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

Abstract

Abstract. The most profound consequences of the presence of Ca–Mg carbonates (CaCO3–MgCO3) in the Earth's upper mantle may be to lower the melting temperatures of the mantle and control the melt composition. Low-degree partial melting of a carbonate-bearing mantle produces CO2-rich, silica-poor melts compositionally imposed by the melting relations of carbonates. Thus, understanding the melting relations in the CaCO3–MgCO3 system facilitates the interpretation of natural carbonate-bearing silicate systems. We report the melting relations of the CaCO3–MgCO3 system and the partition coefficient of trace elements between carbonates and carbonate melt from experiments at high pressure (6 and 9 GPa) and temperature (1300–1800 ∘C) using a rocking multi-anvil press. In the absence of water, Ca–Mg carbonates are stable along geothermal gradients typical of subducting slabs. Ca–Mg carbonates (∼ Mg0.1–0.9Ca0.9–0.1CO3) partially melt beneath mid-ocean ridges and in plume settings. Ca–Mg carbonates melt incongruently, forming periclase crystals and carbonate melt between 4 and 9 GPa. Furthermore, we show that the rare earth element (REE) signature of Group-I kimberlites, namely strong REE fractionation and depletion of heavy REE relative to the primitive mantle, is resembled by carbonate melt in equilibrium with Ca-bearing magnesite and periclase at 6 and 9 GPa. This suggests that the dolomite–magnesite join of the CaCO3–MgCO3 system might be useful to approximate the REE signature of carbonate-rich melts parental to kimberlites.

Stability of magnesite in the presence of hydrous fluids up to 12 GPa: Insights into subduction zone processes and carbon cycling in the Earth’s mantle

“…Magnesite + periclase occur in the colder region, while brucite + periclase along with significant pore space exist in the hotter region of the capsule. This hourglass texture is commonly observed in cases involving thermal gradients as seen in melting experiments with the same experimental setup (Sieber et al 2020;Sieber et al 2022), experiments involving large amount of fluids (e.g. Deon et al 2011) and experiments on brucite dehydration (Johnson and Walker 1993).…”

“…At 1760 ℃ (run 121), the sphere remained at the top, but the texture of elongated magnesite grains and dendritic brucite crystals indicates the presence of a melt (Figure 2b). Periclase crystals are found at the bottom of the capsule, indicating their crystallization in the lower temperature region, a common phenomenon for subliquidus phases (Fukui et al 2005;Sieber et al 2020;Sieber et al 2022;Inoue 1994). The dendritic texture of brucite crystals in run 121 is attributed to quenching from a MgO-H 2 O-bearing melt.…”

“…Earlier studies reported congruent melting of magnesite at pressures ≥2.7 GPa (Huang and Wyllie 1976;Shatskiy et al 2016;Katsura and Ito 1990). Sieber et al (2022) recently revised the melting reaction, demonstrating incongruent melting for magnesite in the anhydrous system at 6 and 9 GPa.…”

“…Details of the multi-anvil press at GFZ are reported in Deon et al 2011. An 14/8 assembly with double stepped graphite heater was used in the Walker multi-anvil press (GFZ) and an 14/8 assembly with Re-heater in the offline experiments in the Aster-15 LVP (for technical drawings used in the offline experiments see Sieber et al 2022). Inside the capsules, a Pt-sphere was placed at the top of the starting mixtures but well below the lid.…”

“…Recently, Sieber et al (2022) demonstrated that incongruent melting of magnesite persists up to 6 and 9 GPa. Our in situ experiment confirm this finding, as periclase formation associated with the breakdown of magnesite is detected by EDXRD.…”