Optimization of the in-situ UePb age dating method via LA-Quadrupole-ICP-MS with applications to the timing of Ue-Zr-Mo mineralization in the Poços de Caldas Alkaline Complex, SE Brazil

Abstract

The high spatial resolution of the LA-ICP-MS systems allows rapid extraction of vital isotopic information from individual growth zones of minerals. This paper describes in detail the optimization of a relatively inexpensive LA-ICP-MS system consisting of a UV 213 Laser Ablation and a Quadrupole ICP-MS. The results of optimization take into account laser energy, beam diameter, frequency and ICP-MS gas conditions. The optimized conditions were tested for precision and accuracy on a number of wellcharacterized zircons, commonly used as primary and secondary quality control standards. The acquisition of the UePb data is carried out in automated mode (pre-set points) for up to 12 h/day with only minimal operator presence. Individual UePb zircon analysis lasts 80 s. The 2s uncertainties of the standards ranged between 1.4 and 8.2%, and overall their relative deviations ranged from 0.02 to 0.87%. The results are comparable to techniques that use more complex and time-consuming approaches such as LA-MC-ICP-MS and ion-microprobe. We have applied this method to obtain ages of numerous granitoid rocks from the Southern S~ ao Francisco Craton and a well-known Archean granitoid of the Kaapvaal Craton, South Africa. We furthermore provide the first results of UePb age dating of UeZreMo mineralization in the Poços de Caldas Alkaline Complex, SE Brazil, with a UePb age of 85 ± 3 Ma for zircon-bearing hydrothermal veins.