ISOTOPE GEOTHERMOMETERS: APPLICATIONS AND LIMITATIONS

Abstract

This paper considers applications and limitations of the new branch in isotope geochronology that is concerned with the estimation of the temperature conditions of superimposed postcrystallization processes from the analysis of partial loss of radiogenic isotopes. The dependence of the loss on time and temperature allows one to reconstruct the metamorphic history of rocks and minerals. The closure temperature of isotopic systems, Tc (Dodson, 1973), is currently used as the main parameter connecting T-t evolution with the measured apparent age, although this is a secondary parameter. Activation energy (E) and the frequency factor C0 (or diffusion coefficient D0) are more appropriate primary parameters. The kinetic parameters E and D0 are used to define the "diffusion loss constant λd", which controls the character of temperature-time evolution. A combination of kinetic parameters for high-temperature (U-Pb for zircon and monazite) and low-temperature (K-Ar for feldspars) systems embraces practically the whole temperature interval including the peak of metamorphism, retrograde stage, and cooling from ∼1000 to ∼200°C and lower, if fission-track and U-He methods are involved. Examples are presented for the use of thermochronology to reconstruct the T-t history of various regions from Alpine to Archean age. The wide application of thermochronology is strongly hindered by the dependence of kinetic parameters including closure temperature on the fluid regime of metamorphism. There is a need for further E, D, and Tc measurements in various isotopic systems and minerals within a range of experimental P-T-x conditions and in various metamorphic environments (natural) from "dry" to fluid-bearing. At early stages of the application of thermochronological methods, one should choose rocks corresponding to dry metamorphic conditions and the isotopic systems that are least dependent on fluid conditions.