Sharp 660-km discontinuity controlled by extremely narrow binary post-spinel transition

Abstract

The Earth's mantle is characterised by a sharp seismic discontinuity at a depth of 660-km that can provide insights into deep mantle processes. The discontinuity occurs over only 2km -or a pressure difference of 0.1 GPa -and is thought to result from the post-spinel transition, that is, the decomposition of the mineral ringwoodite to bridgmanite plus ferropericlase. Existing high-pressure-temperature experiments have lacked the pressure control required to test whether such sharpness is the result of isochemical phase relations or chemically distinct upper and lower mantle domains. Here, we obtain the isothermal pressure interval of the Mg-Fe binary post-spinel transition by applying advanced multianvil techniques with in situ X-ray diffraction with help of Mg-Fe partition experiments. It is demonstrated that the interval at mantle compositions and temperatures is only 0.01 GPa, corresponding to 250 m. This interval is indistinguishable from zero at seismic frequencies. These results can explain the discontinuity sharpness and provide new support for whole mantle convection in a chemically homogeneous mantle. The present work suggests that distribution of adiabatic vertical flows between the upper and lower mantles can be mapped based on discontinuity sharpness. Main:The 660-km seismic discontinuity (D660) is the boundary between the upper and lower mantles. Seismological studies based on short-period P-wave reflections (Pʹ660Pʹ-PʹPʹ) have demonstrated that D660 is extremely sharp and less than 2 km thick 1 , which is in striking contrast to the 7-km-thick 410-km discontinuity 1 . Understanding the nature of D660 from a perspective of mineral physics provides important clues to open questions about the structure and dynamic processes in the Earth's mantle, such as slab subduction and upwelling of hot plume.Geochemical studies suggest that the Earth's upper mantle consists of ca. 60% atom-

Segregated oceanic crust trapped at the bottom mantle transition zone revealed from ambient noise interferometry

“…7a, b ). One plausible conclusion is that on average a sharper 660-km discontinuity exists beneath this region in line with mineralogical prediction 29 , 30 , compared with the 410-km discontinuity. A consistent conclusion was also obtained through fitting the waveform of P410P and P660P in NCFs 17 , which agrees with Lawrence and Shearer 31 who suggest a ~3 times thicker 410-km discontinuity relative to the 660-km discontinuity.…”

Sharpness of the paired 660-km discontinuity beneath the Izu-Bonin area

“…The sharp 660-km discontinuity seen in this study is consistent with an early work by Castle and Creager (2000) and a recent work by Zhang M et al (2019), which suggest that the discontinuity is at most 10 km thick, and it is marginally consistent with a previous Sto-P conversion study that concluded the discontinuity is ≤5 km thick (Yamazaki and Hirahara, 1994). A sharp 660-km discontinuity resulting from the post-spinel transition was also revealed in a recent mineral physics experiment by Ishii et al (2019). The broadened 660-km discontinuity observed before (e.g., Wang BS and Niu FL, 2010;Li J et al, 2013) was explained by the multiple phase transitions associated with dissolution of the olivine and garnet components (Wang BS and Niu FL, 2010), whereas the sharp 660-km discontinuity observed in this study referred only to the phase change of the olivine component.…”

Effect of Water on Lattice Thermal Conductivity of Ringwoodite and Its Implications for the Thermal Evolution of Descending Slabs

“…The water‐bearing phase in a slab located in the MTZ are expected to consist of nominally anhydrous minerals (e.g., wadsleyite and ringwoodite) and dense hydrous magnesium silicates (DHMS, such as superhydrous Phase B or Phase D) (Ohtani et al, 2004). Once ringwoodite reaches the base of the MTZ, it decomposes into LM assemblage (e.g., Ishii et al, 2018; Ishii, Huang, et al, 2019; Ishii, Kojitani, & Akaogi, 2019; Litasov et al, 2005), which is expected to host only ≈1,000 ppm of water (Fu et al, 2019; Litasov et al, 2003). Due to the release of water around 660‐km depth, this decomposition most likely causes major slab dehydration (Schmandt et al, 2014).…”