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

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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.