Rare minerals reveal huge impact crater

Challenge

When meteorites hit the Earth, a crater forms violently and very fast, lifting deep-seated rocks from the centre of the impact area to the surface in seconds. Measuring the consequent geological deformation at different scales in these complex impact structures is tricky. If microscale mineral deformation can be correlated with macroscale crater-formation processes, we can better understand how the Earth’s surface behaves during meteorite impact.

RESEARCH

The Woodleigh crater near Shark Bay, WA was created by a meteorite impact about 360 million years ago. It is buried below younger sedimentary rocks that obscure its macroscale features, including its size. Estimates of the crater diameter vary between 60 and 120km. Research student Morgan Cox from Curtin University, under the supervision of Dr Aaron Cavosie, is studying core samples from the Woodleigh crater. The samples were collected by the Geological Survey of Western Australia from 300m below what is thought to be the centre of the crater.

Electron microscopy, including electron backscatter diffraction analysis, in the Microscopy Australia Linked Lab at Curtin, revealed tiny areas of a mineral named reidite. This extremely rare mineral only occurs in rocks subjected to the incredible pressure created when space rocks slam into the Earth’s crust. Reidite forms when the common mineral zircon is transformed during the massive pressure of impact.

Electron diffraction image of a shocked zircon that was partially transformed to reidite. Red lines are deformation twins; reidite is shown in purple.

The researchers also found microscopic structures called deformation twins that only form in zircon grains shocked by impact. Analysis revealed that reidite formed first during the initial shock-compression stage. The twins then formed during the post-compression stage, when a lower pressure shock wave lifted the formerly compressed crust rapidly upwards.

The discovery of reidite near the base of the core suggests a relatively large crater. The research team is now using numerical modelling to refine the size of Woodleigh. If its diameter is greater than 100km, as now seems likely, it will be the largest-known impact crater in Australia, and the fourth largest known on Earth. The third largest is the Mexican crater (Chicxulub, largely underwater), formed by the meteorite impact that led to the extinction of the dinosaurs.

Morgan Cox in the field.

Impact

• First physical evidence that links different cratering stages to the predicted theoretical and numerical models of crater formation.
• This analysis provides a new benchmark against which other craters can be analysed.
• Large economic mineral deposits are currently being mined close to the surface in impact craters: gold at the world’s largest impact crater in South Africa (half the worlds’s gold ever mined has come from here); and huge nickel deposits at the world’s second largest impact crater in Canada. It is not currently known if such deposits are present at Woodleigh.
• Shock-twinned zircons from Apollo mission lunar impact samples may now be linked to central uplift regions of complex craters and/or impact basins on the Moon.
• Presence of reidite indicates the crater’s size and is therefore a measure of the resulting environmental catastrophe. This impact is likely to have been an extinction-level event, even though it is smaller than the impact that  caused the dinosaurs to be wiped out.
• The rare presence of reidite enables investigation of its properties and evaluation of its potential as a new material for future technologes.

Morgan A. Cox, Aaron J. Cavosie, Phil A. Bland, Katarina Miljković, Michael T.D. Wingate; Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia. Geology ; 46 (11): 983–986. doi: https://doi.org/10.1130/G45127.1