Constraints on source-forming processes of West Greenland kimberlites inferred from Hf-Nd isotope systematics.

Amy M. Gaffney (1) , Janne Blichert-Toft (2) , Bruce K. Nelson (1) , Martin Bizzarro (3) , Minik Rosing (4) , Francis Albarède (2)

1 Department of Earth and Space Sciences [Seattle]
2 LST - Laboratoire de Sciences de la Terre
3 Geological Museum [Copenhagen]
4 NORDCEE - Nordic Centre of Earth Evolution

Kimberlites from West Greenland have Hf-Nd isotope as well as major and trace element compositions that are similar to other Group I kimberlites, but that are distinctive in the spectrum of magmas sampled at Earth's surface. The West Greenland kimberlites have εNdi that ranges from +1.6 to +3.1 and εHfi that ranges from −4.3 to +4.9. The samples exhibit ubiquitous negative ΔεHfi (deviation from the ocean island basalt εHf-εNd reference line), ranging from −1.8 to −11.2. The kimberlites are characterized by steep heavy rare earth element patterns, positive Ta-Nb anomalies and negative Hf-Zr anomalies. These chemical signals are consistent with the presence of ancient, subducted oceanic crust in the kimberlite source region. In the model we present, dewatering and possibly partial melting of rutile-bearing oceanic crust during subduction results in characteristic trace element patterns in the residual crust. During aging, the Hf-Nd isotopic composition of this dewatered/partially melted EMORB-type crust evolves to negative ΔεHfi values. Metasomatic fluids derived from this ancient subducted oceanic crust infiltrate and impart their trace element and isotopic signal on proximal peridotitic mantle. Melting of this metasomatized mantle peridotite results in kimberlite magmas.

Domaines: Sciences de la Terre

Amy M. Gaffney, Janne Blichert-Toft, Bruce K. Nelson, Martin Bizzarro, Minik Rosing, et al.. Constraints on source-forming processes of West Greenland kimberlites inferred from Hf-Nd isotope systematics.. Geochimica et Cosmochimica Acta, 2007, 71 (11), pp.2820-2836. ⟨10.1016/j.gca.2007.03.009⟩. ⟨hal-00337857⟩