Strength of slightly serpentinized peridotites: Implications for the tectonics of oceanic lithosphere

We deformed cores of peridotite with similar to 10%-15% lizardite and chrys otile serpentine to determine the influence of serpentine content on the st rength and the style of deformation. The strength, the pressure dependence of strength, and the nominally nondilatant mode of brittle deformation of s lightly serpentinized peridotites are comparable to those of pure serpentin ites. These results indicate that deformation is accommodated primarily by serpentine, white olivine, despite being the more abundant component, remai ns nominally undeformed. On the basis of these data and previous work, we d etermine that the transition from a "strong," dilatant dunite rheology to a "weak," nondilatant serpentinite rheology is not a linear function of the degree of serpentinization. Instead, an abrupt transition in strength is ob served at low degrees of serpentinization. The pressure of the transition f rom localized to distributed deformation also decreases abruptly, from > 10 00 MPa to 150-350 MPa. The change in rheological behavior occurs at a serpe ntine content of 10%-15% or less, which corresponds to published compressio nal seismic velocity of >7.8-7.5 km/s at a pressure of 200 MPa. The seismic velocity of the oceanic lithosphere, particularly of that formed at slow s preading ridges, can thus provide constraints on its mechanical properties at depth. Because slightly serpentinized peridotites have a rheology simila r to that of pure serpentinite, significant lithospheric weakening may occu r after the onset of alteration near or at the ridge axis.