Constraining core-mantle interaction

在地球表面以下约3000公里处，温度跃升2000 K，周围环境从固体硅酸盐矿物变成液态铁合金。这是地核-地幔边界。它是我们星球上最深刻的不连续性，在很小的距离（<200米）内发生巨大的物理和化学变化。边界两侧的温度差异一直被认为是加热地核上方地幔的原因，使其密度降低，并作为羽流上升，形成夏威夷等海洋岛屿。然而，这种长期持有的“羽流范例”越来越受到质疑，需要一些确凿的证据来证明形成这些海洋岛屿的物质的最终深层起源。地震学是想象地球内部的传统工具，它很难清楚地分辨海洋岛屿下的狭窄羽流。地球化学可以提供关键的证据，如果一个化学印记的居住旁边的组成非常独特的核心可以发现在大洋岛屿熔岩。最近的研究表明，有这样的签名。最初的工作集中在提高元素Os的同位素比值，186 Os/188 Os。在夏威夷的熔岩中发现了较高的186 Os/188 Os比率，并且可以用其来源地核的贡献来解释。然而，对于这一观察结果还有其他合理的解释。另一个不那么模棱两可的指纹来自另一个同位素比率，这次是W元素的182 W/184 W。这个比例只在地球~ 4550 My历史的前50 My发生了变化，因此只反映了最初建立地核和地幔之间成分差异的最古老的过程。钨同位素系统对地幔中加入的地核物质也高度敏感，但初步研究表明，在显示最极端的Os同位素组成的夏威夷样品中没有地核钨同位素特征。本文的工作旨在解决这一问题。一个合理的替代品需要找到升高的186 Os/188 Os和反对提出的W同位素示踪剂的可靠性需要充分解决。此外，这种方法需要进一步使用，而不是目前的数据库，从一个单一的海洋岛屿的三个样本。该项目将研究地球物理或地球化学证据表明岩浆活动最终有深部来源的大范围地点。此外，还将使用一种新的示踪剂，即Mo。利用钼及其同位素的浓度分析，可以清楚地区分几种相互竞争的假设，以解释对比鲜明的W和Os同位素数据。这项研究的结果将提供一个明确的答案，即我们是否可以在地球表面对地球最深处的水库进行化学取样。如上所述，这不是毫无意义的推测，而是将帮助我们了解构造板块中间的火山活动是如何发生的，以及在地球上最引人注目的边界发生了什么过程。

Some 3000km beneath the surface of the Earth the temperature jumps 2000K and the surroundings change from solid silicate minerals to liquid iron alloy. This is the core-mantle boundary. It is our planet's most profound discontinuity, with huge physical and chemical changes occuring over a very small distance (<200m). The temperature contrast across the boundary has long been held responsible for heating the mantle above the core such that it becomes less dense and upwells as plumes to create oceanic islands such as Hawaii. However, this long held 'plume paradigm' has been increasingly questioned and some hard evidence for the an ultimately deep origin of the material that forms these oceanic islands is needed. The traditional tool for imagining the Earth's interior, seismology, has difficulty in clearly resolving narrow plumes beneath ocean islands. Geochemistry could provide the crucial evidence if a chemical inprint of residence next to the compositonally highly distinctive core could be found in oceanic island lavas. Recent work has suggested that there are such signatures. Initial work focussed on elevated values of an isotope ratio of the element Os, 186Os/188Os. Elevated 186Os/188Os ratios were found in lavas from Hawaii and can be attractively explained by a contribution from the core in their source. However there are other plausible explanations of this observation. A much less equivocal fingerprint comes from another isotope ratio, this time of the element W, 182W/184W. This ratio changed only in the first 50My of Earth's ~4550My history and so only reflects the most ancient processes that originally established the compositional differences between core and mantle. The W isotope system is also highly sensitive to the addition of core material to the mantle but an initial study indicated no core W isotope signature in the samples from Hawaii which showed the most extreme Os isotope compositions. The work here proposes to resolve this empasse. A plausible alternative needs to be found for the elevated 186Os/188Os and objections raised to the reliability of the W isotope tracer need to be fully addressed. Moreover, the approach needs to be used further than the current database of three samples from a single ocean island. This project will study a large range of locations where geophysical or geochemical evidence has suggested an ultimately deep source for magmatism. In addition a new tracer will be used, namely Mo. Using analyses of the concentration of Mo and its isotopes it is possible to clearly distinguish between several competing hypotheses to account for the contrasting W and Os isotopic data. The outcome of the study will provide a definitive answer as to whether we can chemically sample the Earth's deepst reservoir at the planet's surface. As indicated above, this is not idle speculation but will help us understand how volcanism in the middle of tectonic plates occurs and indeed what processes occur at the most dramatic boundary on Earth.