IN SITU HIGH PRESSURE INFRARED STUDY OF THE CARBON ENVIRONMENT IN (MG,FE)CO3 CARBONATE

Carbonates are likely to be the main carbon-bearing phase in the Earth’s mantle, and therefore knowledge of their mineral physics down to core mantle boundary conditions is critical for understanding the deep carbon cycle. (Mg,Fe)CO₃ has been the focus of many recent high pressure studies which indicate several crystallographic changes. An electronic spin transition in the iron end-member has been reported at approximately 45 GPa. As a result, a change in the volume and the equation of state, and moreover a change in the rate of C-O bond distortion were described by X-ray diffraction (XRD) studies (B. Lavina et al., 2009; 2010). At higher pressures, above 80 GPa, we have observed the transformation of (Mg,Fe)CO₃ carbonate into a new high-pressure high-temperature phase by in situ XRD (Boulard et al., 2011). Investigation of the carbon environment had previously been limited to ex situ studies at ambient conditions after releasing the pressure on the sample. Spectroscopy on the carbon C-k edge indicated a potential change in the carbon environment, and a transformation of the carbonate trigonal CO₃ groups into CO₄ tetrahedra had been proposed (Boulard et al., 2011). However this interpretation is still under debate. To follow the evolution of C-O bonds and clarify the existence of CO₄ tetrahedra in high-pressure carbonate phases, we combined in-situ infrared spectroscopy with theoretical calculations. Mid-infrared spectroscopy, performed at high pressure before and after laser heating at U2A, NSLS, BNL show several changes in the (Mg,Fe)CO₃ spectrum after laser heating at 103 GPa. We will discuss the interpretation of these new spectroscopic signatures and the possibility of a dramatic change in the carbon environment.

References:

Boulard, E. et al., (2011). New host for carbon in the deep Earth. PNAS, 108(13), 5184–5187.

Lavina, B. et al., (2009). Siderite at lower mantle conditions and the effects of the pressure-induced spin-pairing transition. Geophysical Research Letters, 36(23), 1–4.

Lavina, B. et al., (2010). Structure of siderite FeCO3 to 56 GPa and hysteresis of its spin-pairing transition. Physical Review B, 82(6), 1–7.