Evolution of water reservoirs in the early solar system through their oxygen isotopic composition

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