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 behavior, 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.
Fractionation of Parent and Daughter
In order to be able to quantitatively determine the ratio of the parent nuclide (P) to a stable daughter nuclide (D) we need to analyses materials that have high proportions of P relative to D at their formation such that the ingrowth of D overwhelms that any amount of initial D isotope
Robust, Understandable Mineral/Rock System
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.
Titanite (CaTiSiO5) is a common accessory magmatic mineral in intermediate and felsic igneous rocks. It also occurs in metamorphic rocks. Occasionally detrital titanite can be found in unmetamorphosed sedimentary rocks. Titanite is most commonly used to date the cooling of metamorphic events using U-Pb isotopes. Although it can accommodate several 100’s of ppm U in the crystal lattice, it also incorporates variable amounts of Pb. Often this results in several analyses of the same unzoned titanite grain that define a discordia (on a Tera-Wasserburg concordia diagram) anchored with a 207Pb/206Pb ratio of common-Pb on one end and the closure age. 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. The relatively high Th and U content allow for monazite to be dated using Th-Pb and U-Pb decay schemes. Like titanite, monazite also incorporates variable amounts of Pb. Monazite is commonly zoned which is generally visualized by trace element variations (e.g. Yb, U, Ca). These zonations can preserve growth events separated by billions of years of Earth history.
Sanidine (a high temperature form of potassium feldspar (K,Na)(Si,Al)4O8))