Abstract

Understanding carbon speciation in Earth materials is important to unravel the geochemical evolution of the Earth’s atmosphere, the composition of mantle partial melts, and the overall distribution of carbon in the deep mantle. In an effort to provide the systematic protocols to characterize carbon-bearing fluid inclusions and other carbon-bearing species using high-resolution 13C solid-state NMR, one of the element-specific probes of local structure around carbon, we explore the atomic configurations around the carbon species formed during the reaction between 13C-enriched amorphous carbon and MgSiO3 enstatite synthesized at 1.5 GPa and 1400 °C using 13C MAS NMR spectroscopy and Raman spectroscopy. The Raman spectra for the fluid inclusion show the presence of multiple molecular species (e.g., CO2, CO, CH4, H2O, and H2) and reveal heterogeneous distribution of these species within the inclusion. 13C MAS NMR results show that the sharp peak at 125.2 ppm is dominant. While the peak could be assigned to either molecular CO2 in the fluid phase or fourfold-coordinated carbon ([4]C), the peak is likely due to fluid CO2, as revealed by Raman analyses of micrometer-sized fluid inclusions in the sample. The peaks at 161.2, 170.9, and 173.3 ppm in the 13C NMR spectrum correspond to the carbonate ions (CO32−) and additional small peak at 184.5 ppm can be attributed to carbon monoxide. Based on the established relationship between 13C abundance and peak intensity in the 13C MAS NMR, the estimated 13C amounts of CO2, CO32−, and CO species are much larger than those estimated from carbon solubility in the crystals, thus, indicating that those carbon species are from external phases. The 13C NMR spectrum for amorphous carbon showed a peak shift from ~130 to ~95 ppm after compression, thereby suggesting that the amorphous carbon underwent permanent pressure-induced densification, characterized by the transition from sp2 to sp3 hybridization and/or pressure-induced changes in sp2 carbon topology. While direct probing of carbon species in the crystalline lattice using NMR is challenging, the current results and method can be utilized to provide quantitative analysis of carbon-species in the fluid-inclusions in silicates, which is essential for understanding the deep carbon cycle and volcanic processes.

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