Utilizing Pedogenic Iron-oxides for Cosmogenic 3He Geochronology of Soils

Directly dating the formation of pedogenic iron-oxides using cosmogenic ³He has the potential to constrain soils ages and can lead to insight into iron-oxide formation mechanisms. Iron-oxide particles in soils commonly have diameters of tens of nanometers. We show through diffusion modeling that hematite particles down to ~10 nm can quantitatively retain helium without diffusive loss for at least 1 Ma at Earth-surface conditions. Since ³He is ejected upon production by cosmic rays, any cosmogenic helium contained in a particle smaller than the ejection distance must have been implanted from the matrix, the composition of which determines the overall production rate. In order to test whether pedogenic iron-oxides can be used for geochronology, we studied a vertical profile of a soil developed on an alluvial fan terrace offset by the San Andreas fault at Whitewater Hill, California. Profiles of ¹⁰Be and ²⁶Al in detrital quartz agree well with an exponential decrease in cosmogenic nuclide production and they yield an exposure age of 52.4±2.2 ka, assuming no erosion. The vertical profile of ³He concentrations in pedogenic iron-oxides shows a decrease with depth, but concentrations are higher than expected in the 40-100 cm depth range. We interpret this as vertical movement of iron-oxides in the soil associated with downward migration of clay particles. These observations are consistent with a soil age of around 200 ka, showing that the no erosion assumption is invalid. Downward migration leads to preferential preservation of iron-oxides relative to larger quartz grains, which remain in-situ. Therefore, cosmogenic ³He in pedogenic iron-oxides can provide additional age constraints. It is also a potential tool to study the timescales of formation and migration of iron-oxide particles in the soil column.