Iron, IAB complex, main group
standby for canyon diablo photo
Found 1891
35° 3' N., 111° 2' W.

A history revealed

Approximately 49,500 years ago an iron meteorite measuring ~100–150 feet (46–66 m) in diameter (solid body) to 217 feet in diameter (tight swarm of fragments) in diameter and weighing at least 100,000 tons (100 million kg, up to 1.2 billion kg), and which is conjectured to have been infalling along a southwest to northeast (Rhinehart, 1958; Artemieva and Pierazzo, 2011) trajectory, was catastrophically disrupted at an altitude of 8.5 miles, forming a pancake-like debris cloud measuring ~400 feet across (Passy and Melosh [Separated Fragments model], Chyba et al., 1993 [Pancake model], Melosh and Collins, 2005; Artemieva, 2006; Artemieva and Pierazzo, 2007, 2009 [SOVA hydrocode model]). This mass of interacting fragments is believed to have struck the Earth at an angle of 45° at an estimated velocity of at least 33,500 mph (15–16 km/s, possibly up to 20 km/s) and experienced considerable ablation and melting.

The resulting 2.5 megaton explosion created a crater one mile in diameter and 600 feet deep, with a rim over 150 feet high. The event excavated 175 million metric tons of rock (Kring, 2006) from 40 m deep in the case of melt material, and up to 100 m deep in the case of non-melt material (Artemieva and Pierazzo, 2011). This created an organized inverted strata with Coconino Sandstone overlying Toroweap Limestone, overlying Kaibab Limestone, overlying units of the Moenkopi Formation (Hagerty et al., 2010). The total energy released by the entire meteorite during its descent from ~9 miles altitude to the surface was calculated to have been as high as 6.5 MT (equivalent to 6.5 million metric tons of TNT; 1 MT = 4.184 × 1015 J), including an intense airblast near the ground. As a result the initial projectile was ejected and dispersed by the plume in the form of solids (26–30%), melt (45–50%), and vapor (20–29%) (Artmieva and Pierazzo, 2011).

After melting/ablation of the meteoroid, ~30% (solid body) to 70% (fragmented swarm) of the mass survived as fragments that fell over an area ~6 miles in diameter centered on the crater, and many were heated to temperatures high enough to alter the Thomson (Widmanstätten) structure of the meteorites of the rim location. The fragments were rapidly cooled in less than two minutes, which created the iron–carbon alloy martensite. The shock waves created pressures inside the fragments greater than 600 kilobars (60 GPa), which injected metal into large graphite nodules, while transforming other graphite into microscopic diamonds and lonsdaleite. All of the diamond-bearing fragments have been recovered from the crater rim with the exception of one plains specimen, and all rim specimens are strongly shocked. The remaining plains specimens are 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. The intense shock forces also acted on the local Coconino sandstone to produce coesite and stishovite, establishing the first discovery in nature of these two high-pressure silica polymorphs (both of which were discovered and named by Dr. Edward Ching-Te Chao, with the latter named after the Russian physicist Sergei Stishov who first synthesized it in a high-pressure laboratory experiment).

By relating known relationships among noble gas isotope ratios, the cosmic ray exposure age can be ascertained for the Canyon Diablo object. The oldest isochron provides evidence for a collision in space 540 m.y. ago, while a secondary isochron of 170 m.y. is suggestive of a more recent collision. 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. Most H chondrites, representing the largest group of stony meteorites found on Earth, suffered intense shock and reheating about 520 m.y. ago. These events might represent the breakup of one or more sizable asteroids with diameters of at least 80 km and masses of 1015 tons. The asteroids today can be associated with others into about 30 families having similar orbits. Each family could represent the debris from the breakup of individual asteroids. Four families that are in Mars-crossing orbits are prime candidates for supplying the Earth with those meteorites in the 500–600 m.y. cosmic ray age group.

The cosmic ray exposure ages of the Canyon Diablo fragments can be correlated with the 3He and 59Ni isotope abundances 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 on the rim or on the surrounding plains. The rim specimens had originally resided at a depth of ~3–6 feet (<3 m) within the projectiles rear surface area, while about half of the plains specimens had been closer to the surface of the projectile. The conclusion can be made that the more deeply buried fragments experienced greater shock, the shock produced diamonds from existing graphite, and these heavily shocked fragments (shrapnel) were ejected with low velocity to land on or near the rim. The mm-sized molten metallic spherules have a low content of the cosmogenic nuclide 59Ni, which is consistent with this melt material originating from the inner portion of the projectile.

Some studies have calculated that all of the surviving material was likely derived from the rearmost 6 feet of the trailing hemisphere of the impactor, all of which constitute only about 15% of the original mass; these fragments were located in areas such as corners, humps, edges, or projections where cancellation between primary and reflected shock waves occurred. Using these parameters, of the more than 300,000 tons comprising the main mass, about 30,000–45,000 tons escaped melting/vaporization. More recent hydrocode modeling by Artemieva and Pierazzo (2007) indicates that over 50% of the impactor remained solid. Only about 2,000 tons can be accounted for today in meteorite fragments, shale balls, metallic spherules, and other oxidation products. Isolated meteorite fragments account for only 30 tons of this material, much of it likely being transported from the site in ancient times, although fragments have only been described dating from ~1860.

The early history of the Canyon Diablo asteroid can also be described. Based on W- and Sm-isotopic data obtained by Schulza et al. (2012), accretion of the IAB parent body occurred ~2 m.y. after solar system formation. Silicate melting and metal segregation to form a core occurred ~3 m.y. later. During the next 0.5–1.5 b.y. the iron cooled through the temperature range of 700–400°C at a rate of about 1°C per m.y., creating the Thomson (Widmanstätten) structure of crystal formation. This cooling rate would be consistent with the asteroidal body being between 250 and 500 km in diameter, which is between one-third and two-thirds the size of the largest known asteroid, Ceres. To learn more about the relationship between this and other iron chemical groups, click here.