Publication 121
Area selection for diamonds using magnetotellurics: Examples from southern Africa.
Jones, A.G., R.L. Evans, M.R. Muller, M.P. Hamilton, M.P. Miensopust, X. Garcia,
P. Cole, T. Ngwisanyi, D. Hutchins, C.J.S Fourie, H. Jelsma, T. Aravanis, W. Pettit, S. Webb,
J. Wasborg, and The SAMTEX Team
Abstract
Southern Africa, particularly the Kaapvaal Craton, is one of the world’s best natural
laboratories for studying the lithospheric mantle given the wealth of xenolith and seismic data
that exist for it. The Southern African Magnetotelluric Experiment (SAMTEX) was launched
to complement these databases and provide further constraints on physical parameters and
conditions by obtaining information about electrical conductivity variations laterally and with
depth. Initially it was planned to acquire magnetotelluric data on profiles spatially coincident
with the Kaapvaal Seismic Experiment, however with the addition of seven more partners to
the original four through the course of the experiment, SAMTEX was enlarged from two to
four phases of acquisition, and extended to cover much of Botswana and Namibia. The
complete SAMTEX dataset now comprises MT data from over 675 distinct locations in an
area of over one million square kilometres, making SAMTEX the largest regional-scale MT
experiment conducted to date.
Preliminary images of electrical resistivity and electrical resistivity anisotropy at 100
km and 200 km, constructed through approximate one-dimensional methods, map resistive
regions spatially correlated with the Kaapvaal, Zimbabwe and Angola Cratons, and more
conductive regions spatially associated with the neighbouring mobile belts and the Rehoboth
Terrain. Known diamondiferous kimberlites occur primarily on the boundaries between the
resistive or isotropic regions and conductive or anisotropic regions.
Comparisons between the resistivity image maps and seismic velocities from models
constructed through surface wave and body wave tomography show spatial correlations
between high velocity regions that are resistive, and low velocity regions that are conductive.
In particular, the electrical resistivity of the sub-continental lithospheric mantle of the
Kaapvaal Craton is determined by its bulk parameters, so is controlled by a bulk matrix
property, namely temperature, and to a lesser degree by iron content and composition, and is
not controlled by contributions from interconnected conducting minor phases, such as
graphite, sulphides, iron oxides, hydrous minerals, etc. This makes quantitative correlations
between velocity and resistivity valid, and a robust regression between the two gives an
approximate relationship of Vs [m/s] = 0.045*log(resistivity [ohm.m]).
Source
Lithos, 112S, 83-92. (Note: In Special Issue of Volume 112 devoted to 9th IKC.)
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Alan G Jones / 12 February 2010 /
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