Results challenge core knowledge
15 Oct 2007 by hjcmtbbcwq.d hjcmtbbcwq.d
Scientists have discovered that the Earth’s core behaves very differently than predicted by models in the first experiments able to mimic the crushing, searing conditions found in Earth’s lower mantle.
Scientists have discovered that the Earth’s core behaves very differently than predicted by models in the first experiments able to mimic the crushing, searing conditions found in Earth’s lower mantle.
Using a laser-heated diamond anvil cell to heat and compress samples, scientists from the Carnegie Institution’s Geophysical Laboratory in the US subjected ferropericlase - the iron laden material found in the lower mantle - to almost 940,000 atmospheres and 1726°C.
Co-author of the paper, Viktor Struzhkin said: “Under these extreme conditions, the atoms and electrons of the rocks become squeezed so close together that they interact very peculiarly. In fact, spinning electrons in iron, which is prevalent throughout the inner Earth, are forced to pair up. When this spin state changes the density, sound velocities, conductivity, and other properties of the materials can change. Understanding these conditions helps scientists piece together the complex puzzle of the interior/surface interactions.”
In the new study both spin states of iron - unpaired electrons (called a high-spin state) and paired electrons (a low-spin state) - coexisted in the same crystal structure. The spin transition was also continuous over a large pressure range, indicating that the mineral is in a complex state over a large range in the deep core.
“We were expecting to find a transition zone, but did not know how extended it may be in the Earth’s mantle,” said Struzhkin. “Our findings suggest that there is a region or ‘spin-transition zone’ from about 620 miles to 1,365 miles deep, where high spin, unpaired electrons, transition to low spin, paired electrons. The transitioning appears to be continuous over these depths. At pressures representing a lower depth of about 1,365 miles the transition stops and ferropericlase is dominated by low-spin electrons.”
The existence of this transition zone may also account for seismic-wave behaviour at those depths. The fact that the lowermost area is dominated by denser low-spin material could also affect the temperature stability of mantle upwellings - the generators of volcanic hotspots, such as those in Hawaii.
