CONTOURING REACTION SPACE IN P-T-X: CALIBRATION FROM EXPERIMENTAL STUDIES

Experimental studies (Liou et al. 1974; Apted & Liou 1983) provide compositional data on amphibole, chlorite & plagioclase coexisting with epidote, quartz and H2O, from 400 to 675oC at 2, 5 & 7 kbar at controlled fO2 (QFM, NNO, HM). This five-mineral phase assemblage [Amp], [Chl], [Plg], [Epi], [Qtz] is common to greenschist, blueschist, and amphibolite facies metamorphism of mafic rocks which usually lose H2O during prograde metamorphism. The oxide components NCFMASHO are necessary to describe the modal and chemical changes of this mineral assemblage during metamorphism open or closed to H2O. In the system NCMASH there are three independent net-transfer reactions (NTR) by which these changes can be measured. FeMg-1 exchange occurs between [Amp]=[Chl], and FeAl-1 exchange occurs among [Epi]=[Amp]=[Chl]. One additional NTR can be written to relate “O”=Fe2O3 [in Epi] – 2FeO [in Amp, Chl].

Useful reaction spaces have been constructed that use as three orthogonal axes NTR’s that buffer NaAl-1Si-1 (a), Al2Mg-1Si-1 with NaSiCa-1Al-1 (b), and Al2Mg-1Si-1 with H2O (c). Although the two experimental studies used different samples, their reaction spaces overlap sufficiently so as to be correlatable. The measured mineral compositions were used to locate the experimental P-T data points. In b-c, temperature increases primarily by decreasing modal [Epi] & increasing [Plg] at 2kbar QFM and with decreasing modal [Chl] & increasing [Amp] at 7kbar NNO and QFM. Decreasing modal [Plg] and increasing [Amp] dominate the greenschist-blueschist transition. In a-b and a-c the temperature interval of the experiments did not show a miscibility gap in NaAl-1Si-1 but evidence for one in Al2Mg-1Si-1 is seen between 550 and 600oC. These reaction progress sections through reaction space are inverse aspects which we have compared with forward aspects of calculations of thermodynamic equilibria in P-T-X space for known fH2O and fO2. P-T space has been contoured for selections of continuous NTR’s along the compositional vectors NaAl-1Si-1 [Amp], Al2Mg-1Si-1 [Amp],[Chl] & NaSiCa-1Al-[Amp],[Plg].