CO2 quantification in magmatic systems – Innovative coupling of X-ray micro-tomography, in-situ microanalysis and thermodynamic modelling
IPGP - Îlot Cuvier
Université du Maine
Carbon is a key element for life and critical for maintaining a habitable environment, but our understanding of the carbon cycle is far from complete. A better quantification of the CO2 during magmatic processes is a starting point to improve our understanding. Indeed, during magmatic processes, CO2 can stay within the mantle and metasomatize it, or can go back to the Earth surface by advective migration with volcanism. Since five-year investigation, I develop and apply a new methodology in order to quantify CO2 in magmatic processes. The innovative developed approach couples X-ray micro-tomography, in-situ microanalysis and thermodynamic modelling. It was successfully applied to two different problematics: 1) Initial composition determination of crystallized silicate melt inclusions (SMI), and 2) CO2 quantification in the Pannonian basin (PB) lithospheric mantle. 1) SMI trapped in minerals and carried up to the surface by volcanism are routinely studied in order to determine the pre-eruptive volatile budgets of volcanic systems. The volatile contents of SMI that are affected by post-entrapment processes, such as crystallization (PEC) and/or bubble nucleation during cooling, are generally difficult to interpret. Therefore, there is a general preference to select SMI that experienced minimal post-entrapment effects. Conversely, SMI can be homogenized at high temperature and quickly quenched. This method is controversial because heating may induce leakage of water from the silicate melt during experiments in addition to loss by diffusion. The aforementioned developed method was applied to this problematic and a mathematical reconstruction of the SMI was performed with the following equation: XSMI = ?[VMineral(i) * XMineral(i)] + [VGlass * XGlass] + [VBubble * XBubble]. The PEC of large minerals observed inside SMI is consistent with slow cooling rates, meaning that exsolved volatiles diffused toward the bubble in equilibrium with the melt before the glass transition. The volatile pressure determined in XSMI and using Rhyolite-MELTS is therefore equal to the pressure of the bubble. The volatile mass in the bubble is then determined and added to the bulk volatile content of the SMI. 2) Subduction of carbonated crust is widely believed to generate a flux of carbon into the base of the continental lithospheric mantle, which in turn is the likely source of widespread volcanic and non-volcanic CO2 degassing in active tectonic intracontinental settings such as rifts, continental margin arcs and back-arc domains. However, the magnitude of the carbon flux through the lithosphere and the budget of stored carbon held within the lithospheric reservoir are both poorly known. New constraints on the CO2 budget of the lithospheric mantle below the PB have been provided through the study of a suite of mantle xenoliths. A quantitative estimate of the CO2 budget of the mantle below the PB using the aforementioned methodology was obtained. The CO2/silicate melt mass ratios in the metasomatic agent that percolated through the lithospheric mantle below the PB were estimated to be between 9.0 and 25.4 wt.%. A first-order estimate of 2000 ppm was proposed as the minimal bulk CO2 concentration in the lithospheric mantle below the PB. The developed methodology coupling X-ray micro-tomography, in-situ microanalysis and thermodynamic modelling can therefore be used for a wide range of samples and may significantly improve our knowledge of volatile degassing processes and on CO2 geological cycle.