The join Mg2Si2O6-CaMgSi2O6at 30 kilobars pressure and its application to pyroxenes from kimberlites

Abstract

At 30 kb pressure the join Mg2Si2O6-CaMgSi2O6 is binary. The liquidus descends in a smooth curve from the melting point of enstatite (1840°C) to that of diopside (1715°C). The solidus is intersected by a transition loop at 1765°C forming a peritectic where the compositions of the solid and liquid phases in equilibrium are: L = en65di35, Diss = en79de21, Enss = en92di8 (mole per cent). The diopside-rich limb of the transition loop (diopside solvus) is sharply inflected toward diopside at about 1500°C. This inflection is attributed to a metastable miscibility gap between clinoenstatite and diopside solid solutions hidden within the transition loop. At temperatures above the inflection the range of diopside solid solutions is much greater than at atmospheric pressure. However, at temperatures below 1400°C the diopside solvus at 30 kb is very close to the boundary of the two-pyroxene field at atmospheric pressure. Ca-rich and Mg-rich pyroxene pairs from peridotite nodules in kimberlites are relatively poor in R2O3, alkalis, and Fe. To a first approximation they can be treated as phases in the system Mg2Si2O6-CaMgSi2O6. Seven modern analyses of Ca-rich pyroxenes from such nodules show a range of solid solution corresponding to a range of equilibration temperatures of about 950–1300°C. At the present time we can only speculate whether this range is primary or whether some exsolution occurred during eruption.