Deep mantle plume osmium isotope signature from West Greenland Tertiary picrites

Picrites from Nuussuaq Peninsula and Qeqertarssuaq (Disko) Island, West Greenland, preserve trace element and isotopic signatures reflecting the composition of the Icelandic plume head. Os isotope ratios are low (Os-187/ Os-188((i)) = 0.1272-0.1371) in terms of global plume related magmatism, and this 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 between initial plume head compositions and two discrete types of West Greenland continental crust. One crustal component is of a local sedimentary origin, and the other is typical of ancient felsic crust. The low Os isotopic signatures of the picritic units are considered to be those of the initial mantle plume. The fact that such low Os isotope ratios occur in sequences with high He-3/He-4, and the nonsystematic variation in Os isotopes with indices of fractionation/accumulation and Pb isotopes, argue against mixing with depleted MORE mantle or ancient subcontinental lithospheric mantle. The high Os concentrations in the picrites are attributed to high degrees of partial melting (> 25%) of mantle containing no residual sulphide. This is consistent with models for plume heads in which anomalous mantle temperatures initiate melting at high pressures generating large degrees of partial melting. Unradiogenic Os and radiogenic Nd components in plume-related CFB magmatism may preserve contributions from a reservoir which is sampled only occasionally in young oceanic basalts. Such a reservoir shares Os isotopic features with the primitive upper mantle (PUM), which may also be manifest in the 'Kea' component of Hawaiian magmatism. Therefore, portions of the West Greenland continental flood basalt province arguably represent the first direct sampling of unmodified plume head material derived from the lower mantle or lower portions of the upper mantle. (C) 2000 Elsevier Science B.V. All rights reserved.