Tuesday, 26 March 2013

Impact crater took up-to one million years to cool

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Crater goatpaddockThe team is planning a campaign for the Goat Paddock site and will then focus on Woodleigh and Yarrabubba. Image: Goat Paddock Crater in the Kimberley by Jesse Allen and Robert Simmon NASA. TWO WA scientists have published a study based on analyses of impact-molten rock samples from the 76-million-year-old Lappajärvi crater in Finland, and now they are shifting their focus to craters much closer to home.

Curtin University’s WA Argon Isotope Facility Director Dr Fred Jourdan and UWA School of Earth and Environment’s Dr Martin Schmieder were the co-authors of ‘The Lappajärvi impact structure (Finland): Age, duration of hydrothermal crater cooling, and implications for life’, published in Geochimica et Cosmochimica Acta.

According to the study, earlier estimates of the cooling of small impact craters suggested a relatively short cool-down period (about 10,000 years after impact).

However, the recent study suggests that the Lappajärvi crater did not cool down as rapidly as expected and may have taken anywhere between 100,000 to one million years.

This may also add an extra dimension in the debate regarding the potential extraterrestrial origin of microbial life on early Earth.

Dr Schmieder says there is no conclusive data on this issue and the idea is heavily disputed, however, cooling impact craters such as Lappajärvi are natural laboratories.

“Some meteorites are carbonaceous chondrites which could be considered the potential basis for life delivered from the outside, but this is only speculation and very hard to prove,” he says.

After publishing the results on the Lappajärvi study, the team has turned attention to the 13 known crater impacts in WA.

In order to confirm a crater impact structure, each site needs to show evidence of ‘shock metamorphic’ effects, which is found in the rock itself.

“We find ‘shatter cones’—they are conical fractures in the rock which indicate the likelihood of being an ancient impact site,” Dr Schmieder says.

“Then we do microscopic studies after retrieving rock samples from these sites. And once we find specific features, like 'planar deformation features' in quartz grains for example, it is confirmed.”

Dr Schmieder says the team is planning a campaign for the Goat Paddock site, and will then focus on Woodleigh and Yarrabubba.

Yarrabubba is potentially the oldest and largest crater impact site in the State.

In order to conduct their research on impact craters, the researchers use the argon-argon dating technique, based on the natural radioactive decay of potassium to argon to measure the age of different minerals formed on impact.

In this technique, samples are usually irradiated to produce 39Ar from 39K, and then degassed in a high-vacuum mass spectrometer.

Dr Schmieder says Perth is a world-renowned leader in the argon-argon dating technique applied to impact craters.

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