The compositions of Fluids in and on the Earth

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Water in and on the Earth facilitates plate tectonics, controls the redistribution of elements to make ore deposits and determines the availability of (trace) elements essential for life. Knowledge of the compositions of these fluids is therefore crucial. Unfortunately, opportunities to sample and quantify fluids directly are rare, espe- cially for sites deep in the Earth and for its earliest history. However, minerals with preserved compositions are readily available, sampling environments >200 km in depth and back to 4.2 billion years ago. Minerals store a chemical fingerprint of the fluid from which they grow, and if the distribution of elements between mineral and fluid is known, this fingerprint can be used to reconstruct the composition of the fluid. The mineral record is therefore an attractive source of information and in this research program I will establish a methodology to reconstruct fluid compositions from the composition of minerals. The research will use Lattice Strain Theory, which relates the misfit in size and charge of an element in the structure of a mineral to its partition coefficient, where a large misfit limits mineral uptake of the element resulting in a low partition coefficient. This theory allows for trends in partitioning to be modeled and for a predictive model of element distribution to be developed. Partitioning experiments at varying temperature and pressure will be combined with computer simulations to investigate the response of partitioning to changing conditions, and how this links to changes in the geometry and elastic/electric properties of the mineral lattice. The resulting model will be applied to natural samples to determine the compositions of fluids involved in formation of gold deposits, to understand the cycling of elements through the Earth, and to reconstruct the compositions of the Early Earth oceans in which life developed. In fact, this approach is applicable to any environment where minerals grow from a fluid and can therefore also be used to study industrial processes from wastewater treatment to ore mineral processing. Few other techniques are able to provide information on fluid compositions deep within the Earth and back in time, and none are applicable to the range of environments that can be covered by this approach.