Geological Society of America

T288 Gemological Research in the 21st Century—Gem Minerals and Localities

Wednesday, 25 September 2019
1:30 PM - 5:30 PM
Room 132ABC, North Building

FRAGMENTS SAMPLED AT EARTH'S TRANSITION ZONE AND LOWER MANTLE BOUNDARY BY A TYPE IAB DIAMOND

Paper number: T288-2
Program: Technical Sessions
Time: 1:45 PM – 2:00 PM

Authors

Tingting Gu Presenting Author
Gemological Institute of America, New York, NY
Martha G. Pamato
University of Padova, Padova
Davide Novella
University of Cambridge, Cambridge
Fabrizio Nestola
University of Padova, Padova
Matteo Alvaro
University of Pavia, Pavia, VA
John H. Fournelle
University of Wisconsin, Madison, WI
Wuyi Wang
Gemological Institute of America, New York, NY

Abstract

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 transition^1-4, in a pyrolitic mantle, or chemical stratification^5-7, 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 (Mg_0.93Fe_0.07SiO₃) and ferropericlase (Mg_0.84Fe_0.16O) in equilibrium with ringwoodite, supporting the breakdown of ringwoodite at the 660 km discontinuity. The presence ofH in the ringwoodite inclusion provides definitive confirmations of the deep-water cycle and the hydrous nature of the TZ.

References

Ringwood, A. E. Composition and Petrology of the Earth’s Mantle. (McGraw-Hill, London, New York and Sydney, 1975).
Ito, E. & Takahashi, E. J. Geophys. Res. Solid Earth 94, 10637–10646 (1989).
Phipps Morgan, J. & Shearer, P. M. Nature 365, 506–511 (1993).
van der Hilst, R. D., Widiyantoro, S. & Engdahl, E. R. Nature 386, 578–584 (1997).
Allègre, C. J., Staudacher, T., Sarda, P. & Kurz, M. Nature 303, 762–766 (1983).
Hofmann, A. W. Nature 385, 219–229 (1997).
Ita, J. & Stixrude, L. J. Geophys. Res. 97, 6849–19 (1992).

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-335372

© Copyright 2019 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.