EXPERIMENTALLY DETERMINED FE-MG EXCHANGE BETWEEN SYNTHETIC STAUROLITEAND GARNET IN THE SYSTEM MGO-FEO-AL2O3-SIO2-H2O

The Fe-Mg exchange between synthetic staurolite and garnet was determi ned experimentally as a function of pressure, temperature and bulk com position in the range of 5-25 kbar/600-750 degrees C on the basis of t he reaction 1/3 pyrope + 1/4 Fe-staurolite = 1/3 almandine + 1/4 Mg-st aurolite (Eq. (1)). The results of the exchange runs clearly indicate that the Fe-Mg fractionation between staurolite and garnet does not de pend much on pressure but on temperature and bulk composition. The com positional data on coexisting grt(ss) and st(ss), as determined by mic roprobe, were processed by a nonlinear least-squares procedure using d ifferent mixing models for staurolite solid solutions and the asymmetr ic solution model for garnet (Hackler and Wood, 1989). In all the mode ls, staurolite solid solutions show large negative deviation from idea lity. The experimental results are best described by a temperature-dep endent symmetric mixing model with W-s(st) = -79 J/K (one-site model). Such a large negative W-S-value, however, is rather unrealistic, espe cially considering, that only a relatively small temperature interval has been investigated. Therefore a symmetric mixing model with W-G(St) = (-11539 +/- 830) J (one-site model) is recommended for use in therm odynamic calculations. The expression for the derived geothermometer i s based on the temperature-dependent mixing parameters, because these describe the Fe-Mg fractionation between staurolite and garnet very we ll. The geothermometer is valid for staurolites with x(Fe) > 0.6 and i s T = [-18.02 - 0.044 P + RT ln gamma(Fe)(grt) - ln gamma(Mg)(grt) ((-87.78) + 0.057P)*(x(Fe)(st))(2)-(X-Mg(st))(2)) + 12.55(x(Ca)(grt) + x(Mn)(grt))]/[-R10(-3)*ln K-D - 0.021 + ((-0.079)*((x(Fe)(st))(2)-( x(Mg)(st))(2)))], where pressure (P) is in kbar and temperature in deg rees C. (C) 1997 Elsevier Science B.V.