Map and table showing isotopic age data in Alaska

Abstract

The source of the data reported here is a compilation of radiometric ages maintained in conjunction with the Alaska Mineral Resource Assessment Program (AMRAP) studies for Alaska. The symbol shape plotted at each location is coded for rock type, whether igneous, metamorphic, or other; the color of the symbol shows the geologic era or period for the Sample(s) at each locale. A list of references for each quadrangle is given to enable the user to find specific information including analytical data for each sample dated within a particular quadrangle. At the scale of this map, the very large number of Samples and the clustering of the samples in limited areas prevented the showing of individual sample numbers on the map. Synthesis and interpretation of any data set requires the user to evaluate the reliability or value of each component of the data set with respect to his or her intended use of the data. For geochronological data, this evaluation must be based on both analytical and geological criteria. Most age determinations are published with calculated estimates of analytical precision, Replicate analyses are infrequently performed; therefore, reported analytical precision is based on estimates of the precision of various components of the analysis and often on an intuitive factor to cover components that may have not been considered. Analytical accuracy is somewhat more difficult to determine; it is not only dependent on the actual measurement, it is also concerned with uncertainties in decay and abundance constants, uncertainties in the isotopic composition and size of the tracer for conventional K-Ar ages, and uncertainties in the Original isotopic composition of the sample, Geologic accuracy of a date is Variable; the interpretation of the meaning of an age determination, is important in the evaluation of its geologic accuracy. Potassium-argon, rubidium-strontium, and uranium-lead age determinations on a single sample can differ widely yet none or all may be wrong Given that the basic Conditions of each dating method were met, each method determines an age based on the equilibration of its particular isotopic system, yet these are different systems and they react to heat, pressure, and recrystallization in different ways. This map is a compilation and not a synthesis or interpretation. Its purpose is to help the user determine the dating coverage of areas of Alaska and gain access to the available data for the state or a project area. Interpretation of that data and evaluation of its suitability for use with any particular project is left to the user. Compilations, with sample data, have been published for much of the state; and are as follows: Wilson, and others (1979), southeastern Alaska; Wilson (1981), Aleutian Islands and Alaska Peninsula, Shew and Wilson (1981), southwestern Alaska; Wilson and others (1985), Yukon Crystalline terrane; Grybeck and others (1977), northern Alaska; Dadisman (1980), south-central Alaska.