The activity of silica in kimberlites, revisited

Abstract

The activity of silica in a silicate liquid \( (a_{{{\text{SiO}}_{2} }}^{\text{liq}} ) \) in equilibrium with olivine and orthopyroxene decreases with increasing pressure. In contrast, the activity of silica in an unbuffered silicate liquid changes little with pressure. Although the implications of these pressure dependencies have been considered by previous authors in terms of inferring pressures of origin of magmas, less consideration has been given to the implications of these dependencies on the evolution of the magma en route to the surface, or to the mantle through which the magma passes. In this paper, a combination of Schreinemakers’ analysis in isothermal \( P - \log (a_{{{\text{SiO}}_{2} }}^{\text{liq}} ) \) section and calculated reactions in \( P - T - \log (a_{{{\text{SiO}}_{2} }}^{\text{liq}} ) \) space is used to (a) rationalize the absence of orthopyroxene xenocrysts in kimberlites and the relative abundance of olivine “megacrysts” therein, (b) propose another reason for the paucity of xenocrystic mantle-derived carbonates in kimberlites, (c) explain why clinopyroxene is much less reactive in the kimberlite melt than is orthopyroxene, and (d) explore the implications of the relative stabilities of olivine, orthopyroxene, and clinopyroxene in kimberlitic magma for the mantle through which the magma transits.