Microanalysis of impactors of the earth and impact products from the moon : the origin and evolution of micrometeorites, and tracking bombardment history through chemical and radioisotopic memories of lunar impact spherules

Abstract

This thesis consists of two topics, the study of terrestrially-collected micrometeorites and lunar impact spherules. The purpose of the first project was primarily method development, although a study of micrometeorite petrography, chemistry, atmospheric alteration, and potential origin ensued. A classification system was developed with a focus on transitional internal textures to highlight the sequence of alteration occurring during atmospheric heating. Micrometeorite composition was used to identify potential parent bodies which are likely to be chondritic. More importantly, it was found that iron metal micrometeorites are likely to form due to separation from a stony micrometeorite body due to immiscibility and, following separation, oxidise to form a FeO glassy cap. Failing that they fully oxidise to form iron oxide micrometeorites. The main topic is the lunar impact spherules which are impact-melts that record their original target composition (major and trace elements), formation age, and integrated exposure age. This is the first study to integrate all these data for individual impact spherules. A method was developed to cater for the requirements of this project and several analytical techniques were tested. The major and trace elements were employed to understand the petrographic characteristics of the impact spherules including the formation of volatile and siderophile crusts and their composition, and how composition affects colour. The compositional data were also used to determine the provenance of the impact spherules and whether they are likely to be produced during impacts into soils or rocks. The first impact spherule age distribution for low potassium, locallyderived impact spherules has also been developed in this study consists of 26 impact spherules. The main features of the formation age distribution are a peak in spherules with ages <400Ma and a minima between 2Ga and 3.2Ga. However, the integrated exposure ages do not exceed 500Ma regardless of the formation age. The greatest achievement of this project is the use of these integrated compositional and age data to constrain selection effects that act on impact spherule records. The contribution of exotic spherules and production of duplicate impact spherules in single impact events was found to contribute minimally to the first order interpretations. Exotic spherules were few in number and duplicate impact spherules more commonly have ages >1Ga indicating that the <400Ma peak in spherule production is not due to overrepresentation. In essence, the results indicate that the impact spherule record in its current state is not representative of the lunar impact history, but is the product of various selection effects, some of which can be constrained using the techniques developed in this study.