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.
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.
The advantage of two independent chronometers in the same mineral is that it is possible to detect small amounts of open system behavior such as Pb loss or the inheritance of an older mineral.
This is a major factor in our ability to make reliable, high-precision age determinations as we can evaluate whether a number of analyses represents a single time of mineral growth. Although all three dates can be calculated from most published analyses, the relative precisions are related to the analytical technique employed (see section 5 for further discussion of age uncertainties).
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.
The different techniques types discussed in more detail below.
Isotope dilution where minerals are dissolved in the presense of tracer isotopes Two separate dates for a zircon based on each individual decay scheme may be calculated and plotted on a concordia diagram (Fig. On a conventional (Wetherill) concordia diagram the X and Y axis are the U ratios respectively, and the concordia curve represents the simultaneous solution of the decay equations for a given age.
This method effectively “mines out” or preferentially dissolves the higher U parts of the zircon that have been damaged by radiation and are thus susceptible to fast-pathway diffusion of Pb from the zircon crystal.
This method seems to offer the promise of the effective elimination of open system behaviour in most zircon.
Where this regression (or discordia) crosses the concordia at its lower-age intercept is interpreted as the age at which the mineral began to accumulate radiogenic-Pb (or became a closed system).