Age of blueschist does not indicate the origin of plate tectonics

Age of blueschist is not an indicator of the date of emergence of plate tectonics
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. Credit: © Richard White

One of the big mysteries in the history of the Earth is the development of plate tectonics. When exactly did these processes begin? Scientific opinion varies widely.

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 mineral 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 and therefore does not date . 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 in the Alps, in Japan, on the west coast of the USA and other places. 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. High pressure and relatively of 200 to 500 degrees Celsius are required for its formation. As such conditions have only prevailed in 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 is due 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.

Age of blueschist is not an indicator of the date of emergence of plate tectonics
Greenschist takes its name from the actinolite and chlorite minerals it contains. Also present are quartz and epidote. Credit: Richard White

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.

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Earth's crust was unstable in the Archean eon and dripped down into the mantle

More information: Richard M. Palin et al. Emergence of blueschists on Earth linked to secular changes in oceanic crust composition, Nature Geoscience (2015). DOI: 10.1038/NGEO2605
Journal information: Nature Geoscience

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Dec 15, 2015
Naively one would ask if there was a beginning to plate tectonics at all. When the entire earth was entirely molten it would have convected. Locations on the surface which were rising or falling would be more fluid than those sludgy places inbetween the rising and falling zones. That's techtonics. To determine when those sludgy bits become "plates" seems to be like trying to decide exactly how high is the upper limit of the atmosphere. To answer the question accurately you have to give an error-bar related to the density. Changing the error-bar on the 'sludge' would shift the geological time significantly. The easy, and likely accurate, answer is that it always had plate techtonics even when the plates couldn't support a solid mass. So I began with the word "naively" and I leave with the naive notion that pursuing this question naively approaches a Zeno's Paradox - it's answered when you decide it is.

Dec 15, 2015
I wonder if this early 'hot' crust was the result of having the lithosphere ripped off to create the Moon. Much less lithosphere equals thin crust enabling 'plate tectonics,'

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