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

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 eNdi that ranges from +1.6 to +3.1 and eHfi that ranges from 4.3 to +4.9. The samples exhibit ubiquitous negative DeHfi (deviation from the ocean island basalt eHf–eNd 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 rutilebearing 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 DeHfi 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. 2007 Elsevier Ltd. All rights reserved.