Subterranean Production Of Argon 39 And Implications For Doe Canyon Well Gas
Creators: name: Sramek, Ondrej
name: McDonough, William F.
Publisher: name: Zenodo
Year: 2018

description: Low-radioactivity argon sources are desired by the WIMP dark matter experimental particle physics community. Accurate understanding of the subsurface production rate of the radionuclide <sup>39</sup>Ar is also necessary for argon dating techniques and noble gas geochemistry of the shallow and the deep Earth.

Our new calculations of subsurface production of neutrons, <sup>21</sup>Ne, and <sup>39</sup>Ar (Šrámek et al., 2017) 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, <sup>21</sup>Ne, and <sup>39</sup>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<sup>7</sup>–10<sup>10</sup> alpha particles are produced in one kilogram of rock per year (order of 1–10<sup>3</sup> kg<sup>−1</sup> s<sup>−1</sup>); the number of produced neutrons is 6 orders of magnitude lower, <sup>21</sup>Ne production rate drops by an additional factor of 15–20, and another one order of magnitude or more is dropped in production of <sup>39</sup>Ar. Calculated <sup>39</sup>Ar production rates span a great range from 29 ± 9 atoms per kg-rock per year in the K–Th–U-enriched Upper Continental Crust to (2.6 ± 0.8) × 10<sup>−4</sup> atoms per kg-rock per year in the Depleted Upper Mantle. Nucleogenic <sup>39</sup>Ar production exceeds the cosmogenic production below ∼ 700 m depth in the Earth.

Recent report by the DarkSide-50 Collaboration (Agnes et al., 2016) puts the concentration of <sup>39</sup>Ar in Doe Canyon, SW Colorado, deep CO<sub>2</sub> well gas at 1400±200 times lower compared to the atmospheric value. While it was argued that the Doe Canyon gas is derived from the Earth’s upper mantle, it shows some counter intuitive isotopic characteristics, such as extremely low <sup>3</sup>He/<sup>4</sup>He suggesting crustal origin, while at the same time extremely low <sup>39</sup>Ar/<sup>40</sup>Ar indicating a source low in K, Th, U abundances. As a possible solution to this puzzle, we envisage a sizeable (i.e., low surface to volume ratio) gas reservoir in the shallow crust where mantle gas, contaminated by crust-derived gases (<sup>4</sup>He, <sup>21</sup>Ne, and N<sub>2</sub>), accumu- lates for sufficient time (&gt; 10<sup>4</sup> years) so that the <sup>39</sup>Ar activity drops to the observed low value.

Overall, the noble gas observations (esp. of helium, neon, argon), both in terms of out- gassing rates from the Earth and the gas’ isotopic composition, present a major challenge to geoscientists who strive to formulate a coherent story of the Earth’s formation, evolution, and current state. A link between underground noble gas production and decay of long lived radionuclides (<sup>40</sup>K, <sup>232</sup>Th, <sup>238</sup>U) ties noble gas geochemistry to dynamic models of thermal evolution of the Earth and to questions about deep Earth’s composition and architecture.

<em>References: </em>

Agnes et al. (2016): “Results from the first use of low radioactivity argon in a dark matter search.” Phys. Rev. D, 93(8): 081101, doi:10.1103/PhysRevD.93.081101 (arXiv:1510.00702)

Gilfillan et al. (2008): “The noble gas geochemistry of natural CO<sub>2</sub> gas reservoirs from the Colorado Plateau and Rocky Mountain provinces, USA,” Geochim. Cosmochim. Acta, 72 (4), 1174–1198, doi:10.1016/j.gca.2007.10.009

Šrámek et al. (2017): “Subterranean production of neutrons, <sup>39</sup>Ar and <sup>21</sup>Ne: Rates and uncertainties.” Geochim. Cosmochim. Acta 196, 370–387, doi:10.1016/j.gca.2016.09.040 (arXiv:1509.07436)

 

This presentation was used for the Low-Radioactivity Underground Argon Workshop held at Pacific Northwest National Laboratory in Richland, Washington on March 19 - 20, 2018.