eEarth
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1
1
2006
10.5194/ee-1-15-2006
http://www.electronic-earth.net/1/15/2006/
http://www.electronic-earth.net/1/15/2006/ee-1-15-2006.html
http://www.electronic-earth.net/1/15/2006/ee-1-15-2006.pdf
15
21
2006-10-23
What olivine, the neglected mineral, tells us about kimberlite petrogenesis
N. T. Arndt
arndt@ujf-grenoble.fr
A. M. Boullier
J. P. Clement
M. Dubois
D. Schissel
OSUG, Univ. de Grenoble, BP 53, Grenoble cedex 38401, France
U.F.R. des Sciences de la Terre, UMR 8110 Processus et Bilans des Domaines Sédimentaires, 59655 Villneuve d'Ascq, France
BHP Billiton World Exploration, Inc., 1400–1111 West Georgia Street, Vancouver, B.C. V6E 4M3, Canada
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.
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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
CO<sub>2</sub>-rich fluid and mantle and that this interaction controlled the
geochemical and isotopic composition of kimberlites.
Arndt, N. T.: Komatiites, kimberlites and boninites, J. Geophys. Res., 108(B6), 2293, doi:10.1029/2002JB002157, 2003.
Becker, M. and Le Roex, A.: Geochemistry of South African on- and off-craton, group I and group II kimberlites: petrogenesis and source region evolution, J. Petrol., 47, 673–703, 2006.
Bernstein, S., Kelemen, P. B., and Brooks, C. K.: Depleted spinel harzburgite xenoliths in Tertiary dykes from East Greenland: Restites from high degree melting, Earth Planet. Sci. Lett., 154, 221–235, 1998.
Dalton, J. A. and Wood, B. J.: The partitioning of Fe and Mg between olivine and carbonate and the stability of carbonate under mantle conditions, Contrib. Mineral. Petrol., 114, 501–509, 1993.
Guéguen, Y.: Dislocations in mantle peridotite nodules, Tectonophysics, 39, 231–254, 1977.
Kelemen, P. B.: Reaction between ultramafic wall rock and fractionating basaltic magma: Part I, Phase relations, the origin of calc-alkaline magma series, and the formation of discordant dunite, J. Petrol., 31, 51–98, 1990.
Khazan, Y. and Fialko, Y.: Why do kimberlites from different provinces have similar trace element patterns?, Geochem. Geophys. Geosyst., 6, Q10002, doi:10.1029/2005GC000919, 2005.
Köhler, T. P. and Brey, G. P.: Calcium exchange between olivine and clinopyroxene calibrated as a geothermobarometer for natural peridotites from 2 to 60 kb with applications, Geochim. Cosmochim. Acta, 54, 2375–2388, 1990.
Le Roex, A. P., Bell, D. R., and Davis, P.: Petrogenesis of group I kimberlites from Kimberley, South Africa: Evidence from bulk-rock geochemistry, J. Petrol., 44, 2261–2286, 2003.
Price, S. E., Russell, J. K., and Kopylova, M. G.: Primitive magma from the Jericho Pipe, N.W.T., Canada: Constraints on primary kimberlite melt chemistry, J. Petrol., 41, 789–808, 2000.