HR: 1340h
AN: V13E-0588    [Abstracts]
TI: Origin of Paleoproterozoic Komatiites at Jeesiörova, Kittilä Greenstone Complex, Finnish Lapland
AU: * Gangopadhyay, A
EM: amitava@magnet.fsu.edu
AF: Department of Geology, University of Maryland, College Park, MD 20742 United States
AU: * Gangopadhyay, A
EM: amitava@magnet.fsu.edu
AF: Geochemistry Division, National High Magnetic Field Laboratory, Tallahassee, FL 32310 United States
AB: Komatiites from the Jeesiörova area in the ~ 2 Ga Kittilä Greenstone Complex in Finnish Lapland have subchondritic Al₂O₃/TiO₂ ratios, similar to those in Al-depleted komatiites from the Barberton Mountainland, South Africa. The Jeesiörova rocks, however, are distinct in that their Al abundances are higher than those at given MgO contents of Al-depleted rocks, yet similar to those in Al-undepleted komatiites from elsewhere. In addition, the Al and Ti contents in chromites in the Jeesiörova rocks are distinctly higher than in chromites found in Al-depleted komatiites. Consequently, the Jeesiörova komatiites were likely Al-undepleted, but rich in Ti relative to typical Al-undepleted komatiites. Some similarly incompatible elements, including Zr , Eu, and Gd are also enriched to about the same extent in these rocks. Neither majorite fractionation in the source of the komatiites nor hydrous melting in a supra-subduction zone setting is likely to have produced these komatiites. The high concentrations of Ti and similarly incompatible elements in the Jeesiörova rocks may have resulted from contamination of their parental melt via interaction with Ti-rich metasomatic mineral assemblages in the lithospheric mantle or enrichment of the mantle source in basaltic melt components. Preferential incorporation of Ti-rich phases would likely have led to a collateral enrichment of Nb that is not observed in these rocks. The Re-Os isotope results for chromite separates from the Jeesiörova rocks yield an average initial ¹⁸⁷Os/¹⁸⁸Os of 0.1131 ± 0.0006 (2σ; γ_Os(I) = 0.1 ± 0.5). These results, coupled with an initial ε_Nd of ~+4 indicate that melt parental to the komatiites likely had minimal chemical interaction with ancient lithospheric mantle. If the mantle source for the komatiites was enriched in a basaltic component, the combined Os-Nd isotopic results limit the enrichment process to within approximately 200 Ma prior to the formation of the komatiites. The Os-Nd isotopic composition of these rocks is consistent with their derivation dominantly from the contemporaneous convective upper mantle.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005