Compositional Constraints on Dehydration Embrittlement in Serpentinized Peridotite

Double seismic zones (DSZ) which have two parallel planes of seismicity separated by 15-40 km are a global feature of subduction zones in the 70-250 km depth range (Brudzinski et al., 2007). While the physical mechanism of lower plane seismicity is still controversial, the leading hypotheses currently are associated with dehydration of antigorite serpentine within the subducting mantle plate (Peacock, 2001; Jung et al., 2004). In this study, we are conducting high-pressure (1-3GPa), high-temperature (720-750 Celsius), deformation experiments on specimens of varying compositions of serpentine plus peridotite in our 4GPa Modified Griggs apparatus. Using samples composed of interlayered thin discs of antigorite and harzburgite, we find that dehydration embrittlement occurs down to less than ~30 vol % antigorite. Interlayered mineralogy was impractical at lower antigorite fractions so we prepared homogeneous mixtures of powders of the two rock types (35-75 μm grain-size) and "warm" pressed them to a coherent solid with little porosity. Subsequent deformation of these specimens extended the faulting regime to as little as ~8 vol % antigorite. In summary, we find that faulting occurs during dehydration in a wide range of serpentinized peridotite compositions but not during dehydration of nearly pure serpentinite nor nearly pure peridotite. We suggest that the lack of faulting in nearly pure peridotite is a consequence of too little H2O production and the lack of faulting in nearly pure serpentine is due to extensive crystal plasticity.