Cosmogenic radionuclide production in terrestrial rocks: Accelerator mass spectrometry measurements and Monte Carlo simulations
Abstract
Cosmogenic nuclides produced in situ in terrestrial rocks provide an important tool for dating and determining erosional histories of landforms. Reliable interpretation of nuclide contents in rocks requires a good understanding of the fundamentals of nuclide production mechanisms. This work completes an investigation of nuclide production dependence on depth and rock composition and also describes a new method for dating eroding landforms. Depth profiles of $\rm\sp{36}Cl$, $\rm\sp{10}Be$, and $\rm\sp{26}Al$ in rocks shielded and suddenly exposed to cosmic rays were measured by accelerator mass spectrometry. The depth dependence of $\rm\sp{36}Cl$ produced by neutron capture was determined by measuring $\rm\sp{36}Cl$ in quartz containing fluid inclusions. The results show a maximum for nuclide production below the top surface of a boulder and are in good agreement with predictions by Monte Carlo simulation of low-energy neutrons near air/rock boundaries. The depth dependence of nuclide production by spallation was determined by measuring $\rm\sp{10}Be$ and $\rm\sp{26}Al$ in quartz. The production remains constant in the top 20 g/cm$\sp2$ and subsequently decreases exponentially with depth. The result supports the model prediction of Masarik and Reedy (1994). The dependence of neutron-capture-produced $\sp{36}$Cl on elemental composition and water content of rocks was determined by Monte Carlo simulation of low-energy neutrons near the air/rock boundary. Chlorine-36 production varies with composition to a lesser extent than does the thermal neutron flux because a significant amount of production occurs at epi-thermal energies. Scaling factors for $\sp{36}$Cl production were determined for rock composition and water content. The effect of snow cover and vegetation on $\sp{36}$Cl production was also determined. A model was developed to determine exposure age of an eroding landform using a whole-rock $\sp{36}$Cl depth profile. The exposure age of a granodiorite glacial erratic boulder were determined to be 93 $\pm$ 6 ka. The exposure age determined by this method agrees well with that from a $\sp{10}$Be depth profile (85 $\pm$ 5 ka). The average erosion rate of the boulder was 0.3 $\pm$ 0.2 mm/ka.
Degree
Ph.D.
Advisors
Elmore, Purdue University.
Subject Area
Geophysics|Nuclear physics|Geology
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