Collaborative Research: Probing zircon reactivity in aqueous solutions at solubility equilibrium using isotope tracers

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The mineral zircon provides an invaluable source of information on Earth's history that extends further back in time than any other minerals (4.1 billion years) due to zircon's extraordinary stability. However, the rock record provides evidence of occasional zircon instability in the Earth's crust and mantle caused by radiation damage and possibly interaction with fluids, resulting in zircon recrystallization. While it has been hypothesized that hot fluids can cause zircon to recrystallize, this hypothesis has not been tested in the laboratory. Because recrystallized zones in zircon can be dated, knowledge of how those zones formed can aid in the interpretation of measured ages, allowing for a more accurate reconstruction of the history of Earth and other planets from which we retrieve zircon-bearing rocks. Knowledge of the mechanisms and conditions of zircon stability should also help evaluate the feasibility of using zircon to safely store highly toxic and radioactive contaminants in geological repositories. STEM workforce training in experimental and analytical techniques will be provided to diverse participants ranging in experience from high school to post-doctoral researchers in laboratories and meeting workshops.This study aims to simulate in the laboratory the process of zircon recrystallization by conducting innovative isotope tracer experiments on zircon-fluid mixtures at high pressures and temperatures. The experimental materials will be characterized using state-of-the-art microanalysis techniques. The following hypotheses will be tested: 1) zircon recrystallizes in aqueous fluid via a dissolution – reprecipitation mechanism that can be detected through measurable Si and O isotope exchange; 2) at the same conditions, small zircon grains will recrystallize, while large grains will not; 3) development of a reaction rim will hinder further reaction, slowing the rate of growth of the reaction rim; 4) large grains of metamict zircon will be replaced by crystalline zircon at a measurable rate, and 5) the replacement of metamict zircon by crystalline zircon proceeds at a constant Si concentration, but the recrystallization rates can be measured with temporal changes in Si and O isotopes.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.