Profiles of in situ 10Be and 26Al at great depths at the Macraes Flat, East Otago, New Zealand

The production of cosmic ray-produced 10Be and 26Al in vein quartz was investigated from near the surface to 49,300 g cm−2 depth at three locations in Macraes gold mine, East Otago, New Zealand. The measured 26Al/10Be ratios are dependent on depth, which indicates that the production mechanisms of these nuclides change with depth. The transition from neutron-induced production to muon-induced production was observed at about 1000 g cm−2. The measured attenuation lengths of 10Be and 26Al from the surface down to 400 g cm−2 of this study are much larger than steady state apparent neutron attenuation length at near surface. The large measured attenuation lengths at shallow depths could be caused by deep erosion, which results in a large muon contribution to the production of the isotopes at all depths. Using revised production rates, the average model surface exposure age of the Macraes Flat was determined to be 25.0±3.3 kyr. The erosion rate of the sampling site was estimated to be at least 12 m Myr−1 using the depth profiles of 10Be and 26Al of this study. Production systematics below ∼5000 g cm−2 are poorly known, and a model is proposed for those depths. We emphasize the importance of using depth profiles of cosmogenic nuclides in understanding surface processes and past geological histories.