LIMELIGHT · The contribution of magma-carbonate interactions in the upper crust to volcanic CO2 emissions: quantitative constraints from experimental petrology, isotope geochemistry and thermal modelling

The contribution of magma-carbonate interactions in the upper crust to volcanic CO2 emissions: quantitative constraints from experimental petrology, isotope geochemistry and thermal modelling.

State of the art and goals of the project Since the beginning of the 20th century, the question of whether the interaction of ascending magmas with their wall-rock can change their geochemical behaviour and differentiation paths has been debated (e.g. Daly 1910; Rittmann 1933; Shand 1945). Such contamination is most often identified via the use of trace elements behaviour or isotope fingerprints (DePaolo 1981). Little attention has been paid, however, on how such a mechanism may affect the volatile budget of magmas. Volatiles released by magmas play a fundamental role on a variety of processes at different time scales, such as the origin and evolution of the atmosphere or the transport and deposition of metallic elements in the crust. A growing body of evidence indicates that the emission of thermogenic gases (i.e. deriving from the heating of sedimentary wall-rocks by the magma) probably contributed to periods of global climate change and mass extinctions that punctuate the evolution of life on earth (e.g. Svensen et al. 2004, 2009; Ganino and Arndt 2009). The assimilation of country rock boosts the volatile content of magmas, and by implication, influences all processes related to volatile transport, including ore deposition around or within intrusive magma bodies. The present project has been devoted to evaluate the role of magma-sediment interactions on magma evolution, degassing and ore formation in magmatic-volcanic environments: it has initially focused on the interaction between magmas and carbonate rocks in the upper continental crust and then has been extended to the interaction of magmas with other sedimentary rocks, i.e. organic matter-bearing sediments and evaporites. Work carried out Several experimental studies were carried out at magmatic pressure and temperatures in internally heated pressure vessels to characterize magma-carbonate interaction and related processes involving magmatic volatiles. The experimental products were characterized as following: textural analysis and phase identification of the run products by scanning electron microscope; mineral and melt compositions determined by electron microprobe; determination of the amounts of volatile dissolved in the melt by infrared spectroscopy and mass spectrometry. The experimental results were elaborated and compared to available data on natural products. Semi-theoretical modelling of experimental data was also carried out. Interactions between magmas and organic matter-bearing rocks were studied using a theoretical approach: thermodynamic calculations of melt-fluid-solid equilibria, involving C-H-S-O species. The theoretical results were applied in particular to the case of the Siberian large igneous province (Siberian Traps), for which the composition and the fluxes of the gases produced by a single magmatic event were estimated. The atmospheric evolution and dispersion of the estimated gas emissions were explored using a regional 3D atmospheric model including transport of trace gases and tropospheric chemistry, which revealed possible short and long term environmental consequences of produced gas emissions.

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