Mobilisation and Loss of Noble Gas and Halogen Elements During the Impact Melting of Ordinary Chondrites

Abstract

Impacts and the resultant shock features they generate are ubiquitous within extra-terrestrial materials. Shock-induced impact melt generation is cited as important for developing some planetary volatile element signatures. This thesis aims to extend our understanding of the role of impact melting in developing planetary volatile element signatures by investigating the noble gas and halogen contents of ordinary chondrite host and impact melt fractions. This is coupled with detailed petrographic descriptions to relate halogen and noble gas behaviours to sample petrography. The petrography, geochronology and noble gas and halogen abundances and isotope compositions of the Chico L6 chondrite were examined to investigate the role of large-scale melting, analogous to accretionary impact melting, in developing planetary halogen and noble gas signatures. Millimetre-scale chemical variations are observed in the impact melt indicating a previously unreported heterogeneity in impact melt chemistry. It has been suggested that the Bulk Silicate Earth (BSE) halogen signature is purely derived from the accretion of chondritic material (due to the similarity of chondritic and BSE halogen signatures). Both the Chico host and impact melt have chondritic elemental halogen ratios indicating large-scale impact melting has not caused considerable elemental fractionation. This indicates that the accretion of chondritic material containing significant amounts of whole-rock impact melt to the Earth could still have developed the BSE halogen signature. However, large-scale impact melting altered impact melt elemental halogen ratios to better agree with those of the BSE. This suggests accretionary impact melting could be an important process involved in developing the BSE halogen signature. The impact melt has retained significantly more primordial noble gases than the host due to a longer characteristic diffusion length, however, no significant isotopic fractionations took place during impact melting. This suggests that neither the delivery of large amounts of chondritic whole-rock impact melt nor accretionary impact melting are likely important processes in developing the terrestrial isotopic noble gas signature. The Chelyabinsk LL5 meteorite was investigated to understand the role of smaller scale in situ melting in developing planetary halogen and noble gas signatures. Halogen elements were not fractionated during smaller scale in situ melting, suggesting the delivery of chondritic material containing significant amounts of small-scale in situ melt to Earth would have helped develop the BSE halogen signature. As for large scale-melting, smaller-scale impact melt retained more primordial noble gases than the host and isotopic fractionations were largely absent. Therefore, the delivery of small-scale in situ chondritic impact melts during planetary evolution is likely not a significant process for developing planetary isotopic noble gas signatures. The petrography and noble gas contents of the host and impact melt of the meteorites Chergach H5, Gao-Guenie H5, and Kilabo LL6 were examined to investigate noble gas behaviour in a wide range of impact melting conditions. Most samples retained a greater concentration of primordial noble gases in the melt over the host and significant isotopic fractionations were absent. This suggests impact melting of any degree is not a significant process in developing planetary isotopic noble gas signatures. Noble gas behaviour during impact melting is complex, however, some coherent behaviour, such as a common trend in elemental noble gas ratios between the host and impact melt, is observed during larger scale impact melting. The results of this work showcase the complexity of volatile element behaviour during impact melting and further our understanding of the role of impact melting in determining planetary volatile signatures.

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Raymond Burgess (Supervisor), Greg Holland (Supervisor) & Rhian Jones (Supervisor)