Experimental Investigation of Soapstone and Granite Rocks as Energy-Storage Materials for Concentrated Solar Power Generation and Solar Drying Technology

The intermittence of solar energy resource in concentrated solar power (CSP) generation and solar drying applications can be mitigated by employing thermal energy storage materials. Natural rocks are well recommended thermal energy storage materials as they are efficient for CSP generation. This study explores the potential of soapstone rock and also the influence of the sites' geo-tectonic setting to soapstone and granite rocks as thermal energy storage materials. Experimental characterization was done to investigate the thermo-chemical properties (thermal stability (TGA), crystalline phases (XRD), petrographic imaging and chemical composition (XRF), and high temperature test); the thermo-physical properties (density, porosity, specific and thermal capacity (DSC), thermal diffusivity, and conductivities (LFA)); and the thermo-mechanical properties (Young's modulus) of the rocks. Consequently, the rock with the most desired properties for thermal energy storage was the soapstone rock from the Craton geo-tectonic setting and it had a Young's modulus of 135 GPa at room temperature. At solar drying and CSP temperatures it had thermal capacities of 3.28 MJ/(m³·K) and 4.65 MJ/(m³·K); densities of 2.785 g/cm³ and 2.77 g/cm³; and conductivities of 2.56 W/(m·K) and 2.43 W/(m·K) respectively, and had weight loss of 0.75% at 900 °C. At high temperatures, only granite from Craton had visible cracks while the other 3 rocks did not show visible signs of fracture. Conclusively, soapstone and granite from Craton in the Dodoma region and Usagaran in the Iringa geo-tectonic settings exhibit significant differences in most thermo-properties.

Plume-driven recratonization of deep continental lithospheric mantle

Cratons are Earth's ancient continental land masses that remain stable for billions of years. The mantle roots of cratons are renowned as being long-lived, stable features of Earth's continents, but there is also evidence of their disruption in the recent1-6 and more distant7-9 past. Despite periods of lithospheric thinning during the Proterozoic and Phanerozoic eons, the lithosphere beneath many cratons seems to always 'heal', returning to a thickness of 150 to 200 kilometres10-12; similar lithospheric thicknesses are thought to have existed since Archaean times3,13-15. Although numerous studies have focused on the mechanism for lithospheric destruction2,5,13,16-19, the mechanisms that recratonize the lithosphere beneath cratons and thus sustain them are not well understood. Here we study kimberlite-borne mantle xenoliths and seismology across a transect of the cratonic lithosphere of Arctic Canada, which includes a region affected by the Mackenzie plume event 1.27 billion years ago20. We demonstrate the important role of plume upwelling in the destruction and recratonization of roughly 200-kilometre-thick cratonic lithospheric mantle in the northern portion of the Slave craton. Using numerical modelling, we show how new, buoyant melt residues produced by the Mackenzie plume event are captured in a region of thinned lithosphere between two thick cratonic blocks. Our results identify a process by which cratons heal and return to their original lithospheric thickness after substantial disruption of their roots. This process may be widespread in the history of cratons and may contribute to how cratonic mantle becomes a patchwork of mantle peridotites of different age and origin.

Deflection of mantle plume material by cratonic keels

Lithosphere that formed in Archaean and possibly early Proterozoic time is thicker, more buoyant, and geochemically distinct from lithosphere that formed after about 2.3 Ga. Mantle xenolith and seismic data indicate that some cratonic roots, or ‘keels’, extend to depths of c. 250 km, compared with normal continental lithosphere of thickness 150 km or less; yet many cratons have experienced uplift, dyking and kimberlite emplacement, suggesting interactions with hot, rising asthenosphere referred to as mantle plumes. Plumes supply additional heat to the base of the lithospheric plates, whose base can be heated and entrained in the flow (thermal erosion). How have these cratonic keels persisted despite their interactions with mantle plumes? The geometry of cratonic keels during their interactions with mantle plumes is a critical factor controlling keel preservation. To a laterally spreading plume head, cratonic keels appear as major obstacles, and the hot, buoyant plume material ponds beneath thinner lithosphere. Our model simulations show that deep keels deflect mantle plume material and that steep gradients at the lithosphere-asthenosphere boundary between Archaean keels and ‘normal’ lithosphere will focus flow, leading to localized adiabatic decompression melting. Plume processes can lead to a reduction in the breadth of a cratonic root where the plume rises beneath the craton, regardless of the initial breadth of the craton. Where the plume rises beneath a craton the hot plume material will spread laterally beneath the keel and attain thicknesses of tens of kilometres. This transfers heat to the base of the lithosphere and could generate small volumes of melt at considerable depth, depending on the composition of the lower lithosphere. We have used model simulations of plumes beneath Africa to predict the magnitude and direction of seismic anisotropy caused by lateral flow of hot plume material beneath and around a cratonic keel. The shear-wave splitting in our models is greatest at the edge of the cratonic keel, and its azimuth is parallel to the plume flow direction.