CANYON DIABLO GRAPHITE NODULE


Iron, IAB complex, main group
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Found 1891
(see the Canyon Diablo page for further details)

An investigation of the Canyon Diablo graphite nodules by Kurat et al. (2000) revealed that trace element contents and platinum group elements within the metal fraction of these nodules are highly variable on a sub-mm scale. They considered that this heterogeneity required a low-temperature origin because a high-temperature origin from an igneous melt, which was followed by relatively slow cooling rates, would have homogenized the metal composition. Additional chemical and isotopic constraints led them to conclude that the metal was probably formed by a chemical vapor deposition process. Subsequently, both the graphite and silicate components were incorporated with the metal fraction in the solid state. Matsuda et al. (2005) found that the retention of various noble gases in the graphite nodules supports such a low-temperature history.

New chemical analyses and experimental modeling of the metal veins within the Canyon Diablo graphite nodules were conducted by Hilton et al. (2020). Based on the abundances of 16 siderophile elements and the modeling results, they concluded that the most likely formation scenario involved the complete melting of the host metal during an episode, or sometimes multiple episodes, of impact shock heating. They determined that the present siderophile element abundances in the metal veins were the result of 60% fractional crystallization of the host melt. Peak temperatures occurred at the graphite–metal interface, which resulted in melting of the host metal and its injection into the graphite while leaving the noble gas complement unaffected. Hilton et al. (2020) also employed Re–Os chronometry to derive a likely age for the metal veins of >4 b.y. (see diagram below). An ancient age is also supported by the presence of excess 129Xe in the graphite–metal nodules which attests to formation while the nebula was still primitive. Further studies are needed to determine whether these metal veins were formed prior to or after separation of the Canyon Diablo host meteoroid from the IAB-MG parent body.

Rhenium–Osmium Systematics
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Diagram credit: Hilton et al., MAPS, vol. 55, #4, pp. 771–780 (2020)
(https://doi.org/10.1111/maps.13464)

Green crystals were identified in a Canyon Diablo graphite nodule by Dr. Laurence Garvie (see image below). He has identified the mineral as either kosmochlor (NaCrSi2O6) or krinovite (NaMg2CrSi3O10).

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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 which destroyed 99.999% of the half-million ton mass and created a spectacular crater. The photo shown above is a 47.3 g graphite nodule end section, while that below shows melting of the outer surface.

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