New High‐Pressure Phase of CaCO₃ at the Topmost Lower Mantle: Implication for the Deep‐Mantle Carbon Transportation

Abstract

In this study, we have investigated the stability of CaCO3 at high pressures and temperatures using synchrotron X‐ray diffraction in laser‐heated diamond anvil cells. Our experimental results have shown that CaCO3 in the aragonite structure transforms into CaCO3‐VII (P21/c) at 27 GPa and 1,500 K with a negative Clapeyron slope of −4.3(9) MPa/K. CaCO3‐VII is stable between 23 and 38 GPa at 2,300 K and transforms into post‐aragonite at 42 GPa and 1,400 K. Furthermore, it reacts with stishovite, an abundant form of SiO2 in subducted oceanic crust, forming CaSiO3‐perovskite. The occurrence of CaSiO3‐perovskite via the reaction of CaCO3‐VII and stishovite provides an explanation for the observation of the high concentrations of CaSiO3‐perovskite and some amount of CaCO3 in deep‐mantle inclusions. CaCO3‐VII is thus an important carbon‐bearing phase at the topmost lower mantle and may provide necessary carbon to produce deep‐mantle diamonds.

Stability and Reactions of CaCO3 Polymorphs in the Earth's Deep Mantle

“…As shown in Figure , within the uncertainties, the predicted phase regions agree quite well with those experimentally determined for the postaragonite phase (Ono et al, ). For the CaCO 3 ‐ P 2 1 / c ‐l phase, its upper boundary is predicted in good agreement with recent experiments (Gavryushkin et al, ; Li et al, ), while its lower boundary is slightly lower than that determined by Li et al () and remarkably lower than that observed by Gavryushkin et al (). The recent experimental observation of the CaCO 3 ‐ P 2 1 / c ‐h phase (Lobanov et al, ) lies right at the predicted phase regime in this study, while the onset of the transition pressure (~103 GPa at ~2000 K) is higher than the predicted boundary here (~87.4 ± 6.6 GPa).…”

“…The transition pressures between the phase CaCO 3 ‐ P 2 1 / c ‐l and its neighborhoods (aragonite at the lower pressure side and postaragonite at the higher pressure side) are relatively insensitive to temperature. The Clapeyron slopes of the boundaries are slightly negative and are around −1.97(4) MPa/K, which is quite close to the most recent experimental measurements (Li et al, ). The upper phase boundary of the postaragonite phase, on the other hand, is very sensitive to temperature, with an average Clapeyron slope of 15.81(6) MPa/K.…”

“…Comparisons between the simulations in this study and the experiments in the literature (Gavryushkin et al, ; Li et al, ; Li et al, ; Litasov et al, ; Lobanov et al, ; Martinez et al, ; Merlini et al, ; Ono et al, ; Ono et al, ): (a) aragonite at 300 and 973 K (inset); (b) other polymorphs at 300 K. The dashed curves and open stars (at 973 K) are the raw results by LD simulations and MD simulations, respectively. We apply the rescaling corrections to these raw results and obtain the solid curves and filled stars.…”

“…Thus, CaCO 3 could be restabilized and become the main host of carbon in the lowermost mantle. This is opposed to extrapolations from experiments at modest pressures (Biellmann et al, ; Li et al, ). Since these previous simulations were carried out under static conditions ( T = 0), it is unclear whether or not the predicted restabilization of CaCO 3 still hold at high temperatures.…”

“…Pickard and Needs () carried out calculations using ab initio random structure searching technique and discovered some additional monoclinic CaCO 3 ‐polymorphs that are more stable at the pressures of the topmost and lower part of the lower mantle. Through state‐of‐the‐art in situ experimental techniques, these new phases or their analogues have been observed in several most recent studies (Gavryushkin et al, ; Li et al, ; Lobanov et al, ). Despite the findings of these high‐pressure polymorphs, there are still significant uncertainties as to the stability of CaCO 3 ‐polymorphs at simultaneously high pressures and high temperatures and remarkable discrepancies still exist in the phase boundaries constrained from different approaches (Li et al, ).…”