Experimental Study of Chemical Counterdiffusion of Major Components (SiO2, Al2O3, Na2O, CaO, MgO, and FeO) and \({\text{CO}}_{3}^{{2 - }}\) Anion during Interaction of Basalt and Kimberlite Melts under Upper Mantle P–T Conditions

Abstract

This paper reports new experimental results on the chemical counterdiffusion of major components (SiO₂, Al₂O₃, Na₂O, CaO, MgO, and FeO) and the \({\text{CO}}_{3}^{{2 - }}\) anion during interaction of basalt and kimberlite melts under upper-mantle pressure. The method of diffusion couples was employed on a BARS split-sphere apparatus at 5.5 GPa and 1850°C. It was shown that the rates of chemical counterdiffusion of all major melt species (SiO₂, Al₂O₃, Na₂O, CaO, MgO, and FeO) and the \({\text{CO}}_{3}^{{2 - }}\) anion are almost identical during interaction of model basalt and carbonate-bearing kimberlite melts and approximately an order of magnitude higher than the diffusion rates of these components during melt interaction under moderate pressures (100 MPa). The equal diffusion rates of CaO and \({\text{CO}}_{3}^{{2 - }}\) indicate that molecular CaCO₃ diffusion from the kimberlitic to basaltic melt (model and natural) occurs also at the high pressure. The diffusion patterns are dramatically different during interaction of natural magnesian basalt and model kimberlite, which was observed for the interaction of these melts at moderate pressure. In addition to the molecular diffusion of CaCO₃ into the magnesian basalt, the diffusion rates of other melt species increase significantly. All diffusing components show weak exponential dependence on concentrations approaching D_i = const, similar to that observed during interaction of such melts at moderate pressures.