Seismic Evidence for Partial Melt at the Base of Earth's Mantle

Williams, Quentin; Garnero, Edward J.

doi:10.1126/science.273.5281.1528

Cited by 540 publications

(449 citation statements)

References 48 publications

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“…If the low-velocity layer, however, is caused by partial melting, an S velocity drop of about 30% will be expected for a P velocity drop of 10% [Williams and Garnero, 1996]. In that case, the converted S to P phase at the upper boundary of ultralow-velocity layers becomes discernible in synthetics, unlike the data.…”

Section: Q(• •) -(•• -•-• (5)mentioning

confidence: 67%

A two‐dimensional P‐SV hybrid method and its application to modeling localized structures near the core‐mantle boundary

Wen¹,

Helmberger²

1998

J. Geophys. Res.

102

105

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Abstract. A P-SV hybrid method is developed for calculating synthetic seismograms involving two-dimensional localized heterogeneous structures. The finite difference technique is applied in the heterogeneous region and generalized ray theory solutions from a seismic source are used in the finite difference initiation process. The seismic motions, after interacting with the heterogeneous structures, are propagated back to the Earth's surface analytically with the aid of the Kirchhoff method. Anomalous long-period SKS-SPdKS observations, sampling a region near the core-mantle boundary beneath the southwest Pacific, are modeled with the hybrid method. Localized structures just above the core-mantle boundary, with lateral dimensions of 250 to 400 km, can explain even the most anomalous data observed to date if S velocity drops up to 30% are allowed for a P velocity drop of 10%. Structural shapes and seismic properties of those anomalies are constrained from the data since synthetic waveforms are sensitive to the location and lateral dimension of seismic anomalies near the core-mantle boundary. Some important issues, such as the density change and roughness of the structures and the sharpness of the transition from the structures to the surrounding mantle, however, remain unresolved due to the nature of the data.

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Section: Q(• •) -(•• -•-• (5)mentioning

confidence: 67%

A two‐dimensional P‐SV hybrid method and its application to modeling localized structures near the core‐mantle boundary

Wen¹,

Helmberger²

1998

J. Geophys. Res.

102

105

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“…A third ULVZ has been reported beneath Tristan by Ni and HeImberger [2001]. The usual interpretation of ULVZs is that they are partial melt zones [Williams and Garnero, 1996]. This interpretation has gained strength with the recent shockwave data [Akins et al, 2003], suggesting that melt from perovskite probably sinks.…”

Section: Distance (Deg)mentioning

confidence: 68%

Seismic modeling constraints on the South African super plume

Helmberger¹,

Ni²

2005

Earth's Deep Mantle: Structure, Composition, and Evolution

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Tomographic studies of the structure of the lower mantle beneath South Africa reveal large-scale low velocities above the core-mantle boundary. Predicted SKS delay patterns (up to 3 s) for some of these models fit observations (Kaapvaal Array data) quite well except for magnitude level, explaining less than one-halfthe observed anomaly. Moreover, the sharpness in travel-time offsets and waveform complications require that nearly vertical walls separate the anomalous structure from the normal preliminary reference Earth model (PREM) mantle. We present numerous record sections along with 2D and 3D synthetics displaying multipathing of arrivals (Sd' SKS, SKKS, S, and ScS), based on a large-scale 3D structure. This kidney-shaped structure has one apex beneath the Indian Ocean (Kerguelen) and the other extending beneath the Mid-Atlantic (Cape Verde). The structure is about 1200 km wide beneath South Africa and extends upward to at least 1000 km through the lower mantle, similar to Grand's model but with an average uniform velocity decrease of about 3% relative to PREM. We have not found any evidences for ultra-low-velocity zones (ULVZ) beneath the main structure but ample evidence at some locations near the edges. We also analyzed Pd and the differentials between PcP travel times and P travel times (PcP-P) along the same great circle paths from the same events. The P-velocity is not very anomalous, perhaps -0.5%. The sharpness of the lateral boundaries (walls) and the large contrast in P and S velocities can be used in arguments for a thermochemical origin.

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“…Seismic data indicate that it has a thickness of 5 -40 km and P and S wave velocity reductions of the order of 5 -30%, suggesting a possible partial melting [Garnero and Helmberger, 1995;Mori and Helmberger, 1995;Williams and Garnero, 1996;Revenaugh and Meyer, 1997;Vidale and Hedlin, 1998;Wen and Helmberger, 1998]. …”

Section: Ultralow-velocity Zonementioning

confidence: 99%

Small‐scale convection in the D″ layer

Solomatov

Moresi

2002

J. Geophys. Res.

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[1] Small-scale convection has been suggested as a possible explanation for seismic heterogeneities in the D 00 layer of the Earth's mantle. Recently developed scaling laws for convection with realistic viscosities allow quantitative assessment of this hypothesis. Large temperature and viscosity contrasts across the thermal boundary layer at the core-mantle boundary suggest that small-scale convection starts at the bottom of the thermal boundary layer and propagates upward before the thermal boundary layer as a whole becomes unstable. The convection boundary is likely to become a chemical boundary as a result of mixing within the convective layer. This implies that the D 00 discontinuity can represent both convective and chemical boundary and that the presence or absence of small-scale convection can be responsible for the observed intermittent nature of the D 00 discontinuity. Most lateral heterogeneities in the D 00 layer are concentrated near the convection boundary, consistent with seismic data. The length scale of lateral temperature variations, the topography of the core-mantle boundary, and the viscosity of the D 00 layer are in agreement with observational constraints. The thickness of the secondary thermal boundary layer formed at the bottom of the convective layer is similar to the thickness of the ultralow-velocity zone. The magnitude of lateral variations in temperature can only marginally be responsible for lateral variations in seismic velocities. Variations in the topography of the convection boundary and chemical heterogeneities are likely to be more important factors. A large seismic velocity drop in the ultralow-velocity zone cannot be explained by thermal effects alone and must be caused by other factors such as partial melting.

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Seismic Evidence for Partial Melt at the Base of Earth's Mantle

Cited by 540 publications

References 48 publications

A two‐dimensional P‐SV hybrid method and its application to modeling localized structures near the core‐mantle boundary

A two‐dimensional P‐SV hybrid method and its application to modeling localized structures near the core‐mantle boundary

Seismic modeling constraints on the South African super plume

Small‐scale convection in the D″ layer

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