THE POTENTIAL OF FINE-GRAINED PEDOGENIC IRON-OXIDES FOR COSMOGENIC 3HE DATING

We are in the process of developing a new tool for cosmogenic dating using fine-grained iron-oxides, which are ubiquitous in most soils. We have recently found evidence suggesting that fine-grained pedogenic iron-oxides retain helium (Hofmann et al., 2017), permitting the measurement of cosmogenic ³He. This technique has the advantage of dating minerals created during soil formation, which is generally not possible using other methods. This new technique would limit the possibility of contamination by inheritance and could also provide new insights into the processes of soil formation. Our case study focuses on a relict soil at Whitewater Canyon near San Gorgonio Pass, California, USA. A well-developed pedon, rich in iron-oxides and quartz, is developed in a fanglomerate deposit. We analyzed fine-grained iron-oxides in the soil for ³He. Preliminary measurements of ³He concentrations show an exponential decrease downwards. The ³He concentration at the surface is around 31 Mat/g. This corresponds to an exposure age between 215 ka and 292 ka. In order to show that this could be used as a reliable dating tool, these concentrations will be compared against ²⁶Al and ¹⁰Be concentrations measured in quartz. We have compared the preliminary ³He dating of pedogenic Fe oxides to age estimation based on the degree of soil development, and the correlation of that pedon development to nearby, dated chronosequences, and find that the results are consistent. In a simple setting we expect both cosmogenic nuclide accumulation systems to be comparable, enabling the calibration of our new approach against an established system.