The Os and Nd isotopic systematics of c. 2.44 Ga Akanvaara and Koitelainenmafic layered intrusions in northern Finland

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 intrusion s which are part of the extensive Paleoproterozoic, ultramafic to felsic, m antle plume-related magmatism in the Fennoscandian Shield. The initial E,, in the layered sequences of both intrusions fall within a limited range aro und -2.0, being compatible with results from other coeval layered intrusion in the shield. The initial Os isotopic ratios in chromitites are near-chon dritic: the total range of gamma (Os)(T) is from -1 to +6 with the highest values reported for low-Os chromites. No systematic differences are found i n the osmium isotopic compositions of the chromites between the intrusions or relative to the stratigraphic position of the sample. The studied intrus ions share similar isotopic and geochemical features with rocks from younge r flood basalt provinces, and also the same problem of the origin of the li thospheric source component, viz. crustal assimilation versus melt generati on within lithospheric mantle. The observed Nd-Os isotopic systematics of t he intrusions require that the magmas had negative epsilon (Nd) values alre ady before emplacement and can best be accounted for by 15-25% deep-crustal contamination of a komatiitic melt initially having a near-chondritic Os i sotopic composition, coupled with additional minor in situ assimilation of country rock material. This hypothesis is supported by the existence of coe val, crustally contaminated komatiitic volcanism on the Fennoscandian Shiel d. (C) 2001 Elsevier Science B.V. All rights reserved.