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

Bruno Reynard, Jay D Bass, Jon Brenizer

Research output: Contribution to journalArticle

Abstract

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.

Original languageEnglish (US)
Pages (from-to)557-566
Number of pages10
JournalGeophysical Journal International
Volume181
Issue number1
DOIs
StatePublished - Apr 2010

Fingerprint

seismic property
softening
orthopyroxene
upper mantle
Earth mantle
phase transition
mid-ocean ridges
Phase transitions
enstatite
mantle
Rocks
Temperature
Shear waves
anelasticity
Acoustic wave velocity
sound velocity
attenuation
Aluminum
rocks
low velocity zone

Keywords

  • Composition of the mantle
  • Elasticity and anelasticity
  • Phase transition
  • Seismic attenuation

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

Cite this

High-temperature elastic softening of orthopyroxene and seismic properties of the lithospheric upper mantle. / Reynard, Bruno; Bass, Jay D; Brenizer, Jon.

In: Geophysical Journal International, Vol. 181, No. 1, 04.2010, p. 557-566.

Research output: Contribution to journalArticle

@article{4b1297e0fd8542e8a97f6476f7be3587,
title = "High-temperature elastic softening of orthopyroxene and seismic properties of the lithospheric upper mantle",
abstract = "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.",
keywords = "Composition of the mantle, Elasticity and anelasticity, Phase transition, Seismic attenuation",
author = "Bruno Reynard and Bass, {Jay D} and Jon Brenizer",
year = "2010",
month = "4",
doi = "10.1111/j.1365-246X.2010.04524.x",
language = "English (US)",
volume = "181",
pages = "557--566",
journal = "Geophysical Journal International",
issn = "0956-540X",
publisher = "Wiley-Blackwell",
number = "1",

}

TY - JOUR

T1 - High-temperature elastic softening of orthopyroxene and seismic properties of the lithospheric upper mantle

AU - Reynard, Bruno

AU - Bass, Jay D

AU - Brenizer, Jon

PY - 2010/4

Y1 - 2010/4

N2 - 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.

AB - 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.

KW - Composition of the mantle

KW - Elasticity and anelasticity

KW - Phase transition

KW - Seismic attenuation

UR - http://www.scopus.com/inward/record.url?scp=77952556182&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=77952556182&partnerID=8YFLogxK

U2 - 10.1111/j.1365-246X.2010.04524.x

DO - 10.1111/j.1365-246X.2010.04524.x

M3 - Article

AN - SCOPUS:77952556182

VL - 181

SP - 557

EP - 566

JO - Geophysical Journal International

JF - Geophysical Journal International

SN - 0956-540X

IS - 1

ER -