[meteorite-list] Meteorites from the bottom of the ocean - Part 2 of 2

[bernd.pauli at paulinet.de]

Sky & Telescope, March 1999, p. 22: Piece of a Killer Asteroid ?

Like finding a stray bullet at a crime scene, a researcher believes he has uncovered
a long-sought chunk of the impactor thought to have snuffed out 70 percent of the
species of life on Earth 65 million years ago. Scientists found the "smoking gun" in
1990: a 180-kilometer-wide circular structure centered beneath the town of Puerto
Chicxulub on the coast of Mexico's Yucatán Peninsula. But no piece of the impactor
had surfaced.
Geochemist Frank T. Kyte (University of California, Los Angeles) has been studying
a core sample from the bottom of the Pacific Ocean containing dark clay marking the
boundary of the Cretaceous and Tertiary periods (the K-T boundary). As Kyte
describes in Nature for November 19, 1998, the clay layer included a 4-millimeter-wide
piece of lighter-colored clay. Upon splitting open the nugget, he discovered a fossil
meteorite. More detailed examination of this sedimentary pearl revealed that it contains
high concentrations of iron oxides, principally hematite.
While the mineralogy of the fossil meteorite has undoubtedly changed over time, Kyte
reports that the amounts of iron, chromium, and iridium are nevertheless close to the
ranges seen in carbonaceous chondrites, a common meteorite type. Yet the specimen
has one significant compositional oddity: it has 1,000 times more gold than chondritic
meteorites commonly have, a curiosity that Kyte finds puzzling.
Because the ocean-floor sediments at the K-T boundary accumulated over perhaps as
much as 500,000 years, there is no way to prove that this truly is a piece of the
K-T impactor. However, a meteoritic impact is most consistent with Kyte's analysis;
he largely discounts the possibilities that the material is interplanetary dust or
cometary debris. Moreover, he thinks it quite conceivable that a piece of the asteroid
that struck the Yucatán Peninsula survived the blast and landed 9,000 kilometers away.