[meteorite-list] Mystery Minerals Formed in Fireball from Colliding Asteroid That Destroyed the Dinosaurs

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Mar 23 18:12:39 2005
Message-ID: <200503232312.j2NNCHq18947_at_zagami.jpl.nasa.gov>

http://www.eurekalert.org/pub_releases/2005-03/uoc-mmf032105.php

Public release date: 23-Mar-2005

Contact: Steve Koppes
skoppes_at_uchicago.edu <mailto:skoppes@uchicago.edu>
773-702-8366
University of Chicago <http://www.uchicago.edu>

Robin Lloyd
lloyd_at_amnh.org <mailto:lloyd@amnh.org>
212-496-3419
American Museum of Natural History <http://www.amnh.org>

Ann Cairns
acairns_at_geosociety.org <mailto:acairns@geosociety.org>
303-357-1056
Geological Society of America <http://www.geosociety.org>

Mystery minerals formed in fireball from colliding asteroid that
destroyed the dinosaurs

Scientists at the American Museum of Natural History and the University
of Chicago have explained how a globe-encircling residue formed in the
aftermath of the asteroid impact that triggered the extinction of the
dinosaurs. The study, which will be published in the April issue of the
journal Geology, draws the most detailed picture yet of the complicated
chemistry of the fireball produced in the impact.

The residue consists of sand-sized droplets of hot liquid that condensed
from the vapor cloud produced by an impacting asteroid 65 million years
ago. Scientists have proposed three different origins for these
droplets, which scientists call "spherules." Some researchers have
theorized that atmospheric friction melted the droplets off the asteroid
as it approached Earth's surface. Still others suggested that the
droplets splashed out of the Chicxulub impact crater off the coast of
Mexico's Yucatan Peninsula following the asteroid's collision with Earth.

But analyses conducted by Denton Ebel, Assistant Curator of Meteorites
at the American Museum of Natural History, and Lawrence Grossman,
Professor in Geophysical Sciences at the University of Chicago, provide
new evidence for the third proposal. According to their research, the
droplets must have condensed from the cooling vapor cloud that girdled
the Earth following the impact.

Ebel and Grossman base their conclusions on a study of spinel, a mineral
rich in magnesium, iron and nickel contained within the droplets.

"Their paper is an important advance in understanding how these impact
spherules form," said Frank Kyte, adjunct associate professor of
geochemistry at the University of California, Los Angeles. "It shows
that the spinels can form within the impact plume, which some
researchers argued was not possible."

When the asteroid struck approximately 65 million years ago, it rapidly
released an enormous amount of energy, creating a fireball that rose far
into the stratosphere. "This giant impact not only crushes the rock and
melts the rock, but a lot of the rock vaporizes," Grossman said. "That
vapor is very hot and expands outward from the point of impact, cooling
and expanding as it goes. As it cools the vapor condenses as little
droplets and rains out over the whole Earth."

This rain of molten droplets then settled to the ground, where water and
time altered the glassy spherules into the clay layer that marks the
boundary between the Cretaceous and Tertiary (now officially called the
Paleogene) periods. This boundary marks the extinction of the dinosaurs
and many other species.

The work that led to Ebel and Grossman's Geology paper was triggered by
a talk the latter attended at a scientific meeting approximately 10
years ago. At this talk, a scientist stated that spinels from the
Cretaceous-Paleogene boundary layer could not have condensed from the
impact vapor cloud because of their highly oxidized iron content. "I
thought that was a strange argument," Grossman said. "About half the
atoms of just about any rock you can find are oxygen," he said,
providing an avenue for extensive oxidation.

Grossman's laboratory, where Ebel worked at the time, specializes in
analyzing meteorites that have accumulated minerals condensed from the
gas cloud that formed the sun 4.5 billion years ago. Together they
decided to apply their experience in performing computer simulations of
the condensation of minerals from the gas cloud that formed the solar
system to the problem of the Cretaceous-Paleogene spinels.

UCLA's Kyte, who himself favored a fireball origin for the spinels, has
measured the chemical composition of hundreds of spinel samples from
around the world.

Ebel and Grossman built on on Kyte's work and on previous calculations
done by Jay Melosh at the University of Arizona and Elisabetta Pierazzo
of the Planetary Science Institute in Tucson, Ariz., showing how the
asteroid's angle of impact would have affected the chemical composition
of the fireball. Vertical impacts contribute more of the asteroid and
deeper rocks to the vapor, while impacts at lower angles vaporize
shallower rocks at the impact site.

Ebel and Grossman also drew upon the work of the University of Chicago's
Mark Ghiorso and the University of Washington's Richard Sack, who have
developed computer simulations that describe how minerals change under
high temperatures.

The resulting computer simulations developed by Ebel and Grossman show
how rock vaporized in the impact would condense as the fireball cooled
from temperatures that reached tens of thousands of degrees. The
simulations paint a picture of global skies filled with a bizarre rain
of a calcium-rich, silicate liquid, reflecting the chemical content of
the rocks around the Chicxulub impact crater.

Their calculations told them what the composition of the spinels should
be, based on the composition of both the asteroid and the bedrock at the
impact site in Mexico. The results closely matched the composition of
spinels found at the Cretaceous-Paleogene boundary around the world that
UCLA's Kyte and his associates have measured.

Scientists had already known that the spinels found at the boundary
layer in the Atlantic Ocean distinctly differed in composition from
those found in the Pacific Ocean. "The spinels that are found at the
Cretaceous-Paleogene boundary in the Atlantic formed at a hotter,
earlier stage than the ones in the Pacific, which formed at a later,
cooler stage in this big cloud of material that circled the Earth," Ebel
said.

The event would have dwarfed the enormous volcanic eruptions of Krakatoa
and Mount St. Helens, Ebel said. "These kinds of things are just very
difficult to imagine," he said.

The results in this paper strengthen the link between the unique
Chicxulub impact and the stratigraphic boundary marking the mass
extinction 65 million years ago that ended the Age of Dinosaurs. The
topic will be explored further in a new groundbreaking exhibition,
"Dinosaurs: Ancient Fossils, New Discoveries," set to open at the
American Museum of Natural History on May 14. After it closes in the New
York, the exhibition will travel to the Houston Museum of Natural
Science (March 3-July 30, 2006); the California Academy of Sciences, San
Francisco (Sept. 15, 2006-Feb. 4, 2007); The Field Museum, Chicago
(March 30-Sept. 3, 2007); and the North Carolina State Museum of Natural
Sciences, Raleigh (Oct. 26, 2007-July 5, 2008).

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Received on Wed 23 Mar 2005 06:12:17 PM PST