The orthorhombic to high‐P monoclinic phase transition in Mg‐Fe Pyroxenes: Can it produce a seismic discontinuity?

The orthorhombic to high‐ P monoclinic phase transition in (Mg,Fe)SiO 3 pyroxene with a mantle‐relevant composition (X Fs = 0.1) is expected to occur at ∼300 km depth [ Woodland and Angel , 1997]. However, the divariant nature of the phase transition in the Mg‐Fe system leaves the question open as to whether this transition occurs over a narrow enough pressure interval to cause a seismic discontinuity. New experimental results with binary Mg‐Fe pyroxenes constrain the divariant loop to be 0.2 GPa wide at the composition of X Fs = 0.4 and on the order of 0.15 GPa for a mantle‐relevant composition. This implies that the phase transition will be complete over a depth interval of about 5–6 km in the mantle and it is concluded that the divariant loop of the orthorhombic to high‐ P monoclinic phase transition in (Mg,Fe)SiO 3 pyroxene is indeed narrow enough to produce a “jump” in seismic velocities. The experimentally observed metastable behavior of orthopyroxene could further reduce the effective depth interval of this phase transition. The expected location of this phase transition coincides with a small magnitude seismic discontinuity, the “X‐discontinuity”, occasionally observed in seismic profiles at ∼300 km depth, and thus provides a viable petrologic explanation for the origin of this discontinuity, if it truly exists.