Understanding Cosmogenic Nuclide Production Underground

Abstract

This PhD study was undertaken to understand geological and geophysical phenomena or events using the cosmogenic nuclides 10Be, 14C, and 26Al. Two major studies have been accomplished using samples from two geological sites: Stable gold mine surfaces in Macraes Flat, East Otago and marine terraces in Fiordland in New Zealand. The production of cosmic ray-induced 10Be, and 26Al in vein quartz was investigated at various depths from near surface down to a depth of 183 m (49,300 g cm-2) for Macraes Flat. In situ production of 14C for samples from the surface down to a depth of 1.5 m (400 g cm-2) was investigated. The depth profiles of 10Be and 26Al of this study show the neutron, muon capture, and fast muon-induced production of 10Be and 26Al underground within the above mentioned depth range. The transition from neutron-induced production to fast muon-induced production of 10Be and 26Al was observed at a depth of ~ 1,000 g cm-2. The production ratio of 26Al/10Be was found to be depth dependent. The muon contribution to the surface samples was relatively high compared to any of the previously published papers on the subject. After elimination of muon-induced concentrations of 10Be and 26Al from the measured total concentrations of 10Be and 26Al, neutron-induced concentrations of 10Be and 26Al were obtained. These values were used for surface exposure age calculation. It was found that the surface of the sampling site. Deep Dell, has been exposed to cosmic rays for the last 22.000 ~ 23.000 years. In addition, the depth profiles of 10Be and 26Al indicate that the Macraes Flat surface exposure does not represent a steady state condition. In the case of the Fiordland marine terrace study, it was found that samples located at both low altitude and high altitude show signs of pre-irradiation during the past glacial periods, i.e. low sea levels. All samples had unexpectedly low measured 26Al/10Be ratios, which were found to be due to burial by the ocean after exposure. Underground production and pre-exposure by neutron spallation may be related to sea level changes in the past. All exposure ages modelled in this study should be considered as either overestimated or underestimated depending on whether samples have experienced significant accumulation of cosmogenic nuclides delivery pre-exposure or show a low signature of pre-exposure concentration due to high erosion or burial above sea level. The Fiordland study was a pilot study to determine the feasibility of surface exposure dating in obtaining the ages of the formation for different marine terraces. The results of this study will assist further investigations, toward sampling techniques and data interpretation.