The ultramafic dikes, that are the object of this study, occur as swarms along the western coast of the Bothnian Gulf from the mainland around Kalix in the north to the Luleå archipelago in the south. The dikes were emplaced at ca. 1.1 Ga (Kresten et al. 1997) into Paleoproterozoic igneous and meta-sedimentary rocks. In earlier petrographic and geochemical studies, Kresten et al. (1977, 1981) described the Kalix dikes as picritic (olivine-rich), and alnöitic (micaceous) lamprophyres, and silico-carbonatites. Rare occurrences of carbonatites (beforsites) have also been reported by Kresten et al. (1981), Griffin & Kresten (1987), and Åhman et al (1990).
For the present study, samples from a total of 6 dikes were collected on the mainland around Kalix, in Storön and in Ryssbält, and in the Kalix archipelago on the islet of Västra Gräddmanhällan. The samples were analyzed for the isotopic composition of strontium and neodymium at the Geological Survey of Finland, and for that of carbon and oxygen at the Department of Geology in the University of Helsinki.
Based on trace element geochemistry, Kresten et al. (1981) proposed an upper mantle origin for the parental magma to the Kalix dikes. Our Sr and Nd isotopic data, with relatively invariant initial (1.1 Ga) 87Sr/86Sr between 0.7026 and 0.7034 (n=7) and Nd values between +4 and +5 (n=6), support this interpretation by indicating a long-term depleted source for Sr and LREE’s in the studied dikes. However, the isotope ratios of C and O in carbonate samples (n=22) from the dikes are more ambiguous, with 13C and 18O values ranging from -6 to -1‰ (PDB) and from +9 to +17‰ (SMOW), respectively. The observed variations could be explained by mixing of mantle-derived material with that from a crustal source, possibly containing Paleoproterozoic sedimentary carbonate rocks that are abundant in the region.