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Project Progress

⁴⁰Ar/³⁹Ar ages of lunar impact glasses (Zellner, Delano, Swindle)

Forty-five (45) lunar glasses from Apollo 15 have been selected and chemically analyzed for subsequent ⁴⁰Ar/³⁹Ar age determinations by collaborators, Nicolle Zellner and Timothy Swindle. Figure 1 shows that these glasses define a compositional trend for MgO (wt%) versus TiO₂ (wt%) within which droplets of glass dominate the high-MgO end and broken glass fragments dominate the low-MgO end of the trend. All of these glasses are >300-microns in size and have K-abundances (>2000 ppm) that will make them suitable for 40Ar/39Ar dating to improve the constraints on the bombardment history of the Moon (and Earth).

Argon diffusion in lunar impact glasses (Baldwin, Watson, Roberge, Swindle, Delano)

The interpretation of ⁴⁰Ar/³⁹Ar ages on lunar impact glasses assumes that argon was not lost from the glasses since the impact event that formed them. Argon diffusion experiments were performed on eight large (>1 mm diameter) lunar impact spherules to test this assumption and to determine kinetic parameters (log(D0/a²)=-6.20 ± 0.17 s^-1 and E_a=19.03 ± 0.55 kcal mol^-1). The fractional loss of radiogenic Ar from lunar impact spherules was modeled using experimentally derived values and assuming thermal histories relevant to residence on the lunar surface, burial in the lunar regolith, and transient thermal events such as those caused by impacts. Models predict that large impact spherules (>0.6 mm) will remain nearly closed to Ar loss, while smaller glasses (<~150 μm) exhibit open system behavior due to temperature variations associated with lunar diurnal cycles. Such loss of radiogenic Ar, predicted from these models, results in ⁴⁰Ar/³⁹Ar ages that are younger than the age of the impact event that formed them, requiring reassessment of published ⁴⁰Ar/³⁹Ar ages.

Lunar plagioclase crystals (Baldwin, Delano, Watson)

Eighteen (18) lunar plagioclase crystals selected from an Apollo 17 regolith have been chemically analyzed (An90-98) for Ar diffusion experiments and ⁴⁰Ar/³⁹Ar age determinations. Data will be used to explore the potential for individual plagioclase crystals in a regolith to constrain the ages of specific events (impacts and volcanism) in a region. This study is a strategy for evaluating whether individual K-bearing crystals from a sieved sample of soil from other planets (especially Mars) could be used to constrain a region’s history of impact and volcanism. The value of the Apollo 17 sample is that this region of the Moon can provide a test of ground-truth since its history has been well-constrained from 40-years of rock analyses.