Melting curve of CaCO3 with implications for the presence of melt in the transition zone

Abstract:

Carbonates are important carriers for carbon exchange between the surface and interior of the Earth and therefore their melting behavior bears directly on the deep carbon cycle. Previous studies showed that the solidus temperature of carbonated silicate spans a wide range of temperature due to composition variations in the starting materials. In order to provide fundamental data for thermodynamic calculations, the melting curve of pure CaCO₃ was measured up to 22 GPa using an in-situ electrical method in this study. The measurement was realized by monitoring abrupt changes in electrical conductivity upon melting. The results showed that the melting temperature of CaCO₃ increases rapidly from 2 to 8 GPa, consistent with the existing data (Irving and Wyllie, 1975; Suito et al., 2001). Our new data at high pressures reveal that the CaCO₃ melting curve flattens around 8GPa, starts to decrease until 15 GPa, and then increases again from 15 to 22 GPa. The changes in the melting slope reflect the evolution of the melt compressibility with pressure and the sub-solidus structural phase transformations. The inferred CaCO₃ melt density from melting curve slope is slightly smaller than basaltic melt at same pressureand temperature. The unusual negative slope melting curve between 8 and 15 GPa likely indicates the presence of melt around 400-500 Km depth, which may explain some of observed low-velocity zones in the transition zone (Vinnik & Farra, 2006).

Reference:

Irving AJ, Wyllie PJ (1975) Subsolidus and melting relationshipsfor calcite, magnesite and the join CaCO3-MgCO3 to 36 kb. GeochimCosmochimActa 39: 35-53.

Suito K, Namba J, Horikawa T, Taniguchi Y, Sakurai N, Kobayashi M, Onodera A, Shimomura O, Kikegawa T (2001) Phase relations of CaCO3 at high pressure and high temperature. Am Mineral 86(9):997-1002.

Vinnik L. & Farra V. (2006) S velocity reversal in the mantle Transition Zone. Geophysical Research Letters 33, L18316.