This project aims to develop and test a new geochemical method for quantifying the temperatures and exposure histories of rocks and sediments exposed at the Earth's surface during the past few million years. The technique is based on the measurement of trace isotopes of the noble gases helium and neon that are produced in rock and mineral grains by cosmic-ray bombardment at the Earth's surface. These concentrations reflect both the length of time the rocks have been exposed at the surface and, because the rate of diffusive loss of these gases depends on temperature, the temperature they experienced during exposure. First, we will use controlled laboratory experiments on both natural and artificially-produced samples of the common minerals quartz and feldspar to establish the mechanism, rate, and temperature dependence of helium and neon diffusion in these minerals. These results will, in principle, allow us to predict concentrations of these gases in natural geological samples whose exposure and temperature histories are already known from other evidence. Thus, the second part of this project will be to test and validate our laboratory results and theoretical framework by comparing predicted with actual concentrations in natural rock samples.
This research is important because reconstructing the past temperatures and surface exposure histories of surface materials is valuable for a broad range of scientific research. First, measuring exposure durations of surface rocks is important in understanding geologic processes that act to form and change Earth's surface, including surface erosion, sediment transport, and earthquake-related surface deformation. Second, measuring past temperatures is important for understanding Earth's natural climate variability during the last few million years. For example, such information is important to establish how past changes in environmental conditions influenced biota at various regions across the globe, and how past climate changes were potentially controlled by natural phenomena such as the gradual development of mountain ranges. However, our ability to quantify past temperatures is currently limited to a small number of geochemical techniques. If this new method is successfully developed and tested, it will provide an independent test of existing methods, and potentially benefit a broad range of research sub disciplines including quantitative geomorphology, landscape evolution studies, late Cenozoic climate changes, glacier and ice sheet change, and, potentially, paleo-elevation of actively uplifting landscapes. This basic geochemistry research will potentially enable a wide array of earth science researchers to address both longstanding and completely new questions, thus benefiting the broader science community. In addition, this project will support a graduate student at UC Berkeley, thus contributing to Earth science education and the development of human resources for geochemistry and broader science research.
