HR: 1340h
AN: MR23A-0048    [Abstracts]
TI: Effects of Fe spin transition in the Earth's lower mantle
AU: * Speziale, S
EM: speziale@uclink.berkeley.edu
AF: University of California, Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AU: Lee, V E
EM: vlee@eps.berkeley.edu
AF: University of California, Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AU: Clark, S M
EM: SMClark@lbl.gov
AF: Lawrence Berkeley National Laboratory, Advanced Light Source, Lawrence Livermore National Laboratory, Berkeley, CA 94720 United States
AU: Pasternak, M P
EM: moshepa@post.tau.ac.il
AF: Tel Aviv University, School of Physics and Astronomy, Tel Aviv University, Tel Aviv, 69978 Israel
AU: Jeanloz, R
EM: jeanloz@berkeley.edu
AF: University of California, Department of Earth and Planetary Science, University of California, Berkeley, CA 94720-4767 United States
AB: Knowing the properties of the Earth's lower-mantle minerals is crucial for interpreting seismological and other geophysical observations, and hence understanding the constitution, state and evolution of this region that makes up the bulk of our planet's interior. The behavior of Fe, the most abundant transition element, is especially important at deep-Earth conditions, with past work predicting that it should collapse in size due to a transition from high-spin (HS) to low-spin (LS) configurations at mantle pressures. Recent experimental observations of the Fe spin transition in both (Mg,Fe)SiO₃-perovskite and ferropericlase (Mg,Fe)O at high-pressure impact our understanding of the stability, chemical partitioning and transport properties of the two most abundant minerals of the Earth's mantle. We focus on the structural and density effect of the high-spin to low-spin transition in ferropericlase (Mg_1-x,Fe_x)O by performing high-pressure x-ray diffraction experiments on compositions (x < 0.25) relevant to the lower mantle. We obtain high-resolution measurements by simultaneously monitoring the unit-cell volumes of our target sample along with those of a different composition of (Mg,Fe)O that shows the spin transition at higher pressures. Our new results, compared with previous M”ssbauer data on a large range of (Mg,Fe)O compositions, confirm that the transition is gradual with pressure at room temperature, and it involves an overall 3 ± 1% volume decrease over a pressure range starting at 40 GPa and extending up to as much as 80 GPa. By combining our results with those of independent studies of comparable compositions [Lin et al., Science, 2005], we infer a 6 ± 1% increase of the bulk sound velocity, which could cause visible seismic anomalies in regions of the lower mantle. In addition to the change in density at the spin transition, more fundamental issues emerge from our and other groups' results. The coexistence of ^HSFe and ^LSFe species poses questions about the interpretation of the Mg-Fe-O phase diagram. In fact, at relevant pressures this system is more correctly defined by four components, including both the Fe species having different spin configurations. If we then consider the coexisting (Mg,Fe)SiO₃, the effects of the electronic transition on Fe partitioning, including the possible segregation of independent ^LSFe enriched phases, open the possibility that the phase diagram and trace-element partitioning of the lower mantle are far richer then previously recognized.
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3900 MINERAL PHYSICS
DE: 3909 Elasticity and anelasticity
DE: 3924 High-pressure behavior
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005