Velocity Changes Across the High-Spin Low-Spin Transition in Ferropericlase at High Pressures up to 61 GPa
Abstract
An electronic transition in Fe, from a high-spin state to low-spin state is promoted by pressure and has been observed in the Fe-bearing lower mantle minerals ferropericlase and silicate perovskite. This phase transition is accompanied by a volume change and presumably by changes in the elastic properties. Previous light scattering measurements have suggested that there is softening of the bulk and shear moduli in the mixed phase region across the HS-LS transition in ferropericlase. The softening was reported to be evident as decreased Vp and Vs within the mixed phase region, but has thus far not been verified by independent experiments. The electronic spin transition of iron in FP can occur above 40 to 90 GPa at room temperature, depending on the Fe content. Its effect on physical properties may profoundly affect our current understanding of lower mantle average composition and heterogeneity. Al will be incorporated into ferropericlase in realistic lower-mantle mineral assemblages, and could cause significant changes of its physical properties. However, the effect of Al on the HS-LS transition and properties of ferropericlase are unknown. We have performed Brillouin scattering experiments on Al-bearing ferropericlase at high pressures up to 61GPa. At ambient condition, incorporation of aluminum into ferropericlase results in a slight drop in the shear modulus, and no observed change in bulk modulus. Our results show a "softening" effect of the spin transition on the bulk modulus of Al-bearing ferropericlase at pressures over 40 GPa. This suggests that the incorporation of aluminum increases the pressure of spin transition. Our high pressure single-crystal velocity data shows that softening is most pronounced in the elastic modulus C12, which decreases significantly within transition region with mixed spin states. C11 is also likely to soften, although the effect appears to be less pronounced than for C12, whereas C44 does not display any softening within the pressure range of our observations. The shear modulus and Vs do not soften across the transition, within the resolution of our measurements.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2009
- Bibcode:
- 2009AGUSMDI73A..08B
- Keywords:
-
- 1040 Radiogenic isotope geochemistry;
- 1212 Earth's interior: composition and state (7207;
- 7208;
- 8105;
- 8124);
- 7207 Core (1212;
- 1213;
- 8124);
- 7208 Mantle (1212;
- 1213;
- 8124);
- 8105 Continental margins: divergent (1212;
- 8124)