Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Age of blueschist is not an indicator of the date of emergence of plate tectonics

15.12.2015

Formation of blueschist is determined by changes to the chemical composition of oceanic crust

One of the big mysteries in the history of the Earth is the emergence of plate tectonics. When exactly did the processes of plate tectonics begin that today involve the subduction of oceanic plates? Scientific opinion varies widely as to this.


A typical blueschist rock: Blueschist is named for its blue-violet color that is due to the presence of the mineral glaucophane; the green mineral in the rock is called epidote.

photo/©: Richard White


A typical greenschist rock: Greenschist takes its name from the actinolite and chlorite minerals it contains; also present are quartz and epidote.

photo/©: Richard White

The dominant view is that oceanic plates have been pushing under other plates and sinking into the Earth's mantle – a process known as subduction – since the beginning of the Hadean eon, more than four billion years ago. Others date the onset of plate tectonic movements to the Neoproterozoic era of 500 to 1,000 million years ago.

This hypothesis is based on the fact that the rock called blueschist began to appear 700 to 800 million years ago. Geoscientists at Johannes Gutenberg University Mainz (JGU) in Germany have now shown that the appearance of blueschist is connected to long-term changes in the composition of the oceanic crust and therefore does not provide evidence of when plate tectonics began. The study has been published in the eminent journal Nature Geoscience.

Blueschist is a blue-violet colored rock that is relatively rare and is found, among other places, in the Alps, in Japan, and on the west coast of the USA. The oldest blueschist found originated in the Neoproterozoic era and is 700 to 800 million years old.

This metavolcanic rock is created during the subduction of oceanic crust. Required for its formation are high pressure and relatively low temperatures of 200 to 500 degrees Celsius. As such conditions have only prevailed in subduction zones in the recent past, blueschist provides evidence of when subduction-driven plate tectonics occurred. The reason why there was no blueschist present on Earth during its first 3.8 billion years is a hotly contested topic among geologists.

"We know that the formation of blueschist is definitely linked to subduction," explained Professor Richard White of the Institute of Geosciences at Mainz University. "The fact that the oldest blueschist is only 700 to 800 million years old does not mean, however, that there were no subduction processes before then, as is sometimes claimed," added Dr. Richard Palin.

In their study, the two researchers have now managed to demonstrate for the first time that the absence of blueschist in the earliest geological periods goes back to a change in the chemical composition of the ocean's crust in the course of the Earth's history, which in turn is a result of the gradual cooling of the Earth's mantle since the Archean eon.

The oceanic crust that formed on the early, hot Earth was rich in magnesium oxide. Using computer models, Palin and White have been able to show that it was not possible for blueschist to form from this magnesium oxide-rich rock during subduction. Instead, the subduction of the magnesium oxide-rich oceanic crust led to the formation of rock similar to greenschist, which is a metamorphic rock that is formed today at low temperatures and low pressure.

Since these greenschist rocks can hold more water than most blueschist, more fluid was able to enter the early Earth's mantle than today, a factor that has an effect on the formation of magmas, which is one of the topics being studied by the Volcanoes and Atmosphere in Magmatic Open Systems (VAMOS) research unit at Johannes Gutenberg University Mainz.

Publication:
Richard M. Palin, Richard W. White
Emergence of blueschists on Earth linked to secular changes in oceanic crust composition
Nature Geoscience, 14 November 2015
DOI: 10.1038/NGEO2605

Further information:
Professor Dr. Richard White
Head of the Metamorphic Geology work group
Institute of Geosciences
Johannes Gutenberg University Mainz
D 55099 Mainz, GERMANY
phone: +49 6131 39-24781
fax: +49 6131 39-23071
e-mail: rwhite@uni-mainz.de
http://www.geowiss.uni-mainz.de/840_ENG_HTML.php

Weitere Informationen:

http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2605.html - link to the Nature Geosciences article ;
http://www.geowiss.uni-mainz.de/index_ENG.php - Institute of Geosciences at Johannes Gutenberg University Mainz ;
http://www.geowiss.uni-mainz.de/482_ENG_HTML.php - Metamorphic Geology work Group ;
http://www.vamos.uni-mainz.de/ – Research Unit on Volcanoes and Atmosphere in Magmatic Open Systems (VAMOS)

Petra Giegerich | idw - Informationsdienst Wissenschaft

More articles from Earth Sciences:

nachricht New research calculates capacity of North American forests to sequester carbon
16.07.2018 | University of California - Santa Cruz

nachricht Scientists discover Earth's youngest banded iron formation in western China
12.07.2018 | University of Alberta

All articles from Earth Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: First evidence on the source of extragalactic particles

For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.

To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...

Im Focus: Magnetic vortices: Two independent magnetic skyrmion phases discovered in a single material

For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.

Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...

Im Focus: Breaking the bond: To take part or not?

Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.

A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...

Im Focus: New 2D Spectroscopy Methods

Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.

"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....

Im Focus: Chemical reactions in the light of ultrashort X-ray pulses from free-electron lasers

Ultra-short, high-intensity X-ray flashes open the door to the foundations of chemical reactions. Free-electron lasers generate these kinds of pulses, but there is a catch: the pulses vary in duration and energy. An international research team has now presented a solution: Using a ring of 16 detectors and a circularly polarized laser beam, they can determine both factors with attosecond accuracy.

Free-electron lasers (FELs) generate extremely short and intense X-ray flashes. Researchers can use these flashes to resolve structures with diameters on the...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

VideoLinks
Industry & Economy
Event News

Leading experts in Diabetes, Metabolism and Biomedical Engineering discuss Precision Medicine

13.07.2018 | Event News

Conference on Laser Polishing – LaP: Fine Tuning for Surfaces

12.07.2018 | Event News

11th European Wood-based Panel Symposium 2018: Meeting point for the wood-based materials industry

03.07.2018 | Event News

 
Latest News

World’s Largest Study on Allergic Rhinitis Reveals new Risk Genes

17.07.2018 | Life Sciences

Electronic stickers to streamline large-scale 'internet of things'

17.07.2018 | Information Technology

Behavior-influencing policies are critical for mass market success of low carbon vehicles

17.07.2018 | Power and Electrical Engineering

VideoLinks
Science & Research
Overview of more VideoLinks >>>