Argon solubility in metamorphic muscovite: determination of partition coefficients and implications for crust:mantle recycling

地质学家利用稀有气体（氦、氖、氩、氪、氙）的稀有性和化学惰性，将其用作地幔演化过程的示踪剂。稀有气体可以帮助回答有关地幔中挥发性物质（如水）的数量和来源以及这些物质如何随时间变化的问题。例如，日本最近的一项研究表明，来自地幔的岩石含有类似于海洋的惰性气体特征，这表明惰性气体可能从地球表面进入构造板块碰撞的地幔。但是，哪些矿物负责将惰性气体输送到地幔中，这些矿物可以输送多少气体？这项提议的目的是量化一种常见的地壳矿物白云母可以运输的氩的数量。稀有气体也被用来测量地质过程的速率和时间尺度，因为铀的放射性衰变产生氦，钾的衰变产生氩。特别是，地质年代学家利用白云母中钾衰变为氩来确定山脉隆起和侵蚀的时间和速度。在板块碰撞带，埋在100公里或更深的地壳岩石在达到最大埋深后的几百万年内就暴露在地表。由于这些垂直速度与水平板块构造速度相似，地质学家希望更多地了解岩石如何在这些区域内移动和相互作用。然而，在某些矿物中，特别是那些通常与这些深埋岩石有关的矿物中，存在“额外的氩”，这使得K-Ar（或更常见的Ar/Ar）年龄看起来人为的老。我们的目标是表明，这种复杂的地质年代学家实际上可能是一个帮助地幔地质学家提供了一种方式来运输氩从地壳到mantle.One这个项目的主要目的是测量有多少氩可以成为被困在白云母，并确定是否溶解度随压力和白云母的晶体“类型”的变化。为了做到这一点，我们将在伦敦大学学院的高压实验中，在富氩流体中培养白云母。由此产生的晶体将在开放大学的高精度Ar/Ar和稀有气体实验室进行分析。最初的实验将用于测试我们是否可以种植合适的大而纯的白云母。随后的实验将测试压力和晶体类型对结晶过程中捕获多少氩气的影响。该项目将产生的结果对测量快速掩埋和隆起地壳的地质时间以及对我们了解稀有气体如何以及如何快速地从地球表面再循环到地幔深处具有重大意义。

By taking advantage of their rarity and chemical inertness, geologists use the noble gases (helium, neon, argon, krypton, xenon) as tracers for the processes responsible for the evolution of the mantle. Noble gases can help answer questions about the amount and source of volatiles such as water in the mantle and how these have changed over time. For example a recent study from Japan showed that rocks from the mantle contain a noble gas signature similar to that of the oceans, suggesting that noble gases may be transported from the Earth's surface into the mantle in zones where tectonic plates collide. But which minerals are responsible for transporting the noble gases into the mantle and how much gas can these minerals transport? This proposal aims to quantify the amount of argon that a common crustal mineral, muscovite, can transport.Noble gases are also used to measure the rates and time scales of geological processes since radioactive decay of uranium produces helium, and decay of potassium produces argon. In particular, geochronologists use the decay of potassium to argon in muscovite to determine the timing and rate of mountain uplift and erosion. In plate collision zones, crustal rocks buried to depths of 100km or more are known to have been exposed on the surface within a few million years of reaching their maximum burial depth. As these vertical speeds are similar to horizontal plate tectonic speeds, geologists want to understand more about how rocks move and interact within these zones. In some minerals, however, and specifically ones commonly found associated with these deeply buried rocks, there is "extra argon" which makes the K-Ar (or, more usually, the Ar/Ar) age appear artificially old. We aim to show that this complication for geochronologists may actually be a help to mantle geologists by providing a way to transport argon from the crust into the mantle.One of the main aims of this project is to measure how much argon can become trapped in muscovite, and to determine whether the solubility changes with pressure and crystal "type" of muscovite. To do this we will grow muscovite in an argon-rich fluid in high pressure experiments at University College London. The resulting crystals will be analysed for incorporated argon in the Open University's high-precision Ar/Ar and Noble Gas Laboratory. Initial experiments will be used to test whether we can grow suitably large and pure muscovite. Subsequent experiments will test the effect of pressure and crystal type on how much argon is trapped during crystallisation. This project will produce results which have major implications for measuring geological time in rapidly buried and uplifted crust, and for our understanding of how and how quickly noble gases may be recycled from the Earth's surface into the deep mantle.