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Subterranean production of neutrons, $^{39}$Ar and $^{21}$Ne: Rates and uncertainties

Accurate understanding of the subsurface production rate of the radionuclide $^{39}$Ar 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, $^{21}$Ne, and $^{39}$Ar 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, $^{21}$Ne, and $^{39}$Ar 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 $10^7$-$10^{10}$ α particles are produced in one kilogram of rock per year (order of 1-$10^3$ kg$^{-1}$ s$^{-1}$); the number of produced neutrons is lower by $\sim6$ orders of magnitude, $^{21}$Ne production rate drops by an additional factor of 15-20, and another one order of magnitude or more is dropped in production of $^{39}$Ar. Our calculation yields a nucleogenic $^{21}$Ne/$^4$He production ratio of $(4.6\pm0.6) \times 10^{-8}$ in Continental Crust and $(4.2\pm0.5) \times 10^{-8}$ in Oceanic Crust and Depleted Mantle. Calculated $^{39}$Ar production rates span a great range from $29\pm9$ atoms kg-rock$^{-1}$ yr$^{-1}$ in the K-Th-U-enriched Upper Continental Crust to $(2.6\pm0.8) \times 10^{-4}$ atoms kg-rock$^{-1}$ yr$^{-1}$ in Depleted Upper Mantle. Nucleogenic $^{39}$Ar production exceeds the cosmogenic production below $\sim700$ meters depth and thus, affects radiometric ages of groundwater.