Melting relations of carbonates and trace element partitioning between carbonates and carbonate liquid in the Earth's upper mantle

We examined the supra-solidus phase relations of the CaCO₃-MgCO₃ system and established trace element partition coefficient between carbonates and carbonate melt by conducting high pressure (6 and 9 GPa) and temperature (1300-1800 ^oC) experiments with a rocking multi-anvil press. It is well known that the major element composition of initial melts derived from low-degree partial melting of the carbonated mantle strongly depends on the melting relations of carbonates (e.g. 1, 2 and reference therein). Understanding the melting relations in the CaCO₃-MgCO₃ system is thus fundamental in assessing low-degree partial melting of the carbonated mantle. We show here to which extent the trace element signature of a pure carbonate melt can be used as a proxy for the trace element signature of mantle-derived CO₂-rich melts such as kimberlites.

Our results support that, in the absence of water, Ca-Mg-carbonates are thermally stable along geothermal gradients typical at subduction zones. Except for compositions close to the endmembers (~Mg_0-0.1Ca_1-0.9CO₃; Ca_0-0.1Mg_1-0.9CO₃), Ca-Mg-carbonates will partially (to completely) melt beneath mid‑ocean ridges and in plume settings. Ca-Mg-carbonates melt incongruently to dolomitic melt and periclase above 1450 ^oC and 9 GPa making the CaCO₃-MgCO₃ a (pseudo-) ternary system as the number of components increases. Further, our results show that the rare earth element signature of a dolomitic melt in equilibrium with magnesite is similar to those of Group I kimberlites, namely that HREE are depleted relative to primitive mantle signatures. This implies that dolomite-magnesite solid solutions might be useful to approximate melting relations and melt compositions of low-degree partial melting of the carbonated mantle.

References

1  Yaxley, Ghosh, Kiseeva, Mallik, Spandler, Thomson, and Walter, CO₂-Rich Melts in Earth, in Deep Carbon: Past to Present, Orcutt, Daniel, and Dasgupta, Editors. 2019, Cambridge University Press: Cambridge. p. 129-162.

2  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.