The pressure and temperature dependence of antigorite polysomatism was investigated in the P, T-range of 350–710°C, 0.2–5.0 GPa within the system MgO-SiO2-H2O (MSH). TEM-study indicated that increasing temperature and decreasing pressure of MSH-antigorite formation are correlated with a shorter a-cell periodicity, i.e. smaller m, value (m = number of tetrahedra in a single chain along the wavelength a). For the P, T-conditions investigated, the compositional m-range of antigorite is rather narrow (14–18) corresponding to Mg2.79Si2O5(OH)3.57 Mg2.83Si2O5(OH)3.67. The change in the crystal structure of antigorite is combined with a gradual partial dehydration process and loss of MgO.
For the formulation of reactions that include antigorite, the P, T-dependence of the chemical composition of antigorite has to be considered. However, our results support the conclusion of Mellini et al. (1987) of the limited applicability of a modulation-dependent geothermobarometer, because equilibrium states of antigorite modulation are rarely observed in natural occurrences and have not been reached in our experiments.
The successive partial dehydration of antigorite during ongoing subduction of serpentine-bearing oceanic lithosphere probably influences its rheological properties.