A ubiquitous low‐velocity layer at the base of the mantle transition zone

Shen, Xuzhang; Yuan, Xiaohui; Li, Xueqing

doi:10.1002/2013gl058918

Cited by 36 publications

(50 citation statements)

References 34 publications

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“…Up to our knowledge, there exists no known mineralogical phase change that would produce the negative signal before P660s. Other studies have also observed similar negative impedance features and have interpreted them as accumulated oceanic crust at the base of the TZ (Eagar et al, 2010;Shen and Blum, 2003;Shen et al, 2008Shen et al, , 2014. This signal is not used in the following analysis, and therefore, we do not further explore its origin.…”

Section: Negative Amplitude Precursory Signalsmentioning

confidence: 83%

The upper-mantle transition zone beneath the Ibero-Maghrebian region as seen by teleseismic Pds phases

et al. 2015

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Section: Negative Amplitude Precursory Signalsmentioning

confidence: 83%

The upper-mantle transition zone beneath the Ibero-Maghrebian region as seen by teleseismic Pds phases

et al. 2015

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“…It is also associated with a velocity reduction (−20% of the reflection amplitude at the '410'). Its origin remains mysterious although Shen et al (2014) suggest that this discontinuity is a global feature associated with delamination and accumulation of oceanic crust at the top of the lower mantle.…”

Section: Resultsmentioning

confidence: 99%

Seismically deduced thermodynamics phase diagrams for the mantle transition zone

Tauzin

Ricard

2014

Earth and Planetary Science Letters

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Seismic discontinuities at 410 and 660 km depth are usually attributed to solid phase changes within the olivine component of the mantle. The Clapeyron slopes γ 410 and γ 660 , i.e. the thermal dependence of the depths of reactions, have been shown experimentally to be of opposite signs. Yet, their values are not well constrained by laboratory measurements. Seismological observations have not been more precise due to the difficulty to separate the competing effects of background wave-velocities and of temperature on the topography of discontinuities. In this study we use conversion imaging of interfaces under western US. We propose a new approach to derive a seismological estimate of the Clapeyron slopes with respect to γ 410 for the major and minor phase changes of the transition zone. We obtain γ 660 ≈ −3 MPa K −1 for γ 410 ≈ +3 MPaK −1 . We construct "seismic phase diagrams" of the transition zone that can be directly compared with experimental phase diagrams. We also apply a " Z -Γ " transform to better constrain the Clapeyron slopes γ of the minor phase changes. Although tenuous, signals in seismic phase diagrams suggest that minor phase transitions, both in the olivine and the non-olivine component of the mantle, have visible seismic expressions. They can tentatively be described as follows. The '410' is overlaid at low temperature by an interface corresponding to a decrease of velocity with depth and γ ≈ +3 MPaK −1 .The '660' widens at high temperature and is preceded at low temperature by an interface, the '620', with γ ≈ +7 MPaK −1 . A '520' is suggested with γ ≈ 2-3 MPa K −1 . These last two interfaces correspond to velocity increases with depth. At last, near 590 km depth, an interface may be associated with a velocity reduction showing a weak dependence on temperature (γ ∼ 0 MPa K −1 ).

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“…The mantle transition zone (MTZ) is an ∼250‐km‐thick boundary layer delimited by two seismic interfaces, the so‐called 410‐ and 660‐km discontinuities, which are attributed to solid‐solid phase changes from olivine (ol) to wadsleyite (wa) and ringwodite (rw) to perovskite+magnesiowustite (ppv+mw; Bina & Helffrich, ). An enigmatic patchwork of low‐seismic velocity zones (LVZs) was reported at various scales and depths within or surrounding the MTZ (e.g., Shen et al, ; Tauzin et al, , Tauzin, Kim, & Kennett, ; Vinnik & Farra, ). These LVZs might be distinctive features of a well‐mixed convective mantle that preserves small‐scale compositional heterogeneities.…”

Section: Introductionmentioning

confidence: 99%

“…These LVZs might be distinctive features of a well‐mixed convective mantle that preserves small‐scale compositional heterogeneities. They are usually attributed to the volatile content (H 2 O, CO 2 ; Bercovici & Karato, ; Hier‐Majumder & Tauzin, ) or to remnants of subducted oceanic crust in the MTZ (Shen & Blum, ; Shen et al, ). High‐resolution mapping of these LVZs at regional scale is thus crucial for our understanding of the recycling of geochemical heterogeneities within large‐scale limbs of thermally driven mantle convection (Tauzin, Kim, & Kennett et al, ; Wei & Shearer, ).…”

Section: Introductionmentioning

confidence: 99%

Multiple Phase Changes in the Mantle Transition Zone Beneath Northeast Asia: Constraints From Teleseismic Reflected and Converted Body Waves

Tauzin

Kim²,

Afonso

2018

JGR Solid Earth

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We reassess the mantle transition zone structure below the northeast Asia margin in the context of subduction of the Pacific plate below the Eurasian continent. We use two independent approaches of teleseismic imaging, namely, compressional‐to‐shear converted waves (receiver functions) and shear wave underside reflections (SS precursors), and compare them within their statistical uncertainties. We find localized complexity in the interfaces marking solid phase changes in mantle minerals, in terms of both apparent topography and reflectivity. The 660‐km discontinuity is doubled, with ∼80‐km maximum vertical distance between the interfaces, over an 890 × 350 km2 region between 36–44°N and 130–133°E at the tip of the subducted Pacific plate. A similar complexity exists on the 410‐km discontinuity, coinciding with the presence of a deep cluster of seismicity below the Japan Sea. Both methods suggest the presence of low‐velocity zones atop the 410, within the mantle transition zone, and below the 660. This complex seismic signature is related to the Pacific plate and interpreted in light of the subduction thermal regime and phase equilibria for a pyrolitic mantle composition. Phase changes manifest themselves as broad zones of velocity gradients with localized doubled or multiple first‐order discontinuities, associated with transitions in the olivine, pyroxene, and garnet systems. An average pyrolitic composition and local temperatures of 1000–1300 K can explain the observed velocity gradients and multiple discontinuities. We show that the dissolution of stishovite, a high‐pressure polymorph of SiO2, into the higher‐pressure perovskite mineral, is a possible explanation for the low‐velocity zones at the top of the lower mantle.

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A ubiquitous low‐velocity layer at the base of the mantle transition zone

Cited by 36 publications

References 34 publications

The upper-mantle transition zone beneath the Ibero-Maghrebian region as seen by teleseismic Pds phases

The upper-mantle transition zone beneath the Ibero-Maghrebian region as seen by teleseismic Pds phases

Seismically deduced thermodynamics phase diagrams for the mantle transition zone

Multiple Phase Changes in the Mantle Transition Zone Beneath Northeast Asia: Constraints From Teleseismic Reflected and Converted Body Waves

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