Shock Effects Recorded by the U-Pb Radiometric System in Zircons of Apollo 17 Impact Breccias and by Raman Spectroscopy in Enstatite Meteorites

Bidong Zhang, The University of Western Ontario

Abstract

Impact cratering processes and formations are ubiquitous on the rocky planets within the Solar System. The Earth-Moon system was formed by a massive planetary impact, and numerous impacts afterwards significantly reshaped the topography and lithology of the Moon surface. This dissertation focuses on studying some specific geochemical, geochronological, and geophysical signatures of impact processes preserved in lunar rocks and asteroidal meteorites.

The zircon U-Pb radiometric system is one of the most reliable geochronometers in measuring geological time. Recently, unsupported radiogenic Pb enrichment has been reported to compromise the accuracy and precision of zircon U-Pb geochronometer in terrestrial rocks. In Chapter 2, we report the first discovery of such Pb enrichment in a set of Apollo 17 lunar zircons. Two ion-microprobe analytical sessions for these zircons show inconsistent ages on the same spots. In these sessions, ²⁰⁷Pb spikes produce unsupported ages for the zircons. We infer that these spurious ages result from the presence of nanometre-scale radiogenic Pb clusters, which are induced by the shock-related deformation of the zircon grains.

The Apollo 17 poikilitic impact melt breccias (IMBs) are regarded as a product of the Serenitatis basin-forming impact. In Chapter 3, we report a suite of zircons from Imbrium ejecta in the Apollo 73155,69 poikilitic IMB. By using the ion microprobe technique, these zircons are found to have an age, texture, and trace-element concentrations similar to those from some of the Imbrium-originated breccias. The 73155,69 zircons are evidence for Imbrium-ejecta components incorporated into the poikilitic breccias. The result highlights that the Apollo 17 poikilitic IMBs are polygenetic.

Optical features modified by opaque minerals and post-shock annealing in meteorites may hamper their petrological classification of shock stages. In Chapter 4, we introduce a method of using Raman spectra to assess the shock stages of enstatite-rich meteorites. The Raman peak widths of almost pure enstatite crystals in enstatite chondrites demonstrate a linear relationship (r² >0.94) with their progressive shock stages, while their peak positions are not affected by shock effects. We conclude that Raman peak widths can be used as a calibrator for establishing the shock stages of enstatite-rich meteorites.