Intellectual Merit: A substantial number of hotspots (concentrations of seafloor volcanoes that are believed to be associated with upwelling plumes of magma from the Earth's mantle) that are located within the interiors of oceanic plates (e.g., Kerguelen, Reunion, Galapagos) appear to directly influence the chemistry of the basalts found at mid-ocean ridges located at the boundaries of the plate, as evidenced by similarities in the chemistry of basalts produced at the hotspot and the proximal portion of the ridge, and anomalous crustal thickness along the ridge. However, the pattern of flow in the upper mantle that would allow such interaction remains poorly understood. This project will characterize mantle flow and melting in off-axis hotspot - ridge systems using a multidisciplinary modeling approach in which melting models are coupled to numerical geodynamic models to allow prediction of melt chemistry. The resulting coupled model will be applied particularly to the Easter - Salas y Gomez Seamount Chain (ESC) - Easter Microplate (EMP) system, with synthetic melt compositions used along with model-predicted geophysical parameters (e.g., crustal thickness, gravity) to constrain mantle flow through direct comparison to observational data. The ESC-EMP system is an ideal natural laboratory for studying interaction between a hotspot and a nearby mid-ocean ridge: the geometry of the ESC-EMP system is simple, there is recent volcanism along the portion of the ESC between the hotspot and the spreading center, and a comprehensive geochemical database for the region already exists. The project consists of two primary tasks. The first task is to model the existing geochemical data to constrain source and melting parameters relevant to the ESC-EMP system. These results will then be incorporated into a 3-D geodynamic model of hotspot-ridge systems to examine mantle flow in the ESC-EMP system. Geodynamic model predictions (e.g., spatial variations in melt chemistry, crustal thickness, gravity) will then be compared to observational data to constrain the temperature and composition of the Easter-Salas y Gomez hotspot as well as the manner in which material flows from the hotspot to the ridge axis (i.e., radial dispersion vs. channeled flow) . As a byproduct of the geodynamic modeling exercise, a series of 3-D benchmark tests relevant to mantle flow will be developed and applied to two numerical modeling packages
Broader Impacts: The inherently cross-disciplinary approach outlined in this proposal, coupling geochemistry with geodynamic modeling, will contribute to the professional development of the Principal Investigators, who are both recent PhDs. The project will also provide funding for a Boston University graduate student, who, while advised by Hall, would interact extensively with Kingsley, presenting the student with an excellent opportunity to work at the interface between geodynamics and geochemistry from the very start of their career. Results of benchmarking of the models for free- and forced-convection scenarios relevant to the upper mantle will be made available to the community at large through publication and online documentation. Furthermore, the simpler benchmark scenarios will become an integral part of an advanced undergraduate/introductory graduate level course on computational fluid dynamics in the Earth sciences currently being developed by Hall, in conjunction with fellow BU professor Sergio Fagherazzi. In addition, Hall will use the results of the project as a case study highlighting a multidisciplinary approach to understanding the Earth's interior, in the undergraduate geodynamics course he will be teaching at BU. Finally, the algorithms developed to model melting and predict melt chemistry, as well as to post-process these results, will be made available to the community through a website hosted at BU and possibly also through the Computational Infrastructure for Geodynamics (CIG) program
