The Boundary Layer Model

Don L. Anderson
Friday, May 6, 2014

The canonical model of mantle geochemistry assumes that the boundary layer above the core is the largest, hottest, most accessible, least degassed  and most “primordial’ (defined as having ³He/⁴He ratios higher than average MORB)  part of the mantle and therefore the logical source of intraplate volcanoes such as Hawaii, Samoa and Yellowstone (e.g., Humphrey & Schmandt; DePaolo & Manga, 2003; Hoffman & Hart, 2007). These assumptions, in turn, are based on the premise that upper mantle temperatures are entirely subsolidus and cannot exceed ~1400°C, or the temperatures inferred for subridge mantle,  (McKenzie & Bickle, 1988). They are also based on the assumption that ³He/⁴He ratios higher than those in average MORB are due to excess ³He, and that most of the mantle supports an adiabatic gradient and that it is not cooling with time.

The great thickness and high temperatures of the surface boundary layer have been confirmed by surface-wave and laboratory data. Slabs, apparently, are less than half the thickness of the surface boundary layer, much of which has long-term strength and buoyancy. Basalt isotope chemistry is largely controlled by binary mixing of several components and involves fractionation during cooling and formation of OIB via the contamination of MORB (see Theory of the Earth & New Theory of the Earth). However, both mixing relations and isotope evolution trajectories are indifferent as to the location, depth, size and absolute temperatures of the components. This means that mixing between a cold, high-³He/⁴He, low-[3He] component, long resident in the shallow mantle, and a high-[3He], low-³He/⁴He component such as
MORB, can satisfy the same equations as the canonical model does (where the high ³He/⁴He component is hot and comes from a deep, high-[3He] reservoir).

Mass balance requires that if the tubes connecting the surface boundary layer with the core-mantle boundary are less than 200 km in diameter they must rise rates of meters/year. If the upwellings are >1000 km in diameter they are rising at cm/yr and they simply represent normal mantle convection; the upwellings in this case must be primarily passive rather than driven by their own buoyancy.

References

• DePaolo, D. J., and M. Manga (2003), Deep origin of hotspots – the mantle plume model, Science, 300, 920-921.
• Hofmann, A.W and S.R. Hart, Another nail in which coffin?, Science, 315, 39-40, 2007.
• Humphreys, E.D. and B. Schmandt (2011), Looking for mantle plumes, Physics Today, 64, 8, 34-39