Towards a global test of the mantle plume paradigm: Shona and Discovery Seamount Chains, South Atlantic - plume tracks or not?

We address here one of the most important fundamental controversies in Earth Sciences: do mantle plumes exist? For many years, mantle plumes have provided an elegant explanation for the origin of oceanic hotspot trails, and more recently, Large Igneous Provinces (LIPs). The existence, source depth, composition and fixity of mantle plume upwellings are extremely important issues whose clarification will have a significant impact on geodynamic models for the mantle and the motions of tectonic plates. Major South Atlantic seamount chains remain virtually unsampled and unknown, leaving a large gap in our understanding of how hotspot chains fit the predictions of the plume hypothesis. Without such information it will remain impossible to properly test the mantle plume paradigm on a global scale. As a basis for this project we helped initiate the ANT-XXIII/5 R/V Polarstern international research expedition (April-June 2006) to dredge-sample seamounts and ridges linked to the proposed Shona and Discovery mantle plumes. Our strategy was to sample multiple hotspot chains stretching across a very broad region of the southern South Atlantic as an innovative way of testing 1) the new thinking that plumes may differ from the classic concept of a ?head-tail? structure and 2) our evolving hypothesis that hotspot melting anomalies may be much broader than suggested by individual chains of seamounts and ridges. The ANT-XXIII/5 samples are the first recovered from the Shona and Discovery seamounts, given their remoteness, prevailing bad weather conditions and the associated challenges of dredging old manganese coated seamounts and ridges. Since we helped lead the petrology sampling during the expedition we are fully aware of the large number of high quality samples available for the purposes of this project. The main analytical goal of this project is to measure incontrovertible ages for a representative suite of samples from each Shona and Discovery dredge haul by systematically applying our new microphenocryst techniques for dating seawater altered rocks. We aim at integrating these 40Ar/39Ar incremental heating mineral ages with geochemical and geophysical data sets provided by our international project partners in order to establish the long-term temporal, spatial and geochemical development of major South Atlantic bathymetric anomalies. These data will provide the first test of how well hotspot volcanism in the remote southern South Atlantic fits the predictions of the mantle plume hypotheses. This information will fill a major gap in our global understanding of the history, distribution and composition of hotpot chains, essential for testing the mantle plumes hypothesis, possible variations and alternative models. Moreover, this project will also test our hypothesis that hotspot melting anomalies are far broader than currently assumed. Our results will also have significant implications for other approaches to testing the plume hypothesis and possible alternatives, e.g. mantle tomography, numerical modelling of the mantle, plate reconstructions, and paleolatitude of seamounts and ridges.