EUROPROBE News 9
KIMBERLITE
Structure and Evolution of Cratonic Lithospheric Roots
by Stephan Sobolev (Moscow, Strasbourg), Karl Fuchs
(Karlsruhe) and Kimberlite colleagues
The Precambrian cratons are characterized by deep lithospheric roots; the
latter are apparent as geoid highs and as positive velocity anomalies in global seismic
tomographic images down to a depth of 200-300 km. These roots are explained by a variety
of hypotheses, ranging from cooling of an old lithosphere to chemical differentiation.
EUROPROBE's KIMBERLITE project aims at clarifying the origin and evolution of the deep
cratonic lithosphere, combining geochemical and petrological investigation of xenoliths
from kimberlites and basalts with geophysical studies for definition and characterization
of deep structures.
Kimberlites and lamproites seem to be always related to thick Archaean and
Palaeoproterozoic cratonic lithosphere. Their ages are mainly Phanerozoic, but range back
to the Mesoproterozoic, eruptions occurring often in relatively small areas over a long
time span. The magmatic rocks as well as the xenoliths shed light on the mineralogical
composition, fluid content and thermal evolution of the lithosphere and asthenosphere at
depths down to at least 250 km. Geophysical, especially different seismic methods, are
able to define structures in the deep lithosphere, such as shear zones or chemical
layering, which may give clues to the origin of the lithospheric roots and kimberlite
magmatism.
The Yakutian province in Siberia and the Arkhangelsk province in northern
Europe are major kimberlite areas. The former offers an outstanding data base of xenoliths
and exceptionally detailed crustal and uppermost mantle seismic data. The KIMBERLITE
project therefore focuses on studies in this area, exploiting existing and contributing
new data. Highlights of KIMBERLITE studies include:
Reinvestigation
of the detailed seismic images of the crust and upper mantle, showing azimuth-independent
very high velocities (8.8-9.0 km/s).
Exploitation
of very long-range seismic profiles sourced by peaceful nuclear explosions (PNE), giving
high resolution information about upper mantle structures, including the transition zone
down to c. 670 km depth, and characterization of small scale heterogeneities in the
scattered wave fields.
Search
for anisotropy (both structural and mineralogical) in the upper mantle, as an indicator of
processes in the lithosphere, asthenosphere and deeper levels, by observation of
birefringence of seismic S-waves in broad band recordings.
Study
of a large number of diamondiferous kimberlite and lamproite pipes, which are commercially
exploited, ranging in age from 450 to 150 Ma, with numerous crustal and mantle xenoliths,
derived from depths of down to 250 km.
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