Quartz shielding of sub-10μm zircons from radiation damage-enhanced Pb loss: An example from a metamorphosed mafic dike, northwestern Wyoming craton

• Published in: Earth and planetary science letters 2012-07, Vol.339-340, p.57-66
• Main Authors: Ault, Alexis K., Flowers, Rebecca M., Mahan, Kevin H.
• Format: Article
• Language: English
• Subjects: Cratons; Damage; Dikes; in situ SIMS U–Pb geochronology; Lead (metal); mafic dike; Pb loss; Quartz; radiation damage; Secondary ion mass spectrometry; shielding; Uranium; Zircon
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2012.04.025

The coupling of two new approaches enabled acquisition of in situ U–Pb data from zircons as small as 5μm in a metamorphosed mafic dike from the Northern Madison Range, Montana. Despite negligible zircon yield by mineral separation, automated mineralogy rapidly identified >375 sub-20μm zircons in a single thin section. Subsequently, zircon crystals were dated using modifications to the conventional SIMS technique to preferentially collect secondary ions emitted from a domain a few microns in size within the ∼20μm diameter analysis pit. This approach allowed analysis of zircons too small to be dated by standard SIMS and TIMS methods. U–Pb data define a discordia array with upper and lower intercepts of 1753±9Ma and 63±8Ma, respectively (1σ error, MSWD=1.5). We interpret the upper intercept to reflect zircon growth during high-temperature and high-pressure metamorphism (800°C, 1.2GPa) based on textural relationships between the dated zircons and the peak metamorphic assemblage. The lower intercept is attributable to the thermal pulse associated with emplacement of the nearby ca. 75Ma Tobacco Root batholith. Percent discordance is linked with both textural setting and U concentration. Zircons located along grain boundaries or within fractured host grains display a positive correlation between U (1049–2817ppm) and percent discordance (12–82%) that is consistent with radiation damage-enhanced Pb loss. In contrast, three zircons housed completely within unfractured quartz yield the most concordant analyses of the dataset (≤7% discordant), despite U concentrations comparable to highly discordant matrix grains. This relationship suggests that the included zircons were shielded from Pb loss by the encapsulating quartz crystals. The results imply that targeting zircons located completely within unfractured host phases may aid in isolating earlier portions of geologic histories. The primary geological implication of this dataset is to increase the documented extent of Paleoproterozoic high-grade metamorphism in the northwestern Wyoming craton. ► Used automated mineralogy and refined SIMS U–Pb geochronology. ► Date mafic dike zircons small as 5μm in situ; U–Pb data define discordia array. ► Matrix zircons are discordant due to radiation damage-enhanced Pb loss. ► Zircons in unfractured quartz are concordant due to shielding from Pb loss. ► Document high-pressure, high-temperature event at ca. 1.75Ga in NW Wyoming craton.