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Project Progress

This study focuses on obtaining geochemical and ⁴⁰Ar/³⁹Ar (argon) chronological data on lunar impact glasses, which are pieces of melted regolith (lunar soil) created by energetic impacting events on the Moon. The aim of the study is to understand the ages of impact glasses, along with their compositions, in order to gain a full interpretation of the impact history of the Moon.

A very important outcome of our work that is currently in review for publication in Geochimica et Cosmochimica Acta is that not all lunar impact glasses are created equal. All previous investigators (e.g., Hui 2011; Hui et al. 2010; Zellner et al., 2009a,b; Delano et al., 2007; Levine et al., 2005; Culler et al., 2000) have implicitly assumed that lunar impact glasses are highly retentive of radiogenic ⁴⁰Ar during prolonged residence in the shallow lunar regolith that is subjected to diurnal temperature variations, but this is not the case. Using experimental data (Gombosi et al. 2012) and applying it to our lunar impact glass data, we have found that composition and size of the glass are important components to consider when evaluating whether or not the ⁴⁰Ar/³⁹Ar age is a true age (or just an apparent one, as the result of argon diffusion). We revisited hundreds of ⁴⁰Ar/³⁹Ar ages of lunar impact glasses and report that only a subset reflect a true formation age of the glass.

We also investigated the observation (e.g., Norman et al. 2012; Hui 2011; Hui et al. 2010; Levine et al. 2005; Culler et al. 2000) that lunar impact glass spherule ages appear to represent an increase in the recent (i.e., <500 Ma) impact flux. We conclude that lunar impact glass spheres may be short-lived. Lunar impact glasses contain high thermal stresses (Ulrich, 1974; strain exotherms) caused by rapid quenching from hyperliquidus temperatures. These thermal stresses would make impact-generated glass spheres susceptible to breaking into shards. Thus, investigators who preferentially select lunar impact glass spheres and who reported an increase in the recent (< 500 Ma) impact flux (i.e., Norman et al. 2012; Levine et al. 2005; Culler et al. 2000) are biasing their lunar impact flux curves to young ages. If we are correct in our suggestion that impact glass spheres are easily broken as a result of regolith gardening, then the high rate of occurrence of lunar impact glass spheres with ages <500 Ma, as reported in those studies, need not require a substantial increase in the impact flux during the last 500 Ma.

These are strong working hypotheses, but more data are needed. In August 2014, Zellner traveled to the Geochronology Center at the University of Arizona to use the argon mass spectrometer to obtain ⁴⁰Ar/³⁹Ar ages on ~30 impact glasses. Almost 100 glasses are still awaiting ⁴⁰Ar/³⁹Ar dating. These analyses will be concluded in 2015.