4) The model makes sense from crystallographic and chemical point of view
3.2. Brief outlook and perspectives
Natural evidences, theoretical calculation models and experiments at extreme conditions they all support the transportation of carbonates via subduction into the deep Earth. From all carbonates, the rhombohedral family is the protagonist. The depth to which they would travel into the Earth and whether they will survive the reducive mantle intact or decompose in formation of other C-bearing species (diamond, CO2, carbides etc…) and oxides strongly depends to their chemistry. The present thesis has demonstrated that the presence of some metals (Ca2+, Mn2+, Fe2+), commonly incorporated in
Figure 10. The crystal structure of (Fe0.4Mg2.6)C3O9.
112 rhombohedral carbonates, grant unique characteristics to the mineral’s high-pressure behavior.
Therefore, by studying each endmember carbonate separately, we can set constrains on the behavior of the natural compositions and understand the effect of impurity elements, but also explain some of the discrepancies between studies. In addition, this thesis explored the formation of carbonates with tetrahedrally coordinated carbon. Our results show that not all carbonates can form tetracarbonates (at least at the investigated pressure and temperature ranges); it appears that Fe, Mg and Mn play an important role in the polymerization of carbon. Summing up all our knowledge on the high pressure and high temperature behavior of carbonates, we explored the possibilities of their seismic detectability. Our results suggest that this is not very likely, but we have set certain constrains and proposed candidate locations on Earth for further examination.
In order to accurately untangle the complex chemistry of carbonates at extreme conditions and determine the thermoelastic properties of Fe-bearing samples we used the SCXRD and the NIS methods.
This thesis is among the few examples that demonstrate the feasibility of SCXRD experiments in laser-heated DACs and highlight the merits of the method. The same holds for the NIS method; it is a very promising technique that deserves more attention in the future.
Although in the time framework of this thesis we answered several questions and proposed the use of specific analytical techniques to solve common problems in the high-pressure research of carbonates, there are still more paths to explore. Dolomitic compositions seem to be on the spotlight today. For long time they received little attention due to their rich chemistry (Ca(Fe,Mg)(CO3)2). However, using SCXRD in combination with Raman spectroscopy and DFT calculations, new dolomite high pressure polymorphs are ongoing discoveries (e.g. dolomite-V, C2/c, Figure 11) [16]. In addition, the strong velocity anisotropies of Fe-bearing samples needs to be described further. We know that NIS measurements are sensitive to crystal orientations (Figure 12). This is still an ongoing project. Other ongoing projects involve
Figure 11. The crystal structure of a new dolomite high-pressure polymorph along the c-axis. (blue Ca forms CaO10; orange Mg forms MgO6; brown C forms CO3).
Figure 12. Partial DOS functions of an FeCO3 crystal oriented along a) the c-axis and b) the a*-axis. The data are derived after NIS measurements at 5 GPa. The different velocities reflect the anisotropic nature of carbonates.
113 the chemical reactions of carbonates with mantle silicates and the stability fields of carbonates in the MgCO3-FeCO3 system. These and many more are projects that require the expertise of various analytical techniques. In this framework, research units such as CarboPaT (Carbonates at high Pressures and Temperatures) were formed, that invite collaborations between several universities and geo-institutes around Germany (and not only). Thus, many new exciting discoveries for the deep carbon cycle await us in the near future.
3.3. References
[1] V. Cerantola, C. McCammon, I. Kupenko, I. Kantor, C. Marini, M. Wilke, L. Ismailova, N. Solopova, A.I.
Chumakov, S. Pascarelli, and L. Dubrovinsky, (2015) High-pressure spectroscopic study of siderite (FeCO3) with focus on spin crossover. American Mineralogist, 100, 2670-2681.
[2] M. Merlini, M. Hanfland, and M. Gemmi, (2015). The MnCO3-II high pressure polymorph of rhodochrosite. American Mineralogist, 100, 11-12.
[3] R.D. Shannon, C.T. Prewitt (1969) Effective ionic radii in oxides and fluorides. Acta Crystallographica Section B, 25, 925-946.
[4]M. Isshiki, T. Irifune, K. Hirose, S. Ono, Y. Ohishi, T. Watanuki, E. Nishibori, M. Takata, and M. Sakata, (2004), Stability of magnesite and its high-pressure form in the lowermost mantle. Nature, 427, 60-63.
[5] V. Cerantola, E. Bykova, I. Kupenko, M. Merlini, L. Ismailova, C. McCammon, M. Bykov, A.I. Chumakov, S. Petitgirard, I. Kantor, V. Svitlyk, J. Jacobs, M. Hanfland, M. Mezouar, C. Prescher, R. Rüffer, V.B.
Prakapenka, and L. Dubrovinsky, (2017) Stability of iron-bearing carbonates in the deep Earth’s interior. Nature Communications, 8, 15960.
[6] C. McCammon, R. Caracas, K. Glazyrin, V. Potapkin, I. Kantor, R. Sinmyo, C. Prescher, I. Kupenko, A.I.
Chumakov, and L. Dubrovinsky, (2016) Sound velocities of bridgmanite from density of states determined by nuclear inelastic scattering and first-principles calculations. Progress in Earth and Planetary Science, doi:10.1186/s40645-016-0089-2.
[7] D.M. Vasiukov, L. Ismailova, I. Kupenko, V. Cerantola, R. Sinmyo, K. Glazyrin, C. McCammon, A.I.
Chumakov, L. Dubrovinsky, and N. Dubrovinskaia, (2018) Sound velocities of skiagite–iron–majorite solid solution to 56 GPa probed by nuclear inelastic scattering. Physics and Chemistry of Minerals, 45, 397-404.
[8] C. Biellmann, P. Gillet, F. Guyot, J. Peyronneau, and B. Reynard, (1993) Experimental evidence for carbonate stability in the Earth’s lower mantle. Earth and Planetary Science Letters, 118, 31-41.
114 [9] R. Dasgupta, M.M. Hirschmann, and A.C. Withers, (2004) Deep global cycling of carbon constrained by solidus of anhydrous, carbonated eclogite under upper mantle conditions. Earth and Planetary Science Letters, 227, 73–85.
[10] T. Plank, and C.H. Langmuir, (1998) The chemical composition of subducting sediments and its consequences for the crust and mantle. Chemical Geology, 145, 325-394.
[11] Y. Fukao, and M. Obayashi, (2013) Subducted slabs stagnant above, penetrating through, and trapped below the 660 km discontinuity. Journal of Geophysical Research: Solid Earth, 118, 5920-5938.
[12] B. Lavina, P. Dera, R.T. Downs, O. Tschauner, W. Yang, O. Shebanova, and G. Shen, (2010) Effect of dilution on the spin pairing transition in rhombohedral carbonates. High Pressure Research, 30, 224-229.
[13] A.R. Oganov, S. Ono, Y. Ma, C.W. Glass, A. Garcia, (2008) Novel high-pressure structures of MgCO3, CaCO3 and CO2 and their role in Earth's lower mantle. Earth and Planetary Science Letters, 273, 38-47.
[14] E. Boulard, D. Pan, G. Galli, Z. Liu, and W.L. Mao, (2015) Tetrahedrally coordinated carbonates in Earth’s lower mantle. Nature Communications, 6, 6311.
[15] M. Merlini, V. Cerantola, G.D. Gatta, M. Gemmi, M. Hanfland, I. Kupenko, P. Lotti, H. Müller, and L.
Zhang, (2017) Dolomite-IV: Candidate structure for a carbonate in the Earth’s lower mantle.
American Mineralogist, 102, 1763-1766.
[16] J. Binck, S.Chariton, M. Stekiel, L. Bayarjargal, W. Morgenroth, L. Dubrovinsky, and B. Winkler (2020) High-pressure, high-temperature phase stability of iron-poor dolomite and the structures of dolomite-IIIc and dolomite-V. Physics of the Earth and Planentary Interiors, 299, 106403.
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