[meteorite-list] Huge Asteroid Impacts Predate Dinosaurs

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:23:45 2004
Message-ID: <200303052005.MAA00665_at_zagami.jpl.nasa.gov>

http://dsc.discovery.com/news/briefs/20030303/impact.html

Huge Asteroid Impacts Predate Dinosaurs
By Larry O'Hanlon
Discovery News
March 4, 2003

The three most terrible asteroid impacts in the Earth's
history are also the oldest, say geologists
working on frozen blobs of melted rock
ejected from impacts more than 3.2 billion
years ago.

Any craters from the impacts were erased
long ago by Earth's everchanging crust.
What does remain, however, are deposits
of rock spherules in South Africa's
Barberton Greenstone Belt region that
were once a fiery rain of molten material
blasted from horrific impacts.



"The bottom line is that I think they were
bigger than the K/T impact," said geologist
Frank Kyte of the University of California at
Los Angeles, referring to the impact of a
ten-kilometer (seven-mile) wide asteroid
that killed the dinosaurs 65 million years
ago.

Kyte and a number of other asteroid impact
researchers published their report in the
March issue of the journal Geology.

Since they were discovered in 1986, the
South African spherules have sparked
scientific debate over whether they were
caused by impacts or some purely
earthbound process. The work by Kyte and
his colleagues might finally settle the
debate because they have found in the
spherules something only found in
extraterrestrial rocks - an unusual
abundance of a rare type of chromium.

For reasons that are buried in the early
history of the solar system, meteorites tend
to have more chromium-53 and chromium-54 than
rocks from the Earth, moon or Mars, which tend
to have more chromium-52 (Martian meteorites
supplied the information about Mars).

Using a new and painstaking technique to
analyze the South African spherules, Kyte and
his colleagues were able to determine that the
rocks contained amounts of chromium-53 and
chromium-54 that pegged them as relatives of a
relatively rare kind of meteorite, a
carbonaceous chondrite.

Factors they used to estimate the size of the
asteroids that spawned the spherules include the
thickness of the spherule beds and the likely
assumption that the debris rained down over the
entire Earth. All that seems to indicate that
the Archean asteroids ranged from one to seven
times the size of the K/T asteroid.

In other words, the discovery not only confirms
the impact origins of the rocks, but it suggests
that the asteroids 3.2 billion years ago were a
bit different than those that are common today,
said planetary scientist Alan Hildebrand of the
University of Calgary.

They are rough estimates, based on comparisons
to similar beds from the much better preserved
K-T impact. Factors they used to estimate the size
of the asteroids include the thickness of the
spherule beds and the likely assumption that the
debris rained down over the entire Earth. All
that seems to indicate that the Archean asteroids
ranged from one to seven times the size of the KT
asteroid.

"In my opinion this is quite a significant result,"
said Hildebrand. "It should settle the debate on
origins."

What isn't settled is just what was going on 3.2
billion years ago to cause three great impacts in
just 20 million years. Were asteroid impacts more
common throughout the early history of the Earth
or was this just a spike of impacts?

Only the discovery of more such spherules of other
ages will tell, said Kyte.
Received on Wed 05 Mar 2003 03:05:32 PM PST