Experimental Investigations on the Variation of the Water Content of Antigorite and Resulting Geodynamic Implications for Subducting Processes

The serpentine-variety antigorite is proposed to be the predominant phase for carrying large amounts of water into the upper mantle during subduction of hydrated ultrabasic rocks. Water-induced processes within subduction zones, like triggering of partial melting, metasomatic alteration or even intermediate depth seismicity (80-150 km), seem to be strongly associated with the decomposition of antigorite. Antigorite forms a series of discrete compositions, which can be expressed by the general formula M_3m-3T_2mO_5m(OH)_4m-6 (T = tetrahedral cations like Si, Al; M = octahedral cations like Fe²⁺ Mg; m = number of tetrahedra in a single chain defined by the wavelength a). According to Mellini et al. (1987), typical compositions of natural antigorites are in a narrow range and the m-value seems to decrease with increasing metamorphic grade. The aim of this experimental study was to investigate the P,T-dependence of the antigorite polysomatism and and its water-content. As starting materials brucite and talc were mixed in the stoichiometric proportions of antigorite with m = 17 plus 20 wt.% additional water. Piston-cylinder and hydrothermal experiments were performed over a wide P,T-range (350-710° C, 0.2-5.0 GPa) in the pure system MgO-SiO₂-H₂O. Run products were characterized by X-ray diffraction and by using TEM. For the determination of the a-lattice modulation (and corresponding m-values) of antigorite, lattice fringe spacings were deduced from diffraction patterns of selected (010) sections in the lattice fringe image. Our study indicated that increasing temperature and decreasing pressure of antigorite formation is correlated with a smaller m-value. For the P,T-conditions investigated, the compositional m-range of antigorite is rather narrow (14-18). The change in the crystal structure of antigorite from high to low m-values is combined with a gradual partial dehydration process. Therefore, during an ongoing subduction of serpentine-bearing lithosphere a successively partial dehydration of antigorite might occur. Fluids set free by this process might influence the rheological properties of surrounding rocks: Fluid migration into pores or along interfaces, thus changing the adhesion of the grains, might reduce the effective confining pressure. As a consequence, for a better understanding of rheological processes like e.g. dehydration embrittlement, which is thought to be a possible reason for seismicity within subduction zones, the proceeding dehydration process of antigorite has to be considered. Additionally, for any formulation of reactions (e.g.within internally consistent thermodynamic datasets) under the participation of antigorite, the variation of its chemical composition with pressure and temperature has to be considered.