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Canyon Diablo

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. Utilizing the short-lived 182Hf–182W chronometer, corrected for neutron capture by 182W due to galactic cosmic rays, Hunt et al. (2018) derived the timing of metal–silicate separation of all genetically-related IAB irons (at least the MG and sLL subgroup [possibly also the sLM subgroup] along with the ungrouped Caddo County [Udei Station grouplet] and Livingstone [Algarrabo duo]) to 6.0 (±0.8) m.y. after CAIs. Based on the constraints provided by the timing of metal segregation, they modeled the early history of the 120(+)-km-diameter IAB parent body as outlined in the following diagram: standby for IAB formation history diagram
Diagram credit: Hunt et al., EPSL, vol. 482, pp. 497 (2018, open access link)
‘Late metal–silicate separation on the IAB parent asteroid: Constraints from combined W and Pt isotopes and thermal modelling’
(https://doi.org/10.1016/j.epsl.2017.11.034)’
Dey et al. (2019) employed 17O and ε54Cr values for several irons and their associated silicates/oxides to investigate i) if each iron and its associated phases originated on a common parent body (i.e., an endogenous mixture of core and mantle vs. an exogenous mixture through impact), and ii) if any genetic connection exists between the irons and other meteorite groups (e.g., IAB with winonaites, IIE with H chondrites, and Eagle Station pallasites with CK chondrites). Three IAB irons were employed in the study, and it was demonstrated on a coupled diagram that although the ε54Cr values for the iron component plot in the winonaite field, values for the silicate component plot in a distinct region on an O–Cr coupled diagram (see diagram below). From these results they ascertained that the the IAB silicated irons formed through an impact-generated mixture comprising iron from a winonaite-related parent body and silicate from an unrelated and otherwise unsampled parent body. Incorporation of the silicates into the FeNi-metal host took place at a depth greater than 2 km, allowing time for a Thomson (Widmanstätten) structure to develop during a long cooling phase. Fractional crystallization occurred in some large molten metal pools, followed by very slow cooling, to produce the broad range of features found in certain IAB meteorites (e.g., silicate-poor, graphite–troilite-rich inclusions and extremely high Ni contents). Other results from their study can be found on the Miles and Eagle Station pages. 17O vs. ε54Cr for Irons and Pallasites
standby for o-cr isotope diagram
click on photo for a magnified view

 

Diagrams credit: Dey et al., 50th LPSC, #2977 (2019)
To learn more about the relationship between this and other iron chemical groups, click here. The specimen of Canyon Diablo shown above is a shrapnel fragment that was shaped during the violent impact event.

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Odessa

Iron, IAB complex, main group
standby for odessa photo
Found before 1922
31° 43′ N., 102° 24′ W. Over 1,000 kg of meteoritic material have been recovered in and around a small rimmed crater measuring 165 m in diameter and its associated impact holes. While the original crater, which was formed ~50,000 years ago, is thought to have measured ~180 m wide and ~30 m deep, the crater that remains today is only ~2 m deep with a rim ~2 m high.

Odessa is a coarse octahedrite assigned to the main group of the IAB complex (Wasson and Kallemeyn, 2002). Troilite–graphite–silicate aggregates are widespread in Odessa specimens, and cohenite is common. These inclusions are chondritic in nature, contain planetary rare gas elements, and were likely formed within partial melts generated through impact events that occurred very early in Solar System history at a location near 1 AU. Odessa has a very high CRE age of 875 (±70) m.y. (Voshage and Feldmann, 1979).

Information on the formation of the IAB complex silicated irons can be found on the Landes page. Pictured above are two ‘Baby Odessa’ individual fragments having a combined weight of 6.5 grams.


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DaG 962

Iron, IAB complex
(Stony iron-ung in MetBull 86)
(Achondrite-ung recommended in MetBull DB)

standby for dag 962 photo
Found October 1999
27° 11.88′ N., 16° 24.51′ E. While hunting in a well-searched area of the southeastern Dar al Gani plateau, Libya, the Pelisson’s of Sahara Meteorite Prospecting found a 130 g stony-iron meteorite. Although they returned to the area, they failed to find any other pieces of this unique meteorite (R. Pelisson, pers. comm.). It is their opinion that the high density of this meteorite might be responsible for a deeper penetration into the soil; luckily, this single individual was only half buried. Another factor that could frustrate attempts to locate more fragments of this meteorite is its proximity to a basaltic massif, which consists completely of black rocks, likely rendering any future searches futile.

In a collaboration among research groups (Cole and Sipiera of the Schmitt Meteorite Research Group; Jerman and Hoover of the NASA Marshall Space Flight Center; Dod of the Mercer University Department of Physics; the Pelisson’s of SaharaMet), an analysis of DaG 962 published in an abstract at the 65th MetSoc (2002) resulted in a classification of anomalous mesosiderite.

This meteorite is composed of 55 vol% metallic matrix and 45 vol% angular silicate clasts (MetBull 86). The silicates are primarily magnesian olivine (Fa1.0) and enstatite (Fs0.84), which are often intruded by veinlets of FeNi-metal (kamacite and taenite). Minor clinopyroxene and plagioclase are present, as well as troilite. Newly conducted electron microprobe studies conducted by Kuehner et al. (2011) identified accessory phases of daubreelite, schreibersite, and magnesiochromite. The internal structure is more similar to the pallasites than the mesosiderites in having a continuous network of FeNi-metal enclosing silicates, consistent with an igneous origin in a core–mantle boundary. However, the silicates are more Mg-rich than those in main-group pallasites or any other pallasites; they are more similar to irons of group IAB. O-isotope data collected by D. Rumble III (Carnegie Institution, Washington DC) also plot near the IAB iron complex and winonaites. Dar al Gani 962 may be comparable to the Landes IAB silicated iron, and shows some similarity to Woodbine.

The photo above shows a 1.3 g partial slice of DaG 962, the mirror slice of the portion used for the thin section that was utilized for the analysis. The top photo below shows the meteorite in situ, while the bottom photo shows the main mass, illustrating the staining on the portion previously embedded below soil level. standby for dag 962 photo
standby for dag 962 photo
Photos courtesy of R & R Pelisson—Sahara Meteorite Prospecting


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Canyon Diablo Graphite Nodule

Iron, IAB complex, main group
standby for graphite nodule photo
Found 1891 Studies have shown that trace element contents as well as platinum group elements within the metal fraction of group IAB meteorites are highly variable on a sub-mm scale. This inhomogeneity requires a low-temperature origin since a high-temperature origin from an igneous melt, followed by such slow cooling rates, would have homogenized the metal. Additional chemical and isotopic constraints lead to the conclusion that the metal was probably formed by a chemical vapor deposition process. The graphite and other silicates were then combined with the metal fraction while in the solid state. In support of a low-temperature history for the graphite–metal inclusions is the presence of various noble gas components (Matsuda et al., 2005). Furthermore, the presence of excess 129Xe in the graphite–metal inclusions is consistent with their formation and inclusion within the metal host while the nebula was still primitive. Green mineral crystals, either kosmochlor (NaCrSi2O6) or krinovite (NaMg2CrSi3O10), were identified in a graphite nodule by Dr. Laurence Garvie.
Kosmochlor or krinovite crystals in a Canyon Diablo graphite nodule; FoV 2.5 mm
Photo shown courtesy of Dr. Laurence Garvie, Center for Meteorite Studies, ASU These graphite nodules survived the incredible 40-megaton blast that occurred around 50,000 years ago in Arizona, destroying 99.999% of the half-million ton mass and creating a spectacular crater. Extreme heat and pressure forced metal in the form of kamacite into the graphite nodules creating ribbons of metal in a jet-black carbon matrix. The above specimen is a 47.3 g end section. Below is a photo of the impact-melted backside of this specimen.

standby for graphite nodule photo