Dr. Emily M. Peterman has been granted the NSF Earth Sciences Postdoctoral Fellowship to carry out a research and education plan at Stanford University. This research will develop and calibrate a new technique to determine particle paths within the crust from a single key mineral that is present in many rocks. Monazite is a uranium- and thorium-bearing phosphate that can be analyzed by multiple methods to provide isotopic age information that bear upon both the timing of high-temperature crystallization and subsequent lower-temperature cooling during crustal residence. Furthermore, the trace element and isotopic composition of monazite can be linked to growth conditions within the crust. By combining all of this information, the newly developed approach will improve our ability to quantitatively reconstruct the spatial and temporal nature of crust-forming processes.
This comprehensive analysis of the use of monazite as a chronometer for elucidating the formation and evolution of continental crust will impact a broad range of geoscience disciplines, including tectonics, geomorphology, metamorphic petrology, landscape evolution and continental dynamics. By improving the accuracy and precision of monazite isotopic age analysis, this research will improve our ability to constrain geodynamic models of crustal evolution. The details of the new methodology - including the calibration of compositionally dependent effects - may have application to other techniques, and should benefit the geoochronologic community at large. Research methods developed in this project will be integrated into a field-based course to be taught by Peterman in Death Valley. Through this course, undergraduate and graduate students will be taught how to collect samples, analyze data, and publish a paper that constrains the timing of deformation along a major fault that exposes some of the deepest-known exposures of southwestern North American crust. The results from this study will also be incorporated into an outreach demonstration that uses coupled monazite chronometry to provide an animated module that illustrates the mechanics, timing and rates of processes responsible for creating dynamic orogens such as the Himalaya.
Many mountainous landscapes provide exposures of large-scale faults (i.e. tens to hundreds of kilometers) that are currently inactive. These exposures prompt questions such as how fast did these large-scale faults move? Did the rate of fault motion change over time? The Catalina Mountains near Tucson, AZ and Monarch Canyon in Death Valley provide excellent opportunities to investigate faults that moved millions of years ago (Ma). As these faults slip, they transport deep rocks towards the surface. As these deep rocks move towards the surface and cool down, minerals undergoing radioactive decay within the rock sequentially record the amount of time that has elapsed since they were at elevated temperatures. Most minerals record only a single episode of cooling and the temperature at which the mineral begins to record this cooling (the closure temperature) is assumed based upon experimentally determined closure temperatures. Therefore, extracting the timing and tempo of large-scale faults over geologic time requires a sequence of co-existing minerals to record temperature-time information. Unfortunately, this type of analysis can be time and labor intensive because it requires the analysis of multiple minerals, some of which do not always co-exist. The main objectives of this research project were to: 1) Develop a new technique to quantify the timing and tempo at which large-scale faults moved using a single mineral (monazite). In contrast to many commonly used chronometers, monazite has two radioactive decay systems that have different closure temperatures—one at high-temperature (~750°C), the other at moderate-temperature (~250 to 300°C). Therefore, monazite should record the elapsed time since the mineral cooled through both high- and moderate-closure-temperatures. 2) Determine methods that address the effect of composition on closure temperature. Monazite varies compositionally, which can affect the closure temperature by up to 100°C. Therefore, new methods must directly measure the temperature at which the mineral begins to record elapsed time. 3) Teach a course in which students apply these newly developed techniques to a geologic problem by conducting field and laboratory work; 4) Provide professional development for an early career female scientist; 5) Create opportunities for underrepresented students to improve their performance and skills in the geosciences; retain these students in the geosciences. 1) During the early phases of this research project, the PI and advisor successfully developed and rigorously evaluated new techniques for quantifying the timing and tempo of fault motion using the mineral monazite. To assess the accuracy of our results from a natural sample, we collected a rock that contains both monazite and a suite of commonly used minerals for temperature-time analysis from the Catalina Mountains near Tucson, AZ. We measured both high- and moderate-temperature results from monazite and the complementary temperature-time information from coexisting minerals. The data yield a self-consistent thermal history indicating that two pulses of motion exhumed these rocks. The monazite data fit excellently with the other chronometers, thereby validating the methods we developed (see attached figure). 2) Previous studies suggested that monazite would not work as a chronometer because it is compositionally variable among samples. Because the methods developed as part of this research directly measure both temperature and time, compositional variation is inherently accounted for and does not preclude the use of monazite as a chronometer. 3) As part of this project, the PI developed a course on thermochronology (measuring temperature-time information from minerals) at Stanford for both undergraduate and graduate students. During the classroom component of this course, students learned the methods and theory of thermochronology. They then applied this knowledge through a weeklong field component in Monarch Canyon, Death Valley, where they examined the timing and tempo of large-scale faulting. All graduate students in this course used newly developed thermochronology techniques in their Masters and PhD projects. 4) The PI gained expertise in a new research direction that naturally stems from her prior work. She fostered collaborations to continue this new direction of work in local and international field areas, thereby jump-starting her career as a new tenure-track faculty member at a liberal arts college (Bowdoin College). She has presented and published the findings of this work at both national and international conferences (e.g. Geological Society of America, American Geophysical Union). 5) The PI also mentored high school, undergraduate and graduate students and learned strategies for recruiting underrepresented students to work with her. Over the duration of this research project, the PI mentored ~20 students, in formal and informal settings. Approximately 80% of these mentees are pursuing career paths in geosciences. For example, a graduate student mentee was awarded an NSF postdoc and is now a tenure-track faculty member. Two undergraduate students traveled to Nevada to collect samples, process samples and conduct "authentic research." These experiences motivated one of the students to design an honors project in geoscience and apply to graduate school to pursue a career in the geosciences.
