Erroneous determination of the duration of metamorphism from analysis of overlapping pre-sputtered areas during SIMS U–Pb dating of zircon

Abstract

The duration of geological events has distinct implications for geological processes. Secondary ion mass spectrometry (SIMS) U–Pb dating of inherited zircon cores and metamorphic rims is the most common method used to determine the age of a protolith, and the timing and duration of metamorphism. This study investigates the age deviation induced by overlapping pre-sputtered areas during SIMS zircon core–rim U–Pb dating, using eclogite and country-rock granitic gneiss samples from the Shuanghe area in the Dabie Orogen, China. First, we analyzed only the zircon core or rim on each grain. Concordant ages of igneous zircon cores in two orthogneisses are 744 ± 10 and 768 ± 7 Ma, demonstrating that their protoliths were Neoproterozoic igneous rocks. The metamorphic zircon rims in the two granitic gneisses yielded a consistent weighted-mean 206Pb/238U age of 216 ± 2 Ma (2 SE; n = 12 and 17). Zircon grains in the studied eclogite are all metamorphic and yielded a weighted-mean 206Pb/238U age of 224 ± 2 Ma (2 SE; n = 21). The different metamorphic ages of the eclogite and orthogneisses suggest that they might be juxtaposed by later tectonic events. After being repolished, zircon grains with apparent core–rim structures from the two granitic gneisses were re-analyzed in the cores and rims of each grain. Due to the small size of the zircon grains, the pre-sputtered areas of the two spots in each zircon overlapped. Compared with the analyses in the first session, the overlapping pre-sputtered areas affacted apparent dates for zircon rims by −8.2% to +13.2%. The age deviations of the second analyses correlate with the relative position of the overlaps. The apparent 206Pb/238U dates are younger when delta X < 0 (i.e., the X coordinate of the second analysis is smaller than the first analysis), while the apparent 206Pb/238U dates are older when delta X > 0. This study proves that erroneous SIMS U–Pb dates can be produced when dating metamorphic zircon rims after analyses of the cores with overlapping pre-sputtered areas. It will lead to an apparently longer and incorrect duration of a metamorphic event.