This project is supporting the updating of Lehigh University's laboratory for argon-argon and uranium-thorium-helium dating. These techniques for dating minerals (termed "noble-gas thermochronology") are relevant to the geosciences because they allow estimates to be made of the time and temperature history of rocks over the range 50 degrees C to 500 degrees C. This is an important temperature range that has bearing on many geological processes, as heat is an important driving agent for many geologic processes (e.g. ore and oil formation) and in turn many of these processes cause temperatures to change within the Earth (e.g., due to erosion in mountainous areas). Noble-gas dating techniques are also used to determine precise ages for geological materials, such as volcanic ashes and lavas, thus allowing calibration of the age of sedimentary sections that contain records of biological evolution, climate variations, and other environmental changes, over a range from 50,000 to more than 500 million years. Knowledge of time and temperature is critical to the understanding of Earth resources, natural hazards, and the basic functioning of Earth systems.
The laboratory upgrades are contributing to research infrastructure by facilitating greatly simplified and higher-purity sample preparation, full automation of both the helium- and argon-dating systems to permit higher sample throughput, and improvements in pumping and gas clean-up that will improve the precision of individual analyses. These upgrades are supporting studies of mountain and landscape development in Tibet, Alaska, Central America, and Mongolia by researchers at Lehigh University, as well as collaborative studies with industry and academic researchers at other institutions. These upgrades are providing more ready access to the laboratory by graduate and undergraduate students, exposing more students to modern methods of geological research, thus contributing to teaching and learning.
The goal of this project was to upgrade portions of the systems used at Lehigh University to process and analyze geological materials for the purpose of noble-gas geochronology. The timing and rates of geological processes are critical for understanding how parts of the Earth system work, for example, faulting and deformation of rocks, growth and erosion of mountains, or accumulation and development of natural resources such as hydrocarbons. Beyond simply determining the age of materials, it is also possible to learn about the temperature history of a rock by interpreting the apparent measured age of a sample's component minerals as reflecting the net diffusion history of certain daughter isotopes produced by radioactive decay. In the case of the U-Th/He and K-Ar methods used at the noble-gas laboratory at Lehigh, the helium or argon contents of various minerals will depend on (a) the temperature history of a mineral and (b) the kinetic properties of the mineral that determine the diffusion of argon or helium. For many minerals, the kinetic information is known or can be measured in the laboratory, leaving the thermal history as the only unknown. Obtained age and diffusion information requires first processing rock samples into pure aliquots of their component minerals, and then usually heating of these aliquots under vacuum under carefully controlled conditions to release either helium or argon. This as is then analyzed using a mass spectrometer long with standards. This project involved equipment upgrades that addressed all those processes. We purchased a highly efficient puck mill that can be easily cleaned and that quickly crushes rocks, with perhaps a 10X increase in efficiency compared to previous machines that were larger and required tedious cleaning between uses. In our vacuum systems, we have replaced larger resistance furnaces for heating samples with small laser microfurnaces that can heat samples with great precision and accuracy, and do this nearly instantly, greatly improving our sample throughtput and our ability to monitor sample temperature. Finally, equipment purchased with this grant has alowed us to automate more of the sample-measuring process, including the introduction of the gas aliquots that we use as standards. Such automation leads to analyses that are considerably more precise than those obtainable using manual operation. As noted above, determination of ages or thermal histories plays a key part in a wide variety of studies of Earth systems, across many time scales. Upgrade of our laboratory better positions us to participate and support such work. Involvment of graduate students in this laboratory and training them in the most modern techniques of geochemical noble-gas analysis also contributes to this broader mission.
