[meteorite-list] Ancient Meteorite Leaves Telltale Traces in Australia

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri Dec 3 13:39:15 2004
Message-ID: <200412031839.KAA14736_at_zagami.jpl.nasa.gov>

http://abc.net.au/science/news/space/SpaceRepublish_1255646.htm
 
Ancient meteorite leaves telltale traces
Anna Salleh
ABC Science Online (Australia)
December 2, 2004

The first convincing evidence of a massive meteorite impact that
occurred 2.63 billion years ago has been found in northwestern
Australia's Pilbara region.

Dr Birger Rasmussen of the University of Western Australia
and Associate Professor Christian Koeberl of the University of
Vienna report their discovery in the latest issue of the
journal Geology.

Some scientists have long suspected that certain features of the
Jeerinan Formation in the area known as the Pilbara Craton were signs of
a large meteor that hit the Earth in the Precambrian period.

But Rasmuassen and Koeberl said the only evidence for this had been
small splatters of melted rock called spherules, between 3.4 and 2.6
billion years old.

Until now, the area had lacked the indisputable hallmarks of a meteorite
impact: high iridium levels and shocked quartz.

Earth's own iridium is locked away in the planet's core but meteorites
are full of the metal and thus a spike in iridium levels is used to
detect debris from meteorite impacts.

Quartz crystals in rocks display a particular texture known as shocked
quartz, which can only be formed by the impact of a bomb or a meteorite.

Until now, such evidence had not been reported in the Pilbara rocks. But
Rasmussen and Koeberl have for the first time published evidence of
shocked quartz in layers of the rock also containing melt spherules and
high iridium levels.

They said their findings provided "compelling evidence" for an
extraterrestrial impact, which was likely to be on land.

But where's the crater?

While they said this was among the largest yet documented impacts in the
Precambrian rock record, they do know exactly where the crater is.

By studying comparative rocks in South Africa, they argue the meteorite
would have caused a blanket of material to be thrown out across 32,000
square kilometres covering South Africa and Australia, which were joined
at the time.

Earth sciences Professor Richard Arculus of the Australian National
University in Canberra, described the research
as "another notch on the record of confirmed impacts that have happened
to Earth".

"It's a nice piece of detective work," he told ABC Science Online.

He said the only life forms present at the time of the impact were
single-celled photosynthetic algae. But there was not enough
palaeontological evidence to know what effect the meteorite impact would
have had on these.

Arculus said the impact was not "as big a deal" as the meteorite that
hit at the end of the Cretaceous period that has been implicated in the
extinction of the dinosaurs.
Received on Fri 03 Dec 2004 01:39:08 PM PST