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Tetrahedrally Coordinated Carbonates in Earth’s Lower Mantle Eglantine Boulard 1,2 , Ding Pan 3 , Giulia Galli 3 , Zenxhian Liu 4 and Wendy L. Mao 1,5 1 Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. 2 PSICHE Beamline, Synchrotron SOLEIL, L'Orme des Merisiers, 91190 Saint-Aubin, France 3 Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA. 4 Geophysical Laboratory, Carnegie Institution of Washington, Washington, District Of Columbia 20015, USA. 5 Photon Science, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA. Carbonates such as ferromagnesite ((Fe,Mg)CO 3 ) are likely to be the carbon-bearing phases in the deep mantle [e.g. 1]. Determination of their stability, thermodynamic properties and phase diagrams are therefore of great interest for understanding the global carbon cycle. Although these results may indicate a possible new local environment for C in HP-HT structures, this drastic change is still controversial and further in situ characterizations are needed. We address this problem with in situ IR spectroscopy on (Mg,Fe)CO 3 at high pressure. Sid CO 3 2- C 3 O 9 6 - CO C K d) HP carb Energy loss (eV) 285 295 305 290 300 2-Theta, deg 2-Theta, deg Counts Counts 4 4 6 6 8 8 10 10 12 12 14 14 16 16 18 18 Previous in situ X-Ray Diffraction (XRD) study shows transformation of (Mg, Fe)CO 3 into a new structure above 80 GPa - 2300 K that is quenchable at room temperature (RT) [Ref. 2]. However, no Rietveld refinements could be performed and therefore no atom positions were proposed. When quenched to ambient pressure and temperature conditions, the transformed (Mg, Fe)-rich carbonate presents a C-K edge different from a carbonate suggesting a change of carbon environment [Ref.2]. Possible new local environment for C ? Introduction in situ XRD pattern of (Mg, Fe)CO 3 at 80 GPa-2300 K [Ref. 2] ex situ EELS spectra at the C-K edge of recovered samples. Sid : Siderite; HP carb : high pressure phase [Ref. 2] IR spectra collected at RT upon compression and decompression IR spectra collected at RT upon decompression after transformation at 103 GPa - 2100 K by laser heating Synchrotron based infrared (IR) spectroscopy was performed on natural (Mg 0.25 ,Fe 0.75 )CO 3 sample. Spectra were collected at high pressure (HP) and room temperature (RT) through diamond anvil cells. In situ Infrared spectroscopy IR ruby sample KBr IR spectra collected upon decompression after transformation at HT. Several new infrared bands are observed and can be follow down to 43 GPa, i.e.: * Method: 3000 2500 2000 1500 1000 500 Wavenumber (cm -1 ) 0 GPa 43 GPa 58 GPa 81 GPa 85 GPa 103 GPa 1 * * * * * * ν 2 ν 3 ν 4 Absorbance 3000 2500 2000 1500 1000 500 Wavenumber (cm -1 ) 0.5 58 GPa 54 GPa 0 GPa ν 4 ν 3 ν 2 ν 4 ν 3 ν 2 (Mg,Fe)CO 3 IR active modes : * * Absorbance Ø X Z Y X Z Y C O Mg, Fe Density functional theory (DFT) Absorbance 2000 1500 1000 500 0 Wavenumber (cm -1 ) 0.5 IR spectra calculated for a pure MgCO 3 without allowing for any phase transition: IR spectrum calculated for the postmagnesite structure: R-3c structure of magnesite postmagnesite structure measured at 82 GPa - 2,300 K [Ref. 2] 1.37 Å 1.39 Å Conclusions Implications on C reservoirs and fluxes in the lower mantle : ● Chemical and physical properties of carbonates ● Increase in carbonate melt viscosity due to the ability of CO 4 to form polymerizable networks. This would inhibit mobility of carbonate melts in the lower mantle and might lead to the presence of deep carbon reservoirs. 4 1 2 3 1 2 3 3 (A) (B) 5 4 5 4 5 The IR spectra of the high-pressure phase can be assigned to sp 3 hybridized configurations. trigonal magnesite structure : sp 2 Carbon bonds New mode at ~1300 cm -1 : IR Fingerprint of tetrahedrally bonded carbon: New carbon - oxygen bonds: References : Present work: Boulard E., Pan D. et al. 2015, Nature Communications, 6, 6311 for comparison, the C-O length in the CO 3 ion is 1.30 Å at ambient conditions [1] Wood, B.J. et al., 1996, Phil. T. R. Soc. Lond. A, v. 354, p. 1495-1511 [2] Boulard, E et al., 2011, PNAS, v. 108, no. 13, p. 5184-5187 [email protected]
