Doctor of Philosophy
Shieh, Sean R.
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.
Summary for Lay Audience
This dissertation has three independent chapters investigating the influence of hypervelocity impacts on chemical and physical attributes of the rocks from space.
Geochemists use radiometric isotopes as geochronometers to measure geological time. One of the most reliable geochronometers is the uranium-lead (U-Pb) system in a refractory mineral – zircon. However, the reliability of the zircon U-Pb geochronometer may be compromised if the Pb ions migrate from their original structural sites and form unsupported clusters in zircons. In Chapter 1, we study a set of zircons from a lunar rock (Apollo 17 Civet Cat norite) and find that these zircons have Pb clusters. These clusters result in (up to) 300 million years older ages for these zircons than their actual formation ages. The Pb clusters are thought to be produced by shock deformation during a major impact event.
The Apollo 17 poikilitic impact melt breccias (IMBs) have been widely accepted to exclusively be associated with the formation of the Serenitatis impact basin on the Moon. Therefore, these breccias have been used to date the formation of this basin. In Chapter 3, we find that zircons in the Apollo 17 poikilitic IMB 73155,69 have age, textural and geochemical characteristics linked to the zircons found in ejecta from the Imbrium impact basin. We thus infer that the 73155,69 zircons originated from the Imbrium ejecta instead. Together with previous geochemical studies and remote-sensing observations, we conclude that Imbrium ejecta have been mixed with or added to the Apollo 17 poikilitic IMBs.
Polarizing microscopes are the main tool for assessing the degree of shock effects (i.e., shock stage) in meteorites. Shock-stage assessments using optical microscopy may be hampered when meteoritic minerals are darkened by other opaque materials. In Chapter 4, we introduce a method using the spectra of laser Raman spectroscopy to assess the shock stage of a group of meteorites enriched in enstatite. We find that the peak widths of enstatite broaden as the shock stages increase. This study highlights that Raman peak widths of enstatite have the potential to be a new non-destructive calibrator for assessing the shock stage of meteorites.
Zhang, Bidong, "Shock Effects Recorded by the U-Pb Radiometric System in Zircons of Apollo 17 Impact Breccias and by Raman Spectroscopy in Enstatite Meteorites" (2019). Electronic Thesis and Dissertation Repository. 6609.