6.4 ZHANG Zhigang

Stability of CaCO3 under Deep Mantle Conditions: Constraints from First Principles Simulations

Zhigang Zhang1,2,3*, Zhu Mao4, Xi Liu5, Yigang Zhang1,2,3 and John Brodholt7

1 Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China.

2 Institutions of Earth Science, Chinese Academy of Sciences, Beijing, 100029, China.

3 University of Chinese Academy of Sciences, Beijing 100049, China.

4 Laboratory of Seismology and Physics of Earth’s Interior, School of Earth and Planetary Sciences, University of Science and Technology of China, Hefei, Anhui, China

5 Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, Peking University, Beijing 100871, China

6 School of Earth and Space Sciences, Peking University, Beijing 100871, China

7 Department of Earth Sciences, University College London, Gower Street, London, WC1E 6BT, United Kingdom

* Corresponding author: Zhigang Zhang (zgzhang@mail.iggcas.ac.cn)

CaCO3 is an important component of carbonates that is often involved in complex geochemical processes such as mantle metasomatism, redox melting and formation of super-deep diamonds. To understand the Ca-bearing carbonates in the Earth’s deep mantle, a new phase diagram of CaCO3 is established in the present study based on extensive first principles simulations. Relevant reactions with MgCO3, MgO, SiO2 and MgSiO3 polymorphs have also been systematically calculated under mantle conditions. We find the increasing pressure induces more variety of CaCO3-polymophs and strongly enhances the stability of CaCO3. In particular, CaCO3 shows an elevated melting temperature of at least 2500 K at 40 GPa and over 3500 K at 80 GPa, which is much higher than the other carbonate components. In addition, CaCO3 is predicted to be more stable than MgCO3 when the pressure is increased to ~100 GPa at room temperature. Furthermore, the temperature effects on the stability of CaCO3 are found to be significant. The boundary of the tetrahedrally structured CaCO3-polymorph is found to be sensitive to temperature with an average Clapeyron slope of 15.81(6) MPa/K. The delayed occurrences of the high pressure polymorphs destabilizes the CaCO3 component and increases the partitioning of Ca into silicates. Our results show that CaCO3 is less stable than MgCO3 over the whole mantle pressures (to ~136 GPa) at temperatures above ~1500 K.