Continents collide

November 11, 2015

Curtin University geoscientists use cutting-edge techniques to tell the story of Earth’s ancient past.

Continents collide

Collecting rock samples at 5200 m on a recent trip to the Tibetan Plateau, Professor Simon Wilde, from the Department of Applied Geology at Curtin University, was pleased to have avoided the symptoms of altitude sickness. The last time he conducted fieldwork in a similar environment had been about 20 years before in Kyrgyzstan, Central Asia, and he’d managed then to also avoid altitude headaches. Nonetheless, he says, Tibet was tough. Due to the atmospheric conditions, the Sun was intensely strong and hot but the ground was frozen. “It’s a strange environment,” he says.

Wilde was invited by scientists at the Guangzhou Institute of Geochemistry, part of the Chinese Academy of Sciences, to collect volcanic rock samples at the Tibetan site. The region is geologically significant because it is where the Indian tectonic plate is currently “driving itself under the Eurasian plate”, he explains. During their recent field trip, Wilde and his Chinese colleagues collected about 100 kg of rocks, which were couriered back to Guangzhou and Curtin for study. The researchers will be drawing on a variety of geochemistry techniques to analyse the material as they try to paint a picture of what happens when two continents collide, gaining insight into the evolution of Earth’s crust.

“We’re trying to unravel a mystery in a sense,” says Wilde. “We don’t have the full information, so we’re trying to use everything we can to build up the most likely story.”

The Guangzhou geochemists will be analysing trace elements in the rock samples to uncover information about their origins and formation. Back at Curtin, Wilde is working on determining the age of zircon crystals collected from the site, using a technique called isotopic analysis. This involves measuring the ratios of atoms of certain elements with different numbers of neutrons (isotopes) to reveal the age of crystals based on known rates of radioactive decay.

It’s work that’s providing a clearer picture of Earth’s early crustal development and is an area in which Wilde is internationally renowned (see profile, p18).

Gaining an idea of the past distribution of Earth’s continental crust has implications for the resources sector, Wilde explains. “It’s important for people working in metallogeny [the study of mineral deposits] to see where pieces of the crust have perhaps broken off and been redistributed,” he says. “There could be continuation of a mineral belt totally removed and on another continent.”

Continents collide: Copper in demand

Professor Brent McInnes, Director of the John De Laeter Centre for Isotope Research, is also interested in the collision of tectonic plates – to help supply China’s increasing demand for domestic copper. “The rapid urbanisation of China since the 1990s has created a significant demand for a strategic supply of domestic copper, used in air conditioners, electrical motors and in building construction,” explains McInnes. Most of the world’s supply of copper comes from a specific mineral deposit type known as porphyry systems, which are the exposed roots of volcanoes formed during tectonic plate collisions.

McInnes’ research involves taking samples from drill cores, rock outcrops and mine exposures in mountainous regions around the world to be studied back in the lab. Specifically, he and his research team are able to elucidate information about the depth, erosion and uplift rate of copper deposits using a technique called thermochronology – a form of dating that takes into account the ‘closure temperature’, or temperature below which an isotope is locked into a mineral. Using this information, scientists can reveal the temperature of an ore body at a given time in its geological history. This, in turn, provides information with important implications for copper exploration, such as the timing and duration of the mineralisation process, as well as the rate of exposure and erosion.

“Institutions such as the Chinese Academy of Sciences have been awarded large research grants to investigate porphyry copper deposits in mountainous terrains in southern and western China, and have sought to form collaborations with world-leading researchers in the field,” says McInnes.

“We’re trying to unravel a mystery, in a sense. We don’t have the full information, so we’re trying to use everything we can to build up the most likely story.”

Continents collide: Interpreting species loss

Professor Kliti Grice, founding Director of the WA-Organic and Isotope Geochemistry Centre, researches mass extinctions. As an organic and isotope geochemist, Grice (see profile, p12) studies molecular fossils in rock sediments from 2.3 billion years ago through to the present day, also known as biomarkers. These contain carbon, oxygen, hydrogen, nitrogen, or sulphur – unlike the rocks, minerals and trace elements studied by inorganic geochemists Wilde and McInnes.

Grice uses tools such as tandem mass spectrometry, which enables the separation and analysis of ratios of naturally occurring stable isotopes to reconstruct ancient environments. For example, carbon has two stable isotopes – carbon-12 and carbon-13 – and one radioactive isotope, carbon-14. The latter is commonly used for dating ancient artefacts based on its rate of decay. A change in carbon-12 to carbon-13 ratios in plant molecules, however – along with a change in hydrogen – can reveal a shift in past photosynthetic activity.

Grice has uncovered the environmental conditions during Earth’s five mass extinction events and has found there were similar conditions in the three biggest extinctions – the end-Permian at 252 million years ago (Ma), end-Triassic at 201 Ma and end-Devonian at 374 Ma. Among other things, there were toxic levels of hydrogen sulphide in the oceans. Grice discovered this by studying molecules from photosynthetic bacteria, which were found to be using toxic hydrogen sulphide instead of water as an electron donor when performing photosynthesis, thereby producing sulphur instead of oxygen.

“The end-Permian and end-Triassic events were almost identical in that they are both associated with massive volcanism, rising sea levels and increased run-off from land, leading to eutrophication,” Grice explains. Eutrophication occurs when introduced nutrients in water cause excessive algal growth, reducing oxygen levels in the environment. “There were no polar ice caps at these times, and the oceans had sluggish circulations,” she adds.

In 2013, Grice co-authored a paper in Nature Scientific Reports documenting that fossils in the Kimberley showed that hydrogen sulphide plays a pivotal role in soft tissue preservation. This modern day insight is valuable for the resources sector because these ancient environments provided the conditions for many major mineral and petroleum systems. “When you have these major extinction events associated with low oxygen allowing the organic matter to be preserved – along with certain temperature and pressure conditions over time – the materials break down to produce oil and gas,” Grice says.

For example, the Permian-Triassic extinction event – during which up to 95% of marine and 70% of terrestrial species disappeared – produced several major petroleum reserves. That includes deposits in Western Australia’s Perth Basin, says Grice, “and probably intervals in the WA North West Shelf yet to be discovered.”

Gemma Chilton

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