Core-mantle chemical issues

Publication Type  Journal Article
Year of Publication  2005
Authors  Walker, D.
Journal Title  Canadian Mineralogist
Volume  43
Pages  1553-1564
Journal Date  Oct
ISBN Number  0008-4476
Accession Number  ISI:000235226100010
Key Words  earth's core; mantle geochemistry; redox; oxygen solubility; electrochemistry; titration; crustal recycling; reflux digestion; platinum-group elements; ni-s system; earths core; high-pressure; siderophile elements; molten iron; sulfide liquid; basalt melt
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, sulfides, 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 similar to 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 refluxing core material into the mantle.

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