Melt inclusion Pb-isotope analysis by LA-MC-ICPMS: assessment of analytical performance and application to OIB genesis

It is widely acknowledged that olivine-hosted melt inclusions retain compositional information unavailable from the study of bulk-rock samples alone. Whether or not the compositions of melt inclusions are truly representative of geologically significant melt bodies has, however, been called into question; isotopic data are critical to resolving this debate but the rare existing data are contradictory. Previous studies of Pb isotope ratios suggest that large compositional variations are preserved by melt inclusions whereas Sr isotope data apparently do not. A new and extensive laser ablation Pb-isotope database is presented here and displays a degree of isotopic heterogeneity in key samples from Mangaia and the Pitcairn seamounts significantly less than previously reported. More than 95% of the inclusions analysed, including results for the low abundance Pb-isotope, 204Pb, which has previously proven difficult to measure, are within error of bulk-rock analyses from these locations. Trace element measurements on two inclusions of different isotopic character from the Pitcairn Seamounts are closely similar to each other, and do not easily support models in which melt inclusions from this locality represent mixing between the Pitcairn mantle plume and the local MORB magmas or lithosphere. Instead, a second component, likely derived from within the plume, is required to explain the isotope and trace element variations observed. In terms of isotopic compositions then, melt inclusions may in most cases be representative of geochemical conditions prevailing within the magmatic plumbing system. The range of isotopic compositions found in a single sample likely includes the composition of the transporting melt (groundmass of the rock) and compositions previously trapped in crystals in the magmatic plumbing system.