Why do we still find rocks from the Archean, one of the earliest geological eons on Earth dating from about 3.8 to 2.5 billion years ago? This is an apt question as our planet is one of the most dynamic in the solar system. Earths crust has been constantly destroyed and created throughout its 4.5-billion-year history.
Tectonic plates are generated at mid-oceanic ridges and sink at their edges in subduction zones or mountain collisions. Continents assemble and break up in 100-hundred-million-year cycles. Yet the ancient cores of continents, called cratons, have survived this violent past. Their old rocks provide a window into the earliest days of Earths geological history.
Cratons resemble icebergs floating in an ocean. Their deep mantle roots, to a depth of 200 km (124 miles) or more, are largely unaffected by the asthenosphere into which they project. The asthenosphere, a zone of Earth's mantle that lies beneath the tectonic plates and consists of several hundred kilometers of deformable rock, flows like putty and drives plate tectonics. Why these roots are not destroyed by the tectonic plate engine is a puzzle.
In the Sept. 2 issue of the journal Nature, Anne Peslier, a Jacobs Technology scientist working at NASAs Johnson Space Center in Houston, and her colleagues, Alan Woodland and Marina Lazarov from the University of Frankfurt, and David Bell from Arizona State University, published key results on rocks from the deepest part of a cratonic root that offer an answer to this conundrum.
These researchers analyzed water in samples found in diamond mines of southern Africa, where the Kaapvaal craton was pierced during the Cretaceous era (when dinosaurs roamed) by explosive magmas called kimberlites. These magmas soared through the mantle and crust via deep fractures, bringing with them pieces of the rocks traversed, including diamonds.
The mantle rocks analyzed by Peslier and colleagues were transported from as deep as 200 km below the surface, where they had been since their formation around 3 billion years ago. These rocks are among the deepest and oldest that can be found on Earth.
It has long been suspected that the composition and temperature of the cratons played a crucial role in their survival throughout geological times. A lot of magma was extracted from the cratonic mantle early in Earths history, which removed much of its iron, aluminum and calcium. These depletions make the cratonic roots less dense and enable them to float on the asthenosphere.
Cratons are also relatively cold compared to the asthenosphere, said Lazarov. This provides a stiffness that contributes to their resistance and makes them less likely to be destroyed in the plate tectonic cycles.
Still, with only temperature and buoyancy contributing to their stability, scientists have had a hard time explaining why cratons have survived for so long surrounded by the hot and dynamic asthenosphere.
The water content of the main mineral of the mantle, olivine, is the key to cratonic root survival, said Peslier.
Water is present in the crystal structure of minerals from Earths mantle and it acts to soften the most abundant one, olivine. Peslier and colleagues found that at the very base of the cratons, at the boundary with the asthenosphere, olivines contain hardly any water. That makes these olivines very hard to deform or break up and helps explain why cratonic roots do not get removed by the asthenosphere: their dry olivines make them strong and resistant.
Why the bottom of the cratonic mantle has dry olivines remains a matter of speculation.
The peculiar chemical and physical conditions at these high pressures may render the fluids present at these depths rich in methane instead of water, said Woodland.
Bell suggests that melts generated in the asthenosphere may pick up water while passing through the base of the cratonic root and bring it to the overlying shallow mantle. Knowing how much water is present deep in terrestrial planets and moons, like Mars or Earth or its moon, is crucial to understanding their history, dynamics and volcanism.
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