A new approach to Ar/Ar thermochronology: investigating the effects of recrystallisation and deformation on alkali feldspar crystals

地球科学家利用铀和钾等常见元素的放射性衰变测量了地球历史上许多最重要事件的年龄。因此，我们知道地球形成于大约4.5亿年前，生命出现在大约5.4亿年前，一颗陨石在6550万年前撞击了墨西哥的地球，同时印度发生了巨大的火山喷发。然而，事件的绝对年龄只是故事的一小部分，地球过程发生的速度对于我们理解地球是如何运行的更重要。虽然人们普遍认为地质过程是缓慢的，但许多现代地球科学正在研究发生在比一百万年短得多的时间尺度上的过程。事实上，对于理解固体地球与海洋和大气的相互作用，以及控制我们的环境，真正重要的是地质过程的速率。然而，地质作用的速度比火山喷发等事件的绝对年龄要难得多。这些特别的过程甚至挑战了我们目前的知识状态，包括在遥远的过去塑造地球表面的过程，如流经岩石的流体、山脉海拔、断层运动和沉淀物流入深海盆地的速度。量化这些过程对于理解我们过去和现在的环境与固体地球的相互依赖至关重要。在过去的几年里，我们在开放大学开发了一项技术，使用一种技术来确定非常小的样品的日期，这项技术涉及使用聚焦的紫外线激光束在直径不到十分之一毫米的地方钻孔。有了这项技术，我们已经能够提前测定上面提到的一些过程的年代。我们已经使用一系列类似的技术测量了火成岩侵入过程中岩石的加热、造山事件的速度、沉积物的扩散速度和断层运动的年龄。特别是，一个涉及本小组所有成员的项目测量了古流体在油气储集层砂岩中的流动速度。这项研究发表在《科学》杂志(Mark等人)上。2005年)，并表明通过结合单个沙粒上矿物过度生长的日期和研究流体的微小包裹体，我们可以确定流体流动的时间、时间和温度。在这项工作中，我们意识到，用于理解矿物过度生长的技术组合也可以用来提高我们对其他碱性长石的理解，这些碱性长石存在于大多数地壳岩石中，但具有复杂的微结构，在过去被证明很难使用。我们认为，通过应用在早期项目中开发的技术，我们可以逐步改变测量地壳岩石中地质过程的速度，从而解决不断变化的环境与固体地球之间相互作用中的一些最重要的过程。

Earth Scientists have measured the age of many of the most important events in Earth history using radioactive decay of common elements such as uranium and potassium. As a result, we know the Earth formed some 4550 million years ago, life emerged around 540 million years ago and a meteorite hit the Earth in Mexico 65.5 million years ago, at the same time as huge volcanic eruptions in India. However, the absolute ages of events are only a small part of the story, the rate at which Earth processes happen is more important for our understanding of how the Earth works. Although it is a common conception that geological processes are slow, a great deal of modern Earth Science is studying processes which take place over timescales considerably shorter that a million years. In fact it is the rates of geological processes which really matter in understanding the interaction of the solid Earth with the oceans and atmosphere, and control our environment. The rates of geological processes are however far more difficult to measure than the absolute ages of events such as volcano eruptions. The particular the processes which challenge even our present state of knowledge include the very processes that have shaped the Earth's surface in the distant past, processes such as fluid flow through rocks, mountain elevation, fault movements, and the rates of sediment flow into deep ocean basins. Quantifying such processes is crucial to understanding the interdependence of our past and present environments with the solid Earth. I the last few years we have developed techniques at The Open University to date very small samples using a technique that involves drilling holes less than one tenth of a millimetre across using a focussed UV laser beam. With this technique we have been able to advance dating of some of the very processes mentioned above. We have measured the heating of rocks during igneous intrusion, the rates of mountain building events, the rates of sediment dispersal, and the ages of fault movements using a range of similar techniques. In particular, a project involving all the members of the present team, measured the rates of ancient fluid flow in hydrocarbon reservoir sandstones. The work was published in the journal 'Science' (Mark et al. 2005), and showed that by combining dates for mineral overgrowths on single sand grains and studying minute inclusions of fluid, we could determine when fluid flowed, for how long and at what temperatures. During this work we realised that the combination of techniques used to understand the mineral overgrowths could also be applied to improve our understanding of other alkali feldspars which are found in the majority of crustal rocks but have complex microstructure and have proved difficult to use in the past. We believe that by applying techniques developed during the earlier project we can make a step change in measuring the rates of geological processes in crustal rocks and thus address some of the most important processes in the interaction between the changing environment and the solid Earth.