Seminar Cinzia Farnetani

Abstract

The spatio-temporal association between Large Igneous Provinces (LIPs) and carbonatites poses key questions about the physical processes leading to the eruption of small volumes of carbonatites before the main LIP volcanism. Models that invoke partial melting of a carbonate-metasomatized lithosphere heated by the plume rely on a shaky physical assumption, namely that diffused heat reaches the lithosphere before buoyant melts.

Here we depart from existing models in several ways: First, we consider that the plume, not the lithosphere, is the source of carbon. Second, we take into account the ionic nature of carbonatitic melts. Third, our numerical simulations provide a fluid dynamically consistent evolution of plume-lithosphere system. We explore a plausible range of carbon concentrations in the plume source (196 ≤ C ≤ 440 ppm) and different depths of melting of carbonated peridotite, depending on the redox state of the mantle. Carbonatitic fluids are highly mobile and separate from the solid matrix even at low melt fracions. We calculate their ascent velocity and follow their 3D-trajectories by using millions of tracers. The term “precursory carbon” indicates plume-carbon that reaches the arbitrary depth of 140 km before the onset of anhydrous silicate melting. Our result showt hat if carbonatitic fluids are produced at depth ≥ 200 km, then the “precursory carbon” flux can be up to 6×10e12 mol(C)/yr and the mass up to 2×10e17 kg(C). Thesevalues are high and indicate that plume-carbon can reach shallow depths 1-2 Myr before the production of mafic melts. The discovery of “precursory carbon” provides a newframework to interpret not only the existence of carbonatite complexes predating LIPs, but also a puzzling observation : for several mid-Phanerozoic LIPs, including the Central Atlantic Magmatic Province, a negative C isotope excursion, synchronous with the mass extinction, slightly predates the tholeiitic volcanism.