Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmmr23a..01o&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #MR23A-01
Other
1025 Composition Of The Mantle, 3621 Mantle Processes (1038), 3630 Experimental Mineralogy And Petrology, 3924 High-Pressure Behavior, 8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105)
Scientific paper
Recent experimental and theoretical studies revealed that there are spin transitions in the lower mantle minerals. Spin transition in ferropericlase occurs at 40-50 GPa at room temperature and broadening of the transition was reported at high temperature both theoretically and experimentally. Spin transition in perovskite is complicated due to existence of both ferrous and ferric iron in its structure. The spin transition pressures in perovskite varies in ferric and ferrous irons [1, 2, 3]; ferric iron shows a wide pressure interval of spin transition between about 60-160 GPa, whereas ferrous iron is in a high spin state at all mantle pressures. The theoretical studies revealed that ferric iron on the Mg-site in post-pervskite is in the high-spin state [2], and ferrous iron also remains in a high-spin state in post-perovskite at all mantle pressures [3]. These complex features of the spin transitions of the lower mantle minerals might affect the Mg-Fe partitioning behaviors of the lower mantle minerals. We have studied the Mg-Fe partitioning between ferropericlase and perovsksite and ferropericlasse, and post-perovskite and ferropericlase to the pressure of the base of the lower mantle at 2000 K . The Mg-Fe partition coefficient between Al-free perovskite and ferropericlse, KD (Pv/Fp) =(FeO/MgO) Pv /(FeO/MgO) Fp is around 0.25 at around 40-70 GPa and decreases to 0.1 at around 100 GPa. We observed a compositional dependence in KD(Pv/Fp) at 75 GPa, i.e., a gentle decrease of the partition coefficient with increasing bulk FeO content in the system, which is consistent with the compositional dependency reported by Katsura et al. at 23 GPa. The compositional and pressure dependencies of the partition coefficient of Al-free perovskite and ferropericlase are generally consistent with the partition coefficients reported by previous authors [4, 5, 6]. We also determined the Mg-Fe partition coefficient KD(PPv/Fp) between Al-free post-perovskite and ferropericlase at about 150 GPa and 2000 K. FeO is preferentially partitioned to ferropericlase, and KD (PPv/Fp) shows a strong compositional dependency, i.e., it decreases from about 0.7 to 0.1 with increasing bulk iron contents. The partitioning coefficient is consistent with the previous works by Kobayashi et al.[4], Murakami et al.[5], and Auzende et al. [6] by taking into account of the strong compositional dependency. The present observation of the pressure dependency of KD (Pv/Fp) is consistent with the spin transition in ferropericase occurring in the broad pressure range from 50-100 GPa at high temperature. On the other hand, the effect of the spin state in perovskite and post-perovskite phase on KD is not well understood due to the complex nature of the spin states in ferric and ferrous irons in these structures. [1] Li et al., 2006. Phys. Chem. Minerals. 33:575. [2] Zhang and Oganov, 2006. Earth Planet. Sci. Lett. 249, 436. [3] Stackhouse et al., 2007. Earth Planet. Sci. Lett. 253, 282. [4] Kobayashi et al., 2007. Earth Planet. Sci. Lett. 260, 564. [5] Murakami et al., 2005. Geophys. Res. Lett. 32, L03304, doi: 10.1029/2004GL021956. [6] Auzende et al., 2008. Earth Planet. Sci. Lett., 269, 164.
Ohtani Eiji
Sakai Tôru
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