Introduction to Geochronology/Geochronometers
Radioactive decay schemes are suitable for dating minerals and rocks and are listed in Table 1. All of these systems are based upon the radioactive decay of a parent nuclide to a stable daughter nuclide. Obtaining accurate information from these decay systems for the purposes of determining the age of a mineral or rock requires: (1) the decay constant of the parent nuclide is accurately and precisely determined; (2) closed system behaviour, which can be simply stated to mean that the Parent/daughter ratio has only changed by radioactive decay; and (3) the initial daughter nuclide, if present, can be precisely and accurately accounted for. In this section we outline the basic principles of the various radio-isotopic geochronometers, differentiating the U-Pb system applied to U-bearing accessory minerals from the isochron geochronometers (Re-Os, Lu-Hf, Pb-Pb etc.) applied to chemical precipitates and organic residues.
Zircon (ZrSiO4) is a common accessory mineral in silicic volcanic rocks ranging from lavas to air-fall tuffs to volcaniclastic sedimentary rocks and is a nearly ubiquitous component of most clastic sedimentary rocks. The refractory and durable nature of zircon over a wide range of geological conditions means that it is likely to retain its primary crystallization age even through subsequent metamorphic events. Silicic air-fall tuffs are the most common volcanic rocks in fossil-bearing sequences and are found in layers that range in thickness from a millimeter to many meters and are commonly preserved in marine settings. In most of these rocks the primary volcanic ash has been altered, probably soon after deposition, to clay minerals in a process that does not affect zircon.
Zircon is ideal for U-Pb dating because U has a similar charge and ionic radius to Zr it substitutes readily into the zircon crystal structure (in modest amounts, typically in the 10’s to hundreds of parts per million (ppm) range) whereas Pb has a different charge and larger ionic radius leading to its effective exclusion from the crystal lattice. 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). 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.
Sanidine (a high temperature form of potassium feldspar (K,Na)(Si,Al)4O8))