Geodynamical simulations underpin our understanding of upper-mantle processes, but their predictions require validation against observational data. Widely used geophysical datasets provide limited constraints on dynamical processes into the geological past, whereas under-exploited geochemical observations from volcanic lavas at Earth's surface constitute a valuable record of mantle processes back in time. Here, we describe a new peridotite-melting parameterization, BDD21, that can predict the incompatible-element concentrations of melts within geodynamical simulations, thereby providing a means to validate these simulations against geochemical datasets. Here, BDD21's functionality is illustrated using the Fluidity computational modelling framework, although it is designed so that it can be integrated with other geodynamical software. To validate our melting parameterization and coupled geochemical-geodynamical approach, we develop 2-D single-phase flow simulations of melting associated with passive upwelling beneath mid-oceanic ridges and edge-driven convection adjacent to lithospheric steps. We find that melt volumes and compositions calculated for mid-oceanic ridges at a range of mantle temperatures and plate-spreading rates closely match those observed at present-day ridges. Our lithospheric-step simulations predict spatial and temporal melting trends that are consistent with those recorded at intra-plate volcanic provinces in similar geologic settings. Taken together, these results suggest that our coupled geochemical-geodynamical approach can accurately predict a suite of present-day geochemical observations. Since our results are sensitive to small changes in upper-mantle thermal and compositional structure, this novel approach provides a means to improve our understanding of the mantle's thermo-chemical structure and flow regime into the geological past.

Several of Earth’s intra-plate volcanic provinces are difficult to reconcile with the mantle plume hypothesis. Instead, they exhibit characteristics that are better explained by shallower processes involving the interplay between uppermost mantle flow and the base of Earth’s heterogeneous lithosphere. The mechanisms most commonly invoked are edge-driven convection (EDC) and shear-driven upwelling (SDU), both of which act to focus upwelling flow, and the associated decompression melting, adjacent to steps in lithospheric thickness. In this study, we undertake a systematic numerical investigation, in both 2-D and 3-D, to quantify the sensitivity of EDC, SDU and their associated melting to several key controlling parameters. Our simulations demonstrate that the spatial and temporal characteristics of EDC are sensitive to the geometry and material properties of the lithospheric step, in addition to the depth-dependence of upper mantle viscosity. These simulations also indicate that asthenospheric shear can either enhance or reduce upwelling velocities and predicted melt volumes, depending upon the magnitude and orientation of flow relative to the lithospheric step. When combined, such sensitivities explain why step changes in lithospheric thickness, which are common along cratonic edges and passive margins, only produce volcanism at isolated points in space and time. Our predicted trends of melt production suggest that, in the absence of potential interactions with mantle plumes, EDC and SDU are viable mechanisms only for Earth’s shorter-lived, lower-volume intra-plate volcanic provinces.

Several of Earth’s intra-plate volcanic provinces are hard to reconcile with the mantle plume hypothesis. Instead, they exhibit characteristics that are more compatible with shallower processes that involve the interplay between uppermost mantle flow and the base of Earth’s heterogeneous lithosphere. The mechanisms most commonly invoked are edge-driven convection (EDC) and shear-driven upwelling (SDU), both of which act to focus upwelling flow and the associated decompression melting adjacent to steps in lithospheric thickness. In this study, we undertake a systematic numerical investigation, in both 2-D and 3-D, to quantify the sensitivity of EDC, SDU, and the associated melting to key controlling parameters. Our simulations demonstrate that the spatio-temporal characteristics of EDC are sensitive to the geometry and material properties of the lithospheric step, in addition to the magnitude and depth-dependence of upper mantle viscosity. These simulations also indicate that asthenospheric shear can either enhance or reduce upwelling velocities and the associated melting, depending upon the magnitude and orientation of flow relative to the lithospheric step. When combined, such sensitivities explain why step changes in lithospheric thickness, which are common along cratonic edges and passive margins, only produce volcanism at isolated points in space and time. Our predicted trends of melt production suggest that, in the absence of potential interactions with mantle plumes, EDC and SDU are viable mechanisms only for Earth’s shorter-lived, lower-volume intra-plate volcanic provinces.

Several of Earth's intra-plate volcanic provinces occur within or adjacent to continental lithosphere, with many believed to mark the surface expression of upwelling mantle plumes. Nonetheless, studies of plume-derived magmatism have generally focussed on ocean-island volcanism, where the overlying rigid lithosphere is of uniform thickness. Here, we investigate the interaction between mantle plumes and heterogeneous continental lithosphere using a series of geodynamical models. Our results demonstrate that the spatio-temporal magmatic expression of plumes in these continental settings is complex and strongly depends on the location of plume impingement, differing substantially from that expected beneath oceanic lithosphere. Where plumes ascend beneath thick continental cratons, the overlying lid locally limits decompression melting. However, gradients in lithospheric thickness channel plume material towards regions of thinner lithosphere, activating magmatism away from the plume conduit, sometimes simultaneously at locations more than a thousand kilometres apart. This magmatism regularly concentrates at lithospheric steps, where it may be difficult to distinguish from that arising through edge-driven convection, especially if differentiating geochemical signatures are absent, as implied by some of our results. If plumes impinge in regions of thinner lithosphere, the resulting asthenospheric flow regime can force material downwards at lithospheric steps, shutting off pre-existing edge-related magmatism. In addition, under certain conditions, the interaction between plume material and lithospheric structure can induce internal destabilisation of the plume pancake, driving complex time-dependent magmatic patterns at the surface. Our study highlights the challenges associated with linking continental magmatism to underlying mantle dynamics and motivates an inter-disciplinary approach in future studies.