Unravelling zircons, and how they have helped determine the rates of mass extinctions, correlate mass extinctions with volcanic eruptions.
BY G B S N P VARMA
Thierry Adatte scrambles up an incline. In the dirt, his shoes lose traction, sliding and then sliding again, one step up, two steps down. He makes it with a final effort. The land in the Rajahmundry quarries is covered with shrubs and rocks, gaping holes and scoured basalts.
Adatte is a research scientist at the University of Lausanne in Switzerland and is on his fifth field trip here. The others are Gerta Keller, Princeton University paleontologist and geologist, and Maricella Coronado, one of her undergraduate students, Princeton geochronologist professor Blair Schoene, his PhD student Kyle, and Professor Syed F. R. Khadri of Amravati University.
As they converge on Adatte, you hear their shoes scrunch on the dirt, deep breaths, and a mélange of German, Swiss, English and Spanish.
A primal compass guides Adatte to a thin green layer between two seams of basalt. The layer is a remnant of a time when lava flowed here and a shallow sea lapped the shores. He pokes the dirt surrounding the layer, makes an indentation underneath, bringing the layer into greater relief so that others see it. The layer could contain zircons.
Zircons (zirconium silicate, ZrSiO4) are what geochronologists call an accessory mineral. They show up in different types of rock in small abundances, around 0.1 per cent, and they’re very small, a couple of hundred microns (1,000 microns =1mm). The good thing for geochronology—the science of determining the ages of events in earth’s history—is that when they crystallise in molten rock or lava, they incorporate a lot of uranium in their structure, making it possible to date volcanic eruptions accurately.
Uranium decays to lead through radioactivity, and the rate of decay allows you to calculate age. It is the founding principle of radiometric dating, or geochronology. Zircons tell when different rocks formed and different earth processes happened. They are totems of deep time and nature’s time capsules.
After his fingers work in the dirt, Adatte calls Schoene. “God of zircons,” Keller ribs Schoene.
Schoene plunks himself in the dirt, scrapes the green layer with his fingers, and contemplates it for some time. He takes his notebook, and sketches the layer. Then he picks up and pours the dirt into a ziplock bag, seals it shut, and folds it, involuntarily bringing it to his mouth as if to kiss.
Zircons “are really pretty under the microscope,” Schoene says. They’re ancient and hardy, they don’t erode or melt, and can survive conditions that the surrounding rock cannot. What’s more, they provide clues about how life might have originated in the Hadean period—the first 700 million years of earth.
An analysis by Mark Harrison et al in the journal Nature in 2008 suggests that plate tectonics had come into play by then, and a chemical cocktail was already brewing for the evolution of life. That new analysis sharply contrasts with the old view that life started after the meteors stopped falling. The researchers got their data after analysing zircons from Jack Hills in Western Australia.
Zircons have helped determine the rates of mass extinctions, correlate mass extinctions with volcanic eruptions, not just at the KT boundary, but other mass extinctions in the past 500 million years.
The key is the uranium inside them. The half life of one uranium isotope is 4.5 billion years, and that of another is 700 million years. Because uranium has that sort of range, it allows the dating of things as old as the earth itself, and things that are younger, a few hundred thousand years old. Carbon dating is limited to about 50,000 years because its half-life is short.
So the story zircons could tell at Rajahmundry is when the basalts erupted.
You would think that the straightforward way is to take the basalts, crush them, find the zircons, and age-date them. But Schoene says the problem for geochronologists is that basalts typically don’t contain zircons.
“You need high silica and high zirconium because they are the stuff zircons are made out of. Basalts have low silica and low zirconium.”
Occasionally, however, you get little patches of melt with high silica and high zirconium. Alternatively, instead of dating basalts directly, the team is looking for ash beds between the basalts—ash beds or what are called red boles may contain ash material.
Red boles are ancient soils, or paleosols, basalts exposed to the atmosphere and weathering down to form soils. The hope then is that the volcanic ash bed fell into those soils.
“You’re literally dating the time at which other volcanoes erupted and put ash within the sequence of basalts. That tells you the time at which the thin layer erupted and if you have enough of those beds you can put together the sequence of basalt eruptions as well. It is an indirect way of dating them,” Schoene says.
It’s the paleosols they found in the Western Ghats in 2013 at the base of the Deccan Traps that helped date the main phase of Deccan eruptions, which led to a paper in Science in December 2014. They report that “the main phase of eruptions initiated 250,000 years before the end-Cretaceous mass extinction, and that > 1.1 million km3 of basalt erupted in 750,000 years.”
The volume of basalt is hundreds of thousands to a million times bigger than most eruptions in historical records, and covers the entire western half of India. “Being able to say when they began relative to the end-Cretaceous mass extinction allows us to ask more pointed questions about their role in ecosystem deterioration and eventually extinction,” Schoene says.
Keller observes: “The new high-precision zircon data demonstrates that massive lava eruptions preceded mass extinction by just 2,50,000 years, a time interval known for rapid global warming and extreme stress for life on earth leading up to the mass extinction. This dating lays to rest the old controversy that Deccan volcanism was unrelated to mass extinction.”
“What we still lack,” she continues, “is additional high-precision dating of major lava flows to reveal the timing and tempo of eruptions including those associated with the mass extinction. Most importantly, we would like to put the finger on the killer lava flows and final punch that caused the mass extinction.”
“The controversy over the impact and volcanism [theories] has been resolved by the new data of volcanic ash and related materials collected from the red/green boles/ segregations which has provided precise and accurate information that led to the confirmation of dinosaur deaths mainly due to huge volcanic activity that led to the release of poisonous gases,” says Syed Khadri.
The zircons have been key to the new answers but finding those ash beds and paleosols in huge flows and lava in the Western Ghats has not been easy. When you’re searching for a needle in a haystack, the last thing you want is more hay.
Is luck involved, or did some cosmic force intervene to guide them to one small niche in the pile?
“Call it luck, call it skill, call it cosmic force,” Schoene smiles. Or, “Call it thinking on the fly in the field and coming up with the best you can.”
In the Rajahmundry quarries, they hope to find zircons in the paleosols. Hence the thin green layer. All of them gather and contemplate the layer.
The talk turns to the ages of rocks, ash beds and soils. They talk of millions of years as minutes or hours in a day.
Geologists and their ilk are tactile people, holding the rock—time petrified—in their hands and communing with it. Their biological periphery extends to their hammers. If their fingers don’t open up the rock, they give it a whack from the hammer, and they’re looking at a brief history of time.
“Gerta,” he calls out to Keller. “After two weeks of strict diet, here is breakfast,” he says handing over the rock full of fossils. That’s paleontological humour for you.
They can laugh because the bio-stratigraphic record already exists in the Rajahmundry quarries and ONGC well cores. Foraminifera, or forams, existed here. Interestingly, 250,000 years before the extinctions—66 million years ago—the stratigraphy shows environmental changes. A number of papers suggest that extinction for many mammals started much before the mass extinction at the end of the Cretaceous.
For example, Bandana Samant from Nagpur University, and Dananjay Mohabey from the Geological Survey of Nagpur (2014) report “a sharp decline in diversity of titanosauriform-abelisaurid dominated dinosaurian fauna … and with the onset of main Deccan volcanic activity … only a single taxon surviving.” They linked it to the onset of Deccan volcanism in C29r.
“Here,” Schoene says, sweeping his hand across, “it’s interesting because in the upper and lower sequence of basalt flows you have bio-stratigraphy between them.” Using that, “we have some idea what the age of these things should be.” What U-Pb geochronology does is to give the age more precision.
The goal, he says, is to determine which flows in the Rajahmundry quarries correlate back to the flows in the Western Ghats. In short, which flows came oozing down from the Western Ghats to this place. Next is correlating that age with biostratigraphic sequences around the world whose dates are well known.
In his Princeton lab, Schoene and his graduate student Kyle will separate individual zircons from the rock. You smash every mineral down to about less than 500 microns, and separate them using magnetic properties and density. And you hopefully end with a vial full of zircons, he says.
“It’s pretty standard technique but it’s cool to check the rock like that.” The heavy minerals can be separated using heavy liquids that are denser than most rocks. In those liquids, zircons sink and other minerals float. You can also use magnets to separate minerals.
Other things you can do with zircons is that you can slice half way through them and look at their internal zonations, he says. You take a bunch of them, and mount them on epoxy, slice half way through them, and use various technologies to image the inside of zircons.
“They could have different stories told by the zonation within the zircons, It tells you something about how it grew,” he says.
For example, he explains, “volcanic zircons—the kind we are interested in here—have typical growth patterns. You can see the textures within them to help confirm if they are actually volcanic.”
The team walks off the spot, happy and sure that the slice of the day yielded a layer that “looks promising.”
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