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

Abstract

International audienceMineralogical 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 V(S) 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 MgSiO(3) 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 V(S) near MOR, hot spots and evolved continental rifts, where it can explain part of the non-linear high-temperature decrease of V(S) 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

Low seismic velocities below mid‐ocean ridges: Attenuation versus melt retention

“…Which interpretation we choose does not just bear on our understanding of subridge structure, but on asthenospheric structure in general. Q formulations with a strong temperature dependence, like Q F and Q g , do not only match low asthenospheric velocities below ridges but also reproduce the range and depth distribution of seismic velocities with age under older oceans [ Behn et al , 2009; Faul and Jackson , 2005; Priestley and McKenzie , 2006; Reynard et al , 2010; Shapiro and Ritzwoller , 2004b]. Furthermore, similar anelasticity formulations have also yielded reasonable temperature estimates below continents, where V S , V P models, surface heat flow data and xenolith thermobarometry can only be reconciled with the same thermal structure if Q S is as temperature sensitive as models Q F and Q g [ Goes et al , 2000; Priestley and McKenzie , 2006; Röhm et al , 2000; Shapiro and Ritzwoller , 2004a, 2004b].…”

“…Other mechanisms may play a role. For example, Reynard et al [2010]showed that substantial elastic weakening is associated with a high‐temperature phase transition in orthopyroxene. Although this mechanism likely only affects lithospheric and not asthenospheric velocities [ Reynard et al , 2010], other mineral phase transitions may have similar effects in the asthenospheric P, T range.…”

Elasticity of Orthoenstatite at High Pressure and Temperature: Implications for the Origin of Low VP/VS Zones in the Mantle Wedge

“…The single‐crystal elastic moduli and sound velocities of opx at ambient conditions (0 GPa and 300 K) have been measured by ultrasonic measurements (Frisillo & Barsch, ; Kumazawa, ; Webb & Jackson, ; Weidner, Wang, & Ito, ) and Brillouin scattering experiments (Bass & Weidner, ; Duffy & Vaughan, ; Jackson, Sinogeikin, & Bass, ; Weidner & Vaughan, ). Several studies have also measured the elasticity of opx at high pressure and room temperature (Angel & Hugh‐Jones, ; Angel & Jackson, ; Chai et al, ; Flesch et al, ; Frisillo & Barsch, ; Hugh‐Jones & Angel, ; Kung et al, ; Li et al, ; Nestola et al, ; Webb & Jackson, ; Zhang et al, ; Zhang & Bass, ) and at high temperature and ambient pressure (Frisillo & Barsch, ; Jackson et al, ; Jackson, Sinogeikin, & Bass, ; Kung, Jackson, & Liebermann, ; Reynard et al, ; Zhao et al, ). First principle calculations of the elasticity of opx have been conducted at static conditions (Li et al, ), but there are no experimental or calculated elastic data of opx for mantle conditions.…”

Brillouin Scattering and its Application in Geosciences

“…High temperature investigations of Mg-SiO 3 orthoenstatite show evidence for elastic softening associated with the onset of a displacive phase transition (Jackson et al 2004a(Jackson et al , 2007. Implications of orthopyroxene elasticity data for interpretation of seismic data in the crust and lithosphere have been discussed by Wagner et al (2008) and Reynard et al (2010).…”