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
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.
Richard M. Palin, Richard W. White
Emergence of blueschists on Earth linked to secular changes in oceanic crust composition
Nature Geoscience, 14 November 2015
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
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
Wandering greenhouse gas
16.03.2018 | Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung
Unique Insights into the Antarctic Ice Shelf System
14.03.2018 | Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar- und Meeresforschung
Animal photoreceptors capture light with photopigments. Researchers from the University of Göttingen have now discovered that these photopigments fulfill an...
On 15 March, the AWI research aeroplane Polar 5 will depart for Greenland. Concentrating on the furthest northeast region of the island, an international team...
The world’s second-largest ice shelf was the destination for a Polarstern expedition that ended in Punta Arenas, Chile on 14th March 2018. Oceanographers from...
At the 2018 ILA Berlin Air Show from April 25–29, the Fraunhofer Institute for Laser Technology ILT is showcasing extreme high-speed Laser Material Deposition (EHLA): A video documents how for metal components that are highly loaded, EHLA has already proved itself as an alternative to hard chrome plating, which is now allowed only under special conditions.
When the EU restricted the use of hexavalent chromium compounds to special applications requiring authorization, the move prompted a rethink in the surface...
At the ILA Berlin, hall 4, booth 202, Fraunhofer FHR will present two radar sensors for navigation support of drones. The sensors are valuable components in the implementation of autonomous flying drones: they function as obstacle detectors to prevent collisions. Radar sensors also operate reliably in restricted visibility, e.g. in foggy or dusty conditions. Due to their ability to measure distances with high precision, the radar sensors can also be used as altimeters when other sources of information such as barometers or GPS are not available or cannot operate optimally.
Drones play an increasingly important role in the area of logistics and services. Well-known logistic companies place great hope in these compact, aerial...
16.03.2018 | Event News
13.03.2018 | Event News
08.03.2018 | Event News
19.03.2018 | Trade Fair News
19.03.2018 | Life Sciences
19.03.2018 | Life Sciences