[S-IT25] Deep Carbon: Diamond formation and carbon speciation in Earth and planetary processes

[SIT25-P01] Revision of the CaCO₃–MgCO₃ phase diagram at 3 and 6 GPa

Anton Shatskiy¹, Ivan Podborodnikov², Anton Arefiev², Daniil Minin¹, Artem Chanyshev¹, *Konstantin Litasov¹ (1.V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia, 2.Novosibirsk State University, Novosibirsk, Russia)

Keywords:carbonate, mantle, high pressure, phase diagram

Subsolidus and melting relationships in the system CaCO₃–MgCO₃ have been reexamined using a Kawai-type multianvil apparatus at 3 and 6 GPa in graphite capsules (Fig.1). Phase boundaries were delineated according to the chemical composition of phases measured by electron microprobe in energy dispersive mode and identification of crystal phases by Raman spectroscopy (Shatskiy et al., 2018).
At 3 GPa, the dolomite-magnesite solvus intersects the melting loop at about 1250 °C, and the isothermal three-phase line so produced represents the peritectic reaction: dolomite (Ca# 43) = magnesite (Ca# 13) + liquid (Ca# 48), where Ca# = 100×Ca/(Ca+Mg). The melting loop for the CaCO₃–MgCO₃ join extends from 1515 °C (CaCO₃) to 1515 °C (MgCO₃) through a liquidus minimum at 1230 °C (near 53 mol% CaCO₃). Starting from 1425 °C at £30 mol% CaCO₃ in the system, the liquid quenches to dendritic carbonate and periclase and contains rounded voids, indicating an incongruent melting reaction: MgCO₃ (magnesite) = MgO (in liquid) + CO₂ (fluid and/or liquid).
At 6 GPa, aragonite + magnesite assemblage is stable up to 1000 °C. The reaction aragonite + magnesite = dolomite locates between 1000 and 1050 °C. The presence of dolomite splits the system into two partial binaries: aragonite + dolomite and dolomite + magnesite. The dolomite-magnesite solvus intersects the melting loop between 1400 and 1450 °C, and the isothermal three-phase line so produced represents the peritectic reaction: dolomite (Ca# 31) = magnesite (Ca# 21) + liquid (Ca# 57). The melting loop for the CaCO₃–MgCO₃ join extends from 1660 °C (CaCO₃) to 1780 °C (MgCO₃) through a liquidus minimum at 1400 °C and 62 mol% CaCO₃.
The compositions of carbonate crystals and melts from the experiments in the carbonated eclogite (Yaxley and Brey 2004) and peridotite (Dalton and Presnall 1998) systems are consistent with the geometry of the CaCO₃-MgCO₃ melting loop at 3 and 6 GPa: Ca-dolomite melt coexists with Mg-calcite in eclogite and peridotite at 3 GPa and dolomite melt coexists with magnesite in peridotite at 6 GPa.
This work is financially supported by Russian Science Foundation (project No 14-17-00609-P).
References
Dalton, J.A., Presnall, D.C. (1998) Contrib. Mineral. Petrol., 131: 123-135.
Shatskiy, A., Podborodnikov, I.V., Arefiev, A.V., Minin, D.A., Chanyshev, A.D., Litasov, K.D. (2018) Amer. Mineral., 103(3), doi: 10.2138/am-2018-6277.
Yaxley, G.M., Brey, G.P., (2004) Contrib. Mineral. Petrol., 146: 606-619.

Fig. 1. Isobaric T-X diagrams for the system CaCO₃-MgCO₃ at 3 and 6 GPa. Arg – aragonite, Cal or Ca-Dol – Mg-bearing calcite or Ca-rich dolomite, Dol – dolomite, Mgs – magnesite, F – CO₂ fluid, L – liquid. Open and grey circles indicate composition of solid phases and liquid. Grey numbers denote eutectic and peritectic compositions in mol% CaCO₃.