Core Mantle Chemical Issues

Abstract

Is the core's chemical isolation from the mantle complete? This is an important question for the balancing of the persistently enigmatic budgets of the highly siderophile elements and PGE, to which Fleet's work has contributed so much. On redox criteria alone, it is clear that the core and the mantle cannot be in bulk equilibrium, and therefore that there are chemical incentives for core-mantle exchange. Recent Os isotope evidence suggests that the core may indeed be leaking chemically (although some W isotope evidence does not encourage this view). Possible mechanisms of chemical transfer between core and mantle include the following.(1) Cooling of the Earth provides a temperature perturbation that affects solubility in the outer core of the oxides, sulfi des, carbides, hydrides, and silicates with which inevitably the outer core becomes saturated. Cooling and buoyancy lead to a progressive transfer of liquid (or crystalline) precipitates to the mantle. Solubilities of nonmetals in the core need determination.(2) Electromagnetic effects arising from core dynamo action can also drive core-mantle transfers. Interesting phase-changes, electrowetting, and dramatic trace-element chemical fractionation effects are driven by ~1 V potentials. The electronic character of, and voltages across, the core-mantle boundary (CMB) need determination. (3) Chemical transfers can be forced by emplacement of chemically incompatible assemblages into the CMB region. The redox imbalance between the core and oxidized slab material subducted to the CMB will certainly require chemical re-adjustments, most plausibly leading to core-to-mantle transfer of mass. The current debate about whether the chemical signature seen in some plumes is recycled crust or core need not have an exclusive "either/or" resolution. Recycled material could be key in refl uxing core material into the mantle.Keywords: Earth's core, mantle geochemistry, redox, oxygen solubility, electrochemistry, titration, crustal recycling, refl ux digestion. SOMMAIREL'isolation chimique du noyau en présence du manteau est-elle complète? Il s'agit d'une question importante pour le bilan des budgets toujours enigmatiques des éléments fortement sidérophiles et des éléments du groupe du platine, domaines auxquels Michael Fleet a tellement contribué. En considérant seuls les critères de redox, il semble évident que le noyau et le manteau ne peuvent pas globalement être à l'équilibre, et donc il y aurait des incitations chimiques favorisant les échanges entre noyau et manteau. D'après les données récentes sur les isotopes d'osmium, le noyau pourrait bien être en train de subir des fuites, quoique certaines observations au sujet des isotopes de W ne favorisent guère ce point de vue. Parmi les mécanismes possibles de tranfert chimique entre noyau et manteau se trouvent les possibilités suivantes. (1) Un refroidissement de la Terre provoque une perturbation thermique qui affecte la solubilité dans le noyau externe des oxydes, sulfures, carbures, hydrides, et silicates avec lesque...

Core–Mantle Interactions

“…Jeanloz (1990) suggested that chemical reactions and transport of light elements into the core could have occurred over most of the Earth's history. Subduction of oxidized ocean crust may also drive reactions by altering chemical conditions at the base of the mantle (Walker, 2005). In either case, chemical reactions could disturb the composition of the core by permitting a flux of mass between the mantle and the core.…”

“…Alternatively, the excess concentration could drive back reactions at the CMB, removing excess light element and precipitating a solid 'sediment' to the base of the mantle . Walker (2005) had also discussed the possibility of electrochemical reactions at the CMB (Kavner and Walker, 2006) and reactions with oxidized oceanic crust. There is no shortage of mechanisms that can cause chemical interactions between the core and the mantle, and there are good reasons for suspecting that many of these operate within the Earth.…”

Diffusion of Au, Pd, Re, and P in FeNi alloys at High Pressure

“…Perhaps the area of most active research regarding this issue is that related to anomalous Os isotope ratios seen in some plume basalts, and thought to represent chemical exchange between the outer core and the lower mantle (Walker et al, 1995;Brandon et al, 1998;Brandon and Walker, 2005;Lassiter, 2006;Walker, 2005). The anomaly might be explained if the inner core fractionates Os from Re and Pt.…”

Core–Mantle Interactions

“…The reaction products are thought to include iron alloys, such as FeO and FeSi, and iron-depleted silicate minerals (Knittle and Jeanloz, 1989;Goarant et al, 1992). Subduction of oxidized ocean crust may also drive reactions by continually altering chemical conditions at the base of the mantle (Walker, 2005). On the other hand, convection in the mantle exposes fresh surfaces of unreacted material Mantle Inner core Equipotential surfaces Figure 7 Schematic illustration of the heterogeneous Earth viewed on an equatorial cross section.…”

“…Differences in redox state imply that the core and the mantle cannot be in bulk equilibrium (Walker, 2005). Differences in redox state imply that the core and the mantle cannot be in bulk equilibrium (Walker, 2005).…”