A thermodynamic model for Fe-Mg aluminous chlorite using data from phase equilibrium experiments and natural pelitic assemblages in the 100 degrees to 600 degrees C, 1 to 25 kb range

The purpose of this study is to derive a solid solution model for aluminous (Si < 3 a.p.f.u.) chlorites encountered in metapelites over a wide range o f P-T conditions. A compilation of chlorite compositions in quartz-bearing rocks led us to propose a four-thermodynamic-component (Mg-amesite, clinoch lore, daphnite, and Mg-sudoite) solid solution model that accounts for the Tschermak, Fe-Mg, and di/trioctahedral substitutions observed in nature. A new feature emerging from this compilation is the contrasting effect of tem perature and pressure variations on the Al-IV and vacancy contents in chlor ites. A 3-site mixing model with symmetric Margules parameters and ideal in ter-site interaction has been adopted to model these compositional changes. In contrast to previous models, the relevant thermodynamic data (Mg-amesit e and daphnite standard state properties as well as W-AlMg, W-AlFe, W squar e (Fe), W square (Mg), and W square (Al) on M1) are calibrated with indepen dent sets of published experiments conducted in the MASH and FMASH systems (similar to 60 reversals) as well as about 200 natural data involving chlor ite + quartz + (carpholite or chloritoid) assemblages. Moreover, the constr aints span a wide range of pressure and temperature conditions (100 degrees -850 degrees C, 0.5-20 kb), so that no extrapolation outside the calibrati on range is needed for P-T thermobarometric purposes. The calculated thermo dynamic data are compatible with the thermodynamic data of clinochlore from Berman (1988), Mg-sudoite and Mg carpholite data from Vidal and others (19 92), Fe-chloritoid from Vidal and others (1994), and the chlorite-chloritoi d Fe-Mg exchange thermometer of Vidal and others (1999). The chlorite solut ion model seems to be consistent also with the solid solution properties fr om Berman (1990) for garnet, Fuhrman and Lindsley (1988) for plagioclase, a nd Evans (1990) for epidote, although additional work is required to explai n the large discrepancies observed between the temperatures obtained from e mpirical garnet-chlorite Fe-Mg exchange thermometers and the temperatures c alculated in the present study. The use of several chlorite endmembers makes the estimation of paleo-pressu re and -temperature conditions possible for high-variance parageneses (> 1) which is not possible when using only one chlorite endmember (classically clinochlore). In particular, reliable pressure estimates can be made for th e common chlorite-quartz-carpholite or chloritoid or garnet bearing rocks d evoid of aluminosilicates, whereas such estimates are impossible when using only one chlorite endmember. In the most favorable cases, temperature cond itions can be estimated from the location of the temperature-dependent equi librium 2 clinochlore + 3 Mg-sudoite = 4 Mg-amesite + 7 quartz + 4 H2O, tha t is from the composition of chlorite associated with quartz. Our chlorite solution model predicts that at fixed pressure and (XMg)(chlorite), the loc ation of this equilibrium is shifted toward higher temperature when decreas ing the Si, Al-VI, and vacancy contents and increasing the Al-IV content. T his result is compatible with the classical empirical thermometers based on the Al-IV and vacancy contents in chlorite. However, the calculated effect of pressure is an increase of the Al-IV, Al-VI, and vacancy contents. This explains why the empirical chlorite thermometers (based on the Al contents in chlorite) derived from low-T samples cannot be used at high pressure co nditions.