Dating zircon grains
These situations are enigmatic, given the dramatic effect of similar temperatures during contact metamorphism.
In some published studies, the inherited zircons are 5-10 times "older" than those matching the accepted ages of granites—1753 Ma in a 21 Ma Himalayan granite found unsupported (or excess) radiogenic Pb in a zircon crystal in an Antarctic gneiss, identified as such because the radiogenic Pb thus produced anomalously high "ages." Similar situations also result in "ages" hundreds of millions of years more than expected and are interpreted as due to excess radiogenic Pb, the origin of which is either explained as mixing from older source materials and/or due to migration as a result of fluids, temperature, and pressure.
Isotopic ratios were also measured on the same crystal faces of 47 baddeleyite crystals but at different orientations over a 180° range, revealing a striking approximately sinusoidal variation with orientation in U apparent ages.
In this way, even different growth zones in individual crystals can be analyzed and thus "dated." An alternative procedure is to take all the zircon grains liberated from a rock sample, and if they are of uniform composition, chemically digest them into solution for standard mass spectrometer analysis.
This dating method has become very popular for dealing with Precambrian terranes where it can often be difficult to resolve relationships between rock units and the geological history. It must be assumed that when the zircon grains crystallized, no radiogenic Pb was in them, and that all the radiogenic Pb now measured was derived by radioactive decay from U and Th.
In the laboratory, rock samples are crushed and the zircon grains are separated from the other minerals by heavy liquid and other mineral separation techniques.
After being mounted, the crystals can be analyzed using an instrument such as a SHRIMP (Sensitive High mass Resolution Ion Micro Probe) which focuses a very narrow ion beam onto the grains so that mass spectrometers can measure the ratios of the isotopes vaporized from the targeted spot.
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.
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.