The mechanism of the alteration of metamict zircon in aqueous solutions: 2H and 18O tracer experiments
Zusammenfassung der Projektergebnisse
Zircon is one of the most important mineral in geochronology and has been proposed as a nuclear waste form to safely encapsulate weapons-grade plutonium. However, actinlde-bearing zircon becomes increasingly radiation-damaged with time, resulting in an unusually open microstructure that is assumed to make radiationdamaged zircon prone to aqueous alteration. To predict long-term behaviour as nuclear waste containment and/or the reliability of geochemical data derived from radiation-damaged and hydrothermally altered zircon, the underlying reaction mechanism of the re-crystallisation process needs to be known. Here we report results from oxygen and hydrogen isotope hydrothermal tracer experiments with a heavily radiation-damaged zircon crystal from Sri Lanka. The experiments were conducted in 18-O/2H-enriched water and in an enriched 0.1 M HCl solution, as well as in milliQ water at temperatures between 100 and 700 °C and pressures ranging from 1 to 1.7 kbar for 1 to 141 days. Zircon grains treated in the 18-O-enriched solutions show up to two distinct alteration zones of re-crystallized zircon as revealed by different contrasts in BSE/CL imaging. Furthermore, Raman spectroscopic measurements from the secondary alteration zones reveal a significant red-shift of the v3(SiO4) band when compared to hydrothermally altered grains from previous studies using non-enriched solutions and dry annealing experiments, indicating a mass-related frequency shift of the v3(SiO4) band due to the incorporation of significant amounts of 18-O from solution. NanoSIMS analyses verify the increase in heavy oxygen isotopes in these areas. The Raman measurements also indicate temperature-dependent structural recovery processes inside the altered areas which are accompanied by partial loss of radiogenic Pb, as revealed by SHRIMP analyses. The occurrence of two distinct alteration zones with different 18-O/16-O ratios cannot be explained by a dissolution-reprecipitation process, but rather by a diffusion-controlled process causing re-crystallisation of radiation-damaged zircons during aqueous alteration. Surprisingly, we were not able to detect significant amounts of heavy hydrogen species within the altered zircons by NanoSIMS measurements. The oxygen isotope composition of zircon is often used to unravel its (pre-)magmatic history, including the presence of liquid water on the surface of the Hadean Earth. Recent discussion about whether zircons retain their primary oxygen isotope signature during alteration and metamorphism will be stirred up by our experimental results. SIMS analyses of samples treated in milliQ water indicate an oxygen isotope exchange between external fiuid and metamict zircon, which means that the oxygen isotope information about the pristine zircon and its host rock will be lost permanently during alteration.
