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

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 ε 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 γ 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 ε 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.