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.
This has therefore become the principal material used in UPb to yield the radiogenic fractions.
For zircons with very low common Pb contents, an adequate correction may be made by estimating common Pb from a general terrestrial Pb evolution model (e.g.
Zircon rim concordia ages derived from the U–Pb age maps are consistent with independent secondary ion mass spectrometry (SIMS) and thermal ionisation mass spectrometry (TIMS) constraints.
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.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.When compositions yielding such concordant ages are plotted graphically (Fig.5.4) they define a curve which was termed the concordia by Wetherill (1956a).These situations are enigmatic, given the dramatic effect of similar temperatures during contact metamorphism.