Quaternary Geochronology | 2019
Cosmogenic and nucleogenic 21Ne in quartz in a 28-meter sandstone core from the McMurdo Dry Valleys, Antarctica
Abstract
Abstract We measured concentrations of Ne isotopes in quartz in a 27.6-m sandstone core from a low-erosion-rate site at 2183\u202fm elevation at Beacon Heights in the Antarctic Dry Valleys. Surface concentrations of cosmogenic 21Ne indicate a surface exposure age of at least 4.1\u202fMa and an erosion rate no higher than ca. 14\u202fcm\u202fMyr−1 21Ne concentrations in the upper few centimeters of the core show evidence for secondary spallogenic neutron escape effects at the rock surface, which is predicted by first-principles models of cosmogenic-nuclide production but is not commonly observed in natural examples. We used a model for 21Ne production by various mechanisms fit to the observations to distinguish cosmic-ray-produced 21Ne from nucleogenic 21Ne produced by decay of trace U and Th present in quartz, and also constrain rates of subsurface 21Ne production by cosmic-ray muons. Core samples have a quartz (U-Th)/Ne closure age, reflecting cooling below approximately 95\u202f°C, near 160\u202fMa, which is consistent with existing apatite fission-track data and the 183\u202fMa emplacement of nearby Ferrar dolerite intrusions. Constraints on 21Ne production by muons derived from model fitting are consistent with a previously proposed value of 0.79\u202fmb\u202fat 190\u202fGeV for the cross-section for 21Ne production by fast muon interactions, but indicate that 21Ne production by negative muon capture is likely negligible.