Great volumes of water are carried downward into the mantle transition zone (MTZ, 410-670 km depth) by subducting slabs. If this water is later drawn upward, the resulting mantle melting may generate intraplate volcanism (IPV). Despite its importance, the amount and spatial distribution of water within the MTZ, and its impact on IPV, are poorly constrained. Here we use a series of plate tectonic reconstructions to estimate rates and positions of water injection into the MTZ by subducted slabs during the past 400 Myr. This allows us to construct maps of heterogeneous MTZ hydration, which we then compare to IPV locations since 200 Ma. We find a statistically significant correlation between wet regions of the MTZ and locations of IPV at the surface, but only if water remains stored in the MTZ for periods of 30-100 Myr after being carried there by slabs. We find that 42-68% of IPV is underlain by wet MTZ, with higher correlations associated with longer MTZ residence time, slower slab sinking rates, and longer time periods between MTZ hydration and IPV eruption. The correlation is highest during the Jurassic, when more extensive slab interaction with the MTZ caused a wider area of the MTZ to become hydrated. Parts of the MTZ near the western Pacific, southern Africa, and western Europe, have remained dry by avoiding wet slabs. Hydrous upwellings rising from the MTZ, some driven by interactions with subducting slabs, may be responsible for IPV rising from wet MTZ regions.

The widespread High Arctic Large Igneous Province (HALIP) exhibits prolonged melting over more than 50 Myr, an observation that is difficult to reconcile with the classic view of large igneous provinces and associated melting in plume heads. Hence, the suggested plume-related origin and classification of HALIP as a large igneous province have been questioned. Here, we use numerical models that include melting and melt migration to investigate a rising plume interacting with variable lithosphere thickness, i.e. an extended-basin-to-craton setting. Models reveal significant spatial and temporal variations in melt volumes and pulses of melt production, including protracted melting for at least about 30-40 Myr, but only if migrating melt transports heat upwards and enhances local lithospheric thinning. Plume material deflected from underneath the Greenland craton can then re-activate melting zones below the previously plume-influenced Sverdrup Basin, even though the plume is already ~500 km away. Hence, melting zones may not represent the location of the deeper plume stem at a given time. Plume flux pulses associated with mantle processes or magma processes within the crust may alter the timing and volume of secondary pulses and their surface expression. Our models suggest that HALIP magmatism is expected to exhibit plume-related trace element signatures throughout time, but potentially shift from mostly tholeiitic magmas in the first pulse towards more alkalic compositions for secondary pulses, with regional variations in timing of magma types. We propose that the prolonged period of rejuvenated magmatism of HALIP is consistent with plume impingement on a cratonic edge.

Asthenospheric shear causes some minerals, particularly olivine, to develop anisotropic textures that can be detected seismically. In laboratory experiments, these textures are also associated with anisotropic viscous behavior, which should be important for geodynamic processes. To examine the role of anisotropic viscosity for asthenospheric deformation, we developed a numerical model of coupled anisotropic texture development and anisotropic viscosity, both calibrated with laboratory measurements of olivine aggregates. This model characterizes the time-dependent coupling between large-scale formation of lattice-preferred orientation (i.e., texture) and changes in asthenospheric viscosity for a series of simple deformation paths that represent upper-mantle geodynamic processes. We find that texture development beneath a moving surface plate tends to align the a-axes of olivine into the plate-motion direction, which weakens the effective viscosity in this direction and increases plate velocity for a given driving force. Our models indicate that the effective viscosity increases for shear in the horizontal direction perpendicular to the a-axes. This increase should slow plate motions and new texture development in this perpendicular direction, and could impede changes to the plate motion direction for 10s of Myrs. However, the same well-developed asthenospheric texture may foster subduction initiation perpendicular to the plate motion and deformations related to transform faults, as shearing on vertical planes seems to be favored across a sub-lithospheric olivine texture. These end-member cases examining shear-deformation in the presence of a well-formed asthenospheric texture illustrate the importance of the mean olivine orientation, and its associated viscous anisotropy, for a variety of geodynamic processes.

Mantle viscosity controls a variety of geodynamic processes such as glacial isostatic adjustment (GIA), but it is poorly constrained because it cannot be measured directly from geophysical measurements. Here we develop a method that calculates viscosity using empirical viscosity flow laws coupled with mantle parameters (temperature and water content) inferred from seismic and magnetotelluric (MT) observations. We find that combining geophysical constraints allows us to place significantly tighter bounds on viscosity estimates compared to using seismic or MT observations alone. In particular, electrical conductivity inferred from MT data can determine whether upper mantle minerals are hydrated, which is important for viscosity reduction. Additionally, we show that rock composition should be considered when estimating viscosity from geophysical data because composition directly affects seismic velocity and electrical conductivity. Therefore, unknown composition increases uncertainty in temperature and water content, and makes viscosity more uncertain. Furthermore, calculations that assume pure thermal control of seismic velocity may misinterpret compositional variations as temperature, producing erroneous interpretations of mantle temperature and viscosity. Stress and grain size also affect the viscosity and its associated uncertainty, particularly via their controls on deformation regime. Dislocation creep is associated with larger viscosity uncertainties than diffusion creep. Overall, mantle viscosity can be estimated best when both seismic and MT data are available and the mantle composition, grain size and stress can be estimated. Collecting additional MT data probably offers the greatest opportunity to improve geodynamic or GIA models that rely on viscosity estimates.

While hotspot tracks beneath thin oceanic lithosphere are visible as volcanic island chains, the plume-lithosphere interaction for thick continental or cratonic lithosphere often remains hidden due to the lack of volcanism. To identify plume tracks with missing volcanism, we characterize the amplitude and timing of surface heat flux anomalies following a plume-lithosphere interaction using mantle convection models. Our numerical results confirm an analytical relationship in which surface heat flux increases with the extent of lithosphere thinning, which is primarily controlled by on the viscosity structure of the lower lithosphere and the asthenosphere. We find that lithosphere thinning is greatest when the plate is above the plume conduit, while the maximum heat flux anomaly occurs about 40-140\,Myr later. Therefore, younger continental and cratonic plume tracks can be identified by observed lithosphere thinning, and older tracks by an increased surface heat flux, even if they lack extrusive magmatism.

Plate reconstructions show that the plume feeding today’s Icelandic volcanism passed beneath the continental lithosphere of Greenland between 50 and 100 million years ago. While there has been ample volcanism on the margins of Greenland, both on the west and the east coasts, the thickness of the Greenland craton prevented surface eruptions within Greenland, leaving details of the plume track unclear. However, the passage of the plume is expected to leave a scar in the lower continental lithosphere of Greenland, where the hot plume material interacts with and erodes the stiff cratonic root. This interaction should cause increased surface heat flux and reduced lithosphere thickness along the plume track, and can potentially be constrained by observations that constrain heat flux (e.g., magnetic data and ice flow rates) and lithospheric structure (e.g., seismic tomography and magnetotelluric modeling). While most seismic tomography models indicate that there might be an east-west trending corridor of reduced lithosphere thickness, recent heat flux maps inferred from magnetic data show a northwest-southeast trending anomaly of high heat flux, suggesting an alternative direction of the passage of the Iceland plume. In order to resolve the discrepancy between the suggested hotspot tracks, we use numerical models to investigate how the surface heat flux would evolve in response to the passage of a plume beneath thick cratonic lithosphere. Our work focuses both on the temporal evolution (e.g. the onset and duration), and the shape of the heat flux anomaly, as well as lithospheric thinning related to convective erosion of the cratonic root by hot plume material. We show that both the onset and the duration of the heat flux anomaly, as well as the degree of lithosphere thinning, depend on various parameters, especially the viscosity structure of lithosphere and asthenosphere and the plume strength. A comparison between observations from Greenland and our model predictions should provide new and better constraints on the subsurface structure of Greenland, and how it was modified by its interaction with the Iceland plume.

The prominent seismic low-velocity zone (LVZ) in the oceanic low-viscosity asthenosphere is approximately coincident with a zone of high seismic attenuation (or low seismic Q). Small grain sizes in the asthenosphere could link these seismic and rheological properties as small grain sizes reduce viscosity and also lower seismic velocity and seismic Q. Because grain-size is reduced by rock deformation, the asthenosphere’s seismic properties can place constraints on asthenospheric deformation or flow. To determine dominant flow patterns, we develop a selfconsistent analytical 1-D channel flow model that accounts for upper mantle rheology and its dependence on flow-modified grain-sizes, water content and melt fraction, both for flow driven by surface plate motions (Couette flow) and/or by horizontal pressure gradients (Poiseuille flow). From our flow models, Couette flow dominates if the upper mantle is dry, and plug flow (a Poiseuille flow for power law rheology) if it is wet. A plug flow configuration spanning the upper 670 km of the mantle best explains the low seismic Q zone in the asthenosphere, which can be attributed to significant grain-size reduction due to extensive shearing across the asthenosphere. Below the asthenosphere, high water content and minimal shear deformation promote large grain sizes and high seismic Q. We suggest that asthenospheric low-Q and LVZ can be largely explained by grain-size variations associated with plug flow in the wet upper mantle.

While the passage of a plume beneath a thin oceanic lithosphere is usually connected to a hot spot track, and thus can easily be traced back in time, the interaction between a plume and thick continental or cratonic lithosphere is less obvious. Although volcanic eruptions are a common feature for plumes reaching the surface beneath continents, especially in the form of large igneous provinces associated with the arrival of a plume head, it is unlikely that the entire plume track is marked by extrusive volcanism. The thickness of continental lithosphere, and especially cratonic lithosphere, may prohibit the eruption of magma in many places, and can re-focus extrusive volcanism towards places where the lithosphere is thinner or more permeable due to preexisting fault structures. However, even though no magma might be erupted, the passage of a continent over a hot mantle upwelling will be visible in the surface heat flux, even millions of years after the plume passage. In this study, we use numerical models to investigate how a plume passing beneath continental or cratonic lithosphere affects surface heat flux over time, and which parameters of the plume and subsurface structure are the most relevant for determining the size of the respective heat flux anomaly. We show that any kind of surface heat flux anomaly is associated with an erosional thinning of the base of the lithosphere, and greater thinning leads to larger heat flux anomalies. While the maximum of lithosphere thinning is observed at a position and time a few million years after the plate passes over the plume, heat flux anomalies related to conduction continue to increase, reaching a maximum about 80-150 Myr after passage over the plume. In the case of stagnant (stationary) plates, the delays between lithosphere thinning and heat flux anomaly are smaller and the observed anomalies are larger. Amplitudes of both thinning and heat flux anomalies are most sensitive to the viscosity of the asthenosphere and the lower lithosphere, because a lower viscosity facilitates basal erosion and thus increases heat fluxes. Also important are the interaction time between plume and plate, i.e. plate velocity or plume life time, and the plume strength / excess temperature. These results have important implications for understanding plume-lithosphere interactions in polar settings, e.g. Greenland and Antartica, and for various places in Africa, North America and China.