Partition coefficients of trace elements between carbonates and melt and suprasolidus phase relation of Ca-Mg-carbonates at 6 GPa

Abstract

The presence of Ca-Mg-carbonates affects the melting and phase relations of peridotites and eclogites in the mantle, and (partial) melting of carbonates liberates carbon from the mantle to shallower depths. The onset and composition of incipient melting of carbonated peridotites and carbonated eclogites are influenced by the pure CaCO3-MgCO3-system making the understanding of the phase relations of Ca-Mg-carbonates fundamental in assessing carbon fluxes in the mantle. By performing high-pressure and high-temperature experiments, this study clarifies the suprasolidus phase relations of the nominally anhydrous CaCO3-MgCO3-system at 6 GPa showing that Ca-Mg-carbonates will (partially) melt for temperatures above ~1300 °C. A comparison with data from thermodynamic modeling confirms the experimental results. Furthermore, partition coefficients for Li, Na, K, Sr, Ba, Nb, Y, and rare earth elements between calcite and dolomitic melt, Ca-magnesite and dolomitic melt, and magnesite and dolomitic melt are established. Experiments were performed at 6 GPa and between 1350 to 1600 °C utilizing a rotating multi-anvil press. Rotation of the multi-anvil press is indispensable to establish equilibrium between solids and carbonate liquid. Major and trace elements were quantified with EPMA and LA-ICP-MS, respectively. The melting temperature and phase relations of Ca-Mg-carbonates depend on the Mg/Ca-ratio. For instance, Ca-rich carbonates with a molar Mg/(Mg+Ca)-ratio (XMg) of 0.2 will transform into a dolomitic melt (XMg = 0.33–0.31) and calcite crystals (XMg = 0.19–0.14) at 1350–1440 °C. Partial melting of Mg-rich carbonates (XMg = 0.85) will produce a dolomitic melt (XMg = 0.5–0.8) and Ca-bearing magnesite (XMg = 0.89–0.96) at 1400–1600 °C. Trace element distribution into calcite and magnesite seems to follow lattice constraints for divalent cations. For instance, the compatibility of calcite (XMg = 0.14–0.19) for Sr and Ba decreases as the cation radii increases. Ca-Mg-carbonates are incompatible for rare earth elements (REEs), whereby the distribution between carbonates and dolomitic melt depends on the Mg/Ca ratio and temperature. For instance, at 1600 °C, partition coefficients between magnesite (XMg = 0.96) and dolomitic melt (XMg = 0.8) vary by two orders of magnitudes from 0.001 to 0.1 for light-REEs to heavy-REEs. In contrast, partition coefficients of REEs (and Sr, Ba, Nb, and Y) between magnesite (XMg = 0.89) and dolomitic melt (XMg = 0.5) are more uniform scattering marginal between ~0.1–0.2 at 1400 °C.

Melting relations of Ca–Mg carbonates and trace element signature of carbonate melts up to 9 GPa – a proxy for melting of carbonated mantle lithologies

“…The 6 GPa experiment confirms the earlier established phase relations (Fig. 4b) (Sieber et al, 2020).…”

“…Periclase is sometimes found only in high-resolution transmission electron microscopy (TEM) images, implying that previous studies may have missed small periclase grains in experimental run products. Furthermore, we determine partition coefficients between Ca-magnesite and dolomitic melt and between Mg-calcite and dolomitic melt at 9 GPa, extending our previous results at 6 GPa (Sieber et al, 2020). The trace element signature of dolomitic melt in equilibrium with Ca-magnesite and periclase could be used to approximate CO 2 -rich liquids parental to kimberlite magmas.…”

“…Figure 4 summarizes the sub-and supra-solidus phase relations in the CaCO 3 -MgCO 3 -system between 1 and 9 GPa after Byrnes and Wyllie (1981), Sieber et al (2020), Shatskiy et al (2018), Irving and Wyllie (1975), and this study. Byrnes and Wyllie (1981) and Irving and Wyllie (1975) investigated the subsolidus and melting relations of the CaCO 3 -MgCO 3 -system at 1 and 3 GPa, respectively.…”

“…The multi-anvil press was rotated throughout the entire run (stopping the rotation 30 s before quenching), enhancing equilibrium between solids and a mobile phase (Schmidt and Ulmer, 2004). Our earlier experiments investigating the same chemical system in the same matter also point toward equilibrium (Sieber et al, 2020).…”

Comprehensive Density Functional Theory Studies of Vibrational Spectra of Carbonates

“…Carbonates form an extensive class of chemical compounds containing the carbonate ion as the main structural element [ 1 , 2 ]. In nature, carbonates are found in many minerals and play a crucial role in the carbon exchange of our planet [ 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. Carbonate compounds are widely used in the construction industry, optics, and nanotechnology [ 9 , 10 , 11 , 12 , 13 , 14 , 15 ].…”