Chemical boundary layer

Figure 9.16 Kinetic fractionation during crystal growth. Steady-state distribution of melt concentrations in the vicinity of a solid growing at the rate v for trace elements with different solid-liquid fractionation coefficients [equation (9.6.5), Tiller et al. (1953)]. The stippled area indicates the steady-state chemical boundary-layer with thickness <5 = <5>/v.

A convenient estimate of the anomalous layer thickness (chemical boundary layer) is given by 8 = 3>/v. Indeed, the excess or deficit M of diffusing substance is equal to the area limited by the concentration profile and the initial distribution C0 in the liquid... 

For kinetic disequilibrium partitioning of trace elements, equation (9.6.6) after Burton et al. (1953) is commonly presented as an alternative to equation (9.6.5) due to Tiller et al. (1953) (e.g., Magaritz and Hofmann, 1978 Lasaga, 1981 Walker and Agee, 1989 Shimizu, 1981). However, the relative values of viscosity and chemical diffusivity in common liquids and silicate melts make the momentum boundary-layer (i.e., the liquid film which sticks to the solid) orders of magnitude thicker than the chemical boundary layer. It is therefore quite unlikely that, except for rare cases of transient state, liquid from outside the momentum boundary-layer may encroach on the chemical boundary-layer, i.e., <5 may actually be taken as infinite. As a simple description of steady-state disequilibrium fractionation, the model of Tiller et al. (1953) has a much better physical rationale. A more elaborate discussion of these processes may be found in Tiller (1991a, b). 

However, the molecules percolating up into the boundary layer from beneath the soil surface tend to become trapped in the stagnant laminar sublayer of the boundary layer. This sublayer is usually much thinner than the overall turbulent boundary layer, since it is dominated by viscous and surface tension forces, rather than by velocity. Phelan and Webb call this the chemical boundary layer and state categorically that there will generally be no chemical signature above this chemical boundary layer [1, p. 52],... 

The air and chemical boundary layers tend to keep chemical vapor close to the surface farther from the surface dilution becomes the dominant factor. 

Turbulence is not the asset in water that it is in air. In air, we saw that some turbulence is required to bring the molecules out of the chemical boundary layer. That may also be needed in water to move molecules away from the bottom surface. Turbulence away from the surface tends to break up the plumes of molecules that are diagrammed in Figure 4.4. Those plumes are the key to successful detection of an underwater object that is releasing the molecules of interest. One result of the ONR experiment at San Clemente Island, off San Diego, California, was a better understanding of the formation, persistence, and dissipation of these plumes. When a well-formed plume is available, it often becomes possible to follow it to its source see Chapters 5 and 6. 

Aside from the core-mantle boundary region, a pyrolite lower-mantle composition appears to be consistent with seismological constraints. Silica enrichment of the lower mantle can be accommodated if the lower mantle is hotter than expected for a simple adiabat rooted at the 660 km y— pv + mw transition (Figure 9). Because any chemical boundary layer between the upper and lower mantle would be accompanied by a corresponding thermal boundary layer, such a model... 

Jellinek A. M. and Manga M. (2002) The influence of a chemical boundary layer on the fixity, spacing and lifetime of mantle plumes. Nature 418, 760-763. 

Jordan, T. H. 1981. Continents as a chemical boundary layer. In The Origin and Evolution of the Earth s Continental Crust. Transactions of the Royal Society of London, London, 301, 359-373. 

The boundary layer at the core-mantle boundary (CMB) has been explored as a reservoir for high He/" He ratio He, in the context of whole mantle convection. It has been suggested that subducted oceanic crust could accumulate there and form a distinct chemical boundary layer, accounting for the properties of the D" layer (Christensen and... 

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