Phase relations and melting in the system CaCO3–MgCO3 at high pressures and temperatures

Calcite and magnesite are among the most common carbonates of the Earth. A deeper insight into the phase relations and melting behavior of the anhydrous and hydrous CaCO3–MgCO3 system is crucial to understand the carbon cycle in the Earth' mantle and project 8 of the Research Unit is dealing with this topic.

The project is divided in three parts:

In part 1 we will determine the melting curve of magnesite under anhydrous conditions in the pressure range of 4 to 13 GPa by in situ measurements of the electrical conductivity change in a multi-anvil (MA) press when magnesite melts. Further we will apply in situ impedance spectroscopy as an independent control on the melting point of magnesite.

In part 2 we will perform quenched MA experiments to (a) reinvestigate the super-solidus phase relations of the anhydrous CaCO3–MgCO3 system at 6 GPa and (b) establish the sub- and super-solidus phase relations of the CaCO3–MgCO3 system at 9 GPa and high temperature (starting from 900 °C) under anhydrous conditions and establish the phase relations in the super-solidus P-T space of the hydrous CaCO3–MgCO3. Furthermore, partition coefficients for Li, K, Na, Sr, Ba, Pb and REE between carbonate and melt will be gained. We will use a rotating multi-anvil press to overcome quenching problems as observed in previous studies.

In part 3 we will examine the pressure dependence of the melting curve of magnesite under hydrous conditions by means of in situ X-ray diffraction measurements with the newly installed multi-anvil apparatus at the synchrotron storage ring PETRA III in Hamburg. Here melting at high pressures and temperatures will be established by the occurrence of diffuse scattering in the X-ray diffraction pattern. This different experimental approach to determine the melting relation in the hydrous system is necessary as the presence of water would falsify the results on the electrical conductivity.