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

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.

Who will benefit from this research?

Although the experiments, analytical procedures and results are not expected to make any major social or economic impact to the non-academic community the UK, we feel that this short project provides an excellent opportunity to shoot and produce two 5 minute "Life in the Lab" videos of our activities in the experimental and analytical labs at UCL and the OU.

The videos will be used in numerous ways - school and science festival outreach, advertisement for research students and for university teaching and research advertising purposes. The impact beneficiaries will therefore be the students who are involved in the making of these videos, colleagues involved in outreach at our respective universities, prospective students thinking about studying Earth Sciences at undergraduate and postgraduate level at our institutions and in general, and the general public who are interested in the kinds of activities that "real scientists" get involved with and where their tax money is going.

How will they benefit from this research?

This project will provide the students with insight into "life in the lab" at two different universities, as well as training them in science communication using audio-visual media. In this way, undergraduate students will both gain insight into academic research life, and develop the science communication skills that are becoming increasingly sought-after by employers and institutions.

As inter-disciplinary collaborative ventures are progressively becoming important, these short videos provide a method of communicating our research with colleagues in ways that will generate discussion and innovation. The project will also provide re-usable outreach and teaching content for schools, UCL and the OU, and will be available to a wide audience through the internet.

Prospective students will benefit by gaining an insight into the infrastructure, research environment and research outputs at UCL and the OU, as well as research-intensive institutions in general.

By showcasing these videos at local events, UCL and the OU will be able to engage with their local neighbours, allowing an insight into the kinds of research that their "local university" is engaged in.