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However, similar significant differences related to orientation were not detected in zircon or monazite crystals (within the analytical statistics).

Nevertheless, monazite crystals contain random sub-microscopic blotchy patches that can vary up to 700 Ma in "age," Clearly, the results of U-Th-Pb mineral dating are highly dependent on the investigator's interpretations.

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Zircon geochronology is a critical tool for establishing geologic ages and time scales of processes in the Earth's crust.

However, for zircons compromised by open system behavior, achieving robust dates can be difficult.

Radiogenic Pb is easily lost by diffusion from some crystals and the process is accelerated by heat, water, radiation damage, and weathering, while in other crystals it is inherited in excess. Williams, "Pb-Loss Patterns in Zircons from a High-Grade Metamorphic Terrain as Revealed by Different Dating Methods: U-Pb and Pb-Pb Ages for Igneous and Metamorphic Zircons from Northern Sri Lanka," A Cocherie, O.

Apparent ages vary significantly within crystals at sub-microscopic scales, and on different crystal faces and at different crystal orientations.

Chemical abrasion (CA) is a routine step prior to thermal ionization mass spectrometry (TIMS) dating of zircon to remove radiation-damaged parts of grains that may have experienced open system behavior and loss of radiogenic Pb.

While this technique has been shown to improve the accuracy and precision of TIMS dating, its application to high-spatial resolution dating methods, such as secondary ion mass spectrometry (SIMS), is relatively uncommon.

For instance, the classical Discordia line was discovered by Ahrens (1955) in Zimbabwe. To interpret the Discordia line, conventional theories have proposed: 1) this line was caused by Pb loss or U gain after formation of the host mineral [9] [11] -[17] , 2) the upper intersection of the Discordia and Concordia lines represents the crystallization age of the mineral [12] and 3) the lower intersection of the Discordia and Concordia lines represents the metamorphic age of the mineral [14] .

However, previous theories are not tenable when used in the following cases:1) the lower intercept point is negative or2) no upper intercept point exists. (2012) (Figure 1), all zircons in YX1 from Yingxian lamproites were found to be discordant and yielded a lower intercept age of −370 ± 690 Ma.

In our efforts to U-Pb date zircons from the late Eocene Caetano caldera by SIMS (SHRIMP-RG: sensitive high resolution ion microprobe, reverse geometry), some grains yielded anomalously young U-Pb ages that implicated Pb-loss and motivated us to investigate with a comparative CA and non-CA dating study.

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