Silicate -carbonate Systems with Bearing on the Origin and Crystalli· Zation of Carbonatites

Experimental approaches attempt to determine the routes by which crustal carbonatite magmas can be derived from C02-bearing peridotite at high pressures. The intimate association of kimberlite and carbonatite on a small scale il· lustrates links in the processes. Carbonation of peridotite by C02 can cause the generation of near-solidus carbonatitic liquid from mantle peridotite at depths greater than about 85 km, the depth depending on the geotherm. 61 For lithosphere peridotite with C02 and H20 and composition suitable for the stabilization of phlogopite, the vapor phase composition is buffered by reactions involving carbonate and phlogopite. The vapor-phase composition at the solidus controls the near-solidus liquid composition in the asthenosphere , but if carbonate persists to the solidus, the liquid is probably carbonatitic, with Ca/Mg > 1, and enriched in alkalis. For high H20/C02 ratios the upper asthenosphere liquid could be a silicate magma, but an immiscible alkali-rich carbonatitic liquid may separate from it. Experimental evidence at crustal pressures supports the concept of nepheline-normative alkaline magmas immiscible with Ca-Na carbonatite magmas, both coexisting with fluids capable of fenitizing the country rocks. Other evidence demonstrates that fractional crystallization of a carbonated alkaline peridotite parent magma from the mantle can produce sequences of mineral assemblages very similar to those developed in alkaline complexes with carbonatites. Accessory minerals such as apatite and sulfide have limited solubility in synthetic carbonatite liquids, and therefore, they are likely to be co-precipitated with calcite through wide temperature intervals in carbonatite magmas. The very low viscosity of carbonate-rich liquids promotes fractionation and the segregation of minerals in carbonatites. Mineral textures in some carbonatites are duplicated in experimental runs quenched from fields for the coexistence of crystals +liquid. The phase equilibrium data are consistent with magmatic origin and history for carbonatites, although there is no doubt that for may carbonatites the clarity of the evidence has been at least partly obscured by metasomatism and recrystallization caused by deformation.