Constraining the source of mantle plumes

Abstract

In order to link the geochemical signature of hot spot basalts to Earth's deep interior, it is first necessary to understand how plumes sample di↵erent regions of the mantle. Here, we investigate the relative amounts of deep and shallow mantle material that are entrained by an ascending plume and constrain its source region. The plumes are generated in a viscous syrup using an isolated heater for a range of Rayleigh numbers. The velocity fields are measured using stereoscopic Particle-Image Velocimetry, and the concept of the 'vortex ring bubble' is used to provide an objective definition of the plume geometry. Using this plume geometry, the plume composition can be analysed in terms of the proportion of material that has been entrained from di↵erent depths. We show that the plume composition can be well described using a simple empirical relationship, which depends only on a single parameter, the sampling coe cient, s c . High-s c plumes are composed of material which originated from very deep in the fluid domain, while low-s c plumes contain material entrained from a range of depths. The analysis is also used to show that the geometry of the plume can be described using a similarity solution, in agreement with previous studies. Finally, numerical simulations are used to vary both the Rayleigh number and viscosity contrast independently. The simulations allow us to predict the value of the sampling coe cient for mantle plumes; we find that as a plume reaches the lithosphere, 90% of its composition has been derived from the lowermost 260 750 km in the mantle, and negligible amounts are derived from the shallow half of the lower mantle. This result implies that isotope geochemistry cannot provide direct information about this un-sampled region, and that the various known geochemical reservoirs must lie in the deepest few hundred kilometres of the mantle.

Subduction initiation from a stagnant lid and global overturn: new insights from numerical models with a free surface

“…To account for the dynamical effect of narrow mantle plumes (e.g. Davies 1994;Morgan 1972), which predominantly form at the CMB and sample the lowermost lower mantle (Cagney et al 2016), it is important to cover the whole mantle depth. The experiments are, therefore, calculated in a larger domain than the previous regional studies and that comprises the whole mantle depth: They are either in 2D Cartesian or in a 3D full spherical shell geometry.…”