The most recent and dramatic example of the extraordinary ability of zircon to retain its U-Pb systematics is from studies of zircons from the Chicxulub impact structure of the Yucatan Peninsula 58 в The shocked zircons were exhumed from the Chicxulub basement rock during meteorite impact and dispersed in the fine dust of the impact cloud.
Discordancies in the U-Pb systematics e. Uraniumвlead dating studies of shocked zircons in the fine-grained ejecta deposited in areas as wide apart as Colorado, Saskatchewan, and Haiti at the Cretaceous-Tertiary boundary have a predominant age of Ma, in agreement with dates for shocked zircons from the Chicxulub crater 59 в Only a material of remarkable durability, such as zircon, could preserve the isotopic signature of an event of such extreme conditions Discordant ages are common, mainly because of Pb-loss and less frequently because of U-loss.
Additionally, under extreme geologic conditions, e. Recent studies have investigated the incorporation of Pb into zircon 66 and the mechanisms by which Pb may be lost In summary, there are abundant data from geochronologic studies to show that natural zircons can quantitatively retain Pb for billions of years in the absence of episodic, thermally induced Pb loss The minor alteration of zircon over long periods of time and under rather extreme conditions stands in contrast to the observations of other potential ceramic nuclear waste forms such as pyrochlore 68 for which the data of the type summarized above are simply not presently available.
Elements may preferentially segregate into aperiodic, damaged regions that have inherently higher solubilities than crystalline ceramics 70and the annealing kinetics of recrystallization of damaged domains may substantially change the response of a material to the damage accumulation process The development of a waste form for weapons Pu requires a knowledge of diffusion rates of the actinides, as well as elements with high neutron-capture cross-sections to control criticality e.
Natural zircons can contain up to several thousand parts per million rare earths, including up to ppm of Gd, and zircon exhibits nearly complete solid solution with hafnon, HfSiO 4. Thus, neutron absorbing nuclides may be incorporated into the zircon structure; however, the neutron absorber must remain within atomic scale proximity of the fissile radionuclide Pu or U. This is why fine-scale chemical zoning and isotopic signatures are preserved in the inherited cores of zircon crystals that have experienced protracted thermal events in boat tachometer hook up past history.
In contrast, the rare-earth elements REE diffuse at rates 4в5 orders of magnitude faster than the tetravalent cations Other phases that have been suggested as waste form phases, e.
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