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)SiO3, pyroxene with a mantle-relevant composition (X-Fs = 0.1) is expected to occur at similar to 300 km depth [Woodland and Angel, 1997]. Howeve r, the divariant nature of the phase transition in the Mg-Fe system le aves the question open as to whether this transition occurs over a nar row enough pressure interval to cause a seismic discontinuity. New exp erimental 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 or der of 0.15 GPa for a mantle-relevant composition. This implies that t he 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)SiO3 pyro xene is indeed narrow enough to produce a ''jump'' in seismic velociti es. The experimentally observed metastable behavior of orthopyroxene c ould further reduce the effective depth interval of this phase transit ion. The expected location of this phase transition coincides with a s mall magnitude seismic discontinuity, the ''X-discontinuity'', occasio nally observed in seismic profiles at similar to 300 km depth, and thu s pro- vides a viable petrologic explanation for the origin of this di scontinuity, if it truly exists.