Age and geochemistry of the mafic sills, ODP site 1276, Newfoundland margin

Site 1276, Leg 210 of the Ocean Drilling Program, was located on the Newfoundland margin in a seismically-defined ∼ 128 Ma “transitional” crust just west of the presumed oceanic crust, and the M3 magnetic anomaly. The goal of drilling on this non-volcanic margin was to study the rifting, nature of basement, and post-rift sedimentation in the Newfoundland–Iberia rift. Drilling of this 1739 m hole was terminated 90–160 m above basement, in the lower of a doublet of alkaline diabase sills. We have carried out geochemical studies of the sill complex, in the hopes that they will provide proxy information regarding the nature of the underlying basement. Excellent 40Ar/39Ar plateau ages were obtained for the two sills: upper sill ∼ 105.3 Ma; lower sill ∼ 97.8 Ma. Thus the sills are substantially younger than the presumed age of the seafloor at site 1276 (∼ 128 Ma), and were intruded beneath substantial sediment overburden (250 m for the upper, older sill, and 575 m for the lower younger sill). While some of the geochemistry of the sills has been compromised by alteration, the “immobile” trace elements show these sills to be hawaiites, differentiated from an enriched alkaline or basanitic parentage. Sr, Nd and Pb isotopes are suggestive of an enriched hotspot/plume mantle source, with a possible “added” component of continental material. These sills unequivocally were not derived from typical MORB (asthenospheric) upper mantle.