P. Schmoldt, PhD - MTNet - DIAS

Compound Formula
5.2. Variation of electric conductivity with depth
Whole crust Oceanic Continental
(A) (B) (C) (D) (E) (F)
Silica SiO2 59.71 47.8 63.3 58.0 57.3 60.6
Alumina Al2O3 15.41 12.1 16.0 18.0 15.9 15.9
Lime CaO 4.90 11.2 4.1 7.5 7.4 6.4
Magnesia MgO 4.36 17.8 2.2 3.5 5.3 4.7
Sodium oxide Na20 3.55 1.31 3.5 3.7 3.1 3.1
Iron(II) oxide FeO 3.52 9.0 7.5 3.5 9.1 6.7
Potassium oxide K2O 2.80 0.03 1.5 2.9 1.1 1.8
Iron(III) oxide Fe2O3 2.63 - - 1.5 - -
Water H2O 1.52 1.0 - 0.9 - -
Titanium dioxide TiO2 0.60 0.59 0.8 0.6 0.9 0.7
Phosphorus pentoxide P2O5 0.22 - - - - 0.1
Manganese oxide MnO - - 0.14 - - -
Tab. 5.2.: Models of the Earth’s crust bulk composition, in weight-percent (major elements >0.01%). A: [Clarke, 1889]; B: [Elthon,
1979], C: [Condie, 1982], D: [Taylor and McLennan, 1985] (Andesite model), E: [Taylor and McLennan, 1985] (Theoretical model)
in Anderson [2004], F: Rudnick and Gao [2003].
crustal depth. Massive ore bodies and interconnected sulphide and graphite phases in
shear zones can result in a vast increase of local conductivity at crustal depth, e.g. a conductivity
of approximately 2 – 5 S/m is inferred for the North American Central Plains
conductivity anomaly (NACP) [Jones and Craven, 1990] and conductivities of less then
1 Ωm are inferred by Korja et al. [1996] for the Lapland Granulite Belt. As for fluid phases
in rock matrices, extension and connectivity of the conducting phase in an ore body is a
fundamental factor (cf. Fig. 3.5). The effect of pressure on the conductivity of the Earth’s
crust, in the absence of temperature changes, is mainly due to resulting changes in connectivity
of a good conductor in its host medium, namely (i) closing fractures, (ii) changing
the geometry of dry and fluid saturated porous media, or (iii) connecting areas with a high
content of water or metal [e.g. Brace et al., 1965; Duba, 1976; Shankland et al., 1997;
Wanamaker and Kohlstedt, 1991]. Such effects are highly dependent on the configuration
of the composite structure and therefore extremely non-linear and localised.
5.2.2. The Earth’s mantle
The Earth’s mantle describes the zone between the Moho and core–mantle boundary
(CMB) at approximately 2890 km, which is further divided into upper mantle, mantle
transition zone (MTZ), and lower mantle according to their chemical and rheological
properties. Moreover, the upper mantle is commonly subdivided by the so-called
lithosphere–asthenosphere boundary (LAB) into a lithospheric part (also referred to as uppermost
mantle) and an asthenospheric part, referring to the rheological strong and weak
layers, respectively. Except for extraordinary regions like MOR’s, the LAB is situated in
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