Hf isotope constraints on mantle sources and shallow‐level contaminants during Kerguelen hot spot activity since ∼120 Ma

We report new Hf isotopic data for basalts from the Kerguelen large igneous province (LIP) obtained using high‐precision multicollector inductively coupled mass spectrometry (MC‐ICP‐MS) analysis. All drill sites from the southern Indian Ocean Kerguelen Plateau–Broken Ridge, in addition to two volcanic suites from the Kerguelen Archipelago, are represented. These new data are integrated with other recently reported geochemical data for this LIP. We examine the geochemical signatures of the mantle sources and shallow‐level contaminants present during the past ∼120 Ma history of Kerguelen hot spot activity. Our results highlight the contribution of distinct mantle source compositions during Cretaceous (Kerguelen Plateau and Broken Ridge) and Cenozoic (northernmost Kerguelen Plateau and Kerguelen Archipelago) magmatism arising from melting of the Kerguelen plume head and plume tail, respectively. The Cretaceous Kerguelen plume basalts have primitive mantle‐like Pb, Sr, and Nd isotopic compositions and moderately depleted Hf isotopic compositions and are different from the Cenozoic plume basalts, which extend to more radiogenic Pb isotopic compositions. Neodymium and Hf isotopes are decoupled in Kerguelen plume‐derived rocks, and this, in combination with their Pb isotopic compositions, implies that the Kerguelen plume contains small amounts of ancient pelagic sediment mixed with old, recycled enriched oceanic crust. Different contributions from pelagic sediments relative to oceanic crust are able to account for the distinction between the isotopic compositions of the Cretaceous and Cenozoic mantle plume sources. Assimilation of shallow‐level continental crust, left stranded in the Indian Ocean during Gondwana breakup, by plume‐derived magmas was the dominant process recorded in Cretaceous Kerguelen Plateau basalts. During the Cenozoic, magmas from the Kerguelen plume mixed to varying degrees with local, Indian Ocean depleted upper mantle and assimilated the overlying Cretaceous Kerguelen Plateau lithosphere. Despite the geochemical heterogeneity of the Kerguelen LIP, we find evidence for a finite number of components involved in the genesis of Kerguelen Plateau–Broken Ridge and Kerguelen Archipelago rocks.