Subterranean production of neutrons, ³⁹Ar and ²¹Ne: Rates and uncertainties

Accurate understanding of the subsurface production rate of the radionuclide ³⁹Ar is necessary for argon dating techniques and noble gas geochemistry of the shallow and the deep Earth, and is also of interest to the WIMP dark matter experimental particle physics community. Our new calculations of subsurface production of neutrons, 21Ne, and ³⁹Ar take advantage of the state-of-the-art reliable tools of nuclear physics to obtain reaction cross sections and spectra (TALYS) and to evaluate neutron propagation in rock (MCNP6). We discuss our method and results in relation to previous studies and show the relative importance of various neutron, 21Ne, and ³⁹Ar nucleogenic production channels. Uncertainty in nuclear reaction cross sections, which is the major contributor to overall calculation uncertainty, is estimated from variability in existing experimental and library data. Depending on selected rock composition, on the order of 10⁷–10¹⁰ α particles are produced in one kilogram of rock per year (order of 1–10³ kg⁻¹ s⁻¹); the number of produced neutrons is lower by ∼6 orders of magnitude, 21Ne production rate drops by an additional factor of 15–20, and another one order of magnitude or more is dropped in production of ³⁹Ar. Our calculation yields a nucleogenic 21Ne/4He production ratio of (4.6±0.6)×10^-8 in Continental Crust and (4.2±0.5)×10^-8 in Oceanic Crust and Depleted Mantle. Calculated ³⁹Ar production rates span a great range from 29 ± 9 atoms kg-rock⁻¹ yr⁻¹ in the K–Th–U-enriched Upper Continental Crust to (2.6 ± 0.8) × 10^-4 atoms kg-rock⁻¹ yr⁻¹ in Depleted Upper Mantle. Nucleogenic ³⁹Ar production exceeds the cosmogenic production below ∼700 m depth and thus, affects radiometric ages of groundwater. The ³⁹Ar chronometer, which fills in a gap between 3H and 14C, is particularly important given the need to tap deep reservoirs of ancient drinking water.