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An additional factor that makes zircon a robust chronometer is its high closure temperature (900°C) to Pb diffusion (Cherniak and Watson, 2003), or the temperature below which U and Pb do not undergo significant thermally activated volume diffusion.This means that zircons tend to preserve their primary ages even in volcanic rocks metamorphosed to amphibolite-facies conditions.Much of this understanding comes from experimental work in the realms of crystallography, mineral chemistry, and diffusion.A good geochronometer must either have little to no initial incorporation of the daughter isotope or must have a fixed ratio from which the radiogenic and nonradiogenic proportions can be determined.The amount of time required for half of a given quantity of a parent radioactive element to decay into the daughter product is referred to as the half-life.It is important however, that the half-life of an element is defined in terms of probability and is not the time required for exactly 50% of a given quantity to decay (e.g.The relatively high Th and U content allow for monazite to be dated using Th-Pb and U-Pb decay schemes.

Therefore at the time of crystallization (t0) there is effectively no Pb present in a crystal (although mineral and fluid inclusions may contain Pb) and the present day Pb is the direct product of in-situ U decay since t0 (see section 3.1 for further details).In order to calculate a date using the age equation it is required to determine the ratio of the parent isotope (P) to it's respective daughter isotope (D).This requires quantitative determination of the relative proportions of both P and D isotopes, and often requires correction for the inclusion of non-radiogenic D.Additionally, several workers have documented metamorphic titanite that preserves multiple generations of mineral growth thus highlighting the importance for detail chemical imaging and multiple analyses per grain to fully characterize the history of mineral growth.Monazite ([Ce, La, Nd, Sm, Gd, Th]PO4) is phosphate mineral common in metamorphic and igneous rocks.




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