¹Daiki Yamamoto,²Noriyuki Kawasaki,³Shogo Tachibana,⁴Lily Ishizaki,⁴Ryosuke Sakurai,²Hisayoshi Yurimoto
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2024.04.014]
¹Department of Earth and Planetary Sciences, Kyushu University, Motooka, Fukuoka 819-0395, Japan
²Department of Natural History Sciences, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
³UTokyo Organization for Planetary Space Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan
⁴Department of Earth and Planetary Science, The University of Tokyo, Hongo, Tokyo 113-0033, Japan
Copyright Elsevier

The reaction mechanism and kinetics of oxygen isotope exchange between tens of nanometer-sized amorphous silicate grains with forsterite composition (amorphous forsterite) and low-pressure carbon monoxide (CO) gas (PCO) of 0.05–1 Pa at 643–883 K were examined to investigate oxygen isotopic evolution in the protosolar disk that led to the mass-independent oxygen isotopic variation of planetary materials. Both CO gas supply- and diffusion-controlled isotope exchange reactions were observed. At 753–883K and PCO of 0.05–1 Pa, the supply of CO gas controls the isotope exchange reaction, and its rate is 2–3 orders of magnitude smaller than that of the H2O supply-controlled isotope exchange reaction. The diffusion-controlled isotope exchange occurred at 643–703 K and PCO of 0.3 Pa, and the reaction rate of D (m2/s) = (3.1 ± 2.3) × 10−23 exp[−41.7 ± 9.6 (kJ mol−1) R−1 (1/T − 1/1200)] was obtained.

We found that the oxygen isotope exchange rates of amorphous forsterite with CO and H2O gases are larger than those of gaseous isotope exchange between CO and H2O gases at a wide range of temperatures, wherein amorphous forsterite crystallization does not precede the isotope exchange reaction of amorphous forsterite with these gases. The most sluggish isotope exchange rate between H2O and CO in the gas phase suggests that amorphous forsterite would play a role in accelerating gaseous isotopic equilibrium through the isotope exchange of amorphous forsterite with both CO and H2O. We found that the oxygen isotopic equilibrium between 0.1 μm-sized amorphous forsterite, CO, and H2O would be accomplished through the isotope exchange of amorphous forsterite at temperatures as low as ∼600–700 K in the dynamically accreting protosolar disk, which is significantly lower than expected for the case of gaseous isotope exchange (>∼800 K).

^1,2Justin Y. Hu,³Ingo Leya,¹Nicolas Dauphas,²Auriol S.P. Rae,²Helen M. Williams
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.04.013]
¹Origins Laboratory, Department of the Geophysical Sciences and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637, USA
²Department of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK
³Space Sciences and Planetology, University of Bern, Bern 3012, Switzerland
Copyright Elsevier

The regolith evolution of airless bodies, like the Moon, is primarily controlled by impact cratering. Since the Apollo Era, measurements of cosmic ray exposure (CRE)-induced Sm and Gd isotopes in lunar drill cores have provided insights into the secondary neutron spectra in the lunar regolith. Since the production and transport of secondary neutrons vary with the regolith’s chemical composition and depth, the neutron fluence profile can be employed to track the evolution of lunar and asteroidal regolith. We developed a stochastic model that incorporates state-of-the-art cosmogenic production rate calculations for Sm and Gd isotopes in an effort to understand regolith evolution in the presence of meteoroid bombardments. By comparing the simulated depth profiles to those observed in the lunar drill cores from the Apollo 15, 16, and 17 missions, we find that the deviations from a static profile are due to continuous surface meteoroid bombardments. These bombardments result in the formation of nuclear-reworked zones near the lunar surface. Based on the surface neutron fluence of lunar rocks and regolith, our modeling shows that the regolith surface is reset by large impact-induced excavation and deposition of blanket ejecta every few hundred million years.