Thermochemical structure and evolution of cratonic lithosphere in central and southern Africa

The thermochemical structure of the subcontinental mantle holds crucial information on its origin and evolution that can inform energy and mineral exploration strategies, natural haz-ard mitigation and evolutionary models of the Earth1−4. However, imaging the fine-scale thermochemical structure of continental lithosphere remains a major challenge. Here we combine multiple land and satellite datasets via thermodynamically-constrained inversions to obtain a high-resolution thermochemical model of central and southern Africa. Results reveal diverse structures and compositions for cratons, indicating distinct evolutions and responses to geodynamic processes. While much of the Kaapvaal lithosphere retained its cra-tonic features, the western Angolan-Kasai shield and the Rehoboth block have lost their cra-tonic keels. The lithosphere of the Congo Craton has been affected by metasomatism, increas-ing its density and inducing its conspicuous low-topography, geoid and magnetic anomalies. Our results reconcile mantle structure with the causes and location of volcanism within and around the Tanzanian Craton, whereas the absence of volcanism towards the north is the result of local asthenospheric downwellings, not to a previously-proposed lithospheric root connecting with the Congo Craton. Our study offers greatly improved integration of man-tle structure, magmatism and the evolution and destruction of cratonic lithosphere and lays the groundwork for new evolutionary models and exploration frameworks for the Earth and other terrestrial planets.

The statistical properties of protostellar discs and their dependence on metallicity

We present the analysis of the properties of large samples of protostellar discs formed in four radiation hydrodynamical simulations of star cluster formation. The four calculations have metallicities of 0.01, 0.1, 1 and 3 times solar metallicity. The calculations treat dust and gas temperatures separately and include a thermochemical model of the diffuse interstellar medium. We find the radii of discs of bound protostellar systems tend to decrease with decreasing metallicity, with the median characteristic radius of discs in the 0.01 and 3 times solar metallicity calculations being ≈20 and ≈65 au, respectively. Disc masses and radii of isolated protostars also tend to decrease with decreasing metallicity. We find that the circumstellar discs and orbits of bound protostellar pairs, and the two spins of the two protostars are all less well aligned with each other with lower metallicity than with higher metallicity. These variations with metallicity are due to increased small scale fragmentation due to lower opacities and greater cooling rates with lower metallicity, which increase the stellar multiplicity and increase dynamical interactions. We compare the disc masses and radii of protostellar systems from the solar metallicity calculation with recent surveys of discs around Class 0 and I objects in the Orion and Perseus star-forming regions. The masses and radii of the simulated discs have similar distributions to the observed Class 0 and I discs.

Experimental and Numerical Analysis of a Low Environmental Impact Pyro-Gasification System for the Energetic Valorization of Waste through a Biomass Steam Power Plant

This paper addresses the study of a pyro-gasification plant designed, built, and operated to recover inert metals from different types of solid waste. Experimental tests were carried out using pulper as the solid waste. However, while a reliable composition analysis of the produced syngas was carried out, a precise composition evaluation of the pulper used during the experimental activities was not performed and the related data were characterized by unacceptable uncertainty. Therefore, with the aim of reliably characterizing the plant operation, a thermochemical model of the gasification process was setup to simulate the equilibrium operation of the plant and a vector optimization methodology was used to calibrate the numerical model. Then, a decision-making problem was solved to identify the most suitable optimal solution between those belonging to the Pareto optimal front, thus obtaining reliable composition data for the adopted pulper waste. In particular, four different identification criteria were applied for the selection of small subset of solutions over the 3138 dominant solutions found. Among them, the solution (i.e., set of calibration parameters) that minimizes the experimental-numerical difference between the lower heating value of the produced syngas seemed to provide the most reliable approximation of the real plant operation. Finally, a possible plant configuration is proposed for the energetic valorization of the pulper waste and its overall conversion process efficiency is estimated.

Gas versus dust sizes of protoplanetary discs: effects of dust evolution

Context. The extent of the gas in protoplanetary discs is observed to be universally larger than the extent of the dust. This is often attributed to radial drift and grain growth of the millimetre grains, but line optical depth produces a similar observational signature. Aims. We investigate in which parts of the disc structure parameter space dust evolution and line optical depth are the dominant drivers of the observed gas and dust size difference. Methods. Using the thermochemical model DALI with dust evolution included we ran a grid of models aimed at reproducing the observed gas and dust size dichotomy. Results. The relation between Rdust and dust evolution is non-monotonic and depends on the disc structure. The quantity Rgas is directly related to the radius where the CO column density drops below 1015 cm−2 and CO becomes photodissociated; Rgas is not affected by dust evolution but scales with the total CO content of the disc. While these cases are rare in current observations, Rgas/Rdust > 4 is a clear sign of dust evolution and radial drift in discs. For discs with a smaller Rgas/Rdust, identifying dust evolution from Rgas/Rdust requires modelling the disc structure including the total CO content. To minimize the uncertainties due to observational factors requires FWHMbeam < 1× the characteristic radius and a peak S/N > 10 on the 12CO emission moment zero map. For the dust outer radius to enclose most of the disc mass, it should be defined using a high fraction (90–95%) of the total flux. For the gas, any radius enclosing >60% of the 12CO flux contains most of the disc mass. Conclusions. To distinguish radial drift and grain growth from line optical depth effects based on size ratios requires discs to be observed at high enough angular resolution and the disc structure should to be modelled to account for the total CO content of the disc.