Measurement of ancient cosmogenic 21Ne in quartz from the 28 Ma Fish Canyon Tuff, Colorado

Measurement of 21Ne in quartz from the 28 Ma buried Fish Canyon Tuff of Colorado yielded a maximum of 2.75±0.57×106 21Ne atoms (g SiO2)−1 above nucleogenic and modern concentrations. This quantity represents cosmogenic 21Ne produced on the Fish Canyon Tuff surface prior to the deposition of the overlying Carpenter Ridge Tuff. Ancient 21Ne was isolated through the analysis and identification of (1) background 21Ne produced from α-particle interactions or inherited; and (2) cosmogenic 21Ne produced from modern radiation. Multiple samples of Fish Canyon Tuff shielded during both exposure periods, 28 Ma ago and in modern time, yielded the background that was then subtracted from all samples. Likewise, samples of Fish Canyon exposed today, but well below the Fish Canyon–Carpenter Ridge contact and therefore shielded 28 Ma ago, yielded a modern component that was also removed from all sample concentrations. The presence of an ancient cosmogenic isotope in a buried surface has several potential applications, including the quantification of paleoelevation, paleolatitude, and pre-Quaternary erosion rates.