Evidence for a thick and localized ultra low shear velocity zone at the base of the mantle beneath the central Pacific

To, Akiko; Fukao, Yoshio; Tsuboi, Seiji

doi:10.1016/j.pepi.2010.10.015

Cited by 21 publications

(19 citation statements)

References 34 publications

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“…Therefore, the strong secondary peaks visible in these data unlikely represent source or receiver effects, or contamination by other phases. We conclude that these peaks can result from more complex (i.e., 2‐D or 3‐D) nature of the ULVZ [ Wen and Helmberger , , ; To et al, ; Thorne et al, ; Jensen et al, ]. While extension to 2‐D and/or 3‐D models is desirable, computational limitations in the Bayesian sampling approach currently prevent such generalizations and are not considered in this work.…”

Section: Resultsmentioning

confidence: 99%

Bayesian inference for ultralow velocity zones in the Earth's lowermost mantle: Complex ULVZ beneath the east of the Philippines

2014

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Ultralow velocity zones (ULVZs) are small-scale structures with a sharp decrease in S and P wave velocity, and an increase in the density on the top of the Earth's core-mantle boundary. The ratio of S and P wave velocity reduction and density anomaly are important to understanding whether ULVZs consist of partial melt or chemically distinct material. However, existing methods such as forward waveform modeling that utilize 1-D and 2-D Earth-structure models face challenges when trying to uniquely quantify ULVZ properties because of inherent nonuniqueness and nonlinearity. This paper develops a Bayesian inversion for ULVZ parameters and uncertainties with rigorous noise treatment to address these challenges. The posterior probability density of the ULVZ parameters (the solution to the inverse problem) is sampled by the Metropolis-Hastings algorithm. To improve sampling efficiency, parallel tempering is applied by simulating a sequence of tempered Markov chains in parallel and allowing information exchange between chains. First, the Bayesian inversion is applied to simulated noisy data for a realistic ULVZ model. Then, measured data sampling the lowermost mantle under the Philippine Sea are considered. Cluster analysis and visual waveform inspection suggest that two distinct classes of ScP (S waves converted to, and reflected as, P waves) waves exist in this region. The distinct waves likely correspond to lateral variability in the lowermost mantle properties in a NE-SW direction. For the NE area, Bayesian model selection identifies a two-layer model with a gradual density increase as a function of depth as optimal. This complex ULVZ structure can be due to the percolation of iron-enriched, molten material in the lowermost mantle. The results for the SW area are more difficult to interpret, which may be due to the limited number of data available (too few waveforms to appropriately reduce noise) and/or complex 2-D and 3-D structures that cannot be explained properly by the 1-D models required by our inversion approach. In particular, the complex waveforms require highly layered 1-D models to fit the data. These models appear physically unreasonable and suggest that the SW region cannot be explained by 1-D structure.

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Section: Resultsmentioning

confidence: 99%

Bayesian inference for ultralow velocity zones in the Earth's lowermost mantle: Complex ULVZ beneath the east of the Philippines

2014

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“…Internal reflections or P-to-S or S-to-P conversions within the ULVZ layer can result in additional delayed arrivals (post-cursors) relative to the main phase (Figures 2b-2d). Detailed waveform modeling utilizing 2-D and 3-D models helps in the imaging, but introducing a larger model space can introduce more trade-offs (e.g., Cottaar & Romanowicz, 2012;Thorne et al, 2013;To et al, 2011). Diffracted phases encounter the CMB either once (e.g., P diff and S diff ) or twice (e.g., SP d KS and PKKP ab_diff , Figures 2e and 2h).…”

Section: Ulvz Seismic Probesmentioning

confidence: 99%

Ultralow Velocity Zone Locations: A Global Assessment

Garnero

2018

Geochem Geophys Geosyst

124

120

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We have compiled all previous ultralow velocity zone (ULVZ) studies, and digitized their core‐mantle boundary (CMB) sampling locations. For studies that presented sampling locations based on infinite frequency ray theory, we approximated Fresnel zones onto a 0.5° × 0.5° grid. Results for these studies were separated according to wave type: (1) core‐reflected phases, which have a single location of ULVZ sampling (ScS, ScP, PcP), (2) core waves that can sample ULVZs at the core entrance and exit locations of the wave (e.g., SPdKS, PKKP, and PKP), and (3) waves which have uncertainties of ULVZ location due to long CMB sampling paths, e.g., diffracted energy sampling over a broad region (Pdiff, Sdiff). For studies that presented specific modeled ULVZ geographical shapes or PKP scatter probability maps, we digitized the regions. We present summary maps of the ULVZ coverage, as well as published locations arguing against ULVZ presence. A key finding is that there is not a simple mapping between lowermost mantle reduced tomographic velocities and observed ULVZ locations, especially given the presence of ULVZs outside of lowermost mantle large low velocity provinces (LLVPs). Significant location uncertainty exists for some of the ULVZ imaging wave types. Nonetheless, this compilation supports a compositionally distinct origin for at least some ULVZs. ULVZs are more likely to be found near LLVP boundaries, however, their relationship to overlying surface locations of hot spots are less obvious. The new digital ULVZ database is freely available for download.

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“…Whereas for today's CMB temperature the vertical extent of ULVZs is below 80 km [ To et al ., ; Williams and Garnero , ], a higher CMB temperature in the early history of the Earth might have allowed for a vertical extent of a basal magma ocean of hundreds of kilometers, as postulated by Labrosse et al . [].…”

Section: Discussionmentioning

confidence: 99%

“…Our results indicate that the maximum vertical extent of ULVZs is severely restricted by the strongly negative Clapeyron slope associated with the large density increase of lower mantle mineral assemblages on melting. The maximum vertical extent of ULVZs in our simulations remains, for realistic parameter sets, always below 80 km, a height consistent with seismic observations of ULVZs [ To et al ., ; Williams and Garnero , ]. Due to a dynamic and thermal feedback mechanism, the presence of low viscosity ULVZs raises the average mantle temperature by a few percent (O (100 K)).…”

Section: Discussionmentioning

confidence: 99%

“…[2] Ultralow velocity zones (ULVZs) are generally up to 40 km (but in a few places also up to 80 km, see To et al [2011]) thick regions in the lowermost mantle directly above the core-mantle boundary (CMB), which exhibit marked seismic P-and S-velocity reductions of typically up to~10% and ~30%, respectively, with respect to the ambient mantle [Williams and Garnero, 1996]. A straightforward explanation for the observed seismic velocity reduction is the presence of melt fractions in ULVZs [Garnero and Helmberger, 1998;Lay et al, 2004;Williams and Garnero, 1996].…”

Section: Introductionmentioning

confidence: 99%

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A melting model for the lowermost mantle using Clapeyron slopes derived from experimental data: Consequences for the thickness of ultralow velocity zones (ULVZs)

Beuchert

Schmeling

2013

Geochem Geophys Geosyst

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[1] Ultralow velocity zones (ULVZs) are relatively thin regions directly above the core-mantle boundary (CMB) that exhibit marked seismic P-and S-velocity reductions. A viable explanation for the reduction is the presence of melt fractions within ULVZs. Since melt was found to be denser than solid in melting experiments at lowermost mantle pressures, partially molten ULVZs should exhibit negative buoyancy. Using published experimental data, we present a melting model based on Clapeyron slopes for the formation of ULVZs as partially molten regions above the CMB and apply the resulting melting curves and latent heat effects in fully dynamic, regionally constant viscosity convection simulations of the lowermost mantle. We find that the height of the ULVZs depends only moderately on Rayleigh number but strongly decreases with increasing excess density of melt over solid. The models predict excess density of at least 1% to explain observed heights. The combined effect of topography and latent heat of melting reduces the vigor of mantle convection only very slightly, while if combined with a decrease of the ULVZ viscosity, mantle flow velocities are significantly enhanced near the CMB, and overall mantle temperatures are notably increased. ULVZ heights are found to be insensitive to ULVZ viscosity (for the range isoviscous to 1/100 the viscosity of the ambient mantle).

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Evidence for a thick and localized ultra low shear velocity zone at the base of the mantle beneath the central Pacific

Cited by 21 publications

References 34 publications

Bayesian inference for ultralow velocity zones in the Earth's lowermost mantle: Complex ULVZ beneath the east of the Philippines

Bayesian inference for ultralow velocity zones in the Earth's lowermost mantle: Complex ULVZ beneath the east of the Philippines

Ultralow Velocity Zone Locations: A Global Assessment

A melting model for the lowermost mantle using Clapeyron slopes derived from experimental data: Consequences for the thickness of ultralow velocity zones (ULVZs)

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