Cinzia G. Farnetani

Some large igneous provinces (LIPs) are associated with precursory carbonatite eruptions. Models for precursory carbonatites invoking partial melting of a carbonate metasomatized lithosphere appear implausible, given the long time needed for heat conduction from the plume head. Instead, we consider that the plume, not the lithosphere, is the source of carbon. We explore a range of carbon concentrations in the plume source (196 C 440 ppm) and various melting depths of carbonate‐bearing peridotite, depending on the mantle redox state. Our numerical simulations provide a fluid dynamically consistent evolution of the plume‐lithosphere system and enable us to calculate the ascent velocity of carbonate melts. Moreover, we advect millions of passive tracers to follow the 3‐D trajectories of melts. The term “precursory‐C” indicates plume carbon that has reached sub‐lithospheric depths (140 km) before the onset of anhydrous silicate melting. For models with carbonatite melt produced at great depths (200 km), we find that the precursory‐C flux can be up to 6 × mol(C)/yr during a time period of ∼2 Myr preceding the onset of anhydrous silicate melting. The considerable mass of precursory‐C, up to 2 × kg(C), can explain carbonatite complexes predating main phase volcanism in the Paraná province and the Deccan Traps. For some LIPs, environmental and biotic perturbations, indicated by negative carbon isotope excursions and mass extinctions, predate the main phase LIPs eruptions. For the Central Atlantic Magmatic Province (CAMP) we suggest that precursory‐C could represent a non‐negligible carbon source for the earliest volcanism, likely synchronous with the end‐Triassic extinction.

Thermal evolution of telluric planets is mainly controlled by secular cooling and internal heating due to the decay of radioactive isotopes, two processes that are equivalent from the standpoint of convection dynamics. In a fluid cooled from above and volumetrically heated, convection is dominated by instabilities of the top boundary layer and the interior thermal structure is non-isentropic. Here we present innovative laboratory experiments where microwave radiation is used to generate uniform internal heat in fluids at high Prandtl number ( ) and high Rayleigh–Roberts number (ranging from to ), appropriate for planetary mantle convection. Non-invasive techniques are employed to determine both temperature and velocity fields. We successfully validate the experimental results by conducting numerical simulations in three-dimensional Cartesian geometry that reproduce the experimental conditions. Scaling laws relating key characteristics of the thermal boundary layer, namely its thickness and temperature drop, to the Rayleigh–Roberts number have been established for both rigid and free-slip boundary conditions. A robust conclusion is that for rigid boundary conditions the internal temperature is significantly higher than for free-slip boundary conditions. Our scaling laws, coupled with plausible physical parameters entering the Rayleigh–Roberts number, enable us to calculate the mantle potential temperature for the Earth and Venus, two telluric planets with different mechanical boundary conditions at their surface.