Fabric development in (Mg,Fe)O during large strain, shear deformation

Implications for seismic anisotropy in Earth's lower mantle

Daisuke Yamazaki, Shun Ichiro Karato

Research output: Contribution to journalArticle

102 Citations (Scopus)

Abstract

Large strain, shear deformation experiments were performed on (Mg1-x,Fex)O (x = 0.25, 1.0), one of the important minerals in Earth's lower mantle. Deformation experiments were made on coarse-grained (~15-20 μm grain-size) hot-pressed aggregates at conditions of T/Tm ~ 0.46-0.65 (T: temperature, Tm: melting temperature) and σ/μ ~ 0.4 × 10-3 to 0.9 × 10-3 (σ: differential stress, μ: shear modulus) up to the shear strain of ~7.8. Under these conditions, deformation occurs by dislocation creep. The microstructural development in (Mg,Fe)O is found to be sluggish and the complete dynamic recrystallization and nearly steady-state fabric (lattice preferred orientation) are achieved only after shear strains of γ ~ 4. At nearly steady-state, (Mg,Fe)O shows strong fabrics characterized by the axes being parallel to the shear direction and the poles of the {1 0 0} planes (and to a lesser extent the poles of the {1 1 1} planes) normal to the shear plane. The seismic anisotropy corresponding to the deformation fabrics in (Mg,Fe)O was calculated. The nature of anisotropy corresponding to a given flow geometry changes significantly with strain as a result of fabric evolution. Anisotropy changes also with depth (pressure) due to the large variation of elastic anisotropy of (Mg,Fe)O with depth. Seismic anisotropy caused by the deformation fabric of (Mg,Fe)O is very weak in the shallow lower mantle (SH > VSV anisotropy (assuming that (Mg,Fe)O occupies ~20% volume fraction of the lower mantle) and little shear wave splitting of vertically travelling waves, a result that is consistent with the seismological observations in the D″ layer of the circum-Pacific regions. Thus, the deformation fabric of (Mg,Fe)O is a vital candidate of the cause of seismic anisotropy in these regions. Anisotropy caused by the lattice preferred orientation of (Mg,Fe)O has a distinct azimuthal anisotropy with a strong 4θ term (θ: azimuth): the direction of propagation of the fastest (slowest) SH (or P) wave is parallel (perpendicular) to the flow direction and the slowest (fastest) SH (or P) wave is at 45° from these two directions.

Original languageEnglish
Pages (from-to)251-267
Number of pages17
JournalPhysics of the Earth and Planetary Interiors
Volume131
Issue number3-4
DOIs
Publication statusPublished - Aug 30 2002
Externally publishedYes

Fingerprint

seismic anisotropy
shear strain
lower mantle
Earth mantle
anisotropy
preferred orientation
SH waves
P-wave
P waves
dislocation creep
poles
wave splitting
shear modulus
shear
flow geometry
azimuth
elastic anisotropy
D region
fabric
S-wave

Keywords

  • (Mg,Fe)O
  • D" layer
  • Seismic anisotropy
  • Simple shear
  • Slip system

ASJC Scopus subject areas

  • Geophysics
  • Space and Planetary Science

Cite this

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title = "Fabric development in (Mg,Fe)O during large strain, shear deformation: Implications for seismic anisotropy in Earth's lower mantle",
abstract = "Large strain, shear deformation experiments were performed on (Mg1-x,Fex)O (x = 0.25, 1.0), one of the important minerals in Earth's lower mantle. Deformation experiments were made on coarse-grained (~15-20 μm grain-size) hot-pressed aggregates at conditions of T/Tm ~ 0.46-0.65 (T: temperature, Tm: melting temperature) and σ/μ ~ 0.4 × 10-3 to 0.9 × 10-3 (σ: differential stress, μ: shear modulus) up to the shear strain of ~7.8. Under these conditions, deformation occurs by dislocation creep. The microstructural development in (Mg,Fe)O is found to be sluggish and the complete dynamic recrystallization and nearly steady-state fabric (lattice preferred orientation) are achieved only after shear strains of γ ~ 4. At nearly steady-state, (Mg,Fe)O shows strong fabrics characterized by the axes being parallel to the shear direction and the poles of the {1 0 0} planes (and to a lesser extent the poles of the {1 1 1} planes) normal to the shear plane. The seismic anisotropy corresponding to the deformation fabrics in (Mg,Fe)O was calculated. The nature of anisotropy corresponding to a given flow geometry changes significantly with strain as a result of fabric evolution. Anisotropy changes also with depth (pressure) due to the large variation of elastic anisotropy of (Mg,Fe)O with depth. Seismic anisotropy caused by the deformation fabric of (Mg,Fe)O is very weak in the shallow lower mantle (SH > VSV anisotropy (assuming that (Mg,Fe)O occupies ~20{\%} volume fraction of the lower mantle) and little shear wave splitting of vertically travelling waves, a result that is consistent with the seismological observations in the D″ layer of the circum-Pacific regions. Thus, the deformation fabric of (Mg,Fe)O is a vital candidate of the cause of seismic anisotropy in these regions. Anisotropy caused by the lattice preferred orientation of (Mg,Fe)O has a distinct azimuthal anisotropy with a strong 4θ term (θ: azimuth): the direction of propagation of the fastest (slowest) SH (or P) wave is parallel (perpendicular) to the flow direction and the slowest (fastest) SH (or P) wave is at 45° from these two directions.",
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author = "Daisuke Yamazaki and Karato, {Shun Ichiro}",
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T1 - Fabric development in (Mg,Fe)O during large strain, shear deformation

T2 - Implications for seismic anisotropy in Earth's lower mantle

AU - Yamazaki, Daisuke

AU - Karato, Shun Ichiro

PY - 2002/8/30

Y1 - 2002/8/30

N2 - Large strain, shear deformation experiments were performed on (Mg1-x,Fex)O (x = 0.25, 1.0), one of the important minerals in Earth's lower mantle. Deformation experiments were made on coarse-grained (~15-20 μm grain-size) hot-pressed aggregates at conditions of T/Tm ~ 0.46-0.65 (T: temperature, Tm: melting temperature) and σ/μ ~ 0.4 × 10-3 to 0.9 × 10-3 (σ: differential stress, μ: shear modulus) up to the shear strain of ~7.8. Under these conditions, deformation occurs by dislocation creep. The microstructural development in (Mg,Fe)O is found to be sluggish and the complete dynamic recrystallization and nearly steady-state fabric (lattice preferred orientation) are achieved only after shear strains of γ ~ 4. At nearly steady-state, (Mg,Fe)O shows strong fabrics characterized by the axes being parallel to the shear direction and the poles of the {1 0 0} planes (and to a lesser extent the poles of the {1 1 1} planes) normal to the shear plane. The seismic anisotropy corresponding to the deformation fabrics in (Mg,Fe)O was calculated. The nature of anisotropy corresponding to a given flow geometry changes significantly with strain as a result of fabric evolution. Anisotropy changes also with depth (pressure) due to the large variation of elastic anisotropy of (Mg,Fe)O with depth. Seismic anisotropy caused by the deformation fabric of (Mg,Fe)O is very weak in the shallow lower mantle (SH > VSV anisotropy (assuming that (Mg,Fe)O occupies ~20% volume fraction of the lower mantle) and little shear wave splitting of vertically travelling waves, a result that is consistent with the seismological observations in the D″ layer of the circum-Pacific regions. Thus, the deformation fabric of (Mg,Fe)O is a vital candidate of the cause of seismic anisotropy in these regions. Anisotropy caused by the lattice preferred orientation of (Mg,Fe)O has a distinct azimuthal anisotropy with a strong 4θ term (θ: azimuth): the direction of propagation of the fastest (slowest) SH (or P) wave is parallel (perpendicular) to the flow direction and the slowest (fastest) SH (or P) wave is at 45° from these two directions.

AB - Large strain, shear deformation experiments were performed on (Mg1-x,Fex)O (x = 0.25, 1.0), one of the important minerals in Earth's lower mantle. Deformation experiments were made on coarse-grained (~15-20 μm grain-size) hot-pressed aggregates at conditions of T/Tm ~ 0.46-0.65 (T: temperature, Tm: melting temperature) and σ/μ ~ 0.4 × 10-3 to 0.9 × 10-3 (σ: differential stress, μ: shear modulus) up to the shear strain of ~7.8. Under these conditions, deformation occurs by dislocation creep. The microstructural development in (Mg,Fe)O is found to be sluggish and the complete dynamic recrystallization and nearly steady-state fabric (lattice preferred orientation) are achieved only after shear strains of γ ~ 4. At nearly steady-state, (Mg,Fe)O shows strong fabrics characterized by the axes being parallel to the shear direction and the poles of the {1 0 0} planes (and to a lesser extent the poles of the {1 1 1} planes) normal to the shear plane. The seismic anisotropy corresponding to the deformation fabrics in (Mg,Fe)O was calculated. The nature of anisotropy corresponding to a given flow geometry changes significantly with strain as a result of fabric evolution. Anisotropy changes also with depth (pressure) due to the large variation of elastic anisotropy of (Mg,Fe)O with depth. Seismic anisotropy caused by the deformation fabric of (Mg,Fe)O is very weak in the shallow lower mantle (SH > VSV anisotropy (assuming that (Mg,Fe)O occupies ~20% volume fraction of the lower mantle) and little shear wave splitting of vertically travelling waves, a result that is consistent with the seismological observations in the D″ layer of the circum-Pacific regions. Thus, the deformation fabric of (Mg,Fe)O is a vital candidate of the cause of seismic anisotropy in these regions. Anisotropy caused by the lattice preferred orientation of (Mg,Fe)O has a distinct azimuthal anisotropy with a strong 4θ term (θ: azimuth): the direction of propagation of the fastest (slowest) SH (or P) wave is parallel (perpendicular) to the flow direction and the slowest (fastest) SH (or P) wave is at 45° from these two directions.

KW - (Mg,Fe)O

KW - D" layer

KW - Seismic anisotropy

KW - Simple shear

KW - Slip system

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