The stability and density of Fe-bearing akimotoite and implications for subduction

Akimotoite (Mg,Fe)SiO3 is a stable mineral in the Earth's transition zone and uppermost lower mantle [1-2]. Previous studies showed that akimotoite is a major component of subducted harzburgite, which contains less than 1 wt.% Al2O3 and may be several hundreds of degrees colder than the ambient mantle [1, 2]. The garnet-akimotoite and akimotoite-bridgmanite phase transitions are associated with steep density and sound velocity increases and may be responsible for the multiple seismic discontinuities near the 660 km depth [1, 2]. Moreover, akimotoite is strongly anisotropic and its temperature-dependent crystallographic preferred orientation pattern has been used to explain the seismic anisotropy of the Tonga slab [3]. To date, most studies have focused on MgSiO3 akimotoite, without considering the influences of iron on its stability and density. We have studied the pressure-temperature stability field of Fe3+-bearing akimotoite and the partitioning of iron among akimotoite and coexisting mantle minerals using a multi-anvil apparatus up to 25 GPa. To determine its equation of state and the effect of pressure on its c/a axial ratio, we conducted powder and single-crystal synchrotron XRD measurements on iron-rich akimotoite using diamond anvil cells. At the pressures of the 660 km depth ( 24 GPa), akimotoite is enriched in ferric iron with a partitioning coefficient between akimotoite and bridgmanite (DFeAk/Bdg) approaching 8. The Fe3+-rich akimotoite is stable for a wider range of pressures than MgSiO3 akimotoite [1], whereas FeSiO3 is not expected to occur in the structure of akimotoite between 1 bar and 25 GPa [4]. Our new data suggest that Fe3+-bearing akimotoite may play an important role in subduction, with implications for subduction regions such as the Pacific plate beneath southern California and the Tonga slab, where multiple discontinuities were detected near the 660 km depth [5, 6]. References1 M. Akaogi, A. Tanaka, E. Ito. Phys Earth Planet Int 135, 291 (2003).2 Y. Wang, T. Uchida, J. Zhang, M. Rivers, S. Sutton. Phys Earth Planet Int 143, 57 (2004).3 R. Shiraishi, E. Ohtani, K. Kanagawa, A. Shimojuku, D. Zhao. Nature 455, 657 (2008).4 L. Stixrude, C. Lithgow-Bertelloni. Geophys. J. Int. 184, 1180 (2011).5 F. Niu, H. Kawakatsu. J Phys Earth 44, 701 (1996).6 V. Vavrycuk. Phys Earth Planet Int 155, 63 (2006).