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Illusory Late Heavy Bombardments.

Patrick Boehnke¹, T Mark Harrison¹

¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095 pboehnke@gmail.com tmark.harrison@gmail.com.

Proceedings of the National Academy of Sciences of the United States of America

|September 14, 2016

Summary

The Late Heavy Bombardment (LHB) may be an artifact of data interpretation. Analysis suggests that argon-40/argon-39 plateau ages can create false impact spikes, misrepresenting lunar bombardment history.

Keywords:

40Ar/39Ar Late Heavy Bombardment geochronology impacts planetary science

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Area of Science:

Planetary Science
Geochronology
Lunar Geology

Background:

The Late Heavy Bombardment (LHB) is a key concept in lunar exploration, supported by argon-40/argon-39 (40Ar/39Ar) plateau age data.
Apollo-era lunar samples exhibit age spectrum disturbances due to magmatism and impacts, potentially affecting 40Ar/39Ar dating.

Purpose of the Study:

To investigate if partial 40Ar* resetting in lunar samples biases interpretations of bombardment histories.
To test the reliability of impact histories derived from 40Ar/39Ar plateau age histograms.

Main Methods:

Development of a physical model for 40Ar* diffusion in Apollo samples.
Inversion of plateau age histograms to reconstruct impact histories.
Analysis of age spectrum disturbances in lunar samples.

Main Results:

Plateau age histograms can generate apparent age peaks even without a true impact spike.
Monotonically declining impact histories can be misinterpreted as LHB-type events.
The assignment of 40Ar/39Ar plateau ages involves significant subjectivity.

Conclusions:

The LHB may be an artifact of 40Ar/39Ar dating methodologies and histogram interpretation.
Illusory impact spikes can arise from the analysis of plateau ages, complicating our understanding of lunar bombardment.
Re-evaluation of lunar impact histories based on 40Ar/39Ar plateau data is warranted.