The superciritical fluid and the origin of carbonatites

The carbonatites have distinct signatures that imply its origin in the upper mantle. A recent study  using boron isotope indicated that recycled crustal components may be sampled by carbonatite melts.

Fenitization is a common phenomenon observed in association with carbonatites, and the metasomatization of the host rock forms fenites by the reaction of alkali-rich aqueous fluids and volatiles that are left-out during crystallisation of carbonatites. Thus, the constituents of the alkali-rich aqueous fluids and volatiles are the original constituents of the carbonatite magma. As early as 1972, the carbonatite melt was thought to be a supercritical fluid. The supercritical vapour saturation of silicate magma releases volatiles, that can interact with subducting sedimentary carbonates, and result in the generation of carbonatite melt in skarn systems.

Further work in the Wasaki area of western Kenya demonstrated that the apatite in the carbonatites has formed from supercritical fluids leading us to believe that carbonatites have their origin in supercritical fluids.  The supercritical fluids control the mobility of the elements possibly by an increase in the solubility of H₂O with the pressure. Under such supercritical conditions, the solubility of carbonates increases due to a significant change in the dielectric constant of water whereby the carbonates may be separated out from the subducting lithosphere by "miscibility gap" at the upper mantle and then get injected into the lithosphere resulting in the formation of "carbonatites". Thus, most of the subducting carbonates might return to the lithosphere and whatever carbon is left-over in the mantle is expected to be in its elemental form.

Thus, the sedimentary carbonates might recycle between the crust and the mantle. The behaviour of carbonates and silicates as "supercritical fluids" is very much important in the understanding of the evolution of carbonatites. Under supercritical conditions, the solubility of carbonates in silicate melt should be higher. However, with a reduced pressure of the ascending carbonatite magma, a "miscibility gap" might arise that results in the crystallisation of carbonatites while the remaining fluids and volatiles react with the host rock to form fenites.

Silicages as a tool for separation process.

The reactivity inside a microenvironment is often different from bulk solution - and that's why nano-geochemistry is more relevant in the study of many processes in the sediments and hydrothermal fluids. 

I hope that the microenvironment provided by the "silicages" micelles of 10-nm-diameter dodecahedral silica enclosures may be used for separation of isotopes in a flow-through column. The key to successful separation lies with the selection of a suitable complexing agent for the chosen element, and a suitable medium with appropriate polarity for the micelle. When the solution containing the complexes of two different isotopes of an element passes through the column filled with the silicages, isotope fractionation should occur through diffusion. The lighter isotope shall diffuse faster and get enriched inside the micelles. Then, comparatively heavier isotope shall become enriched in the column. During this process, the stability of the micelles may be disturbed, and that's where the challenge lies. Either the silicages may have to be stabilised after removing the micelles or, suitable starting material like vanadium oxide, gold or silver has to be used for forming the nanometer-sized cages.

Apart from this, the silicages can also be used for the separation and purification of organics from the sediments in the study of organic geochemistry.  Similarly, the silicages enclosures may form a better reaction chamber for microbial growth, wastewater treatment, and many other processes.