Testing the Validity of Muscovite as a Continuous History Thermochronometer

Lehigh University, Bethlehem PA

Investigators

Abstract

Temperature change is important to a rock's chemical evolution. The tectonic deformation and erosion that shape landscapes cause temperature changes that are recorded in rocks. Ore deposit formation and the creation of hydrocarbon accumulations are in part temperature-driven. Over the past 50 years, geologists have developed ways use radioactive decay products found in minerals to decode temperature histories. While decay products accumulate at a well-known regular rate, they can also be lost from minerals at elevated temperatures. Over the life span of a mineral grain, the net remaining decay product and thus the measured mineral age represents the outcome of these competing processes of steady gain and temperature-dependent loss. Studying different minerals with different responses to temperature can be powerful. Researchers need a broader range of alteration-resistant minerals to use for as wide a range of temperatures as possible. This team will determine whether the common white mica can be used for temperature-history studies. This mineral is easily dated by the potassium-argon method, is resistant to weathering, and shows promise in being able to work over the 300 ˚C to 400˚C temperature range, an interval not well covered by other minerals. In addition to presentations and publications, this team will develop outreach videos for students and the general public about mineral dating and how it supports basic research about the Earth. The goal of this project is to develop a new means of determining the temperature histories of crustal rocks across the range 275˚C to 425˚C. Several published reports suggest that 40Ar/39Ar analyses of the mineral muscovite can address this range. To test how useful muscovite might be as not just a dating target but also as a means to measure temperatures during rock cooling, the researchers will carry out diffusion and dating experiments on a suite of well characterized muscovite samples that have known temperature histories that can be compared to predictions from their experiments. They will also choose samples for reheating experiments designed to examine the response of muscovite samples to shorter timescale, higher-temperature thermal pulses. What makes muscovite a particularly appealing target for study are suggestions that rather than behaving as a simple diffusing system, muscovite grains show multi-diffusion-domain behavior, such that each grain of muscovite is in effect a collection of subsamples having partially overlapping retentivities for argon. If so, an individual sample could record a continuous segment of a thermal history that extends over some 100˚C or more. Moreover, it has been proposed that sample-specific information about muscovite diffusion properties can be obtained as a byproduct of 40Ar/39Ar laboratory step heating analysis, which would support the recovery of more accurate thermal histories than would be possible by applying generic kinetic data obtained from a limited number of published diffusion studies. The research will test both the proposal that muscovite analyses can reveal a range of thermal-history information and that these analyses can also yield accurate diffusion-kinetics data. The work involved will form part of a Ph.D. dissertation as well as an undergraduate project. To convey their work broadly, the team will develop instructional materials targeted at the undergraduate level on such geochronology topics as what sort of diverse people can do this kind of work, why they do it, and how. These materials will be integrated with a series of videos prepared in collaboration with animation professionals, and tested in classes before release. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.