Melting-induced dehydration and plume-lithosphere interaction at intra-plate hotspots

While the dramatic effect of water on the rheology of olivine aggregates has been demonstrated experimentally [Hirth and Kohlstedt, 1996], the geodynamic implications of variations of water content within the mantle have yet to be fully explored. In particular, because water behaves as a highly incompatible element during partial melting of peridotite, the water content of the mantle will decrease rapidly with the onset of melting, resulting in an abrupt increase in the viscosity of the residual peridotite. Due to preferential removal of Fe relative to Mg during melting, the residual will also be slightly buoyant. The existence of this buoyant, viscous residual mantle has been shown to strongly effect mantle flow and melting at mid-ocean ridges [Braun et al., 2000; Hall and Parmentier, 2000], at ridge-centered mantle plumes [Ito et al., 1999] and at off-axis mantle plumes [e.g., Hall and Kincaid, 2003; 2004], and it has been proposed as the cause of the large bathymetric swell associated with the Hawaiian hotspot [Phipps Morgan and Morgan, 1995]. We present results from a series of numerical geodynamic experiments undertaken to evaluate the generation and fate of depleted, buoyant residual mantle within plumes rising in a purely intra-plate setting, and its effects on the pattern of mantle flow and melt generation. Experiments were conducted using the CitcomCU finite element package to model mantle flow associated with a thermally buoyant plume rising beneath an oceanic plate within a 3-D model domain corresponding to the upper mantle. Melting and dehydration are incorporated using a Lagrangian particle method [Hall and Kincaid, 2003]. The velocity of the overriding plate, plume temperature, plume water content and plume diameter were varied systematically between experiments to fully characterize the system. Results indicate that dehydration due to melting within the upwelling plume results in the growth of a viscous plug that extends downward from the base of the lithosphere. This plug alters the flow of upwelling plume material at the base of the lithosphere and results in variations in the spatial distribution of melting relative to scenarios without dehydration. We suggest that some of the spatial complexity in volcanism at individual hotspots may reflect the existence of a plug of viscous, buoyant residual mantle.