Transformations and Decomposition of MnCO3 at Earth's Lower Mantle Conditions

Abstract

Carbonates have been proposed as the principal oxidized carbon-bearing phases in the Earth's interior. Their phase diagram for the high pressure and temperature conditions of the mantle can provide crucial constraints on the deep carbon cycle. We investigated the behavior of MnCO 3 at pressures up to 75 GPa and temperatures up to 2200 K. The phase assemblage in the resulting run products was determined in situ by X-ray diffraction (XRD), and the recovered samples were studied by analytical transmission electron microscopy (TEM) and X-ray absorption near edge structure (XANES) imaging. At moderate temperatures below 1400 K and pressures above 50 GPa, MnCO 3 transformed into the MnCO 3 -II phase, with XANES data indicating no change in the manganese oxidation state in MnCO 3 -II. However, upon heating above 1400 K at the same pressure conditions, both MnCO 3 and MnCO 3 -II undergo decomposition and redox reactions which lead to the formation of manganese oxides and reduced carbon.

Poroelastic response to rapid decarbonatisation as a mechanism of the diamonds formation in the mantle wedge of Kamchatka

“…Equilibrium diagram of the decarbonatisation reactions in the upper mantle. Decarbonatisation of sideritic dolomites is plotted in accordance with experimental data by Martin and Hammouda [2011], dolomite plus silica reaction parameters are from [Luth, 1995], siderite stability from [Kang et al, 2015] and extrapolated MnCO 3 stability from [Boulard et al, 2016]. Cross-hatched rectangle outlines conditions of the eclogitic diamonds [Smith et al, 2015].…”

“…is close to the equilibrium in reaction (3) and intersects with graphite -diamond transition line at 5 GPa, 1280 ∘ C close to the mantle adiabate and melting point of siderite. At = 2 GPa decomposition curve of FeCO 3 and the stability boundary of the ferrous dolomite and silica approaches each other at = 840 ∘ C. The PT conditions of MnCO3 decomposition are experimentally constrained at atmospheric and rather high (above 15 GPa) pressures [Boulard et al, 2016]. With linear extrapolation from 15 to 2 GPa, the equilibrium temperature is 920 ∘ C, which is higher than for the ferrous dolomite [Martin and Hammouda, 2011].…”

“…It should be noted that in the reactions (25), (26) and (24), carbon isotopes are fractionated with enrichment of CO and, finally, C (diamond) with light 12 C isotope and carbon dioxide with 13 C. The efficiency of the fractionation of carbon isotopes in a pair of CO 2 -CO is higher than in a pair of CO 2 -CH 4 [Chacko et al, 2001]. As a result, at a high temperature of 800-900 ∘ C, an equilibrium isotopic shift between CO 2 and diamond is expected to be about −(10 − 17) o / oo .…”

Formation of Spessartine and CO2 via Rhodochrosite Decarbonation along a Hot Subduction P-T Path

“…The greatest amount of Mn is contained in the oceanic crust in ferromanganese nodules as oxides and in marine sediments as Mn-carbonate (rhodochrosite) [5]. As shown in a number of modern experimental works on the behavior of rhodochrosite and Mn-oxides at high temperatures and pressures, these minerals can be thermodynamically stable to ultrahigh pressure (P) and temperature (T) conditions (Figure 1) [6][7][8][9][10][11][12]. However, during subduction of the oceanic crust, Mn-rich oxides and rhodochrosite not only are transported to the mantle but interact with mantle rocks, leaving characteristic chemical "traces" in mantle rocks, most pronounced in garnet-bearing assemblages.…”

“…The main goal of this study is to experimentally simulate the rhodochrosite-involving decarbonation reactions, resulting in the formation of spessartine and CO2fluid, in a wide range of pressures and temperatures, with implications to the Mn-carbonate stability and spessartine formation under hot subduction conditions. [12,28]. MnCO3-I-rhombohedral structure (rhodochrosite), MnCO3-II-triclinic structure high-pressure polymorph.…”