New Insights into the Lithospheric Mantle Carbon Storage in an Intra-Continental Area: A Geochemical and 3D X-Ray Micro-Tomography Study

Abstract:

The Pannonian Basins situated in a context of lithospheric fluxing by mantle CO₂-rich fluids, as evidenced by Plio-Pleistocene alkaline basalts and Basin gas geochemical data [1]. Such type of intracontinental CO₂-fluxes remain poorly constrained at the scale of the global C-cycle. We report here the first quantification of the CO₂ volumes stored in the lithospheric mantle, by coupling geochemical and 3D micro-tomography studies of lherzolitic and harzburgitic mantle xenoliths. The Pannonian Basin xenolith peridotites present numerous signs of melt/fluid migration. The compositions of glasses found in the peridotites vary from sub-alkaline (Na₂O + K₂O = 3.8 wt. %) to alkaline (Na₂O + K₂O = 12.6 wt. %) and from mafic (SiO₂ = 48.2 wt. %) to more felsic (SiO₂ = 62.1 wt. %) compositions and differ markedly from the host basalts of the xenoliths. Microthermometric and Raman spectroscopic studies on fluid inclusions (n = 115) show pure CO₂ compositions with densities range between 0.6 and 0.9 g.cm³ [290 to 735 MPa (PCO₂)], corresponding to deep fluid trapping on both sides of the Moho. High-resolution synchrotron X-ray micro-tomography (Micro-CT), together with laboratory micro-CT were performed to obtain information about structure, volume and density of each phase (minerals, melts and fluids). Fluids and melts are mainly located at grain boundaries and secondary trails cut off the grain boundaries, which implies a contemporary introduction of such fluids [Figure 1]. The amount of fluid inclusions in xenoliths is heterogeneous and varied from 0.79 ± 0.15 to 4.58 ± 0.54 vol % of the peridotite. The carbon-dioxide content stored in the lithospheric mantle, due to the percolation of asthenospheric melts produced in the mantle beneath the Pannonian Basin, can be estimated by the combination of 3D reconstruction (Micro-CT) and CO₂ pressures from inclusions.

[1] B. Sherwood Lollar et al., 1997. Geochim. Cosmochim. Acta, vol. 61, no. 11, pp. 2295–2307