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| Titel |
Morphology and surface features of olivine in kimberlite: implications for ascent processes |
| VerfasserIn |
T. J. Jones, J. K. Russell, L. A. Porritt, R. J. Brown |
| 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 (2014-05-16), S.313-326 |
| Datensatznummer |
250115270
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| Publikation (Nr.) |
 copernicus.org/se-5-313-2014.pdf |
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| Zusammenfassung |
| Most kimberlite rocks contain large proportions of ellipsoidal-shaped
xenocrystic olivine grains that are derived mainly from disaggregation of
peridotite. Here, we describe the shapes, sizes and surfaces of olivine
grains recovered from kimberlite lavas erupted from the Quaternary Igwisi
Hills volcano, Tanzania. The Igwisi Hills kimberlitic olivine grains are
compared to phenocrystic olivine, liberated from picritic lavas, and mantle
olivine, liberated from a fresh peridotite xenolith. Image analysis,
scanning electron microscopy imagery and laser microscopy reveal significant
differences in the morphologies and surface features of the three crystal
populations. The kimberlitic olivine grains form smooth, rounded to
ellipsoidal shapes and have rough flaky micro-surfaces that are populated by
impact pits. Mantle olivine grains are characterised by flaked surfaces and
indented shapes consistent with growth as a crystal aggregate. Phenocrystic
olivine exhibit faceted, smooth-surfaced crystal faces. We suggest that the
unique shape and surface properties of the Igwisi Hills kimberlitic olivine
grains are products of the transport processes attending kimberlite ascent
from mantle source to surface. We infer that the unique shapes and surfaces
of kimberlitic olivine grains result from three distinct mechanical
processes attending their rapid transport through the thick cratonic mantle
lithosphere: (1) penetrative flaking from micro-tensile failure induced by
rapid decompression; (2) sustained abrasion and attrition driven by
particle–particle collisions between grains within a turbulent,
volatile-rich flow regime; and (3) higher-energy particle–particle
collisions producing impact cavities superimposed on decompression
structures. The combination of these processes during the rapid ascent of
kimberlite magmas is responsible for the distinctive ellipsoidal shape of
olivine xenocrysts found in kimberlites worldwide. |
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