IODP Expedition 330: assessing the mantle plume component in the Louisville Seamount Trail, SW Pacific Ocean

One of the main products from melting of the Earth's interior is basalt. Basalt underlies the ocean basins, forms the bulk of ocean islands, and is found throughout the continents. It is the closest we have to providing a window into the mantle, the vast volume of rock between the crust and core of the Earth. The Earth's mantle convects on a range of scales, and comprises several chemical reservoirs. Basalts formed by melting of these reservoirs provide information about the evolution of the Earth's interior. For example, mid-ocean ridge magmas tap the shallow, chemically-depleted upper mantle reservoir, whereas ocean island magmas tap mantle derived from deeper, more enriched reservoirs. This simple model has, however, been questioned. For example, some magmas erupted in Iceland, a classic hotspot/plume region, have been found to be chemically depleted. Similarly, some magmas erupted at the distal end of the Emperor Seamount Chain also are depleted, despite being associate with the long-lived Hawaiian hotspot and plume. Are these unusual compositions due to interaction between a deeply-sourced plume and the upper mantle, or is the depleted character inherent in the plume itself? Because of their different histories, we may expect these different depleted components (within plume mantle or upper mantle) to develop characteristic isotopic features, and indeed this has been argued on the basis of Hf isotopes.
The Louisville Seamount Trial, recently drilled during IODP Expedition 330, provides an opportunity to test the idea that depleted components may be incorporated in plumes. Expedition 330 recovered >800 m of rock from 5 seamounts at the older end of the Trail, emplaced during the late Cretaceous. Importantly, the seamounts were emplaced through lithosphere of different ages and thicknesses. This is crucial, because the lithosphere controls the extent of melting in the ascending plume, which in turn controls the composition of the magmas. Plume mantle ascending beneath thin lithosphere melts more extensively; the melts sample the mantle more comprehensively, including more refractory, depleted components. Plume mantle ascending beneath thick lithosphere (e.g., present-day Hawaii) has a restricted melting regime and the melting samples the more fusible, enriched components. Results from Iceland and the older Emperor Seamounts concur with this; where the plume interacts with thin lithosphere, we see more of the 'depleted' mantle components in the erupted basalts. The Louisville Seamount Trail provides a similar opportunity.
Why is this important? Knowledge of the composition and evolution of the deep mantle reservoirs is fundamental to understanding the source of heat energy within the planet, as well as its long-term evolution. If large volumes of the deep mantle are chemically depleted, this would imply that they also have lower-than-predicted contents of radioactive isotopes (U, Th, K) and heat-generating capacity.
We propose to analyse a range of basaltic samples from the Louisville Seamount Trail, in order to assess the temporal and spatial variations in elemental and isotopic composition. We shall use the trace and major element data to determine the extent and depth of melting of the mantle source, and the isotope (especially Hf and Nd isotopes) and highly-incompatible trace element data to assess the relative contribution from enriched and depleted mantle sources.

This project follows on from an exceptionally successful IODP Expedition to explore the Louisville Seamount Trail. The research proposed here has two academic impacts. Firstly, it contributes to an effort to understand the origin and evolution of the LST itself. This first output of the research project will likely be published alongside other investigations on the LST by shipboard scientists in a special, dedicated high impact issue on the Louisville Seamounts. Secondly, this research has a far wider impact on the understanding of the earth's mantle and plume magmatism. We anticipate that this research will attract attention from the wider community working on these large, earth system problems and therefore attendance at both national and international conferences is planned to disseminate the results in the coming year. The outcome of this wider research will be published in high-ranking peer reviewed journals.
Furthermore, this project not only allows for a strengthening of previous collaborations between the PI and the University of Edinburgh but also opens up new international collaborations. The isotopic work is being undertaken as part of a new collaboration forged whilst onboard by Williams with Dr Hamelin at the world-renowned Institute of Geophysics, Paris. This work is part of a wider isotopic study of the Louisville Seamounts, involving researchers from the USA, Japan and China. These collaborations open up new avenues of research for the University and potential research opportunities for Williams. As her first PDRA position, this project offers an excellent opportunity for further research training and participation in a large, international research group. Williams also plans on attending the NERC Communicating Science to the Public workshop (2011) to maximise her ability to utilise the university and science press as well as other training workshops offered through the University's 'Skills for the Professional Researcher' programme.
Williams already organises and delivers outreach activities for various schools nationwide as part of her position at the University of Leicester. She also participated weekly in video conferences with schools from around the world whilst onboard the Expedition. The ideas and experience gained from these videoconferences will feed into future events that will include activities based upon IODP and can be used to highlight the UK involvement in this internationally important programme. In addition to this, talks are planned to both the student community and the general public through both the student seminar series and the well-attended Leicester Literary and Philosophical Society.