Scientists reveal likely origin of Earth's oldest crystals (Update)

April 28, 2016
Scanning electron microscope picture of a zircon crystal from the Sudbury crater. Credit: Gavin Kenny, Trinity College Dublin.

New research suggests that the very oldest pieces of rock on Earth—zircon crystals—are likely to have formed in the craters left by violent asteroid impacts that peppered our nascent planet, rather than via plate tectonics as was previously believed.

Rocks that formed over the course of Earth's history allow geologists to infer things such as when water first appeared on the planet, how our climate has varied, and even where life came from. However, we can only go back in time so far, as the only material we have from the very early Earth comes in the form of tiny, naturally occurring zircon crystals.

Naturally then, the origin of these crystals, which are approximately the width of a human hair and more than four billion years old (the Earth being just over four and a half billion years old), has become a matter of major debate. Fifteen years ago these crystals first made headlines when they revealed the presence of water on the surface of the Earth (thought to be a key ingredient for the origin of life) when they were forming.

Ten years ago, a team of researchers in the US1 argued that the ancient zircon crystals probably formed when tectonic plates moving around on the Earth's surface collided with each other in a similar fashion to the disruption taking place in the Andes Mountains today, where the ocean floor under the Pacific Ocean is plunging under South America.

Shatter cones (pyramid-like structures) formed from the shock wave of the impact, and can be seen as that wave migrated through the rock from the bottom up. Credit: Gavin Kenny, Trinity College Dublin.

However, current evidence suggests that plate tectonics—as we know it today—was not occurring on the early Earth. So, the question remained: Where did the crystals come from?

Recently, geologists suggested these grains may have formed in huge impact craters produced as chunks of rock from space, up to several kilometres in diameter, slammed into a young Earth. To test this idea, researchers from Trinity College Dublin decided to study a much younger to see if zircon crystals similar to the very old ones could possibly have formed in these violent settings.

In the summer of 2014, with the support of the Irish Reseach Council (IRC) and Science Foundation Ireland (SFI), the team collected thousands of zircons from the Sudbury impact crater, Ontario, Canada - the best preserved large impact crater on Earth and the planet's second oldest confirmed crater at almost two billion years old.

After analysing these crystals at the Swedish Museum of Natural History in Stockholm, they discovered that the crystal compositions were indistinguishable from the ancient set.

PhD Researcher in Trinity's School of Natural Sciences, Gavin Kenny, is first author of the article which explains these findings, and which has just been published in leading international journal, Geology.

He said: "What we found was quite surprising. Many people thought the very ancient couldn't have formed in impact craters, but we now know they could have. There's a lot we still don't fully understand about these little guys but it looks like we may now be able to form a more coherent story of Earth's early years—one which fits with the idea that our planet suffered far more frequent bombardment from asteroids early on than it has in relatively recent times."

Gavin Kenny recently travelled to the annual Lunar and Planetary Science Conference (LPSC) in Houston, Texas, to present these findings to the space science community.

He added: "There was a lot of enthusiasm for our findings. Just two years ago a group2 had studied the likely timing of impacts on the early Earth and they suggested that these impacts might explain the ages of the ancient zircons. They were understandably very happy to see that the chemistry of the zircons from the Canadian impact crater matched the oldest crystals known to man."

Explore further: Study questions dates for cataclysms on early moon, Earth

More information: Kenny GG, Whitehouse MJ, Kamber BS. Differentiated impact melt sheets may be a potential source of Hadean detrital zircon. Geology. 2016; DOI: 10.1130/G37898.1

References:

1: Watson EB, Harrison TM. Zircon thermometer reveals minimum melting conditions on earliest Earth. Science. 2005; 308:841-844. DOI: 10.1126/science.1110873

2: Marchi S, Bottke WF, Elkins-Tanton LT, Bierhaus M, Wuennemann K, Mordibelli A, Kring DA. Widespread mixing and burial of Earth's Hadean crust by asteroid impacts. Nature. 2014; 511:578-582. DOI: 10.1038/nature13539

Correction: The word 'likely' was added to title at the request of the authors of the publication.

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rhugh1066
3.8 / 5 (6) Apr 28, 2016
"...the chemistry of the zircons from the Canadian impact crater matched the oldest crystals known to man."
If the 2 billion year old zircons match the 4+ billion year old zircons, how does science tell the younger ones from the older ones?
TheGhostofOtto1923
3 / 5 (4) Apr 28, 2016
"...the chemistry of the zircons from the Canadian impact crater matched the oldest crystals known to man."
If the 2 billion year old zircons match the 4+ billion year old zircons, how does science tell the younger ones from the older ones?
Is that a real question or are you trying to make some point or other?

Have you thought about what they might mean by 'match'?
rhugh1066
4.4 / 5 (7) Apr 28, 2016
Exactly. Ever wonder what percentage of this site's readership are non-scientists? Like me? I do, and when I read articles written like this I know that a great many people must draw entirely wrong conclusions simply because of the choices of phrase the writers make. The chemistry matches? Then they must be identical, no? Not so fast, but then I'm not a student of this and so off I go on my merry way, wrong conclusions firmly in hand. Time wasted.
antigoracle
5 / 5 (5) Apr 28, 2016
how does science tell the younger ones from the older ones?

They could infer age by the rocks in which they were found.
RealScience
5 / 5 (9) Apr 28, 2016
@rhugh1066:

The chemistry of the zircons refers to which elements were originally present in which quantities and in which configurations.

The dating is done by which isotopes of which atoms have decayed since the initial zircon crystal formation.

The article appears to be un-paywalled - try:
www.hou.usra.edu/...2473.pdf

From the article it can be seen that the chemistry refers to the use of Titanium in the zircons as a thermometer for the temperature at which they were formed. The dating to the Sudbury impact was confirmed by the post-formation decay of uranium isotopes to lead.

RealScience
5 / 5 (9) Apr 28, 2016
@rhugh1066: Honest quests for knowledge are welcome. The article is not particularly clear that it is the ORIGINAL chemistry at the time of formation of the crystal that matches (in particular, the titanium content that indicates the temperature of formation and the water content of the original melt - see paper linked above).

Since more of the uranium will have have changed to lead in the older crystals, the complete chemistry at the current time will NO LONGER match (although the titanium content still will).

To add a bit of a wrinkle, the uranium ISOTOPE ratio 4 billion years ago was not the same as it was 2 billion years ago, but for atoms as heavy as uranium isotopes are generally ignored in chemistry. (For very light atoms the number of neutrons has a noticeable chemical effect, particularly with deuterium, but life forms have selective chemistry that they sometimes even treat C, N or O isotopes a bit differently.)
torbjorn_b_g_larsson
5 / 5 (5) Apr 28, 2016
RealScience, that isn't the original article as such, but can be a conference synopsis of it.

Well, I don't buy it. Kenny et al present a minor opinion, but their data is shady. I found an article describing that the zircons they choose as associated to the Sudbury by age is ~ 2 % of the total, claiming that the older ones are from earlier impacts. But those do not have the "just so" Ti signature, which Valley's 2015 review show is present in *all* zircons older than 3.5 Ga.

Also, the 3.5+ Ga zircons do not show the typical impact fractures that only ~ 15 % of zircons do. [See the photo for an example.] This problem for the LHB is by the way one reason Kenny et al may pursue their line of work.

[There are other, more arguable problems. The oldest rocks is the ~ 4.4 Ga Nuvvuaggituq according to O'Neill, the expert on them. They do not have zircons. O'Neills explanation is that they are older than the first subduction events.

But that is in itself a minority opinion.]
Hoyasaur
not rated yet May 03, 2016
"In the beginning God created the heaven and the earth...

...and a moment later He created Plate Tectonics...."

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