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

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Earth and Planetary Science Letters





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The coupling of two new approaches enabled acquisition of in situ U–Pb data from zircons as small as 5 μm in a metamorphosed mafic dike from the Northern Madison Range, Montana. Despite negligible zircon yield by mineral separation, automated mineralogy rapidly identified >375 sub-20 μm 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 ∼20 μm 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±9 Ma and 63±8 Ma, respectively (1σ error, MSWD=1.5). We interpret the upper intercept to reflect zircon growth during high-temperature and high-pressure metamorphism (800 °C, 1.2 GPa) 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. 75 Ma 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 (1049–2817 ppm) 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.

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