About 49,500 years ago a 60 to 80 meter iron meteorite struck the Earth at over 25,000 mph.The resulting explosion created a crater one mile in diameter and 600 feet deep with a rim over 150 feet high. The fragments that survived vaporization were heated to temperatures high enough to alter the Widmanstatten pattern of the meteorites of the rim location, and they were rapidly cooled in less than two minutes creating the iron-carbon alloy martensite. The shock waves created pressures inside the fragments greater than 600 kilobars which transformed graphite into diamonds. All of the diamond-bearing fragments were recovered from the crater rim with the exception of one plains specimen, and all were strongly shocked. The remaining plains specimens were only lightly to moderately shocked and contain no diamonds. This is consistant with other evidence supporting the theory that the diamonds were formed upon impact with the Earth. The graphite particles present in the meteorite were transformed by the compression waves into droplets of liquid carbon and then frozen into tiny diamonds when decompressed by the rarefaction wave.
By relating known relationships among noble gas isotope ratios, the cosmic ray exposure age can be determined for the Canyon Diablo object. The oldest exposure date is 540 m.y. ago, with a secondary collision occurring 170 m.y. ago. One fragment shows evidence of a third collision 15 m.y. ago. More than half of all iron meteorites found on Earth have exposure ages of between 500 and 600 m.y. ago. Most H chondrites, representing the largest group of stony meteorites found on Earth, suffered intense shock and reheating about 520 m.y. ago.
The cosmic ray exposure ages of the Canyon Diablo fragments can be correlated with the Helium 3 isotope abundance in the fragments to determine the depth at which individual fragments were residing in the main body before Earth impact. This depth was correlated with the location at which each specimen had been collected, either the rim or the plains. The rim specimens had originally been at a depth of more than a meter within the main mass, and about half of the plains specimens had been closer to the surface. The conclusion can be made that the more deeply buried fragments experienced greater shock, the shock produced diamonds from graphite, and these heavily shocked fragments were ejected with low velocity landing on the rim. The surviving fragments all came from the outer two meters of the rear of the object; probably located in areas such as corners, humps, edges, or projections where cancellation between primary and reflected shock waves occurred. Of the two-million ton mass of the original body, only about 50,000 tons of material making up the rearmost 2 meters could have escaped vaporization. Of this amount, only about 2,000 tons can be accounted for in meteorite fragments, shale balls, and other oxidation products. Isolated meteorite fragments account for only 30 tons of this amount. Thus, only 4% of the rear 2 meters or .1% of the entire impacting body survived.
The early history of the Canyon Diablo object can also be described. About 4.55 b.y. ago, inside the parent body, a melting process separated the nickle-iron alloy from the silicates with which it was originally associated. During the next .5 to 1.5 b.y. the iron cooled through the temperature range from 700°C to 400°C at a rate of about 1°C per m.y., creating the Widmanstatten pattern of crystal formation. This cooling rate would be consistant with the asteroidal body being between 250 and 500 km diameter. This is between a third and two-thirds that of the largest known asteroid, Ceres.
