The complicating effects of galactic chemical evolution for age determinations have been largely ignored in our discussions. The basic equations on which such studies are based are those shown in Section 3. There exists a very substantial literature concerning chemical evolution effects on age dating, including considerations of varied prescriptions for the star formation history, and of the consequences of infall and outflow of gas from the star forming regions. This literature has been reviewed most recently by Cowan, Thielemann, & Truran (1991a, b), and their Table 3 summarizes the galactic ages obtained from both r-process and other chronometers, by a variety of authors. These, again, are generally quite model-dependent age determinations.
Meyer & Schramm (1986), extending the early work by Schramm & Wasserburg (1970) sought to provide a `model independent' age determination. They noted that the general expression we have derived for Ni(T) is dependent upon the effective nucleosynthesis rate e, such that any age determination requires information about galactic chemical evolution. In an attempt to minimize such dependence, they choose to expand the equation for Ni(T) in moments of the production function about the mean age tm given by (Tinsley 1980)
In the limit of long-lived chronometers (T << 1), they derive a simple expression for the age that is approximately independent of galactic evolution effects. Proceeding in this manner, they obtained a `nearly model-independent' range for TGalaxy of 8.7 TGalaxy 28.1 Gyr. In fact, this range is quite compatible with the different age determinations by other authors. That is, model dependent age estimates, with parameters that otherwise fit general galactic evolution abundance trends, yield ages in the range ~ 10-20 Gyr.
In general, the observational and theoretical considerations briefly reviewed in this paper allow the following conclusions: