Schmucker-Weidelt Lecture Notes, Aarhus, 1975 - MTNet

parameter u and that the pressure dependence of o and A should
0 0
not be the determining factor for the conductivity down the depth
of several hundred kilometers.
7 (, "C The u-T dependence of Olivin (Mg,
1200 3 b0 60 0 11
Fe ) Si04 has been extensively
T--- 2
investigated by various authors
for temperaturs up to 1400~~ and
pressures up to 30 kbar. This
mineral is thought to be the main
constituent of the upper mantle
with a Mg : Fe ratio of 9 : 1 :
I Si04. The diagram
(Mg0.9 Fe~.l 2
shows the range of u(T)--curves
as obtained for Olivin with 90%
Forsterite CMg2Si041. It is beli.eved that the discrepancies among
-the curves over orders of magni:tude are mainly due to different
degrees of oxydati-on of ~e" to Fe 'I within the olivLn samples
used for the measurement. In fact, the samples may have been oxy-
dized in some cases during the heating experiment as evidenced by
the irreproducibility of the u (TI-curves. However, the r>ar.ge of
possible conductivities for olivin with 10% forsterite is greatly
narrowed in at the high temperature end, where we may expec-t a
resistivity of 10 to 100 Qm for 1400°c, corresponding to a depth
of 100 to 200 km within the mantle.
2.:Electrolytic conduction through salty solutions, filling pores and
cracks of unconsolidated rocks, gives clastic sediments resistivi-
ties from 1 t o 50 Qm. The resistivity of sea water with 3.5 gr
NaCl/liter is 0.25 am. There should be an increase of resistivity
by one or two orders of magnitude at the top of the crystalline
basement beneath sedimentary basins.
Near to the surface the conduction in crystalline rocks is also
electrolytic. Their resistivity may be here as low as 500 Qm, but
it increases rapidly with depth, when the cracks and pores close
under increasing pressure. Below the Mohorovicic-discontinuity
the conduction through grains can be expected to be more effective
than the conduction through pore fluids. If, however, partial