Geocheminar spring-2016

Apparent Late Heavy Bombardments

The Late Heavy Bombardment is one of the major hypotheses to emerge from Apollo-era scientific work. While it was originally formulated based off a handful of Rb/Sr dates and Pb isotope analyses, the majority of the evidence now marshaled for its existence comes from compilations of 40Ar/39Ar “plateau” ages. However, 40Ar/39Ar analyses of extra-terrestrial samples show pervasive evidence for 40Ar loss due to later reheating events (e.g., subsequent impacts) which when interpreted in a “plateau” age framework leads to misleadingly young ages. In order to test the reliability of impact histories derived from 40Ar/39Ar “plateau” ages, we construct a first principles diffusion model for 40Ar. Our model is constrained to a compilation of ages derived from the early heating steps from 40Ar/39Ar analyses of Apollo samples, as they constrain the timing of the last reheating event, and a lunar crustal age distribution derived from zircon 207Pb-206Pb ages. The model is then used to generate monotonic impact histories that fit the constraints, and we compile model “plateau” age histograms to evaluate the degree to which these histograms accurately reflect the input impact history. We relate our model to 40Ar/39Ar step-heating analyses of Apollo samples and while it is broadly applicable, it does not consider the complications arising from the presence of multiple activation energies for 40Ar diffusion. Our modeling shows that the 40Ar/39Ar system as currently interpreted is prone to showing bombardment episodes that are more apparent than real. Furthermore, the complicated nature of 40Ar diffusion in Apollo samples underscores the inability of the current 40Ar/39Ar interpretative framework (i.e., “plateau” ages) to recover the true impact history.