Precambrian Research 109 (2001) 73 – 102 The Os and Nd isotopic systematics of c. 2.44 Ga Akanvaara and Koitelainen mafic layered intrusions in northern Finland E. Hanski a, *, R.J. Walker b , H. Huhma c , I. Suominen d a Geological Surey of Finland, P.O. Box 77, FIN-96101 Roaniemi, Finland b Isotope Geochemistry Laboratory, Department of Geology, Uniersity of Maryland, College Park, MD 20742, USA c Geological Surey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland d Department of Geology, Uniersity of Helsinki, P.O. Box 11, FIN-00014, Helsinki, Finland Received 18 September 2000; accepted 29 January 2001 Abstract We have determined Os and Nd isotopic compositions of various rock types in and adjacent to the c. 2.44 Ga Akanvaara and Koitelainen layered intrusions which are part of the extensive Paleoproterozoic, ultramafic to felsic, mantle plume-related magmatism in the Fennoscandian Shield. The initial  Nd in the layered sequences of both intrusions fall within a limited range around -2.0, being compatible with results from other coeval layered intrusion in the shield. The initial Os isotopic ratios in chromitites are near-chondritic: the total range of  Os (T ) is from -1 to +6 with the highest values reported for low-Os chromites. No systematic differences are found in the osmium isotopic compositions of the chromites between the intrusions or relative to the stratigraphic position of the sample. The studied intrusions share similar isotopic and geochemical features with rocks from younger flood basalt provinces, and also the same problem of the origin of the lithospheric source component, viz. crustal assimilation versus melt generation within lithospheric mantle. The observed Nd – Os isotopic systematics of the intrusions require that the magmas had negative  Nd values already before emplacement and can best be accounted for by 15–25% deep-crustal contamination of a komatiitic melt initially having a near-chondritic Os isotopic composition, coupled with additional minor in situ assimilation of country rock material. This hypothesis is supported by the existence of coeval, crustally contaminated komatiitic volcanism on the Fennoscandian Shield. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Layered intrusions; Chronology; Sm/Nd; Re/Os; Paleoproterozoic; Finland www.elsevier.com/locate/precamres 1. Introduction Different igneous and post-magmatic processes were most likely involved in the formation and subsequent evolution of mafic layered intrusions * Corresponding author. Fax: +358-2055014. E-mail address: eero.hanski@gsf.fi (E. Hanski). 0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0301-9268(01)00142-5