Dating zircon grains
However, there are several lines of evidence that indicate radiogenic Pb can be inherited during crystallization of the mineral grains, and that open-system behavior is common, with radiogenic Pb lost by diffusion due to the way the Pb is held in the crystal lattice.Even as early as 1960 Tilton reported that Pb diffuses from zircon and U-bearing minerals at temperatures as low as 50°C. This is dramatically illustrated by the contact metamorphic effects of a Tertiary granite stock on zircon crystals in surrounding regionally metamorphosed Precambrian sediments and volcanics.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. 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.
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.
demonstrated that unzoned crystals can be the result of recrystallization of zoned crystals accompanied by loss of U, Th, and Pb, and "resetting" of the U-Pb "ages." Such recrystallization can be due to subsequent regional metamorphism. found that high-grade metamorphism of granitic and related rocks reduced their U-Pb zircon "ages" from 1000 Ma down to 540 Ma, with zircons even from a single sample yielding U-Pb "ages" between 1072 Ma and 539 Ma. Yet another significant problem for zircon U-Pb "dating" is the discovery in some metamorphic and granitic rocks of zircon crystals that yield much older "ages" than the accepted ages of the rocks.
In the case of metamorphic rocks this has been interpreted as inheritance of those zircon grains from the original sources of the sediments, the zircons somehow surviving metamorphism without resetting of the U-Pb isotopic system.
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.