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

Details 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 pyroxenes: Can it produce a seismic discontinuity?

Apr 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998georl..25.1241w&link_type=abstract

Geophysical Research Letters, Volume 25, Issue 8, p. 1241-1244

Physics

21

Mineralogy And Petrology: Experimental Mineralogy And Petrology, Mineral Physics: High-Pressure Behavior, Seismology: Lithosphere And Upper Mantle, Tectonophysics: Earth'S Interior-Composition And State

Scientific paper

The orthorhombic to high-P monoclinic phase transition in (Mg,Fe)SiO3 pyroxene with a mantle-relevant composition (XFs=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 XFs=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)SiO3 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.

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