Comparative deformation behavior of minerals in serpentinized ultramafic rock: Application to the slab-mantle interface in subduction zones

The layer-structure minerals serpentine, brucite, and talc are postulated to form in the mantle wedge above a subdueting slab as a result of progressive hydration and silica metasomatism. Tectonic mixing at the slab-mantle interface generates serpentinite melanges that contain blocks of high-pressure (HP) or ultrahigh -pressure (UHP) metamorphic rock derived from the subducting slab. Such serpentinite melanges may provide a means of exhumation of HP/UHP metamorphic rocks, and may define the lower limit of locked regions on the subduction interface that fail in large earthquakes. We review recently obtained frictional strength data for brucite and talc over the temperature range 25-400 degrees C at 100 MPa effective normal stress and compare them with new data for antigorite. These minerals respond to heating in different ways, causing their frictional strengths to diverge. Water-saturated antigorite strength increases toward the fixed dry value of mu approximate to 0.75-0.80 with heating: mu approximate to 0.50 at 25 degrees C and mu > 0.60 at 400 degrees C. The difference in mu between dry and watersaturated talc gouge also decreases with increasing temperature, but both the dry and watersaturated values of mu are lower at elevated temperatures. For dry talc, mu decreases from 0.35 to 0.25 between 25 degrees and 300 degrees C, whereas for water-saturated talc, mu is approximately 0.20 at 25 degrees C and 0.100.15 at elevated temperatures. Weakening of the interlayer bond of talc with heating may be responsible for the overall reduction in its frictional strength. The strength of dry brucite also is fixed at mu = 0.45-0.50, but the water-saturated value of mu decreases from approximate to 0.30 at 25 degrees C to 0.20-0.25 at 200 degrees-400 degrees C. The water-saturated brucite gouge has extensively recrystallized along the shear surfaces, and its weakening may be attributable to solution-transfer processes. Because the serpentine minerals become stronger at elevated temperatures, to achieve low frictional strength in a serpentinized mantle wedge or serpentine-rich melange would require some other cause such as nearly lithostatic fluid pressures or the addition of lower-strength minerals. Increasing abundance of brucite and talc in serpentinite at constant physical conditions would progressively reduce its frictional strength. The concentration of talc along a metasomatic front at the edge of the mantle wedge or in reaction zones surrounding HP/UHP blocks in serpentinite melange should tend to localize shear in this extremely weak material.