The Open University ’ s repository of research publications and other research outputs Carbon chemistry of giant impacts

Impact diamonds were found in several inipactites from the Ries crater. Geriiiaiiy including fallout and fallback (crater f i l l ) suevites. a glass bomb, impact melt i-ock and shocked gneiss. These diamonds formed two distinct grain size populations: 50-300 pii apographitic. platy aggregates with surface ornamentation and etching that \vere observed using optical and scanning electron microscopy and 5-20 pm diaiiionds which displayed two different inorphologies identified using traiisinission electron microscopy and selected area electron diffraction. These 5-20 pm grains comprised apographitic. platy gi-nins with stacking faults, etching and graphite intergrowths together with elongate skeletal grains with prefemd orientations to the individual crystallites. Thermal annealing of stacking fa~i l is and surface features was also detected. Stepped combustion coiiibined with static mass spectrometry to give carbon isotopic analysis of iiidividunl diamonds. graphite and acid-residues indicate that the priinai-y carbon source is graphite. This graphite was found to be %depleted with i-cspcct to similar samples firom the Popigai impact crater. The admixture of presumably carbonate derived carbonaceous material is wzyxteù to account for the "C-enriched 6°C coinpositions encountered in whole-i-ock we\.ites known to include carbonate melts. 011 the busis of moiphology. iiineralogicnl associations. diamoncVgraphite ratios aiid carbon isotopic compositions three possible formation mechanisms for impact dimionds are suzgested: fast. high temperature conversion of graphite following the pissage of the shock wave, a vapour phase condensation or growth within substrate iiiiiicrnls 01an orientated stress field and the incomplete translomiation of a mixture of aiiiorphous and iiitcriiiediary carbyne phases cannot be discounted. r a l h e graphite. Further niore exotic mechanisms such as linpact diamonds, 1-5 pni in size. were also identified in suevite residues and a hlack matrix lithic breccia from the Gardnos impact crater. Norway. The carbon isotopic Jennifer i Abbott ABSTRACT conipositions are in agreement with previous measurements of whole rock saiiiplcs with ;i riiiall "C-cnriched component probably representing diamond. Jennifer I Abhott ABSTRACT