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Project Progress

(a) Ar diffusion in lunar impact glasses
Diffusion of radiogenic Ar (⁴⁰Ar*), produced from the decay of 40K, may occur in lunar impact glasses over geologic timescales, and result in apparent ⁴⁰Ar/³⁹Ar ages younger than the age of glass formation. Previous studies have assumed that glass has a remained a closed system and the ages obtained have been interpreted to date the time of an impact event . ⁴⁰Ar/³⁹Ar ages on lunar glasses have also been used to infer changes in the rate of impactor flux to the Earth-Moon system over time. The objective of this study is to determine the diffusion kinetics of Ar in Apollo 16 lunar impact glasses and to use results to predict the amount of ⁴⁰Ar* lost in a given impact glass based on numerical models. An assessment of the potential for diffusive Ar loss in lunar impact glasses is required to correctly interpret ⁴⁰Ar/³⁹Ar ages obtained on lunar impact glasses.

Eight 1-2 mm glass spherules from Apollo 16 sample 61502,13 were irradiated for 35 hours with fast neutrons to generate ³⁹Ar from 39K, and subsequently step heated in a double vacuum resistance furnace attached to a noble gas extraction line and mass spectrometer. Reactor-produced ³⁹Ar was used as a diffusant to determine Ar diffusivities and results are plotted on an Arrhenius plot (Fig. 1). Segments of non-linearity on the Arrhenius diagram correlate over similar temperatures to those observed in terrestrial basaltic glass Ar diffusion experiments suggesting that lunar glasses pass through the supercooled liquid transition at similar temperatures to terrestrial basalts. This transition occurs at ~300°C, so all linear extrapolations to determine kinetics must be performed below this transition temperature. These experiments are analytically challenging because very small fractions of the total quantity of ³⁹Ar (<0.3%) are released at temperatures < 300°C, necessitating heating steps on the order of days to generate signals above blank levels. Results of Ar diffusion experiments indicate activation energies of 17.5-24.8 kcal/mole and log (D₀/a²) values of -9.1 – 0.40 s^-1. Ages determined from these glasses are generally young, either Eratosthenian or Copernican, and yield complex age spectra. Several of the glasses have near blank levels of ⁴⁰Ar* indicating a <100Ma age likely derived locally from North or South Ray craters near the Apollo 16 landing site.

Two modeling programs are in the process of being written; the first to simulate diffusion profiles in single grains, and a second to simulate how diffusion affects populations of glasses dated by ⁴⁰Ar/³⁹Ar geochronology.

(b) Geochronology of lunar impact glasses
Twenty-four lunar impact glasses from the Apollo 14, 16, and 17 regolith samples have been selected for ⁴⁰Ar/³⁹Ar dating. These glasses were chosen on the basis of their K₂O abundance (to ensure that that sample gives a high signal) and on their similarity to compositions of glasses that have previously been Ar/Ar dated. These glasses are currently in the irradiation process and will undergo analysis in 2011/12. The ages will help us refine the rate of impacts in the Earth-Moon system. We expect to describe results in detail in next year’s report.

In a new development, Nicolle Zellner is working with Dave Gombosi and Georgiana Kramer (LPI) to compare sample ages and compositions from the Levine et al. (2005) study of lunar impart glasses. These authors report an increase in the recent (<500 Ma) global lunar impact rate, based on the ages of Apollo 12 impact glasses, but our samples do not show this, and we are attempting to reconcile these findings.

Reference:
Levine, J., et al., 2005, Geophysical Research Letters, 32, L15201