Testing the mantle plume hypothesis through IODP drilling on the Louisville Seamounts

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Mantle plumes are solid-state convective mantle upwellings that provide the principal means by which heat and material are transferred from the deep Earth to the surface. They are responsible for much of the world's intraplate volcanism (e.g. Hawaii) and their initiation is accompanied by massive outpourings of magma on a scale not seen today. These massive volcanic episodes produce large igneous provinces (LIPs), two of the largest of which coincided with global mass-extinction events at the Permian-Triassic (Siberia) and Cretaceous-Palaeocene (Deccan) boundaries. The largest of all Earth's LIPs, the Ontong Java Plateau (OJP), was erupted entirely below sea level and consequently had only a limited environmental impact. Testing the mantle plume hypothesis and, if plumes do indeed exist, understanding their nature and origin, are first-order goals of the Earth Sciences, and have provided the motivation for several DSDP, ODP and IODP deep-sea drilling expeditions including IODP Expedition 330 which is the basis of the present proposal. One of the early predictions of the mantle plume hypothesis is that plumes remain fixed in the mantle for tens of millions of years and that their surface manifestation as hotspot trails (chains of ocean islands and seamounts) can be used to define absolute plate motions. This prediction was tested during ODP Leg 197 on the Hawaii-Emperor seamount chain (HESC) and shown to be invalid. Palaeomagnetic studies on the cores showed that the hotspot drifted about 15 degrees southwards between 81 and 47 Ma, implying motion of the early Hawaiian plume of about 40 mm/yr with respect to the Earth's rotation axis. Although this finding doesn't necessarily weaken the plume hypothesis, it does show that plumes aren't always fixed. One possible explanation for the motion is that it reflects a general southward mantle flow (mantle 'wind') over this period. IODP Expedition 330 is designed to test the mantle 'wind' hypothesis by carrying out a similar study on the part of the Louisville seamount chain (LSC) that formed over the same period. The LSC is located in the south Pacific Ocean, runs roughly parallel to the HESC, and is the only hotspot chain on which the hypothesis can be tested. My role in IODP Expedition 330 will be to study the petrology and geochemistry of the drill core, and I propose to test the mantle plume hypothesis in three ways. (1) I propose to use the major element composition of basalt to estimate the mantle temperature at the time of seamount formation and (2) trace element data to assess the composition of the mantle source. It has been proposed that the LSC were produced from a mantle plume whose initiation phase was responsible for the giant OJP LIP. Trace element data, coupled with isotope studies being carried out by other shipboard scientists, should be allow us to test this hypothesis. The OJP was drilled during ODP Leg 192, on which I served as Co-Chief Scientist. Finally (3) I propose to use helium-isotope data (obtained in collaboration with Dr Finlay Stuart at SUERC) to look for evidence of deep-mantle involvement in the formation of the LSC. High 3He/4He has been found in basalt from the HESC (and in other possibly plume-related basalts) and is thought to imply an input of primordial 3He, either from the deep mantle or possibly the core.