The composition of the lower mantle -comprising 56% of Earth's volume -remains poorly constrained. Among the major elements, Mg/Si ratios ranging from ∼0.9-1.1, such as in rocky solar-system building blocks (or chondrites), to ∼1.2-1.3, such as in upper-mantle rocks (or pyrolite), have been proposed. Geophysical evidence for subducted lithosphere deep in the mantle has been interpreted in terms of efficient mixing and thus homogeneous Mg/Si across most of the mantle. However, previous models did not consider the effects of variable Mg/Si on the viscosity and mixing efficiency of lower-mantle rocks. Here, we use geodynamic models to show that large-scale heterogeneity with viscosity variations of ∼20×, such as due to the dominance of intrinsically strong (Mg,Fe)SiO 3 −bridgmanite in low-Mg/Si domains, are sufficient to prevent efficient mantle mixing, even on large scales.Models predict that intrinsically strong domains stabilize degree-two mantle-convection pat-1 terns, and coherently persist at depths of ∼1,000-2,200 km up to the present-day, separated by relatively narrow up-/downwelling conduits of pyrolitic material. The stable manifestation of such "bridgmanite-enriched ancient mantle structures" (BEAMS) may reconcile the geographical fixity of deep-rooted mantle-upwelling centers, and fundamental geophysical changes near 1,000 km depth (e.g. in terms of seismic-tomography patterns, radial viscosity increase, lateral deflections of rising plumes and sinking slabs). Moreover, these ancient structures may provide a reservoir to host primordial geochemical signatures.State-of-the-art seismic-tomography models are difficult to reconcile with a mantle that is homogeneous (pyrolitic) on large length-scales. For example, most recently-subducted slabs flatten appearing to stagnate at either ∼660 km or ∼1,000 km depth 1 . Many mantle plumes are inferred to be deflected at similar depths 2, 3 . In particular, deflections of mantle up-/downwellings in the uppermost lower mantle remain enigmatic. A viscosity increase near 1,000 km depth, consistent with geoid inversions, has been invoked to explain these observations 4, 5 . However, there is no candidate phase transition to account for a sharp viscosity jump that could markedly affect mantle flow. Alternatively, compositional layering has been proposed 6 , but the effects of coupled largescale compositional and rheological heterogeneity on mantle dynamics remain poorly understood. Composition-induced viscosity variations in the lower mantleLateral heterogeneity in lower-mantle composition can give rise to rheological contrasts. Heterogeneity involving SiO 2 -enriched rocks has been put forward to balance the Earth's Si budget relative to the sun and chondrites, also given limitations to dissolve Si in the present-day 2 outer core 7 . SiO 2 -enriched rocks with CI-chondritic Mg/Si of ∼0.9-1.1 should host ∼87-97% (Mg,Fe)SiO 3 −bridgmanite (Br) and only ∼0-10% (Mg,Fe)O-ferropericlase (Fp), in addition to a minor amount of Ca-perovskite (∼3%). In contrast, pyrolitic rocks w...

The boundary between Earth's rigid lithosphere and the underlying, ductile asthenosphere is marked by a distinct seismic discontinuity 1 . A decrease in seismic-wave velocity and increase in attenuation at this boundary is thought to be caused by partial melt 2 . The density and viscosity of basaltic magma, linked to the atomic structure 3,4 , control the process of melt separation from the surrounding mantle rocks 5-9 . Here we use high-pressure and high-temperature experiments and in situ X-ray analysis to assess the properties of basaltic magmas under pressures of up to 5.5 GPa. We find that the magmas rapidly become denser with increasing pressure and show a viscosity minimum near 4 GPa. Magma mobility-the ratio of the melt-solid density contrast to the magma viscosityexhibits a peak at pressures corresponding to depths of 120-150 km, within the asthenosphere, up to an order of magnitude greater than pressures corresponding to the deeper mantle and shallower lithosphere. Melts are therefore expected to rapidly migrate out of the asthenosphere. The diminishing mobility of magma in Earth's asthenosphere as the melts ascend could lead to excessive melt accumulation at depths of 80-100 km, at the lithosphere-asthenosphere boundary. We conclude that the observed seismic discontinuity at the lithosphereasthenosphere boundary records this accumulation of melt.Along the axial zone of mid-ocean ridges (MORs), asthenospheric mantle rises in response to the diverging motion of oceanic lithosphere and experiences decompression melting. Depending on the volatile content and temperature of the upper mantle, peridotite partial melting initiates at depths of about 80-130 km (ref. 10). The resulting basaltic magmas are buoyant and mobile, percolating upward to form the crust, and leaving a refractory residuum that forms the oceanic lithosphere. Along the more than 50,000-km-long global MOR system, roughly 60,000 tons of magma are processed per minute 11 , replenishing the entire ocean floor in ∼100 Myr. This process is the primary engine for present-day geochemical fractionation of our planet.Structural changes in basaltic magmas with pressure (or depth) play a central role in controlling magma mobility and melting. Pressure-dependent structural changes in silicate melts associated with transformations in the coordination of aluminium ions have been suggested from nuclear magnetic resonance spectroscopic studies of quenched glasses 3 . Such structural changes usually

No abstract

The stagnation of ~1000-km deep slabs indicates that dense basalt may be more abundant in the lower mantle than in the upper mantle.

No abstract

No abstract

Some oceanic volcano chains violate the predictions of the hotspot hypothesis for geographic age progressions. One mechanism invoked to explain these observations is small‐scale sublithospheric convection (SSC). In this study, we explore this concept in thermo‐chemical, 3D‐numerical models. Melting due to SSC is shown to emerge in elongated features (∼750 km) parallel to plate motion and not just at a fixed spot; therefore volcanism occurs in chains but not with hotspot‐like linear age progressions. The seafloor age at which volcanism first occurs is sensitive to mantle temperature, as higher temperatures increase the onset age of SSC because of the stabilizing influence of thicker residue from previous mid‐ocean ridge melting. Mantle viscosity controls the rate of melt production with decreasing viscosities leading to more vigorous convection and volcanism. Calculations predict many of the key observations of the Pukapuka ridges, and the volcano groups associated with the Line, Cook‐Austral, and Marshall Islands.

[1] Many volcano chains in the Pacific do not follow the most fundamental predictions of hot spot theory in terms of geographic age progressions. One possible explanation for non-hot spot intraplate volcanism is small-scale sublithospheric convection (SSC), and we explore this concept using 3-D numerical models that simulate melting with rheology laws that account for the effects of dehydration. SSC spontaneously self-organizes beneath relatively mature oceanic lithosphere. Whenever this lithosphere is sufficiently young and thin, SSC replaces the shallow layer of harzburgite, which was formed by partial melting at the mid-ocean ridge, with fresh peridotite. This mechanism enables magma generation without any preexisting thermochemical anomalies. However, the additional effect of melting-induced dehydration to stiffen the harzburgite requires lower background viscosities to allow for vigorous SSC, overturn of the compositional stratification, and related magmatism. The intrinsic stiffness of the dehydrated harzburgite furthermore restricts penetration of SSC into very shallow and cooler levels. On the one hand, such a restriction precludes high degrees of melting, but on the other hand, it slows asthenospheric cooling and thus prolongs the duration of melting (to $25 Ma). Volcanism over such an elongated melting anomaly continues for at least 10-20 Ma and occurs on seafloor ages of $20 to $60 Ma. These seafloor ages increase with increasing mantle temperature due to the effect of forming a thicker harzburgite layer from more extensive mid-ocean ridge melting. The long durations of volcanism predicted reconcile observations of extended activity of individual seamounts and synchronous activity over great distances along some volcanic chains. SSC thus gives an explanation for previously enigmatic volcano ages along the Line Islands and the Gilbert and Pukapuka ridges, as well as along the individual subchains of the Wakes, Marshalls, and CookAustrals.