The stability of merwinite in the system CaO-MgO-SiO2-H2O-CO2 with CO2-poor fluids

The stability of merwinite (Mw) and its equivalent assemblages, akermanite (Ak)+calcite (Cc), diopside (Di)+calcite, and wollastonite (Wo)+monticellite (Mc)+calcite was determined at T=500–900° C and P_f=0.5–2.0 kbar under H₂O–CO₂ fluid conditions with X_CO2=0.5, 0.1, 0.05, and 0.02. Merwinite is stable at P_f=0.5 kbar with T>700° C and X_CO2<0.1. At P_f=2.0 kbar, the assemblage Di+Cc replaces merwinite at all T and X_CO2 conditions. At intermediate P_f=1 kbar, the assemblage Ak+Cc is stable above 707° C and Wo+Mc+Cc is stable below 707° C. The univariant curve for the reaction Di+Cc=Wo+Mc+CO₂ is almost parallel to the T axis and shifts to low P_f with increasing X_CO2, with the assemblage Di+Cc on the high-P_f side. The implications of the experimental results in regard to contact metamorphism of limestone are discussed using the aureole at Crestmore, California as an example.