GENESIS OF PRESOLAR DIAMONDS - COMPARATIVE HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY STUDY OF METEORITIC AND TERRESTRIAL NANO-DIAMONDS

Nano-diamonds isolated from acid dissolution residues of primitive car bonaceous meteorites (Allende and Murchison) were studied using high-r esolution transmission electron microscopy. To discriminate among thei r most likely formation mechanisms, high-pressure shock-induced metamo rphism or low-pressure vapor condensation, the microstructures of pres olar diamond crystallites were compared to those of (terrestrial) synt hesized nano-diamonds. The synthesized diamonds used for comparison in this study were produced by high-pressure shock waves generated in co ntrolled detonations and by direct nucleation and homoepitaxial growth from the vapor phase in low-pressure chemical vapor deposition (CVD)- type processes. Microstructural features were identified that appear u nique to shock metamorphism and to nucleation from the vapor phase, re spectively. A comparison of these features to the microstructures foun d in presolar diamonds indicates that the predominant mechanism for pr esolar diamond formation is a vapor deposition process, suggesting a c ircumstellar condensation origin. A new presolar grain component has a lso been identified in the meteoritic residues, the (2H) hexagonal pol ytype of diamond (lonsdaleite).