Spin Transition Zone in Earth’s Lower Mantle

Mineral properties in Earth’s lower mantle are affected by the electronic states of iron, but these states have not yet been probed in the laboratory at the relevant pressures and temperatures until now. Livermore scientists, in collaboration with colleagues at several institutions in the U.S. and Europe, measured the spin states of iron in the mineral ferropericlase up to 95 gigapascals (GPa) and 2,000 kelvins. The team found that a gradual spin transition of iron occurs over a pressure-temperature range that corresponds to a zone in the lower mantle from about 1000 kilometers in depth and 1,900 kelvin to 2,200 kilometers and 2,300 kelvins. Because low-spin ferropericlase exhibits higher density and faster sound velocity relative to the high-spin form, the observed increase in the low-spin mineral at these conditions would manifest itself seismically in Earth’s lower-mantle as a spin transition zone characterized by a steeper-than normal density gradient.

The experiments were conducted at the GeoSoilEnviro Consortium for Advanced Radiation Sources (GSECARS) sector of the Advanced Photon Source at Argonne National Laboratory. The samples had a composition of (Mg0.75,Fe0.25) O and measured ~12 micrometers thick and 70 micrometers in diameter. Samples were loaded into diamond anvil cells (DACs) with beryllium gaskets. Dried sodium chloride layers acted as thermal insulators between the sample and diamond anvils, as well as the pressure medium and the pressure calibrant. A near-infrared laser beam was used to heat the sample from both sides of the DAC. A monochromatic x-ray beam with energy of 14 kiloelectronvolts was used to probe the samples. The x-ray emission spectra (XES) of the Kβ fluorescence of iron were collected as a function of pressure and temperature up to 95 GPa and 2,000 kelvins, respectively. X-ray diffraction patterns were collected from some of the samples before, during, and after laser heating at high pressures.

The researchers performed an integrated absolute difference analysis of the x-ray emission spectra to determine the ratio of the high-spin and low-spin states in the sample. The derived fractions of the high-spin state were used to construct the spin-crossover phase diagram of iron in ferropericlase. Whereas the XES results revealed an electronic spin transition with a mixed population of high-spin and low-spin states of iron at high pressures and temperatures, the x-ray diffraction patterns show that the (Mg0.75,Fe0.25)O sample is structurally stable before, during, and after the laser heating of the DAC, up to 95 GPa and 2,000 kelvins. High temperatures significantly affect the fraction of the high-spin state between 50 and 95 GPa where the spin crossover occurs. The researchers found that the observed spin transition is readily reversible with temperature. However, its width in ferropericlase is much narrower than that predicted by existing theoretical models.

This research, which was published in the September 21, 2007, issue of Science, has led to improved understanding of the properties of Earth’s lower mantle at depths below 2000 kilometers.

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Jung-Fu Lin