Lead isotope signatures of Kerguelen plume-derived olivine-hosted melt inclusions: Constraints on the ocean island basalt petrogenesis

The nature of magmatic sources reflected by isotopic composition of the ocean island basalt (OIB) remains an ongoing question in igneous geochemistry. To constrain the magmatic sources for OIB related to the Kerguelen plume activity, we performed detailed microanalytical investigation of the 21.4 Ma picritic basalt (MD109-D6-87) dredged during the "Marion Dufresne" cruise on a seamount between Kerguelen Archipelago and Heard Island. Lead isotope compositions of olivine-hosted melt inclusions and matrix glasses were measured by Laser Ablation Multiple Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICP-MS) and Secondary Ion Mass Spectrometry (SIMS). We also performed major and trace element microanalyses and mapping of the inclusions and the host olivine phenocrysts by electron microprobe (wavelength-dispersive X-ray spectroscopy, WDS). The observed significant major element (K2O/P2O5, Al2O3/TiO2) and Pb isotope (Pb-207/Pb-206 and Pb-208/Pb-206) heterogeneities of parental melts (MgO = 7-10 wt.%) during early high pressure crystallisation stage (200-300 MPa, Fo(82-86) mol%), and relative homogeneity at later lower-pressure crystallisation stage (<100 MPa, Fo(75-80) mol%) are interpreted by mixing between "Plume" and "Assimilant" melts during magma residence and transport. Lead isotope composition of the parental basaltic melts was inherited from both heterogeneous mantle and the Kerguelen Plateau crust. High K2O/P2O5 (>4), Al2O3/TiO2 (>4) ratios are attributed to assimilation of the plateau basaltic crust (>= 50 wt.%) by the melts in the magma chamber at palaeodepths from 6 to 9 km. The crustal assimilation may have happened through plagioclase dissolution. The large chemical and isotopic heterogeneity of the parental OIB melts found by in situ microanalyses in this study suggests that the bulk rock chemistry alone cannot provide enough information to constrain the nature of the magmatic sources. (C) 2014 Elsevier B.V. All rights reserved.