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

Abstract

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.