Investigation of a Dating Technique

Samples are gathered, crushed, sieved, and sorted magnetically to divert problematic interference of mafic rock. The remaining fine-grained felsic material is etched to remove all but the strong mineral structure of the quartz crystal. These crystals are transitioned back and forth between aqueous and solid states using acids and evaporation, as per Bierman's CosmoLab Procedures . This Be is stored with a known concentration of the stable Be-9, and then measured with a mass spectrometer. The mass spectrometer accelerates the atoms and uses magnetics to separate and measure the constituent elements of a sample. In this case it provides a ratio of Be-10 to Be-9 – allowing a calculation of Be-10 content, and thereby a date of exposure based on the half-life 1.39 x10^6 years.

The main assumptions necessary in cosmogenic dating techniques is that of inherited nuclides. Cosmic rays and their associated free neutrons only penetrate a few meters of the Earth’s Surface. However, if an exposed rock was buried, and subsequently exhumed again (relatively quickly), it could have a leftover signal of unstable 10-Be. Drainage basins erode at an average rate of 2.18m Kyr⁻¹ with a global median of 0.54 m Kyr⁻¹; and outcrops erode at an average of 0.12 m Kyr⁻¹ with a global median 0.054 m Kyr⁻¹, (Portenga and Bierman. 2011). Glaciers, while effective erosion agents, do not have as strong constraints on erosional rates (its hard to measure under all that ice). Suggestions range from 0.07 – 100m Kyr⁻¹ (Koppes and Montgomery, 2009).