Petrogenetic grid for ultrahigh‐pressure metamorphism in the model system CaO‐MgO‐SiO2‐CO2‐H2O

Abstract

Abstract Petrogenetic grids for ultrahigh‐pressure (UHP) metamorphism were calculated at different Xco2 conditions in the model system CaO‐MgO‐SiO2‐CO2‐H2O involving coesite (Co), diopside (Di), dolomite (Do), enstatite (En), forsterite (Fo), magnesite (Ms), quartz (Qz), talc (Tc), tremolite (Tr) using a published internally consistent thermodynamic data set. Two P‐T grids at Xco2= 0.01 and 0.5 are described. In the calculated P‐T grid at Xco2= 0.01, four out of 10 stable invariant points, Co‐En‐Ms‐Tc, Co‐Di‐En‐Tc‐Tr, Co‐Di‐Ms‐Tc‐Tr and Di‐En‐Ms‐Tc‐Tr lie within the stability field of coesite. If the fluid phase has Xco2= 0.5, no invariant point is stable under UHP conditions. Some magnesite‐bearing assemblages are stabilized by the following three reactions: Di + Ms = Do + Fo + CO2, Ms + Tr = Do + Fo + CO2+ H2O and Ms + Tc = Fo+ CO2+ H2O at Xco2= 0.01 and by reaction Ms + Tc = Fo + CO2+ H2O together with these three at Xco2= 0.5. Ten possible UHP assemblages for mafic and ultramafic compositions at very low Xco2 conditions include the following: Co‐Do‐Ms, Co‐Di‐Ms, Co‐Di‐Tc, Di‐Ms‐Tc, Di‐En‐Tc‐, Di‐En‐Ms, Co‐Di‐En, Di‐En‐Fo, Di‐Fo‐Ms, Di‐Do‐Fo. Among them, talc‐bearing assemblages are restricted to Xco2 < 0.02 and their high‐P limit is 31.7 kb (749°C) at Xco2= 0.01. Dolomite‐magnesite‐silica assemblages have large P‐T stability fields even if Xco2 is as low as 0.1, and could occur in cold subduction zones with very low geothermal gradients. Reported UHP coesite‐dolomite assemblage is restricted only to a calc‐silicate rock interlayered with marble where Xco2 is relatively higher; no such assemblage appears for mafic and ultramafic rocks with low Xco2 evidenced by the occurrence of diopside (or omphacite) at the expense of dolomite + coesite. The effect of Xco2 on the stability of coesite‐dolomite‐magnesite, diopside‐enstatite‐magnesite, diopside‐talc assemblages is examined and the occurrence of coesite‐dolomite, magnesite‐bearing and talc‐bearing assemblages in the Dabie UHP rocks are interpreted by employing the calculated P‐T grids.