THERMAL HISTORIES FROM THERMOCHRONOLOGY: WHERE DO WE STAND?
The kinetics and systematics of noble-gas diffusion and fission-track annealing are well known and there is no doubt that temperature is the major control on measured mineral ages. To first order, it is possible to invert such ages for thermal information with confidence, so long as geological context and the role of other mineralogical processes is kept in mind. Using multiple methods, it is now possible to constrain temperatures over the range 40˚C to over 500 ˚C, and in many cases extract continuous thermal histories rather than point estimates.
Fifty years on, the question at this point is the degree of accuracy and precision with which we can now practice thermochronology. Using data from an ongoing intercomparison study at the classic Little Devil’s Postpile contact zone in Yosemite National Park, we can illustrate both the sensitivity and broad accuracy of current techniques, as well as raise questions about the relative performance of a number of commonly used thermochronometers, including zircon and apatite (U-Th)/He and fission track and K-feldspar MDD. As an example, in their overall pattern of resetting, apatites from the Cathedral Peak granite provide strong constraints on intrusion geometry, but in detail partial resetting has led to a spread in intra-sample grain ages correlated with eU, as predicted by the RDAAM model.
Beyond fission-track and (U-Th)/He dating of apatite and MDD analysis of some K-feldspars, our community’s database of kinetic information and diffusion behavior is not deep and limits precise determination of paleotemperatures. Resolution of this limitation would greatly increase the precision and robustness of sophisticated inverse models that use thermochronological data. Distribution of laboratory kinetic standards and community maintenance of an intercalibrated kinetic database would be an important next step.