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

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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.