December 31, 2013
Dense Earth Crust Was Recycled Into The Mantle During Archean Eon
redOrbit Staff & Wire Reports - Your Universe Online
The temperature of the Earth’s mantle during the Archean eon some four billion years ago was significantly higher than it is today, causing the crust to become unstable and drip back down into the mantle, according to research published this month in Nature Geoscience.
As part of the study, Dr. Tim Johnson of the Institute of Geosciences at Johannes Gutenberg University Mainz (JGU) and his colleagues created model calculations that determined that the extreme density of the primary crust caused it to subside back into the mantle.
Conversely, modern tectonic plates primarily move laterally, not vertically, with oceanic lithosphere becoming recycled in subduction zones. The study helps shed new light on “how cratons and plate tectonics, and thus also the Earth's current continents, came into being,” according to a Science 2.0 news report.
Since the mantle temperatures were so much higher during the Archean eon, the planet’s primary crust that formed during that time would have to have been extremely thick and quite rich in magnesium, the investigators said. However, Dr. Johnson’s team found that precious little of this crust had been preserved, implying that the majority of it must have been recycled into the Earth’s mantle.
“Furthermore, Archean crust exposed today is composed mostly of tonalite–trondhjemite–granodiorite (TTG), indicative of a hydrated, low-magnesium basalt source, suggesting that they were not directly generated from a magnesium-rich primary crust,” the authors wrote.
In their paper, the team added that they presented “thermodynamic calculations that indicate that the stable mineral assemblages expected to form at the base of a [29-mile-thick], fully hydrated and anhydrous magnesium-rich crust are denser than the underlying, complementary residual mantle.”
Using two-dimensional geodynamic models, they indicated that the base of this overly-thick, magnesium-rich crust would have been gravitationally unstable at mantle temperatures exceeding 2,732 degrees to 2,822 degrees Fahrenheit. The dense crust would drip down into the mantle, creating a return flow of asthenospheric mantle that would melt and create additional primary crust.
“The conclusion is that these pieces of crust cannot be the direct products of an originally magnesium-rich primary crust,” the university explained in a statement. “These TTG complexes are among the oldest features of our Earth's crust. They are most commonly present in cratons, the oldest and most stable cores of the current continents.”
With the help of collaborators from Yale University, the University of Maryland and the University of Southern California (USC), Dr. Johnson and his fellow JGU researchers used thermodynamic calculations to determine that mineral collections that formed at the base of a 29-mile-thick magnesium-rich crust were denser than the underlying layer of mantle. With the assistance of geophysicists from Mainz University, they were able to use that data to create models simulating the conditions present when the Earth was relatively young.
“Continued melting of over-thickened and dripping magnesium-rich crust, combined with fractionation of primary magmas, may have produced the hydrated magnesium-poor basalts necessary to provide a source of the tonalite–trondhjemite–granodiorite complexes,” the Science 2.0 news report added. “The dense residues of these processes, which would have a high content of mafic minerals, must now reside in the mantle.” Mafic minerals are rich in magnesium and iron.