GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 288-1
Presentation Time: 1:45 PM


GU, Tingting1, PAMATO, Martha G.2, NOVELLA, Davide3, NESTOLA, Fabrizio2, ALVARO, Matteo4, FOURNELLE, John H.5 and WANG, Wuyi1, (1)Gemological Institute of America, 50 west 47 Street, New York, NY 10036, (2)Department of Geosciences, University of Padova, Padova, 35131, Italy, (3)Department of Earth Sciences, University of Cambridge, Cambridge, CB3 0EZ, United Kingdom, (4)Department of Earth and Environmental Sciences, University of Pavia, Pavia, VA 27100, Italy, (5)Department of Geoscience, University of Wisconsin, Madison, WI 53706

The transition zone (TZ) - lower mantle boundary at 660 km depth in the Earth’s interior controls the transfer of mass and heat within the planet and, in turn, global dynamics and the evolution of the planet. However, the exact origin of this discontinuity is still debated, raising fundamental doubts on the mineralogy, chemistry and convective style of the deep mantle. Indirect experimental, seismological and geochemical constraints have provided evidence in support of a phase transition1-4, in a pyrolitic mantle, or chemical stratification5-7, in a compositionally layered mantle, as the cause of the 660 km discontinuity. Natural samples from extreme depths within the Earth’s TZ and lower mantle are extremely rare but remain the only direct way to unravel the nature of the 660 km boundary. Here we report the discovery of a polyphasic inclusion composed of hydrous ringwoodite + ferropericlase + enstatite (MgSiO3) within a type IaB gem diamond from the Karowe mine (Botswana). Considering enstatite as the product of the back-transformation from bridgmanite, this discovery provides a unique fragment of the TZ - lower mantle boundary sampled at depths of approximately 660 km. For the first time, to our knowledge, we show the direct evidence of a pyrolytic1 composition at the top of the lower mantle, with bridgmanite (Mg0.93Fe0.07SiO3) and ferropericlase (Mg0.84Fe0.16O) in equilibrium with ringwoodite, supporting the breakdown of ringwoodite at the 660 km discontinuity. The presence of H in the ringwoodite inclusion provides definitive confirmations of the deep-water cycle and the hydrous nature of the TZ.


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