CARBONATE STABILITY IN THE EARTHS MANTLE - A VIBRATIONAL SPECTROSCOPIC STUDY OF ARAGONITE AND DOLOMITE AT HIGH-PRESSURES AND TEMPERATURES

The Raman spectra of aragonite and dolomite have been measured to pressures of 23 and 28 GPa, respectively, and the infrared spectra of aragonite measured to 40 GPa. Results have also been obtained probing the spectra of aragonite at high pressures following laser-heating to temperatures in excess of 2000 K, and of compressed dolomite during external heating to 800 K. Our spectral range, between 100 and 1400 cm-1, encompasses vibrations characteristic of both stretching and bending motions of the carbonate group, as well as lower frequency bands associated with lattice vibrations. The pressure shifts of the aragonite bands vary from -0.3 cm-1/GPa for the carbonate out-of-plane bend to 3.4 cm-1/GPa for a lattice mode, while those of dolomite lie between 1.1 cm-1/GPa (in-plane carbonate bend) and 4.4 cm-1/GPa (lattice mode). Bands associated with vibrations of the carbonate ions have systematically smaller Gruneisen parameters than those associated with cation displacements, and we infer from the mode shifts of these low frequency vibrations that the bulk of the compression of these crystals is produced by the deformation and compaction of the metal ion environment. Indeed, the carbonate group remains stable and relatively undistorted throughout this pressure range, as manifested by the amount of splitting of the carbonate asymmetric stretching vibration (approximately 90 cm-1) at the highest pressures of these measurements (41 GPa). Neither aragonite nor dolomite are observed to undergo phase transitions over the pressure and temperature range of this study, documenting that these phases are notably stable under compression. Thus, carbon is likely to remain stably bound, possibly within carbonate phases, over a pressure range spanning those present in the upper mantle and shallow lower mantle.