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| Titel |
Petrophysical constraints on the seismic properties of the Kaapvaal craton mantle root |
| VerfasserIn |
V. Baptiste, A. Tommasi |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1869-9510
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| Digitales Dokument |
URL |
| Erschienen |
In: Solid Earth ; 5, no. 1 ; Nr. 5, no. 1 (2014-01-29), S.45-63 |
| Datensatznummer |
250115253
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| Publikation (Nr.) |
 copernicus.org/se-5-45-2014.pdf |
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| Zusammenfassung |
| We calculated the seismic properties of 47 mantle xenoliths from 9
kimberlitic pipes in the Kaapvaal craton based on their modal composition,
the crystal-preferred orientations (CPO) of olivine, ortho- and
clinopyroxene, and garnet, the Fe content of olivine, and the pressures and
temperatures at which the rocks were equilibrated. These data allow
constraining the variation of seismic anisotropy and velocities within the
cratonic mantle. The fastest P and S2 wave propagation directions and
the polarization of fast split shear waves (S1) are always subparallel
to olivine [100] axes of maximum concentration, which marks the lineation
(fossil flow direction). Seismic anisotropy is higher for high olivine
contents and stronger CPO. Maximum P wave azimuthal anisotropy (AVp)
ranges between 2.5 and 10.2% and the maximum S wave polarization
anisotropy (AVs), between 2.7 and 8%. Changes in olivine CPO
symmetry result in minor variations in the seismic anisotropy patterns,
mainly in the apparent isotropy directions for shear wave splitting. Seismic
properties averaged over 20 km-thick depth sections are, therefore, very
homogeneous. Based on these data, we predict the anisotropy that would be
measured by SKS, Rayleigh (SV) and Love (SH) waves
for five endmember orientations of the foliation and lineation. Comparison to
seismic anisotropy data from the Kaapvaal shows that the coherent fast
directions, but low delay times imaged by SKS studies, and the low azimuthal
anisotropy with with the horizontally polarized S waves (SH)
faster than the vertically polarized S wave (SV) measured using
surface waves are best explained by homogeneously dipping (45°)
foliations and lineations in the cratonic mantle lithosphere. Laterally or
vertically varying foliation and lineation orientations with a dominantly
NW–SE trend might also explain the low measured anisotropies, but this model
should also result in backazimuthal variability of the SKS splitting data,
not reported in the seismological data. The strong compositional
heterogeneity of the Kaapvaal peridotite xenoliths results in up to 3%
variation in density and in up to 2.3% variation of Vp, Vs, and
Vp / Vs ratio. Fe depletion by melt extraction increases Vp and Vs,
but decreases the Vp / Vs ratio and density. Orthopyroxene enrichment due
to metasomatism decreases the density and Vp, strongly reducing the
Vp / Vs ratio. Garnet enrichment, which was also attributed to
metasomatism, increases the density, and in a lesser extent Vp and the
Vp / Vs ratio. Comparison of density and seismic velocity profiles
calculated using the xenoliths' compositions and equilibration conditions to
seismological data in the Kaapvaal highlights that (i) the thickness of the
craton is underestimated in some seismic studies and reaches at least
180 km, (ii) the deep sheared peridotites represent very local modifications
caused and oversampled by kimberlites, and (iii) seismological models
probably underestimate the compositional heterogeneity in the Kaapvaal mantle
root, which occurs at a scale much smaller than the one that may be sampled
seismologically. |
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