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.