Stability of RSiO₃ phases in the manganese rich portion of the system MnO–MgO–FeO–SiO₂–CO₂

抄録

Experimental investigation in the Mn-rich portion of the system MnO–MgO–FeO–SiO₂–CO₂ has been carried out to study the effect of X_Fe on the phase relation in the ternary system MnSiO₃–MgSiO₃–FeSiO₃. The experiments were conducted with oxygen and/or graphite buffers in the system C–O–H at pressure-temperature conditions corresponding to normal crustal metamorphism to understand the evolution of mineral assemblages from Ca-poor Mn–Mg–Fe carbonates admixed with silica.
Stability field of pyroxmangite solid solution containing Fe can appear at lower temperature than the MnSiO₃ end member. The appearance of single phase pyroxmangite_ss is insensitive to variations in X_Mn of the bulk in the range ∼0.8 and above. With increasing X_Mg and X_Fe in the bulk composition, carbonate_ss+quartz react to produce pyroxmangite_ss+Mn-clinopyroxene_ss and Mn-clinopyroxene_ss successively. All the reaction boundaries are insensitive to pressure, at least up to 8 kb, and have gentle slope in temperature. These reactions are also independent of the X_CO2 of the fluid phase in the region X_CO2=0.21−0.53 above moderate temperatures. The slope of the lnX_CO2−10⁻³/T K relationship of these reactions is similar to that of the pure pyroxmangite forming reaction. Mn-clinopyroxene_ss can be stabilized at a low temperature than pyroxmangite_ss from favourable bulk composition [X_(Mg+Fe)=0.4]. With increasing Fe content, stability field of Mn-clinopyroxene_ss extends toward higher X_Mn in region as compared to that in Fe-free system.