Geochemistry, Mineralogy, Volcanology Meeting Report

Better Tools for Tracing the Thermal History of Rocks

Thermo2016: The 15th International Conference on Thermochronology; Maresias, Brazil, 18–23 September 2016

By , Roderick Brown, and Peter Hackspacher

Temperature is an important control on many geological processes like mountain building, fluid flow, and development of natural resources. Over the past 4 decades, the subdiscipline of thermochronology has emerged as an important tool in measuring temperature histories.

Thermochronology extends the reach of conventional geochronology by marrying the decay of naturally occurring radioactive elements and the temperature-sensitive diffusion of their daughter products. Examples include the decay of uranium and thorium to produce helium, lead, and crystal defects called fission tracks.

Superimposed on a micrograph of a fluorapatite crystal is a schematic showing a helium diffusion pathway through the crystal structure.
Understanding factors controlling helium diffusion in minerals was a major theme at Thermo2016. Superimposed on a micrograph of a fluorapatite crystal is a schematic showing a helium diffusion pathway through the crystal structure. Credit: Cécile Gautheron and Jérôme Roques

Last September, more than 130 delegates attended a biennial international thermochronology workshop in the beachside setting of Maresias, Brazil. The workshop focused on developments in the theory, methods, application, and interpretation of thermal history analysis. The agenda included discussions, presentations, and a midweek field trip to facilitate informal debate. Early-career scientists and students were actively engaged in presenting keynote talks and papers. Industrial sponsors and Brazilian academic and scientific organizations provided financial support.

Low-temperature thermochronology was a major focus of the workshop. Breakout sessions facilitated discussions on the need for standardization in reporting data from fission track methods and uranium-thorium/helium (U-Th/He) dating, excess dispersion in U-Th/He single-crystal ages, and requirements for robust thermal history modeling.

These discussions spilled into informal evening debate and culminated in a lively plenary session. One outcome will be the writing of papers recommending standards for data reporting, which for fission track dating requires an update now that new methods of analysis are being used.

Age dispersion (variability in age estimates beyond analytical uncertainty) was the subject of lengthy discussion and was a theme of several talks presented at the workshop. Single-crystal apatite and zircon U-Th/He ages from the same sample are often dispersed well beyond analytical uncertainty, particularly in older samples having complex thermal histories.

Considerable progress has been made in understanding the causes for dispersion, which include variations in diffusion radius, the effect of analyzing broken grains, and modification of helium diffusion due to radiation damage accumulation. Theoretical approaches predict that variations in a mineral’s chemical composition should affect how helium diffuses through the mineral, but workshop participants noted that experimental evidence is lacking. Because these factors systematically alter diffusion, they can provide additional information about thermal history, attendees agreed. This observation led Rod Brown of Glasgow University to comment that “dispersion is beautiful.”

However, less alluring cases still persist in which excess age dispersion remains unexplained. Participants agreed that more research is needed to better understand helium diffusion in natural samples, including the effects of crystal imperfections of all types and scales. This work could also have important implications for noble gas diffusion in other phases and diffusion of geochronologically important daughter products in general, workshop attendees noted. Renewed work on the nature of fission track and alpha-recoil damage was also an emergent theme.

Thermo2016 also hosted a meeting of the International Standing Committee on Thermochronology. The committee moved to solidify its informal status by creating a website that will report on its work and serve as a clearinghouse for the international thermochronology community. The website will also include information about future meetings and awards.

The committee approved the formation of an early-career award in thermochronology, and it presented student prizes for the best talks and posters. The committee awarded the Laslett Prize for fission track dating to Charles Naeser, and Peter Zeitler received the Dodson Prize for thermochronology.

Finally, the committee gave final approval to the organizers of Thermo2018, which will be held in Quedlinburg, Germany, from 16 to 21 September 2018.

—Peter Zeitler (email: [email protected]), Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pa.; Roderick Brown, School of Geographical and Earth Science, University of Glasgow, UK; and Peter Hackspacher, Laboratório de Geoquímica Isotópica, Instituto de Geociencias e Ciencias Exatas, Universidade Estadual Paulista, Rio Claro, São Paulo, Brazil

Citation: Zeitler, P., R. Brown, and P. Hackspacher (2017), Better tools for tracing the thermal history of rocks, Eos, 98, Published on 18 May 2017.
© 2017. The authors. CC BY 3.0