Zircon Recrystallization History as a Function of the U-Content and Its Geochronologic Implications: Empirical Facts on Zircons from Romanian Carpathians and Dobrogea

Abstract

IntroductionAccording to Davies et al. (2003), "…the age of Earth and the time scale of pre-human events are central to a civilization's sense of origin and purpose. Therefore, the quest for precise and reliable geochronometers has had a scientific and cultural importance that few other enterprises can match". In this respect, since the beginning of the last century it has been recognized that long-lived radioactive decay systems provide the only valid means of quantifying geologic time.One of the most reliable Earth's timekeeper has proven to be the mineral zircon, since it records the ages of Earth's earliest evolution stages, the oldest sediments, extinction episodes, mountain-building events and supercontinents' coalescence and dispersal (e.g. Rubatto and Hermann, 2007;Harley et al., 2007). Its widespread use in geochronology is based on the decay of uranium (U) and thorium (Th) to lead (Pb). They provide three distinct radioactive decay series involving the parent isotopes 238 U, 235 U and 232 Th and their daughter isotopes 206 Pb, 207 Pb and 208 Pb, respectively. Through the incorporation of U and Th at the time of growth, every zircon grain hosts three different clocks. In an ideal closed system, the three estimates would agree with each other within the analytical errors of measurements. However, in natural systems the zircon grains are not equally closed for Th and U with respect to post-crystallization effects. The usual approach in zircon geochronology is to consider the U-Pb system alone, as there is no natural non-nuclear ways of fractionating 235 U from 238 U. As the modern day-ratio of 235 U/ 238 U is well known (1/137.88) the need to actually analyze very low abundances of 235 U is obviated. Besides U and Th, zircon can incorporate some other incompatible elements such as P, Sc, Nb, Hf, Ti, and REE in trace (up to thousands of ppm) or minor (up to 3% wt) amounts. The primary control factor on the substitutions is the ionic radii of the substituting cations compared with Zr 4+ and Si 4+ cations. Substitutions that minimize strain effects on either or both sites will be favored. The crystal-chemical limitations are that Zr 4+ in 8-fold coordination has an ionic radius of 84*10 -3 nm and Si 4+ , in tetrahedral coordination, has an ionic radius of 26*10 -3 nm. U 4+ (ionic radius of 10*10 -2 nm in 8-fold coordination) and Th (105*10 -3 nm in 8-fold coordination) can be accommodated in the Zr 4+ sites. Uranium concentrations are usually www.intechopen.com Recrystallization 304 less than 5000 ppm and Th concentration less than 1000 ppm. Because of its ionic radius of 129*10 -3 nm (8-fold coordination), Pb 2+ is highly incompatible with growing zircon crystal lattice and therefore is not incorporated more than ppb levels, which is crucial in geochronology. Because of the same reason, Pb 2+ can easily escape from zircon lattice when some conditions are fulfilled.Based on idea of concordant ages between 235 U/ 207 Pb and 238 U/ 206 Pb, Wetherill (1956) has developed the Concordia diagram. Quite soon thou...