Towards a unified theory for element uptake by minerals over the full range of element concentrations

点击翻译按钮获取中文摘要

Element mobility in the Earth drives global geochemical cycles and concentrates elements into ore deposits. This mobility can rarely be measured directly, especially not for the deep Earth or the earliest history of our planet. Instead, it has to be reconstructed from the rock record, in particular from the chemistry of minerals that formed in the process of interest. Despite huge progress in developing tools to read the mineral record, there is still major ambiguity in derived data because of a lack of understanding in how minerals get their composition. Indeed, no model correctly predicts the uptake of elements by minerals from the tens to thousands of mg/kg. At trace levels, Lattice-Strain Theory (LST) provides a framework to predict the uptake of elements from melts and fluids, showing it to be controlled by an element's mismatch in the crystal lattice in terms of size and charge. However, LST is only valid at "infinite dilution" and strong deviations are observed at higher concentrations. As a result, LST does not provide meaningful concentration data in modeling ore forming systems to their extreme enrichment. Thermodynamic solid solution models allow for modeling of element incorporation at high concentrations from the Gibbs free energy of element substitutions (e.g. Ca+Al = Na+Si in plagioclase). However, uncertainties in thermodynamic data do not permit extrapolations to low concentrations. There is therefore an urgent need for a model to predict element uptake at all concentrations. This research program aims to develop a unified theory to explain and predict element uptake over the full range of concentrations by gaining fundamental insights into how elements are accommodated in minerals. Experiments will be conducted to characterise the response of mineral lattices to mismatching element incorporation. This will be combined with direct measurements of crystal lattice dimensions and elasticity, and with predictive modelling of the local lattice around mismatching elements at the atom-scale by atomistic simulations. The elemental and isotopic composition of minerals contains a treasure trove of information on pressure, temperature, age, and element mobility in the minerals' growth environment. The fundamental insights developed in this research program will permit us to extract this information without the ambiguity that currently cripples this approach. This allows us to address key material and geoscience questions, from subduction zone element cycling, to the compositions of the Early Earth crust and oceans, to ore formation. Moreover, it has direct applications in industry where the desired behaviour of a material, for example as a catalyst, is directly linked to the abundance of functional elements. This research thereby addresses direct concerns to the Canadian general public, including developing novel materials for green technologies, remediating waste, addressing dwindling natural resources, and the impact of climate change.