In situ X-ray diffraction of pyrolite to 40 GPa using Kawai-type apparatus with sintered diamond anvils: possibility for the existence of iron-rich metallic particles in the lower mantle

Abstract

We investigated phase relations in pyrolite at ∼33–40 GPa and ∼1800–2150 K by in situ X-ray diffraction using Kawai-type apparatus with sintered diamond anvils. The results demonstrated that MgSiO₃-rich orthorhombic perovskite (mpv), CaSiO₃-rich cubic perovskite (cpv) and (Mg,Fe)O ferropericlase (fp) are the stable phases in pyrolite bulk composition at the conditions corresponding to the lower mantle. However, chemical analyses of a run product recovered from ∼ 34 GPa by an analytical transmission electron microscope showed the coexistence of metallic iron particles with mpv, fp, and SiO₂-rich amorphous phase. Also, Fe/Mg partitioning coefficient between mpv and fp was found to be 0.66(31), which is consistent with previous results for pyrolite bulk composition at 26–30 GPa and ∼1900 K. These results indicate that iron-rich metallic particles can exist in the lower mantle as a stable phase to the depth of at least ∼900 km.

Keywords:

• pyrolite
• lower mantle
• iron
• perovskite
• in situ X-ray diffraction

Acknowledgements

We thank Sho Yokoshi, Hitoshi Yamada, Yoshitaka Aizawa, and Takashi Yoshino for experimental assistance. We also thank Chikako Oka and Yukari Shimizu for technical assistance, and Shun-ichiro Karato for discussion. Comments from an anonymous reviewer greatly helped in improving the manuscript. In situ X-ray diffraction experiments were conducted at SPring-8 (No: 2001B0304-ND-np, 2002A0149-CD2-np, 2002B0045-CD2-np, 2003A0585-ND2-np). This work was financially supported by Research Fellowships (to A.K.) from the Japan Society for Promotion of Science for young scientists.