R2: Crystal structure, crystal chemistry, physical properties of minerals, crystal growth and applied mineralogy

[R2-P-11] Crystal chemistry of bridgmanite with Al/Fe contents expected in MORB component

*Akihiko NAKATSUKA¹, Hiroshi FUKUI², Seiji KAMADA³, Naohisa HIRAO², Makio OHKAWA⁴, Kazumasa SUGIYAMA⁵, Takashi YOSHINO⁶ (1. Yamaguchi Univ. Sci. Tech. Innov., 2. JASRI, 3. AD Science Inc., 4. Hiroshima Univ. Adv. Sci. Eng., 5. Tohoku Univ. IMR, 6. Okayama Univ. IPM)

Keywords:Bridgmanite, MORB, Single-crystal X-ray diffraction, Synchrotron Mössbauer

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 Mg_0.662Fe_0.338Si_0.662Al_0.338O₃ 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.