12:30 〜 14:00
[R2-P-11] MORB組成から期待されるAl/Fe含有量をもつブリッジマナイトの結晶化学
キーワード：ブリッジマナイト、MORB、単結晶X線回折、放射光メスバウアー
Bridgmanite (simplified formula MgSiO3) is the most abundant constituent in the Earth’s lower mantle. The effects of the incorporation of Fe and Al into bridgmanite can have a large effect on the physical properties and rheology of the lower mantle. Bridgmanite formed from a mid-ocean ridge basalt (MORB) component of subducting slabs contains larger amounts of Fe and Al than that formed from a pyrolytic composition. This difference in bridgmanite composition can cause a difference in the incorporation mechanism of Fe and Al into the crystal structure between subducting slabs and their surrounding lower mantle. This should cause heterogeneity in the physical properties and rheology of the lower mantle. Elucidating the crystal chemistry of bridgmanite formed from the MORB composition is a key to resolving this issue. The precise crystal chemistry examined employing a single crystal is, therefore, significant for gaining a detailed understanding of lower-mantle dynamics. In particular, the use of 57Fe-Mössbauer spectroscopy is indispensable for distinguishing the valence and spin states of Fe, which cannot be directly observed by X-ray diffraction. For this purpose, we characterize Mg0.662Fe0.338Si0.662Al0.338O3 bridgmanite single-crystal, with the Fe and Al contents expected in MORB, by a combination of single-crystal X-ray diffraction, synchrotron 57Fe-Mössbauer spectroscopy conducted at SPring-8 BL10XU, and electron probe microanalysis.
The present study reveals that the charge-coupled substitution AMg2+ + BSi4+ <---> AFe3+(high-spin) + BAl3+ is predominant in the incorporation of Fe and Al into the practically eightfold-coordinated A-site and the sixfold-coordinated B-site in bridgmanite structure. The incorporation of both cations via this substitution enhances the structural distortion due to the tilting of BO6 octahedra, yielding the unusual expansion of mean <A-O> bond-length due to flexibility of A-O bonds for the structural distortion, in contrast to mean <B-O> bond-length depending reasonably on the ionic radius effect. Moreover, we imply the phase transition behavior and the elasticity of bridgmanite in slabs subducting into deeper parts of the lower mantle, in terms of the relative compressibility of AO12 (practically AO8) and BO6 polyhedra.
The present study reveals that the charge-coupled substitution AMg2+ + BSi4+ <---> AFe3+(high-spin) + BAl3+ is predominant in the incorporation of Fe and Al into the practically eightfold-coordinated A-site and the sixfold-coordinated B-site in bridgmanite structure. The incorporation of both cations via this substitution enhances the structural distortion due to the tilting of BO6 octahedra, yielding the unusual expansion of mean <A-O> bond-length due to flexibility of A-O bonds for the structural distortion, in contrast to mean <B-O> bond-length depending reasonably on the ionic radius effect. Moreover, we imply the phase transition behavior and the elasticity of bridgmanite in slabs subducting into deeper parts of the lower mantle, in terms of the relative compressibility of AO12 (practically AO8) and BO6 polyhedra.