11) GPa with = 13 (2), and = 127 (2) GPa if is constrained to a value of 4. Analysis of axial compressible moduli shows an apparent compression anisotropy of Sm(CO 3 )OH: = 215 (5) GPa and = 265 (5) GPa.Raman spectra of the synthesized quenchable crystal of Sm(CO 3 )OH displayed no detectable phase transition at the pressure range from ambient to 20.6 GPa, and no noticeable phase transition when the temperature and pressure were increased from ambient conditions to 473 K and 5.9 GPa, respectively.These results suggest that hydroxylbastnsite-(Sm) phase may play a potential role as a conveyor for the migration of rare earth elements (REE), carbon (C) and water (-OH) to the deep Earth during plate