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Geochemical evidence suggests that at least some kimberlite (Smith et al., 1985) and carbonatite magmas (Nelson et al., 1988; Harmer and Gittins, 1998) are derived by melting of the asthenospheric mantle. On account of the high concentrations of incompatible trace elements in carbonatites and kimberlites, generation by very small degrees of melting is inferred (Nelson et al., 1988). Experiments to determine the phase equilibria involving incipient melting of upper mantle peridotite are technically a difficult task. Because of the small amount of melt present, it is hard to maintain equilbrium during the experiments, and the melts generated are almost invariably modified upon quenching and impossible to analyze because of their very small volume. However, by reducing the number of components, and hence the variance of the system, the task becomes reasonable in scope. The system CaOMgO-Al2O3-SiO2-CO2 (CMAS-CO2) is a realistic model system to investigate the phase equilbria involving mantle peridotite in equilibrium with CO2bearing melts (Dalton and Presnall, 1998a,b). In the CMAS-CO2 system, the garnet lherzolite phase assemblage coexists with melt along a divariant surface in P-T space (Fig. 1). On the low-temperature side, the divariant surface is bounded by a univariant solidus curve, along which the garnet lherzolite phase assemblage is joined by dolomite or magnesite. On the high-temperature side, the surface is delimited by the CO2-free garnet lherzolite solidus. As the P-T slope of the CO2-free solidus is considerably greater than that of the carbonate-bearing solidus, the temperature difference between these curves increases with pressure. At a fixed pressure and temperature, the garnet lherzolite plus liquid phase assemblage is invariant and compositions of all phases are uniquely defined, independent of the bulk composition of the system. This means that with proper starting compositions the phase equilibrium data can be obtained in experiments with large amount of melt present. Once the phase equilibrium data have been acquired, algebraic methods allow calculation of melting paths for any arbitrary mantle composition in the CMAS-CO2 system (Presnall, 1986), including melting paths where very small amounts of liquid are present.

Continuous gradations among primary kimberlitic, carbonatitic, melilititic, and komatiitic melts in equilibrium with garnet lherzolite at 3-8 GPa