Tag Archives: zircon

World_geologic_provinces

The Earth had continental crust much earlier than thought — potentially life, too

The Earth might have developed its continental crust much earlier than believed, new research reveals. The findings could have major implications for how we think about the evolution of life on our planet.

World_geologic_provinces

Map showing the world’s geologic provinces.
Image credits United States Geological Survey.

Strontium atoms locked in rocks from northern Canada might rewrite the history of life on Earth. According to new research from the University of Chicago, they suggest that continental crust developed hundreds of millions of years earlier than previously assumed.

Crustally fit

“Our evidence, which squares with emerging evidence including rocks in western Australia, suggests that the early Earth was capable of forming continental crust within 350 million years of the formation of the solar system,” says first author Patrick Boehnke.

“This alters the classic view, that the crust was hot, dry and hellish for more than half a billion years after it formed.”

There are two types of crust covering the Earth: oceanic, which is basically solidified magma, and continental, which is less dense and has a different chemical make-up — most notably, a much higher content of silica. We know that all crust starts out as the oceanic kind, and continental crust later develops on top of this. Geologists have been trying to determine how and at what point continental crust first appeared ever since we’ve known there is such a thing as ‘continental crust’.

However, that’s easier asked than answered. Part of the problem is that the Earth’s crust is continuously recycled over geological timescales — it sinks, melts down, and reforms. This also destroys the evidence geologists would need in order to back-track the process of continental crust formation.

Some fragments of these ancient bits of crust can still be found today, embedded into young rocks as flakes of the mineral apatite. But if they’re not perfectly insulated, they will degrade over time through oxidation, interaction with water, or other chemical and mechanical means.

Luckily, some of the younger minerals also include some that are very durable, such as zircons. These are hardy materials, similar to diamonds, that are very weather-resistant. Even better for a geologist with a mission, zircon can be dated.

“Zircons are a geologist’s favorite because these are the only record of the first three to four hundred million years of Earth. Diamonds aren’t forever — zircons are,” Boehnke said.

The team used strontium isotope analysis to date rocks retrieved from sites in Nuvvuagittuq, northern Canada to determine their age and the amount of silica present as it was forming. Because the flakes of rock they recovered were incredibly tiny — about as thick as a strand of spider silk, five microns across — the team had to use chili.

More specifically, they had to use CHILI (all capitalized). This unique instrument, the Chicago Instrument for Laser Ionization, came on-line last year. It uses laser beams that can be tuned to pick out and ionize strontium atoms, allowing the team to count them. The results of this counting process suggested plenty of silica was present when it formed.

The chemical composition of the crust tells us a lot about the state of the Earth at the time — our planet is like one huge chemistry jar, and every component interacts with all others. Crustal composition directly affects the atmosphere, for example, mostly through oxidation effects. It also alters the composition of seawater and dictates what nutrients are available to any potential organisms. The fact that Earth sported continental crust that early, and that is was so chemically similar to that of today, suggests that conditions at the time weren’t that different from those today. That doesn’t mean the continents looked like they do today (because they didn’t) but geochemical conditions should have been pretty similar to those today.

It could also be a sign that fewer meteorites hit Earth at this time than we assumed — these would pummel the planet, making it hard for continental crust to form.

The findings also suggest we need to take a second look at the processes we believe create continental crust: if the team’s findings are true, they need to work much faster than current models assume.

The paper “Potassic, high-silica Hadean crust” has been published in the journal Proceedings of the National Academy of Sciences.

Mistastin lake.

Gemstones prove asteroid impact was the hottest event ever recorded on Earth

Snow-clad Canada boasted, for the briefest of moments, the highest temperature ever recorded on Earth. The blistering event, clocking in at some 2370°C (4300°F), was the product of a chunk of space rock crashing down on the planet. Researchers looking into the event say we had no idea “real rocks can get that hot,” underscoring how devastating meteorite impacts can be.

Mistastin lake.

Image credits Jcmurphy / Wikimedia.

Some 40 million years ago in what we now call Canada, an otherworldly visitor was dropping in for a visit. This meteorite crashed with such speed and force that it heated rocks at the impact site to over half the temperature of the Sun’s surface — some 2370°C (4300°F). This event, which has the distinction of being the highest temperature witnessed on our planet, was recorded in gemstones formed under that immense release of heat.

Falling skies

Impacts between space-faring matter and the Earth release a monumental amount of energy — which leads to some mind-bogglingly high temperatures developing in the collision zone. In fact, the more energetic of such impacts (those which involved larger pieces of space rock) shaped the planet into what we know today. They affect the chemical makeup of the atmosphere and crust, directly playing a part in Earth’s habitability, and could even have created the Moon.

Pinning an actual number on these temperatures, however, is a tricky proposition. For one, notable impacts took place a very long time ago, on the order of millions of years. Secondly, because they’re near apocalyptic events, shock waves released during impacts tend to literally vaporize both the meteorites and the rocks they hit. Which is really bad news if you’re trying to analyze those rocks and estimate how much heat they were subjected to.

So researchers have an idea of the upper extremes these temperatures can reach — estimated to be well over 2000°C (3632°F) — but no way to refine that estimate since there’s no geological evidence to test it against. After all, you’d need something that can shrug off an event of such magnitude it turns rocks into thin air.

Thankfully, a team led by Nicholas Timms, a senior researcher and lecturer of Geology at the Curtin University in Perth, Australia, has found one such substance. Their work focused on the 28-kilometer (17.4 miles) wide Mistastin Lake crater in Labrador, Canada, estimated to have been the site of a violent impact some 40 million years ago. The team reports that there was enough energy released during the impact to fuse rock-borne zircon into gem-like cubic zirconia, whose minimum formation temperature is 2370 °C.

“These new results underscore just how extreme conditions can be after asteroids strike,” Timms says.

“Nobody has even considered using zirconia as a recorder of temperatures of impact melts before. This is the first time that we have an indication that real rocks can get that hot.”

The finding showcases how extreme conditions can become in the few minutes after an asteroid impact, and offer us a sort of benchmark for shelters if we’re ever faced with such an event — for which we may be long overdue.

It also helps offer us a glimpse into the environment of early Earth, which was constantly and repeatedly bombarded from space. The team says these impacts could have churned the crust enough to keep hydrogen, carbon, and sulfur in circulation in the atmosphere. These elements are fundamental to life as we know it (you need to mix both hydrogen and oxygen to get water). However, they point out that too severe a bombardment would have negatively impacted the planet’s climate and chemical balance, making it less habitable in the long run.

The paper “Cubic zirconia in >2370 °C impact melt records Earth’s hottest crust” has been published in the journal Earth and Planetary Science Letters.

A 4.4 billion-year-old zircon crystal from the Jack Hills region of Australia. Photo: University of Wisconsin

This is the oldest known piece of our planet – a 4.4 billion-year-old gem

Using two different dating technique, geologists have come across what they believe to be the oldest piece of Earth discovered thus far. The zircon crystal, found on a sheep ranch in Western Australia , was confirmed to be 4.4 billion years old and offers tantalizing clues and insights on how our planet must have looked like in its infancy. To put things into perspective, our planet is believed to have formed 4.5 billion years ago.

A 4.4 billion-year-old zircon crystal from the Jack Hills region of Australia. Photo: University of Wisconsin

A 4.4 billion-year-old zircon crystal from the Jack Hills region of Australia. Photo: University of Wisconsin

The geological relic indicates, for one, that Earth’s crust formed shortly after the planet stabilized and formed. John Valley, a University of Wisconsin geoscience professor who led the research, said the findings suggest that the early Earth was not as harsh a place as many scientists have thought.

No doubt, this is an extraordinary find, however, the untrained eye would have surely missed it. Measuring about  200 by 400 microns, or roughly two times the width of a human hair, the tiny gem was luckily retrieved by geologists in 2001 from a rock outcrop in Australia’s Jack Hills region.

“Zircons can be large and very pretty. But the ones we work on are small and not especially attractive except to a geologist,” Professor Valley said. “If you held it in the palm of your hand, if you have good eyesight you could see it without a magnifying glass.”

The zircon fragment was dated using two separate techniques. The first one was the conventional dating through  the study of radioactive decay of uranium to lead in a mineral sample.  Because some scientists voiced that this method is unreliable for samples this old because of the possible movement of lead atoms within the crystal over time, a second technique was employed to date the sample.

The oldest piece of Earth

Using what’s known in the field as atom-probe tomography, the researchers identified individual atoms of lead in the crystal and determined their mass. Indeed, the analysis confirmed that the gem was genuinely 4.4 billion years old.  This suggests that Earth’s crust formed just 100 million years after the planet stabilized under the form of a giant molted ball (or was it?). This also means that the crystal came into existence just 160 million years after the solar system was formed. Yes, this rock right here has seen a few!

Geological timeline of Earth. Photo: University of Wisconsin

Geological timeline of Earth. Photo: University of Wisconsin

Interestingly enough, the zircon crystal seems to support the “cool early Earth” theory that states that during the Hadean eon, when the Earth was exposed to hellish conditions like meteorite bombardment and an initially molten surface, it’s possible that the planet might have hosted oceans and even life even by then.

“One of the things that we’re really interested in is: when did the Earth first become habitable for life? When did it cool off enough that life might have emerged?” Professor Valley said.

“We have no evidence that life existed then. We have no evidence that it didn’t. But there is no reason why life could not have existed on Earth 4.3 billion years ago,” he added.

To our knowledge, the oldest fossil record is  3.4 billion years old – stromatolites produced by an archaic form of bacteria. The gem was covered in a paper published in the journal Nature Geoscience. 

Ancient, long-lost continent found under the Indian Ocean

Evidence of drowned remnants of an ancient microcontinent have been found in sand grains from the beaches of a small Indian Ocean island, according to a new research.

Zircons and volcanoes

mauritia

This evidence was found in Mauritius, a volcanic island 900 kilometres east of Madagascar which serves as an exotic destination for many tourists. Basaltic rocks from the island have been dated to approximately 9 million years ago, but now, an international research team analyzed the beaches and found fragments of zircon that are much older, between 600 million and 2 billion years old.

Bjørn Jamtveit, a geologist at the University of Oslo explained that the zircons had crystallized within granites or other acidic igneous rocks (basalts being basic, non acidic). He believes that rocks containing these minerals came from a long-submerged landmass that was once wedged between India and Madagascar in a prehistoric supercontinent known as Rodinia; geologically recent volcanic eruptions brought the rocks up to the surface, where they were eroded, resulting in the shards they picked up. Most of the rocks were melted by the high temperatures, but some grains of zircons survived and were frozen into the lavas, rolling towards the Mauritian surface.

“When lavas moved through continental material on the way towards the surface, they picked up a few rocks containing zircon,” study co-author Bjørn Jamtveit, a geologist at the University of Oslo in Norway, explained in an email.

rodinia

The tectonic plates are mobile in geologic time – the surface of the Earth didn’t always look like this. As a matter of fact, the further down you go on the time scale, the more different it looks like. According to plate tectonic reconstructions, Rodinia existed between 1.1 billion and 750 million years ago; virtually all of the Earth’s landmass was concentrated in this single supercontinent which started to split 3/4 billion years ago.

The study also analyzed the gravity field and as it turns out, something really interesting happened to the remains of Rodinia in that area. As India and Madagascar began to drift apart some 85 million years ago, the landmass just sinked, Atlantis style. The cause was tectonic rifting and sea-floor spreading sending the Indian subcontinent surging northeast, sinking the fragments of Mauritia (how the researchers named this microcontinent).

The variations in the gravitational field observed in some areas in Mauritius, the Seychelles, and the Maldives is pretty much a smoking gun suggesting a thick crust supporting the long-lost continent theory, with the continent being “tucked” under the Indian Ocean.

gravity

A non-geologic accident?

The only weak point, is that the study, thorough as it is, relies mostly on those zircons; couldn’t they be just some sort of non-geologic accident?

“There’s no obvious local source for these zircons,” says Conall Mac Niocaill, a geologist at the University of Oxford, UK, who was not involved in the research.

It also doesn’t look like they were brought there by winds.

“There’s a remote possibility that they were wind blown, but they’re probably too large to have done so,” adds Robert Duncan, a marine geologist at Oregon State University in Corvallis.

Also, the samples were picked up from remote sites, where it’s quite unlikely that humans would have brought them there. However, Jérôme Dyment, a geologist at the Paris Institute of Earth Physics in France, is not convinced. He believes that a number of non-geologic processes could have brought the minerals there, as part of ship ballast or modern construction material for example.

“Extraordinary claims require extraordinary evidence, which are not given by the authors so far,” said Dyment, who did not participate in the research. “Finding zircons in sand is one thing, finding them within a rock is another one … Finding the enclave of deep rocks that, according to the author’s inference, bring them to the surface during an eruption would be much more convincing evidence.”

He makes an even more convincing argument, explaining that if remains of such a continent were to exist, evidence for its existence should have been found as part of an ongoing experiment that installed deep-sea seismometers to investigate Earth’s mantle around Réunion Island, which is situated about 200 kilometers from Mauritius.

So is this compelling evidence, or is it more of an educated assertion? But Conall Mac Niocaill, a geologist at the University of Oxford in the U.K. who was also not involved in the study, is spot on: “the lines of evidence are, individually, only suggestive, but collectively they add up to a compelling story.”, he says. Particularly, the geophysic (gravimetric) evidence is highly consistent with the researchers’ claims. All in all, it paints a consistent picture which makes sense in a tectonic context, but as almost always in geology, you can’t just draw a line and say “This is so”; one thing’s for sure though: oceanic basins worldwide may very well host similarly submerged remains of “ghost continents”.

Via Nature Geoscience