Tracing pollution of the mantle with isotopically anomalous Mo and U

在45亿年的地球历史过程中，地球的大部分硅酸盐部分（即除了核心之外的所有固体行星）可能已经通过地球表面的板块构造循环进行了处理。在地球表面，海水的蚀变在板块通过俯冲回到地幔之前赋予了板块顶部独特的化学特征。追踪旧盘子的化学痕迹是一项重要的任务。如果我们能在地表再次喷发的物质中识别出这些特征，那就能让我们了解地幔是如何被搅动的。虽然有改进的地幔对流模型，但这是一个复杂的问题，需要通过观测进行校准。一个主要的问题仍然是地幔的上部和下部混合得有多好。过去人们认为它们是相互隔离的。地震学的最新研究使这幅图景变得站不住脚，但仍然不能确定顶部和底部之间到底有多少交流。我们试图通过观察两种元素（Mo和U）来解决这个问题，这两种元素以独特的同位素标记返回地幔。如果它们没有很好地混合，它们也会以足够的丰度“再循环”，从而能够在上地幔和下地幔中产生不同的同位素特征。利用来自洋脊和洋岛的熔体可以有效地对上地幔和下地幔进行取样。如果整个地幔混合良好，它们将具有相同的Mo和U同位素特征，如果不是，我们预测小但显着的差异。这项工作是新的，因为直到最近才有可能以足够高的精度测量适合这个问题的元素的同位素比率。我们的初步工作表明，上地幔和下地幔之间的Mo和U存在明显差异，但这些结果需要进行更详细的研究，以充分相信其意义和影响。这是我们提出的工作。

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.