The thermal boundary-layer interpretation of D'' and its role as a plume source

Details The thermal boundary-layer interpretation of D'' and its role as a plume source The thermal boundary-layer interpretation of D'' and its role as a plume source

Aug 1983

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1983pepi...33...45s&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 33, Issue 1, p. 45-55.

Physics

126

Scientific paper

The ``anomalous'' layer in the lowermost mantle, identified as D'' in the notation of K.E. Bullen, appears in the PREM Earth model as a 150 km-thick zone in which the gradient of incompressibility with pressure, dK/dP, is almost 1.6, instead of 3.2 as in the overlying mantle. Since PREM shows no accompanying change in density or density gradient, we identify D'' as a thermal boundary layer and not as a chemically distinct zone. The anomaly in dK/dP is related to the temperature gradient by the temperature dependence of Ks, for which we present a thermodynamic identity in terms of accessible quantities. This gives the numerical result (∂Ks/∂T)P=-1.6×107 Pa K-1 for D'' material. The corresponding temperature increment over the D'' range is 840 K. Such a layer cannot be a static feature, but must be maintained by a downward motion of the lower mantle toward the core-mantle boundary with a strong horizontal flow near the base of D''. Assuming a core heat flux of 1.6 × 1012 W, the downward speed is 0.07 mm y-1 and the temperature profile in D'', scaled to match PREM data, is approximately exponential with a scale height of 73 km. The inferred thermal conductivity is 1.2 W m-1 K-1. Using these values we develop a new analytical model of D'' which is dynamically and thermally consistent. In this model, the lower-mantle material is heated and softened as it moves down into D'' where the strong temperature dependence of viscosity concentrates the horizontal flow in a layer ~ 12 km thick and similarly ensures its removal via narrow plumes.

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