Two views of Hawaiian plume structure

International audienceFundamentally 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 view holds that the regional isotopic pattern of the volcanoes reflects large-scale heterogeneities in the basal thermal boundary layer of the mantle. These are drawn into the rising plume conduit, where they are vertically stretched and ultimately sampled by volcanoes. The alternative view is that the plume resembles a uniformly heterogeneous plum pudding, with fertile plums of pyroxenite and/or enriched peridotite scattered in a matrix of more refractory peridotite. In a rising plume, the plums melt before the matrix, and the final melt composition is controlled significantly by the bulk melt fraction. Here we show that the uniformly heterogeneous plum pudding model is inconsistent with several geochemical observations: (1) the relative melt fractions inferred from La/Yb ratios in shield-stage basalts of the two parallel (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 to uniformly depleted Nd-isotopic compositions during rejuvenated volcanism. We extend our previous numerical simulation to the low melt production rates calculated far downstream (200-400 km) from shield volcanism. Part of these melts, feeding rejuvenated volcanism, are formed at pressures of approximate to 5 GPa in the previously unmelted underside of the plume, from material that originally constituted the uppermost part of the thermal boundary layer at the base of the mantle

Thermochemical convection and helium concentrations in mantle plumes

International audienc

Dynamique d'un manteau terrestre hétérogène (implications pour les observables actuels)

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Stabilité et instabillité de la lithosphère continentale

PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

How double volcanic chains sample geochemical anomalies from the lowermost mantle

International audienc

Displaced helium and carbon in the Hawaiian plume

International audienc

Spin state transition and partitioning of iron: Effects on mantle dynamics

International audienc

Dynamics of rheological heterogeneities in mantle plumes

The geochemical record of Hawaiian basalts has been interpreted to reflect vertically stretched, partly filament-like heterogeneities in the Hawaiian plume, but one alternative interpretation has been that this record reflects intra-conduit mixing, caused by rheological contrasts across the conduit. Here we present numerical simulations of a mantle plume carrying rheological heterogeneities λ times more viscous than the surrounding fluid. Our first objective is to quantify how the heterogeneity deforms during upwelling. We find a full spectrum of shapes, from stretched filaments to nearly undeformed blobs, and we map the respective stability domain as a function of the viscosity ratio λ and of the flow characteristics, including the plume buoyancy flux. Our second objective is to test the hypothesis that a rheological heterogeneity can cause intra-conduit mixing. Although horizontal velocities do appear across the plume conduit, we have not found any toroidal “doughnut-shaped swirl” mode. Instead we show that perturbations of the flow trajectories are a local phenomenon, unable to cause permanent mixing. Our third objective is to determine over which time-scales a rheological heterogeneity crosses the magma capture zone (MCZ) beneath a hotspot volcano. For a blob-like heterogeneity of radius 30–40 km and viscosity ratio 15–20, the crossing time-scale is less than 1 Myr. Contrary to a stretched filament, a blob can entirely fill the MCZ, thereby representing the unique source rock of partial melts feeding a volcano. If the heterogeneity has a distinct isotopic fingerprint (or a distinct fertility), surface lavas will then record an isotopic fluctuation (or a fluctuation in melt productivity) lasting 0.5–0.8 Myr. Our simulations predict that such fluctuations should occur preferentially in low buoyancy flux hotspots, where blob-like rheological heterogeneities are more easily preserved than in the vigorous Hawaiian plume

Convection in an internally heated stratified heterogeneous reservoir

International audienceThe Earth's mantle is chemically heterogeneous and probably includes primordial material that has not been affected by melting and attendant depletion of heat-producing radioactive elements. One consequence is that mantle internal heat sources are not distributed uniformly. Convection induces mixing, such that the flow pattern, the heat source distribution and the thermal structure are continuously evolving. These phenomena are studied in the laboratory using a novel microwave-based experimental set-up for convection in internally heated systems. We follow the development of convection and mixing in an initially stratified fluid made of two layers with different physical properties and heat source concentrations lying above an adiabatic base. For relevance to the Earth's mantle, the upper layer is thicker and depleted in heat sources compared to the lower one. The thermal structure tends towards that of a homogeneous fluid with a well-defined time constant that scales with is the Rayleigh-Roberts number for the homogenized fluid. We identified two convection regimes. In the dome regime, large domes of lower fluid protrude into the upper layer and remain stable for long time intervals. In the stratified regime, cusp-like upwellings develop at the edges of large basins in the lower layer. Due to mixing, the volume of lower fluid decreases to zero over a finite time. Empirical scaling laws for the duration of mixing and for the peak temperature difference between the two fluids are derived and allow extrapolation to planetary mantles

Fundamentals of laminar free convection in internally heated fluids at values of the Rayleigh–Roberts number up to

International audienc