The effect of chemical and physical imperfections in zircon grains in influencing the U-Pb age analyses: Insights from zircon fission track etching

AVANÇOS METODOLÓGICOS NA DATAÇÃO POR TRAÇOS DE FISSÃO E U-Pb EM ZIRCÃO

“…Regarding the isotopic characterization, the ages 207 Pb/ 235 U and 206 Pb/ 238 U are discordant in A and concordant in B. This discrepancy may be attributed to: i) a mineral inclusion in the ablated region; ii) mineral phases and inclusions in the grain structure that can cause the opening of the U-Pb system and iii) the leaching and metamictization phenomena that create a drastically damaged crystallographic area in the mineral, which opens the U-P system (Resende et al, 2019). Therefore, the FCT sample can not be used as standard for the U-Pb dating method.…”

“…The behaviour of a closed system can be analyzed using a Concordia diagram, which provides additional information about the geological history of minerals when quantifying the behaviour of an open system (Schoene, 2014 U-Pb radiometric dating was performed on zircon grains belonging to the Fish Canyon Tuff (FCT) sample after the etching process in micro-areas with and without fission tracks (Resende et al, 2019). According to Curvo et al (2013) the chemical etching used in FTM dating process helps to better position the laser on the grain surface to avoid reaching regions with mineralogical mixtures, thus improving the closeness of agreement between the U-Pb ages.…”

“…According to Curvo et al (2013) the chemical etching used in FTM dating process helps to better position the laser on the grain surface to avoid reaching regions with mineralogical mixtures, thus improving the closeness of agreement between the U-Pb ages. The FCT sample comes from a pyroclastic rock formed from an intense volcanic extrusion during the Tertiary (~30 Ma) in the volcanic mountains in San Juan during the collapse of La Garita Boiler, United States, and since then, it has not suffered any reactivation (Resende et al, 2019). Heavy minerals, zircon and apatite, from the FCT sample are used as natural standards for FTM dating.…”

“…In cases the U-Pb ages are discordant in the same grain, they can be misinterpreted as distinct ages of metamorphic (re)crystallization (Resende et al, 2019). In fact, this may be related to lack of preserved crystalline structure of the mineral, which leads to the opening of the U-Pb isotopic system.…”