The Earth's mantle is characterized by an abrupt velocity increase at a depth of 660-km, referred to as the 660-km discontinuity. Mineral physics shows that ringwoodite decomposes to bridgmanite plus ferropericlase around a pressure corresponding to the 660-km depth, referred to as the post-spinel transition. It has been therefore considered that the 660-km discontinuity is caused by the post-spinel transition.
One of the prominent feature of the 660-km discontinuity is that it occurs over an interval less than 2 km only, corresponding to 0.1 GPa in pressure. This extreme sharpness was not able to be duplicated by any laboratory experiment. One reason is that the pressure resolutions in previous studies were limited to 0.2~0.5 GPa. Another is that sample pressures dropped more than 2 GPa even with keeping temperature and press load. We developed high-precision pressure measurement to 0.05 GPa by in situ X-ray diffraction, and attempted to keep sample pressures constant by finely controlling press load. By combining such state-of-the-art high-pressure experiments and thermochemical calculation, we have demonstrated that the pressure interval is only 0.01 GPa, corresponding to the discontinuity thickness of 250 m (Fig. 1.).
These results are more than enough to interpret the discontinuity sharpness, and provide new support for whole mantle convection in a chemically homogeneous pyrolitic mantle. The present work also propose a new method to detect flows between the upper and lower mantles based on discontinuity sharpness, because the interval should increase by a rapid flow across the 660-km discontinuity.
Fig. 1. Phase relations in the system Mg2SiO4-Fe2SiO4 at a temperature of 1700 K. Brg: bridgmanite, fPc: ferropericlase, Rw: ringwoodite, St: stishovite. At the mantle composition, a width of the Rw + Brg + fPc loop is only 0.01 GPa in pressure, corresponding to 250 m in depth. This width becomes even smaller at the mantle temperature of 2000 K. The horizontal axis is the fraction of Mg2SiO4 component in percent.
Takayuki Ishii, Rong Huang, Robert Myhill, Hongzhan Fei, Iuliia Koemets, Zhaodong Liu, Fumiya Maeda, Liang Yuan, Lin Wang, Dmitry Druzhbin, Takafumi Yamamoto, Shrikant Bhat, Robert Farla, Takaaki Kawazoe, Noriyoshi Tsujino, Eleonora Kulik, Yuji Higo, Yoshinori Tange, Tomoo Katsura, 2019. Sharp 660-km discontinuity controlled by extremely narrow binary post-spinel transition, Nature Geoscience, 12, https://doi.org/10.1038/s41561-019-0452-1
Strictly necessary cookies guarantee functions without which this website would not function as intended. As a result these cookies cannot be deactivated. These cookies are used exclusively by this website and are therefore first party cookies. This means that all information stored in the cookies will be returned to this website.
Functional cookies enable this website to provide you with certain functions and to store information already provided (such as registered name or language selection) in order to offer you improved and more personalized functions.
Performance cookies gather information on how a web page is used. We use them to better understand how our web pages are used in order to improve their appeal, content and functionality.
Marketing / Third Party Cookies originate from external advertising companies (among others) and are used to gather information about the websites visited by you, in order to e.g. create targeted advertising for you.