Carbon chemistry of giant impacts
Impact diamonds were found in several impactites from the Ries crater, Germany including fallout and fallback (crater fill) suevites. a glass bomb, impact melt rock and shocked gneiss. These diamonds formed two distinct grain size populations: 50-300?m apographitic. platy aggregates with surface ornamentation and etching that were observed using optical and scanning electron microscopy and 5-20?m diamonds which displayed two different morphologies identified using transmission electron microscopy and selected area electron diffraction. These 5-20?m grains comprised apographitic. platy grains with stacking faults, etching and graphite intergrowths together with elongate skeletal grains with preferred orientations to the individual crystallites. Thermal annealing of stacking faults and surface features was also detected. Stepped combustion combined with static mass spectrometry to give carbon isotopic analysis of individual diamonds. graphite and acid-residues indicate that the primary carbon source is graphite. This graphite was found to be 13C depleted with respect to similar samples from the Popigai impact crater. The admixture of presumably carbonate derived carbonaceous material is suggested to account for the 13C-enriched ?13C compositions encountered in whole-rock suevites known to include carbonate melts. On the basis of morphology. mineralogical associations diamond/graphite ratios and carbon isotopic compositions three possible formation mechanisms for impact diamonds are suggested: fast. high temperature conversion of graphite following the passage of the shock wave, a vapour phase condensation or growth within substrate minerals or an orientated stress field and the incomplete transformation of a mixture of amorphous and crystalline graphite. Further more exotic mechanisms such as intermediary carbyne phases cannot be discounted. Impact diamonds, 1-5?m in size. were also identified in suevite residues and a black matrix lithic breccia from the Gardnos impact crater. Norway. The carbon isotopic compositions are in agreement with previous measurements of whole rock samples with a small 13C-enriched component probably representing diamond.