An isochron approach to 21Ne cosmic ray exposure dating by activation with deuteron–deuteron fusion neutrons

This paper describes an alternative approach to cosmogenic 21Ne (21Necos) exposure dating that entails irradiating Mg-bearing samples in a deuteron–deuteron (D–D) neutron generator to produce 22Ne by the reaction 25Mg(n,α)22NeMg. Following irradiation and provided that the 22NeMg/22Necos ratio is greater than ~ 103, one can identify cosmogenic 21Ne using a 21Ne/20Ne vs. 22Ne/20Ne isochron. The method is analogous to identifying radiogenic 40Ar using a conventional 40Ar/36Ar vs. 39ArK/36Ar isochron. With knowledge of the 21Ne production rate and the relationship between 22NeMg and Mg content (determined by co-irradiating a standard of known Mg concentration), the slope of the isochron (21Necos/22NeMg) can be translated into an exposure age. In this way a 21Ne exposure age is calculated without making assumptions regarding the isotopic composition of the trapped or cosmogenic components. This technique, if proven to be viable, will be applicable to minerals that contain a stoichiometric abundance of Mg (e.g., orthopyroxene, clinopyroxene, and olivine).