Do Plumes Suck?

NASA Astrophysics Data System (ADS)

Geophysical observations at plumes, ridges, and arcs indicate that the the volcanic accretionary zone is much narrower than the inferred melt production region in the upwelling mantle. For ridges and arcs, lateral pressure gradients induced by advection of viscous asthenospheric mantle have been proposed as a potential mechanism for focusing melts to the accretionary center [Phipps Morgan, 1987; Spiegelman and McKenzie, 1987]. For ridges and arcs with asthenospheric viscosities >=1021 Pa?s, the magnitude of the lateral pressure gradients associated with viscous corner flow are comparable to vertical melt buoyancy (? ? g). Plumes, however, differ from ridges and arcs in that mantle flow is driven primarily by buoyancy of the upwelling solid as opposed to viscous drag induced by surface plate motions. This difference in driving forces changes the relationship between the solid flow field and the resulting pressure gradients. We use numerical models to examine the influence of lateral pressure gradients from solid advection in plumes. We calculate the stream function and pressure field in the solid induced by a buoyant cylinder beneath a stationary lithosphere using the method of Ribe and Christensen [1999] after Pozrikidis [1997]. Initial results suggest that lateral pressure gradients may draw melt into the top of the plume towards the flow stagnation point. However, the largest flow-induced pressure gradients are oriented vertically within the buoyant plume. Compression where the plume impinges on the lithospheric lid has the potential to impede the vertical migration of melt within the plume. The magnitude of the flow-induced pressure gradients scales with the strength of the buoyant upwelling. However, unlike ridges and arcs, asthenospheric viscosity has little effect on the pressure gradients, because velocity and viscosity of plume material are interdependent. We explore the possible role of these pressure gradients in melt migration at plume and ridge-plume environments. Phipps Morgan, J., Melt migration beneath mid-ocean spreading centers, Geophys. Res. Lett., 14 (12), 1238-1241, 1987. Pozrikidis, C., Introduction to theoretical and computational fluid dynamics, 675 pp., Oxford University Press, New York, 1997. Ribe, N.M., and U.R. Christensen, The dynamical origin of Hawaiian volcanism, Earth and Planet. Sci. Lett., 171, 517-531, 1999. Spiegelman, M., and D. McKenzie, Simple 2-D models for melt extraction at mid-ocean ridges and island arcs, Earth and Planetary Science Letters, 83 (1-4), 137-152, 1987.

Braun, M. G.; Sohn, R. A.; Ribe, N. M.

2001-12-01