Critique of paper by N. L. Carter and G. C. Kennedy, ‘Origin of diamonds in the Canyon Diablo and Novo Urei meteorites’

Carter and Kennedy claim to have found convincing evidence against a shock origin of meteoritic diamonds and suggest that they formed under high hydrostatic pressures instead. We have examined their key exhibit, an ‘unheated,’ diamond-bearing Canyon Diablo specimen. It contains ample evidence of shock in the range of 400–750 kb. A few other clarifications and corrections are in order. (1) The correlation between shock and diamonds holds without exception. (2) The probability that diamonds are equally common among rim and plains specimens is 4×10−54. (3) Canyon Diablo was not a composite fall. (4) The polycrystallinity of meteoritic diamonds has not been duplicated by any laboratory treatment. Only shock-loading or flash-heating of graphite (without catalyst) produces diamonds of similar polycrystallinity. (5) The degree of graphitization of meteoritic diamonds is not inconsistent with a shock origin. (6) Graphite rims around troilite nodules originated in the solid state, not from the melt. (7) If diamonds had formed from the melt, they would have floated to one end of the nodule, which is contrary to observation. (8) Diamonds formed from the melt would have redissolved in the metal during traversal of the γ loop. (9) Diamonds were not ‘chemically isolated’ from the metal. (10) A diamond ancestry of cliftonite is contradicted by at least four lines of evidence. (11) We did not propose in our 1961 paper that meteoritic diamonds originated by solid-state decomposition of cohenite. (12) The state of deformation of the silicates in Novo Urei (and other ureilites) is consistent with severe shock, and their asterism actually suggests such shock. (13) The preferred orientation of the ureilite diamonds resembles that of shock-produced diamonds. (14) There is no mineralogical evidence for static high pressures in the ureilites. (15) The Widmanstätten pattern definitely formed at low pressures. (16) Moon-sized objects would not cool as required; they would heat up instead. (17) There is no obvious way to break up and disperse moon-sized objects. (18) There is no obvious way to make moon-sized objects vanish from the asteroid belt. (19) The collisions required in the Carter-Kennedy cosmogony must have resulted in severe shock effects, contrary to hypothesis.