Serpentinites, Peridotites, and Seismology

Understanding the physical properties of ultramafic rocks is important for evaluating the wide variety of petrologic models for the Earth's upper mantle and lower oceanic crust. At comparable temperatures and pressures, velocities of compressional and shear waves in ultramafic rocks decrease with increasing serpentinization. A major factor affecting these velocities is the variety of serpentine present. Antigorite, the serpentine species stable at high temperatures, has higher velocities and a lower Poisson's ratio than the serpentine polymorphs lizardite and chrysotile. In addition, seismic properties of ultramafic rocks vary with their proportion of olivine to pyroxene and abundances of accessory minerals formed during serpentinization, such as brucite, magnetite, magnesite, tremolite, and talc. Seismic anisotropy is an important property of relatively unaltered peridotites. Large, well-exposed ultramafic massifs provide the best information on the nature of upper mantle compressional wave anisotropy and shear wave splitting. Average compressional wave anisotropy in these massifs is approximately 5%. Shear wave splitting magnitudes vary significantly with propagation direction. The major lithologies present in much of the lower ocean crust are metadiabase and gabbro. Lizardite-chrysotile-bearing serpentinites are abundant, however, in regions that have allowed penetration of sea water into the upper mantle. Dehydration of subducting slabs and the rising of released fluids have resulted in hydrothermal alteration and a lowering of velocities in forearc mantle wedges. In addition to serpentinization, high pore pressures and metamorphism producing chlorite are required to explain the seismic properties of the forearc upper mantle.