The assemblage stilpnomelane–chlorite–phengitic mica: a geothermobarometer for blueschist and associated greenschist terranes

Two anhydrous equilibria can be written among the components of stilpnomelane, chlorite, white mica and quartz, namely 89 daphnite+131 Fe‐celadonite+190 quartz=96 stilpnomelane+71 muscovite, and amesite+Mg‐celadonite=muscovite+clinochlore. We assume that the free energy change of reaction, Δ G =Δ G o +Σ RT  ln a i j , is approximated by Δ G = A − BT  + C ( P −1)+Σ RT  ln a i j , where Δ G o is the free energy change of the end‐member components at temperature T  and pressure P , a i is the activity of component i whose coefficient in the equilibrium is j, and A , B and C are constants to be determined. Values of C can be approximated by the change in volume on reaction, namely C =406.517 J/bar for the first reaction and C =0.613 J/bar for the second reaction. Constants A and B were determined by using six occurrences of the assemblage stilpnomelane–chlorite–white mica for which P – T  have been otherwise estimated. Using solution models from the literature, linear regression gives for the first equation A =−6118.269 kJ, B =−4584.09 J/K, and for the second equation A =19.397 kJ, B =66.72 J/K. These values predict P – T  within 0.5 kbar and 25 K for all occurrences, and appear reasonably robust relative to probable analytical errors. P – T  are determined by intersection of the curves generated by given compositions in P – T  space. Fine‐grained and/or zoned chlorite and white mica make application of the geothermobarometer difficult in some instances, but our work in the Bathurst region of New Brunswick suggests that, with patience and care, useful analyses can be obtained, and the database for the geothermobarometer greatly expanded.