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| Titel |
What olivine, the neglected mineral, tells us about kimberlite petrogenesis |
| VerfasserIn |
N. T. Arndt, A. M. Boullier, J. P. Clement, M. Dubois, D. Schissel |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1815-381X
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| Digitales Dokument |
URL |
| Erschienen |
In: eEarth ; 1, no. 1 ; Nr. 1, no. 1 (2006-10-23), S.15-21 |
| Datensatznummer |
250000103
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| Publikation (Nr.) |
 copernicus.org/ee-1-15-2006.pdf |
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| Zusammenfassung |
| We report here the results of a petrographic and geochemical study of
remarkably well-preserved kimberlites from the Kangamiut region in
Greenland. The samples contain between 5 and 45% of olivine in the form
of rounded "nodules", each 1 to 5 mm in diameter. Most originally were
single crystals but many consist of polycrystalline, monomineralic
aggregates. Olivine compositions vary widely from nodule to nodule (from Fo
81–93) but are constant within individual nodules. A thin rim of high-Ca
olivine of intermediate composition (Fo 87–88) surrounds many nodules.
Deformation structures in olivine in the nodules and in the matrix
demonstrate a xenocrystic origin for the olivine: only olivine in the thin
rims is thought to have crystallized from the kimberlite magma. Using major
and trace element data, we show that the kimberlite compositions are
controlled by the addition of xenocrystic olivine into a parental magma that
contained about 24–28% MgO.
The monomineralic character of the olivine nodules is problematic because
dunite is a relatively rare rock in the lithospheric mantle. The source of the
xenocrystic olivine lacked pyroxene and an aluminous phase, which make up
about half of most mantle-derived rocks. It appears that these minerals were
removed from the material that was to become the nodules, perhaps by fluids
that immediately preceded the passage of the kimberlites. We speculate that
this mantle "defertilization" process was linked to interaction between
CO2-rich fluid and mantle and that this interaction controlled the
geochemical and isotopic composition of kimberlites. |
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