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Journal of Petrology Advance Access published online on December 20, 2005
Journal of Petrology, doi:10.1093/petrology/egi093
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oxfordjournals.org
Received November 18, 2003
Accepted November 16, 2005

Article

Origin of Paleoproterozoic Komatiites at Jeesiörova, Kittilä Greenstone Complex, Finnish Lapland

AMITAVA GANGOPADHYAY 1 *, RICHARD J. WALKER 1, EERO HANSKI 2, and PETER A. SOLHEID 3

1 ISOTOPE GEOCHEMISTRY LABORATORY, DEPARTMENT OF GEOLOGY, UNIVERSITY OF MARYLAND, COLLEGE PARK, MD 20742, USA
2 DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF OULU, P.O. BOX 3000, FIN-90014 UNIVERSITY OF OULU, FINLAND
3 INSTITUTE FOR ROCK MAGNETISM, 291 SHEPHERD LABS, 100 UNION STREET S.E., MINNEAPOLIS, MN 55455-0128, USA

* To whom correspondence should be addressed.
AMITAVA GANGOPADHYAY, E-mail: amitava{at}magnet.fsu.edu


  Abstract

Komatiites from the ~2 Ga Jeesiörova area in Finnish Lapland have subchondritic Al2O3/TiO2 ratios like those in Al-depleted komatiites from Barberton, South Africa. They are distinct in that their Al abundances are higher than those of the Al-depleted rocks and similar to levels in Al-undepleted komatiites. Moderately incompatible elements such as Ti, Zr, Eu, and Gd are enriched. Neither majorite fractionation nor hydrous melting in a supra-subduction zone setting could have produced these komatiites. Their high concentrations of moderately incompatible elements may have resulted from contamination of their parental melt through interaction with metasomatic assemblages in the lithospheric mantle or enrichment of their mantle source in basaltic melt components. Re-Os isotope data for chromite from the Jeesiörova rocks yield an average initial 187Os/188Os of 0·1131 ± 0·0006 (2{sigma}), {gamma}Os(I) = 0·1 ± 0·5. These data, coupled with an initial {varepsilon}Nd of ~+4, indicate that melt parental to the komatiites interacted minimally with ancient lithospheric mantle. If their mantle source was enriched in a basaltic component, the combined Os-Nd isotopic data limit the enrichment process to within ~200 Myr prior to the formation of the komatiites. Their Os-Nd isotopic composition is consistent with derivation from the contemporaneous convecting upper mantle.

Keywords: Finnish Lapland; Jeesiörova; komatiites; mantle geochemistry; petrogenesis; redox state; Re/Os isotopes; Ti enrichment.
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