Mantle plumes and geochemistry

There is considerable interest in the extent to which mantle plumes exist, how many there may be, and how best they can be recognized. It has proved unexpectedly difficult to image them consistently from seismology, and it has been suggested that they may be recognized from the geochemistry of rocks erupted at the Earth's surface. The difficulty is that plumes are physical features, and they can only be identified geochemically if they are either derived from, or entrain material that has a distinctive geochemical fingerprint. Too often plumes have been invoked because the erupted basalts have a within plate signature, irrespective of whether the volumes of magma or the rates of eruption can be estimated. U-series isotopes in basalts are sensitive to variations in the upwelling rates of their mantle source rocks, and those can be used to invoke upward movements that may be associated with mantle plumes. A contribution from the Earth's core might provide a possible geochemical tracer of deep-seated mantle plumes, although it is difficult to assess whether core material may be readily entrained in the mantle. Given the marked compositional differences between the core and the mantle, it should be possible to identify material that contains a contribution from the core and hence may be inferred to be from the core–mantle boundary. The core should have high Os and W abundances, and distinctive isotope ratios. Arguments for the recognition of core material using Os and W isotopes are therefore reviewed, as are those relying on rare gases to identify contributions from the lower mantle.