Edward J. Garnero

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

The presence of an intermittent layer at the base of Earths mantle with a maximum thickness near 40 kilometers and a compressional wave velocity depressed by ∼10 percent compared with that of the overlying mantle is most simply explained as the result of partial melt at this depth. Both the sharp upper boundary of this layer (<10 kilometers wide) and the apparent correlation with deep mantle upwelling are consistent with the presence of liquid in the lowermost mantle, implying that the bottom of the thermal boundary layer at the base of the mantle may lie above its eutectic temperature. Such a partially molten zone would be expected to have enhanced thermal and chemical transport properties and may provide constraints on the geotherm and lateral variations in lowermost mantle temperature or mineralogy.

The core-mantle boundary layer and deep Earth dynamics

Recent seismological work has revealed new structures in the boundary layer between the Earths core and mantle that are altering and expanding perspectives of the role this region plays in both core and mantle dynamics. Clear challenges for future research in seismological, experimental, theoretical and computational geophysics have emerged, holding the key to understanding both this dynamic system and geological phenomena observed at the Earths surface.

Structure and Dynamics of Earth's Lower Mantle

Processes within the lowest several hundred kilometers of Earths rocky mantle play a critical role in the evolution of the planet. Understanding Earths lower mantle requires putting recent seismic and mineral physics discoveries into a self-consistent, geodynamically feasible context. Two nearly antipodal large low-shear-velocity provinces in the deep mantle likely represent chemically distinct and denser material. High-resolution seismological studies have revealed laterally varying seismic velocity discontinuities in the deepest few hundred kilometers, consistent with a phase transition from perovskite to post-perovskite. In the deepest tens of kilometers of the mantle, isolated pockets of ultralow seismic velocities may denote Earths deepest magma chamber.

Seismic Detection of the Lunar Core

Reinterpreted Apollo-era seismic data from the Moon reveal a solid inner core and a fluid outer core. Despite recent insight regarding the history and current state of the Moon from satellite sensing and analyses of limited Apollo-era seismic data, deficiencies remain in our understanding of the deep lunar interior. We reanalyzed Apollo lunar seismograms using array-processing methods to search for the presence of reflected and converted seismic energy from the core. Our results suggest the presence of a solid inner and fluid outer core, overlain by a partially molten boundary layer. The relative sizes of the inner and outer core suggest that the core is ~60% liquid by volume. Based on phase diagrams of iron alloys and the presence of partial melt, the core probably contains less than 6 weight % of lighter alloying components, which is consistent with a volatile-depleted interior.

A Post-Perovskite Lens and D'' Heat Flux Beneath the Central Pacific

Temperature gradients in a low-shear-velocity province in the lowermost mantle (D″ region) beneath the central Pacific Ocean were inferred from the observation of a rapid S-wave velocity increase overlying a rapid decrease. These paired seismic discontinuities are attributed to a phase change from perovskite to post-perovskite and then back to perovskite as the temperature increases with depth. Iron enrichment could explain the occurrence of post-perovskite several hundred kilometers above the core-mantle boundary in this warm, chemically distinct province. The double phase-boundary crossing directly constrains the lowermost mantle temperature gradients. Assuming a standard but unconstrained choice of thermal conductivity, the regional core-mantle boundary heat flux (∼85 ± 25 milliwatts per square meter), comparable to the average at Earths surface, was estimated, along with a lower bound on global core-mantle boundary heat flow in the range of 13 ± 4 terawatts. Mapped velocity-contrast variations indicate that the lens of post-perovskite minerals thins and vanishes over 1000 kilometers laterally toward the margin of the chemical distinct region as a result of a ∼500-kelvin temperature increase.

Seismic detection of a thin laterally varying boundary layer at the base of the mantle beneath the central-Pacific

We explore lowermost mantle structure beneath the Pacific with long‐period recordings of the seismic core phases SKS, SP_dKS, and SKKS from 25 deep earthquakes. SP_dKS and SKKS are anomalously delayed relative to SKS for lower mantle paths beneath the southwest Pacific. Late SP_dKS arrivals are explained by a laterally varying mantle‐side boundary layer at the CMB, having P‐velocity reductions of up to 10% and thickness up to 40 km. This layer is detected beneath a tomographically resolved large‐scale low velocity feature in the lower mantle beneath the central‐Pacific. SKS, SP_dKS, and SKKS data for the generally faster‐than‐average circum‐Pacific lower mantle are well‐fit by models lacking any such low‐velocity boundary layer. The slow boundary layer beneath the central Pacific may be a localized zone of partial melt, or perhaps a chemically distinct layer, with its location linked to overlying upwelling motions.

A very slow basal layer underlying large-scale low-velocity anomalies in the lower mantle beneath the Pacific: evidence from core phases

A multi-phase analysis using long-period World Wide Standardized Seismograph Network and Canadian Network data has been conducted using core-phases for deep focus events from the southwest Pacific. These include SKS, S2KS, SVd_(iff), and SP_dKS. The last phase emerges from SKS near 106° and is associated with a P-wave diffracting along the bottom of the mantle. Patterns in S2KS - SKS differential travel times (T_(S2KS-SKS)) correlate with those in SP_dKS - SKS (T_(SPdKS-SKS)). T_(S2KS-SKS) values strongly depend on variations in V_S structure in the lower third of the mantle, whereas T_(SPdKS-SKS) values mainly depend on V_P structure and variations in a thin zone (100 km or less) at the very base of the mantle. Anomalously large T_(S2KS-SKS) and T_(SPdKS-SKS) values (relative to the Preliminary Reference Earth Model (PREM)) are present for Fiji-Tonga and Kermadec events (recorded in North and South America), along with anomalously large SV_(diff) amplitudes well into the cores shadow. More northerly paths beneath the Pacific to North America for Indonesian and Solomon events display both PREM-like and anomalous times. A model compatible with the observations is presented, and contains a thin very-low-velocity layer at the base of the mantle that underlies the large volumetric lower-mantle low-velocity regions in the southwest Pacific. A low-velocity layer of 20–100 km thickness with reductions of up to 5–10% (relative to PREM) can reproduce T_(SPdKS-SKS) as well as SV_(diff) amplitudes. Large-scale (more than 1000 km) lower-mantle V_S heterogeneity (2–4%) can explain long-wavelength trends in T_(S2KS-SKS). The exact thickness and velocity reduction in the basal layer is uncertain, owing to difficulties in resolving whether anomalous structure occurs on the source- and/or receiver-side of wavepaths (at the CMB).

Seismological constraints on a possible plume root at the core–mantle boundary

Recent seismological discoveries have indicated that the Earths core–mantle boundary is far more complex than a simple boundary between the molten outer core and the silicate mantle. Instead, its structural complexities probably rival those of the Earths crust. Some regions of the lowermost mantle have been observed to have seismic wave speed reductions of at least 10 per cent, which appear not to be global in extent. Here we present robust evidence for an 8.5-km-thick and ∼50-km-wide pocket of dense, partially molten material at the core–mantle boundary east of Australia. Array analyses of an anomalous precursor to the reflected seismic wave ScP reveal compressional and shear-wave velocity reductions of 8 and 25 per cent, respectively, and a 10 per cent increase in density of the partially molten aggregate. Seismological data are incompatible with a basal layer composed of pure melt, and thus require a mechanism to prevent downward percolation of dense melt within the layer. This may be possible by trapping of melt by cumulus crystal growth following melt drainage from an anomalously hot overlying region of the lowermost mantle. This magmatic evolution and the resulting cumulate structure seem to be associated with overlying thermal instabilities, and thus may mark a root zone of an upwelling plume.

Lateral variations in lowermost mantle shear wave anisotropy beneath the north Pacific and Alaska

S waves recorded by long-period World-Wide Standardized Seismograph Network and broadband stations in North America for deep northwest Pacific subduction zone earthquakes provide evidence for anisotropy in the lowermost mantle shear velocity structure beneath the north Pacific and Alaska. Systematic delays of up to 4 s are observed between longitudinal components (SV) and transverse components (SH) of motion for core-reflected ScS waves as well as for core-grazing and diffracted S waves. The absence of significant splitting for S waves that have turning points more than a few hundred kilometers above the core-mantle boundary indicates that anisotropy is localized within the D′′ region (the lowermost portion of the mantle). SV-SH differential arrival times for both ScS and Sdiff, along with path length estimates assuming a 250 km thick D′′ region, indicate spatial variations in the strength of shear wave anisotropy. The strongest anisotropy (1–1.5%) is found in the eastern part of the study area, with systematic reduction in magnitude toward the west. A transverse isotropy model can explain the data, with the velocity structure for horizontally polarized waves (VSH) having a 2–3% discontinuous shear velocity increase at the top of D′′ (as proposed in earlier studies of the region) and a similar structure for S wave particle motion in the direction normal to the core-mantle boundary (VSV) but with the velocity jump at the top of D′′ and the velocity within D′′ being reduced from that for VSH by 0.5–1.5 km/s. Large uncertainties exist for velocity gradients above and below the velocity jump, but the requirement of a reduced VSV relative to VSH in D′′ is clear. Synthetic waveforms calculated by using separate isotropic structures for SH and SV match the observations well and constrain the basic anisotropic structure, because the shear waves all traverse the region with near-grazing geometries. The study area exhibits strong lateral variations in lower mantle shear velocity structure and variable thickness of the D′′ layer. Topography of the D′′ layer is not well resolved (because of trade-off with volumetric heterogeneity).

Seismic detection of folded, subducted lithosphere at the core–mantle boundary

Seismic tomography has been used to infer that some descending slabs of oceanic lithosphere plunge deep into the Earths lower mantle. The fate of these slabs has remained unresolved, but it has been postulated that their ultimate destination is the lowermost few hundred kilometres of the mantle, known as the D″ region. Relatively cold slab material may account for high seismic velocities imaged in D″ beneath areas of long-lived plate subduction, and for reflections from a seismic velocity discontinuity just above the anomalously high wave speed regions. The D″ discontinuity itself is probably the result of a phase change in relatively low-temperature magnesium silicate perovskite. Here, we present images of the D″ region beneath the Cocos plate using Kirchhoff migration of horizontally polarized shear waves, and find a 100-km vertical step occurring over less than 100 km laterally in an otherwise flat D″ shear velocity discontinuity. Folding and piling of a cold slab that has reached the core–mantle boundary, as observed in numerical and experimental models, can account for the step by a 100-km elevation of the post-perovskite phase boundary due to a 700 °C lateral temperature reduction in the folded slab. We detect localized low velocities at the edge of the slab material, which may result from upwellings caused by the slab laterally displacing a thin hot thermal boundary layer.