EXPERIMENTAL-STUDY OF CHLORITOID STABILITY AT HIGH-PRESSURE AND VARIOUS FO(2) CONDITIONS

The reaction chloritoid (ctd) = almandine (alm) + diaspore + H2O (CAD) has been reversed using Fe3+ -free synthetic chloritoid and almandine , under fO(2) conditions of the solid oxygen buffer Fe/FeO (CADWI), an d using partially oxidized synthetic minerals under fO(2) conditions o f the solid oxygen buffer Ni/NiO (CADNNO). Experiments have been condu cted between 550 and 700 degrees C, 25 and 45 kbar. The equilibrium pr essure and temperature conditions are strongly dependent on the fO(2) conditions (CADNNO lies somewhat 50 degrees C higher than CADWI). This can be explained by a decrease in aH2O for experiments conducted on t he Fe/FeO buffer, and a decrease in actd and aalm (through incorporati on of ferric iron preferentially in chloritoid) for experiments conduc ted on the Ni/NiO buffer. The H2O activity has been calculated using t he MRK equation of state, and the values obtained checked against the shift of the equilibrium diaspore = corundum + H2O bracketed on the Fe /FeO buffer and under unbuffered fO(2) conditions. For fO(2) buffered by the assemblage Fe/ FeO, aH(2)O increases with pressure from about 0 .85 at 600 degrees C, 12 kbar to about 0.9 at 605 degrees C, 25 kbar a nd 1 above 28 kbar. For fO(2) buffered by the assemblage Ni/ NiO, aH(2 )O = 1. The aH(2)O decrease from Ni/NiO to Fe/ FeO is, however, too sm all to be entirely responsible for the temperature shift between CADNN O and CADWI. In consequence, the amount of ferric iron in almandine an d chloritoid growing in the CADNNO experiments must be significant and change along the CADNNO, precluding calculation of the thermodynamic properties of chloritoid from this reaction. Our experimental data obt ained on the Fe/FeO buffer are combined, using a thermodynamic analysi s, with Ganguly's (1969) reversal of the reaction chloritoid = almandi ne + corundum + H2O (CAC) on the same oxygen buffer. Experimental brac kets are mutually consistent and allow extraction of the thermodynamic parameters H degrees f,ctd and S degrees ctd. Our thermodynamic data are compared with others, generally calculated using Ganguly's bracket ing of CACNNO. The agreement between the different data sets is relati vely good at low pressure, but becomes rapidly very poor toward high p ressure conditions. Using our thermodynamic data for chloritoid and K- D = (Fe3+/ Al)ctd/(Fe3+/Al)alm estimated from natural assemblages, we have calculated the composition of chloritoid and almandine growing fr om CADNNO and CACNNO. The Fe3+ content in chloritoid and almandine inc reases with pressure, from less than 0.038 per FeAl2SiO5(OH)(2) formul a unit at 10 kbar to at least 0.2 per formula unit above 30 kbar. This implies that chloritoid and almandine do contain Fe3+ in most natural assemblages. The reliability of our results compared to natural syste ms and thermodynamic data for Mg-chloritoid is tested by comparing the equilibrium conditions for the reaction chloritoid + quartz = garnet (gt) + kyanite + H2O (COCK), calculated for intermediate Fe-Mg chlorit oid and garnet compositions, from the system FASH and from the system MASH. For 0.65 < (XFe)gt < 0.8, CQKG calculated from FASH and MASH ove rlap for K-D = (Mg/Fe)ctd/(Mg/Fe)gt = 2. This is in good agreement wit h the K-D values reported from chloritoid + garnet + quartz + kyanite natural assemblages.