How iron isotopes were distributed in Earth’s deep interior 4.5 bln yrs ago

June 16th, 2009 - 2:59 pm ICT by ANI  

Washington, June 16 (ANI): Two geochemists have produced the first picture of how different isotopes of iron were initially distributed in the solid Earth 4.5 billion years ago, opening the door to new studies of planet’s geologic history.

The picture was produced by the two UC (University of California) Davis geochemists by using a super-computer to virtually squeeze and heat iron-bearing minerals under conditions that would have existed when the Earth crystallized from an ocean of magma to its solid form 4.5 billion years ago.

Sandwiched between Earth’s crust and core, the vast mantle accounts for about 85 percent of the planet’s volume.

One source of information providing insight into the physics of this viscous mass are the four stable forms, or isotopes, of iron that can be found in rocks that have risen to Earth’s surface at mid-ocean ridges where seafloor spreading is occurring.

Geologists suspect that some of this material originates at the boundary between the mantle and the core some 1,800 miles beneath the surface.

“Geologists use isotopes to track physico-chemical processes in nature the way biologists use DNA to track the evolution of life,” said Qing-zhu Yin, an associate professor of geology, UC Davis.

Because the composition of iron isotopes in rocks will vary depending on the pressure and temperature conditions under which a rock was created, in principle, geologists could use iron isotopes in rocks collected at hot spots around the world to track the mantle’s geologic history.

But in order to do so, they would first need to know how the isotopes were originally distributed in Earth’s primordial magma ocean when it cooled down and hardened.

As a team, Yin and James Rustad, a Chancellor’s fellow and professor of geology, UC Davis, faced the challenge of determining how the competing effects of extreme pressure and temperature deep in Earth’s interior would have affected the minerals in the lower mantle.

Using Rustad’s powerful 144-processor computer, the two calculated the iron isotope composition of two minerals under a range of temperatures, pressures and different electronic spin states that are now known to occur in the lower mantle.

The two minerals, ferroperovskite and ferropericlase, contain virtually all of the iron that occurs in this deep portion of the Earth.

These calculations were so complex that each series Rustad and Yin ran through the computer required a month to complete.

In the end, the calculations showed that extreme pressures would have concentrated iron’s heavier isotopes near the bottom of the crystallizing mantle.

“It will be a eureka moment when these theoretical predictions are verified one day in geological samples that have been generated from the lower mantle,” Yin said. (ANI)

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