Post-Imbrium Pb-Pb isochron ages for Apollo basaltic impact melt samples 14078 and 68415

Area of Science:

• Lunar geology and geochemistry
• Planetary science
• Isotope geochemistry

Background:

• Apollo 14 and 16 missions returned crystalline impact melt samples dated around 3800-3850 Ma.
• Previous interpretations suggested these rocks formed from pre-Imbrium basin impacts.
• Recent studies indicate the Imbrium basin formed around 3920 Ma.

Purpose of the Study:

• To determine precise ages of Apollo 14 and 16 impact melt samples using lead-lead (Pb-Pb) isochron dating.
• To investigate the origin and source characteristics of these lunar samples.
• To re-evaluate the formation events of lunar impact basins.

Main Methods:

• Lead-lead (Pb-Pb) isochron dating applied to lunar samples 14078 and 68415.
• Analysis of lead (Pb) isotopic compositions to determine initial Pb isotopic composition.
• Modeling of Pb isotope evolution to infer source characteristics (e.g., 238U/204Pb ratios).

Main Results:

• Pb-Pb isochron ages determined for sample 14078 (3848 ± 4 Ma) and 68415 (3834 ± 11 Ma).
• Sample 14078 indicates derivation from a source with high 238U/204Pb ratios (approx. 2400), similar to Apollo 14 basalts.
• Sample 68415 suggests derivation from a source with lower 238U/204Pb ratios (approx. 1000).

Conclusions:

• The determined ages and isotopic compositions suggest that samples 14078 and 68415 formed from impacts local to the Apollo 14 and Apollo 16 landing sites, respectively.
• These findings challenge the interpretation of these samples as products of a large pre-Imbrium basin-forming impact.
• The study highlights the importance of precise geochronology and isotopic analysis in understanding lunar impact history.