The origin and evolution of cratons: Age and chemical constraints from the lithospheric root beneath West Greenland.

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Like an iceberg, the main volume of a continent is hidden at depth in 'mantle roots'. Beneath ancient crustal rocks (>2.5 billion years old), known as cratons, many of these roots extend to great depth (at least 250 km and perhaps 400 km), are very ancient and are host to the worlds supply of gem diamonds. Arguably the most interesting aspect of these cratonic mantle roots is that they seem to assist in the long-term survival of continental crust. To understand the mechanisms by which cratons and their roots are formed and stabilised requires information on the bulk composition of various cratonic roots and their age of formation. This is difficult to achieve because samples of the roots are only erupted by deeply derived volcanic rocks in certain areas and many samples are too small to obtain representative estimates of their bulk compositions or obtain age information. The one craton where addressing these issues has been possible in a detailed way, the Kaapvaal craton, S. Africa, appears to have an unusual composition and hence we are in need of an alternative benchmark dataset for cratonic lithosphere, with which to improve our knowledge of how typical cratons form and evolve. The North Atlantic Craton, exposed in S.W. Greenland, together with its surrounding areas of deformed cratonic crust, is intruded by deeply derived volcanic rocks carrying an exceptional inventory of large fragments of the mantle root beneath. Using published data from the experimental melting of mantle at different pressures, together with the analysis of elements that have differing affinities for the melt compared to the solid residue during melting at different pressures, we plan to constrain the depth of melting that formed the mantle root beneath the differing parts of this region. This will enable us to test a model that suggests that cratonic roots are formed by the subduction stacking of older oceanic plates. Mineral analysis of the mantle fragments will allow us to evaluate the depths from which they were derived and hence look at the thickness of the root beneath different areas. We will also use the radiometric decay systems 187Re-1870s and 176Lu-176Hf, on both minerals and the bulk rock, to determine the age of the melt extraction event and to try to determine the age of any subsequent disturbance of the root associated with tectonic episodes responsible for the fragmentation of a much larger craton. In this way we can obtain an understanding of the age and depth of the deep mantle root beneath crustal regions of different ages. This will provide us with a picture of not only how cratons are formed, but how they are affected by major tectonic events and why they sometimes breakup. Lastly, diamonds have been found in the volcanic rocks intruding both the craton and the region surrounding the craton, that has been heavily influenced by tectonic events. Understanding how the age, depth and composition of the cratonic root beneath region changes laterally will also improve our understanding of how diamonds are distributed in the roots in and around cratons and ultimately what processes by cause their formation or destruction.