A Coupled Geochemical‐Geodynamic Approach for Predicting Mantle Melting in Space and Time.

Geodynamic simulations underpin our understanding of upper‐mantle processes, but their predictions require validation against observational data. Widely used geophysical datasets provide limited constraints on dynamic 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 geodynamic simulations, thereby providing a means to validate these simulations against geochemical datasets. Here, BDD21's functionality is illustrated using the Fluidity computational modeling framework, although it is designed so that it can be integrated with other geodynamic software. To validate our melting parameterization and coupled geochemical‐geodynamic 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 with the same conditions. Our lithospheric step simulations predict spatial and temporal melting trends that are consistent with those recorded at intraplate volcanic provinces in similar geologic settings. Taken together, these results suggest that our coupled geochemical‐geodynamic 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. Plain Language Summary: Earth's mantle is a ∼3,000 km thick layer of hot rock that lies between Earth's crust and core. When anomalously hot mantle arrives at shallow depths it begins to melt. These melts are less dense than the surrounding mantle and ascend to the surface to erupt as volcanic lavas. The mantle's slow, creeping, convective motion over billions of years has been integral to Earth's thermal, chemical, tectonic and geological evolution. However, an inability to reproduce observational constraints derived from the composition of volcanic lavas at Earth's surface limits our capacity to validate models of mantle convection back in time. Here, we present a new framework that can predict the volume and composition of melts generated within the mantle. These predictions compare favorably with those recorded by igneous rocks at Earth's surface in two geologic settings: mid‐oceanic ridges, where plates move apart to drive decompression melting, and lithospheric steps, where instabilities associated with changes in the thickness of Earth's rigid outermost shell generate volcanism far from plate boundaries. The approach and tools presented here will allow scientists to better understand the mantle's past structure, dynamics, evolution and impact on Earth's surface. Key Points: We present the BDD21 peridotite‐melt‐chemistry parameterization that can be coupled with geodynamic models to calculate melt compositionBDD21 is applied to simulations of mid‐oceanic ridges to replicate observed patterns of crustal thickness and melt chemistryGeodynamic simulations that incorporate BDD21 can be used to study melting adjacent to lithospheric steps and in other geologic settings [ABSTRACT FROM AUTHOR]