Tracing pollution of the mantle with isotopically anomalous Mo and U

Over the 4.5 billion year course of Earth History most of the silicate portion of the Earth (i.e. all the solid planet except the core) has likely been processed through the plate tectonic cycle at the Earth's surface. While at the Earth's surface, alteration by seawater imparts a distinctive chemical signature to the top of the plate before it is returned by subduction back to the mantle. Tracking such chemical traces of old plates is an important task. If we can identify these signatures in material erupted again at the surface it gives us a means to understand how the mantle has been stirred. Whilst there are improving models of mantle convection, it is a complex problem and needs to be calibrated with observations. A major question remains as to how well the upper and lower portions of the mantle are mixed. It used to be thought that they were isolated from one another. Recent work from seismology has made this picture untenable, but it is nevertheless unsure as to just how much communication there is between top and bottom. We are attempting to solve this problem by looking at two elements (Mo and U) that are returned to the mantle with a distinctive isotopic label. They are also 'recycled' in sufficient abundance to be able to result in a different isotopic signature in the upper and lower mantle, if they are not well mixed . We can effectively sample upper and lower mantle using melts that come from ocean ridges and oceanic island respectively. If the whole mantle is well mixed they will have identical isotopic signatures of Mo and U, if not we predict small but significant differences. This work is new in that it has only recently become possible to measure the isotopic ratios of suitable elements for this problem with high enough accuracy. Our initial work suggests that distinct differences in Mo and U between upper and lower mantle are present, but these results need to be investigated in much greater detail to have full confidence in their significance and implications. This is the work we propose.