High-temperature elastic softening of orthopyroxene and seismic properties of the lithospheric upper mantle

SUMMARY Mineralogical models suggest that low velocity zones of the upper mantle may be accounted for by elasticity of dry rocks along geotherms away from the mid-oceanic ridges (MOR) while, closer to MOR, anelasticity plays a significant role in reducing shear wave velocities VS and generating attenuation. We investigate the potential influence of elastic softening, precursor of high-temperature phase transition in orthopyroxenes, on the seismic properties of the upper mantle. In situ Brillouin and Raman spectroscopy were used to evidence pre-transitional behaviour at high temperature in natural San Carlos orthopyroxene. Pre-transitional behaviour induces a large softening of the acoustic and low frequency modes, resulting in a large anharmonic decrease of sound velocities, similar to that observed in the MgSiO3 end-member orthoenstatite. The high-temperature high-pressure phase diagram of enstatite is revised to account for the new phase transition, and a simple model is developed to evaluate elastic softening effects on the upper-mantle seismic properties, whose results depend much on the as yet poorly constrained pressure and compositional dependence of the phase transition boundary. Within the tested range of parameters, OPx softening is likely to affect the seismic properties of mantle rocks at depths shallower than 80 km in hot regions. Thus elastic softening of pyroxene is unlikely to affect the LVZ or continental lithospheric mantle unless the transition temperature is drastically reduced by incorporation of aluminium in orthopyroxene. It will contribute to a decrease of VS near MOR, hot spots and evolved continental rifts, where it can explain part of the non-linear high-temperature decrease of VS in the lithospheric oceanic mantle. It will affect the magnitude of anelastic effects and the nature of the likely mechanisms of attenuation required to match seismological observations.