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

摘要

A model for the incorporation of alumina in FeO-MgO-Al2O3-SiO2-H2O (FMASH) serpentinites has been developed by considering ideal Tschermak (Al2Mg_(-1)Si_(-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 3.6 wt.% Al2O3 at 20 kbar-650 °C and of 4.5 wt% Al2O3 at 3 kbar-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 (> 1.8 wt%) and is widespread in lherzolites, where it occurs together with diopside or, in a narrow temperature field, with tremolite.