H2O-present melting curve of magnesite and trace element distribution during melting of (dry) magnesite and calcite in the upper mantle

The presence of magnesite (MgCO₃) in the Earth’s mantle plays a fundamental role in reducing the melting point of the mantle [1] and forming carbonate‑rich melts such as kimberlites and carbonatites [2]. The melting curve of (dry) magnesite is well constrained [3, 4], but melting of magnesite in the presence of H₂O, providing the basis for more complex (natural) systems, is poorly understood from some quenched experiments [5]. Also, the distribution of trace elements such as Li, Sr, Pb, and rare earth elements during melting of magnesite is poorly considered in models that evaluate the trace element budget of carbonate‑rich melts parental to kimberlites [6].

Here we report, first, the H₂O‑present melting curve of magnesite between 2 and 12 GPa. The melting curve of magnesite mixed with 16 wt% brucite was established by in ‑ situ X‑ray diffraction measurements using the large volume press at P61B at PETRA III (DESY). Second, we report trace element partitioning data for congruent melting of calcite and incongruent melting of magnesite producing carbonate melt and periclase between 6 and 9 GPa. Those results were obtained from quenched experiments using a rocking multi‑anvil press at the GFZ overcoming equilibrium and quenching problems in previous studies [7].

 

1          Dasgupta and Hirschmann, The deep carbon cycle and melting in Earth's interior. Earth and Planetary Science Letters, 2010. 298(1-2): p. 1-13.

2          Jones, Genge, and Carmody, Carbonate Melts and Carbonatites. Reviews in Mineralogy and Geochemistry, 2013. 75(1): p. 289-322.

3          Solopova, Dubrovinsky, Spivak, Litvin, and Dubrovinskaia, Melting and decomposition of MgCO3 at pressures up to 84 GPa. Physics and Chemistry of Minerals, 2014. 42(1): p. 73-81.

4          Müller, Koch-Müller, Rhede, Wilke, and Wirth, Melting relations in the system CaCO3-MgCO3 at 6 GPa. American Mineralogist, 2017. 102(12): p. 2440-2449.

5          Ellis and Wyllie, Carbonation, hydration, and melting relations in the system MgO-H2O-CO2 at pressures up to 100 kbar. American Mineralogist, 1979. 64(1-2): p. 32-40.

6          Girnis, Bulatov, Brey, Gerdes, and Höfer, Trace element partitioning between mantle minerals and silico-carbonate melts at 6–12GPa and applications to mantle metasomatism and kimberlite genesis. Lithos, 2013. 160-161: p. 183-200.

7          Buob, Experiments on CaCO3-MgCO3 solid solutions at high pressure and temperature. American Mineralogist, 2006. 91(2-3): p. 435-440.