Carbonate stability in the Earth's mantle: A vibrational spectroscopic study of aragonite and dolomite at high pressures and temperatures

Abstract

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 Grüneisen 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 (∼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.

Ternary Ca–Fe–Mg carbonates: subsolidus phase relations at 3.5 GPa and a thermodynamic solid solution model including order/disorder

“…Diamond-anvil experiments demonstrate that magnesite is the only stable carbonate in the lower mantle (Biellmann et al 1993;Gillet 1993) and in situ X-ray diffraction study suggests that MgCO 3 is stable in a hitherto undetermined structure down to the core-mantle boundary (Isshiki et al 2004). Also, CaCO 3 is stable up to [130 GPa (Ono et al 2007) and dolomite to[9 GPa (Luth 2001), persisting at room temperature metastably to [28 GPa (Kraft et al 1991). Nevertheless, because magnesite is the more stable carbonate at pressure beyond 4 GPa in peridotitic systems (Dasgupta and Hirschmann 2006), it is allegedly the only carbonate in the deep mantle.…”

Phase transitions in the system CaCO3 at high P and T determined by in situ vibrational spectroscopy in diamond anvil cells and first-principles simulations

“…P at T Max. T at P This study Adams and Williams (1980) 26.5 GPa/RT RT Kraft et al 1991 took Raman spectra after laser heating to 2100 K at 30 GPa b Kraft et al 1991took Raman spectra after externally heating to 550 K at 10 GPa All experiments in the DAC-HT used NaCl as pressure medium and the cell were relaxed by annelaing using the internal heater to 70 to 100 °C at low pressures b Petroleum jelly was used as pressure medium c Argon was used as pressure medium; if necessary, the cell was annealed at 8 GPa to 120 °C for 0.5 h d "transition" means that the P-T conditions are close to the transformation of cc-III to cc-VI, indicated by disappearance of the Raman band around 120 cm-1 and before the appearance of the two bands at 150 and 200 cm-1, which are indicative for cc-VI e In these experiments, the phase boundary cc-IIIb-cc-VI was crossed several times and on their base the slope of the transition has been determined Table 3 Lattice parameters and unit cell volumes of cc polymorphs from DFT calculations Phase Perdew and Wang (1992) functional (ABINIT option ixc = 7). All other calculations used pseudopotentials with ixc = 2 (LDA according to Perdew and Zunger (1981)).…”

Structural and mechanical properties of dolomite rock under high pressure conditions: A first‐principles study

“…Carbonate minerals have long been considered as important means of carbon sequestration in the deep Earth. In particular, CaCO 3 -aragonite, CaMg(CO 3 ) 2 -dolomite and calcite-structured MgCO 3 -magnesite have each been presented as possible phases where carbon is stored in the mantle [1][2][3][4][5]. Besides carbonates contain about 60% of the World's oil reserves.…”