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Post Info TOPIC: Precambrian Impact


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Precambrian Impact

Precambrian Impact:
The first convincing evidence of a massive meteorite impact that occurred during the Precambrian period, 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.
spacer.gif Pilbara cratons


have long suspected that certain features of the Jeerinan Formation in the area known as the Pilbara Craton were signs Of a large meteor impact.  
However, until now, the only evidence for this had been small splatters of melted rock called spherules, between 3.4 and 2.6 billion years old; the area had lacked the indisputable hallmarks of a Meteorite impact: high iridium levels and shocked quartz.
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.
new2.gif Until now,
spacer.gif Pilbara cratons

Close up

such evidence had not been found in the Pilbara rocks. However, a new search discovered shocked quartz in layers of the rock also containing melt spherules (with ages of ca 3.4 to 2.6 billion years old), high iridium levels and other siderrophile elements in the spherule layer indicate that they contain as much as 2-3 wt% of a chondritic meteorite component.
This provides "compelling evidence" for an extraterrestrial impact, which was likely to be on land.  Geologists have however, not found the crater the meteorite must have left, nor have they found any pieces of the meteorite
. But fallout from the collision lies in layers of the Kaapvaal craton in South Africa and the Pilbara craton in Western Australia, providing a rare glimpse into Earths early meteorite history
But where's the crater? While this was among the largest yet documented impacts in The Precambrian rock record, no one knows exactly where the crater is.
By studying comparative rocks in South Africa, it is shown that 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, is among the largest yet documented in the Precambrian rock record.
This research as "another notch on the record of confirmed impacts that have happened to Earth. It's a nice piece of detective work."
The only life forms present at the time of the impact were single-celled photosynthetic algae. However, not enough palaeontological evidence exists to know what effect the meteorite impact would have had on these.   But 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
The Kaapvaal
spacer.gif Pilbara cratons


and Pilbara cratons contain Earths oldest well-preserved sedimentary and volcanic rocks . They "rest on greatly thickened sections of crust and lithosphere, which have prevented their later subduction, rifting and other major tectonism" .
The Pilbara Craton

in Western Australia has a domainal architecture, which has been interpreted to reflect a history of accretion. The Tabba Tabba Shear Zone is the major division between the East and West Pilbara blocks: this is based on significant differences in the tectono-thermal histories of the bordering terranes.

New laser ablation U-Pb zircon geochronological data, coupled with trace element data for the same core parts of the sampled mineral grains indicate a range of magmatic crystallisation ages for representative igneous rocks emplaced before, during or after shearing.

The Tabba Tabba Shear Zone currently forms the eastern bounding fault of the Mallina Basin. The last major activity in the structure occurred during a major phase of oblique sinistral movement, corresponding to closure of the Mallina Basin. Ages of late syn-kinematic granitic intrusions indicate that this occurred at about 2940 Ma.

These unique locations have kept the formations from experiencing the depths that metamorphose rocks, obscuring their original sedimentary and volcanic features.

The granite
spacer.gif Pilbara cratons


is exposed as batholiths up to a 100 km's in length; these light rocks are intrude into the dark greenstones (metamorphosed basalt). To the south is the Hamersley Range (blue area on map) and the smaller Opthalmia Range (red), bordered on the south by the Ashburton Trough (left) and the Bangemall basin (right). Much of the region contains hills of low relief; the highest area (1235 m) is in the Hamersley Range.



November 19th 2004 An Open University research student revealed her findings on what caused one of the world's 'Big Five' mass extinctions at the Geological Society of America's annual meeting in Denver, USA, this month.
PhD student Charlotte Pearce will input into the debate as to whether a meteorite impact or volcanic eruption caused the Cretaceous Tertiary Boundary (KTB) mass extinction 65-million-years-ago.
Between 50 and 60 per cent of marine and terrestrial life forms became extinct during the KTB extinction, including the dinosaurs.
The cause of this mass extinction has received much attention from scientists over the last 25 years, since the detection of iridium-rich cosmic debris at the boundary layer around the world -- an element known to be rare on earth. This led to the theory that a meteorite impact could have been responsible for the debris and the mass extinction; the 180-km wide Chicxulub impact crater was eventually discovered in the Gulf of Mexico.
However, on the other side of the world, massive volcanic eruptions, known as the Deccan Traps continental flood basalt province, were simultaneously reaching their peak, forming a 2.5km thick pile of lava.

"Both the Chicxulub impact and the Deccan eruptions would have had the potential to induce detrimental environmental changes serious enough to significantly affect terrestrial ecosystems. Dust-induced darkness, acid rain, wild fires and global warming would all have played a role in inducing biospheric trauma, but the timescales over which these were effective would be expected to be different, dependent on the event that caused them.

Charlotte used chemical and isotopic fingerprinting techniques on molecular fossils from North America, and more recently from New Zealand, to investigate patterns which would establish whether there was instantaneous change (as the result of a meteorite impact) or gradual change within the ecosystem (as caused by prolonged volcanism).

"Carbon isotopes can tell us a lot about the stability of an ecosystem and, together with the identification of molecular fossils, enables past variations in habitat, climate and biology to be investigated."

The aim of the project is to compare and contrast samples from several terrestrial and marine KTB successions, at varying palaeogeographical distances from the locations associated with the two putative causes of end-Cretaceous environmental stress. In effect the project examines the effects of these two environmental disasters working outwards from the 'ground zero' locations.
Results taken from samples in the Western Interior of North America show that the ecosystem experienced a short sharp shock at the boundary consistent with a meteorite impact. Early analysis of the New Zealand samples also point to a meteorite impact.

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