Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmpp13c1470k&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #PP13C-1470
Physics
3630 Experimental Mineralogy And Petrology, 3662 Meteorite Mineralogy And Petrology (1028, 6240), 6022 Impact Phenomena (5420, 8136), 6240 Meteorites And Tektites (1028, 3662)
Scientific paper
Research into a proposed extraterrestrial (ET) event 12.9 ka ago at the onset of the Younger Dryas revealed that for impact sediments (the YDB) that were tested across N America and NW Europe, all contain impact- related nanodiamonds ranging in size from 1 to 1700 nm. They appear in bulk sediment, but mostly occur inside carbon spherules and glass-like carbon, which are the charred, melted, amorphous-carbon byproducts of intense, impact-related wildfires. No diamonds were found stratigraphically above or below the impact layers. Selected area electron diffraction (SAED) by transmission electron microscopy (TEM) produced reflections of 2.06, 1.26, 1.07, and 0.89 A, which correspond to the lattice planar d-spacings of cubic diamonds. So-called "forbidden" reflections were also apparent at 1.78, 1.04, and 0.796 A, and these spacings are characteristic of a metastable cubic diamond polymorph called "n-diamond," the dominant form of diamond found in the YDB. N-diamonds have been produced under lab conditions and have been identified in meteorites, but they have never been found associated with mantle-derived diamonds. We have been able to reverse-engineer the process by which the impact-related n-diamonds form. First, various carbon-rich materials (coal, coconut shells, and wood) were charred at about 500°C under low- oxygen conditions. Next, the char was heated to more than 1000°C in a partially sealed vessel, while adding steam or nitrogen at near-atmospheric pressure. As a result, rounded, nanometer-sized domains of graphite formed in the char, to be then transformed into n-diamonds. To prevent combustion of the n- diamonds, the char was quenched under low-oxygen conditions. As it happens, this procedure is identical to the commercial process for producing activated charcoal, and in fact, samples of commercially available activated carbon manufactured by both Calgon Carbon Corporation and Norit Americas, Inc. were found to be enriched with n-diamonds. The process of forming n-diamonds requires conditions unlike any that are normal to the Earth's surface. However, the requirements match the extreme conditions that exist during an ET impact or airburst: (1) transient high temperatures; (2) an oxygen-poor (or steam-rich) atmosphere within the fireball and behind the shock front; and (3) the opportunity for quenching in a low-oxygen environment. Our research confirms that the YD nanodiamonds could not have formed under normal terrestrial conditions, and instead, required conditions consistent with an ET impact. Our research group has filed a provisional patent application for this previously unknown method of producing diamonds.
Kennett James P.
Kimbel D.
West Andrew A.
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