Constraining core-mantle interaction

限制核幔相互作用

• 批准号: NE/D012805/1
• 负责人: Timothy Elliott
• 金额: $ 20.72万
• 依托单位: University of Bristol
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2007
• 资助国家: 英国
• 起止时间: 2007 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FD012805%2F1
• 关键词: Constraining; core; mantle; interaction

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.

在地球表面下方约3000公里的温度下降了2000k，周围环境从固体硅酸盐矿物质变为液态铁合金。这是核心掩护边界。这是我们星球上最深刻的不连续性，距离很小的距离（<200m）发生了巨大的物理和化学变化。长期以来，整个边界的温度对比一直负责将地幔加热到核心上方的地幔，从而使其变得较少，并且像羽毛一样，以创建海洋岛（例如夏威夷）的羽流。但是，这种长期存在的“羽状范式”受到了越来越多的质疑，并且需要一些构成这些海洋岛屿的材料的最终材料起源的艰难证据。想象地球内部的传统工具，即地震学，难以清楚地解决海洋岛屿下的狭窄羽毛。如果在海洋岛熔岩中可以找到占地高度独特的核心，地球化学可以提供至关重要的证据。最近的工作表明有这样的签名。最初的工作着重于元素OS的同位素比的升高值，186OS/188OS。在夏威夷的熔岩中发现了升高的186OS/188OS比率，可以通过核心核心的贡献来吸引人的解释。但是，对这一观察结果还有其他合理的解释。较少的模棱两可的指纹来自另一个同位素比，这段时间为元素W，182W/184W。该比率仅在地球约4550年的前50年代发生变化，因此仅反映了最初确定核心和地幔之间的组成差异的最古老过程。 W同位素系统对向地幔中添加核心材料的添加也很敏感，但初步研究表明，夏威夷样品中没有核心W同位素特征，该样品显示出最极端的OS同位素组成。这里的工作提议解决此Empasse。需要找到一个合理的替代方案，即升高的186OS/188OS以及对W同位素示踪剂的可靠性提出的异议。此外，该方法需要比目前来自一个海洋岛的三个样品的数据库进一步使用。该项目将研究地球物理或地球化学证据最终提出岩浆作用的各种位置。另外，将使用新的示踪剂，即MO。使用MO浓度及其同位素的分析，可以清楚地区分几种竞争假设，以说明对比的W和OS OSOSOTOPIC数据。该研究的结果将为我们是否可以化学对地球表面的地球深层储层进行化学采样提供明确的答案。如上所述，这不是闲置的猜测，而将有助于我们了解构造板中间的火山主义是如何发生的，而实际上是什么过程在地球上最引人注目的边界处发生。

项目成果

High-Precision Mass-Dependent Molybdenum Isotope Variations in Magmatic Rocks Determined by Double-Spike MC-ICP-MS

双尖峰 MC-ICP-MS 测定岩浆岩中与质量相关的高精度钼同位素变化

• DOI: 10.1111/ggr.12109
• 发表时间: 2016
• 期刊: Geostandards and Geoanalytical Research
• 影响因子: 3.8
• 作者: Willbold M

Molybdenum mobility and isotopic fractionation during subduction at the Mariana arc

• DOI: 10.1016/j.epsl.2015.10.006
• 发表时间: 2015-12-15
• 期刊: EARTH AND PLANETARY SCIENCE LETTERS
• 影响因子: 5.3
• 作者: Freymuth, Heye;Vils, Flurin;Elliott, Tim
• 通讯作者: Elliott, Tim