Slab-wedge Coupling Promotes Deep Subduction of Sediment: Geodynamic Insights to Mantle Geochemical Heterogeneity

Radiogenic isotopes - particularly in ocean island basalts - suggest that the mantle is heterogeneous. Recycling and mixing of various lithologies likely forms the variety of observed endmembers. The EM2 (high ⁸⁷ Sr/ ⁸⁶ Sr) endmember composition may consist of terrigenous sediments mixing with the mantle, however the low density of sediment necessitates a mechanism by which to transport it to depth.

Using the finite element code ASPECT, we model the behavior of sediment and the overlying mantle wedge at depths beyond the "subduction factory." The model consists of a 30 km thick section representing the upper portion of the descending slab, a sediment layer, and 100 km of the overlying mantle wedge. We perform a parameter sweep varying sediment layer thickness, sediment density, sediment viscosity, slab angle, and slab velocity. In all cases, the density of the sediment is lower than the overlying mantle. Therefore, sediment diapirs would be expected, however, preliminary results show minimal diapirism in most model runs. As the overlying mantle is cooled by the cold, descending lithosphere, a higher viscosity thermal-boundary layer forms above the sediment layer. This clamps sediment against the plate, both of which are subducted into the deep mantle. Crucially, we find that sediment that descends past the subduction factory will continue to descend with the plate regardless of density contrast.

A sediment layer may present as a thin, low velocity layer atop the subducting slab. Such a layer could be imaged seismically if it serves as a guide for high frequency seismic waves. Using deep earthquakes under South America, we attempt to locate this layer atop the Nazca plate in an effort to constrain the true thickness of a deeply subducted sediment layer.

Additionally, recent work has used geochemical mass balance to argue for the role of mélange mixing and melting in the mantle wedge to explain the isotopic composition of arc basalts. Adopting this model, we extend these mass balance calculations past the subduction factory to validate the isotopic evolution of residual sediments to affirm the origin of the EM2 endmember.