[meteorite-list] Double Impact Crater in Canada Formed in Two Separate Impacts

[Ron Baalke]

http://www.astrobio.net/news-exclusive/double-impact-crater-in-canada-formed-in-two-separate-impacts/

Double Impact Crater in Canada Formed in Two Separate Impacts
By Amanda Doyle 
Astrobiology Magazine
Mar 12, 2015

An asteroid smashing into a planet can dramatically alter the planet's 
habitability by setting back evolution or even encouraging biodiversity.

In order to understand how cosmic impacts influence life and the environment, 
scientists study the craters left behind. Some of these impact craters 
come in pairs, most likely caused by binary asteroids. A binary asteroid 
is two asteroids that are orbiting each other, as well as orbiting the 
Sun.

The Clearwater lakes in Canada are a double crater, but geologist Martin 
Schmieder of the University of Western Australia, and colleagues, now 
believe that the craters were formed in two separate events. Their research 
was recently published in the journal Geochimica et Cosmochimica Acta.

A number of double impact craters exist on Earth. In 1965, researchers 
proposed that the craters forming the Clearwater lakes were the result 
of such a single incident. West Clearwater Lake has a diameter of 36 kilometers 
(22.5 miles), while its eastern cousin is 10 kilometers smaller. During 
an impact, rocks from the Earth's crust can be uplifted to form a central 
peak, or ring, within the center of the crater.

In the West Lake, this is evident as a ring of islands in the middle of 
the lake. The East Lake also has a central peak, but it is below the waters 
of the lake and was only revealed when the Geological Survey of Canada 
drilled into the frozen lake in the 1960s.

Measuring the ages of craters

There are a number of different ways to measure the age of an impact crater. 
Sometimes the layers of rock tell the story as the impact might have occurred 
at the boundary between two geological time periods. Fossils preserved 
within rocks can also help place constraints on the age.

It is also possible to use the decay of radioactive isotopes in samples 
of rocks that were created at the time of the impact to find out the age 
of a crater. Isotopes can be stable or radioactive, and if they are radioactive, 
then they will decay into "daughter" products over a known period of time.

There is evidence that the asteroid that formed the East crated impacted 
a marine environment, which would place the impact during the Ordovician 
period. The West crater was created in the Permian period and impacted 
the landmass Pangaea.

Potassium-40 decays slowly into argon-40, so that the more argon-40 present, 
the older the sample is. However, measuring the ratio of potassium-40 
to argon-40 has the disadvantage of the potassium and argon needing to 
be measured separately. A more reliable variant of this method is to convert 
the potassium into argon-39. The rock sample is heated to release both 
the argon-39 and argon-40, so that the two isotopes can be measured at 
the same time. The amount of argon-39 that it is released indicates how 
much potassium-40 was originally in the rock. For the Clearwater dating 
study, this method was applied at the University of Heidelberg in Germany.

The heating of the sample occurs incrementally, in what is known as "step 
heating." Ideally each argon degassing step should yield the same age, 
so that when all the individual ages are plotted together on a graph, 
the age is constant for the entire sample and yields a plateau. This is 
known as a "plateau age." However, in some cases a plateau age is not 
found. Instead, the individual steps often make up a "u-shaped" or "staircase" 
pattern.

Two separate impacts

The West Clearwater Lake has accurate plateau ages from the argon dating. 
Different rock samples all indicate that the crater was formed around 
290 million years ago. The new argon ages of 286 million years determined 
by Schmieder, and his collaborators also agree with this.

The age of the East Clearwater Lake crater is much more difficult to determine. 
In previous work performed by other scientists, a different isotope method 
was used to measure the age of the crater. The rubidium (Rb) to strontium 
(Sr) ratio suggested that this crater is also around 290 million years 
old, roughly the same age as the West crater. However, this method of 
dating is rather unreliable when it comes to dating impact craters.

"Even as a well-established method, Rb/Sr dating has commonly failed in 
impact crater dating" explains Schmieder. 'This is mostly because rubidium 
is very mobile and the Rb/Sr system is therefore easily disturbed by heating 
and weathering that affect the impact rocks after their formation."

Argon ages for the East Lake also show a u-shaped spectrum, rather than 
a clear plateau age. This makes it more difficult to determine an accurate 
argon age, but suggests a maximum age of around 460 million years, which 
would be far older than the dating of the West Clearwater Lake crater. 
In 1990, researchers initially calculated a 460 million age for the East 
Lake, but then assumed it to be incorrect out of suspicion that excess 
argon was contaminating the sample and mimicking an older age for the 
crater.

However, Schmieder and colleagues also determined an argon age of 460 
to 470 million years for the East crater. They consider it highly unlikely 
that four different rock samples that were collected at different locations 
and depths at the impact melt layer inside the crater would all yield 
the same false age.

"We think that the accurate age for the East Clearwater crater was, in 
fact, already measured back in 1990," says Schmieder.

Further evidence

Another point in favor of the older age of the East crater comes from 
studying the magnetization of rocks. The magnetic field of the Earth can 
be "captured" by certain types of rocks, and this magnetic signature can 
be used to study the Earth’s magnetic field throughout history. The magnetic 
poles of the Earth are not fixed, and pole reversals have occurred many 
times in the past.

The rocks from the West Lake show that they were formed during a "superchron," 
which is an unusually extended period of time where no reversals occurred. 
This superchron, known as the Permo-Carboniferous Reversed Superchron, 
lasted from 316 to 265 million years ago, which agrees with the age found 
by the argon dating.

The rocks from the East Lake tell a different story. They have a number 
of different magnetic polarizations, which indicate viscous remnant magnetization. 
This is magnetization that is acquired slowly over a long period of time. 
The more complex magnetic history points to the rocks being much older 
than the West Lake, as they have had more time to be altered.

The argon-argon age of 460 to 470 million years for the East crater suggests 
that this impact occurred in the Ordovician time period in a near-coastal 
environment, when large parts of eastern Canada were occupied by a shallow 
ocean. There are geological clues that point towards an impact in a shallow 
marine or coastal environment at the East crater. The rocks from the East 
crater have more chlorine in them than the West crater, which might be 
indicative of the presence of sea water. There is also some evidence of 
the increased movement of hot fluids after the East impact, which altered 
the rocks. The West crater was formed during the Permian, when the asteroid 
would have struck the Pangaea landmass.

Despite the fact that it is statistically very unlikely for the two craters 
to have been formed in two separate impact events, the new evidence unearthed 
by Schmieder and his team shows that in this case the more unlikely scenario 
is true.

"Overall, the doublet theory has been so compellingly advocated over the 
decades that alternative scenarios seem to have been abandoned. In our 
view, there is a whole line of geologic evidence that argues against the 
double impact."

The impact on life

Impacts that leave behind a 100 kilometer (62.5 mile) diameter crater 
or less, such as those that struck the Clearwater lakes, are widely thought 
to have no global effects. In fact, impacts can even increase biodiversity. 
For example, the Great Ordovician Biodiversification Event, which saw 
an explosion in the number of animal species around 470 million years 
ago, has been linked to frequent impact events at the time. This is possibly 
due to the fact that an impact could disrupt local life just enough to 
let another species dominate, or because slowly cooling craters can provide 
habitats for life.

Even if the Clearwater Lakes impacts were caused by a double impact, the 
extra energy released by two bodies smashing into the Earth simultaneously 
would have had no significant effect on life. While the fireball and earthquake 
would have decimated any life within a few hundred kilometers, the impacts 
were not big enough to cause much chaos on a global scale.