Feasibility of chemical U–Th–total Pb baddeleyite dating by electron microprobe

Abstract

We evaluate the potential for applying the electron microprobe (EM) chemical dating technique to the mineral baddeleyite (ZrO 2), an accessory phase commonly used to determine crystallization ages of mafic rocks in conventional isotopic U–Pb studies. Baddeleyite is well suited for EM chemical dating because it has relatively high concentrations of U, negligible common Pb, and is highly resistant to Pb loss. EM chemical and isotope dilution U–Pb ages are presented for five baddeleyite reference samples, including the 780 Ma Faber Lake gabbro sheet (Great Bear Magmatic Zone, Canada), the 1109 Ma Moore Lakes gabbro (Churchill Province, Canada), the 1269 Ma Muskox intrusion (Slave Craton, Canada), the 2060 Ma Phalaborwa carbonatite (S. Africa), and the 2410 Ma Binneringie gabbro dyke (Yilgarn Craton, Australia). Overall, the EM chemical ages show good correlation with the conventional U–Pb ages. Three of the reference samples yielded EM ages within ∼30–60 Ma of the conventional ages, well within analytical uncertainty. For reasons that are not known, two of the samples yielded EM chemical ages that are systematically old by ∼140 Ma. We conclude from these data that the EM chemical dating technique provides reasonable first-order age information for typical Precambrian baddeleyite. The systematic variation of U and Pb at the micron scale in baddeleyite, coupled with the excellent spatial resolution of the EM (∼1 μm), allows for determination of chemical U–Th–total Pb isochrons from individual baddeleyite crystals in polished thin section. EM chemical dating offers a non-destructive, time efficient (a matter of hours for each sample), and cost effective way to obtain contextural age information from baddeleyite in mafic rocks with a grain size too small for isolation and conventional U–Pb dating (<20 μm). In reconnaissance geochronological investigations of Precambrian mafic dyke swarms, a limited number of high-precision U–Pb ages could be used in combination with a larger number of EM ages to obtain robust age constraints on a vast number of dykes.