IMPACT AGES, COMPOSITIONS, AND BIO-TECTONIC IMPLICATIONS OF LUNAR REGOLITH GLASSES

The lunar impact record has implications for solar system dynamics and the evolution of planetary and biological systems. Crystalline lunar impact-melt rocks returned by the Apollo missions have ages of 3.75-3.95 Ga and may reflect a increased intensity of bombardment by asteroid-size bodies related to re-alignment of planetary orbits in the outer solar system. Alternatively, this narrow range of ages may be a geological sampling bias imposed by Imbrium ejecta. Either way, the ages of crystalline lunar melt rocks reflect basin-forming events, which ended on the Moon at ~3.7 Ga.

In contrast, smaller impacts over the life of the solar system may have significant implications for dynamical instabilities in the asteroid belt and biological evolution, e.g., the KT event. Evidence for such events may be preserved on the Moon but are less well defined by sample data. A potentially powerful resource for understanding the younger end of the impact spectrum is the lunar regolith, which contains thousands of tiny spherules and fragments of quenched impact melt. New approaches for measuring the compositions and ages of individual impact particles are based on microbeam analysis of geochemical characteristics and/or U-Pb and 40Ar-39Ar isotopic dating. The challenge is to extract maximum information from ~100 micron particles to link ages and target rock compositions, thereby constraining the frequency of impact events. This approach applies electron microprobe, laser ablation ICPMS, ion microprobe, and laser step-heating Ar mass spectrometry to analyze individual particles of quenched impact melt.

Although the data are sparse and subject to interpretation, patterns of lunar regolith glass ages seem to correlate broadly with significant geological and biological transitions on Earth. For example, minima in lunar glass ages coincide with the Great Oxygenation Event on Earth, and the sharp rise in those ages at ~500 Ma coincides with the beginning of the Phanerozoic Era. The current data suggest that it might be possible to correlate detailed impact records from the Moon with the bio-tectonic evolution of Earth. Additional studies of lunar spherule age distributions and a better understanding of their significance for the true impact record could provide additional information about the geological and biological evolution of Earth.