Deep mantle plume osmium isotope signature from West Greenland Tertiary picrites

Picrites from Nuussuaq Peninsula and Qeqertarssuaq (Disko) Island, West Gre enland, preserve trace element and isotopic signatures reflecting the compo sition of the Icelandic plume head. Os isotope ratios are low (Os-187/ Os-1 88((i)) = 0.1272-0.1371) in terms of global plume related magmatism, and th is is coupled with anomalously high Os abundances and radiogenic Nd-143/Nd- 144((i)) isotopes. Crustally contaminated basalts within the West Greenland sequences possess Os and Nd isotopic signatures consistent with mixing bet ween initial plume head compositions and two discrete types of West Greenla nd continental crust. One crustal component is of a local sedimentary origi n, and the other is typical of ancient felsic crust. The low Os isotopic si gnatures of the picritic units are considered to be those of the initial ma ntle plume. The fact that such low Os isotope ratios occur in sequences wit h high He-3/He-4, and the nonsystematic variation in Os isotopes with indic es of fractionation/accumulation and Pb isotopes, argue against mixing with depleted MORE mantle or ancient subcontinental lithospheric mantle. The hi gh Os concentrations in the picrites are attributed to high degrees of part ial melting (> 25%) of mantle containing no residual sulphide. This is cons istent with models for plume heads in which anomalous mantle temperatures i nitiate melting at high pressures generating large degrees of partial melti ng. Unradiogenic Os and radiogenic Nd components in plume-related CFB magma tism may preserve contributions from a reservoir which is sampled only occa sionally in young oceanic basalts. Such a reservoir shares Os isotopic feat ures with the primitive upper mantle (PUM), which may also be manifest in t he 'Kea' component of Hawaiian magmatism. Therefore, portions of the West G reenland continental flood basalt province arguably represent the first dir ect sampling of unmodified plume head material derived from the lower mantl e or lower portions of the upper mantle. (C) 2000 Elsevier Science B.V. All rights reserved.