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1. The morphology, evolution and seismic visibility of partial melt at the core-mantle boundary: Implications for ULVZs

   https://doi.org/10.1093/gji/ggab242  

   Dannberg, Juliane ; Myhill, Robert ; Gassmöller, René ; Cottaar, Sanne ( July 2021 , Geophysical Journal International)

   null (Ed.)

   Seismic observations indicate that the lowermost mantle above the core-mantle boundary is strongly heterogeneous. Body waves reveal a variety of ultra-low velocity zones (ULVZs), which extend not more than 100 km above the core-mantle boundary and have shear velocity reductions of up to 30 per cent. While the nature and origin of these ULVZs remain uncertain, some have suggested they are evidence of partial melting at the base of mantle plumes. Here we use coupled geodynamic/thermodynamic modelling to explore the hypothesis that present-day deep mantle melting creates ULVZs and introduces compositional heterogeneity in the mantle. Our models explore the generation and migration of melt in a deforming and compacting host rock at the base of a plume in the lowermost mantle. We test whether the balance of gravitational and viscous forces can generate partially molten zones that are consistent with the seismic observations. We find that for a wide range of plausible melt densities, permeabilities and viscosities, lower mantle melt is too dense to be stirred into convective flow and instead sinks down to form a completely molten layer, which is inconsistent with observations of ULVZs. Only if melt is less dense or at most ca. 1 per cent more dense than the solid, or if melt pockets are trapped within the solid, can melt remain suspended in the partial melt zone. In these cases, seismic velocities would be reduced in a cone at the base of the plume. Generally, we find partial melt alone does not explain the observed ULVZ morphologies and solid-state compositional variation is required to explain the anomalies. Our findings provide a framework for testing whether seismically observed ULVZ shapes are consistent with a partial melt origin, which is an important step towards constraining the nature of the heterogeneities in the lowermost mantle and their influence on the thermal, compositional, and dynamical evolution of the Earth. 

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2. Influence of the asthenosphere on earth dynamics and evolution

   https://doi.org/10.1038/s41598-023-39973-y  

   Cathles, Lawrence ; Fjeldskar, Willy ; Lenardic, Adrian ; Romanowicz, Barbara ; Seales, Johnny ; Richards, Mark ( December 2023 , Scientific Reports)

   The existence of a thin, weak asthenospheric layer beneath Earth’s lithospheric plates is consistent with existing geological and geophysical constraints, including Pleistocene glacio-isostatic adjustment, modeling of gravity anomalies, studies of seismic anisotropy, and post-seismic rebound. Mantle convection models suggest that a pronounced weak zone beneath the upper thermal boundary layer (lithosphere) may be essential to the plate tectonic style of convection found on Earth. The asthenosphere is likely related to partial melting and the presence of water in the sub-lithospheric mantle, further implying that the long-term evolution of the Earth may be controlled by thermal regulation and volatile recycling that maintain a geotherm that approaches the wet mantle solidus at asthenospheric depths.

    

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3. A Mid‐Lithospheric Discontinuity Detected Beneath 155 Ma Western Pacific Seafloor Using Sp Receiver Functions

   https://doi.org/10.1029/2024GL108347  

   Chen, K. ‐X. ; Forsyth, D. W. ; Fischer, K. M. ( March 2024 , Geophysical Research Letters)

   This study probes the lithosphere‐asthenosphere system beneath 155 Ma Pacific seafloor using teleseismic S‐to‐p receiver functions at the Pacific Lithosphere Anisotropy and Thickness Experiment project ocean‐bottom‐seismometers. Within the lithosphere, a significant velocity decrease at 33–50 km depth is observed. This mid‐lithospheric discontinuity is consistent with the velocity contrast between the background mantle and thin, trapped layers of crystallized partial melt, in the form of either dolomite or garnet granulite. These melts possibly originated from deeper asthenospheric melting beneath the flanks of spreading centers, and were transported within the cooling lithosphere. A positive velocity increase of 3%–6% is observed at 130–155 km depth and is consistent with the base of a layer with partial melt in the asthenosphere. A shear velocity decrease associated with the lithosphere‐asthenosphere boundary at 95–115 km depth is permitted by the data, but is not required.

    

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4. Experimental Evidence Supporting an Overturned Iron‐Titanium‐Rich Melt Layer in the Deep Lunar Interior

   https://doi.org/10.1029/2022GL099066  

   Xu, Man ; Jing, Zhicheng ; Van Orman, James A. ; Yu, Tony ; Wang, Yanbin ( June 2022 , Geophysical Research Letters)

   Dense Fe‐Ti‐rich cumulates, formed as the last dregs of the lunar magma ocean, are thought to have driven a large‐scale overturn of the lunar mantle over 4 Ga ago. Analysis of lunar seismic data has implied that some of the overturned bodies may have reached the lunar core‐mantle boundary and remained there until the present day as a partially molten layer. However, whether such a molten layer could be stable during >4 Ga of post‐magma‐ocean lunar history and explain lunar seismic observations remains poorly constrained. Here, we report the first sound velocity measurements on a Fe‐Ti‐rich lunar melt up to conditions of the lowermost lunar mantle. Our results suggest that a partial melt layer with at least 20% overturned Fe‐Ti‐rich melt can be trapped atop the lunar core‐mantle boundary until the present day, strongly influencing the thermochemical evolution of the lunar interior.

    

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5. Constraints on the Depth, Thickness, and Strength of the G Discontinuity in the Central Pacific From S Receiver Functions

   https://doi.org/10.1029/2019JB019256  

   Mark, H. F. ; Collins, J. A. ; Lizarralde, D. ; Hirth, G. ; Gaherty, J. B. ; Evans, R. L. ; Behn, M. D. ( April 2021 , Journal of Geophysical Research: Solid Earth)

   The relative motion of the lithosphere with respect to the asthenosphere implies the existence of a boundary zone that accommodates shear between the rigid plates and flowing mantle. This shear zone is typically referred to as the lithosphere‐asthenosphere boundary (LAB). The width of this zone and the mechanisms accommodating shear across it have important implications for coupling between mantle convection and surface plate motion. Seismic observations have provided evidence for several physical mechanisms that might help enable relative plate motion, but how these mechanisms each contribute to the overall accommodation of shear remains unclear. Here we present receiver function constraints on the discontinuity structure of the oceanic upper mantle at the NoMelt site in the central Pacific, where local constraints on shear velocity, anisotropy, conductivity, and attenuation down to ∼300 km depth provide a comprehensive picture of upper mantle structure. We image a seismic discontinuity with a Vsv decrease of 4.5% or more over a 0–20 km thick gradient layer centered at a depth of ∼65 km. We associate this feature with the Gutenberg discontinuity (G), and interpret our observation of G as resulting from strain localization across a dehydration boundary based on the good agreement between the discontinuity depth and that of the dry solidus. Transitions in Vsv, azimuthal anisotropy, conductivity, and attenuation observed at roughly similar depths suggest that the G discontinuity represents a region of localized strain within a broader zone accommodating shear between the lithosphere and asthenosphere.

    

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