Mixing and deformations in mantle plumes

The long standing idea that the source of oceanic island basalts includes ancient subducted material is strengthened by recent geochemical observations for Hawaii [Lassiter and Hauri, Earth Planet. Sci. Lett. 164 (1998) 483-496] and Iceland [Kempton et al., Earth Planet. Sci. Lett. 177 (2000) 255-271]. In particular, the isotopic variations in Hawaiian shield lavas indicate the presence of two distinct recycled components: ancient oceanic crust+ sediments, and altered ultramafic lower crust or lithospheric mantle. Lassiter and Hauri [Earth Planet. Sci. Lett. 164 (1998) 483-496] suggest that both components are from the same packet of recycled oceanic lithosphere, thus implying that chemical heterogeneities a few km thick can be preserved in the convecting mantle. In this paper we investigate the role of mantle plumes in stirring mantle heterogeneities and we address the following questions: (1) Is the heterogeneous nature of plumes inherited at the source or does it develop through entrainment? (2) Is stirring more efficient in the plume head or in the long-lived plume tail? (3) Are the geochemical implications consistent with fluid dynamical models? We use a three-dimensional numerical model in Cartesian geometry to simulate the dynamics of an isolated plume. Transport calculations, conducted on a vertical plane of symmetry, allow us to advect passive tracers forward or backward in time to investigate mixing. We also calculate the finite-time Lyapunov exponents in order to quantify the deformations associated to the plume rise. Our results show that: (1) the thermal boundary layer, where the plume forms, is the region most efficiently sampled by a mantle plume. Since the overlying mantle is not entrained in the plume head, we speculate that the geochemically heterogeneous nature of plumes is inherited from the source. Our results also predict the absence of present-day upper mantle, source of MORB, in plume lavas. (2) Heterogeneities initially located in the source region undergo a series of stretching and folding events while rising in the plume head and may be reduced to narrow filaments, We find that stirring is more important in the plume head than in the long-lived plume tail. Therefore, our results predict that distinct geochemical heterogeneities are more likely to be found in hotspot lavas rather than in flood basalt lavas, associated to partial melting of a plume tail and a plume head (Richards et al., Science 246 (1989) 103-107] respectively. (3) High Lyapunov exponents. indicating high deformations, are found at the frontier between the plume head and the sublithospheric mantle surrounding the plume head. We speculate that the arrival of a large plume head could induce seismic anisotropy in the shallow upper mantle. (C) 2002 Elsevier Science B.V. All rights reserved.