Density distribution and constitution of the mantle

Abstract

Major regions of inhomogeneity are present in the mantle at depths less than 1000 km. The thermal gradient also greatly exceeds its adiabatic value at relatively shallow depths. Hence the Williamson‐Adams equation cannot be used in this part of the earth to derive the density variation from seismic data. In this paper the density in the upper mantle is obtained by explicitly introducing the constitution of the material there. In the lower mantle the extended Williamson‐Adams equation is used, and the constitution of this region is deduced from the density curve. Recent seismic results for the upper mantle, particularly those relating to low‐velocity zones, are examined. Significant regional differences are present. Beneath the oceans there is a definite LV zone for S, and possibly one for P as well. Beneath Precambrian shields the LV zone for S is less pronounced, and the LV zone for P seems to be absent. Other continental regions are intermediate between these zones. The LV zone is considered to be due to high thermal gradients with mineralogical and chemical heterogeneity superimposed. Differences between the behavior of P and S are largely due to different temperature coefficients of the two velocities. Regional contrasts arise from regional differences in thermal gradients and mineralogy. Consideration of temperature‐depth relations leads to the conclusion that the mantle is hottest beneath the oceans because of the absence of a thick, radioactive crust, and coolest beneath Precambrian shields because of low heat flow. A self‐consistent model of the mantle requires that the thermal flux at a depth of 400 km be about 0.5 μcal/cm² sec, because geochemical evidence indicates that the K/U ratio in the upper mantle is much smaller than in chondrites. The self‐consistent model requires very high thermal conductivity at high temperatures, such as would be provided by radiative transfer or possibly by movement of material. Petrological models of the upper mantle are constructed on the assumptions of an over‐all pyrolite (ultrabasic) composition and an eclogitic composition. Densities in the mantle are then calculated from known densities, thermal expansions, and compressibilities of minerals inferred to be present. Corrections for the effect of pressure on thermal expansion and compressibility are made from results of the theory of finite strain. The transition zone, at depths between 400 and 1000 km, is the site of a series of major phase transformations leading to close‐packed structures with silicon in sixfold coordination. The density curve in this region is approximated by a linear increase in density with depth. The lower mantle, between 1000 and 2900 km, is considered to be homogeneous, and the density is computed from the Williamson‐Adams equation, modified in some cases to take account of a superadiabatic thermal gradient. The magnitude of the density increase in the transition zone is adjusted to satisfy the restrictions imposed by the total mass and moment of inertia of the earth. A complete density curve ...

Rheological structure of the earth's mantle derived from glacial rebound in Laurentide.

“…Although this assumption may not hold very well, we may use the resultant relation if A is regarded If the rheological parameters obtained by KARATO (1977) and KIRBY and RALEIGH (1973) ( Table 2) are employed, the average temperatures turn out to be about 1,700K and 1,500K, respectively. These estimates are almost independent of the thickness of the low viscosity layer, and are consistent with the geotherms in the earth's mantle obtained by MERCIER and CARTER (1975), RINGWOOD (1964), andAKIMOTO et al (1976 The distribution of the present displacement is rather smooth, and the magnitude these values into (3.50) and (3.51), we have 2.8/HH2sec-1 for the magnitude of estimated to be 4.5, 9, and 13.5 bars for H=200, 400, and 600km, respectively (Table 3). The results of laboratory creep experiments are shown on the plane of strain Fig.…”

Composition of the core and implications for origin of the earth.

“…Meteoritic abundance estimates suggest the presence of about 6% Ni. However, the density of the outer core appears to be about 8-10 percent smaller than would be expected for pure iron under equivalent P,T conditions, whilst the hydrodynamical sound velocity of the core also appears to be substantially higher than expected for pure iron (e.g., BIRCH, 1952BIRCH, , 1972KNOPOFF and MACDONALD, 1960;CLARK and RINGWOOD, 1964;MCQUEEN and MARSH, 1966). These properties have been generally interpreted to imply that the core contains about 10 to 20 percent of an element or elements of low atomic weight, e.g., H, C, N, 0, Mg, Si, S (BIRCH, 1952BRETT, 1976).…”

Electrical Resistivity of Basalts from DSDP Leg 26

“…The higher temperatures are required by the similar surface-heat flow and the higher radioactive heat production in continental crustal rocks (Bullard, 1963;Clark and Ringwood, 1964;Sclater and Francheteau, 1970). A consistent electrical resistivity and thermal model for the crust and upper mantle beneath Iceland has been produced by Hermance and Grillot (1973).…”