Tschermak's substitution in antigorite and consequences for phase relations and water liberation in high-grade serpentinites

A model for the incorporation of alumina in FeO-MgO-Al₂O₃-SiO₂-H₂O (FMASH) serpentinites has been developed by considering ideal Tschermak (Al₂Mg_-1Si_-1) solid solution in antigorite. The antigorite model has been calibrated by fitting the experimental conditions for the decomposition of antigorite to chlorite+olivine+orthopyroxene+fluid in the FMASH system. The antigorite Al-contents predicted with this model are in agreement with natural observations and suggest a maximum alumina solubility in antigorite of 3.6wt.% Al₂O₃ at 20kbar-650°C and of 4.5wt.% Al₂O₃ at 3kbar-560°C. In the assemblage antigorite-olivine-chlorite-fluid, the Al-content of antigorite is buffered and temperature sensitive. This temperature sensitivity is the basis for a serpentinite geothermometer at greenschist, amphibolite and eclogite facies conditions. The buffered assemblage is stable in harzburgite compositions for relatively moderate amounts of Al₂O₃ (>1.8wt.%) and is widespread in lherzolites, where it occurs together with diopside or, in a narrow temperature field, with tremolite.