HR: 17:00h
AN: MR14A-05    [Abstracts]
TI: Spin Transition in Earth's Lower Mantle: (Mg$_{x}$Fe$_{1 - x}$)O Magnesiow\"{u}stite at High Pressures
AU: Milner, A
EM: moshepa@post.tau.ac.il
AF: School of Physics and Astronomy, Tel Aviv University, Tel Aviv, CA 69973 Israel
AU: Pasternak, M
EM: moshepa@post.tau.ac.il
AF: School of Physics and Astronomy, Tel Aviv University, Tel Aviv, CA 69973 Israel
AU: Lee, V E
EM: vlee@eps.berkeley.edu
AF: Dept. Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AU: Speziale, S
EM: speziale@uclink.berkeley.edu
AF: Dept. Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AU: * Jeanloz, R
EM: jeanloz@uclink.berkeley.edu
AF: Dept. Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AB: New experiments confirm W. S. Fyfe's prediction of 45 years ago that Fe$^{2+}$ should undergo a pressure-induced electronic transition deep in the Earth's mantle, from high-spin (paramagnetic) to low-spin (diamagnetic) states. We used $^{57}$Fe M\"{o}ssbauer ($\gamma$-ray absorption) spectroscopy and powder x-ray diffraction with gasketed diamond-anvil cells to characterize the (Mg, Fe)O solid-solution at elevated pressures, because high pressure - temperature experiments on peridotitic (upper mantle-like) bulk compositions indicate that Mg$_{0.80}$Fe$_{0.20}$O magnesiow\"{u}stite is likely the second-most abundant mineral of the Earth's lower mantle. Prior work shows that the Fe$_{0.94}$O w\"{u}stite endmember loses its magnetic moment above $\sim$ 70 to 140 GPa (onset to completion) at 300 K. Because the magnetic-ordering (N\'{e}el) temperature of w\"{u}stite exceeds 300 K above 20 GPa, and increases with increasing pressure as expected, the observed collapse of magnetism offers direct evidence that the iron ion has transformed to the diamagnetic (low-spin) state at deep-mantle pressures. Similar experiments conducted on Mg$_{0.20}$Fe$_{0.80}$O, Mg$_{0.50}$Fe$_{0.50}$O and Mg$_{0.80}$Fe$_{0.20}$O compositions show that the high- to low-spin transition begins at lower pressure with increasing Mg content: 80 ($\pm10$), 60 ($\pm10$) and 40 ($\pm10$) GPa, respectively, are required for inducing significant transformation at 300 K (the corresponding value for w\"{u}stite is 90 $\pm10$ GPa). The variation in transition pressure correlates with the composition-dependence of the (Mg, Fe)O lattice parameter, such that the spin change occurs for all compositions when the metal-oxygen bond length is reduced to 197 ($\pm2$) pm. In the high-spin state, all the magnesiow\"{u}stites we studied were found to be antiferromagntically ordered at low temperatures (zero-pressure N\'{e}el temperatures of 190 K, 140 K, 80 K and 25 K, respectively, for Mg/(Mg + Fe) ratios of 0, 0.20, 0.50 and 0.80), and it is the appearance of a non-magnetic site under pressure that signals the onset of the spin transition. Contrary to prior expectations, there is no evidence of any significant change in volume or bulk modulus across the Fe-spin transition in magnesiow\"{u}stite. Therefore, this important bonding change may cause little if any seismological anomaly, and its occurrence deep in the Earth's mantle can only be inferred through the combination of laboratory experiment and geophysical observation.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7207 Core and mantle
DE: 3924 High-pressure behavior
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 1025 Composition of the mantle
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting