CO 2 as a volatile component of the mantle: The system Mg 2SiO 4SiO 2H 2OCO 2

Abstract

The system Mg 2SiO 4SiO 2H 2OCO 2 has been studied at a pressure of 20 kbar as a model for the melting behavior of peridotite in the presence of two volatile components. In the presence of CO 2 and H 2O, the assemblage forsterite (Fo) + enstatite (En) begins to melt at a higher temperature than in the presence of H 2O alone. The liquids are CO 2- and H 2O-charged, yet show no tendency toward carbonatite immiscibility. Partial melts are generated at temperatures greater than 1400°C when CO 2 H 2O ratios are greater than 45 mole % CO 2 and are silica-undersaturated in the CIPW norm, whereas partial melts produced at lower temperatures and H 2O-rich conditions are silica-oversaturated. Liquids in the vapor-absent region of the join Fo-En-H 2O which are in equilibrium with Fo + En also exhibit this normative change in character at 1400°C at 20 kbar. The chemical change therefore is not a result of changes in melt structure due to CO 2 solution, even though up to about 7 wt.% CO 2 dissolves in En melt at 20 kbar. Rather, CO 2 raises the temperature of the peridotite solidus relative to the beginning of melting with H 2O and reduces activity of H 2O. Hydrous magmas produced in the mantle in general contain much less H 2O than is needed to saturate the melt and hence do not evolve vapor as they rise, until shallow depths are reached. By contrast, magmas containing CO 2 and H 2O may evolve vapor in the mantle or lower crust. The principal cause is the relatively lower solubility of CO 2 in silicate melts, which is less than that of H 2O at high pressure and very much less at pressures less than 10 kbar. Vapor rich in CO 2 dissolves less total silicate than H 2O-vapor and has markedly lower solubility of silica but does dissolve alkalies. The relationship of silica-undersaturated liquid and CO 2-rich vapor may explain some of the characteristics of kimberlite or of magmas associated with kimberlite.