Stability of the scapolite meionite (3CaAl₂Si₂O₂ · CaCO₃) at high pressures and storage of CO₂ in the deep crust

Abstract

The stability field of the end-member scapolite meionite was determined in piston-cylinder apparatus. Meionite has very high thermal stability at high pressures, exceeding 1500° C at 20 kbar. Below 6 kbar and 1270 ° C scapolite breakdown is subsolidus, to an-orthite + gehlenite + wollastonite + CO₂, with a slope of 20 bars/degree. An extrapolation of existing thermodynamic data for CO₂ permits calculation of ΔG ^o_F =-2384.5 kcal/mol for meionite at 1270 ° C, very close to the value for 3 anorthite + calcite. Above 1270 ° C, scapolite begins to melt to An+Geh+Liq+CO₂, and as pressure increases the melting curve steepens, the Geh and An being progressively replaced by Liq+corundum with Al in 6-coordination. At pressures >25kbar dp/dt becomes negative, corundum is the only crystalline product, and CO₂ bubbles disappear from the quenched glass, indicating a solubility of CO₂ under these conditions of about 5 wt. percent in the liquid.

The subsolidus breakdown of meionite at high pressures to grossularite + kyanite + quartz + calcite nearly coincides with the upper pressure limits of anorthite. Thus scapolite is essentially limited to crustal rocks. In view of its great thermal stability, meionite can play a role as a primary mineral in deep-seated basic or intermediate magmatic processes. It is also likely that CO₂ coming from the earth's interior will be captured by reaction with plagioclase and clinopyroxene. Scapolite has been noted in basic granulite inclusions from basaltic pipes in three continents. It seems probable that scapolite acts as a major storage site for CO₂ in the deep crust.