Radioisotope studies show the continental crust formed 3 billion years ago

July 3, 2015 by Heather Zeiger report
Variation in the thickness of juvenile continental crust through time. Credit: (c) 2015 Nature Geoscience, 2015. doi:10.1038/ngeo2466

(Phys.org)—New research sheds light on how and when the modern day continents began to form. Researchers from the University of Bristol analyzed radio isotope abundances in 13,000 samples of continental crust of varying age and found that the continents began to form around 3 billion years ago. This date may coincide with when plate tectonics began. Their research appears in Nature Geoscience.

One of the difficulties with determining the age of the Earth's crust is that the crust itself is constantly undergoing melting and re-deposition due to subduction. This is a larger factor for the ocean floor than in because the continental crust is thick and buoyant, making it less susceptible to subduction. By combining information from various radioisotope pairs found in the continental crust, one can unearth clues as to how the crust was formed and when.

In this paper, Dhuime, et al. studied the abundance of rubidium compared to strontium. Rubidium (87Rb) becomes strontium-87 (87Sr) through . Strontium-87 has a half-life of 48.8 billion years. Strontium-86, the most abundant isotope of Sr, is stable and does not undergo radioactive decay. Therefore, the abundance of 87Sr in the continental crust can be traced to the radioactive decay of 87Rb, and based on its half-life, one can determine the age of a rock sample.

Whenever the Earth's crust re-melts or is formed from magma deposition, the amount of 87Rb and silica increases in the residual melt, but the amount of 86Sr does not. Consequently, 87Rb/86Sr correlates with silica content. The silica content is important because the composition of the continental crust is predominantly silica, while the early Earth's crust was likely mafic, or composed of predominantly iron and magnesium.

To calculate 87Rb/86Sr, Dhuime, et al. first determined the 87Sr-to-86Sr ratio and then determined what the ratio was at the time when the crustal melt crystallized. From this they were able to calculate 87Rb/86Sr and therefore determine the amount of silica present at various time periods.

This calculation was performed for over 13,000 volcanic and plutonic rock samples of varying ages, based on neodymium model ages. Results show that at about 3 billion years ago the Earth's continental crust transitioned from a mafic crust to a silica-rich crust. Silica content increased from 3 billion years to 1 billion years ago. At 1 billion years, the amount of silica started to gradually decrease. This result coincides with previous studies using other radioisotope ratios.

Furthermore, Dhuime, et al.'s data show that there is a positive correlation between the increase in 87Rb/86Sr and silica and crustal thickness. The authors point out that the gradual increase of 87Rb/86Sr from 3 billion years to 1 billion years may indicate that the Earth's continental crust was thickening. They estimate that the average thickness of new continental crust increases from ~20 km at 3 billion years ago to ~40 km at 1 billion years ago, and then decreases to ~30 km to the present.

Since subduction is a result of plate activity, the transition at 3 billion years may also indicate the onset of . The thickness of new continental crust reached a maximum and then began decreasing. The authors point out that this may coincide with the time of the Rodinia supercontinent and mountain formation.

Overall, this study provides compelling evidence that the continental crust formed 3 billion years ago, coinciding with the onset of plate tectonics. Additionally, silica content could be used as a metric for determining crust thickness over time.

Explore further: Earth's crust slowly being destroyed

More information: "Emergence of modern continental crust about 3 billion years ago" Nature Geoscience, 2015. DOI: 10.1038/ngeo2466

Abstract
The continental crust is the principal record of conditions on the Earth during the past 4.4 billion years. However, how the continental crust formed and evolved through time remains highly controversial. In particular, the composition and thickness of juvenile continental crust are unknown. Here we show that Rb/Sr ratios can be used as a proxy for both the silica content and the thickness of the continental crust. We calculate Rb/Sr ratios of the juvenile crust for over 13,000 samples, with Nd model ages ranging from the Hadean to Phanerozoic. The ratios were calculated based on the evolution of Sr isotopes in the period between the TDM Nd model age and the crystallization of the samples analysed. We find that the juvenile crust had a low silica content and was largely mafic in composition during the first 1.5 billion years of Earth's evolution, consistent with magmatism on a pre-plate tectonics planet. About 3 billion years ago, the Rb/Sr ratios of the juvenile continental crust increased, indicating that the newly formed crust became more silica-rich and probably thicker. This transition is in turn linked to the onset of plate tectonics5 and an increase of continental detritus into the oceans.

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19 comments

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richardwenzel987
4.8 / 5 (16) Jul 03, 2015
Oh, no, no, no, no! This is all clearly wrong! Every single premise in this article MUST be wrong! We have it on unimpeachable theological authority that the earth is no more than 6000 years old!
(Heavy sarcasm)
gkam
3 / 5 (6) Jul 03, 2015
Our god made it like that for us to discover.

Hail the Aten!
ogg_ogg
2.3 / 5 (3) Jul 03, 2015
Interesting! Lots here I didn't know. I wonder about the relationship of the crustal growth to the great oxidation event...Nutters out in force, I see. too bad they don't have anything better to do. I guess they're just so lonely they have to make noise without having anything to actually contribute. Mommy! Mommy! Look at Me! Look at Me!
WillieWard
2 / 5 (4) Jul 03, 2015
Rubidium (87Rb) becomes strontium-87 (87Sr) through radioactive decay. Strontium-87 has a half-life of 48.8 billion years. Strontium-86, the most abundant isotope of Sr, is stable and does not undergo radioactive decay. Therefore, the abundance of 87Sr in the continental crust can be traced to the radioactive decay of 87Rb, and based on its half-life, one can determine the age of a rock sample.
Earth's crust is worse than Fukushima. Go to a safer planet, nuclear-phobics.
Solon
not rated yet Jul 03, 2015
You can date the elements, but how can it be shown just when those elements formed into a planet?
Torbjorn_Larsson_OM
5 / 5 (6) Jul 03, 2015
This may be the best evidence yet for a late start of plate tectonics. Many believe it is essential for a rich biosphere, and maybe it is, but having life likely persist through 1/3 of the planets age without implies it isn't essential for a long term biosphere.

Plate tectonic onset ~0.5 billion year before the atmosphere oxygenation may or may not imply a correlation.
Torbjorn_Larsson_OM
5 / 5 (4) Jul 03, 2015
@Solon: I am not sure what you mean.

- Elements can't be dated except hydrogen and helium which emerged when the universe did. The rest are produced in stars, supernovas et cetera continously (from hydrogen and helium).

- We can date rocks with the help of (radioactive) element.

- Similarly the solar system is very well dated by such elements, from the first aggregates in the protoplanetary disk. "The oldest inclusions found in meteorites, thought to trace the first solid material to form in the pre-solar nebula, are 4568.2 million years old, which is one definition of the age of the Solar System.[1]" [ https://en.wikipe...r_System ]

IIRC the error is +/- 0.5 % so it isn't as well dated as the age of the universe which we know +/- 0.1 %.

[tbctd]
Torbjorn_Larsson_OM
5 / 5 (5) Jul 03, 2015
[ctd]

- To date the larger bodies of the solar system it gets more complicated.

-- For the star and gas giants we use star respectively planet formation models.

-- For the terrestrials, which differentiates, isotope ratios can tell the core formation time. Mars formed 3-4 Myrs after the disk.

-- Earth is a special case since the Great Impact that made Earth and Moon from Tellus and Theia contributed the last 10 % of mass and remelted the planet. (I.e. Tellus was Venus massed and Theia Mars massed before the GI.)

Until the last year there were different estimates of Earth age, but now two different methods that use the clock of elements that were replenished by the early bombardment come to the same age. Earth is ~95 +/- 30 Myrs younger than the solar system. I.e. Earth formed ~ 4.47 Gyrs ago, and we know that an ocean and the mafic continental crust the article describes had formed ~ 4.40 Gyrs ago (from dated zircons).
gkam
4 / 5 (4) Jul 04, 2015
"How can the researchers know about the original ratio of these radio isotopes in the crust or when started the radioisotope clocks?"
----------------------------------

You are confusing elements and compounds. The action of the Earth exposes elements to each other, creating minerals and rocks and other compounds. Some elements are created by radioactive decay, then transmute into others in a progression, and we know how long each step takes. The radioactive components decay at a known rate. Pretty simple.
gkam
3.7 / 5 (3) Jul 05, 2015
"3. The process rate must always have been the same. The decay rate must never have changed."
-----------------------------------------
How do you change the decay rate?

I'm waiting.
gkam
2.3 / 5 (3) Jul 05, 2015
"Interesting is the fact that in the Earth's crust can be find isotopes that can not be out there for more than tens thousand years."
-----------------------------------------

That is because they are the daughter products of other decays, which take time, as well. Science is fascinating, and you should look into it.
Captain Stumpy
5 / 5 (5) Jul 05, 2015
@ren
so... half life's are determined by careful observation and measurement (as well as constrained experiments that are designed to remove bias) so you don't like it...

instead, you choose to post complete unsubstantiated conjecture like this
Interesting is the fact that in the Earth's crust can be find isotopes that can not be out there for more than tens thousand years
under the ASSumption that it must be correct because WHY?
re: 1
there is NO such assumption that no other elements are there, but there is a well known observational as well as MEASURED evidence that radioactive materials decay into well known products

this is why RELIGION has no place in science
see also: http://www.ploson...tion=PDF

you continually validate this study every time you post religious diatribe while ignoring the validated evidenciary claims of science
Captain Stumpy
5 / 5 (4) Jul 05, 2015
@ren cont'd
re: 1 - strawman based upon fallacy as well as red herring

re: 2 - another strawman based upon a fallacy. the devay process for multiple radioactive elements is not only known, but OBSERVED and MEASURED

re: 3 - there are no indications that the rate has ever changed. so this is a distraction/red herring

re: 4 - WTF are you trying to get at with this and number 5? that we can never extrapolate data from experiments because they do not mimic real life exactly?
Except that to explain or define a characteristic or series of processes which we KNOW, we must constrain the experiment to allow for specific observations showing what is really happening (see also: Lenski e.coli experiments)

Science is about finding answers - that is why religions interfere with finding those answers. you make ASSumptions and proclamations based upon ignorance (stupidity) or known fallacies and think it is equivalent to scientific research ...

it isn't
gkam
4 / 5 (4) Jul 05, 2015
"How many years of experience have science with different isotopes to accept that the speed of their decay does not change with time?"
-----------------------------------

Gosh, Ren, how do we know the speed of light hasn't changed since the Hadean? Or the speed of sound? Or the Universal Gravitational Constant?

How do we know Xenu didn't change it all when he brought those Thetans to Earth and put them into volcanoes?
Torbjorn_Larsson_OM
5 / 5 (4) Jul 05, 2015
@Ren: ""Strontium-87 has a half-life of 48.8 billion years." How do they now this?"

For pity's sake, with observation of course! If you train to be an engineer, you will likely do it yourself, a fun exercise, with Geiger counters. (And lead blocks and long tongs...)

Besides that it works over sample, over time, with different amount and heritage of radioactive elements, by known processes and their theoretical and observed stability*, et cetera, the different methods (including fossils) and different sediment layers (making many new clocks) cross check each other. You never find a "younger" layer buried under an "older", unless there has been mountain folding which turns the layers around.

* "A number of experiments have found that decay rates of other modes of artificial and naturally occurring radioisotopes are, to a high degree of precision, unaffected by external conditions..." [ https://en.wikipe...ve_decay ]
Torbjorn_Larsson_OM
5 / 5 (3) Jul 05, 2015
@Ren: "Did some microbiologists with a solid reputation in the international scientific community recognize the results of the Lensky experiment".

Way to try to deflect the science of _continental crust formation_. Okay, I'll bite.

"Recent Press about Our Research ...

Carl Zimmer reports in the New York Times on the Scence Article Repeatability and Contingency in the Evolution of a Key Innovation in Phage Lambda by current and former lab members: Justin R. Meyer, Devin T. Dobias, Joshua S. Weitz, Jeffrey E. Barrick, Ryan T. Quick, and Richard E. Lenski (26 January 2012). You can also listen to Justin's Science podcast.

New York Times, Science News, Discover, New Scientist, Nature, and msnbc report our recent findings on evolvability (March 2011)

BEACON, an NSF Center for the Study of Evolution in Action, is funded and announced (17 February 2010) ..."

[ http://myxo.css.msu.edu/ ]

[tbctd]
Torbjorn_Larsson_OM
5 / 5 (4) Jul 05, 2015
[ctd]

"After 25 years and 58,000 bacterial generations, Lenski's
bacteria are still growing, mutating, and evolving. They are
proving as critical to understanding the workings of evolution as
classic paleontology studies such as Stephen Jay Gould's research on
the pace of change in mollusks."

[ "The Man Who Bottled Evolution", Elizabeth Pennisi; http://beacon-cen...nski.pdf ]

PS: To reject radioactivity is like rejecting electricity (especially when your electricity _is_ generated by fission plants =D), stupid and at times dangerous. And it is really easy to grok, the decay law is simpler than Ohm's law even.
Torbjorn_Larsson_OM
5 / 5 (4) Jul 05, 2015
Oops, this was cut, so ctd:

"A new Science paper from the Lenski lab is generating lots of buzz! Facebook's popular page "I F*ing Love Science" sums up the Long-Term Evolution Experiment in this infographic: ..."

[ http://beacon-cen...olution/ ]
TheGhostofOtto1923
3.9 / 5 (7) Jul 07, 2015
How do they now this? By extrapolation? How can the researchers know about the original ratio of these radio isotopes in the crust or when started the radioisotope clocks? And how in the real Earth history various physical factors influenced this clocks? Interesting is the fact that in the Earth's crust can be find isotopes that can not be out there for more than tens thousand blah
Well of course the god who could drop all the photons in their place and send them on their way, in order to make all those stars out there appear to be much older and farther away than they must be in a 6500yo universe, could also manipulate isotope decay rates.

But we can also take this as more evidence that your god is nothing BUT deception. He seeks to trick us at every turn.

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