Cinzia G. Farnetani

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) 4833496] and Iceland [Kempton et al., Earth Planet. Sci. Lett. 177 (2000) 2553271]. 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) 4833496] 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. fl 2002 Elsevier Science B.V. All rights reserved.

Thermochemical convection and helium concentrations in mantle plumes

Compositionally heterogeneous material may exist in the lowermost mantle [van der Hilst and Karason, Science 283 (1999) 1885–1888]. Here we use a numerical model to investigate the dynamics of (i) the subducted oceanic crust and lithosphere, (ii) a deep layer chemically denser, relatively undegassed and enriched in radiogenic elements. Tracers carry U, Th, K, and He concentrations which vary due to radioactive decay and to partial melting and degassing processes. We investigate the stability of the denser layer and find that by considering a depth dependent thermal expansion coefficient and temperature dependent viscosity, a layer with a chemical density excess of 2.4% can remain stable and poorly mixed until present-day time. The calculated helium ratios are in good agreement with 3He/4He observed at ridges and hotspots and show that the large spectrum of helium ratios of OIB can be explained by mixing between undegassed material, recycled oceanic crust and lithosphere. For MORB, the sharp spectrum of helium ratios may be due to a degassed, homogeneous and well mixed shallow mantle.

Lagrangian structures and stirring in the Earth’s mantle

Abstract In this paper we investigate three Lagrangian methods that have been recently proposed to quantify mixing in chaotic and time-aperiodic geophysical flows. The analytical method proposed by Haller [e.g., G. Haller, Chaos 10 (2000) 99–108] has a strong mathematical foundation and seeks to determine the location of stable (i.e., most attracting) and unstable (i.e., most repelling) material lines. The main hyperbolic lines describe the spatial organization of chaotic mixing. The finite-size Lyapunov exponents estimate the local mixing properties of the flow using the finite dispersion of particles, while the finite-time Lyapunov exponents have been often used to locate dynamically distinguished regions in geophysical flows. Mantle stirring is induced by the repeated action of stretching and folding, thus requiring to follow the strain history of a fluid element along a trajectory. We calculate the trajectories of more than half a million passive tracers forward and backward in time. The Eulerian velocity field is computed using a finite element code for solid state convection. We focus on a thermochemical model with a chemically denser layer at the base of the Earth’s mantle. This case allows us to test the ability of the Lagrangian techniques to detect the location of a dynamical barrier that inhibits mass exchanges and delimits domains characterized by different efficiency of stirring. We find that the Lagrangian techniques provide a satisfactory description of the main structures governing stirring, and enlighten different and complementary aspects: the methods based on the Lyapunov exponents provide a clear picture of mantle domains characterized by different strength of stirring, while the method proposed by Haller identifies the skeleton of the main structures around which stirring is organized. Our paper builds toward a more rigorous analysis of the stirring processes in the Earth’s mantle, which is required to understand the existence of geochemical reservoirs under a dynamical prospective.

Two views of Hawaiian plume structure

] Fundamentally contradictory interpretations of the isotopic compositions of Hawaiian basalts persist,even among authors who agree that the Hawaiian hotspot is caused by a deep-mantle plume. One viewholds that the regional isotopic pattern of the volcanoes reflects large-scale heterogeneities in the basalthermal boundary layer of the mantle. These are drawn into the rising plume conduit, where they arevertically stretched and ultimately sampled by volcanoes. The alternative view is that the plumeresembles a ‘‘uniformly heterogeneous plum pudding,’’ with fertile plums of pyroxenite and/or enrichedperidotite scattered in a matrix of more refractory peridotite. In a rising plume, the plums melt before thematrix, and the final melt composition is controlled significantly by the bulk melt fraction. Here we showthat the uniformly heterogeneous plum pudding model is inconsistent with several geochemicalobservations: (1) the relative melt fractions inferred from La/Yb ratios in shield-stage basalts of the twoparallel (Kea- and Loa-) volcanic chains, (2) the systematic Pb-isotopic differences between the chains,and the absence of such differences between shield and postshield phases, (3) the systematic shift touniformly depleted Nd-isotopic compositions during rejuvenated volcanism. We extend our previousnumerical simulation to the low melt production rates calculated far downstream (200–400 km) fromshield volcanism. Part of these melts, feeding rejuvenated volcanism, are formed at pressures of 5GPain the previously unmelted underside of the plume, from material that originally constituted theuppermost part of the thermal boundary layer at the base of the mantle.

Microwave-based laboratory experiments for internally-heated mantle convection

The thermal evolution of terrestrial planets is mainly controlled by the amount of radioactive heat sources in their mantle, and by the geometry and efficiency of solid state thermo-chemical convection within. So far, these systems have been studied using numerical methods only and cross validation by laboratory analogous experiments has not been conducted yet. To fill this gap we perform the first laboratory experiments of mantle convection driven by microwave-generated internal heating. We use a 30×30×5 cm3 experimental tank filled with 0.5 % Natrosol in water mixture (viscosity 0.6 Pa.s at 20°C). The fluid is heated from within by a microwave device that delivers a uniform volumetric heating from 10 to 70 kW/m3; the upper boundary of the fluid is kept at constant temperature, whereas the lower boundary is adiabatic. The velocity field is determined with particle image velocimetry and the temperature field is measured using thermochromic liquid crystals which enable us to charaterize the geometry of the...

Microwave-based, internally-heated convection: New perspectives for the heterogeneous case

The thermal evolution of telluric planets is primarily controlled by the balance between internal heating - due to ra-dioactive decay - and effciency of convective heat transfer in their mantle. In the Earth, the problem is particularly complex due to the heterogeneous distribution of heat sources in the mantle and the non-linear coupling between this distribution and convective mixing. To tackle this issue, we have developed a new technology to produce internally-heated convection based on microwaves absorption. This technology has the unique capability to selectively heat different zones of a convective fluid (heterogeneous convection) through the careful control of the absorption properties of the different fluids. Here we illustrate with two examples the new geophysical perspectives offered by microwave-based internally-heated convection: the problem of lithosphere stability and the evolution of a hidden enriched reservoir in the lowermost mantle.