Dobson, David Peter; (1995) The system potassium carbonate-magnesium carbonate as an analogue to mantle carbonate melts: Experimental determination of phase relations and selected physical properties at high pressure. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Carbonatites are igneous rocks with a major element chemistry based on the carbonate anion. Although they are rare throughout the Earth's history, geochemical evidence suggests that carbonate-rich melts and fluids play an important role within the Earth's mantle, as agents for metasomatism and fluxes to larger volume partial melting. Relatively few experiments have been performed at pressure and temperature conditions pertaining to the mantle. Generating the high pressures necessary to simulate the mantle is a non-trivial task. Such experimental studies have so far been concerned with melting phenomena in carbonated mantle silicate assemblages, rather than employing a systematic approach to characterising the properties (physical and chemical) of carbonate systems at temperatures below melting of the silicate host-rock. The compositions of natural 'fluid'-inclusions in mantle minerals, particularly coated-stones were used to constrain the probable composition of the "primary' mantle carbonate melt. The system K2CO3-MgCO3 was chosen as a first approximation to an end-member mantle carbonaceous metasomatising agent and melting phase relations were studied at pressures up to 5 GPa, corresponding to 150 km depth. In order to assess the mobility and, hence effectiveness of such melts as metasomatic agents, it is important to study their physical properties, so the viscosity and density of various molten carbonate systems were measured in situ using the falling sphere technique. Studies into a possible genetic link between such melts and diamond growth were performed and it was found that some carbonate compositions were effective catalysts to diamond nucleation and growth under mantle pressure and temperature conditions. The experimental evidence leads to the conclusion that molten alkali carbonate fluids are powerful metasomatising agents within the Earth's mantle which are involved in the formation of silica-undersaturated melts, such as kimberlites and silicate- carbonatite igneous complexes, as well as the growth of coated-stone diamond. Mantle carbonate melts are likely to be important in the transport of recycled carbon into the mantle from subducted oceanic crust.

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