The extent of moderately volatile elements (MVE) depletion and its effects on the Moon's evolutionary history remain contentious, partly due to unintentionally biased sampling by the Apollo missions from the Procellarum KREEP Terrane. In this study, we analyzed the Zn and K isotope compositions of a series of lunar basaltic meteorites, which vary in Th content and are likely to represent a broader sampling range than previous studies, including samples from the far side of the Moon. Our findings indicate remarkably consistent Zn and K isotope compositions across all lunar basalt types, despite significant variations in Th content. This consistency suggests a relatively homogeneous isotopic composition of volatile elements within the Moon, unaffected by subsequent impact events that formed major basins. Our results suggest that the estimates of MVE abundance and isotopic compositions from the Apollo returned samples are likely representative of the bulk Moon, supporting a globally volatile-depleted lunar interior.

Space weathering has long been known to alter the chemical and physical properties of the surfaces of airless bodies such as the Moon. The isotopic compositions of moderately volatile elements in lunar regolith samples could serve as sensitive tracers for assessing the intensity and duration of space weathering. In this study, we develop a new quantitative tool to study space weathering and constrain surface exposure ages based on potassium isotopic compositions of lunar soils. We first report the K isotopic compositions of 13 bulk lunar soils and 20 interval soil samples from the Apollo 15 deep drill core (15004-15006). We observe significant K isotope fractionation in these lunar soil samples, ranging from 0.00 %o to + 11.77 %o, compared to the bulk silicate Moon (-0.07 +/- 0.09 %o). Additionally, a strong correlation between soil maturity (Is/FeO) and K isotope fractionation is identified for the first time, consistent with other isotope systems of moderately volatile elements such as S, Cu, Zn, Se, Rb, and Cd. Subsequently, we conduct numerical modeling to better constrain the processes of volatile element depletion and isotope fractionation on the Moon and calculate a new K Isotope Model Exposure Age (KIMEA) through this model. We demonstrate that this KIMEA is most sensitive to samples with an exposure age lower than 1,000 Ma and becomes less effective for older samples. This novel K isotope tool can be utilized to evaluate the surface exposure ages of regolith samples on the Moon and potentially on other airless bodies if calibrated using other methods (e.g., cosmogenic noble gases) or experimental data.

Space weathering alters the surface materials of airless planetary bodies; however, the effects on moderately volatile elements in the lunar regolith are not well constrained. For the first time, we provide depth profiles for stable K and Fe isotopes in a continuous lunar regolith core, Apollo 17 double drive tube 73001/2. The top of the core is enriched in heavy K isotopes (delta 41K = 3.48 +/- 0.05 parts per thousand) with a significant trend toward lighter K isotopes to a depth of 7 cm; while the lower 44 cm has only slight variation with an average delta 41K value of 0.15 +/- 0.05 parts per thousand. Iron, which is more refractory, shows only minor variation; the delta 56Fe value at the top of the core is 0.16 +/- 0.02 parts per thousand while the average bottom 44 cm is 0.11 +/- 0.03 parts per thousand. The isotopic fractionation in the top 7 cm of the core, especially the K isotopes, correlates with soil maturity as measured by ferromagnetic resonance. Kinetic fractionation from volatilization by micrometeoroid impacts is modeled in the double drive tube 73001/2 using Rayleigh fractionation and can explain the observed K and Fe isotopic fractionation. Effects from cosmogenic 41K (from decay of 41Ca) were calculated and found to be negligible in 73001/2. In future sample return missions, researchers can use heavy K isotope signatures as tracers of space weathering effects.

The Moon is depleted in volatile elements and compounds, and lunar samples exhibit a wide range of Cl isotopic compositions, which is believed to result from the volatilization of metal chlorides (e.g., NaCl, KCl, and FeCl2). However, the Cl isotopic fractionation behavior during volatilization is not well constrained, particularly for metal chlorides. Furthermore, the effect of metal chloride evaporation on metal isotopes is poorly known. In the present study, we performed NaCl and KCl sublimation experiments to study Cl and K isotopic fractionations at temperatures ranging from 923 K to 1061 K and at pressures of 7x10-5 bar to 1 bar in an N2 atmosphere. The isotope fractionation factors of 37/35Cl(& alpha;gas-solid) from NaCl sublimation experiments are 0.9985 & PLUSMN;0.0002, 0.9958 & PLUSMN;0.0004, and 0.99807 & PLUSMN;0.00004 at 1, 10-2, and 7x10-5 bar, respectively. Those of 41/39K(& alpha;gas-solid) and 37/35Cl (& alpha;gas-solid) from KCl sublimation experiments are 0.99884 & PLUSMN;0.00004 and 0.9988 & PLUSMN;0.0003 at 1 bar, 0.9977 & PLUSMN;0.0002 and 0.9972 & PLUSMN;0.0003 at 10-2 bar, and 0.9989 & PLUSMN;0.0002 and 0.9989 & PLUSMN;0.0001 at 7x10-5 bar, respectively. Chlorine and K isotopes fractionate more at 10-2 bar than at 7x10-5 bar and 1 bar. The saturation index in all the sublimation experiments was >95%, which resulted in near-equilibrium isotopic fractionation at the sublimation interface. Therefore, the isotopic fractionation was controlled by mass transfer processes in the gas and solid phases. The isotopic fractionation at 10-2 bar was controlled by the chemical diffusion of sublimated gas in an N2 atmosphere with almost no convection effect, (i.e., Pe number close to zero), whereas the isotopic fractionation at 1 bar was suppressed by atmospheric convection with a turbulence factor of 0.4 & PLUSMN;0.1 (i.e., Pe number >1). The extremely high sublimation rate and the very slow diffusion in the sublimating solid at 7x10-5 bar suppressed isotopic fractionations. Based on our experimental results, calculations using Cl/K and Na/K in lunar materials reveal that degassing of KCl contributed very little (0.58%o) during lunar magma ocean degassing. The Cl isotopic fractionation factor from lunar samples is similar to our results at 10-2 bar. This similarity of Cl isotope fractionation indicates that there may have been a transient atmosphere above the lunar magma ocean.