[meteorite-list] Scientists Confirm Age Of The Oldest Meteorite Collision On Earth

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:52:20 2004
Message-ID: <200208222035.NAA02341_at_zagami.jpl.nasa.gov>

http://www.stanford.edu/dept/news/pr/02/impactor911.html

Stanford University
Stanford, California

CONTACT:
Mark Shwartz, News Service
(650) 723-9296; e-mail: mshwartz_at_stanford.edu

COMMENT:
Donald R. Lowe
Geological and Environmental Sciences
(650) 725-3040; lowe_at_pangea.stanford.edu

Joseph L. Wooden
Geological and Environmental Sciences & U.S. Geological Survey
(650) 725-9237; jwooden_at_usgs.gov

August 20, 2002

Scientists confirm age of the oldest meteorite collision on Earth
By Mark Shwartz

A team of geologists has determined the age of the oldest known
meteorite impact on Earth -- a catastrophic event that generated
massive shockwaves across the planet billions of years before a
similar event helped wipe out the dinosaurs.

In a study published in the Aug. 23 issue of the journal Science,
the research team reports that an ancient meteorite slammed into
Earth 3.47 billion years ago. Scientists have yet to locate any
trace of the extraterrestrial object itself or the gigantic
crater it produced, but other geological evidence collected on
two continents suggests that the meteorite was approximately 12
miles (20 kilometers) wide -- roughly twice as big as the one
that contributed to the demise of the dinosaurs some 65 million
years ago.

"We are reporting on a single meteorite impact that has left
deposits in both South Africa and Australia," said Donald R.
Lowe, a Stanford professor of geological and environmental
sciences who co-authored the Science study. "We have no idea
where the actual impact might have been."

To pinpoint when the huge meteorite collided with Earth, Lowe
and his colleagues performed highly sensitive geochemical
analyses of rock samples collected from two ancient formations
well known to geologists: South Africa's Barberton greenstone
belt and Australia's Pilbara block. The two sites include rocks
that formed during the Archean eon more than 3 billion years
ago -- when Earth was "only" a billion years old and single-
celled bacteria were the only living things on the planet.

"In our study, we're looking at the oldest well-preserved
sedimentary and volcanic rocks on Earth," Lowe noted. "They
are still quite pristine and give us the oldest window that
we have on the formative period in Earth's history. There are
older rocks elsewhere, but they've been cooked, heated, twisted
and folded, so they don't tell us very much about what the
surface of the early Earth was really like."

Controversial findings

Lowe and Louisiana State University geologist Gary R. Byerly --
lead author of the Science study -- began collecting samples
from the South African and Australian formations more than
20 years ago. Although thousands of miles apart, both sites
contain 3.5-billion-year-old layers of rock embedded with
"spherules" -- tiny spherical particles that are a frequent
byproduct of meteorite collisions.

"A meteor passes through the atmosphere in about one second,
leaving a hole -- a vacuum -- behind it, but air can't move in
fast enough to fill that hole," Lowe explained. "When the meteor
hits the surface, it instantaneously melts and vaporizes rock,
and that rock vapor is sucked right back up the hole into the
atmosphere. It spreads around the Earth as a rock vapor cloud
that eventually condenses and forms droplets that solidify into
spherules, which rain back down onto the surface."

The meteorite that led to the dinosaur extinction produced
spherule deposits around the world that are less than 2
centimeters deep. But the spherule beds in South Africa and
Australia are much bigger -- some 20 to 30 centimeters thick.
A chemical analysis of the rocks also has revealed high
concentrations of rare metals such as iridium -- rare in
terrestrial rocks but common in meteorites.

In the mid-1980s, when Lowe and Byerly first suggested that
these iridium- and spherule-rich rock layers were produced
by fallout from a meteorite, they were greeted with some
skepticism -- primarily from geochemists, who argued that
the spherules probably did not come from space but were more
likely to have been formed through some kind of volcanic
activity on Earth.

Doubts remained until two years ago, when isotopic studies
confirmed that much of the chromium buried in the rock samples
came from an extraterrestrial source.

"That pretty well laid to rest any lingering doubts of their
impact origin," Lowe recalled.

SHRIMP technology

To narrow down the timeframe when the meteorite impact occurred,
Lowe and Byerly turned to a powerful analytic instrument at
Stanford called the Sensitive High-Resolution Ion MicroProbe
Reverse Geometry -- or SHRIMP RG.

Operated jointly by Stanford and the U.S. Geological Survey
(USGS), the SHRIMP RG rapidly can determine the age of minute
grains of zircon -- one of nature's most durable minerals.

"Of all the minerals on Earth, zircons are the most resistant
to all the things that can happen to rocks," said USGS scientist
Joseph L. Wooden, co-director of the SHRIMP RG and consulting
professor in Stanford's Department of Geological and
Environmental Sciences.

Zircons often contain ancient isotopes of radioactive uranium
that have been trapped for billions of years.

"The SHRIMP RG makes it possible to work with an individual
zircon and quickly determine its age by measuring how much
radioactive decay has occurred," noted Wooden, co-author of
the Science paper. "To dissolve and prepare individual zircon
grains for analysis in a standard lab could take months."

But with the SHRIMP RG, a zircon is simply mounted on a slide,
then exposed to a high-energy beam that determines its age in
about 10 minutes. For the Science study, researchers analyzed
about 50 zircons extracted from South African and Australian
rocks. According to Wooden, it took about one day for the
SHRIMP RG to calculate a more precise age of the zircons --
3.47 billion years, plus or minus 2 million years.

Early Earth

What was Earth like when the ancient collision occurred? No one
is certain, but speculation abounds.

"You'll find that the science of the Archean Earth is full of
personalities and controversies, so you can take your choice,"
Lowe observed.

He and his colleagues point to evidence showing that, 3.5
billion years ago, Earth was mostly covered with water.

"There were probably no large continental blocks like there are
today, although there may have been microcontinents -- very
small pieces of continental-type crust," Lowe said, noting that
if the Archean ocean had the same volume of water as today, it
would have been about 2 miles (3.3 kilometers) deep.

"It would have taken only a second or two for a meteor that's
20 kilometers in diameter to pass through the ocean and impact
the rock beneath," Lowe said. "That would generate enormous
waves kilometers high that would spread out from the impact
site, sweep across the ocean and produce just incredible
tsunamis -- causing a tremendous amount of erosion on the
microcontinents and tearing up the bottom of the ocean."

In addition to the 3.47-billion-year-old impact, Lowe and Byerly
have found evidence of meteorite collisions in three younger
rock layers in the South African formation. According to Lowe,
the force of those collisions may have been powerful enough
to cause the cracks -- or tectonic plates -- that riddle the
Earth's crust today.

"In South Africa, two of the younger layers -- 3.2 to 3.3
billion years old -- coincide with major tectonic changes," he
observed. "How come? Maybe those impacts were large enough to
affect tectonic systems -- to affect the dynamics of the Earth's
crust."

Evolution and meteorites

The impact of these major catastrophes on the evolution of early
life is difficult to determine, Lowe observed.

"The most advanced organisms at the time were bacteria, so there
isn't a big extinction event you can identify as cut-and-dry as
the extinction of the dinosaurs," he said.

He also pointed to controversy about the fossil record, noting
that the oldest known microbial fossils have been found in
rocks 3.4 to 3.5 billion years old -- roughly the same age as
the ancient meteorite collision documented in the Science study.
Could the meteorite somehow have contributed to the origin of
bacterial life on Earth? Lowe has his doubts: "It's quite
possible that life evolved as far back as 4.3 billion years
ago, shortly after the Earth had formed."

He also pointed to uncertainty among scientists about what the
climate of the Archean Earth was really like. In a forthcoming
study, Lowe will present evidence that the average temperature
of the planet back then was very hot -- perhaps 185 F (85 C).

"It's not clear what effect a large meteorite impact would have
on an extremely hot Earth," he explained. "We know in terms
of the present climate that if we had a very large impact, it
would send enormous amounts of dust into the atmosphere and the
climate might cool. Such a scenario may have contributed to the
extinction of dinosaurs. They're really big guys and they're
very strong, but they're actually much more susceptible to
environmental changes than microbes are. Dinosaurs didn't have
anywhere to go -- they couldn't go underground or avoid cold
climates" -- unlike bacteria, which have adapted successfully
to a variety of extreme conditions.

"It looks like what we are seeing is a much greater rate of the
large impacts on the early Earth, certainly than we have today,
and perhaps even a much greater rate than what was suspected,"
Lowe concluded. "I think the effort now will be to try to do
studies like this that will enhance our understanding of the
impactors on early Earth -- to try to find other layers, to
understand the mechanics of impact events and how they
affected early life."

The Science study was supported by grants from the National
Science Foundation Petrology and Geochemistry Program and the
NASA Astrobiology Program. Louisiana State University graduate
student Xiaogang Xie also contributed to the study.

-30-

EDITORS: The Aug. 23 study, "An Archean Impact Layer from the
Pilbara and Kaapvaal Cratons," can be obtained from Science
magazine by calling (202) 326-6440 or by e-mailing
scipak_at_aaas.org .

Photographs can be downloaded at
     http://newsphotos.stanford.edu (slug: "Impactor")

Relevant Web URLs:

* http://shrimprg.stanford.edu/
* http://pangea.stanford.edu/SED/sedgroup.html
* http://www.ucmp.berkeley.edu/precambrian/archaean.html
* http://www.curtin.edu.au/curtin/centre/waisrc/zircons.html
Received on Thu 22 Aug 2002 04:35:50 PM PDT