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528155 
Journal Article 
The mineral isotope composition of two Precambrian carbonatite complexes from the Kola Alkaline Province - Alteration versus primary magmatic signatures 
Tichomirowa, M; Grosche, G; Gotze, J; Belyatsky, BV; Savva, EV; Keller, J; Todt, W 
2006 
1 
Lithos
ISSN: 0024-4937 
91 
1-4 
229-249 
English 
We investigated the isotope composition (O, C, Sr, Nd, Pb) in mineral separates of the two Precambrian carbonatite complexes Tiksheozero (1.98 Ga) and Siilinjarvi (2.61 Ga) from the Karelian-Kola region in order to obtain information on Precambrian mantle heterogeneity. All isotope systems yield a large range of variations. The combination of cathodoluminescence imaging with stable and radiogenic isotopes on the same samples and mineral separates indicates various processes that caused shifts in isotope systems. Primary isotope signatures are preserved in most calcites (O, C, Sr, Pb), apatites (O, Sr, Nd), amphiboles (O), magnetites (O), and whole rocks (Sr, Nd). The primary igneous C and O isotope composition is different for both complexes (Tiksheozero: delta C-13=-5.0%, delta O-18=6.9%; Siilinjarvi: delta C-13=-3.7%, delta O-18=7.4%) but very uniform and requires homogenization of both carbon and oxygen in the carbonatite melt. The lowest Sr isotope ratios of our carbonates and apatites from the Archaean Siilinjarvi (0.70137) and the Palaeoproterozoic Tiksheozero (0.70228) complexes are in the range of bulk silicate earth (BSE). Positive epsilon(Nd) values of the two carbonatites point to very early Archaean enrichment of Sm/Nd in the Fennoscandian mantle. No HIMU components could be detected in the two complexes, whereas Tiksheozero carbonatites give the first indication of Palaeoproterozoic U depletion for Fennoscandia. Sub-solidus exchange processes with water during emplacement and cooling of carbonatites caused an increase in the oxygen isotope composition of some carbonates and probably also an increase of their Sr-s87/Sr-86 ratio. A larger increase of initial Sr isotope ratios was found in carbonatized silicic rocks compared to carbonatite bodies. The Svecofennian metamorphic overprint (1.9-1.7 Ga) caused reset of Rb/Sr (mainly mica) and Pb/Pb (mainly apatite) isochron systems. (C) 2006 Elsevier B.V. All rights reserved. 
carbonatite; C-O-Sr-Nd-Pb isotopes; mantle source; Kola Alkaline; Province; mantle heterogeneity; temperature-dependence; geochemical evolution; baltic shield; oxygen; mantle; calcite; sr; nd; fractionation; pb