UC Berkeley

Cosmogenic nuclides, which are produced in the uppermost few meters of the Earth's crust by cosmic-ray particle interactions with atomic nuclei, are commonly used to quantify the rates and timing of surface processes. Some of the first terrestrial cosmogenic nuclide measurements revealed that the cosmogenic noble gases 3He and 21Ne are diffusively lost at Earth surface temperatures in common silicate minerals like quartz and feldspars. Viewed as a fatal limitation for geologic applications since then, the open-system behavior of cosmogenic noble gases can, in fact, be exploited to quantitatively reconstruct temperatures at the surfaces of Earth and other planetary bodies.

In Chapter 1, I develop a theoretical framework for using cosmogenic noble gases as a paleothermometer based on the principles and mathematics underlying radiogenic noble gas thermochronometry. With this framework and published information on the diffusion kinetics of helium and neon in quartz and feldspars, I demonstrate that cosmogenic 3He–in–quartz measurements could be used to constrain past surface temperatures at high latitudes and elevations on Earth, while 21Ne–in–feldspar measurements could be used to constrain past surface temperatures at lower latitudes and elevations, and on other planetary bodies.

I then explore the applicability of these published diffusion kinetics through a series of stepwise degassing experiments on quartz (Chapter 2) and feldspars (Chapter 3) containing initially uniform distributions of proton-induced 3He and 21Ne. These experiments reveal that 3He and 21Ne diffusion kinetics vary significantly across samples of different geologic origin, and that in many cases quartz and feldspars exhibit complex diffusion behavior mani- fest as nonlinearity in Arrhenius plots. The origin of this complex behavior is indeterminate, but I demonstrate that it is not caused by temperature-dependent structural transformations or anisotropy and that it is not an artifact of proton irradiation. Instead, complex diffusion behavior appears to be controlled by some intrinsic, sample-specific material property. I also demonstrate that we can mathematically model complex diffusion behavior, and use geologic examples with simple exposure and temperature histories to validate this mathematical model.

Having laid out the theoretical and experimental backbone of cosmogenic noble gas paleothermometry, in Chapter 4 I present two applications of the technique to problems in paleoclimate and planetary science. In the first application, I use cosmogenic 3He and 10Be observations in quartz from a series of nested moraines in the Maritime Italian Alps to reconstruct temperatures since the Last Glacial Maximum (LGM). I demonstrate that temperatures reconstructed from the cosmogenic 3He observations are consistent with temperatures expected for this region since the LGM from a global circulation model (GCM) and other proxy data, but that additional constraints are necessary to fully interpret this dataset. In the second application, I use observations of cosmogenic neon isotopes in plagioclase feldspars from lunar sample 76535 to demonstrate that this sample only experienced solar heating during its 142 million year residency at the lunar surface. This constraint on the thermal history of 76535 agrees with existing argon measurements and confirms the fidelity of paleomagnetic measurements in the same sample, which have been used to demonstrate that the Moon had an early core dynamo.