MOLECULAR-DYNAMICS SIMULATIONS OF CACO3 MELTS TO MANTLE PRESSURES AND TEMPERATURES - IMPLICATIONS FOR CARBONATITE MAGMAS

Carbonatite magmas have been suggested to be important agents of metasomatism of the lithospheric mantle. However, the structures and properties of this important class of melt have been only poorly constrained at mantle pressures and temperatures. In the present study a molecular dynamics approach is adopted to constrain carbonatite magma properties and structure, since experimental difficulties preclude the direct study of alkaline carbonate melts at pressure. Simulation results suggests that CaCO3 melt densities increase from 2000 kg m(-3) at P approximate to 0.1 GPa to 2900 kg m(-3) at P approximate to 10.0 GPa. Estimates of the constant pressure heat capacity of 1.65-1.90 J g(-1) K-1, isothermal compressibilities of 0.0120-0.002 kbar(-1) and thermal expansivities of 1.886-0.589 x 10(-4) K-1 for CaCO3 melts to mantle pressures and temperatures are also provided from simulation results. Self-diffusion coefficients, calculated from simulation results, qualitatively suggest that CaCO3 melts have very low viscosities even at high pressures. The molecular dynamics simulations suggest octahedral Ca2+ coordination in carbonatite melts up to at least 11.5 GPa and the presence in the melt phase of associative metal-carbonate clusters. Fluid-flow calculations, based on the derived density data, suggest carbonatite ascent rates of 20-65 m s(-1), which imply that mantle derived carbonatites should be capable of transporting mantle xenoliths of dimensions up to 0.25 that of their conduits.